TWI889779B - Light emitting device - Google Patents

Abstract

The provided light-emitting device includes a self-luminous element; a low-refractive-index layer disposed on a visually observable side of the self-luminous element; and a high-refractive-index adhesive layer laminated in direct contact with the low-refractive-index adhesive layer. The high-refractive-index adhesive layer has a refractive index _n1 greater than 1.570, a total light transmittance of 86% or greater, and a haze value of 3.0% or less.

Description

Light-emitting device

The present invention relates to a light-emitting device, and more specifically, to a light-emitting device having an adhesive layer disposed on the visual side of a self-luminous element. This application claims priority based on Japanese Patent Application No. 2020-052408 filed on March 24, 2020, Japanese Patent Application No. 2020-166428 filed on September 30, 2020, and Japanese Patent Application No. 2021-049061 filed on March 23, 2021, and the entire contents of these applications are incorporated herein by reference.

Generally speaking, adhesives (also called pressure-sensitive adhesives, hereinafter referred to as such) are soft solids (viscoelastics) in the temperature range near room temperature, allowing them to be easily bonded to adherends by pressure. Leveraging this property, adhesives are widely used for bonding, fixing, and protecting components in a wide range of industries, from home appliances to automobiles, various machinery, electrical equipment, and electronic devices. One example of adhesive applications is the bonding of polarizing films, retardation films, window cover components, and other light-transmitting components in display devices such as liquid crystal displays and organic EL displays. Technical literature on adhesives for optical components includes patent documents 1 and 2. Prior Art Literature Patent Literature

Patent Document 1: Japanese Patent Application Publication No. 2014-169382 Patent Document 2: Japanese Patent Application Publication No. 2017-128732

Problem to be Solved by the Invention Patent Documents 1 and 2 propose adhesive compositions based on (meth)acrylate polymers and adhesives formed by crosslinking these adhesive compositions. The (meth)acrylate polymers contain monomers with multiple aromatic rings as monomer units. However, they do not disclose specific adhesives with a refractive index greater than 1.570. On the other hand, a technique for increasing the refractive index by blending particles composed of high-refractive-index inorganic materials (e.g., zirconium oxide particles or titanium oxide particles) into a resin is also known. However, adhesives blended with inorganic particles have difficulty in application in the adhesive field due to the trade-off between refractive index and adhesive properties (e.g., peel strength, flexibility, etc.). For example, when increasing the refractive index of the adhesive layer disposed on the viewing side of the self-luminous element in a light-emitting device, the effect on optical properties (such as total light transmittance and haze) must be considered when adding inorganic particles.

The present invention was created in view of the above situation, and its purpose is to provide a light-emitting device having an adhesive layer of high refractive index and optical quality disposed on the visual side of the self-luminous element.

The present disclosure provides a light-emitting device comprising a self-luminous element; a low-refractive-index layer positioned closer to the viewing side of the self-luminous element; and a high-refractive-index adhesive layer laminated in direct contact with the low-refractive-index layer. The high-refractive-index adhesive layer has a refractive index _n1 greater than 1.570, a total light transmittance of 86% or greater, and a haze value of 3.0% or less.

In some aspects, the ratio of the refractive index _n1 of the high-refractive-index adhesive layer to the refractive index _n2 of the low-refractive-index layer ( _n1 / _n2 ) is preferably greater than or equal to about 1.05.

In some aspects, the arithmetic average roughness Ra of the surface of the high refractive index adhesive layer is preferably less than 100 nm.

In some aspects, the ratio of the thickness _T1 of the high refractive index adhesive layer to the thickness _T2 of the low refractive index layer ( _T1 / _T2 ) is preferably in the range of approximately 0.5-5.

In some aspects, the thickness _T1 of the high refractive index adhesive layer is preferably greater than 5µm.

In some aspects, the laminated sheet (adhesive sheet) composed of the high refractive index adhesive layer and the low refractive index layer preferably has a total light transmittance of 86% or more and a haze value of 3.0% or less.

In addition, inventions that are formed by appropriate combinations of the elements described in this specification may also be included in the scope of the invention sought to be protected by this patent application.

The following describes a preferred embodiment of the present invention. Matters necessary for the implementation of the present invention, other than those specifically mentioned in this specification, are understood by those skilled in the art based on the teachings regarding the implementation of the invention set forth in this specification and the technical common sense at the time of filing. The present invention can be implemented based on the disclosures in this specification and the technical common sense in the art. In addition, in the following figures, components and areas performing the same function are designated by the same reference numerals, and overlapping descriptions may be omitted or simplified. Furthermore, the embodiments depicted in the figures are schematic for the purpose of clarifying the present invention and do not accurately represent the dimensions or scale of the actual product.

In this specification, a self-luminous element refers to a light-emitting element whose luminance can be controlled by the value of the current flowing through it. A self-luminous element can be constructed as a single unit or as a collection of units. Specific examples of self-luminous elements include, but are not limited to, light-emitting diodes (LEDs) and organic ELs. The light-emitting device disclosed herein includes the self-luminous element as a component. Examples of the light-emitting device described above also include, but are not limited to, light source module devices used for lighting (e.g., planar light-emitting modules) or display devices having pixels.

The technical matters disclosed in this specification include: a light-emitting device, a high-refractive-index adhesive layer and an adhesive composition used to form the same, a low-refractive-index layer and a composition used to form the same, a laminate (adhesive sheet) comprising a high-refractive-index adhesive layer and a low-refractive-index layer, and a laminate with a peel-off liner in which the adhesive surface of the laminate is protected by a peel-off liner.

<Example of Light-Emitting Device Configuration> Figure 1 shows an example of the configuration of a light-emitting device provided in this specification. The light-emitting device 100 shown in Figure 1 includes a self-luminous element 70; a low-refractive-index layer 12 disposed on the visual side of the self-luminous element 70; and a high-refractive-index adhesive layer 11 laminated in direct contact with the low-refractive-index layer 12. The light-emitting device 100 may further include a cover window member 80 disposed on the visual side of the high-refractive-index adhesive layer 11. In the light-emitting device 100 shown in Figure 1, a laminate sheet 10 composed of the high-refractive-index adhesive layer 11 and the low-refractive-index layer 12 is disposed between the self-luminous element 70 and the cover window member 80. One, two, or more layers (not shown) may be interposed independently between the self-luminous element 70 and the low-refractive index layer 12, between the high-refractive index adhesive layer 12 and the cover window member 80, and on the more visible side of the cover window member 80. These layers may also be absent. Furthermore, contrary to FIG1 , the high-refractive index adhesive layer 11 may be positioned on the self-luminous element side, while the low-refractive index layer 12 may be positioned on the cover window member side.

In the disclosed technology, the low-refractive-index layer can be adhesive or non-adhesive. In several aspects, the low-refractive-index layer is preferably an adhesive layer, i.e., a low-refractive-index adhesive layer. This allows the laminated sheet (adhesive sheet) of the low-refractive-index adhesive layer and the high-refractive-index adhesive layer to be double-sidedly bonded, thereby improving assembly during the manufacture of the light-emitting device. Before being incorporated into a light-emitting device, the laminate may be in the form of a laminate with a peel-off pad attached, for example, as shown in FIG2 : It is a laminate 10 (a double-sided adhesive sheet 2 without a substrate) composed of a high-refractive-index adhesive layer 11 and a low-refractive-index adhesive layer 12, and the surface (first surface) 10A of the laminate 10 on the high-refractive-index adhesive layer 11 side serves as the first adhesive surface, and the surface (second surface) 10B on the low-refractive-index adhesive layer 12 side serves as the second adhesive surface, and these adhesive surfaces are each protected by a peel-off pad 31 or 32.

<High-Refractive-Index Adhesive Layer> The light-emitting device disclosed herein includes a high-refractive-index adhesive layer, which is laminated directly in contact with the low-refractive-index layer included in the light-emitting device. The high-refractive-index adhesive layer has a refractive index relatively higher than that of the low-refractive-index layer. The high-refractive-index adhesive layer preferably has a refractive index _n1 greater than 1.570, a total light transmittance of 86% or greater, and a haze value of 3.0% or less.

(Refractive Index) The light-emitting device disclosed herein has a high-refractive-index adhesive layer having a refractive index _n1 greater than 1.570. The high-refractive-index adhesive layer can be realized by forming at least one surface (adhesive surface) of the high-refractive-index adhesive layer with an adhesive (viscoelastic material) having a refractive index greater than 1.570.

In addition, in this specification, the refractive index of the adhesive means the refractive index of the surface (adhesive surface) of the adhesive. The refractive index of the adhesive can be measured using a commercially available refractive index measuring device (Abbe refractometer) under the conditions of a measuring wavelength of 589 nm and a measuring temperature of 25°C. The Abbe refractometer can be, for example, the "DR-M4" model manufactured by ATAGO or its equivalent. The measurement sample can use an adhesive layer composed of the adhesive to be evaluated. The refractive index of the adhesive can be specifically measured by the method described in the embodiments described later. The refractive index of the adhesive can be adjusted by, for example, the composition of the adhesive (for example, the composition of the monomer components constituting the base polymer, additives that can be used as needed, etc.).

The technical matters provided by this specification include: an adhesive layer having a refractive index greater than 1.570 (a high-refractive-index adhesive layer), an adhesive composition capable of forming such an adhesive layer, and a laminate comprising such a high-refractive-index adhesive layer. The laminate may be, for example, a laminate comprising the high-refractive-index adhesive layer and a low-refractive-index layer (typically a low-refractive-index adhesive layer), or may be a laminate comprising a high-refractive-index adhesive layer and a low-refractive-index layer laminated sequentially or in reverse order on one side of a support substrate.

In several aspects, the refractive index of the high-refractive-index adhesive layer is preferably 1.580 or greater, more preferably 1.585 or greater, and even more preferably 1.590 or greater (e.g., 1.595 or greater). By having a high-refractive-index adhesive layer with such a refractive index, the behavior of light passing through the high-refractive-index adhesive layer can be effectively controlled by utilizing the relative refractive index relationship between the high-refractive-index adhesive layer and the directly adjacent low-refractive-index layer (typically the low-refractive-index adhesive layer). In several aspects of the disclosed technology, the refractive index of the high-refractive-index adhesive layer can be, for example, 1.600 or greater, 1.605 or greater, or 1.610 or greater. The ideal upper limit of the refractive index of the high-refractive-index adhesive layer may vary depending on the refractive index of the adjacent layer, and is therefore not limited to a specific range. In several aspects, considering the balance between adhesive properties and transparency, the refractive index of the high-refractive-index adhesive layer may be, for example, 1.700 or less, 1.670 or less, or 1.650 or less.

(Total Light Transmittance) In the disclosed technology, the total light transmittance of the high-refractive-index adhesive layer is suitably 86% or higher, preferably 88% or higher, and more preferably 90% or higher (e.g., greater than 90.0%). It can be 90.5% or higher, 93% or higher, or even 95% or higher. The upper limit of the total light transmittance is theoretically the value obtained by 100% exclusion of light loss due to reflection at the air interface (Fresnel loss). Practically, it can be approximately 98% or lower, approximately 96% or lower, or approximately 95% or lower. In some embodiments, depending on the refractive index or adhesive properties, the total light transmittance of the high-refractive-index adhesive layer can be approximately 94% or lower, approximately 93% or lower, or approximately 92% or lower. Total light transmittance is measured in accordance with JIS K 7136:2000 using a commercially available transmittance meter. A transmittance meter that can be used is the "HAZEMETER HM-150" manufactured by Murakami Color Research Laboratory, or an equivalent. More specifically, the total light transmittance of a high-refractive index adhesive layer can be measured, for example, according to the examples described below. The total light transmittance of a high-refractive index adhesive layer can be adjusted by, for example, selecting the composition and thickness of the high-refractive index adhesive layer.

