Adhesives and adhesive sheets

Abstract

This invention provides an adhesive that balances high refractive index and flexibility. The adhesive has a refractive index of 1.55 or higher and a storage modulus G' (0°C) in the range of 1.0 × 10⁴ Pa to 1.0 × 10⁶ Pa at 0°C.

Description

Technical Field

[0001] This invention relates to adhesives and adhesive sheets. This application claims priority based on Japanese Patent Application No. 2021-127818 filed on August 3, 2021 and Japanese Patent Application No. 2022-061161 filed on March 31, 2022, the entire contents of which are incorporated herein by reference. Prior Technology

[0002] Generally, adhesives (also known as pressure-sensitive adhesives, hereinafter the same) are soft solids (viscoelastics) in a temperature range near room temperature, possessing the property of easily adhering to the substrate by pressure. Utilizing this property, adhesives are widely used in various industries, from household appliances to automobiles, various machinery, electrical equipment, and electronic equipment, for purposes such as bonding, fixing, and protection. One example of the use of adhesives is their application in display devices such as liquid crystal displays or organic EL displays, for bonding polarizing films, retardation films, cover window components, and various other light-transmitting components and other parts. Technical literature on adhesives for optical components can be found in Patent Documents 1 and 2. Previous technical documents Patent documents

[0003] Patent Document 1: Japanese Patent Application Publication No. 2014-169382 Patent Document 2: Japanese Patent Application Publication No. 2017-128732 Summary of the Invention

[0004] The problem the invention aims to solve Patent documents 1 and 2 disclose an adhesive composition based on a (meth)acrylate polymer and an adhesive formed by crosslinking the adhesive composition. The (meth)acrylate polymer contains monomers having a plurality of aromatic rings as monomer units. Furthermore, it proposes using monomers having a plurality of aromatic rings to achieve a refractive index of 1.50 or higher, preferably 1.51 or higher, in the adhesive. For example, it is known that materials for which adhesives can be attached, such as optical components, contain materials with high refractive indices. If a general acrylic adhesive is used in the bonding of these high-refractive-index materials, reflection will occur at the interface due to the difference in refractive indices. By using an adhesive with a high refractive index for bonding such high-refractive-index materials, such interface reflection can be prevented or suppressed. Moreover, the refractive index of acrylic adhesives is typically around 1.47.

[0005] Adhesives, depending on their application and usage, can be chosen based on their flexibility. For example, in recent years, foldable or rollable displays have become practical for organic EL displays used in smartphones and other electronic devices. Therefore, adhesives used in these applications must possess flexibility to accommodate repeatedly bent substrates. Adhesives with excellent flexibility also easily conform to and adhere to curved surfaces such as 3D shapes, making them suitable for applications with curved surfaces in electronic devices. Adhesives with high refractive indices are useful for applications requiring flexibility if their flexibility can be improved. However, high refractive index materials used as adhesive polymer monomers or adhesive additives tend to have high glass transition temperatures, such as aromatic rings, which can reduce the flexibility of adhesives. In adhesive design, high refractive index and flexibility are mutually exclusive.

[0006] The present invention was created in view of the above-mentioned circumstances, with the aim of providing an adhesive that can balance high refractive index and flexibility. Another object of the present invention is to provide an adhesive sheet comprising the above-mentioned adhesive.

[0007] The means to solve the problem According to this specification, an adhesive is provided with a refractive index of 1.55 or higher and a storage modulus of elasticity G' (0°C) in the range of 1.0 × 10⁴ Pa to 1.0 × 10⁶ Pa. Because the adhesive has a high refractive index and the storage modulus of elasticity G' (0°C) is kept within a low range, it achieves a balance between high refractive index and flexibility. The adhesive preferably has a high refractive index suitable for applications such as foldable displays and is suitable for bonding, fixing, and protection applications requiring flexibility to withstand repeated bending operations.

[0008] In several samples, adhesives with a glass transition temperature (Tg) in the range of -50°C to 0°C can be used. Adhesives with a Tg in the range of -50°C to 0°C tend to readily achieve good flexibility.

[0009] In several samples, adhesives with a storage modulus of elasticity G'(0°C) to the storage modulus of elasticity G'(80°C) at 80°C (G'(0°C) / G'(80°C)) in the range of 1 to 1000 can be used. Based on adhesives that satisfy the above characteristics, since changes in the modulus of elasticity over a wide temperature range from 0°C to high temperatures can be suppressed, it is easy to exhibit properties (such as softness) that are stable to temperature changes.

[0010] In several ideal samples, adhesives with a storage modulus of elasticity G'(-10℃) at -10℃ to G'(80℃) at 80℃ (G'(-10℃) / G'(80℃)) in the range of 1 to 1000 can be used. Based on adhesives that satisfy the above characteristics, since changes in the modulus of elasticity over a wide temperature range from low to high temperatures can be suppressed, they easily exhibit stable properties (such as softness) to temperature changes, thus achieving an ideal result.

[0011] Furthermore, according to this specification, an adhesive sheet is provided, comprising an adhesive layer composed of any of the adhesives disclosed herein (which may be an adhesive formed from any of the adhesive compositions disclosed herein). The adhesives disclosed herein are configured to form an adhesive sheet, suitable for use in various forms, such as attachment to components constituting a foldable display.

[0012] In several samples, the thickness of the aforementioned adhesive layer ranges from 5 to 75 µm. The technique disclosed herein is suitable for implementation in samples having an adhesive layer with a thickness of 5 to 75 µm.

[0013] In several samples, the product of the aforementioned storage modulus of elasticity G'(0°C) [Pa] and the aforementioned adhesive layer thickness T [µm] (G'(0°C)×T) is in the range of 5.0×10⁴ to 5.0×10⁷. A parameter (G'(0°C)×T) of 5.0×10⁴ or higher means that a storage modulus of elasticity G'(0°C) in the range of 1.0×10⁴ Pa to 1.0×10⁶ Pa can be used in a thin adhesive layer. On the other hand, a parameter of 5.0×10⁷ or lower means that there is an upper limit to the thickness of the adhesive layer and the storage modulus of elasticity G'(0°C). Based on the adhesive sheet that satisfies the above parameters, excellent flexibility can be achieved due to the thinness of the adhesive layer (thickness below a predetermined value) and the storage modulus of elasticity G'(0°C) limited to the range of 1.0×10⁴ Pa to 1.0×10⁶ Pa.

[0014] In several ideal samples, the total light transmittance of the adhesive sheet is above 85%. As described, the highly transparent adhesive sheet is suitable for applications requiring high light transmittance (e.g., optical applications) or applications where the properties of the adhered object must be clearly visible through the adhesive sheet.

[0015] In several ideal samples, the haze value of the adhesive sheet is below 3%. As described, the highly transparent adhesive sheet is suitable for applications requiring high light transmittance (e.g., optical applications) or applications requiring good visibility of the properties of the adhered object through the adhesive sheet.

[0016] Several types of adhesive sheets exhibit a peel strength (adhesion) of 0.1 N / 25 mm or higher to the glass plate. Adhesive sheets with the above-mentioned adhesion strength are suitable for use in applications where the sheets are attached to a substrate.

[0017] Furthermore, any invention that is formed by properly combining the elements described in this specification may also be included within the scope of the invention for which patent protection is sought under this patent application. Simple Explanation of the Diagram

[0018] Figure 1 is a schematic cross-sectional view showing the composition of an adhesive sheet in one embodiment. Figure 2 is a cross-sectional view schematically showing the composition of the adhesive sheet in another embodiment. Figure 3 is a cross-sectional view schematically showing the composition of the adhesive sheet in another embodiment. Implementation

[0019] The following describes the ideal embodiment of the invention. Matters not specifically mentioned in this specification that are necessary for the implementation of the invention can be understood by those skilled in the art based on the teachings regarding the implementation of the invention as described in this specification and common technical knowledge at the time of application. The invention can be implemented based on the content disclosed in this specification and common technical knowledge in the field. Furthermore, in the following figures, components and parts that perform the same function are sometimes given the same symbols for description, and repeated descriptions are sometimes omitted or simplified. Also, the embodiments shown in the figures are schematic for the purpose of clearly illustrating the present invention and do not completely and accurately represent the dimensions or scale of the actual product provided.

[0020] In this specification, the term "base polymer" in an adhesive refers to the main component of the rubber-like polymer contained in the adhesive. The aforementioned rubber-like polymer refers to a polymer that exhibits rubber elasticity in a temperature range near room temperature. Furthermore, in this specification, unless otherwise specified, "main component" means a component comprising more than 50% by weight.

[0021] In this specification, "acrylic polymer" means a polymer comprising monomer units derived from a monomer having at least one (meth)acrylic group per molecule. Hereinafter, a monomer having at least one (meth)acrylic group per molecule is also referred to as an "acrylic monomer." Therefore, the acrylic polymer in this specification is defined as a polymer comprising monomer units derived from acrylic monomers. A typical example of an acrylic polymer is an acrylic polymer in which more than 50% by weight (preferably more than 70% by weight, for example more than 90% by weight) of the monomer composition is an acrylic monomer.

[0022] Furthermore, in this specification, "acrylic monomer" means a monomer having at least one (meth)acrylic group per molecule. Here, "(meth)acrylic" refers to both acrylonitrile and methacrylic. Therefore, the concept of acrylic monomer as used herein can include both monomers having an acrylonitrile (acrylic monomer) and monomers having a methacrylic group (methacrylic monomer). Similarly, in this specification, "(meth)acrylic acid" refers to both acrylic acid and methacrylic acid, while "(meth)acrylate" refers to both acrylate and methacrylate. Other similar terms are also used in the same way.

[0023] Properties of Adhesives (Refractive index) The adhesive disclosed herein is characterized by a refractive index of 1.55 or higher. A refractive index of 1.560 or higher is suitable for the aforementioned adhesive, and preferably greater than 1.570. In several applications, the refractive index of the aforementioned adhesive may be 1.575 or higher, 1.580 or higher, or 1.585 or higher. Based on the adhesive having the aforementioned refractive index, in applications where it is adhered to materials with high refractive indices, light reflection at the interface with the adhered object can be appropriately suppressed. The ideal upper limit of the refractive index of the adhesive may vary depending on the refractive index of the adhered object, and is therefore not limited to a specific range; for example, it may be 1.700 or lower, 1.670 or lower, 1.650 or lower, 1.620 or lower, or 1.600 or lower.

[0024] The refractive index of an adhesive can be adjusted, for example, by its composition (e.g., the composition of the monomer components constituting the acrylic polymer). For example, by using an acrylic polymer or additive (HRO) containing a high content of monomer (Al) in the monomer components, an adhesive exhibiting a predetermined refractive index can be formulated.

[0025] Furthermore, in this specification, the refractive index of the adhesive refers to the refractive index of the adhesive surface (adhesive surface). 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. For example, an Abbe refractometer, such as the ATAGO "DR-M4" or an equivalent, can be used. The test sample can be an adhesive layer composed of the adhesive of the object being evaluated. Specifically, the refractive index of the adhesive can be measured using the method described in the examples below.

[0026] (Storage elasticity modulus G') Furthermore, another characteristic of the adhesive disclosed herein is that its storage modulus of elasticity G' (0°C) is in the range of 1.0 × 10⁴ Pa to 1.0 × 10⁶ Pa. According to the above adhesive, since it has a high refractive index and the range of its storage modulus of elasticity G' (0°C) is suppressed to a low range, it can be a product that balances high refractive index and flexibility. An adhesive with a storage modulus of elasticity G' (0°C) within the above range can be a product that balances high refractive index and flexibility and has the flexibility to withstand repeated bending operations. The above-mentioned storage modulus of elasticity G' (0°C) is preferably 5.0 × 10⁵ Pa or less, but can be 2.0 × 10⁵ Pa or less, 1.0 × 10⁵ Pa or less, 7.0 × 10⁴ Pa or less, 5.0 × 10⁴ Pa or less, or 3.0 × 10⁴ Pa or less. Furthermore, the aforementioned storage elastic modulus G' (0℃) is preferably 2.0 × 10⁴ Pa or higher, more preferably 4.0 × 10⁴ Pa or higher, may be 6.0 × 10⁴ Pa or higher, or may be 1.0 × 10⁵ Pa or higher.

[0027] The storage modulus of elasticity G' (80°C) of the adhesive disclosed herein is not particularly limited. For example, it is suitable to be less than 1.0 × 10⁵ Pa, preferably less than 5.0 × 10⁴ Pa, more preferably less than 3.0 × 10⁴ Pa, and may be less than 1.0 × 10⁴ Pa or even less than 5.0 × 10³ Pa. Adhesives with the above-mentioned storage modulus of elasticity G' (80°C) exhibit good flexibility in high-temperature regions. The lower limit of the above-mentioned storage modulus of elasticity G' (80°C) is not particularly limited. For example, it is suitable to be 1.0 × 10² Pa or more, preferably 5.0 × 10² Pa or more, preferably 1.0 × 10³ Pa or more, more preferably 3.0 × 10³ Pa or more, and may also be 5.0 × 10³ Pa or more. An adhesive with the above-mentioned storage elastic modulus G' (80°C) still has moderate cohesive force even in high-temperature regions and tends to have excellent heat resistance, so it is ideal.

[0028] The storage modulus of elasticity G' (-10°C) of the adhesive disclosed herein is not particularly limited. For example, it may be less than 1.0 × 10⁹ Pa, less than 1.0 × 10⁸ Pa, or less than 1.0 × 10⁷ Pa, preferably less than 5.0 × 10⁶ Pa, less than 1.0 × 10⁶ Pa, less than 5.0 × 10⁵ Pa, or less than 1.0 × 10⁵ Pa. Adhesives with such limited storage modulus of elasticity G' (-10°C) can have superior flexibility. For example, they can have good flexibility in low-temperature regions and flexibility that can withstand repeated bending operations in a wide temperature range including low-temperature regions. The lower limit of the aforementioned storage modulus G' (-10°C) is not particularly limited. For example, it is appropriate for it to be 1.0 × 10² Pa or more, or 1.0 × 10³ Pa or more, preferably 5.0 × 10³ Pa or more, more preferably 1.0 × 10⁴ Pa or more, and possibly 5.0 × 10⁴ Pa or more, possibly 1.0 × 10⁵ Pa or more, or possibly 5.0 × 10⁵ Pa or more. Adhesives having the aforementioned storage modulus G' (-10°C) can be both flexible and possess moderate cohesive strength. Furthermore, adhesives having the aforementioned storage modulus G' (-10°C) tend to easily maintain both high refractive index and flexibility even in low-temperature regions.

[0029] The storage modulus of elasticity G' (-20°C) of the adhesive disclosed herein is not particularly limited. For example, it may be less than 1.0 × 10¹⁰ Pa, less than 1.0 × 10⁹ Pa, and preferably below 5.0 × 10⁸ Pa, below 1.0 × 10⁸ Pa, below 5.0 × 10⁷ Pa, below 1.0 × 10⁷ Pa, below 5.0 × 10⁶ Pa, below 1.0 × 10⁶ Pa, or below 5.0 × 10⁵ Pa. Adhesives with such limited storage modulus of elasticity G' (-20°C) can possess particularly excellent flexibility. For example, they can possess good flexibility in lower temperature regions and flexibility capable of withstanding repeated bending operations in a wide temperature range, including low-temperature regions. The lower limit of the aforementioned storage modulus G' (-20°C) is not particularly limited. For example, it is appropriate for it to be 1.0 × 10² Pa or more, or 1.0 × 10³ Pa or more, preferably 1.0 × 10⁴ Pa or more, more preferably 1.0 × 10⁵ Pa or more, and it can be 5.0 × 10⁵ Pa or more, or even 1.0 × 10⁶ Pa or more. Adhesives having the aforementioned storage modulus G' (-20°C) can be both flexible and possess moderate cohesive strength. Furthermore, adhesives having the aforementioned storage modulus G' (-20°C) tend to easily maintain both high refractive index and flexibility even in low-temperature regions.

[0030] (Storage elasticity modulus ratio) Among several types of adhesives, adhesives with a storage modulus of elasticity G'(0°C) at 0°C to a storage modulus of elasticity G'(80°C) at 80°C (G'(0°C) / G'(80°C)) in the range of 1 to 1000 can be used. According to adhesives that satisfy the above characteristics, since changes in the modulus of elasticity over a wide temperature range from 0°C to high temperatures can be suppressed, they easily exhibit characteristics (such as softness) that are stable to temperature changes. It is appropriate for the above ratio (G'(0°C) / G'(80°C)) to be 300 or less, preferably 100 or less, more preferably 50 or less, possibly 25 or less, possibly 10 or less, or possibly 5 or less. For example, the lower limit of the above ratio (G'(0°C) / G'(80°C)) can be 2 or more, or possibly 3 or more.

[0031] Among several types of adhesives, an adhesive with a ratio (G'(-10℃) / G'(80℃)) of the storage modulus of elasticity at -10℃ to the storage modulus of elasticity at 80℃ in the range of 1 to 1000 can be used. According to an adhesive that satisfies the above characteristics, since the change in modulus of elasticity over a wide temperature range from low to high temperatures can be suppressed, it easily exhibits stable properties (such as softness) to temperature changes, which is ideal. It is appropriate for the ratio (G'(-10℃) / G'(80℃)) to be 300 or less, preferably 150 or less, more preferably 100 or less, and it can be 50 or less, 30 or less, 20 or less, or even 10 or less. For example, the lower limit of the ratio (G'(-10℃) / G'(80℃)) can be 2 or more, or 3 or more.

[0032] Among several types of adhesives, adhesives with a storage modulus of elasticity G'(-20°C) at -20°C to a storage modulus of elasticity G'(80°C) at 80°C (G'(-20°C) / G'(80°C)) in the range of 1 to 1000 can be used. According to adhesives that satisfy the above characteristics, since changes in the modulus of elasticity over a wide temperature range from lower to higher temperatures can be suppressed, they can exhibit stable properties (such as softness) to temperature changes. The aforementioned ratio (G'(-20°C) / G'(80°C)) can be 500 or less, 300 or less, 150 or less, 100 or less, 50 or less, or 30 or less. The lower limit of the aforementioned ratio (G'(-20°C) / G'(80°C)) can, for example, be 5 or more, 10 or more, 50 or more, or 100 or more.

[0033] There is no particular limitation on the glass transition temperature (Tg) of an adhesive. It can be set considering factors such as flexibility at low temperatures or cohesion (heat resistance, etc.) at high temperatures. In several scenarios, the Tg of the adhesive can be, for example, below 30°C, below 15°C, or even below 5°C. In several ideal scenarios, from a flexibility perspective, a Tg below 0°C is preferable to below -5°C, even more so below -10°C, and also below -15°C (e.g., below -20°C). The lower the Tg of the adhesive, the better its adhesive properties, such as adhesion to the substrate. Furthermore, by setting a lower Tg, changes in the modulus of elasticity in temperature regions above Tg can be suppressed. The lower limit of the adhesive's Tg is, for example, above -50°C, above -40°C is appropriate, and above -30°C is also acceptable. Adhesives with the aforementioned Tg tend to readily achieve appropriate cohesive strength. Furthermore, they tend to readily form adhesives that balance high refractive index and low elastic modulus.

[0034] The storage modulus G' and glass transition temperature Tg of the adhesive at the above-mentioned temperatures can be determined by the methods described in the examples below, and the ratio of each storage modulus can be calculated from the results. The storage modulus G', the ratio of each storage modulus, and the glass transition temperature Tg of the adhesive can be adjusted, for example, by selecting the composition of the monomer components constituting the base polymer (e.g., selecting the type and content of monomer (Al), selecting the type or amount of plasticizer, whether or not a crosslinking agent is used and its type and amount, whether or not an additive is used and its type and amount, etc.

[0035] Composition of Adhesives (Base polymer) In the technology disclosed herein, the type of adhesive is not particularly limited. The adhesive may be one or more of various rubber-like polymers that can be used in the field of adhesives, including acrylic polymers, rubber polymers (e.g., natural rubber, synthetic rubber, mixtures thereof), polyester polymers, carbamate polymers, polyether polymers, polysiloxane polymers, polyamide polymers, fluoropolymers, etc., as the adhesive polymer (meaning a structural polymer that can shape the adhesive, hereinafter also referred to as the "base polymer"). From the viewpoint of adhesive performance or cost, adhesives containing acrylic polymers or rubber polymers as the base polymer are suitable. Among these, adhesives using acrylic polymers as the base polymer (acrylic adhesives) are preferred. The technology disclosed herein is suitable for implementation in the case of using acrylic adhesives.

[0036] The following mainly describes acrylic adhesives, but it is not intended to limit the adhesives disclosed herein to acrylic adhesives.

[0037] (Acrylic polymer) The technology disclosed herein is suitable for implementation in the use of acrylic adhesives. The acrylic polymer used as the base polymer of the aforementioned acrylic adhesive is preferably an acrylic polymer containing an aromatic ring monomer (A1) as a monomeric component constituting the acrylic polymer. That is, it is preferably an acrylic polymer containing an aromatic ring monomer (A1) as a monomeric unit. Here, the term "monomeric component constituting the acrylic polymer" in this specification refers to a monomer that constitutes a repeating unit of the acrylic polymer in the adhesive formed from the adhesive composition, whether it is included in the adhesive composition in the form of a pre-formed polymer (which may be an oligomer) or in the form of an unpolymerized monomer. In other words, the monomeric component constituting the acrylic polymer can be included in the aforementioned adhesive composition in any form, including polymer, unpolymerized, and partially polymerized forms. From the perspective of ease of preparation of adhesive composition, among several types of adhesive compositions, it is preferable to include virtually all (e.g., 95% by weight or more, preferably 99% by weight or more) of the monomer components in the form of polymer.

[0038] (Single (A1)) The monomer (A1) can be a compound containing at least one aromatic ring and at least one vinyl unsaturated group in one molecule. The monomer (A1) can be used alone or in combination of two or more of the compounds.

[0039] Examples of the aforementioned vinyl unsaturated groups include (meth)acryl, vinyl, and (meth)allyl. From the perspective of polymerization reactivity, (meth)acryl is preferable, while from the perspective of flexibility or adhesiveness, acrylonitrile is better. From the perspective of inhibiting the reduction of the adhesive's flexibility, the monomer (A1) can be a compound containing one vinyl unsaturated group per molecule (i.e., a monofunctional monomer).

[0040] The number of aromatic rings contained in a molecule of a compound that can be used as a monomer (A1) can be 1 or more. There is no particular upper limit to the number of aromatic rings, for example, it can be 16 or less. In several cases, from the viewpoint of ease of preparation of acrylic polymers or transparency of adhesives, the number of aromatic rings can be, for example, 12 or less, preferably 8 or less, more preferably 6 or less, 5 or less, 4 or less, 3 or less, or even 2 or less.

[0041] The aromatic rings of compounds that can be used as monomers (A1) can also be: benzene rings (which may be benzene rings that form part of a biphenyl or phenanthrene structure); carbon rings such as condensed rings of naphthalene, indene, azurite, anthracene, and phenanthrene rings; or heterocycles such as pyridine, pyrimidine, pyridine, triazine, pyrrole, pyrazole, imidazole, triazole, succinate, isosuccinate, thiazole, and thiophene rings. The heteroatoms contained in the ring-forming atoms of the above-mentioned heterocycles can, for example, be one or more selected from the group consisting of nitrogen, sulfur, and oxygen. In several states, the heteroatoms constituting the heterocycles can be one or both of nitrogen and sulfur. The monomer (A1) can also, for example, have a structure where one or more carbon rings and one or more heterocycles have been condensed, as in the dinaphthothiophene structure.

[0042] The aforementioned aromatic ring (preferably a carbocyclic ring) may have one or more substituents on the ring constituent atoms, or it may not have substituents. When substituents are present, examples of substituents include alkyl, alkoxy, aryloxy, hydroxyl, halogen atoms (fluorine, chlorine, bromine, etc.), hydroxyalkyl, hydroxyalkyloxy, epoxypropoxy, etc., but are not limited thereto. Among the substituents containing carbon atoms, the number of carbon atoms contained in the substituent is preferably 1 to 4, more preferably 1 to 3, and for example, 1 or 2. In several cases, the aforementioned aromatic ring may be an aromatic ring without substituents on the ring constituent atoms, or an aromatic ring having one or more substituents selected from the group consisting of alkyl, alkoxy, and halogen atoms (e.g., bromine atoms) on the ring constituent atoms. Furthermore, the presence of substituents on the ring constituent atoms of the aromatic ring of the monomer (A1) refers to the presence of substituents other than those with vinyl unsaturated groups on the aromatic ring.

[0043] The aromatic ring and the vinyl unsaturated group can be directly bonded or bonded through a linking group. The linking group can be, for example, a group containing one or more structures selected from alkyl, oxoalkyl, poly(oxoalkyl), phenyl, alkylphenyl, alkoxyphenyl, or groups in which one or more hydrogen atoms are replaced by hydroxyl groups (e.g., hydroxyalkyl), oxy (-O-), thiooxy (-S-), etc. Among several options, it is suitable to use aromatic ring monomers with structures in which the aromatic ring is directly bonded to the vinyl unsaturated group, or aromatic ring monomers with structures in which they are bonded through a linking group selected from the group consisting of alkyl, oxoalkyl, and poly(oxoalkyl). The number of carbon atoms in the alkyl and oxoalkyl groups is preferably 1 to 4, more preferably 1 to 3, for example, 1 or 2. The number of repetitions of the oxoalkyl unit in the poly(oxoalkyl) group can be, for example, 2 to 3.

[0044] Examples of compounds suitable as monomers (A1) include aromatic ring (meth)acrylates and aromatic ring vinyl compounds. Aromatic ring (meth)acrylates and aromatic ring vinyl compounds can be used alone or in combination of two or more. Alternatively, one or more aromatic ring (meth)acrylates and one or more aromatic ring vinyl compounds can be used in combination.