(Haze Value) The haze value of the high-refractive index adhesive layer is suitably 3.0% or less, preferably 2.0% or less, more preferably 1.0% or less, and even more preferably 0.9% or less. A low haze value of the high-refractive index adhesive layer is advantageous in applications requiring high light transmittance (such as optical applications) or in applications requiring good visibility of the adherend through the high-refractive index adhesive layer. In several embodiments, the haze value of the high-refractive index adhesive layer can be 0.8% or less, 0.5% or less, or even 0.3% or less. There is no particular lower limit for the haze value of the high-refractive index adhesive layer; from the perspective of improving transparency, a lower haze value is preferred. On the other hand, in several aspects, taking into account the refractive index or adhesive properties, the haze value of the high-refractive-index adhesive layer can be, for example, 0.05% or greater, 0.1% or greater, 0.2% or greater, 0.3% or greater, or 0.4% or greater. These haze values for the high-refractive-index adhesive layer can also be appropriately applied to the haze values of a laminate composed of the high-refractive-index adhesive layer and the low-refractive-index layer described below (typically the low-refractive-index adhesive layer).

Here, "haze value" refers to the ratio of diffuse light transmitted to total light transmitted when the object being measured is illuminated with visible light. It is also called the haze value. The haze value can be expressed as follows: Th (%) = Td / Tt × 100 In the above formula, Th is the haze value (%), Td is the diffuse light transmittance, and Tt is the total light transmittance. The haze value can be measured according to the method described in the examples below. The haze value of the adhesive layer can be adjusted by, for example, selecting the adhesive layer's composition and thickness.

(Storage modulus G') In the technology disclosed herein, the storage modulus G' of the high refractive index adhesive layer at 25°C (hereinafter also referred to as "storage modulus G' _V1 (25)") can be appropriately set according to the purpose of use or usage mode, and is not limited to a specific range. The storage modulus G' _V1 (25) can be, for example, about 700 kPa or less. In several aspects, from the perspective of ease of attachment to the adherend, it is advantageous for the storage modulus G' _V1 (25) to be about 600 kPa or less, preferably 500 kPa or less, and more preferably 400 kPa or less (for example, 350 kPa or less). In several embodiments, from the perspective of improving the flexibility of the high refractive index adhesive layer at room temperature (e.g., 25°C) and facilitating adhesion to the adherend, the storage modulus G' _V1 (25) is advantageously less than about 330 kPa, and preferably less than 300 kPa. In several embodiments where greater emphasis is placed on adhesion or flexibility at room temperature, the storage modulus G' _V1 (25) may be, for example, less than 270 kPa or less than 250 kPa, and less than 200 kPa is advantageous, and preferably less than 180 kPa, more preferably less than 160 kPa (e.g., less than 140 kPa). In several embodiments, the storage modulus G' _V1 (25) may be less than 100 kPa, or less than 90 kPa. The lower limit of the storage elastic modulus G' _V1 (25) is not particularly limited, but from the perspective of processability or handling, it can be, for example, 30 kPa or more, 50 kPa or more, or 70 kPa or more. In several aspects, considering the high refractive index, the storage elastic modulus G' _V1 (25) can be 100 kPa or more, 150 kPa or more, 200 kPa or more, 250 kPa or more, or 300 kPa or more.

In the technology disclosed herein, the storage modulus G' of the high refractive index adhesive layer at 50°C (hereinafter also referred to as "storage modulus G' _V1 (50)") is not particularly limited, and for example, may be less than 100 kPa. In several aspects, it is appropriate for the storage modulus G' _V1 (50) to be less than 60 kPa, and preferably less than 40 kPa, more preferably less than 38 kPa (for example, less than 36 kPa). The high refractive index adhesive layer having a limited storage modulus G' _V1 (50) as described above can easily improve its adhesion to the adherend by appropriately heating as needed, thereby improving its adhesion to the adherend. There is no particular limitation on the lower limit of the storage modulus G' _V1 (50). In several aspects, from the perspective of heat resistance, etc., the storage elastic modulus G' _V1 (50) can be, for example, greater than 10 kPa, greater than 15 kPa, greater than 20 kPa, or greater than 23 kPa.

In several aspects of the technology disclosed herein, the high refractive index adhesive layer preferably meets at least one of the following conditions: (a) the storage elastic modulus G' _V1 (25) is 350 kPa or less (preferably less than 200 kPa _, for example, less than 180 kPa); and (b) the storage elastic modulus G' _V1 (50) is less than 60 kPa (preferably less than 50 kPa, more preferably less than 40 kPa, for example, less than 38 kPa). A high refractive index adhesive layer that meets at least the above condition (a) is ideal from the perspective of adhesion or softness to the adherend at room temperature (e.g., 25°C). A high-refractive-index adhesive layer that satisfies at least condition (b) is preferred because its adhesion to the adherend can be easily improved by heating it to a temperature slightly above room temperature. An adhesive sheet having a high-refractive-index adhesive layer that does not meet condition (a) but meets condition (b) can be used as a heat-activated adhesive sheet. It has excellent initial reworkability (re-adhesion) when attached at room temperature, and its peel strength from the adherend can be effectively improved by heating it to a temperature slightly above room temperature. The heat activation can also be performed by heating the adhesive sheet to a temperature slightly above room temperature when attached to the adherend. The temperature slightly higher than room temperature is, for example, about 60°C or lower, and preferably about 55°C or lower (for example, about 50°C or lower).

In several aspects of the technology disclosed herein, the ratio of the storage elastic modulus G' _V1 (50) [kPa] to the storage elastic modulus G' _V1 (25) [kPa], that is, the storage elastic modulus ratio G' _V1 (50) / G' _V1 (25), is, for example, less than 70%, less than 40%, less than 30%, or less than 20%. An adhesive sheet having a high refractive index adhesive layer with a small G' _V1 (50) / G' _V1 (25) is suitable for use as the above-mentioned heat-activated adhesive sheet. There is no particular restriction on the lower limit of G' _V1 (50) / G' _V1 (25). G' _V1 (50)/G' _V1 (25) is, for example, greater than 5%, but from the perspective of heat resistance, it is preferably greater than 10%, may be greater than 12%, or may be greater than 15%.

The storage elastic modulus G' _V1 (25) and G' _V1 (50) can be obtained by dynamic viscoelasticity measurement, and G' _V1 (50)/G' _V1 (25) can be calculated from the result. The dynamic viscoelasticity measurement can be carried out in a conventional manner using a commercially available dynamic viscoelasticity measurement device, for example, ARES manufactured by TA Instruments or its equivalent, under the following measurement conditions. The test specimen is a sample obtained by adjusting the thickness of the adhesive layer of the evaluation object to about 1.5 mm by lamination as required. [Measurement conditions] Deformation mode: torsion Measurement frequency: 1 Hz Heating rate: 5°C/min Shape: parallel plate 7.9 mm φ

The storage elastic modulus G' _V1 (25), G' _V1 (50) and the storage elastic modulus ratio (G' _V1 (50)/G' _V1 (25)) can be adjusted by selecting the composition of the monomer components of the base polymer constituting the adhesive (for example, selecting the type and content of the monomer (m1) described later), whether or not a crosslinking agent is used, and selecting the type and amount of use, whether or not a refractive index enhancer or plasticizer described later is used, and selecting the type and amount of use, etc. For example, as a monomer (m1), in addition to the first monomer which is the main component of the monomer (m1), a smaller amount of a second monomer having a chemical structure different from that of the first monomer can be used in combination with the above-mentioned first monomer. In this way, in addition to reducing G' _V1 (50) when the first monomer is used alone as the monomer (m1), G' _V1 (50)/G' _V1 (25) can also be reduced.

(Surface Smoothness of the Adhesive Surface) In several aspects, the surface of the high-refractive-index adhesive layer (adhesive surface) preferably has high surface smoothness.

For example, the arithmetic mean roughness Ra of the adhesive surface is preferably limited to below a predetermined value. From the perspective of optical homogeneity, it is desirable to have an adhesive surface designed to have a low arithmetic mean roughness Ra. By limiting the arithmetic mean roughness Ra, for example, in applications where light is captured through the adhesive surface (e.g., in a light-emitting device, the adhesive sheet is positioned closer to the viewpoint than the light-emitting element), it is possible to suppress brightness unevenness caused by the surface condition of the adhesive layer. A low arithmetic mean roughness Ra of the adhesive surface also helps suppress optical strain, and suppressing optical strain also helps improve optical homogeneity. When the laminate sheet (adhesive sheet) disclosed herein is a double-sided adhesive sheet having a first adhesive surface and a second adhesive surface (for example, a laminate sheet composed of a high-refractive index adhesive layer and a low-refractive index adhesive layer), the arithmetic average roughness Ra of at least the first adhesive surface is preferably limited to below a predetermined value, and more preferably, the arithmetic average roughness Ra of both adhesive surfaces is limited to below a predetermined value. By having high surface smoothness on each adhesive surface of the double-sided adhesive sheet, excellent optical homogeneity can be achieved.

In several aspects, the arithmetic average roughness Ra of the adhesive surface is preferably less than or equal to about 70 nm, more preferably less than or equal to about 65 nm, and even more preferably less than or equal to about 55 nm. It may be less than or equal to 50 nm, less than or equal to 45 nm, or even less than or equal to 40 nm. From the perspective of production efficiency, in several aspects, the arithmetic average roughness Ra of the adhesive surface may be, for example, greater than or equal to about 10 nm, greater than or equal to about 20 nm, or greater than or equal to about 30 nm (e.g., greater than or equal to about 40 nm). In aspects in which the laminate sheet has a first adhesive surface and a second adhesive surface, the arithmetic average roughness Ra of the first adhesive surface and the arithmetic average roughness Ra of the second adhesive surface may be equal or different.

Furthermore, for example, the maximum height Rz of the adhesive surface is preferably limited to below a predetermined value. From the perspective of optical homogeneity, it is preferable to have an adhesive surface designed so that the maximum height Rz becomes lower. By limiting the maximum height Rz, for example, in the above-mentioned usage mode of capturing light through the adhesive surface, it is possible to suppress uneven brightness caused by the surface state of the adhesive layer. The fact that the maximum height Rz of the adhesive surface is low is also beneficial for suppressing optical strain. When the laminate disclosed herein is in the form of a double-sided adhesive sheet having a first adhesive surface and a second adhesive surface, it is preferable that at least the maximum height Rz of the first adhesive surface is limited to below a predetermined value, and it is more preferable that the maximum height Rz of both adhesive surfaces is limited to below a predetermined value. The high surface smoothness of each adhesive surface of the double-sided adhesive sheet allows for a bond with excellent optical homogeneity.

In several aspects, the maximum height Rz of the adhesive surface is preferably less than or equal to approximately 600 nm, more preferably less than or equal to approximately 500 nm, more preferably less than or equal to approximately 450 nm, and particularly preferably less than or equal to approximately 400 nm. It may be less than or equal to 350 nm, less than or equal to 300 nm, or even less than or equal to 250 nm. From the perspective of production efficiency, in several aspects, the maximum height Rz of the adhesive surface may be, for example, greater than or equal to approximately 10 nm, greater than or equal to approximately 50 nm, greater than or equal to approximately 100 nm, or greater than or equal to approximately 200 nm. In aspects having a first adhesive surface and a second adhesive surface, the maximum height Rz of the first adhesive surface may be equal to or different from the maximum height Rz of the second adhesive surface.

The arithmetic average roughness Ra and maximum height Rz of the adhesive surface are measured using a non-contact surface roughness measuring device. A non-contact surface roughness measuring device can be an optical interferometer, such as a 3D optical profiler (trade name "NewView7300", manufactured by ZYGO Corporation) or its equivalent. Specifically, the arithmetic average roughness Ra and maximum height Rz can be measured using the following measurement method, or by setting the measurement operation or measurement conditions in a manner that produces results equivalent to or corresponding to those using the measurement method.

Specifically, the surface shape of the test specimens was measured using a 3D optical profiler (trade name "NewView7300," manufactured by ZYGO Corporation) at 23°C and 50% RH under the following conditions. The arithmetic surface roughness Ra was calculated from the measured data in accordance with JIS B 0601-2001. The maximum height Rz was calculated by calculating the sum of the height Rp from the average line of the roughness curve to the highest peak on the upper side and the depth Rv from the average line to the deepest valley on the lower side. Five measurements were performed (i.e., N = 5), and the average value was used. The test specimen can be prepared, for example, by cutting the adhesive layer or adhesive sheet containing the adhesive layer to a length of approximately 150 mm and a width of 50 mm. If the adhesive surface is protected by a release liner, the release liner is removed steadily (e.g., at a tensile speed of 300 mm/min and a peeling angle of 180°) to expose the adhesive surface. It is recommended to allow the exposed adhesive surface to rest for approximately 30 minutes before performing the measurement. [Measurement Conditions] Measurement Area: 5.62mm × 4.22mm (Objective Lens: 2.5x, Internal Lens: 0.5x) Analysis Mode: Remove: Cylinder Data Fill: ON (Max: 25) Remove Spikes: ON (xRMS: 1) Filter: OFF

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

(Water Absorption) In several aspects, the water absorption of the high-refractive-index adhesive layer is preferably limited to below a predetermined value. By limiting the water absorption of the high-refractive-index adhesive layer, the tendency for the high-refractive-index adhesive layer to undergo dimensional changes due to fluctuations in the amount of water in the layer (e.g., absorption and release of moisture from ambient air) is suppressed. This can prevent warping of the high-refractive-index adhesive layer or the optical laminate containing the high-refractive-index adhesive layer caused by dimensional discrepancies between the high-refractive-index adhesive layer and adjacent layers (such as a low-refractive-index layer, a supporting substrate, a release liner, or an adherend). Suppressing fluctuations in the amount of water in the high-refractive-index adhesive layer is also advantageous from the perspective of maintaining the flatness, transparency, and refractive index of the high-refractive-index adhesive layer. Furthermore, a high-refractive-index adhesive layer with a low water absorption rate is less likely to absorb water, making it suitable as a component of components or products that contain elements that dislike water, such as organic EL devices.

In several aspects, the water absorption of the high-refractive index adhesive layer is suitably about 1.0% or less, preferably 0.7% or less, more preferably 0.5% or less (e.g., less than 0.5%), and can be 0.4% or less, 0.3% or less, or even 0.2% or less. The lower limit of the water absorption of the high-refractive index adhesive layer is not particularly limited; from a practical perspective, such as considering adhesive properties, it can be, for example, 0.01% or more, 0.05% or more, 0.1% or more, 0.15% or more, or even 0.25% or more. The water absorption of the low-refractive index layer can be the same as that of the high-refractive index adhesive layer, or it can be different. From the perspective of achieving higher effects, the water absorption of both the high-refractive-index adhesive layer and the low-refractive-index layer should be limited to below a predetermined value.

The water absorption rate (also known as moisture content) of the high-refractive index adhesive layer was measured using the following method. The water absorption rate of the low-refractive index layer was also measured using the same method. [Determination of Moisture Content] The adhesive layer to be evaluated was cut into a 4cm x 5cm ( ^20cm² area) size along with two peel pads placed on one and the other side. After removing the peel pad from one side, the adhesive layer was laminated onto a pre-weighed aluminum foil. The peel pad on the other side of the adhesive layer was then removed and the adhesive layer was placed in a constant temperature and humidity chamber at 60°C and 90% relative humidity. The adhesive layer was removed after 72 hours. After weighing the test piece with the adhesive layer and aluminum foil laminated on it, the moisture content was measured using a moisture meter (Mitsubishi Chemical Analytech CA-200) equipped with a heated vaporization device (Mitsubishi Chemical Analytech VA-200) by Karl Fischer electrometric titration under the following conditions. Anode liquid: AQUAMICRON AKX (Mitsubishi Chemical) Cathode liquid: AQUAMICRON CXU (Mitsubishi Chemical) Vaporization temperature: 150°C

(Gel Fraction) The gel fraction of the high-refractive-index adhesive layer can be appropriately set based on the intended use and usage, and is not limited to a specific range. For example, a gel fraction of approximately 99% or less, with approximately 97% or less being suitable. To achieve a good balance between high refractive index and adhesive properties, in several ideal embodiments, the gel fraction is approximately 95% or less, with approximately 92% or less (e.g., approximately 90% or less) being more preferred. A moderate gel fraction is also preferred because it allows the adhesive to conform to surface irregularities (e.g., irregular structures in light-emitting devices to enhance light extraction efficiency) and thus maintain good adhesion. In several aspects, the gel fraction can be less than about 88%, less than about 75%, or less than about 65%. Furthermore, from the perspective of imparting appropriate cohesiveness to the adhesive to appropriately exhibit adhesive properties, the gel fraction can be, for example, greater than about 10%, with greater than about 20% being appropriate, and can also be greater than about 30%. From the perspective of the adhesive layer's resistance to deformation (preventing overflow due to pressure or bubbles caused by trapped foreign matter), the gel fraction is preferably greater than about 30%, more preferably greater than about 40%, and can be greater than about 45%, greater than about 50%, greater than about 65%, or greater than about 75%. The gel fraction of a laminate comprising a high-refractive index adhesive layer and a low-refractive index layer (typically, a laminate in the form of a double-sided adhesive sheet without a substrate composed of the high-refractive index adhesive layer and the low-refractive index layer) is also preferably within the ranges exemplified above. The gel fraction can be adjusted by adjusting the molecular weight or molecular structure, concentration, and degree of crosslinking of the base polymer. The gel fraction is measured using the following method.

[Gel Fraction Determination] A predetermined amount of adhesive sample (weight Wg ₁ ) is wrapped in a small bag with a porous polytetrafluoroethylene membrane (weight Wg ₂ ) with an average pore size of 0.2 µm. The opening is secured with a kite string (weight Wg ₃ ). The porous polytetrafluoroethylene (PTFE) membrane is Nitoflon (registered trademark) NTF1122 (average pore size 0.2 µm, porosity 75%, thickness 85 µm), available from Nitto Denko Corporation, or an equivalent. The bundle is immersed in a sufficient amount of ethyl acetate and maintained at room temperature (typically 23°C) for 7 days until only the sol in the adhesive has dissolved out of the film. The bundle is then removed and the ethyl acetate attached to the outer surface is wiped off. The bundle is then dried at 130°C for 2 hours and the weight of the bundle is measured (Wg ₄ ). The gel fraction of the adhesive layer can be calculated by substituting the values into the following formula: Gel fraction (%) = [(Wg ₄ -Wg ₂ -Wg ₃ )/Wg ₁ ] × 100

The gel fraction of the high-refractive-index adhesive layer and the low-refractive-index layer can be equal or different. In some embodiments, the gel fraction of the low-refractive-index layer can be lower than that of the high-refractive-index adhesive layer. This configuration facilitates improved overall flexibility due to the contribution of the relatively low-refractive-index layer. This achieves a balance between high refractive index and flexibility.

In several aspects of the disclosed technology, the peak temperature of tan δ of the adhesive forming the high-refractive-index adhesive layer is preferably above approximately -50°C and below approximately 50°C. Here, tan δ (loss tangent) of an adhesive refers to the ratio of the loss modulus G" to the storage modulus G' of the adhesive. In other words, tan δ = G" / G'. The tan δ of an adhesive can be determined by sandwiching a disc-shaped adhesive sample approximately 2 mm thick and 7.9 mm in diameter between parallel plates. A viscoelastic testing apparatus is used to apply shear strain at a frequency of 1 Hz. The adhesive is then subjected to a temperature dispersion test in shear mode over a temperature range of -60°C to 60°C at a heating rate of 5°C/minute. The storage modulus G' (Pa) and loss modulus G" (Pa) at this time are then determined using the following formula: tan δ = G" / G'. The peak temperature of the adhesive's tan δ (hereinafter referred to as Tpeak) can be determined from the variation in tan δ within this temperature range. A viscoelastic testing apparatus such as the TA Instruments ARES or its equivalent can be used.

In several aspects, it is advantageous for the Tpeak of the high refractive index adhesive layer to be below 45°C or below 35°C, and preferably below 30°C (for example, below 25°C), below 20°C, or below 15°C. With an adhesive having a lower Tpeak, it tends to be easier to obtain good initial adhesion or tightness at room temperature. On the other hand, it is ideal for the Tpeak of the adhesive to be not too low from the perspective of imparting appropriate cohesiveness to the adhesive, and it tends to be suitable for taking into account a high refractive index. From this perspective, in several aspects, the Tpeak of the adhesive can be, for example, above -40°C, above -30°C, above -20°C, above -5°C, above 5°C, above 15°C, or even above 25°C. Adhesives with a high Tpeak can be applied to an adherend by heating one or both of the adhesive and the adherend to a temperature slightly above room temperature, as needed. The Tpeak of the adhesive can be adjusted by selecting the adhesive composition (for example, the monomer composition of the base polymer, the presence, type, and amount of refractive index enhancers or plasticizers). The Tpeak of the adhesive described above should preferably apply to at least the high-refractive index adhesive layer, and preferably to both the high-refractive index adhesive layer and the low-refractive index layer. The Tpeak of the high-refractive index adhesive layer and the Tpeak of the low-refractive index layer can be the same or different.

(Base Polymer) In the disclosed technology, the type of adhesive constituting the high-refractive-index adhesive layer is not particularly limited. The adhesive may comprise one or more of various rubbery polymers used in adhesives, such as acrylic polymers, rubber polymers (natural rubber, synthetic rubber, or mixtures thereof), polyester polymers, urethane polymers, polyether polymers, polysilicone polymers, polyamide polymers, and fluoropolymers, as adhesive polymers (the structural polymer that shapes the adhesive, hereinafter referred to as "base polymers"). From the perspectives of adhesive performance and cost, adhesives containing acrylic polymers or rubber polymers as base polymers are preferably used. Among these, adhesives containing acrylic polymers as base polymers (acrylic adhesives) are particularly preferred. The technology disclosed herein is preferably implemented using an acrylic adhesive.

Hereinafter, the high refractive index adhesive layer formed of an acrylic adhesive will be mainly described, but it is not intended to limit the high refractive index adhesive layer in the technology disclosed herein to an acrylic adhesive layer.

In this specification, the term "base polymer" of an adhesive refers to the main component of the rubbery polymer contained in the adhesive, and is not intended to be limiting otherwise. The rubbery polymer is a polymer that exhibits rubbery elasticity in a temperature range near room temperature. Furthermore, in this specification, "main component," unless otherwise specified, means a component comprising greater than 50% by weight. Also, in this specification, an "acrylic polymer" refers to a polymer comprising monomer units derived from a monomer having at least one (meth)acryl group per molecule. Hereinafter, a monomer having at least one (meth)acryl group per molecule is also referred to as an "acrylic monomer." Therefore, an acrylic polymer in this specification is defined as a polymer comprising monomer units derived from an acrylic monomer. Typical examples of acrylic polymers include polymers in which the ratio of acrylic monomers in the total monomers used to synthesize the acrylic polymer is greater than 50% by weight (preferably greater than 70% by weight, for example, greater than 90% by weight). In this specification, "(meth)acryl" refers to both acryl and methacryl groups. Similarly, "(meth)acrylate" refers to both acrylate and methacrylate, and "(meth)acrylic acid" refers to both acrylic acid and methacrylic acid. Therefore, the term "acrylic monomer" as used herein encompasses both monomers having an acryl group (acrylic monomer) and monomers having a methacryl group (methacrylic monomer).

(Acrylic polymer (A)) The high refractive index adhesive sheet disclosed herein is preferably implemented in a state comprising the following acrylic adhesive layer: a refractive index greater than 1.570, a storage elastic modulus G' _V1 at 25°C of 30 kPa to 700 kPa, a total light transmittance of 86% or more, and a haze value of 1.0% or less. The acrylic polymer that is the base polymer of the acrylic adhesive layer is preferably one that contains an aromatic ring-containing monomer (m1) as a monomer component constituting the acrylic polymer. In other words, it is preferably an acrylic polymer that contains an aromatic ring-containing monomer (m1) as a monomer unit. Hereinafter, the acrylic polymer is also referred to as "acrylic polymer (A)". As used herein, the term "monomer component constituting an acrylic polymer" refers to a monomer that constitutes a repeating unit of an acrylic polymer in the adhesive formed from the adhesive composition, regardless of whether the monomer component is included in the adhesive composition in the form of a preformed polymer (which may be an oligomer) or an unpolymerized monomer. In other words, the monomer component constituting the acrylic polymer may be included in the adhesive composition in any of the following forms: polymerized, unpolymerized, or partially polymerized. From the perspective of ease of preparation of the adhesive composition, in some embodiments, it is preferable that substantially all (e.g., 95% by weight or more, preferably 99% by weight or more) of the monomer component be included in the form of a polymer. An adhesive composition containing substantially all monomer components in the form of a polymer is also preferred from the viewpoint of being able to easily form laminates with little strain or warping.