[0045] Among several options, considering the ease with which a high refractive index can be achieved, monomers with two or more aromatic rings (preferably carbon rings) per molecule can be used as monomers (A1). Examples of monomers with two or more aromatic rings per molecule (including monomers with multiple aromatic rings) include: monomers with two or more non-condensed aromatic rings bonded by a linker group; monomers with two or more non-condensed aromatic rings directly (i.e., without being separated by other atoms) chemically bonded; monomers with condensed aromatic ring structures; monomers with benzothiophene structures; monomers with dibenzothiophene structures; and monomers with dibenzothiophene structures. Monomers containing multiple aromatic rings can be used alone or in combination of two or more.

[0046] The linking group mentioned above can be, for example: oxy (-O-), thiooxy (-S-), oxyalkyl (e.g., -O-(CH2)n-yl, where n is 1 to 3, preferably 1), thiooxyalkyl (e.g., -S-(CH2)n-yl, where n is 1 to 3, preferably 1), linear alkyl (i.e., -(CH2)n-yl, where n is 1 to 6, preferably 1 to 3), or groups in which the alkyl groups of the above-mentioned oxyalkyl, thiooxyalkyl, and linear alkyl have been partially or completely halogenated. From the viewpoint of the adhesive's flexibility, suitable examples of the above-mentioned linking group include oxy, thiooxy, oxyalkyl, and linear alkyl groups. Monomers having a structure in which two or more non-condensed aromatic rings are linked by a linker group can be exemplified as follows: phenoxybenzyl (meth)acrylate (e.g., m-phenoxybenzyl (meth)acrylate), thiophenoxybenzyl (meth)acrylate, benzyl (meth)acrylate, etc.

[0047] Monomers with structures consisting of two or more non-condensed aromatic rings directly chemically bonded can be, for example, (meth)acrylates containing a biphenyl structure, (meth)acrylates containing a triphenyl structure, or biphenyls containing a vinyl group. Specific examples include o-phenylphenol (meth)acrylate and biphenyl methyl (meth)acrylate.

[0048] Examples of monomers with condensed aromatic ring structures include naphthyl ring (meth)acrylates, anthracene ring (meth)acrylates, vinylnaphthalene, and vinylanthracene. Specific examples include: 1-naphthylmethyl (meth)acrylate (also known as: 1-naphthylmethyl (meth)acrylate), hydroxyethylated β-naphthol acrylate, 2-naphthylethyl (meth)acrylate, 2-naphthoxyethyl acrylate, and 2-(4-methoxy-1-naphthoxy)ethyl (meth)acrylate.

[0049] Specific examples of monomers with the aforementioned genus structure include 9,9-bis(4-hydroxyphenyl)genus(meth)acrylate and 9,9-bis[4-(2-hydroxyethoxy)phenyl]genus(meth)acrylate. Furthermore, monomers with the genus structure, because they contain a structural portion with two benzene rings directly chemically bonded, are included in the concept of monomers with structures having two or more non-condensed aromatic rings directly chemically bonded, as described above.

[0050] Monomers with the dinaphthothiophene structure mentioned above include dinaphthothiophene containing (meth)propenyl, dinaphthothiophene containing vinyl, and dinaphthothiophene containing (meth)allyl. Specific examples include: (meth)propenyloxymethyl dinaphthothiophene (e.g., compounds with a structure of CH 2CH(R 1)C(O)OCH 2- bonded at the 5 or 6 position of the dinaphthothiophene ring; where R 1 is a hydrogen atom or a methyl group), (meth)propenyloxyethyl dinaphthothiophene (e.g., compounds with a structure of CH 2CH(R 1)C(O)OCH(CH 3)- or CH 2CH(R 1)C(O)OCH 2CH 2- bonded at the 5 or 6 position of the dinaphthothiophene ring; where R 1 is a hydrogen atom or a methyl group), vinyl dinaphthothiophene (e.g., compounds with a structure of vinyl bonded at the 5 or 6 position of the dinaphthothiophene ring), (meth)allyloxy dinaphthothiophene, etc. Furthermore, the monosystem having a dinaphthothiophene structure is also included in the concept of the monomer having a condensed aromatic ring structure by including a naphthalene structure and a structure having a thiophene ring and two naphthalene structures that have undergone condensation.

[0051] Examples of monomers having a dibenzothiophene structure include dibenzothiophene containing (meth)acrylyl and dibenzothiophene containing vinyl groups. Furthermore, monomers having a dibenzothiophene structure are included in the concept of monomers having a condensed aromatic ring structure because they have a structure in which the thiophene ring and two benzene rings have undergone condensation. Furthermore, neither dinaphthothiophene nor dibenzothiophene structures belong to structures with two or more non-condensed aromatic rings directly chemically bonded together.

[0052] In several ideal configurations, the monomer (A1) can be a monomer with one aromatic ring (preferably a carbon ring) per molecule. Monomers with one aromatic ring per molecule (monomers with an odd number of aromatic rings) can, for example, help improve the flexibility of the adhesive, adjust its adhesive properties, or improve transparency. Monomers with an odd number of aromatic rings can be used alone or in combination of two or more. From the viewpoint of improving the refractive index of the adhesive, monomers with one aromatic ring per molecule can also be used in combination with monomers with multiple aromatic rings.

[0053] Examples of monomers containing one aromatic ring in one molecule include: benzyl (meth)acrylate, methoxybenzyl (meth)acrylate, phenyl (meth)acrylate, ethoxylated phenol (meth)acrylate, phenoxypropyl (meth)acrylate, phenoxybutyl (meth)acrylate, toluene (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, chlorobenzyl (meth)acrylate, and other carbon-containing aromatic ring (meth)acrylates; 2-(4,6-dibromo-2-di-butylphenoxy)ethyl (meth)acrylate, 2-(4,6-dibromo-2-isopropylphenoxy)ethyl (meth)acrylate, 6-( 4,6-Dibromo-2-di-butylphenoxy)hexyl (meth)acrylate, 6-(4,6-dibromo-2-isopropylphenoxy)hexyl (meth)acrylate, 2,6-dibromo-4-nonylphenyl acrylate, 2,6-dibromo-4-dodecylphenyl acrylate, and other bromine-substituted aromatic ring (meth)acrylates; vinyl compounds containing carbon aromatic rings such as styrene, α-methylstyrene, vinyltoluene, and tert-butylstyrene; compounds with vinyl substituents on heteroaromatic rings such as N-vinylpyridine, N-vinylpyrimidine, N-vinylpyridine, N-vinylpyrrole, N-vinylimidazolium, and N-vinylpyrazole, etc.

[0054] The monomer (A1) can also be a monomer with a structure in which an oxyethyl chain is sandwiched between the vinyl unsaturated group and the aromatic ring, as described above. As described, a monomer in which an oxyethyl chain is sandwiched between the vinyl unsaturated group and the aromatic ring can be considered an ethoxylated form of the original monomer. The repeating number of the oxyethyl unit (-CH₂CH₂O-) in the oxyethyl chain is typically 1 to 4, preferably 1 to 3, more preferably 1 to 2, for example, 1. Specific examples of ethoxylated aromatic ring monomers include: ethoxylated o-phenylphenol (meth)acrylate, ethoxylated nonylphenol (meth)acrylate, ethoxylated cresol (meth)acrylate, phenoxyethyl acrylate, phenoxydiethylene glycol di(meth)acrylate, etc.

[0055] There is no particular limitation on the content of monomers containing multiple aromatic rings in monomer (A1), for example, it can be 5% by weight or more, 25% by weight or more, or 40% by weight or more. In several states, the content of monomers containing multiple aromatic rings in monomer (A1) can be, for example, 50% by weight or more, and from the viewpoint of easily obtaining a higher refractive index, it is preferable to be 70% by weight or more, it can be 85% by weight or more, it can be 90% by weight or more, or it can be 95% by weight or more. Monomer (A1) can also be substantially 100% by weight of monomers containing multiple aromatic rings. That is, monomer (A1) can also use only one or two or more monomers containing multiple aromatic rings. Furthermore, in several states, considering, for example, high refractive index and low elastic modulus, and further considering the balance with adhesion where necessary, the content of monomers containing multiple aromatic rings in the monomer (A1) can be less than 100% by weight, less than 98% by weight, less than 90% by weight, less than 80% by weight, less than 70% by weight, less than 65% by weight, less than 50% by weight, less than 25% by weight, or less than 10% by weight. The technique disclosed herein can still be implemented even in states where the content of monomers containing multiple aromatic rings in the monomer (A1) is less than 5% by weight. It is also possible to omit the use of monomers containing multiple aromatic rings.

[0056] There is no particular limitation on the content of monomers containing multiple aromatic rings in the monomer components constituting acrylic polymers, and it can be set to achieve an adhesive that balances the desired refractive index and storage modulus G' (0°C). The content of monomers containing multiple aromatic rings in the aforementioned monomer components can, for example, be 3% by weight or more, 10% by weight or more, or 25% by weight or more. From the viewpoint of easily achieving an adhesive with a higher refractive index, the content of monomers containing multiple aromatic rings in the aforementioned monomer components can be, for example, greater than 35% by weight, greater than 50% by weight, preferably greater than 70% by weight, and can be 75% by weight or more, 85% by weight or more, 90% by weight or more, or 91% by weight or more, 92% by weight or more, 93% by weight or more, 94% by weight or more, 95% by weight or more, 96% by weight or more, 97% by weight or more, 98% by weight or more, or 99% by weight or more. The content of monomers containing multiple aromatic rings in the aforementioned monomeric components, taking into account high refractive index and low elastic modulus, and further considering the balance with adhesion where necessary, is advantageously set to approximately 99% by weight or less, preferably 98% by weight or less, more preferably 96% by weight or less, and can be 93% by weight or less, 90% by weight or less, 85% by weight or less, 80% by weight or less, or 75% by weight or less. In several samples, from the viewpoint of easily achieving higher adhesive properties and / or optical properties (e.g., transparency), the content of monomers containing multiple aromatic rings in the aforementioned monomeric components can be 70% by weight or less, 60% by weight or less, 50% by weight or less, 40% by weight or less, 25% by weight or less, 15% by weight or less, or 5% by weight or less. The technology disclosed herein can still be implemented even in samples where the content of monomers containing multiple aromatic rings in the aforementioned monomeric components is less than 3% by weight.

[0057] There is no particular limitation on the content of monomers containing an odd number of aromatic rings in monomer (A1), for example, it can be 5% by weight or more, 25% by weight or more, or 40% by weight or more. In several states, the content of monomers containing an odd number of aromatic rings in monomer (A1) can be, for example, 50% by weight or more, and from the viewpoint of easily obtaining a higher refractive index, it is preferable to be 70% by weight or more, it can be 85% by weight or more, it can be 90% by weight or more, or it can be 95% by weight or more. Monomer (A1) can also be substantially 100% by weight of monomers containing an odd number of aromatic rings. That is, monomer (A1) can also use only one or two or more monomers containing an odd number of aromatic rings. Furthermore, in several states, considering, for example, high refractive index and low elastic modulus, and further considering the balance with adhesion where necessary, the content of monomers containing an odd number of aromatic rings in monomer (A1) can be less than 100% by weight, less than 98% by weight, less than 90% by weight, less than 80% by weight, less than 70% by weight, less than 65% by weight, less than 50% by weight, less than 25% by weight, or less than 10% by weight. The technique disclosed herein can still be implemented even in states where the content of monomers containing an odd number of aromatic rings in monomer (A1) is less than 5% by weight. It is also possible to omit the use of monomers containing an odd number of aromatic rings.

[0058] There is no particular limitation on the content of monomers containing an odd number of aromatic rings in the monomer components constituting acrylic polymers, and it can be set to achieve an adhesive that balances the desired refractive index and storage modulus G' (0°C). The content of monomers containing an odd number of aromatic rings in the aforementioned monomer components can, for example, be 3% by weight or more, 10% by weight or more, or 25% by weight or more. From the viewpoint of easily achieving an adhesive with a higher refractive index, the content of monomers containing an odd number of aromatic rings in the aforementioned monomer components can be, for example, greater than 35% by weight, greater than 50% by weight, preferably 60% by weight or more, more preferably greater than 70% by weight, 75% by weight or more, 85% by weight or more, 90% by weight or more, 95% by weight or more, or 98% by weight or more. The content of monomers containing an odd number of aromatic rings in the aforementioned monomeric components, taking into account high refractive index and low elastic modulus, and further considering the balance with adhesion where necessary, can be set to approximately 99% by weight or less, preferably 98% by weight or less, more preferably 96% by weight or less, possibly 93% by weight or less, possibly 90% by weight or less, possibly 85% by weight or less, possibly 80% by weight or less, or possibly 75% by weight or less. In several samples, from the viewpoint of easily achieving higher adhesive properties and / or optical properties (e.g., transparency), the content of monomers containing an odd number of aromatic rings in the aforementioned monomeric components can be 70% by weight or less, possibly 60% by weight or less, possibly 50% by weight or less, possibly 40% by weight or less, possibly 25% by weight or less, possibly 15% by weight or less, or possibly 5% by weight or less. The technology disclosed herein can still be implemented even in samples where the content of monomers containing an odd number of aromatic rings in the aforementioned monomeric components is less than 3% by weight.

[0059] In several embodiments of the technology disclosed herein, a high-refractive-index monomer may be suitably used as at least a portion of the monomer (A1). Here, "high-refractive-index monomer" means a monomer with a refractive index of, for example, approximately 1.510 or higher, preferably approximately 1.530 or higher, or more preferably approximately 1.550 or higher. There is no particular upper limit to the refractive index of the high-refractive-index monomer; however, from the viewpoint of ease of formulation of acrylic polymers or ease of achieving both suitability as an adhesive and flexibility, it may be, for example, 3.000 or lower, 2.500 or lower, 2.000 or lower, 1.900 or lower, 1.800 or lower, or 1.700 or lower. One high-refractive-index monomer may be used alone or in combination of two or more. Furthermore, the refractive index of the monomer was measured using an Abbe refractometer at a measurement wavelength of 589 nm and a measurement temperature of 25°C. The Abbe refractometer can be the ATAGO "DR-M4" model or an equivalent. If a nominal value for the refractive index at 25°C is provided by the manufacturer, that nominal value can be used.

[0060] The aforementioned high-refractive-index monomers can be appropriately selected from compounds containing the concept of aromatic ring monomers (A1) disclosed herein (such as the compounds and groups of compounds exemplified above). Specific examples include: m-phenoxybenzyl acrylate (refractive index: 1.566, Tg of homopolymer: -35℃), 1-naphthylmethyl acrylate (refractive index: 1.595, Tg of homopolymer: 31℃), ethoxylated o-phenylphenol acrylate (repetition number of oxyethyl units: 1, refractive index: 1.578), benzyl acrylate (refractive index (nD20): 1.519, Tg of homopolymer: 6℃), phenoxyethyl acrylate (refractive index (nD20): 1.517, Tg of homopolymer: 2℃), phenoxydiethylene glycol acrylate (refractive index: 1.510, Tg of homopolymer: 1.510, Tg of homopolymer: 1.510, Tg of homopolymer: 1.510, Tg of homopolymer: 1.510, Tg of homopolymer: 1.510, Tg of homopolymer: 1.519 ... -35℃), 6-propenylioxymethyl dinaphthothiophene (6MDNTA, refractive index: 1.75), 6-methpropenylioxymethyl dinaphthothiophene (6MDNTMA, refractive index: 1.726), 5-propenylioxyethyl dinaphthothiophene (5EDNTA, refractive index: 1.786), 6-propenylioxyethyl dinaphthothiophene (6EDNTA, refractive index: 1.722), 6-vinyl dinaphthothiophene (6VDNT, refractive index: 1.802), 5-vinyl dinaphthothiophene (abbreviation: 5VDNT, refractive index: 1.793), etc., but not subject to such limitations.

[0061] There are no particular restrictions on the content of high-refractive-index monomers (i.e., aromatic ring-containing monomers with a refractive index of approximately 1.510 or higher, preferably approximately 1.530 or higher, and more preferably approximately 1.550 or higher) in monomer (A1). For example, it can be 5% by weight or more, 25% by weight or more, 35% by weight or more, or 40% by weight or more. In several states, from the viewpoint of easily obtaining higher refractive indices, the content of high-refractive-index monomers in monomer (A1) can 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. Monomer (A1) can also be substantially 100% by weight high-refractive-index monomers. Furthermore, in several samples, considering the balance between high refractive index and low elastic modulus, and where necessary, further balancing adhesion, the content of high refractive index monomer in monomer (A1) can be less than 100% by weight, less than 98% by weight, less than 90% by weight, less than 80% by weight, or less than 65% by weight. In several samples, considering adhesive properties and / or optical properties, the content of high refractive index monomer in monomer (A1) can be less than 50% by weight, less than 25% by weight, less than 15% by weight, or less than 10% by weight. The technique disclosed herein can still be implemented even in samples where the content of high refractive index monomer in monomer (A1) is less than 5% by weight. Alternatively, high refractive index monomer may not be used. []

[0062] There are no particular restrictions on the content of high-refractive-index monomers in the monomer components constituting acrylic polymers, and the composition can be set to achieve an adhesive that balances the desired refractive index and elastic modulus. Furthermore, where necessary, the composition can be further adjusted to balance adhesive properties (e.g., adhesion strength) and / or optical properties (e.g., total light transmittance, haze value). The content of high-refractive-index monomers in the aforementioned monomer components can, for example, be 3% by weight or more, 10% by weight or more, or 25% by weight or more. In several cases, the content of high-refractive-index monomers in the monomer components constituting acrylic polymers can, for example, be greater than 35% by weight, and from the viewpoint of easily obtaining higher refractive indices, greater than 50% by weight is advantageous, preferably greater than 70% by weight, and can be 75% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. From the perspective of balancing high refractive index and low elastic modulus, and further considering adhesion where necessary, the content of high refractive index monomers in the aforementioned monomer composition is advantageously set to 99 wt% or less, preferably 98 wt% or less, more preferably 96 wt% or less, and could be 93 wt% or less, 90 wt% or less, 85 wt% or less, 80 wt% or less, or even 75 wt% or less. In several samples, considering adhesive properties and / or optical properties, the content of high refractive index monomers in the aforementioned monomer composition can be 70 wt% or less, 50 wt% or less, 25 wt% or less, 15 wt% or less, or even 5 wt% or less. The technique disclosed herein can still be implemented even in samples where the content of high refractive index monomers in the aforementioned monomer composition is less than 3 wt%.

[0063] In several ideal samples, at least a portion of the monomer (A1) is a homopolymer containing an aromatic ring monomer with a Tg below 10°C (hereinafter referred to as "monomer L"). When the content of the aromatic ring monomer (A1) in the monomer composition (especially the aromatic ring monomer (A1) equivalent to at least one of the monomers containing multiple aromatic rings, monomers containing a single aromatic ring, and high refractive index monomers mentioned above) is increased, the storage elastic modulus G' of the adhesive tends to increase. However, by using monomer L as part or all of the monomer (A1), the increase in the storage elastic modulus G' can be suppressed. This allows for better maintenance of the low elastic modulus and an increase in the refractive index. The Tg of monomer L can be, for example, below 5°C, below 0°C, below -10°C, below -20°C, or below -25°C. There is no particular limitation on the lower limit of the Tg of monomer L. Considering the balance between the refractive index enhancement effect and other factors, in several states, the Tg of monomer L can be, for example, above -70℃, above -55℃, or above -45℃. In other states, the Tg of monomer L can be, for example, above -30℃, above -10℃, above 0℃, or above 3℃. Monomer L can be used alone or in combination of two or more types.

[0064] Monomer L can be suitably selected from compounds containing the concept of aromatic ring monomers (A1) disclosed herein (e.g., the compounds and groups of compounds exemplified above) that have the Tg. Suitable examples of aromatic ring monomers that can be used as monomer L include: m-phenoxybenzyl acrylate (Tg of homopolymer: -35°C), benzyl acrylate (Tg of homopolymer: 6°C), phenoxyethyl acrylate (Tg of homopolymer: 2°C), and phenoxydiethylene glycol acrylate (Tg of homopolymer: -35°C).

[0065] There are no particular restrictions on the content of monomer L in monomer (A1), for example, it can be 5% by weight or more, 25% by weight or more, or 40% by weight or more. Among several options, from the viewpoint of easily obtaining an adhesive that balances high refractive index and low elastic modulus at a higher level, the content of monomer L in monomer (A1) can be, for example, 50% by weight or more, while from the viewpoint of reducing the elastic modulus, it is preferable to be 60% by weight or more, and can be 70% by weight or more, 75% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. Monomer (A1) can also be substantially 100% by weight of monomer L. Furthermore, in several states, for example, from the perspective of balancing high refractive index and low elastic modulus, and further considering adhesion when necessary, the content of monomer L in monomer (A1) can be less than 100% by weight, less than 98% by weight, less than 90% by weight, less than 80% by weight, or less than 65% by weight.

[0066] The content of monomer L in the monomer component constituting the acrylic polymer can be, for example, 3% by weight or more, 10% by weight or more, or 25% by weight or more. Among several samples, from the viewpoint of easily obtaining an adhesive that balances high refractive index and low elastic modulus at a higher level, the content of monomer L in the monomer component can be, for example, greater than 35% by weight, while from the viewpoint of increasing refractive index, greater than 50% by weight is advantageous, preferably greater than 70% by weight, and can be 75% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. From the perspective of balancing high refractive index and low elastic modulus, and further balancing adhesion when necessary, the content of monomer L in the above-mentioned monomer components is advantageously set to approximately 99% by weight or less, preferably 98% by weight or less, more preferably 96% by weight or less, could be 93% by weight or less, could be 90% by weight or less, could be 85% by weight or less, could be 80% by weight or less, or could be 75% by weight or less.

[0067] In several samples, the glass transition temperature Tg A1 is not particularly limited based on the composition of the monomer (A1). From the viewpoint of reducing the elastic modulus, a glass transition temperature Tg A1 of approximately 20°C or lower is advantageous, preferably below 10°C, for example, below 5°C, below 0°C, below -10°C, below -20°C, or below -25°C. There is no particular limitation on the lower limit of the glass transition temperature Tg A1. Considering the balance with the refractive index enhancement effect, in several samples, the glass transition temperature Tg A1 can be, for example, above -70°C, above -55°C, or above -45°C. The technology disclosed herein is still suitable for implementation even in samples with glass transition temperatures Tg A1 of, for example, above -40°C, above -35°C, above -33°C, above -30°C, or above -25°C. In several other states, the glass transition temperature Tg A1 can be above -10℃, above 0℃, or above 3℃.

[0068] Here, the glass transition temperature Tg A1 based on the composition of monomer (A1) refers to the Tg obtained by the Fox formula described later, based solely on the composition of monomer (A1) in the monomeric component constituting the acrylic polymer. The glass transition temperature Tg A1 can be calculated by applying the Fox formula to monomer (A1) in the monomeric component constituting the acrylic polymer, and by combining the glass transition temperature of the homopolymer of each aromatic ring-containing monomer used as monomer (A1) with the weight fraction of each aromatic ring-containing monomer in the total stoichiometry of monomer (A1). In a sample using only one monomer as monomer (A1), the Tg of the homopolymer of that monomer is consistent with the glass transition temperature Tg A1.

[0069] In several formulations, aromatic ring monomers (A1) can be used in combination with monomer L (i.e., aromatic ring monomers with a Tg below 10°C in homopolymers) and monomer H with a Tg above 10°C. The Tg of monomer H can be, for example, above 10°C, above 15°C, or above 20°C. By combining monomer L and monomer H, in adhesives with a high content of aromatic ring monomers (A1) in the monomer composition, a higher level of balance can be achieved between the adhesive's high refractive index and its flexibility suitable for adhesion to the substrate. The ratio of monomer L to monomer H used can be set to appropriately exhibit the aforementioned effects and is not particularly limited. For example, it is preferable to set the ratio of monomer L to monomer H to satisfy any of the aforementioned glass transition temperatures Tg A1.

[0070] Among several options, the aromatic ring monomer (A1) can be suitably selected from compounds that do not contain structures with two or more non-condensed aromatic rings directly chemically bonded (e.g., biphenyl structures). For example, it is preferable to use acrylic polymers composed of monomer components in an amount of less than 5% by weight (preferably less than 3% by weight, or even 0% by weight) of compounds containing structures with two or more non-condensed aromatic rings directly chemically bonded. Limiting the amount of compounds containing structures with two or more non-condensed aromatic rings directly chemically bonded, as described above, is advantageous from the viewpoint of achieving an adhesive with high refractive index and low elastic modulus, and, where necessary, further balancing adhesion.

[0071] There are no particular restrictions on the content of monomer (Al) in the monomer components constituting acrylic polymers. It can be configured to achieve an adhesive that balances desired refractive index and elastic modulus, as well as adhesive properties (e.g., adhesion strength) and / or optical properties (e.g., total light transmittance, haze value). In several samples, the content of monomer (Al) in the aforementioned monomer components can be, for example, 30% by weight or more, preferably 50% by weight or more, more preferably 60% by weight or more, or even 70% by weight or more. In several ideal samples, the content of monomer (Al) in the monomer components constituting acrylic polymers can be, for example, greater than 70% by weight, with 75% by weight or more being appropriate; and from the viewpoint of easily obtaining higher refractive indices, it is preferable to be 80% by weight or more, and can be 85% by weight or more, 90% by weight or more, or 91% by weight or more, 92% by weight or more, 93% by weight or more, 94% by weight or more, 95% by weight or more, 96% by weight or more, 97% by weight or more, 98% by weight or more, or 99% by weight or more. The content of monomer (Al) in the above monomer components is typically less than 100% by weight, and from the viewpoint of balancing high refractive index and low elastic modulus, and further balancing adhesion where necessary, approximately 99% by weight or less is advantageous, preferably 98% by weight or less, more preferably 96% by weight or less, can be 93% by weight or less, or can be 90% by weight or less. In several samples, from the viewpoint of easily achieving higher adhesive properties and / or optical properties (e.g., transparency), the content of monomer (Al) in the above monomer components can be less than 90% by weight, less than 85% by weight, or less than 80% by weight.

[0072] (Single (A2)) In several ideal samples, the monomer components constituting the acrylic polymer may include monomer (A2) in addition to the monomer (A1) described above. The monomer (A2) is a monomer equivalent to at least one of a hydroxyl-containing monomer (hydroxyl-containing monomer) and a carboxyl-containing monomer (carboxyl-containing monomer). The hydroxyl-containing monomer is a compound having at least one hydroxyl group and at least one vinyl unsaturated group per molecule. The carboxyl-containing monomer is a compound containing at least one carboxyl group and at least one vinyl unsaturated group per molecule. Monomer (A2) can help to introduce crosslinking points into acrylic polymers or impart appropriate aggregation properties to adhesives. Monomer (A2) can be used alone or in combination of two or more. Monomer (A2) is typically a monomer without an aromatic ring.

[0073] Examples of vinyl unsaturated groups in monomer (A2) include (meth)acryl, vinyl, and (meth)allyl. From the perspective of polymerization reactivity, (meth)acryl is preferable, while from the perspective of low elastic modulus or adhesiveness, acrylonitrile is better. From the perspective of inhibiting the low elastic modulus of adhesives, monomer (A2) can be a compound containing one vinyl unsaturated group per molecule (i.e., a monofunctional monomer).

[0074] In several states, the monomer (A2) can be a monomer with a relatively long distance between the vinyl unsaturated group (e.g., (meth)acryl) and the hydroxyl and / or carboxyl groups. This allows for the easy acquisition of highly flexible cross-linked structures in the states where the hydroxyl and / or carboxyl groups are used for cross-linking reactions. For example, a compound constituting the chain (linking chain) connecting the vinyl unsaturated group and the hydroxyl and / or carboxyl groups (typically carbon or oxygen atoms) can be used as the monomer (A2) if the number of atoms (typically carbon or oxygen atoms) is 3 or more (e.g., 4 or more, 5 or more, 6 or more, 7 or more, 8 or more, 9 or more, 10 or more, 11 or more, 12 or more, 13 or more, 14 or more, 15 or more, 16 or more, 17 or more, 18 or more, or 19 or more). The upper limit for the number of atoms constituting the aforementioned linking chain is, for example, 45 or less, or 20 or less (e.g., 19 or less, 18 or less, 17 or less, 16 or less, 15 or less, 14 or less, 13 or less, 12 or less, 11 or less, 10 or less, 9 or less, or 8 or less). Furthermore, the number of atoms constituting the linking chain connecting the aforementioned vinyl unsaturated group to the hydroxyl and / or carboxyl groups refers to the minimum number of atoms required to reach the hydroxyl or carboxyl group from the vinyl unsaturated group. For example, when the aforementioned linking chain is composed of a straight-chain extended alkyl group (i.e., -(CH₂)ₙ-yl), the number of 'n' becomes the number of atoms constituting the aforementioned linking chain. As another example, when the aforementioned linking chain is an oxy-extended ethyl group (i.e., -(C₂H₄O)ₙ-yl), the product of the sum of the number of carbon atoms 2 and the number of oxygen atoms 1 constituting the oxy-extended ethyl group (3) and 'n' (3n) becomes the number of atoms constituting the aforementioned linking chain. As the monomer (A2), at least one alkyl unit (e.g., -(CH2)n-) or an oxyalkyl unit (e.g., an oxyalkylene unit where m is 2, an oxypropylene unit where m is 3, or an oxybutylene unit where m is 4) can be used between the ethylene unsaturated group and the hydroxyl and / or carboxyl group, but there is no particular limitation. The number of the alkyl or oxyalkyl units is not particularly limited, and may be 1 or more (e.g., 1 to 15, 1 to 10, 2 to 6, or 2 to 4). Furthermore, n in the formula representing the alkyl unit may be an integer from 1 to 10, and may be 2 or more, 3 or more, 4 or more, 6 or less, or 5 or less. m in the formula representing the oxyalkyl unit may be an integer from 2 or more, for example, an integer from 2 to 4. The monomer (A2) may contain, in addition to the aforementioned vinyl unsaturated groups, hydroxyl and / or carboxyl groups, alkyl units and / or oxyalkyl units, ester or ether bonds, thioether bonds, aromatic rings, aliphatic rings, or heterocycles (e.g., rings containing nitrogen (N) or oxygen (O) and sulfur (S) atoms). Furthermore, the aforementioned alkyl or oxyalkyl units may also have substituents.

[0075] Examples of hydroxyl-containing monomers include: 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-hydroxylaurate (meth)acrylate, and hydroxyalkyl (meth)acrylates such as (4-hydroxymethylcyclohexyl)meth(meth)acrylate, but not limited to these. Suitable examples of hydroxyl-containing monomers include 4-hydroxybutyl acrylate (Tg: -40℃) and 2-hydroxyethyl acrylate (Tg: -15℃). From the viewpoint of improving flexibility in the room temperature range, 4-hydroxybutyl acrylate, with its lower Tg, is preferable. Furthermore, in the case where hydroxyalkyl (meth)acrylate is used as a hydroxyl-containing monomer and the hydroxyl group is utilized in the crosslinking reaction, from the viewpoint of obtaining a highly flexible crosslinked structure, it is preferable to use a monomer with a higher carbon number of the hydroxyalkyl group in the aforementioned hydroxyalkyl (meth)acrylate, and preferably a hydroxyalkyl (meth)acrylate (e.g., 4-hydroxybutyl acrylate) with 3 or more carbons of the aforementioned hydroxyalkyl group (e.g., 3 to 12, preferably 4 to 10). In an ideal case, 50% by weight or more (e.g., greater than 50% by weight, greater than 70% by weight, or greater than 85% by weight) of the monomer (A2) can be 4-hydroxybutyl acrylate. One hydroxyl-containing monomer can be used alone or in combination of two or more.

[0076] In several embodiments using hydroxyl-containing monomers as monomers (A2), the aforementioned hydroxyl-containing monomers may be one or more compounds selected from those without a methacrylic group. Suitable examples of hydroxyl-containing monomers without a methacrylic group include the various hydroxyalkyl acrylates mentioned above. For example, it is preferable that the hydroxyl-containing monomer used as monomer (A2) comprises more than 50% by weight, more than 70% by weight, or more than 85% by weight of hydroxyalkyl acrylate. By using hydroxyalkyl acrylates, it is possible to introduce hydroxyl groups into acrylic polymers, which can help provide crosslinking points or impart moderate aggregation properties, and it is easier to obtain adhesives with good flexibility or adhesion at room temperature compared to using only the corresponding methacrylic hydroxyalkyl group.

[0077] Examples of carboxyl-containing monomers, besides acrylic monomers such as (meth)acrylic acid, (meth)acrylate carboxyethyl ester, and (meth)acrylate carboxypentyl ester, include isoconic acid, maleic acid, fumaric acid, crotonic acid, and isocrotonic acid, but are not limited to these. Suitable examples of carboxyl-containing monomers include acrylic acid and methacrylic acid. Furthermore, from the viewpoint of reducing the elastic modulus of the adhesive, among several options, compounds represented by the following formula (1) are preferable carboxyl-containing monomers. CH 2=CR 1-COO-R 2-OCO-R 3-COOH (1) Here, R1 in formula (1) above is hydrogen or methyl. R2 and R3 are divalent linkages (specifically, organic groups with 1 to 20 carbon atoms (e.g., 2 to 10, preferably 2 to 5), which may be the same or different from each other. R2 and R3 in formula (1) above can be, for example, divalent aliphatic hydrocarbon groups, aromatic hydrocarbon groups, or alicyclic hydrocarbon groups. For example, R2 and R3 above can be alkyl groups with 2 to 5 carbon atoms. Specific examples of the carboxyl-containing monomers shown in formula (1) above include: 2-(meth)propenyloxyethyl hexahydrophthalic acid, 2-(meth)propenyloxyethyl-phthalic acid, 2-(meth)propenyloxyethyl-2-hydroxyethyl-phthalic acid, 2-(meth)propenyloxyethyl-succinic acid, 2-(meth)propenyloxypropyl hexahydrophthalic acid hydrogen ester, 2-(meth)propenyloxypropyl phthalic acid hydrogen ester, 2-(meth)propenyloxypropyl tetrahydrophthalic acid hydrogen ester, etc. One carboxyl-containing monomer can be used alone or in combination of two or more. Hydroxyl-containing monomers and carboxyl-containing monomers can also be used together.

[0078] The content of monomer (A2) in the monomer components constituting acrylic polymers is not particularly limited and can be set according to the purpose. In several samples, the content of the aforementioned monomer (A2) can 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 performance effect, in several samples, the content of the aforementioned monomer (A2) should preferably be 1% by weight or more, can be 2% by weight or more, or can be 4% by weight or more. The upper limit of the content of monomer (A2) in the monomer components is set so that the total content with the monomer (A1) does not exceed 100% by weight. In several samples, it is appropriate to set the content of the aforementioned monomer (A2) to 30% by weight or less or 25% by weight or less, for example. From the viewpoint of making it easier to achieve a high refractive index by having a relatively high content of monomer (A1), it is preferable to set it to 20% by weight or less, more preferably 15% by weight or less, less than 12% by weight, less than 10% by weight, or less than 7% by weight. In several ideal samples, from the viewpoint of making the adhesive have a low elastic modulus, the content of the above monomer (A2) is less than 5% by weight, preferably less than 3% by weight, and can also be less than 1.5% by weight.

[0079] The total content of monomers (A1) and (A2) in the monomer components constituting acrylic polymers may be, for example, 31% by weight or more, preferably 51% by weight or more, possibly 61% by weight or more, or possibly 71% by weight or more. In several cases, from the viewpoint of easily and appropriately exerting the effects of these monomers, the total content of monomers (A1) and (A2) in the monomer components constituting acrylic polymers may be, for example, 76% by weight or more, preferably 81% by weight or more, possibly 86% by weight or more, possibly 91% by weight or more, possibly 96% by weight or more, possibly 99% by weight or more, or substantially 100% by weight.

[0080] (Single A3) In several ideal samples, the monomer components constituting the acrylic polymer may contain, in addition to the aforementioned monomer (A1), alkyl (meth)acrylate (hereinafter also referred to as "monomer (A3)"). Monomer (A3) can help reduce the elastic modulus of the adhesive. Furthermore, it can help improve the adhesive properties such as compatibility or adhesion of additives within the adhesive. Monomer (A3) can be used alone or in combination of two or more.

[0081] The monomer (A3) is suitable for use in alkyl (meth)acrylates with a straight-chain or branched alkyl group having 1 to 20 carbon atoms at the ester terminus (i.e., C1-20). (Meth)acrylate C Specific examples of 1-20 alkyl esters include: methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, butyl methacrylate, tert-butyl methacrylate, pentyl methacrylate, isoamyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, isooctyl methacrylate, nonyl methacrylate, isononyl methacrylate, decyl methacrylate, isodecyl methacrylate, undecyl methacrylate, dodecyl methacrylate, tridecyl methacrylate, tetradecyl methacrylate, pentadecyl methacrylate, hexadecyl methacrylate, heptyl methacrylate, octadecyl methacrylate, isooctadecyl methacrylate, nonadecanyl methacrylate, ecicoacrylate, etc., but are not limited thereto.

[0082] In several formulations, at least a portion of the monomer (A3) may be a homopolymer of alkyl (meth)acrylate with a Tg below -20°C (preferably below -40°C, e.g., below -50°C). This low-Tg alkyl (meth)acrylate can help improve the low elastic modulus of the adhesive. Furthermore, it can help improve adhesive properties such as adhesion strength. There is no particular limitation on the lower limit of the Tg of the aforementioned alkyl (meth)acrylate; for example, it may be above -85°C, above -75°C, above -65°C, or above -60°C. Specific examples of the aforementioned low-Tg alkyl (meth)acrylate include n-butyl acrylate (BA), 2-ethylhexyl acrylate (2EHA), heptaacrylate, octyl acrylate, and isononyl acrylate (iNA). In several other formulations, at least a portion of the monomer (A3) may be a homopolymer of alkyl (meth)acrylate with a Tg above -20°C (e.g., above -10°C). The upper limit of the Tg of the aforementioned alkyl methacrylates is, for example, below 10°C, below 5°C, or below 0°C. Alkyl methacrylates with a Tg within this range can help adjust the elastic modulus of the adhesive. Alkyl methacrylates with the above-mentioned Tg are preferably used in combination with the aforementioned low-Tg alkyl methacrylates, but there are no particular limitations. A specific example of an alkyl methacrylate with the above-mentioned Tg is lauryl acrylate (LA).

[0083] Among the various monomers (A3), C4-8 alkyl methacrylate is preferred as the monomer (A3). C4-8 alkyl acrylate is particularly suitable. One C4-8 alkyl methacrylate can be used alone or in combination of two or more. Using C4-8 alkyl methacrylate tends to easily reduce the elastic modulus of the adhesive and readily achieve good adhesive properties (adhesion strength, etc.). In monomers (A3), the ratio of C4-8 alkyl methacrylate to C4-8 alkyl methacrylate in the monomer composition should ideally be 30% by weight or more, preferably 50% by weight or more, more preferably 70% by weight or more, more preferably 90% by weight or more, and practically 100% by weight.

[0084] Among several states of monomer (A3), C1-6 alkyl methacrylate is suitable as monomer (A3). By using C1-6 alkyl methacrylate, the storage modulus of elasticity in each temperature range can be adjusted. For example, setting the storage modulus of elasticity in the high-temperature range to be relatively high can suppress the widening difference between the storage modulus of elasticity in the low-temperature and high-temperature ranges. Furthermore, C1-6 alkyl methacrylate tends to have excellent copolymerization properties with monomer (A1). C1-6 alkyl methacrylate can be used alone or in combination of two or more. C1-6 alkyl methacrylate is preferably C1-6 alkyl acrylate, more preferably C2-6 alkyl acrylate, and even more preferably C4-6 alkyl acrylate. Among the other several states, C1-6 alkyl esters of (meth)acrylate are preferably C1-4 alkyl esters of (meth)acrylate, more preferably C2-4 alkyl esters of (meth)acrylate, and even more preferably C2-4 alkyl esters of acrylic acid. A suitable example of a suitable C1-6 alkyl ester of (meth)acrylate is BA.

[0085] In the monomer components constituting acrylic polymers, the content of (meth)acrylate C1-6 alkyl esters can 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 several samples, from the viewpoint of low elastic modulus and adhesion, the content of the aforementioned (meth)acrylate C1-6 alkyl esters can be 10% by weight or more, 15% by weight or more, 20% by weight or more, or 25% by weight or more (e.g., 30% by weight or more). The upper limit of the content of (meth)acrylate C1-6 alkyl esters in the monomer components is, for example, less than 50% by weight, or less than 35% by weight. In several samples, from the viewpoint of maintaining a high refractive index, the content of the aforementioned (meth)acrylate C1-6 alkyl esters is, for example, less than 24% by weight, preferably less than 20% by weight, more preferably less than 17% by weight, less than 12% by weight, less than 7% by weight, less than 3% by weight, or less than 1% by weight. The techniques disclosed herein can still be implemented even without the substantial use of (meth)acrylate C1-6 alkyl esters.

[0086] Among the other states of monomer (A3), C7-12 alkyl methacrylates are suitable as monomer (A3). By using C7-12 alkyl methacrylates, the storage modulus of elasticity can be appropriately reduced. C7-12 alkyl methacrylates can be used alone or in combination of two or more. C7-12 alkyl methacrylates are preferably C7-10 alkyl acrylates, more preferably C7-9 alkyl acrylates, and even more preferably C8 alkyl acrylates. Examples of C7-12 alkyl methacrylates include 2EHA, iNA, and LA; 2EHA is a suitable example.

[0087] In the monomer components constituting acrylic polymers, the content of (meth)acrylate C7-12 alkyl esters can 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 several samples, from the viewpoint of low elastic modulus and adhesion, the content of the aforementioned (meth)acrylate C7-12 alkyl esters can be 10% by weight or more, 15% by weight or more, 20% by weight or more, or 25% by weight or more (e.g., 30% by weight or more). The upper limit of the content of (meth)acrylate C7-12 alkyl esters in the monomer components is, for example, less than 50% by weight, or less than 35% by weight. In several samples, from the viewpoint of maintaining a high refractive index, the content of the aforementioned (meth)acrylate C7-12 alkyl esters is, for example, less than 24% by weight, preferably less than 20% by weight, more preferably less than 17% by weight, less than 12% by weight, less than 7% by weight, less than 3% by weight, or less than 1% by weight. The techniques disclosed herein can still be implemented even without substantially using (meth)acrylate C7-12 alkyl esters.

[0088] From the viewpoint of reducing the elastic modulus, at least a portion of the monomer (A3) used in several samples should preferably be alkyl acrylates. The use of alkyl acrylates is also advantageous in terms of adhesive properties such as adhesion strength. For example, alkyl acrylates should preferably comprise at least 50% by weight of the monomer (A3), and the proportion of alkyl acrylates in the monomer (A3) should preferably be at least 75% by weight, more preferably at least 90% by weight. The monomer (A3) can also be substantially 100% by weight of alkyl acrylates. Samples can also be prepared using only one or more alkyl acrylates as monomers (A3) without using alkyl methacrylates.

[0089] In samples containing monomer (A3) (alkyl methacrylate), the content of alkyl methacrylate in the monomer component can be set to appropriately exert its effect. In several samples, the content of the aforementioned alkyl methacrylate can be, for example, 1% by weight or more, 3% by weight or more, 5% by weight or more, or 8% by weight or more. The upper limit of the content of monomer (A3) in the monomer component is set so that the total content of monomers (A1) and (A2) does not exceed 100% by weight, for example, less than 50% by weight or less than 35% by weight. In several samples, the content of the aforementioned monomer (A3) can be, for example, 24% by weight or less. Generally speaking, alkyl methacrylate has a low refractive index; therefore, in order to achieve a high refractive index, it is advantageous to limit the content of monomer (A3) in the monomer component and make the content of monomer (A1) relatively higher. From the aforementioned viewpoint, it is appropriate for the content of monomer (A3) to be less than 23% by weight of the monomer component, preferably less than 20% by weight, more preferably less than 17% by weight, less than 12% by weight, less than 7% by weight, less than 3% by weight, or even less than 1% by weight. The technology disclosed herein can still be appropriately implemented even in a configuration where monomer (A3) is not substantially used.

[0090] (Other monomers) The monomer components constituting acrylic polymers may also include monomers other than those mentioned above (A1), (A2), and (A3) (hereinafter referred to as "other monomers"), depending on requirements. These other monomers may be used, for example, to adjust the Tg of the acrylic polymer, adjust adhesive properties, or improve compatibility within the adhesive layer. One of these other monomers may be used alone or in combination of two or more.

[0091] Examples of other monomers mentioned above include monomers with functional groups other than hydroxyl and carboxyl groups (including functionalized monomers). For example, other monomers that can improve the cohesiveness or heat resistance of adhesives include monomers containing sulfonic acid groups, monomers containing phosphoric acid groups, and monomers containing cyano groups. Furthermore, monomers that can introduce functional groups into acrylic polymers that can serve as crosslinking sites, or that can help improve adhesion to the adherend or improve compatibility within the adhesive, include: monomers containing amide groups (e.g., (meth)acrylamide, N-hydroxymethyl(meth)acrylamide, etc.), monomers containing amine groups (e.g., aminoethyl(meth)acrylate, N,N-dimethylaminoethyl(meth)acrylate, etc.), monomers with nitrogen-containing rings (e.g., N-vinyl-2-pyrrolidone, N-(meth)acrylamide morphofolin, etc.), monomers containing amide groups, monomers containing epoxy groups, monomers containing ketone groups, monomers containing isocyanate groups, and monomers containing alkoxysilicon groups, etc. Furthermore, monomers with nitrogen-containing rings, such as N-vinyl-2-pyrrolidone, are also equivalent to monomers containing amine groups. The relationship between the aforementioned monomers with nitrogen-containing rings and amine-containing monomers is similar.

[0092] Other monomers that can be used besides the functionalized monomers mentioned above include: vinyl acetate and other vinyl ester monomers; non-aromatic cyclic (meth)acrylates such as cyclohexyl methacrylate and isoborneol (meth)acrylate; olefin monomers such as ethylene, butadiene, and isobutene; chlorinated monomers such as vinyl chloride; alkoxy-containing monomers such as methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylate, and ethoxyethoxyethyl (meth)acrylate; and vinyl ether monomers such as methyl vinyl ether. A suitable example of other monomers that can be used to improve the flexibility of adhesives is ethoxyethoxyethyl acrylate (also known as ethyl carbitol acrylate, Tg of homopolymer: -67°C).

[0093] When using the aforementioned other monomers, there are no particular restrictions on their usage, and they can be appropriately set within a range where the total amount of monomer components does not exceed 100% by weight. From the viewpoint of easily achieving the refractive index enhancement effect obtained by using monomer (Al), the content of the aforementioned other monomers in the monomer component can be set to, for example, about 35% by weight or less, about 25% by weight or less (e.g., 0 to 25% by weight), which is appropriate, about 20% by weight or less (e.g., 0 to 20% by weight), about 10% by weight or less (e.g., 0 to 10% by weight), preferably about 5% by weight or less, for example, about 1% by weight or less. The technique disclosed herein can be suitably implemented in a monomer component that substantially does not contain the aforementioned other monomers.

[0094] In several formulations, the monomeric component constituting the acrylic polymer may be a composition in which the amount of methacrylamide monomer used is suppressed to a predetermined level. For example, the amount of methacrylamide monomer used in the monomeric component may be less than 5% by weight, less than 3% by weight, less than 1% by weight, or less than 0.5% by weight. Limiting the amount of methacrylamide monomer used as described is advantageous from the viewpoint of achieving a balance between flexibility or adhesiveness and a high refractive index adhesive. The monomeric component constituting the acrylic polymer may also be a composition that does not contain methacrylamide monomers (e.g., a composition consisting only of acrylamide monomers).

[0095] In several samples, from the viewpoint of inhibiting the coloring or discoloration (e.g., yellowing) of the adhesive, the amount of carboxyl-containing monomers used in the monomer component of the base polymer (e.g., acrylic polymer) constituting the adhesive is limited. The amount of carboxyl-containing monomers used in the monomer component can be, for example, less than 1% by weight, less than 0.5% by weight, less than 0.3% by weight, less than 0.1% by weight, or less than 0.05% by weight. This limitation on the amount of carboxyl-containing monomers is also advantageous from the viewpoint of inhibiting corrosion of metallic materials (e.g., metal wiring or metal films that may exist on the adherend) that can contact or are disposed adjacent to the adhesive disclosed herein. The technique disclosed herein can be implemented in samples where the monomer component does not contain carboxyl-containing monomers. For the same reason, in several samples, the amount of monomers containing acidic functional groups (including sulfonic acid groups, phosphate groups, etc., in addition to carboxyl groups) in the monomer components constituting the adhesive base polymer should be limited. The amount of acidic functional group monomers in the monomer components of the aforementioned samples can be the ideal amount of carboxyl group monomers mentioned above. The technology disclosed herein is suitable for implementation in samples where the monomer components do not contain acidic functional group monomers (i.e., the adhesive base polymer is acid-free).

[0096] (Glass transition temperature Tg T) The monomer components of the base polymer (e.g., acrylic polymer) constituting the adhesive should preferably have a glass transition temperature (TgT) of approximately 15°C or lower, depending on the composition of the monomer components. In several cases, the aforementioned glass transition temperature (TgT) is preferably below 10°C, more preferably below 5°C, even more preferably below 1°C, and may also be below 0°C. In other cases, the aforementioned glass transition temperature (TgT) may be below -10°C, below -20°C, below -25°C, below -30°C, or below -35°C. From the viewpoint of achieving a low elastic modulus in the adhesive, a low glass transition temperature (TgT) is advantageous. Furthermore, the glass transition temperature (TgT) can be, for example, above -60°C, and from the viewpoint of facilitating a high refractive index in the adhesive, it is preferably above -50°C, more preferably above -45°C, and may also be above -40°C. In several ideal samples, the aforementioned glass transition temperature TgT can be higher than -30°C, higher than -20°C, higher than -10°C, or even higher than -5°C. Adhesives that combine high refractive index and low elastic modulus can be suitably formed using a base polymer with a composition having a glass transition temperature TgT within the aforementioned range.

[0097] Here, unless otherwise specified, the glass transition temperature Tg refers to the glass transition temperature obtained by the Fox formula based on the composition of the aforementioned monomer components. The Fox formula, as shown below, is a relationship between the Tg of the copolymer and the glass transition temperature Tgi of the homopolymer of each monomer constituting the copolymer after homopolymerization. 1 / Tg=Σ(Wi / Tgi) In the above Fox formula, Tg represents the glass transition temperature of the copolymer (unit: K), Wi represents the weight fraction of monomer i in the copolymer (weight-based copolymerization ratio), and Tgi represents the glass transition temperature of the homopolymer of monomer i (unit: K). The glass transition temperature of the homopolymer used in the calculation of Tg is the value recorded in known sources such as the "Polymer Handbook" (3rd edition, John Wiley & Sons, Inc., 1989). For monomers with multiple values ​​recorded in the aforementioned Polymer Handbook, the highest value is used. When the Tg of the homopolymer is not recorded in known sources, the value obtained using the measurement method described in Japanese Patent Application Publication No. 2007-51271 is used.

[0098] (Modification method of base polymer) In the techniques disclosed herein, there are no particular limitations on the method for obtaining a base polymer (e.g., an acrylic polymer) composed of the monomer components described above, and known polymerization methods such as solution polymerization, emulsion polymerization, bulk polymerization, suspension polymerization, and photopolymerization can be appropriately employed. For example, solution polymerization is suitable. The polymerization temperature during solution polymerization can be appropriately selected according to the type of monomer and solvent used, the type of polymerization initiator, etc., and can be set to approximately 20°C to 170°C (typically approximately 40°C to 140°C).