(Monomer (m1)) Monomer (m1) can be a compound containing at least one aromatic ring and at least one ethylenically unsaturated group in one molecule. Monomer (m1) can be used alone or in combination of two or more.

Examples of the ethylenically unsaturated group include (meth)acrylic, vinyl, and (meth)allyl groups. From the perspective of polymerization reactivity, a (meth)acrylic group is preferred, while from the perspective of flexibility and adhesiveness, an acryl group is more preferred. From the perspective of suppressing the decrease in the flexibility of the adhesive, the monomer (m1) preferably contains one ethylenically unsaturated group per molecule (i.e., a monofunctional monomer).

The number of aromatic rings contained in the molecule of the compound 1 that can be used as the monomer (m1) may be 1 or 2 or more. The upper limit of the number of aromatic rings contained in the monomer (m1) is not particularly limited and may be, for example, 16 or less. In several aspects, from the perspective of ease of preparation of the acrylic polymer (A) or transparency of the adhesive, the number of aromatic rings may be, for example, 12 or less, preferably 8 or less, more preferably 6 or less, 5 or less, 4 or less, 3 or less, or 2 or less.

The aromatic ring possessed by the compound that can be used as the monomer (m1) may be, for example, a benzene ring (which may be a benzene ring constituting part of a biphenyl structure or a fluorene structure); a carbon ring such as a condensed ring of a naphthalene ring, an indene ring, an azulene ring, an anthracene ring, or a phenanthrene ring; or a heterocyclic ring such as a pyridine ring, a pyrimidine ring, an oxadiazole ring, a pyrrolidine ring, a pyrazole ring, an imidazole ring, a triazole ring, an oxadiazole ring, an isoxadiazole ring, a thiazole ring, or a thiophene ring. The heteroatoms contained in the heterocyclic ring as ring constituent atoms may be, for example, one or more selected from the group consisting of nitrogen, sulfur, and oxygen. In several embodiments, the heteroatoms constituting the heterocyclic ring may be one or both of nitrogen and sulfur. The monomer (m1) may also have a structure in which one or more carbon rings are condensed with one or more heterocyclic rings, such as a dinaphthothiophene structure.

The aromatic ring (preferably a carbocyclic ring) may have one or two or more substituents on the ring-constituting atoms, or may have no substituents. When having a substituent, the substituent may be exemplified by, but is not limited to, an alkyl group, an alkoxy group, an aryloxy group, a hydroxyl group, a halogen atom (fluorine atom, chlorine atom, bromine atom, etc.), a hydroxyalkyl group, a hydroxyalkoxy group, a glycidoxy group, and the like. In a substituent containing carbon atoms, the number of carbon atoms contained in the substituent is preferably 1 to 4, more preferably 1 to 3, and may be, for example, 1 or 2. In several aspects, the aromatic ring may be an aromatic ring having no substituents on the ring-constituting atoms, or may be an aromatic ring having one or two or more substituents selected from the group consisting of an alkyl group, an alkoxy group, and a halogen atom (e.g., a bromine atom) on the ring-constituting atoms. Furthermore, the fact that the aromatic ring of the monomer (m1) has a substituent on a ring-constituting atom means that the aromatic ring has a substituent other than a substituent having an ethylenically unsaturated group.

The aromatic ring and the ethylenically unsaturated group may be bonded directly or through a linking group. The linking group may be, for example, a group comprising one or more structures selected from alkylene groups, alkylene oxide groups, poly(alkylene oxide) groups, phenyl groups, alkylphenyl groups, alkoxyphenyl groups, groups in which one or more hydrogen atoms in these groups are replaced by hydroxyl groups (e.g., alkylene oxide), oxy groups (-O- groups), thiooxy groups (-S- groups), and the like. In several aspects, aromatic ring-containing monomers having a structure in which the aromatic ring and the ethylenically unsaturated group are bonded directly or through a linking group selected from the group consisting of alkylene groups, alkylene oxide groups, and poly(alkylene oxide) groups may be suitably used. The carbon number of the alkylene group and the oxyalkylene group is preferably 1 to 4, more preferably 1 to 3, for example 1 or 2. The number of repetitions of the oxyalkylene unit in the poly(oxyalkylene) group can be, for example, 2 to 3.

Examples of compounds that can be suitably used as the monomer (m1) include aromatic ring-containing (meth)acrylates and aromatic ring-containing vinyl compounds. Aromatic ring-containing (meth)acrylates and aromatic ring-containing vinyl compounds can be used alone or in combination of two or more. Alternatively, one or more aromatic ring-containing (meth)acrylates and one or more aromatic ring-containing vinyl compounds can be used in combination.

The content of the monomer (m1) in the monomer components constituting the acrylic polymer (A) is not particularly limited and can be set to achieve an adhesive layer that takes into account both the desired refractive index and adhesive properties (e.g., peel strength, flexibility, etc.) and/or optical properties (e.g., total light transmittance, haze value, etc.). In several aspects, the content of the monomer (m1) in the above-mentioned monomer components can be, for example, 30% by weight or more, preferably 50% by weight or more, 60% by weight or more, or 70% by weight or more. From the perspective of easily obtaining a higher refractive index, in several ideal aspects, the content of the above-mentioned monomer (m1) can be, for example, greater than 70% by weight, 75% by weight or more, 80% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. The upper limit of the content of the monomer (m1) in the above-mentioned monomer component is 100 weight %. From the perspective of balancing high refractive index with adhesion properties and/or optical properties, it is advantageous to set the content of the above-mentioned monomer (m1) to less than 100 weight %, for example, it is preferably about 99 weight % or less, more preferably 98 weight % or less, it can be 97 weight % or less, or it can be 96 weight % or less. In several aspects, the content of the above-mentioned monomer (m1) can be 93 weight % or less, 90 weight % or less, 80 weight % or less, or 75 weight % or less. In several aspects in which adhesion properties and/or optical properties are more important, the content of the above-mentioned monomer (m1) in the above-mentioned monomer component can be 70 weight % or less, 60 weight % or less, or 45 weight % or less.

Among several aspects of the technology disclosed herein, monomers having two or more aromatic rings (preferably carbon rings) in one molecule are preferably used as monomer (m1) because they can easily achieve a higher refractive index-enhancing effect. Examples of monomers having two or more aromatic rings in one molecule (hereinafter also referred to as "monomers containing multiple aromatic rings") include monomers having a structure in which two or more non-condensed aromatic rings are bonded via a linking group, monomers having a structure in which two or more non-condensed aromatic rings are directly chemically bonded (i.e., not through other atoms), monomers having a condensed aromatic ring structure, monomers having a fluorene structure, monomers having a dinaphthothiophene structure, and monomers having a dibenzothiophene structure. Monomers containing multiple aromatic rings may be used alone or in combination of two or more.

The linking group may be, for example, an oxy group (-O-), a thiooxy group (-S-), an oxyalkylene group (e.g., an -O-(CH ₂ ) _n - group, where n is 1 to 3 and preferably 1), a thiooxyalkylene group (e.g., an -S-(CH ₂ ) _n - group, where n is 1 to 3 and preferably 1), a linear alkylene group (i.e., a -(CH ₂ ) _n - group, where n is 1 to 6 and preferably 1 to 3), or a partially or completely halogenated alkylene group in the above-mentioned oxyalkylene groups, thiooxyalkylene groups, and linear alkylene groups. From the perspective of the adhesive's flexibility, suitable examples of the linking group include an oxy group, a thiooxy group, an oxyalkylene group, and a linear alkylene group. Specific examples of monomers having a structure in which two or more non-condensed aromatic rings are bonded via a linking group include phenoxybenzyl (meth)acrylate (e.g., m-phenoxybenzyl (meth)acrylate), thiophenoxybenzyl (meth)acrylate, and benzylbenzyl (meth)acrylate.

Examples of monomers having a structure in which two or more non-condensed aromatic rings are directly chemically bonded include biphenyl-containing (meth)acrylates, triphenyl-containing (meth)acrylates, and vinyl-containing biphenyls. Specific examples include o-phenylphenol (meth)acrylate and biphenylmethyl (meth)acrylate.

Examples of the monomer having a condensed aromatic ring structure include naphthalene ring-containing (meth)acrylates, anthracene ring-containing (meth)acrylates, vinyl naphthalene-containing, and vinyl anthracene-containing. Specific examples include 1-naphthylmethyl (meth)acrylate (also known as 1-naphthylmethyl (meth)acrylate), hydroxyethylated β-naphthol acrylate, 2-naphthylethyl (meth)acrylate, 2-naphthyloxyethyl acrylate, and 2-(4-methoxy-1-naphthyloxy)ethyl (meth)acrylate.

Specific examples of the monomers having a fluorene structure include 9,9-bis(4-hydroxyphenyl)fluorene(meth)acrylate and 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene(meth)acrylate. Furthermore, since monomers having a fluorene structure include a structure in which two benzene rings are directly chemically bonded, they are included in the concept of monomers having a structure in which two or more non-condensed aromatic rings are directly chemically bonded.

Examples of the monomer having a dinaphthothiophene structure include dinaphthothiophene containing a (meth)acryloyl group, dinaphthothiophene containing a vinyl group, and dinaphthothiophene containing a (meth)allyl group. Specific examples include (meth)acryloyloxymethyl dinaphthothiophene (for example, a compound having a structure of CH2CH( ^R1 )C(O) _OCH2- bonded to the 5-position or 6-position of the dinaphthothiophene ring; here, ^R1 is a hydrogen atom or a methyl group), (meth)acryloyloxyethyl dinaphthothiophene (for example, a compound having a structure of _CH2CH ( _R1 )C(O)OCH(CH3)- or CH2CH( ^R1 )C(O) _OCH2CH2- bonded to the _{5 -} position or 6-position of the _{dinaphthothiophene} ring; here, ^R1 is a hydrogen atom or ^a methyl group), vinyl dinaphthothiophene (for example, a compound having a structure of a vinyl group bonded to the 5-position or 6-position of the naphthothiophene ring), and (meth)allyloxy dinaphthothiophene. Furthermore, a monomer having a dinaphthothiophene structure is also included in the concept of a monomer having a condensed aromatic ring structure because it includes a naphthalene structure and a structure in which a thiophene ring and two naphthalene structures are condensed.

Examples of monomers having a dibenzothiophene structure include dibenzothiophene containing a (meth)acryloyl group and dibenzothiophene containing a vinyl group. Furthermore, since monomers having a dibenzothiophene structure have a thiophene ring condensed with two benzene rings, they are included in the concept of monomers having a condensed aromatic ring structure. Additionally, neither the dinaphthothiophene structure nor the dibenzothiophene structure are structures in which two or more non-condensed aromatic rings are directly chemically bonded.

The monomer (m1) used in the disclosed technology may also contain one aromatic ring (preferably a carbon ring) per molecule. Monomers containing one aromatic ring per molecule can, for example, help improve the flexibility of the adhesive, adjust adhesive properties, and enhance transparency. In some aspects, from the perspective of increasing the refractive index of the adhesive, monomers containing one aromatic ring per molecule are preferably used in combination with monomers containing multiple aromatic rings.

Examples of monomers having one aromatic ring in one molecule include: carbon-containing aromatic ring (meth)acrylates such as benzyl (meth)acrylate, methoxybenzyl (meth)acrylate, phenyl (meth)acrylate, ethoxylated phenol (meth)acrylate, phenoxypropyl (meth)acrylate, phenoxybutyl (meth)acrylate, toluene (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, and chlorobenzyl (meth)acrylate; 2-(4,6-dibromo-2-dibutylphenoxy)ethyl (meth)acrylate, 2-(4,6-dibromo-2-isopropylphenoxy)ethyl (meth)acrylate, 6-( Bromine-substituted aromatic ring (meth)acrylates such as 4,6-dibromo-2-dibutylphenoxy)hexyl (meth)acrylate, 6-(4,6-dibromo-2-isopropylphenoxy)hexyl (meth)acrylate, 2,6-dibromo-4-nonylphenyl acrylate, and 2,6-dibromo-4-dodecylphenyl acrylate; vinyl compounds containing carbon aromatic rings such as styrene, α-methylstyrene, vinyltoluene, and tertiary butylstyrene; and compounds having vinyl substituents on heteroaromatic rings such as N-vinylpyridine, N-vinylpyrimidine, N-vinylpyrrol, N-vinylimidazole, and N-vinyloxazole.

Monomers (m1) may also be monomers having a structure in which an oxyethylene chain is sandwiched between the ethylenically unsaturated group and the aromatic ring, as in the various aromatic ring-containing monomers described above. Monomers having an oxyethylene chain sandwiched between the ethylenically unsaturated group and the aromatic ring as described above can be considered ethoxylated versions of the original monomers. The number of repetitions of the oxyethylene unit (—CH ₂ CH ₂ O—) in the oxyethylene chain is typically 1 to 4, preferably 1 to 3, more preferably 1 to 2, and for example, 1. Specific examples of ethoxylated aromatic ring-containing monomers include ethoxylated o-phenylphenol (meth)acrylate, ethoxylated nonoxyphenol (meth)acrylate, ethoxylated cresol (meth)acrylate, phenoxyethyl (meth)acrylate, and phenoxydiethylene glycol di(meth)acrylate.

The content of the monomer containing multiple aromatic rings in the monomer (m1) is not particularly limited, and may be, for example, 5% by weight or more, 25% by weight or more, or 40% by weight or more. In several aspects, from the perspective of easily achieving an adhesive having a higher refractive index, the content of the monomer containing multiple aromatic rings in the monomer (m1) may be, for example, 50% by weight or more, preferably 70% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. The monomer (m1) may also be substantially 100% by weight of a monomer containing multiple aromatic rings. That is, the monomer (m1) may also use only one or two or more monomers containing multiple aromatic rings. Furthermore, in several aspects, for example, considering the balance between a high refractive index and adhesion and/or optical properties, the content of the monomer containing multiple aromatic rings in the monomer (m1) may be less than 100% by weight, may be less than 98% by weight, may be less than 90% by weight, may be less than 80% by weight, or may be less than 65% by weight. In several aspects, considering adhesion and/or optical properties, the content of the monomer containing multiple aromatic rings in the monomer (m1) may be less than 70% by weight, may be less than 50% by weight, may be less than 25% by weight, or may be less than 10% by weight. The technology disclosed herein can be implemented even when the content of the monomer containing multiple aromatic rings in the monomer (m1) is less than 5% by weight. Monomers containing multiple aromatic rings may also not be used.

The content of the monomer containing multiple aromatic rings in the monomer components constituting the acrylic polymer is not particularly limited and can be set to achieve an adhesive layer that takes into account both the desired refractive index and adhesive properties (e.g., peel strength, flexibility, etc.) and/or optical properties (e.g., total light transmittance, haze value, etc.). The content of the monomer containing multiple aromatic rings in the above-mentioned monomer components can be, for example, 3% by weight or more, 10% by weight or more, or 25% by weight or more. In several aspects, from the perspective of easily achieving an adhesive with a higher refractive index, the content of the monomer containing multiple aromatic rings in the above-mentioned monomer components can be, for example, greater than 35% by weight, preferably greater than 50% by weight, greater than 70% by weight, greater than 75% by weight, greater than 85% by weight, greater than 90% by weight, or greater than 95% by weight. The content of the monomer containing multiple aromatic rings in the monomer component can be 100% by weight. However, from the perspective of balancing high refractive index with adhesive and/or optical properties, it is advantageous to set it to less than 100% by weight. It is preferably about 99% by weight or less, more preferably 98% by weight or less, and can be 96% by weight or less, 93% by weight or less, 90% by weight or less, 85% by weight or less, 80% by weight or less, or even 75% by weight or less. In several aspects, considering adhesive and/or optical properties, the content of the monomer containing multiple aromatic rings in the monomer component can be 70% by weight or less, 50% by weight or less, 25% by weight or less, 15% by weight or less, or even 5% by weight or less. The technology disclosed herein can be implemented even when the content of the monomer containing multiple aromatic rings in the above-mentioned monomer components is less than 3% by weight.

In several aspects of the disclosed technology, a high-refractive-index monomer is preferably used as at least a portion of the monomer (m1). Here, "high-refractive-index monomer" refers to a monomer having a refractive index of, for example, approximately 1.510 or greater, preferably approximately 1.530 or greater, and more preferably approximately 1.550 or greater. The upper limit of the refractive index of the high-refractive-index monomer is not particularly limited, but from the perspective of balancing ease of preparation of the adhesive composition with flexibility as an adhesive, the upper limit may be, for example, 3.000 or less, 2.500 or less, 2.000 or less, 1.900 or less, 1.800 or less, or even 1.700 or less. High-refractive-index monomers may be used alone or in combination of two or more. The refractive index of the monomer is measured using an Abbe refractometer at a wavelength of 589 nm and a temperature of 25°C. An Abbe refractometer such as the "DR-M4" manufactured by ATAGO or its equivalent can be used. If a nominal value of the refractive index at 25°C is provided by the manufacturer, that nominal value may be used.

The high refractive index monomer can be appropriately selected from the compounds included in the concept of the aromatic ring-containing monomer (m1) disclosed herein (e.g., the compounds and compound groups exemplified above) having the refractive index. Specific examples include: m-phenoxybenzyl acrylate (refractive index: 1.566, homopolymer Tg: -35°C), 1-naphthylmethyl acrylate (refractive index: 1.595, homopolymer Tg: 31°C), ethoxylated o-phenylphenol acrylate (number of repetitions of oxyethylene units: 1, refractive index: 1.578), benzyl acrylate (refractive index (nD20): 1.519, homopolymer Tg: 6°C), phenoxyethyl acrylate (refractive index (nD20): 1.517, homopolymer Tg: 2°C), phenoxydiethylene glycol acrylate (refractive index: 1.510, homopolymer Tg: -35°C), 6-acryloyloxymethyl dinaphthothiophene (6MDNTA, refractive index: 1.75), 6-methacryloyloxymethyl dinaphthothiophene (6MDNTMA, refractive index: 1.726), 5-acryloyloxyethyl dinaphthothiophene (5EDNTA, refractive index: 1.786), 6-acryloyloxyethyl 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 are not limited thereto.

The content of the high-refractive-index monomer (i.e., an aromatic-ring-containing monomer having a refractive index of approximately 1.510 or greater, preferably approximately 1.530 or greater, and more preferably approximately 1.550 or greater) in the monomer (m1) is not particularly limited, and may be, for example, 5% by weight or greater, 25% by weight or greater, 35% by weight or greater, or 40% by weight or greater. In several aspects, from the perspective of easily obtaining a higher refractive index, the content of the high-refractive-index monomer in the monomer (m1) may be, for example, 50% by weight or greater, preferably 70% by weight or greater, 85% by weight or greater, 90% by weight or greater, or 95% by weight or greater. Substantially 100% by weight of the monomer (m1) may be the high-refractive-index monomer. Furthermore, in several aspects, for example, from the perspective of balancing high refractive index with adhesion and/or optical properties, the content of the high refractive index monomer in monomer (m1) may be less than 100 wt%, may be less than 98 wt%, may be less than 90 wt%, may be less than 80 wt%, or may be less than 65 wt%. In several aspects, considering adhesion and/or optical properties, the content of the high refractive index monomer in monomer (m1) may be less than 70 wt%, may be less than 50 wt%, may be less than 25 wt%, may be less than 15 wt%, or may be less than 10 wt%. The technology disclosed herein can be implemented even when the content of the high refractive index monomer in monomer component (m1) is less than 5 wt%. It is also possible not to use a high refractive index monomer.

The content of the high refractive index monomer in the monomer components constituting the acrylic polymer is not particularly limited and can be set to achieve an adhesive layer that takes into account both the desired refractive index and adhesive properties (such as peel strength, flexibility, etc.) and/or optical properties (such as total light transmittance, haze value, etc.). The content of the high refractive index monomer in the above-mentioned monomer components can be, for example, 3% by weight or more, 10% by weight or more, or 25% by weight or more. In several aspects, from the perspective of easily achieving an adhesive with a higher refractive index, the content of the high refractive index monomer in the above-mentioned monomer components can be, for example, greater than 35% by weight, preferably greater than 50% by weight, greater than 70% by weight, greater than 75% by weight, greater than 85% by weight, greater than 90% by weight, or greater than 95% by weight. The content of the high-refractive index monomer in the monomer component can be 100% by weight, but from the perspective of balancing the high refractive index with adhesion and/or optical properties, it is advantageous to set it to less than 100% by weight. It is preferably 99% by weight or less, more preferably 98% by weight or less, and can be 96% by weight or less, 93% by weight or less, 90% by weight or less, 85% by weight or less, 80% by weight or less, or even 75% by weight or less. In several aspects, considering adhesion and/or optical properties, the content of the high-refractive index monomer in the monomer component can be 70% by weight or less, 50% by weight or less, 25% by weight or less, 15% by weight or less, or even 5% by weight or less. The technology disclosed herein can be implemented even when the content of the high-refractive index monomer in the monomer component is less than 3% by weight.

In several preferred aspects of the technology disclosed herein, at least a portion of the monomer (m1) is an aromatic ring-containing monomer (hereinafter referred to as "monomer L") having a homopolymer Tg of 10°C or less (preferably 5°C or less or 0°C or less, more preferably -10°C or less, and even more preferably -20°C or less, for example -25°C or less). Increasing the content of the aromatic ring-containing monomer (m1) in the monomer composition (particularly an aromatic ring-containing monomer (m1) corresponding to one or both of the aforementioned monomers containing multiple aromatic rings and high refractive index monomers) tends to increase the storage modulus G' of the adhesive. However, by using monomer L as part or all of the monomer (m1), this increase in the storage modulus G' can be suppressed. This allows for better maintenance of the softness suitable for use as an adhesive and increases the refractive index. There is no particular lower limit for the Tg of monomer L. Considering the balance with the refractive index-enhancing effect, in several aspects, the Tg of monomer L can be, for example, above -70°C, above -55°C, or above -45°C. Monomer L can be used singly or in combination of two or more.

Monomer L can be appropriately selected from compounds encompassed by the concept of the aromatic ring-containing monomer (m1) disclosed herein (e.g., the compounds and compound groups exemplified above) that have the desired Tg. A suitable example of an aromatic ring-containing monomer that can be used as monomer L is m-phenoxybenzyl acrylate (homopolymer Tg: -35°C). Another suitable example is phenoxydiethylene glycol acrylate (homopolymer Tg: -35°C).

The content of monomer L in monomer (m1) is not particularly limited and may be, for example, 5% by weight or greater, 25% by weight or greater, or 40% by weight or greater. In several aspects, from the perspective of easily obtaining an adhesive having a higher level of both high refractive index and softness, the content of monomer L in monomer (m1) may be, for example, 50% by weight or greater, 60% by weight or greater, 70% by weight or greater, 75% by weight or greater, 85% by weight or greater, 90% by weight or greater, or 95% by weight or greater. Monomer (A1) may also comprise substantially 100% by weight of monomer L. Furthermore, in several aspects, for example, from the perspective of balancing flexibility and high refractive index for suitability as an adhesive, the content of monomer L in monomer (m1) may be less than 100 wt %, 98 wt % or less, 90 wt % or less, 80 wt % or less, 70 wt % or less, 50 wt % or less, 25 wt % or less, or even 10 wt % or less. The technology disclosed herein can be implemented even when the content of monomer L in monomer (m1) is less than 5 wt %. Monomer L may also be omitted.