[0099] The solvent used in solution polymerization (polymerization solvent) can be appropriately selected from conventionally known organic solvents. For example, one or a mixture of two or more solvents selected from the following can be used: aromatic compounds such as toluene (typically aromatic hydrocarbons); acetates such as ethyl acetate; aliphatic or alicyclic hydrocarbons such as hexane or cyclohexane; haloalkanes such as 1,2-dichloroethane; lower alcohols such as isopropanol (e.g., monohydric alcohols with 1 to 4 carbon atoms); ethers such as tributyl methyl ether; ketones such as methyl ethyl ketone, etc.

[0100] The initiator used for polymerization can be appropriately selected from 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 used. Other examples of polymerization initiators include: persulfates such as potassium persulfate; peroxide-based initiators such as benzoyl peroxide and hydrogen peroxide; substituted ethane-based initiators such as phenyl-substituted ethane; and aromatic carbonyl compounds. Further examples of polymerization initiators include redox initiators composed of peroxides and reducing agents. One polymerization initiator can be used alone or in combination of two or more. The amount of polymerization initiator used is generally acceptable, for example, it can be selected in the range of approximately 0.005 to 1 part by weight (typically approximately 0.01 to 1 part by weight) relative to 100 parts by weight of the monomer component.

[0101] In the above polymerization, various known chain transfer agents can be used as needed. For example, thiols such as n-dodecylthiol, tridecylthiol, hydrothioacetic acid, and α-thioglycerol can be used. Alternatively, chain transfer agents without sulfur atoms (non-sulfur chain transfer agents) can also 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 terpinene; and styrene-based agents such as α-methylstyrene and α-methylstyrene dimers. One chain transfer agent can be used alone or in combination of two or more. The amount of chain transfer agent used relative to 100 parts by weight of the monomer component can be set to approximately 0.01 to 1 part by weight.

[0102] The weight-average molecular weight (Mw) of the base polymer (e.g., acrylic polymer) is not particularly limited, but is suitable for example, being approximately 30 × 10⁴ or higher, approximately 50 × 10⁴ or higher, approximately 70 × 10⁴ or higher, or approximately 80 × 10⁴ or higher. By using a base polymer with a predetermined Mw value or higher, it is easy to obtain a moderate cohesive force that can achieve the desired adhesive properties. Furthermore, it is possible to contain more additives such as plasticizers, which tends to easily achieve the desired modulus of elasticity. The upper limit of the Mw of the base polymer is, for example, approximately 500 × 10⁴ or lower, and from the point of view of adhesive properties, it is preferable to be in the range of approximately 400 × 10⁴ or lower (more preferably approximately 150 × 10⁴ or lower, for example, approximately 130 × 10⁴ or lower).

[0103] Here, the Mw of the polymer can be obtained by converting it to polystyrene using gel permeation chromatography (GPC). Specifically, the GPC can be determined using a product named "HLC-8220GPC" (manufactured by Tosoh Corporation) under the following conditions. [GPC Measurement Conditions] Sample concentration: 0.2% by weight (tetrahydrofuran solution) Sample injection volume: 10µL Solution: Tetrahydrofuran (THF) Flow rate: 0.6 mL / min Column temperature (measurement temperature): 40℃ Tube column: Sample columns: 1 tube of "TSKguardcolumn SuperHZ-H" + 2 tubes of "TSKgel SuperHZM-H" (manufactured by Tosoh Corporation) Reference tube: 1 tube of "TSKgel SuperH-RC" (manufactured by Tosoh Corporation) Detector: Differential refractometer (RI) Standard sample: polystyrene

[0104] (Plasticizer) In several samples, the aforementioned adhesives (e.g., acrylic adhesives) contain plasticizers in addition to the base polymer. By using plasticizers, the elastic modulus of the adhesive can be effectively reduced. Furthermore, in the form of adhesive sheets, flexibility or adaptability to deformation can be improved. One type of plasticizer can be used alone or in combination of two or more.

[0105] Suitable examples of plasticizers disclosed herein include cyclic unsaturated organic compounds having two or more double-bonded rings. In other words, the plasticizers of the aforementioned suitable examples are compounds having two or more double-bonded rings in one molecule. Therefore, the aforementioned plasticizers have at least a first double-bonded ring and a second double-bonded ring. By having two or more double-bonded rings, the refractive index of the adhesive can be maintained or preserved while also contributing to a lower elastic modulus of the adhesive. From the viewpoint of achieving a plasticizing effect, the number of double-bonded rings in the aforementioned plasticizers should preferably be 6 or less, and may be 4 or less, or may be 3 or less.

[0106] Furthermore, the plasticizer used in the disclosed technology is preferably a compound that is liquid at 30°C. In addition, in this specification, "liquid" means fluidity, and in terms of the state of matter, it refers to a liquid. The compound includes compounds with melting points below 30°C. The aforementioned plasticizer is liquid at 30°C, thereby appropriately exerting its plasticizing effect and effectively achieving a low elastic modulus of the adhesive. The aforementioned plasticizer is preferably a compound that is liquid at 25°C, and more preferably a compound that is liquid at 20°C. For example, by using a compound having two or more rings containing double bonds and being liquid at 30°C as the aforementioned plasticizer, an adhesive that balances high refractive index and low elastic modulus can be appropriately formed.

[0107] In the use of plasticizers having double-bonded rings, the double-bonded rings in the plasticizers can be either conjugated double bonds (typically aromatic rings) or non-conjugated double bonds. The plasticizers can have at least one type of ring selected from aromatic rings and heterocycles as the double-bonded rings. Furthermore, the heterocycles can have a structure incorporated into the aromatic rings or a heterocyclic structure containing double bonds that differs from the aromatic rings. The aforementioned plasticizer may have a double-bonded ring (typically an aromatic ring) that can be a benzene ring (which may be a benzene ring constituting part of a biphenyl or phenanthrene structure); a carbon ring such as a condensed ring of naphthalene, indene, azurite, anthracene, or phenanthrene; or a heterocyclic ring such as a pyridine ring, pyrimidine ring, pyridine ring, triazine ring, pyrrole ring, pyrazole ring, imidazole ring, triazole ring, acetazole ring, isoacetazole ring, thiazole ring, or thiophene ring. The heteroatoms constituting the rings in the aforementioned heterocyclic rings may, for example, be selected from one or more types of the group consisting of nitrogen, sulfur, and oxygen. In several cases, the heteroatoms constituting the heterocyclic rings may be one or both of nitrogen and sulfur. The aforementioned plasticizer may also, for example, have a structure similar to dinaphthothiophene, in which one or more carbon rings have been condensed with one or more heterocyclic rings.

[0108] The aforementioned double-bonded ring (typically an aromatic ring, preferably a carbocyclic ring) may have one or more substituents on the ring constituent atoms, or it may not have substituents. When substituents are present, examples of substituents include alkyl, alkoxy, aryloxy, hydroxyl, halogen atoms (fluorine, chlorine, bromine, etc.), hydroxyalkyl, hydroxyalkyloxy, epoxypropoxy, etc., but are not limited thereto. Among substituents containing carbon atoms, the number of carbon atoms contained in the substituent is preferably 1 to 4, more preferably 1 to 3, and for example, 1 or 2. In several cases, the aforementioned double-bonded ring may be an aromatic ring without substituents on the ring constituent atoms, or it may be an aromatic ring with one or more substituents selected from the group consisting of alkyl, alkoxy, vinyl unsaturated groups (e.g., (meth)acryloxy), hydroxy, and hydroxyalkyl. Substituents may suitably be alkyl, alkoxy, or hydroxyalkyl.

[0109] Among several types of samples, compounds without vinyl unsaturated groups can be appropriately used as plasticizers. This can suppress the deterioration of the adhesive composition due to heat or light (due to gelation or increased viscosity leading to decreased leveling properties), thus improving storage stability. The use of plasticizers without vinyl unsaturated groups is also preferable from the viewpoint that, in adhesive sheets having an adhesive layer containing such plasticizers, changes in elastic modulus, dimensional changes, deformations (warping, undulations, etc.), and optical strain caused by the reaction of vinyl unsaturated groups can be suppressed.

[0110] Plasticizers with a refractive index of approximately 1.50 or higher are suitable. Using high-refractive-index plasticizers allows for a better balance between high refractive index and low modulus of elasticity. From the perspective of maintaining and increasing the refractive index of the adhesive while achieving a low modulus of elasticity, the refractive index of the plasticizer should ideally be approximately 1.51 or higher, preferably approximately 1.53 or higher, even better if it is approximately 1.55 or higher, and can be approximately 1.56 or higher, approximately 1.58 or higher, approximately 1.60 or higher, or approximately 1.62 or higher. Among these options, from the perspective of ease of formulation of the adhesive composition or compatibility within the adhesive, a refractive index of 2.50 or lower is appropriate, 2.00 or lower is advantageous, and 1.90 or lower, 1.80 or lower, or 1.70 or lower are also acceptable. Furthermore, the refractive index of the plasticizer is measured using an Abbe refractometer at a wavelength of 589 nm and a temperature of 25°C, similar to that of the monomer. If a nominal value for the refractive index at 25°C is provided by the manufacturer, that nominal value may be used.

[0111] There is no particular limitation on the molecular weight of plasticizers, but generally, those with a smaller molecular weight than the base polymer (e.g., acrylic polymers) are used. From the viewpoint of easily exhibiting plasticizing effects, a molecular weight of 30,000 or less is suitable, 25,000 or less is advantageous, less than 10,000 (e.g., less than 5,000), or less than 3,000 is acceptable. Among several options, a molecular weight of 2,000 or less is preferable, less than 1,200 is more desirable, less than 900 is even better, less than 600, less than 500, less than 400, less than 300, or less than 250 (e.g., less than 220). A relatively small molecular weight of plasticizer is advantageous from the viewpoint of improving compatibility within the adhesive layer. Furthermore, from the perspective of easily achieving a sufficient plasticizing effect, a molecular weight of 100 or higher for the plasticizer is appropriate, preferably 130 or higher, more preferably 150 or higher, and can be 170 or higher, 200 or higher, 220 or higher, or even 250 or higher. A molecular weight that is not too low is also desirable from the perspective of improving the heat resistance of the adhesive sheet or inhibiting contamination of the adhered body. Among several options, a molecular weight of 300 or higher, 315 or higher for the plasticizer is appropriate, and can also be 350 or higher. Plasticizers with large molecular weights are less prone to vaporization; therefore, by using plasticizers with large molecular weights in adhesives, it is easy to obtain adhesives that exhibit stable properties. Furthermore, plasticizers with large molecular weights do not easily migrate within the adhesive. Therefore, phenomena such as plasticizer migration to the adhesive surface that affects adhesive properties are less likely to occur. The molecular weight of the aforementioned plasticizer should preferably be above 400, more preferably above 450, especially above 500, and may also be above 530. Furthermore, the molecular weight of plasticizers can be calculated based on their chemical structure. When a nominal molecular weight value is provided by the manufacturer, that value can be used.

[0112] Among several states, the plasticizer may be selected from one or more of the following compounds: compounds having a structure in which two or more non-condensed double-bonded rings (typically aromatic rings) are bonded by linker groups; compounds having a structure in which two or more non-condensed double-bonded rings (typically aromatic rings) are directly (i.e., chemically bonded without being separated by other atoms); compounds having a condensed double-bonded ring (typically aromatic ring) structure; compounds having a cyclopentadienyl structure; compounds having a dibenzothiophene structure; and compounds that are liquid at 30°C (e.g., 25°C or 20°C).

[0113] In the case of using a compound having a structure of two or more non-condensed double-bonded rings separated by a linker group as a plasticizer, the linker group may be, for example, an oxy group (-O-), a thiooxy group (-S-), an oxyalkyl group (e.g., -O-(CH2)n-yl, where n is 1 to 3, preferably 1), a thiooxyalkyl group (e.g., -S-(CH2)n-yl, where n is 1 to 3, preferably 1), a linear alkyl group (i.e., -(CH2)n-yl, where n is 1 to 6, preferably 1 to 3), or a partially or fully halogenated group among the above-mentioned oxyalkyl groups, thiooxyalkyl groups, and linear alkyl groups. The linker group may also be a group having a siloxane bond (-SiOR-) or an ester bond. In plasticizers, the linking group connecting the first double-bonded ring (non-condensed ring) and the second double-bonded ring (non-condensed ring) can also be selected from the same type as described above. From the viewpoint of reducing the elastic modulus of the adhesive, suitable examples of the above-mentioned linking groups include oxy-, thiooxy-, oxyalkyl-, and straight-chain alkyl-. The number of atoms of the above-mentioned linking group is not particularly limited, for example, 1 to 30, 1 to 25, 1 to 20, 1 to 18 is appropriate, 1 to 12 is preferable, 1 to 10 is more preferable, 1 to 8 is more preferable, 1 to 5 is particularly preferable, 1 to 3 is acceptable, or 1 or 2 is also acceptable. Furthermore, the number of atoms of the linking group refers to the minimum number of atoms required to move from one non-condensed double-bonded ring to another non-condensed double-bonded ring. For example, when the linking group is composed of a straight-chain alkyl- (i.e., -(CH₂)n-yl), the number of n becomes the number of atoms of the linking group. For example, when the linking group is oxyethyl (i.e., -(C₂H₄O)ₙ-yl), the product of the sum of the number of carbon atoms (2) and the number of oxygen atoms (1) of the oxyethyl group, 3, and n (3n) becomes the number of atoms of the linking group. Suitable examples of the above-mentioned compounds include compounds having a phenoxybenzyl group. Examples of the above-mentioned compounds include phenoxybenzyl (meth)acrylate (e.g., m-phenoxybenzyl (meth)acrylate), phenoxybenzyl alcohol, oxybis[(alkoxyalkyl)benzene] (e.g., 4,4'-oxybis[(methoxymethyl)benzene]), etc. Other examples of the above-mentioned compounds include polysiloxane plasticizers (specifically, siloxane compounds) described later.

[0114] The compounds having a structure with two or more directly chemically bonded rings containing double bonds (non-condensed rings) can be, for example, compounds containing biphenyl structures, compounds containing triphenyl structures, etc. Furthermore, examples of compounds having a condensed ring structure containing double bonds include compounds containing naphthalene rings, compounds containing anthracene rings, etc. Specific examples include 1-naphthyl ketone, etc. In addition, the compounds having the above-mentioned benzo[a]thiophene structure are included in the concept of compounds having a structure with two or more directly chemically bonded rings containing double bonds (non-condensed rings) because they contain a naphthalene structure and a structure with a thiophene ring condensed with two naphthalene structures. The compounds having the above-mentioned dibenzo[a]thiophene structure are included in the concept of compounds having a condensed ring structure containing double bonds because they contain a thiophene ring condensed with two benzene rings.

[0115] Polysiloxane plasticizers can be used as plasticizers in several formulations. By using polysiloxane plasticizers, stable plasticizing effects and high adhesion are easily obtained, thus achieving a balanced improvement in the adhesive's refractive index, flexibility, and adhesion. There are no particular limitations on polysiloxane plasticizers; for example, compounds with one or more rings containing double bonds can be used. Furthermore, polysiloxane plasticizers are preferably compounds that are liquid at 30°C. Specifically, polysiloxane plasticizers are siloxane compounds with one or more Si atoms (typically two or more), with no particular upper limit, for example, around 10 or less. In one molecule of a polysiloxane plasticizer, the Si atoms and the rings containing double bonds may or may not be directly bonded. At least one of the aforementioned Si atoms should preferably be directly bonded to at least one ring containing a double bond. Polysiloxane plasticizers can be used alone or in combination of two or more.

[0116] Among several types, polysiloxane plasticizers may be composed of siloxane compounds having 2 to 5 Si atoms, wherein at least one of the Si atoms has a ring with two or more double bonds bonded to it. Polysiloxane plasticizers composed of siloxane compounds having the aforementioned structure can exert a plasticizing effect based on the flexibility of the siloxane structure, and by having at least one Si atom having 2 to 5 Si atoms bonded to a ring with two or more double bonds, a balance can be achieved between the ease of mixing or compatibility with the target material and the stability of the aforementioned plasticizing effect (e.g., a low rate of increase in elastic modulus during storage under humid heat). From a chemical stability point of view, the aforementioned siloxane compound preferably does not have hydrogen atoms bonded to Si atoms. That is, it is preferably a siloxane compound without Si-H bonds.

[0117] When the number of Si atoms in the aforementioned silicate compounds is 3 or more, the silicate compounds can be chain-like or cyclic. From the viewpoint of suppressing volatilization, chain-like silicate compounds are preferable. The aforementioned chain-like silicate compounds with 3 or more Si atoms can be linear or branched. From the viewpoint of obtaining a higher plasticizing effect, linear chains are preferable. Unless otherwise specified, silicate compounds with 3 or more Si atoms refer to chain-like (typically linear) silicate compounds with 3 or more Si atoms.

[0118] In several states, the polysiloxane plasticizer described above may have a ring containing a double bond that is conjugated (typically an aromatic ring) or a ring containing a non-conjugated double bond. The plasticizer may have at least one ring selected from aromatic rings and heterocycles as the ring containing a double bond. Furthermore, the heterocycle may have a structure incorporated into an aromatic ring or a heterocycle structure containing a double bond that differs from an aromatic ring. The double-bonded ring (typically an aromatic ring) that the plasticizer may have may be a carbon ring such as a benzene ring or a naphthalene ring; or a heterocycle such as a pyridine ring, imidazole ring, triazole ring, acetazole ring, thiazole ring, or thiophene ring. The heteroatoms contained in the heterocycle as ring constituent atoms may, for example, be one or more selected from the group consisting of nitrogen, sulfur, and oxygen. In several states, the heteroatoms constituting the heterocycle may be one or both of nitrogen and sulfur.

[0119] The aforementioned ring containing double bonds (typically an aromatic ring, preferably a carbocyclic ring) may have one or more substituents on the ring constituent atoms, or it may not have substituents. When substituents are present, examples of substituents include alkyl, alkoxy, hydroxy, halogen atoms (fluorine, chlorine, bromine, etc.), hydroxyalkyl, hydroxyalkyloxy, epoxypropoxy, etc., but are not limited thereto. Among the substituents containing carbon atoms, the number of carbon atoms contained in the substituent is preferably 1 to 4, more preferably 1 to 3, and for example, 1 or 2. In several embodiments, each ring containing double bonds in the polysiloxane plasticizer is independently selected from the group consisting of aromatic rings without substituents on the ring constituent atoms and aromatic rings with substituents, wherein the substituents of the aromatic rings with substituents are selected from one or more of the group consisting of alkyl, alkoxy, hydroxy, and hydroxyalkyl (preferably the group consisting of alkyl and alkoxy). For example, the double-bonded ring system of polysiloxane plasticizers is selected from aromatic rings (preferably carbon rings) that do not have substituents on the ring-forming atoms. In several ideal samples, the double-bonded rings of polysiloxane plasticizers are all benzene rings.

[0120] From the viewpoint of facilitating the plasticizing effect and suppressing its stability (e.g., suppressing the increase in elastic modulus due to the volatilization and dissipation of the plasticizer from the material containing it), the aforementioned silicate compound should preferably have 3 or more Si atoms. Furthermore, from the viewpoint of compatibility within adhesives, the aforementioned silicate compound should preferably have 4 or fewer Si atoms, more preferably 3 or fewer. Among these, a polysiloxane plasticizer with 3 Si atoms in the aforementioned silicate compound is particularly desirable, i.e., a polysiloxane plasticizer composed of trisiloxane compounds.

[0121] In several states, the number of double-bonded rings (e.g., benzene rings with or without substituents) in the aforementioned silicate compound is at least 2, and from the viewpoint of heat resistance in terms of plasticizing effect (e.g., low rate of increase in elastic modulus under humid heat storage), it is preferable to have 3 or more, more preferably 4 or more, and may also have 5 or more. Furthermore, when the number of Si atoms in the silicate compound is n, the number of double-bonded rings in the aforementioned silicate compound is typically 2n+2 or less, and from the viewpoint of improving plasticizing effect, 2n+1 or less is appropriate, preferably 2n or less, may be 2n-1 or less, and may also be 2n-2 or less. For example, in the case of the aforementioned siloxane compound being a trisiloxane compound, the number of double-bonded rings in the trisiloxane compound is typically 8 or less, for example, it may be 2 or more but 7 or less, 3 or more but 7 or less, or 4 or more but 7 or less. Preferably, it is a trisiloxane compound with 4 or more but 6 or less (e.g., 4 or 5) double-bonded rings (e.g., unsubstituted benzene rings).

[0122] In several states, at least one of the Si atoms (typically the Si atoms constituting the siloxane chain) contained in the aforementioned siloxane compound is a Si atom with two or more double-bonded rings. From the viewpoint of improving the stability of plasticizing effect, the number of Si atoms with two or more double-bonded rings in the aforementioned siloxane compound can also be two or more. In siloxane compounds with three or more Si atoms, the number of Si atoms with two or more double-bonded rings can be two or more, or three or more. Furthermore, when the number of Si atoms in the aforementioned siloxane compound is n, it can be less than n, less than n-1, or less than n-2. From the perspective of improving plasticizing effect, in the above-mentioned siloxane compound (preferably a siloxane compound with 3 or more Si atoms), at least one of the Si atoms has a double-bonded ring bonded to that Si atom, and the number of such rings is 1 or 0. For example, it is preferably a linear siloxane compound with 3 or more but less than 5 Si atoms, and the Si atoms at both ends each have 2 or 3 (preferably 2) double-bonded rings independently, while the Si atoms other than the two ends each have 1 double-bonded ring or no double-bonded ring structure.

[0123] In several states, the aforementioned siloxane compounds may also contain Si atoms bonded to groups other than those containing double bonds. Examples of such groups other than those containing double bonds include: alkyl, aralkyl, alkoxy, halogen atoms (fluorine, chlorine, bromine, etc.), fluoroalkyl, hydroxy, hydroxyalkyl, hydroxyalkyloxy, epoxy, epoxypropoxy, amino, monoalkylamino, dialkylamino, carboxyl, carboxylalkyl, mercapto, etc., but are not limited thereto. Among the substituents containing carbon atoms, the number of carbon atoms in the substituent is, for example, 1 to 8, preferably 1 to 4, more preferably 1 to 3, for example, 1 or 2. The groups other than those containing double bonds bonded to each Si atom in the siloxane compound may be independently selected from the group consisting of the groups exemplified above.

[0124] Among the various states, the aforementioned silicone compounds preferably do not possess vinyl unsaturated groups (including those containing vinyl double bonds in rings). Polysiloxane plasticizers composed of silicone compounds without vinyl unsaturated groups are advantageous from the viewpoint of the stability of the plasticizing effect provided by such plasticizers. They are also desirable from the viewpoint of the preservation stability of adhesive sheets having adhesive layers containing the aforementioned polysiloxane plasticizers, or from the viewpoint of suppressing changes in elastic modulus, dimensional changes, or deformations (warping, undulations, etc.) and optical strain caused by the reaction of vinyl unsaturated groups.

[0125] In the several polysiloxane plasticizers disclosed herein, from the viewpoint of improving plasticizing effect, at least one (or at least two) of the Si atoms contained in the aforementioned silicate compounds preferably has at least one methyl group on that Si atom. For example, the Si atoms at both ends of the silicate chain preferably each have one or two (preferably one) methyl groups independently. In several ideal samples, each Si atom contained in the aforementioned silicate compound independently has one or two methyl groups. According to the polysiloxane plasticizer composed of silicate compounds with the aforementioned structure, a balance can be struck between the plasticizing effect brought about by the flexibility of the silicate structure and the stability of the aforementioned plasticizing effect brought about by the structure having two or more rings containing double bonds bonded to at least one Si atom.

[0126] In several samples, when the number of Si atoms in the aforementioned siloxane compound is n, the total number of substituents bonded to the same number of Si atoms (hereinafter also referred to as the total number of substituents) is typically 2n+2, and at least two of them are rings containing double bonds. In several samples, the proportion (SR) of rings containing double bonds (preferably aromatic carbide rings, such as benzene rings) in the total number of substituents of the polysiloxane plasticizer is at least 16%, can be more than 20%, and can also be more than 25%. If the above proportion (SR) increases, there is a tendency for the heat resistance of the polysiloxane plasticizer or the stability of the plasticizing effect brought about by the polysiloxane plasticizer to generally improve. In several samples, the above proportion (SR) of 33% or more is advantageous, preferably more than 40%, more preferably more than 50% (e.g., more than 60%), can be more than 65%, and can also be more than 75%. The aforementioned ratio SR can be 100%, but from the viewpoint of ease of mixing or compatibility, it is advantageous to be below 85%, preferably below 80%, below 75%, below 65%, or below 60% (e.g., below 50%).

[0127] From the perspective of the stability of plasticizing effect, among several samples, a molecular weight of 400 or higher for the aforementioned polysiloxane plasticizer (specifically, a silicate compound) is appropriate, 430 or higher is advantageous, preferably 460 or higher, possibly 490 or higher, or possibly 520 or higher. Furthermore, from the perspective of plasticizing effect, ease of mixing, and compatibility, a molecular weight of 900 or lower for the aforementioned silicate compound is appropriate, 850 or lower is advantageous, preferably 700 or lower, more preferably 650 or lower, possibly 600 or lower, possibly 560 or lower, possibly 540 or lower, or possibly 500 or lower.

[0128] The molecular weight of the aforementioned siloxane compounds can be calculated based on their chemical structure or determined using matrix-assisted laser desorption / ionization time-of-flight mass analysis (MALDI-TOF-MS). When a nominal molecular weight value is provided by the manufacturer, that value may be used.