The content of monomer L in the monomer component constituting the acrylic polymer can be, for example, 3% by weight or greater, 10% by weight or greater, or 25% by weight or greater. In several aspects, from the perspective of easily obtaining an adhesive having a higher level of both high refractive index and softness, the content of monomer L in the monomer component can be, for example, greater than 35% by weight, greater than 50% by weight, greater than 70% by weight, greater than 75% by weight, greater than 85% by weight, greater than 90% by weight, or greater than 95% by weight. The content of monomer L in the above-mentioned monomer component can be 100 weight %, but considering the balance between high refractive index and adhesion and/or optical properties, it is advantageous to set it to less than 100 weight %, and is preferably about 99 weight % or less, more preferably 98 weight % or less, can be 96 weight % or less, can be 95 weight % or less, can be 93 weight % or less, can be 90 weight % or less, can be 85 weight % or less, can be 80 weight % or less, and can also be 75 weight % or less. In several aspects, the content of monomer L in the above-mentioned monomer component can be 70 weight % or less, can be 50 weight % or less, can be 25 weight % or less, can be 15 weight % or less, and can also be 5 weight % or less. The technology disclosed herein can be implemented even when the content of monomer L in the above-mentioned monomer component is less than 3 weight %.

In several aspects, from the perspective of the softness of the adhesive, it is advantageous for the glass transition temperature Tg _m1 of the composition of the monomer (m1) to be approximately 20°C or less, preferably 10°C or less (e.g., 5°C or less), more preferably 0°C or less, more preferably -10°C or less, and can be -20°C or less, or -25°C or less. There is no particular restriction on the lower limit of the glass transition temperature Tg _m1 . Considering the balance with the refractive index enhancement effect, in several aspects, the glass transition temperature Tg _m1 can be, for example, -70°C or more, -55°C or more, or -45°C or more. The technology disclosed herein can still be suitably implemented even in aspects where the glass transition temperature Tg _m1 is, for example, -40°C or more, -35°C or more, -33°C or more, -30°C or more, or -25°C or more.

Here, the glass transition temperature Tg _m1 based on the composition of monomer (m1) refers to the Tg obtained by the Fox equation described below based on the composition of only monomer (m1) among the monomer components constituting the acrylic polymer. The glass transition temperature Tg _m1 can be calculated from the glass transition temperature of the homopolymer of each aromatic ring-containing monomer used as monomer (m1) and the weight fraction of each aromatic ring-containing monomer in the total weight of monomer (m1) by applying the Fox equation described below to only monomer (m1) among the monomer components constituting the acrylic polymer. When only one monomer is used as monomer (m1), the Tg of the homopolymer of that monomer coincides with the glass transition temperature Tg _m1 .

In several aspects, the aromatic ring-containing monomer (m1) can be used in combination with a monomer L (i.e., an aromatic ring-containing monomer having a homopolymer Tg of 10°C or less, preferably 5°C or less or 0°C or less, more preferably -10°C or less, more preferably -20°C or less, for example -25°C or less) and a monomer H having a Tg greater than 10°C. The Tg of monomer H can be, for example, greater than 10°C, greater than 15°C, or greater than 20°C. By combining monomer L and monomer H, for example, in a configuration in which the content of the aromatic ring-containing monomer (m1) is relatively high in the monomer component, a higher level of both a high refractive index and softness of the adhesive can be achieved. 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, the usage ratio of monomer L to monomer H is preferably set to satisfy any of the above-mentioned glass transition temperatures Tg _m1 .

In several aspects, the aromatic ring-containing monomer (m1) can be suitably selected from compounds that do not contain a structure in which two or more non-condensed aromatic rings are directly chemically bonded (e.g., a biphenyl structure). For example, an acrylic polymer composed of monomer components is preferably selected such that the content of compounds containing a structure in which two or more non-condensed aromatic rings are directly chemically bonded is less than 5% by weight (preferably less than 3% by weight, and may be 0% by weight). Limiting the amount of compounds containing a structure in which two or more non-condensed aromatic rings are directly chemically bonded, as described above, is advantageous from the perspective of achieving an adhesive that balances flexibility or adhesion with a high refractive index.

(Monomer (m2)) In several aspects of the disclosed technology, the monomer components constituting the acrylic polymer may include, in addition to the monomer (m1), a monomer (m2). The monomer (m2) corresponds to at least one of a monomer containing a hydroxyl group (hydroxyl-containing monomer) and a monomer containing a carboxyl group (carboxyl-containing monomer). The hydroxyl-containing monomer is a compound containing at least one hydroxyl group and at least one ethylenically unsaturated group per molecule. The carboxyl-containing monomer is a compound containing at least one carboxyl group and at least one ethylenically unsaturated group per molecule. Monomer (m2) helps introduce crosslinking points into the acrylic polymer or imparts appropriate cohesiveness to the adhesive. Monomer (m2) may be used alone or in combination of two or more. The monomer (m2) is typically a monomer that does not contain an aromatic ring.

Examples of ethylenically unsaturated groups in monomer (m2) include (meth)acrylic groups, vinyl groups, and (meth)allyl groups. From the perspective of polymerization reactivity, (meth)acrylic groups are preferred, while from the perspective of flexibility and adhesiveness, acrylic groups are more preferred. From the perspective of suppressing the decrease in the flexibility of the adhesive, monomer (m2) containing one ethylenically unsaturated group per molecule (i.e., a monofunctional monomer) is preferably used.

Examples of hydroxyl-containing monomers include, but are not limited to, hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, and (4-hydroxymethylcyclohexyl)methyl (meth)acrylate. Examples of suitable hydroxyl-containing monomers include 4-hydroxybutyl acrylate (Tg: -40°C) and 2-hydroxyethyl acrylate (Tg: -15°C). From the perspective of improving flexibility at room temperature, 4-hydroxybutyl acrylate, due to its lower Tg, is preferred. In an ideal embodiment, 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 (m2) can be 4-hydroxybutyl acrylate. The hydroxyl group-containing monomer can be used alone or in combination of two or more.

In several aspects of using a hydroxyl-containing monomer as the monomer (m2), the hydroxyl-containing monomer may be one or more selected from compounds without a methacrylic group. Suitable examples of hydroxyl-containing monomers without a methacrylic group include the various hydroxyalkyl acrylates described above. For example, it is preferred that greater than 50% by weight, greater than 70% by weight, or greater than 85% by weight of the hydroxyl-containing monomer used as the monomer (m2) be a hydroxyalkyl acrylate. By using a hydroxyalkyl acrylate, hydroxyl groups that help provide crosslinking points or impart moderate cohesiveness can be introduced into the acrylic polymer, and an adhesive having good flexibility or adhesion at room temperature can be more easily obtained than when only the corresponding methacrylic acid hydroxyalkyl group is used.

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

The content of the monomer (m2) in the monomer component constituting the acrylic polymer is not particularly limited and can be set according to the purpose. In several aspects, the content of the above-mentioned monomer (m2) can be, for example, 0.01 weight % or more, 0.1 weight % or more, or 0.5 weight % or more. From the perspective of obtaining a higher use effect, in several aspects, the content of the above-mentioned monomer (A2) is preferably set to 1 weight % or more, can be set to 2 weight % or more, and can also be set to 4 weight % or more. The upper limit of the content of the monomer (m2) in the monomer component is set so that the total content of the monomer (m2) and the content of other monomers does not exceed 100 weight %. In several aspects, it is appropriate to set the content of the above-mentioned monomer (m2) to, for example, less than 30 weight % or less than 25 weight %. From the perspective of making the content of monomer (m1) relatively large and facilitating high refractive index, it is preferably set to less than 20 weight %, more preferably less than 15 weight %, and can be less than 12 weight %, less than 10 weight %, or less than 7 weight %.

In the embodiment using a hydroxyl-containing monomer as the monomer (m2), the content of the hydroxyl-containing monomer in the monomer component is not particularly limited, and can be, for example, 0.01% by weight or greater (preferably 0.1% by weight or greater, more preferably 0.5% by weight or greater). In several embodiments, the content of the hydroxyl-containing monomer is preferably 1% by weight or greater, 2% by weight or greater, or even 4% by weight or greater of the monomer component. The upper limit of the content of the hydroxyl-containing monomer in the monomer component is set so that the total content of the hydroxyl-containing monomer and other monomers does not exceed 100 weight%. For example, it is appropriate to set it to 30 weight% or less or 25 weight% or less. From the perspective of making the content of monomer (m1) relatively large and facilitating a high refractive index, it is preferably set to 20 weight% or less, more preferably to 15 weight% or less, and can be less than 12 weight%, less than 10 weight%, or less than 7 weight%.

In the embodiment of using a carboxyl group-containing monomer as the monomer (m2), the content of the carboxyl group-containing monomer in the monomer component is not particularly limited, for example, it can be 0.01% by weight or more (preferably 0.1% by weight or more, more preferably 0.3% by weight or more). In several embodiments, the content of the above-mentioned carboxyl group-containing monomer can be set to 1% by weight or more, can be set to 2% by weight or more, and can also be set to 4% by weight or more. The upper limit of the content of the carboxyl group-containing monomer in the monomer component is set to a total of no more than 100% by weight together with the amount of other monomers used, for example, it is appropriate to set it to 30% by weight or less or 25% by weight or less, and from the viewpoint of making the content of the monomer (m1) relatively large and easy to high refractive index, it is preferably set to 20% by weight or less, preferably 15% by weight or less, can be less than 12% by weight, and can also be less than 10% by weight. In several aspects, from the perspective of improving the softness of the adhesive, it is advantageous to set the content of the carboxyl group-containing monomer to less than 7% by weight, preferably less than 5% by weight, less than 3% by weight, less than 1% by weight, or less than 0.5% by weight. For example, the technology disclosed herein can be suitably implemented using only hydroxyl group-containing monomers as monomers (m2), that is, without using carboxyl group-containing monomers.

The total content of the monomers (m1) and (m2) in the monomer components constituting the acrylic polymer can be, for example, 31% by weight or more, preferably 51% by weight or more, 61% by weight or more, or 71% by weight or more. In several aspects, the total content of the monomers (m1) and (m2) in the monomer components constituting the acrylic polymer can be, for example, 76% by weight or more, preferably 81% by weight or more, 86% by weight or more, 91% by weight or more, 96% by weight or more, 99% by weight or more, or substantially 100% by weight, from the perspective of facilitating and appropriately exerting the effects of these monomers.

(Monomer m3) The monomer components constituting the acrylic polymer may, if desired, include monomers other than the aforementioned monomers (m1) and (m2). An example of such an optional component is an alkyl (meth)acrylate (hereinafter also referred to as "monomer (m3)"). Monomer (m3) helps adjust the softness of the adhesive or improve its compatibility within the adhesive.

The monomer (m3) can be suitably used in an alkyl (meth)acrylate having a linear or branched alkyl group with 1 to 20 carbon atoms (i.e., _C1-20 ) at the end of the ester. Specific examples of the _C1-20 alkyl (meth)acrylate include: methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, dibutyl (meth)acrylate, tertiary butyl (meth)acrylate, amyl (meth)acrylate, isoamyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecanyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, nonadecanyl (meth)acrylate, eicosyl (meth)acrylate, etc., but are not limited thereto.

In several aspects, at least a portion of the monomer (m3) can be suitably an alkyl (meth)acrylate having a homopolymer Tg of -20°C or less (preferably -40°C or less, for example, -50°C or less). Such a low-Tg alkyl (meth)acrylate helps improve the softness of the adhesive. The lower limit of the Tg of the aforementioned alkyl (meth)acrylate is not particularly limited, and for example, it can be above -85°C, above -75°C, above -65°C, or above -60°C. Specific examples of such low-Tg alkyl (meth)acrylates include n-butyl acrylate (BA), 2-ethylhexyl acrylate (2EHA), and isononyl acrylate (iNA).

In several aspects using monomer (m3), from the perspectives of flexibility and adhesiveness, at least a portion of the monomer (m3) is preferably an alkyl acrylate. For example, it is preferred that alkyl acrylates account for at least 50% by weight (preferably at least 75% by weight, and more preferably at least 90% by weight) of the monomer (m3). Alternatively, only one or two or more alkyl acrylates may be used as monomer (m3), without using an alkyl methacrylate.