[0129] The refractive index of the polysiloxane plasticizer disclosed herein is not particularly limited, and may be in the range of approximately 1.30 to 1.80. From the viewpoint of suppressing the reduction of the refractive index of materials incorporating plasticizers (e.g., adhesives) while simultaneously seeking a low modulus of elasticity, a refractive index of 1.45 or higher is appropriate for the polysiloxane plasticizers in several samples, preferably 1.50 or higher, more preferably 1.52 or higher (e.g., 1.53 or higher or 1.54 or higher), and even more preferably 1.55 or higher (e.g., 1.56 or higher or 1.57 or higher). Furthermore, from the viewpoint of ease of blending or compatibility, the refractive index of the polysiloxane plasticizer may be, for example, 1.70 or lower, 1.65 or lower, or 1.60 or lower.

[0130] In several formulations, the plasticizer may be an ethylene glycol compound having two or more rings containing double bonds in one molecule. The number of oxyethyl units (i.e., -(C₂H₄O)- units in the aforementioned ethylene glycol compound is, for example, 1 to 10, possibly 1 to 6, or even 2 to 4. The aforementioned ethylene glycol compound may be a compound having the following structure: two or more non-condensed rings containing double bonds linked by oxyethyl units (e.g., 1 to 10, ideally 1 to 6, further 2 to 4 oxyethyl units). The compound may have one or more ester groups. Examples of the aforementioned ethylene glycol compounds include compounds having two or more benzoic acids linked to ethylene glycol, diethylene glycol, triethylene glycol, or polyethylene glycol via ester bonds.

[0131] Furthermore, the technology disclosed herein can be implemented without using the aforementioned ethylene glycol compounds as plasticizers, or with limited usage. For example, the content of the aforementioned ethylene glycol compounds in the plasticizer contained in the adhesive can be less than 90% by weight. The content of the aforementioned ethylene glycol compounds in the plasticizer can be less than 50% by weight, less than 10% by weight, less than 3% by weight, or less than 1% by weight, and the adhesive can also substantially not contain the aforementioned ethylene glycol compounds as plasticizers. Similarly, in the adhesive, the amount of the aforementioned ethylene glycol compounds relative to 100 parts by weight of the base polymer (e.g., acrylic polymer) can be set to less than 0.5 parts by weight or less than 0.1 parts by weight.

[0132] In several other samples, liquid rosin esters and other liquid rosin derivatives, as well as liquid camphene phenols, can be used as plasticizers. The aforementioned liquid rosin derivatives (e.g., liquid rosin esters) are equivalent to the compounds described above that have a condensed ring structure containing double bonds.

[0133] Furthermore, one or more of the known plasticizers (such as phthalate esters, terephthalate esters, adipate esters, adipic acid polyesters, ethylene glycol benzoate, etc.) may be used.

[0134] There is no particular limitation on the amount of plasticizer used; it can be set according to the purpose. From the viewpoint of reducing the elastic modulus of the adhesive, the amount of plasticizer used relative to 100 parts by weight of the base polymer (e.g., acrylic polymer) can be, for example, more than 1 part by weight, or more than 10 parts by weight. In several ideal cases, the amount of plasticizer used relative to 100 parts by weight of the base polymer is greater than 15 parts by weight, which can be more than 20 parts by weight, more than 30 parts by weight (e.g., more than 30 parts by weight), preferably more than 40 parts by weight, more preferably more than 50 parts by weight, especially more than 60 parts by weight, more than 75 parts by weight, or more than 90 parts by weight. For example, when using the above-mentioned ethylene glycol-based compound as a plasticizer, it is advisable to use more than 30 parts by weight relative to 100 parts by weight of the base polymer (e.g., more than 40 parts by weight, more preferably more than 50 parts by weight). Furthermore, from the perspective of balancing the high refractive index and low elastic modulus of the adhesive, it is appropriate to set the amount of plasticizer relative to the base polymer at approximately 200 parts by weight or less, preferably 150 parts by weight or less, more preferably 120 parts by weight or less, and could be 100 parts by weight or less, 80 parts by weight or less, or even 70 parts by weight or less. In several cases where adhesive properties are given greater emphasis, the amount of plasticizer relative to the base polymer at 100 parts by weight could be 45 parts by weight or less, or even 35 parts by weight or less.

[0135] (Additive (H RO)) The adhesives disclosed herein may contain organic materials with a higher refractive index than the base polymer (e.g., acrylic polymers) as additives that can be used as desired. Hereinafter, the organic material will be referred to as "additive (H RO)". Here, "H RO" refers to an organic material with a high refractive index. By using additives (H RO), adhesives that better balance refractive index and adhesive properties (peel strength, flexibility, etc.) can be achieved. Organic materials that can be used as additives (H RO) can be polymers or non-polymers. Furthermore, they may or may not have polymerizable functional groups. In addition, in this specification, additives (H RO) are defined as compounds that are different from those that can be used as plasticizers. Therefore, additives (H RO) are specifically not liquid at 30°C (e.g., 25°C or 20°C). Additives (H RO) can be used alone or in combination of two or more.

[0136] The refractive index of the additive (H RO) can be set within an appropriate range relative to the refractive index of the base polymer (e.g., acrylic polymer), and is therefore not limited to a specific range. The refractive index of the additive (H RO) can be, for example, greater than 1.55, greater than 1.56, or greater than 1.57, and can be selected from a range higher than the refractive index of the base polymer. From the viewpoint of increasing the refractive index of the adhesive, among several options, a refractive index of the additive (H RO) of 1.58 or higher is advantageous, preferably 1.60 or higher, more preferably 1.63 or higher, and can be 1.65 or higher, 1.70 or higher, or 1.75 or higher. With an additive (H RO) of a higher refractive index, the target refractive index can be achieved even with a smaller amount of additive (H RO). This is preferable from the viewpoint of suppressing the reduction of adhesive or optical properties. There is no particular upper limit to the refractive index of the additive (H RO). However, from the perspective of compatibility with the adhesive, the ease of achieving a high refractive index and the flexibility suitable for use as an adhesive, it can be, for example, below 3.000, below 2.500, below 2.000, below 1.950, below 1.900, or below 1.850. Furthermore, the refractive index of the additive (H RO) is measured using an Abbe refractometer at a wavelength of 589 nm and a temperature of 25 °C, in the same manner as the refractive index of the monomer. If a nominal value for the refractive index at 25 °C is provided by the manufacturer, that nominal value may be used.

[0137] The difference between the refractive index nb of the additive (H RO) and the refractive index na of the base polymer (e.g., acrylic polymer), i.e., nbna (hereinafter also referred to as "Δn A"), is set to be greater than 0. In several samples, Δn A is, for example, 0.02 or more, 0.05 or more, 0.07 or more, 0.10 or more, 0.15 or more, 0.20 or more, or 0.25 or more. By selecting the base polymer and additive (H RO) in a way that maximizes Δn A, the effect of using the additive (H RO) to increase the refractive index tends to be higher. Furthermore, from the viewpoint of the compatibility of the additive (H RO) within the adhesive, in several samples, Δn A is, for example, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.35 or less.

[0138] In several samples, the difference between the refractive index nb of the additive (H RO) and the refractive index nT of the adhesive containing the additive (H RO), i.e., nbn T (hereinafter also referred to as "Δn B"), is set to be greater than 0. In several samples, Δn B is, for example, 0.02 or more, 0.05 or more, 0.07 or more, 0.10 or more, 0.15 or more, 0.20 or more, or 0.25 or more. By selecting the composition of the adhesive and the additive (H RO) in a way that makes Δn B larger, the effect of using the additive (H RO) to increase the refractive index tends to be higher. Furthermore, from the viewpoint of compatibility within the adhesive or the transparency of the adhesive, in several samples, Δn B is, for example, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.35 or less.

[0139] There is no particular limitation on the molecular weight of the organic material used as an additive (H RO), and it can be selected according to the purpose. From the viewpoint of balancing the effect of high refractive index with other properties (such as suitability for adhesive flexibility, haze, and other optical properties), among several options, a molecular weight of the additive (H RO) of approximately less than 10,000 is appropriate, 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 even less than 400. A relatively low molecular weight of the additive (H RO) is advantageous from the viewpoint of improving its compatibility with the adhesive. Furthermore, the molecular weight of the additive (H RO) can, for example, be 130 or higher, or even 150 or higher. From the perspective of increasing the refractive index of the additive (H RO), among several samples, the molecular weight of the additive (H RO) should preferably be above 170, more preferably above 200, and could be above 230, above 250, above 270, above 500, above 1000, or even above 2000. Among several samples, polymers with a molecular weight of around 1000 to 10000 (e.g., above 1000 and below 5000) can be used as additives (H RO). The molecular weight of the additive (H RO) can be calculated based on the chemical structure of non-polymer or low-polymerization-degree (e.g., around 2-5 polymers) polymers, or determined using matrix-assisted laser desorption / ionization time-of-flight mass analysis (MALDI-TOF-MS). When the additive (H RO) is a polymer with a higher degree of polymerization, the weight-average molecular weight (Mw) based on GPC performed under appropriate conditions can be used. If a nominal molecular weight value is provided by the manufacturer, etc., that nominal value can be used.

[0140] Examples of organic materials that can be used as additives (HRO) include organic compounds with aromatic rings, organic compounds with heterocycles (which may be aromatic or non-aromatic heterocycles), etc., but are not limited thereto.

[0141] The aromatic rings of the aforementioned organic compounds with aromatic rings that can be used as additives (H RO) (hereinafter also referred to as "aromatic ring compounds") can be selected from those with the same aromatic rings as those of compounds that can be used as monomers (A1).

[0142] The aromatic ring described above may have one or more substituents on the ring constituent atoms, or it may not have substituents. When substituents are present, examples of substituents include alkyl, alkoxy, aryloxy, hydroxy, halogen atoms (fluorine, chlorine, bromine, etc.), hydroxyalkyl, hydroxyalkyloxy, epoxypropoxy, etc., but are not limited thereto. Among the substituents containing carbon atoms, the number of carbon atoms contained in the substituent is advantageous, for example, 1 to 10, 1 to 6, preferably 1 to 4, more preferably 1 to 3, for example, 1 or 2. In several cases, the aromatic ring described above may be an aromatic ring without substituents on the ring constituent atoms, or an aromatic ring having one or more substituents selected from the group consisting of alkyl, alkoxy, and halogen atoms (e.g., bromine atoms) on the ring constituent atoms.

[0143] Examples of aromatic ring compounds that can be used as additives (HRO) include: compounds that can be used as monomers (A1); oligomers containing compounds that can be used as monomers (A1) as monomer units; compounds from which a group having an ethylene unsaturated group (which may be a substituent bonded to a ring-forming atom) or the portion of that group constituting an ethylene unsaturated group is removed and replaced with a hydrogen atom or a group without an ethylene unsaturated group (e.g., hydroxyl, amino, halogen, alkyl, alkoxy, hydroxyalkyl, hydroxyalkyloxy, epoxypropoxy, etc.) may be included, and those not equivalent to the plasticizers disclosed herein may be included, but are not limited thereto.

[0144] Among several options, considering the ease of achieving a high refractive index, organic compounds with two or more aromatic rings per molecule (hereinafter also referred to as "compounds containing multiple aromatic rings") are suitable as additives (HRO). Compounds containing multiple aromatic rings may or may not possess polymerizable functional groups such as vinyl unsaturated groups. Furthermore, compounds containing multiple aromatic rings can be polymers or non-polymers. Moreover, the aforementioned polymers can be oligomers containing monomers containing multiple aromatic rings as monomer units (preferably oligomers with a molecular weight of approximately 5000 or less, more preferably approximately 1000 or less; for example, oligomers of approximately 2 to 5 polymers). Examples of such oligomers include: homopolymers of monomers containing multiple aromatic rings; copolymers of two or more monomers containing multiple aromatic rings; copolymers of one or more monomers containing multiple aromatic rings with other monomers, etc. The other monomers mentioned above may be aromatic ring monomers that are not monomers containing multiple aromatic rings, monomers that do not have aromatic rings, or combinations thereof.

[0145] Non-limiting examples of compounds containing a plurality of aromatic rings include: compounds having two or more non-condensed aromatic rings bonded together by a linker group; compounds having two or more non-condensed aromatic rings directly bonded (i.e., without being separated by other atoms); compounds having a condensed aromatic ring structure; compounds having a cyclopentadienyl structure; compounds having a dibenzothiophene structure; and compounds having a dibenzothiophene structure. Compounds containing a plurality of aromatic rings may be used alone or in combination of two or more.

[0146] Examples of heterocyclic organic compounds (hereinafter also referred to as heterocyclic organic compounds) that can be used as additives (HRO) include thioepoxides and compounds with trihalomethane rings. Examples of thioepoxides include the bis(2,3-cyclothiopropyl) disulfide and its polymer (refractive index 1.74) disclosed in Japanese Patent No. 3712653. Examples of compounds with trihalomethane rings include compounds having at least one (e.g., 3 to 40, preferably 5 to 20) trihalomethane rings per molecule. Furthermore, since trihalomethane rings are aromatic, compounds with trihalomethane rings are also included in the above concept of aromatic ring-containing compounds, and compounds having multiple trihalomethane rings are also included in the above concept of compounds containing multiple aromatic rings.

[0147] Among several samples, compounds without vinyl unsaturated groups can be appropriately used as additives (H RO). This can suppress the deterioration of the adhesive composition due to heat or light (due to gelation or increased viscosity leading to reduced leveling properties), thus improving storage stability. The use of additives without vinyl unsaturated groups (H RO) is also advantageous from the perspective of suppressing dimensional changes or deformations (warping, undulations, etc.) and optical strain caused by the reaction of vinyl unsaturated groups in adhesive sheets having an adhesive layer containing such additives (H RO).

[0148] In the case of using oligomers as additives (HRO), the oligomers can be obtained by polymerizing the corresponding monomer components using known methods. When manufacturing the above-mentioned oligomers using free radical polymerization, polymerization initiators, chain transfer agents, emulsifiers, etc., suitable for carrying out free radical polymerization can be appropriately added to the above-mentioned monomer components to carry out polymerization. There are no particular limitations on the polymerization initiators, chain transfer agents, emulsifiers, etc., that can be used for free radical polymerization, and they can be appropriately selected and used. In addition, the weight average molecular weight of the oligomer can be controlled by the amount of polymerization initiator and chain transfer agent used, the reaction conditions, and the amount used should be appropriately adjusted according to the types of these agents. Examples of chain transfer agents include lauryl thiol, epichlorohydrin, mercaptoacetic acid, 2-mercaptoethanol, α-thioglycerol, hydrothioacetic acid, 2-ethylhexyl hydrothioacetic acid, and 2,3-dimercapto-1-propanol. A single chain transfer agent can be used, or two or more can be used in combination. The amount of chain transfer agent used can be determined based on the composition of the monomers used in the synthesis of the oligomer or the type of chain transfer agent, to obtain an oligomer with the desired weight-average molecular weight. In several samples, it is appropriate to use approximately 15 parts by weight or less of the chain transfer agent relative to 100 parts by weight of the total monomers used in the synthesis of the oligomer, which can be 10 parts by weight or less, or approximately 5 parts by weight or less. There is no particular limitation on the lower limit of the amount of chain transfer agent used relative to 100 parts by weight of the total monomers used in the synthesis of the oligomer; for example, it can be 0.01 parts by weight or more, 0.1 parts by weight or more, 0.5 parts by weight or more, or 1 part by weight or more.

[0149] The amount of additive (H RO) relative to the base polymer (e.g., acrylic polymer) is not particularly limited if it is greater than 0 parts by weight (or the sum of equal amounts when using multiple compounds), and can be set according to the purpose. In several samples, the amount of additive (H RO) relative to the base polymer can be set to, for example, 80 parts by weight or less. From the viewpoint of balancing the high refractive index of the adhesive with the inhibition of the reduction of adhesive or optical properties, it is advantageous to set it to 60 parts by weight or less, and preferably to 45 parts by weight or less. In several samples where adhesive or optical properties are of greater importance, the amount of additive (H RO) relative to the base polymer can be, for example, 30 parts by weight or less, 20 parts by weight or less, 15 parts by weight or less, or 10 parts by weight or less. Furthermore, from the perspective of increasing the refractive index of the adhesive, the amount of additive (H RO) relative to 100 parts by weight of the base polymer can be set to, for example, 1 part by weight or more, 3 parts by weight or more, preferably 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.

[0150] (Cross-linking agent) In the technology disclosed herein, the adhesive composition used to form the adhesive may contain a crosslinking agent as needed to adjust the cohesive force of the adhesive. The crosslinking agent can be any crosslinking agent known in the adhesive field, such as isocyanate-based crosslinking agents, epoxy-based crosslinking agents, acrylonitrile-based crosslinking agents, acezoline-based crosslinking agents, melamine-based resins, and metal chelate-based crosslinking agents. Isocyanate-based and epoxy-based crosslinking agents are particularly suitable. Other examples of crosslinking agents include monomers having two or more vinyl unsaturated groups within one molecule, i.e., multifunctional monomers. One type of crosslinking agent can be used alone, or in combination of two or more.

[0151] Isocyanate-based crosslinking agents can use isocyanate compounds with two or more functions, such as: aliphatic polyisocyanates such as trimethylene diisocyanate, butyl diisocyanate, pentamethylene diisocyanate (PDI), hexamethylene diisocyanate (HDI), and diisocyanate; and alicyclic polyisocyanates such as cyclopentyl diisocyanate, cyclohexyl diisocyanate, isoflavone diisocyanate (IPDI), and 1,3-bis(isocyanomethyl)cyclohexane. Isocyanates include: 2,4-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, xylene diisocyanate (XDI), and other aromatic isocyanates; and polyisocyanate modifiers (e.g., HDI's trimerocyanate form, HDI's urea-coke bond, urea bond, carbodiimide bond, urea-cokeimide bond, and acetyltriketone bond) which are formed by modifying isocyanate compounds through urea-coke bonds, biuret bonds, trimerocyanate bonds, urea-cokeimide bonds, and acetyltriketone bonds. Examples of commercially available products include: TAKENATE 300S, TAKENATE 500, TAKENATE 600, TAKENATE D165N, TAKENATE D178N, TAKENATE D178NL (all manufactured by Mitsui Chemicals), Sumidur T80, Sumidur L, Desmodur N3400 (all manufactured by Sumika Bayer Urethane), Millionate MR, Millionate MT, Coronate L, Coronate HL, Coronate HX, Coronate 2770 (all manufactured by Tosoh), and Durnate A201H (all manufactured by Asahi Kasei Corporation). Isocyanate compounds can be used alone or in combination of two or more. They can also be used in combination with difunctional isocyanate compounds and trifunctional or higher isocyanate compounds.

[0152] Examples of epoxy crosslinking agents include: bisphenol A, epichlorohydrin-type epoxy resins, ethyl glycidyl ether, polyethylene glycol diglycidyl ether, glycerol diglycidyl ether, glycerol triglycidyl ether, 1,6-hexanediol glycidyl ether, trimethylolpropane triglycidyl ether, diglycidyl aniline, diaminopropylamine, N,N,N',N'-tetraglycidyl-meta-diamine, and 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, etc. These can be used alone or in combination of two or more.

[0153] Examples of multifunctional 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, neopentyl tetraethylene glycol di(meth)acrylate, neopentyl tetraethylene glycol tri(meth)acrylate, dinepentyl tetraethylene hexa(meth)acrylate, ethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,12-dodecanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane tri(meth)acrylate, allyl methacrylate, ethylene methacrylate, divinylbenzene, bisphenol A di(meth)acrylate, epoxy acrylate, polyester acrylate, carbamate acrylate, butyl glycol di(meth)acrylate, hexyl glycol di(meth)acrylate, etc. Multifunctional monomers can be used alone or in combination of two or more.

[0154] In several samples, at least a portion of the crosslinking agent may be a difunctional crosslinking agent having two crosslinking reactive groups (e.g., isocyanate groups) per molecule. By using a difunctional crosslinking agent, a flexible crosslinked structure can be easily formed. A single difunctional crosslinking agent may be used alone or in combination of two or more. Furthermore, a difunctional crosslinking agent may also be used in combination with a trifunctional or higher crosslinking agent.

[0155] In several states, acyclic crosslinking agents (also known as chain crosslinking agents) without aromatic rings, aliphatic rings, or other cyclic structures are suitable crosslinking agents. For example, among the aforementioned isocyanate-based crosslinking agents, isocyanate compounds without aromatic rings or triisocyanate rings are preferable. By using acyclic isocyanate compounds as crosslinking agents, it is easy to form crosslinking agents with high flexibility. Specific examples of the aforementioned acyclic isocyanates include: aliphatic isocyanate compounds (e.g., PDI or HDI), or modified aliphatic isocyanate compounds (e.g., polyisocyanate modified PDI or HDI modified with urethane bonds, biuret bonds, urea bonds, or carbodiimide bonds). Acyclic crosslinking agents can be used alone or in combination of two or more. In several ideal states, acyclic difunctional crosslinking agents can be used as crosslinking agents. []

[0156] In several samples, a crosslinking agent with a relatively long distance between one crosslinking reactive group (e.g., an isocyanate group) and another crosslinking reactive group in one molecule can be used as a crosslinking agent. This allows the formation of a flexible crosslinked structure with a predetermined length. For example, a compound in one molecule of the crosslinking agent whose number of atoms constituting the linking chain connecting the crosslinking reactive group and the other crosslinking reactive group is 10 or more (e.g., 12 or more, or 14 or more) can be used as a crosslinking agent. The upper limit of the number of atoms constituting the linking chain can be adjusted according to the purpose through polymerization or the like, and is therefore not particularly limited; for example, it can be 2000 or less, 1000 or less, 500 or less, 100 or less, 50 or less, 30 or less, or 20 or less. Furthermore, the number of atoms constituting the linking chain of the aforementioned crosslinking reactive groups refers to the minimum number of atoms required in one molecule of the crosslinking agent to reach other crosslinking reactive groups (when there are three or more crosslinking reactive groups, it is the crosslinking reactive group closest to the aforementioned crosslinking reactive group). Crosslinking agents with the aforementioned linking chains can be used alone or in combination of two or more. In several ideal cases, non-cyclic difunctional crosslinking agents can be used as the aforementioned crosslinking agents. Commercially available examples of the aforementioned crosslinking agents include Coronate 2770 (manufactured by Tosoh Corporation), TAKENATE D178NL (manufactured by Mitsui Chemicals Corporation), and Durnate A201H (manufactured by Asahi Kasei Corporation).

[0157] The amount of crosslinking agent (which may be a multifunctional monomer) used is not particularly limited, and can be set in the range of approximately 0.001 parts by weight to 5.0 parts by weight relative to 100 parts by weight of the base polymer. From the viewpoint of improving adhesion to the adherend, in several samples, the amount of crosslinking agent used relative to 100 parts by weight of the base polymer should preferably be 3.0 parts by weight or less, more preferably 2.0 parts by weight or less, and can be 1.0 parts by weight or less, 0.5 parts by weight or less, or 0.2 parts by weight or less. Furthermore, from the viewpoint of appropriately maximizing the effect of the crosslinking agent, in several samples, the amount of crosslinking agent used relative to 100 parts by weight of the base polymer can be, for example, 0.005 parts by weight or more, 0.01 parts by weight or more, 0.05 parts by weight or more, 0.08 parts by weight or more, 0.1 parts by weight or more, 0.2 parts by weight or more, or 0.4 parts by weight or more.

[0158] To facilitate a more efficient crosslinking reaction, a crosslinking catalyst can be used. Examples of crosslinking catalysts include: tetrabutyl titanate, tetraisopropyl titanate, tetraacetyl acetone zirconium, acetyl acetone iron(III), butyltin oxide, and dioctyltin dilaurate, among other metal-based crosslinking catalysts. Tin-based crosslinking catalysts, such as dioctyltin dilaurate, are preferred. There is no particular limitation on the amount of crosslinking catalyst used. However, considering the balance between the speed of the crosslinking reaction and the shelf life of the adhesive composition, the amount of crosslinking catalyst relative to 100 parts by weight of the base polymer can be set to, for example, approximately 0.0001 parts by weight to 1 part by weight, preferably 0.001 parts by weight to 0.5 parts by weight.

[0159] The adhesive composition may contain a compound capable of generating keto-enol tautomerism as a crosslinking delay agent. This extends the shelf life of the adhesive composition. For example, a compound capable of generating keto-enol tautomerism is suitable for use in adhesive compositions containing isocyanate-based crosslinking agents. Various β-dicarbonyl compounds can be used as compounds capable of generating keto-enol tautomerism. For example, β-diketones (acetylacetone, 2,4-hexanedione, etc.) or acetyl acetates (methyl acetate, ethyl acetate, etc.) are suitable. One or more compounds capable of generating keto-enol tautomerism can be used alone or in combination of two or more. The amount of the compound capable of generating keto-enol tautomerism, relative to 100 parts by weight of the base polymer, can be, for example, 0.1 parts by weight or more and 20 parts by weight, 0.5 parts by weight or more and 10 parts by weight or more and 5 parts by weight or more.

[0160] (Adhesive) The adhesive disclosed herein may also contain a tackifier. The tackifier may be a known tackifying resin such as rosin-based, terpene-based, phenolic, hydrocarbon-based, ketone-based, polyamide-based, epoxy-based, or elastic-system tackifying resins. One or more of these may be used alone or in combination. The amount of tackifying resin used is not particularly limited and can be set according to the purpose and application to achieve appropriate adhesive properties. In several samples, from the viewpoint of refractive index or transparency, it is appropriate to use 30 parts by weight or less of the tackifier relative to 100 parts by weight of the base polymer, preferably 10 parts by weight or less, and most preferably 5 parts by weight or less. The technique disclosed herein can be suitably implemented in samples without the use of a tackifier.

[0161] (Leveling agent) In several formulations, the adhesive composition used to form the adhesive layer may contain a leveling agent to improve the appearance of the adhesive layer formed by the composition (e.g., to improve the uniformity of thickness) or to improve the coatability of the adhesive composition, as needed. Non-limiting examples of leveling agents include acrylic leveling agents, fluorinated leveling agents, and polysiloxane leveling agents. Appropriate leveling agents can be selected from commercially available leveling agents and used using conventional methods.

[0162] In several samples, the above-mentioned leveling agent may be suitable for use with the following polymer (hereinafter also referred to as "polymer (B)"), which is a polymer comprising a monomer having a polyorganosiloxane backbone (hereinafter also referred to as "monomer S1") and an acrylic monomer (hereinafter also referred to as "monomer raw material B"). Polymer (B) may be referred to as a copolymer of monomer S1 and acrylic monomer. Polymer (B) may be used alone or in combination of two or more.

[0163] Monomer S1 is not particularly limited and any monomer containing a polyorganosiloxane backbone can be used. Monomer S1 is suitable for use with a structure having a polymerizable reactive group at one end. Among them, monomer S1 with a polymerizable reactive group at one end and no functional group at the other end that can crosslink with acrylic polymers is suitable. Commercially available examples include one-terminal reactive polysiloxane oils manufactured by Shin-Etsu Chemical Co., Ltd. (e.g., models X-22-174ASX, X-22-2426, X-22-2475, KF-2012, etc.). Monomer S1 can be used alone or in combination of two or more.

[0164] The functional group equivalent of monomer S1 can be, for example, around 100 g / mol to 30,000 g / mol. In several ideal samples, the aforementioned functional group equivalent is, for example, above 500 g / mol, above 800 g / mol, above 1500 g / mol, or above 2000 g / mol. Alternatively, the aforementioned functional group equivalent can be, for example, below 20,000 g / mol, below 10,000 g / mol, below 7,000 g / mol, or below 5,500 g / mol. If the functional group equivalent of monomer S1 is within the above range, it is easier to achieve a good balancing effect. Furthermore, when using two or more monomers with different functional group equivalents 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 that monomer.

[0165] Here, "functional group equivalent" refers to the weight of the main backbone (e.g., polydimethylsiloxane) bonded to each functional group. The unit g / mol is converted to 1 mol of functional group. The functional group equivalent of monomer S1 can be calculated, for example, from the spectral intensity of 1H-NMR (proton NMR) of nuclear magnetic resonance (NMR). The calculation of the functional group equivalent (g / mol) of monomer S1 based on the spectral intensity of 1H-NMR can be performed according to general structural analysis methods for 1H-NMR spectral analysis, and, if necessary, with reference to the description in Japanese Patent No. 5951153. In the functional group equivalent of monomer S1, the aforementioned functional group refers to polymerizable functional groups (e.g., vinyl, allyl, and other vinyl unsaturated groups).

[0166] The content of monomer S1 in monomer raw material B can be an appropriate value within the range that allows the desired effect to be achieved using monomer S1, and is not limited to a specific range. In several samples, the content of monomer S1 in monomer raw material B can be, for example, 5-60% by weight, 10-50% by weight, or 15-40% by weight.

[0167] In addition to monomer S1, monomer raw material B also contains acrylic monomers that can copolymerize with monomer S1. This improves the compatibility of polymer (B) within the adhesive layer. Examples of acrylic monomers that can be used in monomer raw material B include alkyl acrylates. Here, "alkyl" refers to chain-like (including linear and branched) alkyl groups, excluding alicyclic hydrocarbon groups described later. In several embodiments, monomer raw material B may contain at least one of (meth)acrylate C4-12 alkyl esters (preferably (meth)acrylate C4-10 alkyl esters, such as (meth)acrylate C6-10 alkyl esters). In other embodiments, monomer raw material B may contain at least one of (meth)acrylate C1-18 alkyl esters (preferably (meth)acrylate C1-14 alkyl esters, such as (meth)acrylate C1-10 alkyl esters). Monomer raw material B may include, for example, one or more of the following as acrylic monomers: methyl methacrylate (MMA), n-butyl methacrylate (BMA), and 2-ethylhexyl methacrylate (2EHMA).

[0168] Other examples of the aforementioned acrylic monomers include (meth)acrylates having an alicyclic hydrocarbon group. Examples include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, isocamphenyl (meth)acrylate, dicyclopentyl (meth)acrylate, and 1-adamantyl (meth)acrylate. Alternatively, (meth)acrylates without an alicyclic hydrocarbon group may not be used.

[0169] The content of the (meth)acrylate containing the above-mentioned alkyl methacrylate and the above-mentioned alicyclic hydrocarbon group in 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.

[0170] Other examples of monomers that can be included together with monomer S1 in monomer raw material B include: carboxyl-containing monomers, anhydride-containing monomers, hydroxyl-containing monomers, epoxy-containing monomers, cyano-containing monomers, isocyanate-containing monomers, amino-containing monomers, monomers having nitrogen-containing rings, (meth)acrylate aminoalkyl esters, vinyl esters, vinyl ethers, olefins, (meth)acrylates having aromatic hydrocarbon groups, (meth)acrylates containing halogen atoms, etc.

[0171] The Mw of polymer (B) can be, for example, 5,000 or more, preferably 10,000 or more, or even 15,000 or more. Conversely, the Mw of polymer (B) can be, for example, 200,000 or less, preferably 100,000 or less, possibly 50,000 or less, or even 30,000 or less. By setting the Mw of polymer (B) within an appropriate range, good compatibility and leveling properties can be achieved.

[0172] Polymer (B) can be produced by polymerizing the aforementioned monomers using known methods such as solution polymerization, emulsion polymerization, bulk polymerization, suspension polymerization, and photopolymerization. To adjust the molecular weight of polymer (B), chain transfer agents can be used as needed. Examples of chain transfer agents include: compounds with thiol groups such as trididodecylthiol, mercaptoethanol, and α-thioglycerol; thioacetic acid esters such as methyl thioacetate and methyl thioacetate; and α-methylstyrene dimers. There are no particular restrictions on the amount of chain transfer agent used; it can be appropriately set to obtain polymer (B) with the desired molecular weight. In several samples, the amount of chain transfer agent used relative to 100 parts by weight of monomer can 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.

[0173] The amount of polymer (B) used relative to the base polymer (e.g., an acrylic polymer) can be set to, for example, 0.001 parts by weight or more, and from the viewpoint of obtaining a higher performance effect, it can be set to 0.01 parts by weight or more, or 0.03 parts by weight or more. Furthermore, the amount of polymer (B) used can be, for example, 3 parts by weight or less, and from the viewpoint of reducing the influence on the refractive index, setting it to 1 part by weight or less is appropriate, and it can be 0.5 parts by weight or less, or 0.1 parts by weight or less. The technique disclosed herein can be implemented in an adhesive composition that substantially does not contain polymer (B).

[0174] (High-refractive-index particles) The adhesive disclosed herein may contain high refractive index particles as an arbitrary component. Here, high refractive index particles refer to particles that can increase the refractive index of the adhesive by including them. Hereafter, high refractive index particles will be denoted as "particle P HRI". HRI stands for high refractive index.

[0175] For example, the Particle PHRI can be composed of one or more materials having a refractive index of 1.60 or higher, preferably 1.70 or higher (potentially 1.80 or higher, 1.90 or higher, or even 2.00 or higher). There is no particular upper limit to the refractive index of the material constituting the Particle PHRI; it can be, for example, below 3.00, below 2.80, below 2.50, below 2.20, or below 2.00. The refractive index of the material constituting the Particle PHRI is measured for a single-layer film of that material (using a film thickness suitable for refractive index measurement) using a commercially available spectroradiometer at a measurement wavelength of 589 nm and a measurement temperature of 25°C. For example, the spectroradiometer can be the "EC-400" (manufactured by JA.Woolam) or an equivalent.

[0176] There are no particular limitations on the type of Particle PHRI. One or more materials that can increase the refractive index of the adhesive can be selected from metal particles, metal compound particles, organic particles, and organic-inorganic composite particles. Suitable inorganic oxides (e.g., metal oxides) that can increase the refractive index of the adhesive sheet can be used in Particle PHRI. Suitable examples of materials constituting Particle PHRI include inorganic oxides (specifically metal oxides) such as titanium dioxide (TiO₂), zirconium oxide (ZrO₂), aluminum oxide, zinc oxide, tin oxide, copper oxide, barium titanate, and niobium oxide (Nb₂O₅). Particles composed of these inorganic oxides (e.g., metal oxides) can be used alone or in combination of two or more. Particles composed of titanium dioxide or zirconium oxide are preferred, with zirconium oxide particles being particularly desirable. Furthermore, regarding metal particles, materials such as iron-based, zinc-based, tungsten-based, and platinum-based materials can have high refractive indices. Regarding organic particles, particles composed of resins such as styrene-based resins, phenolic resins, polyester resins, and polycarbonate resins have relatively high refractive indices. Organic-inorganic composite particles can be exemplified by composites of the aforementioned inorganic and organic materials, or inorganic particles coated with organic materials such as resins. From the viewpoint of compatibility with adhesive components, Particle PHRI can also utilize the aforementioned organic and inorganic particles treated with surface treatment agents.

[0177] There is no particular limitation on the average particle size of the P HRI particles; particles of an appropriate size that can achieve the desired refractive index enhancement by incorporating them into the adhesive can be used. For example, the average particle size of the P HRI particles can be set to approximately 1 nm or more, and approximately 5 nm or more is appropriate. From the viewpoint of enhancing the refractive index or processability, the average particle size of the P HRI particles should preferably be approximately 10 nm or more, and can be approximately 20 nm or more, or approximately 30 nm or more. Furthermore, from the viewpoint of maintaining adhesive properties, the upper limit of the aforementioned average particle size is, for example, approximately 300 nm or less; while from the viewpoint of enhancing the refractive index, it should preferably be approximately 100 nm or less, more preferably approximately 70 nm or less, even more preferably approximately 50 nm or less, and can also be approximately 35 nm or less (e.g., approximately 25 nm or less).

[0178] Furthermore, the average particle size of the aforementioned PHRI refers to the volume average particle size, specifically, the particle size that represents the 50% cumulative value of the particle size distribution measured using a particle size distribution measuring device based on laser scattering diffraction (hereinafter referred to as D 50). The measuring device can be, for example, the product name "Microtrac MT3000II" manufactured by MicrotracBEL or its equivalent.

[0179] The content of particulate PHRI in the adhesive is not particularly limited. The content of the aforementioned particulate PHRI can vary depending on the target refractive index. For example, the content of the aforementioned particulate PHRI can be appropriately set to a refractive index that is predetermined or higher, taking into account the required adhesive properties. In several samples, the content of particulate PHRI in the adhesive can be set to, for example, about 75% by weight or less in the adhesive, and from the viewpoint of adhesive properties or transparency, it can be set to about 50% by weight or less, or about 30% by weight or less. There is no particular limitation on the lower limit of the particulate PHRI content; for example, it can be greater than 0% by weight, more than 1% by weight, or more than 5% by weight. In several other samples, the content of particulate PHRI in the adhesive can be, for example, less than 10% by weight, less than 1% by weight, or less than 0.1% by weight. The technique disclosed herein can be implemented in samples where the adhesive substantially does not contain particulate PHRI.

[0180] The content of particulate PHRI in the adhesive can also be specified by its relative relationship with the amount of the base polymer (e.g., acrylic polymer) contained in the adhesive. The content of particulate PHRI relative to 100 parts by weight of the base polymer can be, for example, about 100 parts by weight or less, and from the viewpoint of adhesive properties or transparency, it can be about 60 parts by weight or less, or about 40 parts by weight or less. There is no particular limitation on the lower limit of the particulate PHRI content; relative to 100 parts by weight of the base polymer, it can be, for example, greater than 0 parts by weight, more than 1 part by weight, or more than 5 parts by weight. In several samples, the content of particulate PHRI relative to 100 parts by weight of the base polymer can be, for example, less than 30 parts by weight, less than 10 parts by weight, less than 1 part by weight, or less than 0.1 parts by weight.

[0181] (Other additives) In the technology disclosed herein, the adhesive composition used in the formation of adhesives may, as needed, include softeners, colorants (dyes, pigments, etc.), fillers, antistatic agents, anti-aging agents, ultraviolet absorbers, antioxidants, light stabilizers, preservatives, and other known additives that can be used in adhesive compositions, to a extent that does not significantly impair the effects of the present invention. For the various additives mentioned, conventionally known ones can be used in accordance with conventional methods, without specifically imparting any features to the present invention; therefore, detailed descriptions are omitted.

[0182] (Formation of adhesive (layer)) The adhesive disclosed herein can be formed using adhesive compositions. The form of the adhesive composition is not particularly limited, and can take various forms such as: solvent-based adhesive compositions containing adhesive-forming components in an organic solvent; active energy line-curing adhesive compositions that can be hardened by active energy lines such as ultraviolet light or radiation to form an adhesive; water-dispersible adhesive compositions in which the adhesive-forming components are dispersed in water; and hot-melt adhesive compositions that can form an adhesive when applied in a molten state and cooled to near room temperature. The adhesive can be an adhesive formed by hardening solvent-based, active energy line-curing, water-dispersible, or hot-melt adhesive compositions through drying, crosslinking, polymerization, cooling, etc., or it can be a hardened product of the aforementioned adhesive compositions. The hardening method of the adhesive composition (e.g., drying, crosslinking, polymerization, cooling, etc.) can be only one, or two or more can be used simultaneously or in multiple stages. For solvent-based adhesive compositions, the adhesive is typically formed by drying the composition (preferably for further crosslinking). For active energy line curing adhesive compositions, the adhesive is typically formed by irradiating an active energy line to induce polymerization and / or crosslinking reactions. When drying of an active energy line curing adhesive composition is necessary, it can be done by irradiating an active energy line after drying. The adhesives disclosed herein can be suitably formed using solvent-based adhesive compositions, but are not particularly limited thereto. In a configuration having a solvent-based adhesive layer formed from a solvent-based adhesive composition, it is suitable to achieve a balance between high refractive index and low elastic modulus.

[0183] An adhesive layer can be formed by applying (e.g., coating) an adhesive composition to a suitable surface and then allowing the composition to harden. The application of the adhesive composition can be carried out using conventional coating machines such as gravure roller coaters, reverse roller coaters, contact roller coaters, dip roller coaters, bar coaters, doctor blade coaters, and spray coaters.

[0184] <Adhesive sheet> According to this specification, an adhesive sheet having an adhesive layer is provided. The adhesive constituting the adhesive layer is as described above, and may be an adhesive formed from the adhesive composition described above (e.g., a hardened version of the adhesive composition). The aforementioned adhesive sheet can be a substrate-attached adhesive sheet with the adhesive layer in the form of the adhesive layer on one or both sides of a non-releasable substrate (supporting substrate), or a substrate-free adhesive sheet in which the adhesive layer is maintained in the form of a release liner (i.e., an adhesive sheet without a non-releasable substrate, typically an adhesive sheet composed of an adhesive layer). The concept of adhesive sheet mentioned herein can include those referred to as adhesive tape, adhesive label, adhesive film, etc. The adhesive sheet disclosed herein can be in roll form or single sheet form. Alternatively, it can be an adhesive sheet further processed into various shapes and forms.

[0185] Examples of the structure of a double-sided adhesive type substrate-free adhesive sheet (substrate-free double-sided adhesive sheet) are shown in Figures 1 and 2. The adhesive sheet 1 shown in Figure 1 has the following structure: both sides 21A and 21B of the substrate-free adhesive layer 21 are each protected by release pads 31 and 32, which serve as release surfaces at least on the adhesive layer side. The adhesive sheet 2 shown in Figure 2 has a structure in which one surface (adhesive surface) 21A of the substrate-free adhesive layer 21 is protected by release pads 31 on both sides, and can be configured such that when wound, the other surface (adhesive surface) 21B of the adhesive layer 21 abuts against the back of the release pad 31, thereby protecting the other surface 21B from the release pad 31 as well. From the viewpoint of adapting to the flexibility of the adhered object by repeated bending, the technology disclosed here is suitable for implementation in the form of a substrate-free adhesive sheet composed of an adhesive layer. The aforementioned substrate-free adhesive sheet is also desirable from the perspective of reducing the thickness of the adhesive sheet or increasing the transparency of the adhesive sheet.

[0186] The adhesive sheet disclosed herein may, for example, have the cross-sectional structure schematically shown in FIG3. The adhesive sheet 3 shown in FIG3 has a supporting substrate 10, and a first adhesive layer 21 and a second adhesive layer 22, each supported by a first surface 10A and a second surface 10B of the supporting substrate 10. Both the first surface 10A and the second surface 10B are non-peelable surfaces. The adhesive sheet 3 is used by attaching the surfaces of the first adhesive layer 21 (first adhesive surface) 21A and the second adhesive layer 22 (second adhesive surface) 22A to the adhered object. That is, the adhesive sheet 3 is constructed in the form of a double-sided adhesive sheet. Before use, the adhesive sheet 3 has the following configuration: the first adhesive surface 21A and the second adhesive surface 22A are each protected by release liner 31, 32, which are at least peelable surfaces (peelable surfaces) on the adhesive side. Alternatively, it can be configured as follows: omit the release liner 32 and use the two sides that become release surfaces as the release liner 31, and wrap the adhesive sheet 3 so that the second adhesive surface 22A abuts against the inside of the release liner 31, thereby the second adhesive surface 22A is also protected by the release liner 31.

[0187] The technology disclosed herein is suitable for fixing or joining components (e.g., optical components) in the form of double-sided adhesive sheets without or with a substrate, as described above. Alternatively, although the adhesive sheet disclosed herein is not specifically illustrated, it can also be a single-sided adhesive sheet with an adhesive layer on only one side of a non-removable substrate (supporting substrate). As an example of a single-sided adhesive sheet, the configuration shown in FIG3 without either the first adhesive layer 21 or the second adhesive layer 22 can be cited.

[0188] (Adhesive layer) The thickness of the adhesive layer constituting the adhesive sheet is not particularly limited; for example, it can be 3µm or more. In several samples, an adhesive layer thickness of 5µm or more is suitable, but can be 10µm or more, 15µm or more, 20µm or more, 30µm or more, 50µm or more, 70µm or more, or 85µm or more. Increasing the thickness of the adhesive layer tends to increase the adhesive strength. Furthermore, in several samples, the adhesive layer thickness can be, for example, 300µm or less, 250µm or less, 200µm or less, 150µm or less, or 120µm or less. In several ideal samples, an adhesive layer thickness of 100µm or less is preferable, 75µm or less is more preferable, 70µm or less is more preferable, 50µm or less is acceptable, and 30µm or less is also acceptable. The fact that the adhesive layer thickness is not too large is advantageous from the perspective of thinner adhesive sheets. Furthermore, thin adhesive layers tend to have excellent conformability to the substrate. The technology disclosed here is suitable for implementation, for example, when the adhesive layer thickness is in the range of 3µm to 200µm (preferably 5µm to 100µm, and more preferably 5µm to 75µm). Moreover, when the adhesive sheet has a first adhesive layer and a second adhesive layer on the first and second surfaces of the substrate, the thickness of the adhesive layer can be at least the thickness of the first adhesive layer. The thickness of the second adhesive layer can also be selected from the same range. Furthermore, when it is an adhesive sheet without a substrate, the thickness of the adhesive sheet is the same as the thickness of the adhesive layer.

[0189] In several samples, the product of the adhesive's storage modulus of elasticity G'(0℃) [Pa] at 0℃ and the adhesive layer thickness T [µm] (G'(0℃)×T) is suitable, for example, in the range of 5.0×10⁴ to 7.5×10⁷, preferably in the range of 5.0×10⁴ to 5.0×10⁷. Even with a relatively high modulus of elasticity, a thin adhesive layer tends to exhibit good flexibility due to the product (G'(0℃)×T) within the aforementioned range. Furthermore, by limiting the product (G'(0℃)×T) to a predetermined value, the upper limits of the adhesive layer thickness and the storage modulus of elasticity G'(0℃) are limited, thereby easily achieving excellent flexibility. In several ideal samples, the area (G'(0℃)×T) can be 1.0×10⁵ or more, 2.0×10⁵ or more, or 8.0×10⁵ or more. Furthermore, the area (G'(0℃)×T) can be 2.0×10⁷ or less, 1.0×10⁷ or less, or 6.0×10⁶ or less.

[0190] (Haze value) In several samples, the haze value of the adhesive layer constituting the adhesive sheet may be, for example, 5.0% or less, preferably 3.0% or less, more preferably 2.0% or less, even more preferably 1.0% or less, possibly 0.9% or less, possibly 0.8% or less, possibly 0.5% or less, or possibly 0.3% or less. The adhesive sheet having the high transparency described above can be composed of a substrate or without a substrate and is suitable for applications requiring high light transmittance (e.g., optical applications) or applications requiring good visibility of the adhered object's properties through the adhesive sheet. There is no particular limitation on the lower limit of the haze value of the adhesive layer; from the viewpoint of improving transparency, a lower haze value is better. On the other hand, in several samples, considering the refractive index or adhesive properties, the haze value may be, for example, 0.05% or more, or possibly 0.10% or more. The haze values ​​of the adhesive layer can also be applied to the haze values ​​of the adhesive sheet when the technology disclosed herein is implemented in the form of a substrate-free adhesive sheet (typically an adhesive sheet composed of an adhesive layer).

[0191] Here, "haze value" refers to the ratio of diffuse transmitted light to total transmitted light when visible light is shone onto the object being measured. It is also known as the haze 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 embodiments below. The haze value of the adhesive layer can be adjusted by, for example, selecting the composition or thickness of the adhesive layer.

[0192] In several samples, the haze value of the adhesive sheet may be, for example, 5.0% or less, preferably 3.0% or less, more preferably 2.0% or less, even more preferably 1.0% or less, possibly 0.9% or less, possibly 0.8% or less, possibly 0.5% or less, or possibly 0.3% or less. The highly transparent adhesive sheet described above is suitable for applications requiring high light transmittance (e.g., optical applications) or applications requiring good visibility of the adhered material's properties through the adhesive sheet. There is no particular limitation on the lower limit of the adhesive sheet's haze value; from the viewpoint of improving transparency, a lower haze value is better. On the other hand, in several samples, considering the refractive index or adhesive properties, the haze value may be, for example, 0.05% or more, or possibly 0.10% or more. The haze value of the adhesive sheet can be measured using the same method as the method used to measure the haze value of the adhesive layer described above. The aforementioned haze value of the adhesive sheet can be obtained by selecting the composition of the adhesive layer or by selecting the type or thickness of the substrate in the composition having a substrate.

[0193] In several samples, the total light transmittance of the adhesive layer is preferably 85.0% or higher (e.g., 88.0% or higher, 90.0% or higher, or greater than 90.0%). Adhesive sheets with such high transparency as described can be configured with or without a substrate and are suitable for applications requiring high light transmittance (e.g., optical applications) or applications where the properties of the adhered object are readily visible through the adhesive sheet. The upper limit of the total light transmittance is practically, for example, about 98% or less, about 96% or less, or about 95% or less. In several samples, considering refractive index or adhesive properties, the total light transmittance of the adhesive layer can be about 94% or less, about 93% or less, or about 92% or less. The total light transmittance is measured using a commercially available transmittance meter according to JIS K 7136:2000. A transmittance meter can be used, such as the "HAZEMETER HM-150" manufactured by Murakami Color Technology Research Institute or an equivalent. Total light transmittance can be measured according to the method described in the examples below. The total light transmittance of the adhesive layer can be adjusted, for example, by selecting the composition or thickness of the adhesive layer.

[0194] In several samples, the total light transmittance of the adhesive sheet is preferably 85.0% or higher (e.g., 88.0% or higher, 90.0% or higher, or greater than 90.0%). The highly transparent adhesive sheet described above is suitable for applications requiring high light transmittance (e.g., optical applications) or applications where the properties of the adhered object must be clearly visible through the adhesive sheet. The upper limit of the total light transmittance in practical applications can be, for example, about 98% or less, about 96% or less, or about 95% or less. In several samples, considering refractive index or adhesive properties, the total light transmittance of the adhesive sheet can be about 94% or less, about 93% or less, or about 92% or less. The total light transmittance of the adhesive sheet can be measured using the same method as for measuring the total light transmittance of the adhesive layer described above. The total light transmittance of the adhesive sheet can be obtained by selecting the composition of the adhesive layer or by selecting the type or thickness of the substrate in the composition having a substrate.

[0195] (Peel strength) There is no particular limitation on the peel strength of the adhesive sheet to the glass plate. In several examples, the peel strength of the adhesive sheet to the glass plate is, for example, 0.1 N / 25 mm or more, or 0.5 N / 25 mm or more. In several ideal examples, the peel strength to the glass plate is 1.0 N / 25 mm or more, preferably 1.5 N / 25 mm or more, more preferably 2.0 N / 25 mm or more, and can be 3.0 N / 25 mm or more, 5.0 N / 25 mm or more, or 10 N / 25 mm or more. As described, adhesive sheets with a peel strength to the glass plate of a predetermined value or more are suitable for joining or fixing, for example, glass components. There is no particular limitation on the upper limit of the peel strength, for example, it can be 30 N / 25 mm or less, 25 N / 25 mm or less, or 20 N / 25 mm or less.

[0196] Here, the peel strength is determined by pressing the material onto an alkaline glass plate (the adherend) and placing it at 23°C and 50% RH for 30 minutes. The peel strength is then measured at a peel angle of 180 degrees and a tensile speed of 300 mm / min. During the measurement, a suitable substrate (e.g., a polyethylene terephthalate (PET) film with a thickness of approximately 25µm to 50µm) can be attached to the adherend as needed for reinforcement. More specifically, the peel strength can be measured according to the methods described in the examples described later.

[0197] (Thickness of the adhesive sheet) The thickness of the adhesive sheet disclosed herein (adhesive sheet without substrate or adhesive sheet with substrate) may be, for example, 1000µm or less, 350µm or less, 200µm or less, 120µm or less, 75µm or less, or 50µm or less. Furthermore, from the viewpoint of processability, the thickness of the adhesive sheet may be, for example, 5µm or more, 10µm or more, 25µm or more, 80µm or more, or 130µm or more. Furthermore, the thickness of the adhesive sheet refers to the thickness of the portion attached to the adhered body. For example, in the adhesive sheet 3 configured as shown in Figure 3, the thickness refers to the thickness from the first adhesive surface 21A to the second adhesive surface 22A, excluding the thickness of the release liner 31 and 32.