In the embodiment where the monomer component includes an alkyl (meth)acrylate, the content of the alkyl (meth)acrylate in the monomer component can be set so as to appropriately exert its use effect. In several embodiments, the content of the above-mentioned alkyl (meth)acrylate 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 embodiments, the content of the above-mentioned alkyl (meth)acrylate can be 15% by weight or more, 30% by weight or more, or 45% by weight or more. The upper limit of the content of the monomer (m3) in the monomer component is set so that the total content of the monomer (m3) and the content of other monomers does not exceed 100% by weight, for example, it can be less than 50% by weight. In several embodiments, the content of the above-mentioned monomer (m3) can be, for example, less than 35% by weight. Generally speaking, alkyl (meth)acrylates have a low refractive index. Therefore, to achieve a high refractive index, it is advantageous to limit the content of monomer (m3) in the monomer component and increase the content of monomer (m1). From this perspective, the content of monomer (m3) is advantageously 24% by weight or less of the monomer component, preferably less than 23% by weight, more preferably less than 20% by weight, less than 17% by weight, less than 12% by weight, less than 7% by weight, less than 3% by weight, and even less than 1% by weight. Monomer (m3) may also be substantially omitted.

(Other Monomers) The monomer components constituting the acrylic polymer may, if desired, include monomers other than the aforementioned monomers (m1), (m2), and (m3) (hereinafter referred to as "other monomers"). These other monomers may be used, for example, to adjust the Tg of the acrylic polymer, adjust adhesive properties, improve compatibility within the adhesive layer, and so on. These other monomers may be used singly or in combination of two or more.

Examples of the aforementioned other monomers include monomers having functional groups other than hydroxyl and carboxyl groups (functional group-containing monomers). For example, other monomers that can improve the cohesiveness or heat resistance of the adhesive include sulfonic acid group-containing monomers, phosphoric acid group-containing monomers, and cyano group-containing monomers. Examples of monomers that can be used to introduce functional groups into acrylic polymers that can serve as crosslinking points or that can help enhance peel strength or improve compatibility within the adhesive layer include amide-containing monomers (e.g., (meth)acrylamide, N-hydroxymethyl (meth)acrylamide, etc.), amino-containing monomers (e.g., aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, etc.), monomers having a nitrogen-containing ring (e.g., N-vinyl-2-pyrrolidone, N-(meth)acryloylisothiazolinone, etc.), imide-containing monomers, epoxy-containing monomers, ketone-containing monomers, isocyanate-containing monomers, and alkoxysilyl-containing monomers. In addition, monomers having a nitrogen-containing ring, such as N-vinyl-2-pyrrolidone, are also equivalent to amide-containing monomers. The relationship between the above-mentioned monomers having a nitrogen-containing ring and amine-containing monomers is the same.

Other monomers that can be used in addition to the functional group-containing monomers mentioned above include: vinyl ester monomers such as vinyl acetate; non-aromatic ring-containing (meth)acrylates such as cyclohexyl (meth)acrylate and isoborneol (meth)acrylate; olefin monomers such as ethylene, butadiene, and isobutylene; chlorine-containing 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 an additional monomer used to enhance the softness of an adhesive is ethoxyethoxyethyl acrylate (also known as ethyl carbitol acrylate, homopolymer Tg: -67°C).

When the aforementioned other monomers are used, their usage amount is not particularly limited and can be appropriately set within a range where the total amount of the monomer components does not exceed 100 wt%. In several aspects, from the perspective of easily exerting the refractive index enhancement effect obtained by using the monomer (m1), the content of the aforementioned other monomers in the monomer components can be set to, for example, about 35 wt% or less, about 25 wt% or less (e.g., 0-25 wt%) is appropriate, about 20 wt% or less (e.g., 0-20 wt%), about 10 wt% or less, about 5 wt% or less, or about 1 wt% or less. The technology disclosed herein can be appropriately implemented in an aspect where the monomer components do not substantially contain the aforementioned other monomers.

In several aspects, the monomer components comprising the acrylic polymer can be compositions in which the amount of methacryloyl-containing monomers is suppressed to below a predetermined level. For example, the amount of methacryloyl-containing monomers in the monomer components can 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 methacryloyl-containing monomers as described above is advantageous from the perspective of achieving an adhesive that balances flexibility or adhesion with a high refractive index. The monomer components comprising the acrylic polymer can also be compositions that do not contain methacryloyl-containing monomers (e.g., compositions consisting solely of acryl-containing monomers).

In several aspects, the amount of carboxyl-containing monomers used in the monomer component of the base polymer (e.g., acrylic polymer) that constitutes the high-refractive index adhesive layer is preferably limited to prevent discoloration or discoloration (e.g., yellowing) of the adhesive layer. The amount of carboxyl-containing monomers used in the monomer component can be, for example, less than 1% by weight, preferably less than 0.5% by weight, more preferably less than 0.3% by weight, less than 0.1% by weight, or even less than 0.05% by weight. Limiting the amount of carboxyl-containing monomers is also beneficial from the perspective of preventing corrosion of metal materials that may come into contact with or be located adjacent to the high-refractive index adhesive layer (e.g., metal wiring or metal films that may be present on the adherend). The technology disclosed herein can be suitably implemented in embodiments where the monomer component does not contain carboxyl-containing monomers. For similar reasons, in several aspects, the amount of monomers containing acidic functional groups (including, in addition to carboxyl groups, sulfonic acid groups, phosphoric acid groups, etc.) in the monomer components of the base polymer constituting the high-refractive index adhesive layer is preferably limited. The amount of acidic functional monomers in the monomer components of these aspects can be applied to the ideal amount of carboxyl-containing monomers described above. The disclosed technology can also be suitably implemented in embodiments in which the monomer components do not contain acidic functional monomers (i.e., in embodiments in which the base polymer of the high-refractive index adhesive layer is acid-free).

(Glass Transition Temperature ( _Tg ) of Base Polymer) In several aspects, the base polymer (e.g., acrylic polymer) of the adhesive layer preferably has a glass transition temperature (Tg ₎ of approximately 20°C or less, preferably approximately 10°C or less, more preferably 0°C or less, and can be -10°C or less, -20°C or less, -25°C or less, -28°C or less, or even -30°C or less. A lower glass transition temperature (Tg ₎ is advantageous from the perspective of enhancing the flexibility of the adhesive. The glass transition temperature _TgT may be, for example, -60°C or higher. However, from the perspective of facilitating the increase in the refractive index of the adhesive, it is preferably -50°C or higher, more preferably higher than -45°C, and may be higher than -40°C, higher than -35°C, higher than -25°C, higher than -15°C, or even higher than -5°C.

Here, the glass transition temperature (Tg ₎ of a polymer, unless otherwise specified, means the glass transition temperature obtained using the Fox equation based on the composition of the monomer components constituting the polymer. The Fox equation, shown below, relates the Tg of a copolymer to the glass transition temperature (Tgi) of the homopolymer obtained by homopolymerizing the monomers constituting the copolymer. 1/Tg = Σ(Wi/Tgi) In the above Fox equation, Tg represents the glass transition temperature of the copolymer (unit: K), Wi represents the weight fraction (weight-based copolymerization ratio) of monomer i in the copolymer, and Tgi represents the glass transition temperature (unit: K) of the homopolymer of monomer i. The glass transition temperature of the homopolymer used in the calculation of Tg is the value described in publicly known documents such as "Polymer Handbook" (3rd edition, John Wiley & Sons, Inc., 1989). For monomers with multiple values listed in the Polymer Handbook, the highest value was used. When the Tg of a homopolymer is not listed in publicly available data, the value obtained by the measurement method described in Japanese Patent Application Publication No. 2007-51271 was used.

(Base Polymer Preparation Method) In the disclosed technology, the method for obtaining the base polymer (e.g., the acrylic polymer (A) composed of the aforementioned monomer components) for the adhesive layer is not particularly limited. Known polymerization methods such as solution polymerization, emulsion polymerization, bulk polymerization, suspension polymerization, and photopolymerization can be appropriately employed. In several embodiments, solution polymerization is preferably employed. The polymerization temperature during solution polymerization can be appropriately selected based on the types of monomers and solvents used, the type of polymerization initiator, and can be, for example, approximately 20°C to 170°C (typically approximately 40°C to 140°C).

The solvent used for solution polymerization (polymerization solvent) can be appropriately selected from conventionally known organic solvents. For example, any one solvent or a mixture of two or more solvents 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; halides such as 1,2-dichloroethane; lower alcohols such as isopropyl alcohol (e.g., monohydric alcohols with 1 to 4 carbon atoms); ethers such as tertiary butyl methyl ether; ketones such as methyl ethyl ketone, etc.

The initiator used for polymerization can be appropriately selected from among 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 suitably used. Other examples of polymerization initiators include persulfates such as potassium persulfate; peroxide-based initiators such as benzoyl peroxide and hydrogen peroxide; substituted ethane-based initiators such as phenyl-substituted ethane; and aromatic carbonyl compounds. Still other examples of polymerization initiators include redox-based initiators, which are combinations of peroxides and reducing agents. Polymerization initiators can be used alone or in combination of two or more. The amount of the polymerization initiator used may be a conventional amount, for example, it can be selected from the range of about 0.005 to 1 part by weight (typically about 0.01 to 1 part by weight) per 100 parts by weight of the monomer component.

In the above-mentioned polymerization, various conventionally known chain transfer agents may be used as needed. For example, thiols such as n-dodecylmercaptan, tert-dodecylmercaptan, thioglycolic acid, and α-thioglycerol may be used. Alternatively, chain transfer agents that do not contain sulfur atoms (non-sulfur chain transfer agents) may be used. Examples of non-sulfur chain transfer agents include anilines such as N,N-dimethylaniline and N,N-diethylaniline; terpenes such as α-pinene and terpinolene; and styrenes such as α-methylstyrene and α-methylstyrene dimer. Chain transfer agents may be used alone or in combination of two or more. When a chain transfer agent is used, the amount used may be, for example, approximately 0.01 to 1 part by weight relative to 100 parts by weight of the monomer raw material.

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

Here, the polymer Mw can be determined by gel permeation chromatography (GPC) in terms of polystyrene. Specifically, it can be determined using the HLC-8220GPC (manufactured by Tosoh Corporation) GPC analyzer under the following conditions. [GPC Measurement Conditions] Sample Concentration: 0.2 wt% (tetrahydrofuran solution) Sample Injection Volume: 10 µL Eluent: Tetrahydrofuran (THF) Flow Rate: 0.6 mL/min Column Temperature (Measurement Temperature): 40°C Columns: Sample Column: 1 "TSKguardcolumn SuperHZ-H" + 2 "TSKgel SuperHZM-H" (Tosoh Corporation) Reference Column: 1 "TSKgel SuperH-RC" (Tosoh Corporation) Detector: Differential Refractometer (RI) Standard Sample: Polystyrene

(Refractive Index Raising Agent) In several aspects of the disclosed technology, a high-refractive index adhesive layer (e.g., an acrylic adhesive layer) may optionally contain a refractive index raising agent in addition to the base polymer. As used herein, a refractive index raising agent refers to a material whose use increases the refractive index of the adhesive layer. The refractive index raising agent may preferably be a material having a higher refractive index than the refractive index of the adhesive layer containing the refractive index raising agent. Furthermore, the refractive index raising agent may preferably be a material having a higher refractive index than the base polymer (e.g., acrylic polymer (A)) of the adhesive layer containing the refractive index raising agent. By appropriately using a refractive index raising agent, a higher refractive index and practical adhesive performance can be achieved. In several aspects, the refractive index raising agent is preferably an organic material. Organic materials that can be used as refractive index enhancers can be polymers or non-polymers. Furthermore, they can have or not have polymerizable functional groups. Refractive index enhancers can be used singly or in combination of two or more.

The refractive index of the refractive index enhancer (e.g., the additive ( _HRO ) described below) can be set within an appropriate range relative to the refractive index of the base polymer and is not limited to a specific range. The refractive index of the refractive index enhancer is, 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 perspective of increasing the refractive index of the adhesive, in several aspects, the refractive index of the refractive index enhancer is advantageously 1.58 or greater, preferably 1.60 or greater, more preferably 1.63 or greater, 1.65 or greater, 1.70 or greater, and 1.75 or greater. By using a refractive index-raising agent with a higher refractive index, the target refractive index can be achieved even with a smaller amount of agent. This is ideal from the perspective of suppressing a decrease in adhesive or optical properties. There is no particular upper limit on the refractive index of the refractive index-raising agent; however, considering compatibility with the adhesive, ease of achieving a high refractive index, and adaptability as an adhesive, the upper limit can be, for example, 3.000 or less, 2.500 or less, 2.000 or less, 1.950 or less, 1.900 or less, or even 1.850 or less.