[0198] <Supporting substrate> Several types of adhesive sheets can be in the form of adhesive sheets with adhesive layers on one or both sides of a supporting substrate. There are no particular limitations on the material of the supporting substrate, and it can be appropriately selected according to the purpose or application of the adhesive sheet. Non-limiting examples of usable substrates include: polyolefin films with polyolefins such as polypropylene (PP) or ethylene-propylene copolymer as the main component; polyester films with polyesters such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN) as the main component; and polyvinyl chloride films with polyvinyl chloride as the main component, etc.; foamed sheets composed of foams such as polyurethane foam, polyethylene (PE) foam, and polychloroprene foam; woven and non-woven fabrics made of various fibrous materials (such as natural fibers such as hemp and cotton, synthetic fibers such as polyester and vinylon, and semi-synthetic fibers such as acetate) alone or in blends; paper such as Japanese paper, woodfree paper, kraft paper, and crepe paper; and metal foils such as aluminum foil and copper foil. Substrates composed of these composites are also possible. Examples of the composite substrate include substrates with structures formed by laminating metal foil and the aforementioned plastic film, and plastic substrates reinforced with inorganic fibers such as glass cloth.

[0199] Various film substrates can be used in several embodiments. These film substrates can be porous substrates such as foamed films or nonwoven sheets, non-porous substrates, or substrates with a structure consisting of porous layers and non-porous layers. Among these embodiments, the film substrates can preferably include a shape-maintaining (self-supporting or non-dependent) resin film as the base film. Here, "resin film" refers to a non-porous structure, typically a resin film that is substantially free of air bubbles (pores). Therefore, the resin film is a concept that can be distinguished from foamed films or nonwovens. The resin films can preferably be shape-maintaining (self-supporting or non-dependent). The resin film can be a single-layer structure or a multi-layer structure with two or more layers (e.g., a three-layer structure).

[0200] The resin materials constituting the resin film can include, for example: polyester; polyolefins; polycyclic olefins derived from monomers with aliphatic ring structures such as norcamphene; polyamides (PA) such as nylon 6, nylon 66, and partially aromatic polyamides; polyamides (PI) such as transparent polyimide (CPI); polyamide-imide (PAI); polyether ether ketone (PEEK); polyether ether (PES); polyphenylene sulfide (PPS); polycarbonate (PC); polyurethane (PU); ethylene-vinyl acetate copolymer (EVA); polytetrafluoroethylene (PTFE) and other fluoropolymers; acrylic resins; cellulose polymers such as cellulose triacetate (TAC); polyarylates; polystyrene; polyvinyl chloride; polyvinyl chloride and other resins.

[0201] The aforementioned resin film can be formed using a resin material comprising only one type of resin, or it can be formed using a blend of two or more resin materials. The resin film can be unstretched or stretched (e.g., uniaxially or biaxially stretched). For example, suitable materials include PET film, PBT film, PEN film, unstretched polypropylene (CPP) film, biaxially stretched polypropylene (OPP) film, low-density polyethylene (LDPE) film, linear low-density polyethylene (LLDPE) film, PP / PE blend film, cyclic olefin polymer (COP) film, CPI film, and TAC film. From the viewpoint of strength or dimensional stability, ideal examples of resin films include PET film, PEN film, PPS film, and PEEK film. From the viewpoint of ease of acquisition, PET film and PPS film are particularly suitable, with PET film being preferred.

[0202] In the resin film, known additives such as light stabilizers, antioxidants, antistatic agents, colorants (dyes, pigments, etc.), fillers, slipping agents, and anti-adhesion agents can be added as needed, within a range that does not significantly impair the effects of the present invention. The amount of additives added is not particularly limited and can be appropriately set according to the intended use of the adhesive sheet.

[0203] There are no particular limitations on the manufacturing method of resin films. For example, commonly known resin film forming methods such as extrusion, gas molding, T-die casting, and calendering roll forming can be appropriately used.

[0204] The aforementioned substrate may be substantially composed of the base film. Alternatively, the aforementioned substrate may also include auxiliary layers in addition to the base film. Examples of such auxiliary layers include optical property adjustment layers (e.g., coloring layers, anti-reflective layers), printing layers or laminates for imparting a desired appearance to the substrate, antistatic layers, primer layers, release layers, and other surface treatment layers.

[0205] Among several options, a light-transmitting substrate (hereinafter also referred to as a light-transmitting substrate) can be suitable as the supporting substrate. This allows for the construction of an adhesive sheet with the light-transmitting substrate. The total light transmittance of the light-transmitting substrate can be, for example, greater than 50%, or even greater than 70%. In several ideal options, the total light transmittance of the supporting substrate is 80% or greater, preferably 90% or greater, and can also be 95% or greater (e.g., 95-100%). The aforementioned total light transmittance is measured using a commercially available transmittance meter according to JIS K 7136:2000. A transmittance meter can be the Murakami Color Technology Research Institute's product under the trade name "HAZEMETER HM-150" or an equivalent. A suitable example of the aforementioned light-transmitting substrate is a light-transmitting resin film. The aforementioned light-transmitting substrate can also be an optical film.

[0206] The thickness of the substrate is not particularly limited and can be selected according to the intended use or application of the adhesive sheet. For example, the substrate thickness can be 500µm or less, but from the perspective of the adhesive sheet's processing or workability, it is preferable to be 300µm or less, or 150µm or less, 100µm or less, 50µm or less, 25µm or less, or even 10µm or less. A smaller substrate thickness tends to improve adaptability to the surface shape of the adhered object. Furthermore, from the perspective of processability or workability, the substrate thickness can be, for example, 2µm or more, 10µm or more, or even 25µm or more.

[0207] The side of the substrate to which the adhesive layer is to be deposited can also undergo conventional surface treatments as needed, such as corona discharge treatment, plasma treatment, ultraviolet irradiation treatment, acid treatment, alkali treatment, or forming a base coat by applying a primer. These surface treatments can be used to improve the anchoring properties of the adhesive layer to the substrate. The composition of the primer used to form the base coat is not particularly limited and can be appropriately selected from known materials. The thickness of the base coat is not particularly limited, but is generally suitable at around 0.01µm to 1µm, and preferably around 0.1µm to 1µm. Other treatments that can be performed on the substrate as needed include antistatic layer formation treatment, coloring layer formation treatment, and printing treatment. These treatments can be applied individually or in combination.

[0208] <Adhesive sheet with peel-off pad> The adhesive sheet disclosed herein may be in the form of an adhesive article in which the surface (adhesive surface) of the adhesive layer abuts against the release surface of the release liner. Therefore, according to this specification, an adhesive sheet (adhesive article) with a release liner is provided, comprising any of the adhesive sheets disclosed herein and a release liner having a release surface abutting against the adhesive surface of the adhesive sheet.

[0209] Release liner is not particularly limited. For example, it can be used as a release liner with a release layer on the surface of a liner substrate such as resin film or paper (which may be paper laminated with resins such as polyethylene), or it can be made of a resin film formed from a low-adhesion material such as fluoropolymer (polytetrafluoroethylene, etc.) or polyolefin resin (polyethylene, polypropylene, etc.). From the perspective of excellent surface smoothness, release liners with a release layer on the surface of a resin film used as a liner substrate, or release liners made of a resin film formed from a low-adhesion material, are suitable. The resin film is not particularly limited if it is a film that can protect the adhesive layer; examples include polyethylene (PE) film, polypropylene (PP) film, polybutene film, polybutadiene film, polymethylpentene film, polyvinyl chloride film, vinyl chloride copolymer film, polyester film (PET film, PBT film, etc.), polyurethane film, ethylene-ethyl acetate copolymer film, etc. The aforementioned release layer can be formed using known release agents such as polysiloxane-based release agents, long-chain alkyl-based release agents, olefin-based release agents, fluorine-based release agents, aliphatic amide-based release agents, molybdenum sulfide, and silicon dioxide powder.

[0210] <Application> The adhesive sheet disclosed herein has no limited application and can be used for various purposes. The adhesive sheet disclosed herein possesses both a high refractive index and flexibility, thus its characteristics can be utilized in various applications requiring both high refractive index and flexibility. For example, in portable electronic devices, it is suitable as an adhesive sheet for devices such as liquid crystal displays, organic EL (electroluminescent) displays, PDP (plasma display panels), electronic paper displays (image display devices), or touch panels (input devices), especially for foldable or rollable displays. For example, it is suitable for use in foldable and rollable displays as a mechanism for joining, fixing, or protecting components with high refractive index. The adhesive sheet disclosed herein has a high refractive index and simultaneously possesses flexibility capable of withstanding repeated bending operations, thus it can well conform to the repeatedly bent substrate (foldable displays, etc.) when attached to a foldable or rollable display. The adhesive sheet described herein can be used on glass components such as the viewing window or cover glass of foldable or rollable displays. Furthermore, the adhesive sheet disclosed herein readily conforms to and adheres to surfaces with curved shapes, such as 3D shapes, in portable electronic devices, thus making it suitable for applications involving such curved surfaces. Additionally, among several ideal examples, the adhesive not only possesses a high refractive index and flexibility but also exhibits excellent heat resistance. Since these portable electronic devices are sometimes used in high-temperature environments, and their internal spaces can become hot due to the heat generated by electronic components, the use of the aforementioned heat-resistant adhesive sheet offers significant advantages.

[0211] Examples of portable electronic devices mentioned above also include: portable telephones, smartphones, tablets, laptops, various wearable devices (such as wristbands like watches, modular devices attached to the body via clips or slings, eyeglasses (monocular or binocular; including headbands), clothing accessories attached to shirts, socks, hats, etc., and earphones), digital cameras, digital video cameras, audio equipment (portable music players, IC recorders, etc.), computers (electronic computers, etc.), portable game consoles, electronic dictionaries, electronic notebooks, e-books, in-vehicle information devices, portable radios, portable televisions, portable printers, portable scanners, portable modems, etc. Furthermore, in this manual, "portable" means more than just being able to carry; it also refers to a degree of portability that is relatively easy for a person (a standard adult) to move.

[0212] The materials (adhesive materials) that can be attached to the adhesive sheets disclosed herein are not particularly limited, and may include, for example, metallic materials such as copper, silver, gold, iron, tin, palladium, aluminum, nickel, titanium, chromium, zinc, or alloys containing two or more of these, or various resin materials such as polyimide resins, acrylic resins, polyether nitrile resins, polyether tin resins, polyester resins (PET resins, polyethylene naphthalate resins, etc.), polyvinyl chloride resins, polyphenylene sulfide resins, polyether ether ketone resins, polyamide resins (so-called arylamide resins, etc.), polyarylate resins, polycarbonate resins, cellulose polymers such as cellulose diacetate or cellulose triacetate, vinyl butyral polymers, liquid crystal polymers, etc. (typically plastic materials), alumina, zirconium oxide, alkali glass, alkali-free glass, quartz glass, carbon, etc. The adhesive sheet disclosed herein can be used to attach to components (e.g., optical components) made of the aforementioned materials.

[0213] The component or material to which the adhesive sheet disclosed herein is attached (at least one adherend in a double-sided adhesive sheet) may be made of a material with a higher refractive index than that of a typical adhesive (e.g., an acrylic adhesive). The refractive index of the adherend material is, for example, 1.50 or higher, including adherend materials with refractive indices of 1.55 or higher or 1.58 or higher, and also those with refractive indices of 1.62 or higher (e.g., around 1.66). The high-refractive-index adherend material is typically a resin material. More specifically, it may be a polyester resin such as PET, or a polyimide resin, an aramid resin, a polyphenylene sulfide resin, a polycarbonate resin, etc. With such materials, the effect of using the adhesive sheet disclosed herein (suppressing light reflection caused by refractive index differences) can be effectively achieved. The upper limit of the refractive index of the aforementioned adherend material is, for example, 1.80 or lower, and may be 1.70 or lower. The adhesive sheet disclosed herein is suitable for use when attached to a high refractive index substrate (e.g., a component) as described above. Suitable examples of the substrate include resin films with a refractive index of 1.50 to 1.80 (preferably 1.55 to 1.75, for example 1.60 to 1.70). The refractive index can be determined using the same method as that used for the adhesive.

[0214] The component or material to which the adhesive sheet is attached (at least one adherend in a double-sided adhesive sheet) can be light-transmitting. With the adherend, the advantages of the technique disclosed herein (suppression of light reflection at the interface between the adherend and the adhesive sheet) are readily obtained. The total light transmittance of the adherend can be, for example, greater than 50%, preferably 70% or more. In several ideal cases, the total light transmittance of the adherend is 80% or more, preferably 90% or more, and can be 95% or more (e.g., 95-100%). The adhesive sheet disclosed herein is suitable for use on adherends (e.g., optical components) with a total light transmittance of a predetermined value or higher. The total light transmittance is measured using a commercially available transmittance meter according to JIS K 7136:2000. The transmittance meter can be the "HAZEMETER HM-150" or its equivalent manufactured by Murakami Color Technology Research Institute.

[0215] In several ideal configurations, the substrate (e.g., a component) to which the adhesive sheet is attached can have the aforementioned refractive index and total light transmittance. Specifically, the adhesive sheet disclosed herein is suitable for use on substrates, such as components, with a refractive index of 1.50 or higher (e.g., 1.55 or higher, 1.58 or higher, 1.62 or higher, around 1.66, etc.) and a total light transmittance greater than 50% (e.g., 70% or higher, preferably 80% or higher, more preferably 90% or higher, and even more preferably 95% or higher). The effects of the technique disclosed herein are particularly suitable for use when attached to said component.

[0216] One example of a preferred application is optical applications. More specifically, for example, the adhesive sheet disclosed herein can be used as an optical adhesive sheet for bonding optical components (for bonding optical components) or for manufacturing articles having the aforementioned optical components (optical articles).

[0217] The aforementioned optical components refer to components possessing optical properties (such as polarization, refraction, scattering, reflection, transmission, absorption, diffraction, rotation, and resolution). There is no particular limitation on the optical components possessing optical properties; examples include components constituting display devices (image display devices), input devices, and other machines (optical machines), or components used in such machines. Examples include polarizing plates, wavelength plates, phase retardation plates, optical compensation films, brightness enhancement films, light guide plates, reflective films, anti-reflective films, hard coating (HC) films, impact absorption films, anti-fouling films, photochromic films, dimming films, transparent conductive films (ITO films), design films, decorative films, surface protection plates, prisms, lenses, color filters, transparent substrates, or components further laminated from these (sometimes collectively referred to as "functional films"). Furthermore, the terms "plate" and "film" are respectively defined to include plate-like, film-like, and sheet-like forms. For example, "polarizing film" is defined to include "polarizing plate" or "polarizing sheet," while "light guide plate" is defined to include "light guide film" or "light guide sheet." Also, the term "polarizing plate" is defined to include a circular polarizing plate.

[0218] Examples of the aforementioned display devices include liquid crystal displays, organic EL displays, micro LEDs (µLEDs), mini LEDs, PDPs, and electronic paper. Furthermore, examples of the aforementioned input devices include touch panels.

[0219] The aforementioned optical components are not particularly limited, and examples include components made of glass, acrylic resin, polycarbonate, PET, metal films, etc. (e.g., sheet-like, film-like, or plate-like components). In addition, the term "optical component" in this specification also includes components that maintain the visibility of the display device or input device while serving a decorative or protective function (design films, decorative films, or surface protective films, etc.).

[0220] The techniques disclosed herein are suitable for bonding optical thin films, such as thin films or fluorescent thin films, which have one or more functions of light transmission, reflection, diffusion, waveguide, focusing, and diffraction, to other optical components (which may be other optical thin films). In the bonding of optical thin films with at least one function of light waveguide, focusing, or diffraction, the entire bonding layer should preferably have a high refractive index, making it an ideal application of the techniques disclosed herein.

[0221] The adhesive disclosed herein is suitable for bonding optical films such as light guide films, diffusion films, fluorescent films, color-matching films, prisms, cylindrical lenses, and microlens array films. In these applications, from the perspective of miniaturization or high performance of optical components, there is a demand for thinner designs or improved light extraction efficiency. The adhesive disclosed herein is suitable for use as an adhesive that meets these requirements. More specifically, for example, in bonding light guide films or diffusion films, adjusting the refractive index of the adhesive layer as the bonding layer (e.g., increasing the refractive index) can help achieve thinner designs. In bonding fluorescent films, the light extraction efficiency (which can also be considered as luminous efficiency) can be improved by appropriately adjusting the refractive index difference between the fluorescent emitter and the adhesive. In bonding color-matching films, by appropriately adjusting the refractive index of the adhesive to reduce the refractive index difference with the color-matching pigment, the scattering component can be reduced, thus helping to improve light transmittance. When bonding prisms, cylindrical films, microlens array films, etc., adjusting the refractive index of the adhesive appropriately can control light diffraction, which can help improve brightness and / or viewing angle.

[0222] The adhesive sheet disclosed herein is suitable for use when attached to a high-refractive-index substrate (which may be a high-refractive-index layer or component, etc.), and can suppress interfacial reflection with the substrate. The adhesive sheet suitable for use in this configuration is preferably characterized by a small refractive index difference with the substrate and high adhesion at the interface with the substrate. Furthermore, from the viewpoint of improving the uniformity of appearance, the uniformity of the adhesive layer thickness should be high, for example, the surface smoothness of the adhesive surface should be high. When the thickness of the high-refractive-index substrate is small (e.g., 5µm or less, 4µm or less, or 2µm or less), suppressing interfacial reflection is particularly significant from the viewpoint of suppressing color or color unevenness caused by interference of reflected light. As one example of the usage, the following configuration can be used: in a polarizing plate having a polarizing element, a first phase difference layer and a second phase difference layer in sequence, the polarizing element can be bonded to the first phase difference layer and / or the first phase difference layer can be bonded to the second phase difference layer.

[0223] Furthermore, the adhesive sheet disclosed herein is suitable for use with high refractive index, making it suitable for application in situations where it is attached to a light-emitting layer (e.g., a high-refractive-index light-emitting layer mainly composed of inorganic materials) of a photonic semiconductor. By reducing the refractive index difference between the light-emitting layer and the adhesive layer, reflection at their interface can be suppressed, thereby improving light extraction efficiency. The adhesive sheet suitable for use in the aforementioned situation preferably has an adhesive layer with a high refractive index. Moreover, from the viewpoint of improving brightness, the adhesive sheet should preferably be lightly tinted. This is also advantageous from the viewpoint of suppressing unintentional tinting caused by the adhesive sheet.

[0224] Furthermore, in this specification, a self-emissive element refers to a light-emitting element whose brightness can be controlled by the value of the current flowing through it. A self-emissive element can be a single unit or an assembly. Specific examples of self-emissive elements include light-emitting diodes (LEDs) and organic light-emitting diodes (ELs), but are not limited thereto. Also, in this specification, a light-emitting device refers to a device that includes the aforementioned self-emissive element as a constituent element. Examples of the aforementioned light-emitting devices also include light source module devices used for illumination (e.g., planar light-emitting modules) or display devices forming pixels, but are not limited thereto.

[0225] The adhesive disclosed herein can be used in microlenses and other lens components (e.g., microlenses constituting microlens array films or camera microlenses) as constituent components of cameras or light-emitting devices, as a coating layer covering the lens surface, a bonding layer for components facing the lens surface (e.g., components having a surface shape corresponding to the lens surface), and a filling layer filling the space between the lens surface and the component. Because the adhesive disclosed herein is suitable for high refractive index applications, even with high refractive index lenses (e.g., lenses made of high refractive index resin or lenses with a surface layer made of high refractive index resin), the refractive index difference with the lens can be reduced. This is advantageous from the viewpoint of thinning the lens and the product having the lens, and can also help suppress aberrations or increase the Abbe number. The adhesive disclosed herein can also be used itself as a lens resin, for example, by filling the recesses or gaps of a suitable transparent component.

[0226] There are no particular limitations on the manner in which the adhesive sheet disclosed herein is used to attach optical components. For example, it may be in the following manner: (1) attaching optical components to each other using the adhesive sheet disclosed herein; or (2) attaching an optical component to a component other than an optical component using the adhesive sheet disclosed herein; or (3) attaching the adhesive sheet disclosed herein to an optical component or a component other than an optical component by including the shape of an optical component in the adhesive sheet. Furthermore, in the manner of (3) above, the adhesive sheet including the shape of an optical component may be, for example, an adhesive sheet whose support is an optical component (e.g., an optical film). An adhesive sheet that includes an optical component as a support, as described above, may also be regarded as an adhesive type optical component (e.g., an adhesive type optical film). Furthermore, when the adhesive sheet disclosed herein is an adhesive sheet having a support and the aforementioned functional film is used as the support, the adhesive sheet disclosed herein can also be regarded as an "adhesive-type functional film" having an adhesive layer disclosed herein on at least one side of the functional film.

[0227] Based on the above, and according to the technology disclosed herein, a laminate comprising the adhesive sheet disclosed herein and a component to which the adhesive sheet is attached is provided. The component to which the adhesive sheet is attached may have the refractive index of the aforementioned adherend material. Furthermore, the difference between the refractive index of the adhesive sheet and the refractive index of the component (refractive index difference) may be the difference between the refractive indices of the adherend and the adhesive sheet. Regarding the components constituting the laminate, since the components, materials, and adherends have been described above, they will not be described again.

[0228] As can be understood from the above description and the following examples, the matters disclosed in this specification include the following matters. [1] An adhesive having a refractive index of 1.55 or higher and a storage modulus G' (0°C) in the range of 1.0×10⁴ Pa to 1.0×10⁶ Pa at 0°C. [2] The adhesive described in [1] above has a glass transfer temperature in the range of -50℃ to 0℃. [3] For adhesives as described in [1] or [2] above, the ratio of the aforementioned storage modulus G'(0°C) to the storage modulus G'(80°C) at 80°C (G'(0°C) / G'(80°C)) is in the range of 1 to 1000. [4] For any of the adhesives mentioned in [1] to [3] above, the ratio of the storage elastic modulus G'(-10℃) at -10℃ to the storage elastic modulus G'(80℃) at 80℃ (G'(-10℃) / G'(80℃)) is in the range of 1 to 1000. [5] The adhesives described in any of [1] to [4] above contain an acrylic polymer, and the monomer components constituting the aforementioned acrylic polymer contain aromatic ring monomers (A1). [6] The adhesive of any of the above [1] to [5] contains a plasticizer, and the aforementioned plasticizer is a compound having two or more rings containing double bonds and being liquid at 30°C. [7] The adhesive as described in [6] above, wherein the aforementioned compound is a liquid compound at 20°C. [8] The adhesive as described in [6] or [7] above, wherein the aforementioned plasticizer comprises more than 15 parts by weight (more specifically 30 parts by weight) relative to 100 parts by weight of the base polymer. [] [9] An adhesive as described in any of [6] to [8] above, wherein the plasticizer has at least one ring selected from aromatic rings and heterocyclic rings as the aforementioned double-bonded ring.

[10] An adhesive as described in any of [6] to [9] above, wherein the plasticizer has a first double-bonded ring and a second double-bonded ring, and the first double-bonded ring and the second double-bonded ring are linked by a linker numbering 1 to 5 atoms apart.

[11] An adhesive as described in any of [6] to

[10] above, wherein the molecular weight of the aforementioned plasticizer is in the range of 100 to 2000.

[12] The adhesive as described in [5] above, wherein the monomer component constituting the aforementioned acrylic polymer contains, in addition to the aforementioned aromatic ring monomer (A1), a monomer (A2) having at least one of a hydroxyl group and a carboxyl group.

[13] The adhesive as described in [5] or

[12] above, wherein the content of the aforementioned aromatic ring monomer (A1) in the aforementioned monomer components is 60% by weight or more.

[14] An adhesive as described in any of [5],

[12] and

[13] above, wherein more than 50% by weight of the aromatic ring monomer (A1) is a homopolymer with a glass transition temperature of less than 10°C.

[15] The adhesives mentioned in any of [5],

[12] to

[14] above further include crosslinking agents.

[0229]

[16] An adhesive sheet comprising an adhesive layer made of any of the adhesives described in [1] to

[15] above.

[17] As described in

[16] above, the thickness of the aforementioned adhesive layer is in the range of 5 to 75 µm.

[18] As described in

[16] or

[17] above, the product of the storage modulus of elasticity of the adhesive at 0°C G'(0°C) [Pa] and the thickness of the adhesive layer T [µm] (G'(0°C)×T) is in the range of 5.0×10 4 to 5.0×10 7.

[19] The adhesive sheet of any of the above

[16] to

[18] has a total light transmittance of 85% or more.

[20] The adhesive sheet of any of the above

[16] to

[19] has a haze value of less than 3%.

[21] The adhesive sheet of any one of

[16] to

[20] above has a peel strength of 0.1 N / 25 mm or more to the glass plate.

[0230] Example The following describes various embodiments of the present invention, but the specific examples are not intended to limit the invention. Furthermore, in the following description, "parts" and "%" indicating the amount or content used are by weight unless otherwise specified.

[0231] <Example 1> (Preparation of acrylic polymer solutions) In a four-necked flask equipped with a stirring blade, thermometer, nitrogen inlet pipe, and cooler, 95 parts of phenoxybenzyl acrylate (manufactured by Kyoei Chemical Co., Ltd., trade name "LIGHT ACRYLATE POB-A", refractive index: 1.566, Tg of homopolymer: -35℃; hereinafter referred to as "POB-A"), 5 parts of 4-hydroxybutyl acrylate (4HBA), 0.2 parts of 2,2'-azobisisobutyronitrile (2,2'-azobisisobutyronitrile) as polymerization initiator, and 150 parts of ethyl acetate as polymerization solvent were introduced while stirring slowly. The polymerization reaction was carried out for 6 hours while maintaining the liquid temperature in the flask at approximately 60℃, resulting in a 40% solution of acrylic polymer P1. The Mw of acrylic polymer P1 was 500,000.

[0232] (Preparation of adhesive composition) The above-mentioned acrylic polymer P1 solution (40%) was diluted with ethyl acetate to 20%, and 10 parts of 1% ethyl acetate solution of hexamethylene diisocyanate trimeric acid (manufactured by Tosoh Corporation, trade name "Coronate HX", a 3-functional isocyanate compound) as a crosslinking agent (0.1 parts of non-volatile component) was added to 500 parts of this solution (100 parts of non-volatile component), 2 parts of acetoacetone as a crosslinking delay agent, and 1 part of 1% ethyl acetate solution of acetoacetone iron (III) as a crosslinking catalyst (0.01 parts of non-volatile component). The mixture was stirred and mixed to prepare the acrylic adhesive composition of this example.

[0233] (Making of adhesive sheets) The aforementioned acrylic adhesive composition was coated onto the polysiloxane-treated surface of a polyethylene terephthalate (PET) film R1 (50µm thick), which had undergone polysiloxane treatment on one side. The film was then heated at 130°C for 2 minutes to form an adhesive layer with a thickness of 20µm. Next, a polysiloxane-treated PET film R2 (25µm thick) was bonded to the surface of the adhesive layer. A substrate-free double-sided adhesive sheet composed of the aforementioned adhesive layer was thus obtained. Both sides of the adhesive sheet were protected by PET films (release liner) R1 and R2.

[0234] <Example 2> (Preparation of acrylic polymer solutions) In a four-necked flask equipped with a stirring blade, thermometer, nitrogen inlet pipe, and cooler, 99 parts of benzyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd., trade name "Viscoat #160", refractive index: 1.519, Tg of homopolymer: 6℃; hereinafter referred to as "BZA") and 11 parts of 4HBA were added as monomer components, 0.2 parts of 2,2'-azobisisobutyronitrile as polymerization initiator, and 100 parts of ethyl acetate as polymerization solvent. Nitrogen gas was introduced while stirring slowly, and the liquid temperature in the flask was maintained at approximately 60℃ for 6 hours to prepare a solution of acrylic polymer P2 (polymer concentration 50%). The Mw of acrylic polymer P2 was 1 million.

[0235] (Preparation of adhesive composition) The above-mentioned acrylic polymer P2 solution (polymer concentration 50%) was diluted with ethyl acetate to a polymer concentration of 30%. Then, 10 parts of a 1% ethyl acetate solution of hexamethylene diisocyanate trimeric acid (manufactured by Tosoh Corporation, trade name "Coronate HX", a 3-functional isocyanate compound) as a crosslinking agent (0.1 parts of non-volatile component), 2 parts of acetoacetone as a crosslinking delay agent, and 1 part of a 1% ethyl acetate solution of acetoacetone iron (III) as a crosslinking catalyst (0.01 parts of non-volatile component) were added to 334 parts of this solution (100 parts of non-volatile component). The mixture was stirred and mixed to prepare the acrylic adhesive composition of this example.

[0236] (Making of adhesive sheets) Using the obtained acrylic adhesive composition, the adhesive sheet of this example (a substrate-free double-sided adhesive sheet consisting of an adhesive layer) was produced in the same manner as in Example 1.

[0237] <Example 3~Example 5> In the preparation of the acrylic adhesive composition in Example 2, relative to 100 parts of the non-volatile components contained in the acrylic polymer P2 solution, 30 parts (Example 3), 45 parts (Example 4), or 60 parts (Example 5) of POB-A (manufactured by Kyoesha Chemical Co., Ltd., trade name "LIGHT ACRYLATE POB-A", m-phenoxybenzyl acrylate, refractive index: 1.566, liquid at 20°C) as plasticizer A1 were further added. Otherwise, the acrylic adhesive composition of each example was prepared in the same manner as the acrylic adhesive composition in Example 2. In addition to using the obtained acrylic adhesive compositions, the adhesive sheets (substrate-free double-sided adhesive sheets composed of adhesive layers) of each example were prepared in the same manner as the adhesive sheet in Example 1.

[0238] <Example 6> The monomer composition was changed to 99 parts of phenoxyethyl acrylate (manufactured by Osaka Organic Chemicals Co., Ltd., trade name "Viscoat #192", refractive index: 1.517, Tg of homopolymer: 2℃; hereinafter referred to as "PEA") and 1 part of 4HBA. Otherwise, an acrylic polymer P3 solution was prepared using the same method as the acrylic polymer solution in Example 2. The Mw of acrylic polymer P3 was 1 million. Except that the acrylic polymer P3 solution was used instead of the acrylic polymer P2 solution, the acrylic adhesive composition of this example was prepared in the same manner as the acrylic adhesive composition in Example 2. The obtained acrylic adhesive composition was used to produce the adhesive sheet (a substrate-free double-sided adhesive sheet composed of an adhesive layer) of this example in the same manner as in Example 1.

[0239] <Example 7> In the preparation of the acrylic adhesive composition in Example 3, a solution of acrylic polymer P3 was used instead of a solution of acrylic polymer P2. The acrylic adhesive composition of this example was prepared in the same manner as that in Example 3, and the resulting acrylic adhesive composition was used to produce the adhesive sheet (a substrate-free double-sided adhesive sheet consisting of an adhesive layer) of this example in the same manner as in Example 1.

[0240] <Example 8> Except for changing the monomer composition to 99 parts BZA and 1 part acrylic acid (AA), an acrylic polymer P4 solution was prepared in the same manner as the acrylic polymer solution in Example 2. The Mw of acrylic polymer P4 is 1 million. The above-mentioned acrylic polymer P4 solution (polymer concentration 50%) was diluted with ethyl acetate to a polymer concentration of 30%. 0.1 parts of an epoxy crosslinking agent (manufactured by MITSUBISHI GAS CHEMICAL, trade name "TETRAD C", 1,3-bis(N,N-diepoxypropylaminomethyl)cyclohexane) was added to 334 parts of this solution (100 parts of non-volatile components) and stirred to prepare the acrylic adhesive composition of this example. In addition to using the obtained acrylic adhesive composition, the adhesive sheet of this example (a substrate-free double-sided adhesive sheet composed of an adhesive layer) was produced in the same manner as the adhesive sheet in Example 1.

[0241] <Example 9~Example 12> In the preparation of the acrylic adhesive composition in Example 8, relative to 100 parts of the non-volatile components contained in the acrylic polymer P4 solution, 60 parts of POB-A were added as plasticizer A1 (Example 9), 60 parts of 4,4'-oxybis[(methoxymethyl)benzene] (manufactured by Tokyo Chemical Industry Co., Ltd., refractive index: 1.56, liquid at 20°C) as plasticizer A2 (Example 10), 60 parts of 3-phenoxybenzyl alcohol (manufactured by Tokyo Chemical Industry Co., Ltd., refractive index: 1.591, liquid at 20°C; hereinafter sometimes referred to as "POB-AL") as plasticizer A3 (Example 11), or 60 parts of 1-naphthyl ethyl ketone (manufactured by Tokyo Chemical Industry Co., Ltd., refractive index: 1.63, liquid at 20°C) as plasticizer A4 (Example 12). Otherwise, the acrylic adhesive compositions of each example were prepared in the same manner as those in Example 8. In addition to using the obtained acrylic adhesive compositions, adhesive sheets (substrate-free double-sided adhesive sheets consisting of adhesive layers) of each example were produced in the same manner as the adhesive sheet in Example 1.

[0242] <Example 13> Except for changing the composition of the monomer components to 90 parts BZA, 9 parts n-butyl acrylate (BA), and 1 part AA, an acrylic polymer P5 solution was prepared in the same manner as the acrylic polymer solution in Example 2. The Mw of acrylic polymer P5 is 1 million. The above-mentioned acrylic polymer P5 solution (polymer concentration 50%) was diluted with ethyl acetate to a polymer concentration of 30%. 60 parts of the above-mentioned plasticizer A3 (POB-AL) and 0.1 parts of the above-mentioned epoxy crosslinking agent were added to 334 parts of the solution (100 parts of non-volatile components) and then stirred to prepare the acrylic adhesive composition of this example. In addition to using the obtained acrylic adhesive composition, the adhesive sheet of this example (a substrate-free double-sided adhesive sheet composed of an adhesive layer) was produced in the same manner as the adhesive sheet in Example 1.

[0243] <Example 14> In the preparation of the acrylic adhesive composition in Example 13, 15 parts of bis(4-hydroxy-3-methylphenyl)phenol (manufactured by Osaka Gas Chemicals Co., Ltd., 9,9-bis(4-hydroxy-3-methylphenyl)phenol, refractive index: 1.68; hereinafter sometimes referred to as "BCF") were added as an additive, based on the solid content, relative to 100 parts of the non-volatile components contained in the acrylic polymer P5 solution. Otherwise, the acrylic adhesive composition of this example was prepared in the same manner as in Example 13. The above additives were added using a 10% ethyl acetate solution. In addition to using the obtained acrylic adhesive composition, the adhesive sheet of this example (a substrate-free double-sided adhesive sheet composed of an adhesive layer) was produced in the same manner as the adhesive sheet in Example 1.

[0244] <Example 15~Example 16> Except for changing the composition of the monomer components to 90 parts BZA, 9 parts 2-ethylhexyl acrylate (2EHA), and 1 part AA, a solution of acrylic polymer P6 was prepared in the same manner as the acrylic polymer solution in Example 2. The Mw of acrylic polymer P6 is 1 million. In the preparation of the acrylic adhesive compositions in Examples 13 and 14, a solution of acrylic polymer P6 was used instead of a solution of acrylic polymer P5. The remaining acrylic adhesive compositions for Examples 15 and 16 were prepared in the same manner as those in Examples 13 and 14. Using the obtained acrylic adhesive compositions, adhesive sheets (substrate-free double-sided adhesive sheets consisting of adhesive layers) were fabricated in the same manner as in Example 1.

[0245] <Example 17~Example 18> Except for changing the monomer composition to 80 parts BZA, 19 parts BA, and 1 part AA, an acrylic polymer P7 solution was prepared in the same manner as the acrylic polymer solution in Example 2. Furthermore, except for changing the monomer composition to 66 parts BZA, 33 parts BA, and 1 part AA, an acrylic polymer P8 solution was prepared in the same manner as the acrylic polymer solution in Example 2. The Mw of acrylic polymers P7 and P8 is 1 million. In the preparation of the acrylic adhesive composition in Example 13, a solution of acrylic polymer P7 (Example 17) or P8 (Example 18) was used to replace the solution of acrylic polymer P5. The remaining acrylic adhesive compositions for Examples 17 and 18 were prepared in the same manner as those in Example 13. Using the obtained acrylic adhesive compositions, adhesive sheets (substrate-free double-sided adhesive sheets consisting of adhesive layers) were fabricated in the same manner as in Example 1.

[0246] <Example 19> Except for changing the composition of the monomer components to 85 parts POB-A, 14 parts BA, and 1 part AA, a solution of acrylic polymer P9 was prepared in the same manner as the acrylic polymer solution in Example 2. The Mw of acrylic polymer P9 is 1 million. In the preparation of the acrylic adhesive composition in Example 13, a solution of acrylic polymer P9 was used instead of a solution of acrylic polymer P5. The acrylic adhesive composition of this example was prepared in the same manner as that in Example 13, and the resulting acrylic adhesive composition was used to produce the adhesive sheet (a substrate-free double-sided adhesive sheet consisting of an adhesive layer) of this example in the same manner as in Example 1.

[0247] <Example 20> In the preparation of the acrylic adhesive composition in Example 19, 23 parts of 6-acryloxymethyl dinaphthothiophene (trade name "6MDNTA" manufactured by SUGAI Chemical Industry Co., Ltd., dinaphthothiophene-6-methacrylate, refractive index 1.75; hereinafter sometimes referred to as "6MDNTA") were added as an additive, relative to 100 parts of the non-volatile components contained in the acrylic polymer P9 solution. The remaining acrylic adhesive composition was prepared in the same manner as in Example 19 to produce the acrylic adhesive composition of this example, and the resulting acrylic adhesive composition was used to produce the adhesive sheet (a substrate-free double-sided adhesive sheet consisting of an adhesive layer) of this example in the same manner as in Example 1. The above additives were added using a 10% ethyl acetate solution.

[0248] <Example 21~Example 22> Except for changing the composition of the monomer components to 80 parts POB-A, 19 parts BA, and 1 part AA, a solution of acrylic polymer P10 was prepared in the same manner as the acrylic polymer solution in Example 2. The Mw of acrylic polymer P10 is 1 million. In the preparation of the acrylic adhesive composition in Example 13, a solution of acrylic polymer P10 was used instead of a solution of acrylic polymer P5. The remaining steps were performed in the same manner as in Example 13 to prepare the acrylic adhesive composition of Example 21. Furthermore, in the preparation of the acrylic adhesive composition of Example 21, the amount of the epoxy crosslinking agent used was changed to 0.5 parts, relative to 100 parts of the non-volatile components contained in the acrylic polymer P10 solution, to prepare the acrylic adhesive composition of Example 22. Using the obtained acrylic adhesive compositions, adhesive sheets (substrate-free double-sided adhesive sheets consisting of adhesive layers) of each example were prepared in the same manner as in Example 1.

[0249] <Example 23> Except for changing the composition of the monomer components to 99 parts of POB-A and 1 part of 2-acryloyloxyethyl-succinic acid (manufactured by Kyoei Chemical Co., Ltd., trade name "HOA-MS(N)", hereinafter referred to as "HOA-MS"), a solution of acrylic polymer P11 was prepared in the same manner as the acrylic polymer solution in Example 1. The Mw of acrylic polymer P11 is 500,000. The above-mentioned acrylic polymer P11 solution was diluted with ethyl acetate to a polymer concentration of 30%. Then, 40 parts of the above-mentioned plasticizer A3 (POB-AL), 20 parts of trimethylpentaphenyltrisiloxane (manufactured by Shin-Etsu Chemical Industry Co., Ltd., trade name "HIVAC F-5", refractive index: 1.575, liquid at 20°C) as plasticizer A5, and 0.3 parts of the above-mentioned epoxy crosslinking agent (hereinafter also referred to as crosslinking agent C1) were added to 334 parts of this solution (100 parts of non-volatile components). The mixture was stirred and mixed to prepare the acrylic adhesive composition of this example. In addition to using the obtained acrylic adhesive composition, the adhesive sheet of this example (a substrate-free double-sided adhesive sheet composed of an adhesive layer) was produced in the same manner as the adhesive sheet in Example 1.

[0250] <Example 24> Except for changing the composition of the monomer components to 95 parts POB-A, 2 parts lauryl acrylate (LA), 2 parts 2EHA, and 1 part 4HBA, a solution of acrylic polymer P12 was prepared in the same manner as the acrylic polymer solution in Example 23. The Mw of acrylic polymer P12 is 500,000. The above-mentioned acrylic polymer P12 solution was diluted with ethyl acetate to a polymer concentration of 30%. Then, 40 parts of the above-mentioned plasticizer A3 (POB-AL), 20 parts of the above-mentioned plasticizer A5 (HIVAC F-5), 0.3 parts of the above-mentioned non-cyclic difunctional isocyanate crosslinking agent C2 (manufactured by Tosoh, trade name "Coronate 2770", hexamethylene diisocyanate (HDI) urea-formaldehyde ester), 2 parts of acetoacetone as a crosslinking delay agent, and 1 part of a 1% ethyl acetate solution of acetoacetone iron (III) as a crosslinking catalyst (0.01 parts of non-volatile component) were added and stirred to prepare the acrylic adhesive composition of this example. In addition to using the obtained acrylic adhesive composition, the adhesive sheet of this example (a substrate-free double-sided adhesive sheet composed of an adhesive layer) was produced in the same manner as the adhesive sheet in Example 1.

[0251] <Example 25> Except for changing the composition of the monomer components to 90 parts POB-A, 9 parts 2EHA, and 1 part 4HBA, a solution of acrylic polymer P13 was prepared in the same manner as the acrylic polymer solution in Example 23. The Mw of acrylic polymer P13 is 500,000. The above-mentioned acrylic polymer P13 solution was diluted with ethyl acetate to a polymer concentration of 30%. Then, 80 parts of the above-mentioned plasticizer A5 (HIVAC F-5), 0.5 parts of the above-mentioned crosslinking agent C2 (Coronate 2770), 2 parts of acetoacetone as a crosslinking delay agent, and 1 part of a 1% ethyl acetate solution of acetoacetone iron (III) as a crosslinking catalyst (0.01 parts of non-volatile components) were added to 334 parts of this solution (100 parts of non-volatile components). The mixture was stirred and stirred to prepare the acrylic adhesive composition of this example. In addition to using the obtained acrylic adhesive composition, the adhesive sheet of this example (a substrate-free double-sided adhesive sheet composed of an adhesive layer) was produced in the same manner as the adhesive sheet in Example 1.

[0252] <Examples 26~28> Except for changing the composition of the monomer components as shown in Table 3, solutions of acrylic polymers P14 and P15 were prepared in the same manner as the acrylic polymer solution in Example 23. The Mw of acrylic polymers P14 and P15 were both 500,000. The solutions of the above-mentioned acrylic polymers P11, P14 or P15 were diluted with ethyl acetate to a polymer concentration of 30%, and the above-mentioned plasticizer A3 and crosslinking agent C1 or C2 were added to 334 parts of the solution (100 parts of non-volatile components) as shown in Table 3. For Examples 26 and 28, 2 parts of acetone as a crosslinking delay agent and 1 part of 1% ethyl acetate solution of acetone iron (III) as a crosslinking catalyst (0.01 parts of non-volatile components) were further added and stirred to prepare the acrylic adhesive composition of each example. In addition to using the obtained acrylic adhesive composition, each example of adhesive sheet (a substrate-free double-sided adhesive sheet consisting of an adhesive layer) was produced in the same manner as the adhesive sheet in Example 1.

[0253] <Evaluation Methods> (Refractive index) For each example of adhesive layer (double-sided adhesive sheet without substrate), the refractive index was measured using an Abbe refractometer (ATAGO CO., LTD., model "DR-M4") at a measurement wavelength of 589 nm and a measurement temperature of 25 °C.

[0254] (Storage elastic modulus G' and glass transition temperature) Each adhesive laminate was prepared to a thickness of approximately 1.5 mm and used as a test sample. Dynamic viscoelasticity was measured using the "Advanced Rheometric Expansion System (ARES)" manufactured by Rheometric Scientific under the following conditions. The storage modulus G' [Pa] of the adhesive at various temperatures (-20°C, -10°C, 0°C, and 80°C) was determined from the measurement results. Furthermore, the temperature corresponding to the peak temperature of the loss tangent tanδ (loss modulus G" / storage modulus G') in the above dynamic viscoelasticity measurement was taken as the glass transition temperature (Tg) [°C] of the adhesive. [Measurement Conditions] Deformation mode: Torsion Measurement frequency: 1Hz Temperature range: -50℃ to 150℃ Heating rate: 5℃ / minute Shape: parallel plate 7.9mmφ

[0255] (Total light transmittance and haze) Using test pieces on which the adhesive layers of each example have been bonded to alkali-free glass (thickness 0.8~1.0 mm, total light transmittance 92%, haze 0.4%), the total light transmittance and haze of the test pieces were measured using a haze meter (Murakami Color Technology Research Institute "HM-150") at a measurement environment of 23°C. The total light transmittance [%] and haze [%] of the adhesive (layer) were obtained by subtracting the total light transmittance and haze of the alkali-free glass from the measured values. For substrate-free adhesive sheets composed of the above adhesive layers, the total light transmittance [%] and haze [%] of the adhesive layer will become the total light transmittance [%] and haze [%] of the adhesive sheet.

[0256] (Peel strength of glass plate) Under a test environment of 23°C and 50%RH, the release liner was peeled off from one side of the adhesive sheet, and a 50µm thick PET film was laminated as a substrate. The resulting sheet was then cut into test pieces with a width of 25mm and a length of 100mm. The release liner was peeled off from the other side of the test piece, and a 2kg roller was used to press the surface of an alkaline glass plate (Matsunami Glass Industry Co., Ltd., 1.35mm thick, frosted blue plate) back and forth once. After being placed in this environment for 30 minutes, the peel strength (adhesion force) [N / 25mm] was measured using a universal tensile and compression testing machine, following JIS Z 0237:2000, at a tensile speed of 300mm / min and a peel angle of 180 degrees. The universal tensile and compression testing machine used was a "Tensile and Compression Testing Machine, TG-1kN" manufactured by Minebea Corporation. Furthermore, for single-sided adhesive sheets with a substrate, a PET film substrate is not required.

[0257] (Bending test) Each example of an adhesive sheet with a release liner was cut into a rectangle of 2cm × 10cm to obtain a test piece for measurement. A cylindrical rod with a diameter of 4mm was horizontally fixed at a sufficient height for measurement, and the obtained test piece was placed on the rod to flex. Specifically, the central portion of the test piece along its length was placed on the rod to form an inverted U-shape. Next, a clamp (13g) was fixed at both ends located below the test piece, and a 60g weight was suspended and fixed through a 1cm long line on the clamp to apply a load to the flexed portion of the test piece. In this state, the test piece was maintained at a predetermined temperature environment (-20°C, -10°C, or 0°C) for 1 minute. After 1 minute, the test piece was removed from the rod. Then, at this temperature environment, the test piece was placed on a horizontal surface with the convex side of the flexed portion facing down, and left to stand for 10 minutes. The time until the end (short side end) of the test piece touched the horizontal surface after 10 minutes of standing was measured. The test was conducted at -20°C, -10°C and 0°C respectively, and the softness was evaluated according to the following criteria. E (Excellent): In the bending test under all temperature conditions (-20℃, -10℃ and 0℃), the end of the test piece contacts the horizontal surface within 10 minutes. G (Good): In bending tests at temperatures of -10℃ and 0℃, the end of the test piece contacts the horizontal surface within 10 minutes. A (Acceptable): In a bending test at 0°C, the end of the test piece contacts the horizontal surface within 10 minutes. P (Poor): In the bending test under all temperature conditions, the end of the test piece does not come into contact with the horizontal surface or the adhesive layer peels off from the release liner within 10 minutes.

[0258] The summary and evaluation results of the adhesives in each case are shown in Tables 1-3.

[0259] [Table 1]

[0260] [Table 2]

[0261] [Table 3]

[0262] As shown in Tables 1-3, the adhesives in Examples 3-5, 7, and 9-28 have a refractive index of 1.55 or higher, and a storage modulus of elasticity G' (0°C) within the range of 1.0 × 10⁴ Pa to 1.0 × 10⁶ Pa, thus passing the bending test (A or higher). These adhesives combine high refractive index with flexibility. On the other hand, the adhesives in Examples 1-2, 6, and 8 have a storage modulus of elasticity G' (0°C) greater than 1.0 × 10⁶ Pa, resulting in a failing bending test (P), failing to balance high refractive index and flexibility.

[0263] The specific examples of the present invention have been described in detail above, but these are merely illustrative and do not limit the scope of the patent application. The technology described in the patent application includes various modifications and alterations to the specific examples exemplified above.

[0264] 1,2,3: Adhesive sheets 10: Supporting substrate 10A: Page 1 10B: Page 2 21: Adhesive layer, first adhesive layer 21A: Adhesive surface, first adhesive surface 21B: Adhesive Flour 22: Second adhesive layer 22A: Second adhesive surface 31, 32: Peeling off the gasket

Claims

1. An adhesive comprising a plasticizer, wherein the plasticizer is a compound that is liquid at 30°C, and the plasticizer is selected from at least one of the following groups of compounds: a compound having a structure of two or more non-condensed double-bonded rings directly chemically bonded, a compound having a condensed double-bonded ring structure, a compound having a cyclopentadienylene structure, a compound having a dibenzothiophene structure, a compound having a dibenzothiophene structure, or a compound having two or more non-condensed double-bonded rings bonded by a linker group, wherein the linker group is an oxy(-O-), -S-, oxyalkylene, -S-(CH2)n-, a group containing a siloxane bond, or a group containing an ester bond (-COO-), wherein the adhesive has a refractive index of 1.55 or higher, and a storage modulus of elasticity G' (0°C) in the range of 1.0 × 10⁴ Pa to 1.0 × 10⁶ Pa.

2. The adhesive of request item 1 has a glass transfer temperature in the range of -50℃ to 0℃.

3. For adhesives as claimed in item 1 or 2, the ratio of the aforementioned storage modulus of elasticity G'(0°C) to the storage modulus of elasticity G'(80°C) at 80°C (G'(0°C) / G'(80°C)) is in the range of 1 to 1000.

4. For adhesives as requested in item 1 or 2, the ratio of their storage modulus of elasticity G'(-10℃) at -10℃ to their storage modulus of elasticity G'(80℃) at 80℃ (G'(-10℃) / G'(80℃)) is in the range of 1 to 1000.

5. An adhesive sheet comprising an adhesive layer made of an adhesive as claimed in any one of claims 1 to 4.

6. The adhesive sheet as claimed in claim 5, wherein the thickness of the aforementioned adhesive layer is in the range of 5 to 75 µm.

7. The adhesive sheet as claimed in claim 6, wherein the product of the aforementioned storage elastic modulus G'(0℃) [Pa] and the thickness T [µm] of the aforementioned adhesive layer (G'(0℃)×T) is in the range of 5.0×10⁴ to 5.0×10⁷.

8. The adhesive sheet for any of the requests 5 to 7 has a total light transmittance of 85% or more.

9. The adhesive sheet for any of the requests in items 5 to 7 has a haze value of 3% or less.

10. The adhesive sheet of any one of the claims 5 to 7 has a peel strength to the glass plate of 0.1 N / 25 mm or more.

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