In some aspects, the difference between the refractive index _nb of the refractive index-enhancing agent (e.g., the additive ( _HRO ) described below) and the refractive index _na of the base polymer, i.e., _nb - _na (hereinafter also referred to as "ΔnA _" ), is set to be greater than 0. In some aspects, _ΔnA is, for example, greater than 0.02, greater than 0.05, greater than 0.07, greater than 0.10, greater than 0.15, greater than 0.20, or greater than 0.25. By selecting the base polymer and refractive index-enhancing agent to increase _ΔnA , the effect of increasing the refractive index using the refractive index-enhancing agent tends to be enhanced. Furthermore, from the perspective of compatibility within the adhesive layer or transparency of the adhesive layer, in several aspects, Δn _A may be, for example, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.35 or less.

In some aspects, the difference between the refractive index _nb of the refractive index-raising agent (e.g., the additive ( _HRO ) described below) and the refractive index _nT of the adhesive layer containing the refractive index-raising agent, namely _nb - _nT (hereinafter also referred to as "ΔnB _" ), is set to be greater than 0. In some aspects, _ΔnB is, for example, greater than 0.02, greater than 0.05, greater than 0.07, greater than 0.10, greater than 0.15, greater than 0.20, or greater than 0.25. By selecting the adhesive layer composition and the refractive index-raising agent to increase _ΔnB , the effect of increasing the refractive index using the refractive index-raising agent tends to be enhanced. Furthermore, from the perspective of compatibility within the adhesive layer or transparency of the adhesive layer, in several aspects, Δn _B may be, for example, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.35 or less.

The amount of the refractive index enhancer used relative to 100 parts by weight of the base polymer (or the total amount of such enhancers when multiple types are used) is not particularly limited and can be set according to the intended purpose. From the perspective of increasing the refractive index of the adhesive, the amount of the refractive index enhancer used relative to 100 parts by weight of the base polymer can be set to, for example, 1 part by weight or more, advantageously 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 even 20 parts by weight or more. Furthermore, in several aspects, the amount of the refractive index enhancer used relative to 100 parts by weight of the base polymer can be set to, for example, 80 parts by weight or less. From the perspective of balancing increasing the refractive index of the adhesive with suppressing a decrease in adhesion or optical properties, it is advantageous to set it to 60 parts by weight or less, and preferably 45 parts by weight or less. In several aspects where adhesion or optical properties are more important, the amount of the refractive index enhancer used per 100 parts by weight of 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, 10 parts by weight or less, 5 parts by weight or less, or even 3 parts by weight or less. The technology disclosed herein can still be suitably implemented even when the amount of the refractive index enhancer used per 100 parts by weight of the base polymer in the adhesive layer is less than 1 part by weight, or when substantially no refractive index enhancer is used. Here, "substantially no" means at least not intentionally used.

(Additive (H _RO )) In several aspects, the refractive index enhancer may suitably be an organic material having a higher refractive index than the base polymer. Hereinafter, the organic material may be referred to as an "additive (H _RO )." Here, "H _RO " refers to an organic material having a high refractive index. By combining a base polymer (e.g., an acrylic polymer, preferably an acrylic polymer (A)) with an additive (H _RO ), an adhesive can be achieved that more appropriately balances refractive index with adhesive properties (peel strength, flexibility, etc.) and/or optical properties (total light transmittance, haze value, etc.). The organic material that can be used as the additive (H _RO ) may be a polymer or a non-polymer. Furthermore, it may or may not have a polymerizable functional group. The additive (H _RO ) may be used alone or in combination of two or more.

The refractive index of the additive (H _RO ) is measured similarly to the refractive index of the monomer using an Abbe refractometer at a wavelength of 589 nm and a temperature of 25°C. If a nominal refractive index value at 25°C is provided by the manufacturer, it may be used.

The molecular weight of the organic material used as the additive (H _RO ) is not particularly limited and can be selected according to the purpose. The molecular weight of the additive (H _RO ) can be selected, for example, from a range of 30,000 or less. Furthermore, the additive (H _RO ) is preferably a polymer or non-polymer having a molecular weight lower than that of the base polymer. From the perspective of balancing the effect of increasing the refractive index with other properties (such as optical properties such as flexibility and haze suitable for adhesives), in several aspects, the molecular weight of the additive (H _RO ) is preferably less than about 10,000, preferably less than 5,000, more preferably less than 3,000 (for example, less than 1,000), less than 800, less than 600, less than 500, or less than 400. A moderate molecular weight of the additive (H _RO ) is advantageous from the perspective of improving compatibility within the adhesive layer. Furthermore, the molecular weight of the additive (H _RO ) can be, for example, 130 or greater, or 150 or greater. In some aspects, from the perspective of increasing the refractive index of the additive (H _RO ), the molecular weight of the additive (H _RO ) is preferably 170 or greater, more preferably 200 or greater, and can be 230 or greater, 250 or greater, 270 or greater, 500 or greater, 1000 or greater, or even 2000 or greater. In some aspects, a polymer with a molecular weight of approximately 1000 to 10000 (e.g., 1000 or greater and less than 5000) can be used as the additive (H _RO ). The molecular weight of the additive (H _RO ) can be calculated based on the chemical structure of a non-polymer or a polymer with a low degree of polymerization (e.g., dimers to _pentamers ). For polymers with a higher degree of polymerization, the weight-average molecular weight (Mw) determined by GPC under appropriate conditions can be used. If a nominal molecular weight value is provided by the manufacturer, it can be used.

Examples of organic materials that can be selected as the additive (H _RO ) include, but are not limited to, organic compounds having an aromatic ring and organic compounds having a heterocyclic ring (which may be an aromatic ring or a non-aromatic heterocyclic ring).

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

The aromatic ring may have one or two or more substituents on the ring-constituting atoms, or may have no substituents. When having substituents, the substituents may be exemplified by, but are not limited to, alkyl groups, alkoxy groups, aryloxy groups, hydroxyl groups, halogen atoms (fluorine atoms, chlorine atoms, bromine atoms, etc.), hydroxyalkyl groups, hydroxyalkoxy groups, glycidoxy groups, and the like. In substituents containing carbon atoms, the number of carbon atoms contained in the substituent is, for example, 1 to 10, preferably 1 to 6, preferably 1 to 4, more preferably 1 to 3, and may be, for example, 1 or 2. In several aspects, the aromatic ring may be an aromatic ring having no substituents on the ring-constituting atoms, or an aromatic ring having one or two or more substituents selected from the group consisting of alkyl groups, alkoxy groups, and halogen atoms (e.g., bromine atoms) on the ring-constituting atoms.

Examples of aromatic ring-containing compounds that can be used as additives (H _RO ) include, but are not limited to, compounds that can be used as monomers (m1); oligomers containing compounds that can be used as monomers (m1) as monomer units; and compounds having a structure in which a group having an ethylenically unsaturated group (which may be a substituent bonded to a ring-constituting atom) or a portion of the group that constitutes an ethylenically unsaturated group is removed from the compound that can be used as monomer (m1) and replaced with a hydrogen atom or a group that does not have an ethylenically unsaturated group (e.g., a hydroxyl group, an amino group, a halogen atom, an alkyl group, an alkoxy group, a hydroxyalkyl group, a hydroxyalkoxy group, a glyoxypropoxy group, etc.). Non-limiting examples of aromatic ring-containing compounds that can be used as additives (H _RO ) include: benzyl acrylate, m-phenoxybenzyl acrylate, 2-(o-phenylphenoxy)ethyl acrylate, phenoxyethyl acrylate, phenoxydiethylene glycol acrylate, phenoxypolyethylene glycol acrylate, 2-hydroxy-3-phenoxypropyl acrylate, the aforementioned monomers having a fluorene structure, monomers having a dinaphthothiophene structure, monomers having a dibenzothiophene structure, and other aromatic ring-containing monomers; and aromatic ring-containing compounds without ethylenically unsaturated groups, such as 3-phenoxybenzyl alcohol, dinaphthothiophene and its derivatives (e.g., compounds having one or more substituents selected from hydroxyl, carbinol, diethanol, and glyoxypropyl groups bonded to the dinaphthothiophene ring). Furthermore, the aromatic ring-containing compound may be an oligomer (preferably having a molecular weight of approximately 5000 or less, more preferably approximately 1000 or less; for example, a dimer to pentamer) containing the aromatic ring-containing monomer as a monomer unit. Examples of such oligomers include homopolymers of the aromatic ring-containing monomer, copolymers of one or more aromatic ring-containing monomers, and copolymers of one or more aromatic ring-containing monomers with other monomers. The other monomers may be one or more monomers that do not contain an aromatic ring.

In several aspects, organic compounds containing two or more aromatic rings per molecule (hereinafter referred to as "compounds containing multiple aromatic rings") are preferably used as additives ( _HRO ) because they can easily achieve a higher refractive index-enhancing effect. Compounds containing multiple aromatic rings may or may not have polymerizable functional groups such as ethylenically unsaturated groups. Furthermore, compounds containing multiple aromatic rings may be polymers or non-polymers. Furthermore, the polymers may be oligomers containing monomers containing multiple aromatic rings as monomer units (preferably oligomers having a molecular weight of approximately 5000 or less, more preferably approximately 1000 or less; for example, oligomers of approximately dimers to pentamers). The oligomers described above may be, for example, homopolymers of monomers containing multiple aromatic rings; copolymers of one or more monomers containing multiple aromatic rings; copolymers of one or more monomers containing multiple aromatic rings and other monomers. The other monomers described above may be aromatic ring-containing monomers other than the monomers containing multiple aromatic rings, monomers without aromatic rings, or combinations thereof.

Non-limiting examples of compounds containing multiple aromatic rings include compounds having a structure in which two or more non-condensed aromatic rings are bonded via a linking group, compounds having a structure in which two or more non-condensed aromatic rings are chemically bonded directly (i.e., not through other atoms), compounds having a condensed aromatic ring structure, compounds having a fluorene structure, compounds having a dinaphthothiophene structure, and compounds having a dibenzothiophene structure. Compounds containing multiple aromatic rings may be used alone or in combination of two or more.

Specific examples of the above-mentioned compound having a fluorene structure include, in addition to the above-mentioned monomers having a fluorene structure, or oligomers of homopolymers or copolymers of the above-mentioned monomers, 9,9-bis(4-hydroxyphenyl)fluorene (refractive index: 1.68), 9,9-bis(4-aminophenyl)fluorene (refractive index: 1.73), 9,9-bis(4-hydroxy-3-methylphenyl)fluorene (refractive index: 1.68), 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene (refractive index: 1.65), and other 9,9-bisphenylfluorene and its derivatives.

Specific examples of the compound having a dinaphthothiophene structure include, in addition to the monomer having a dinaphthothiophene structure, or an oligomer of a homopolymer or copolymer of the monomer, dinaphthothiophene (refractive index: 1.808); hydroxyalkyl dinaphthothiophene such as 6-hydroxymethyl dinaphthothiophene (refractive index: 1.766); dihydroxy dinaphthothiophene such as 2,12-dihydroxy dinaphthothiophene (refractive index: 1.750); 2,12-dihydroxy dinaphthothiophene (refractive index: 1.766); Dihydroxyalkyloxy dinaphthothiophene, such as hydroxyethyloxy dinaphthothiophene (refractive index: 1.677); diglycidyloxy dinaphthothiophene, such as 2,12-diglycidyloxy dinaphthothiophene (refractive index 1.723); dinaphthothiophene with two or more ethylenically unsaturated groups, such as 2,12-diallyloxy dinaphthothiophene (abbreviation: 2,12-DAODNT, refractive index 1.729), and their derivatives.

Specific examples of the compound having a dibenzothiophene structure include, in addition to the monomer having a dibenzothiophene structure, or an oligomer of a homopolymer or copolymer of the monomer, dibenzothiophene (refractive index: 1.607), 4-dimethyldibenzothiophene (refractive index: 1.617), and 4,6-dimethyldibenzothiophene (refractive index: 1.617).

Examples of heterocyclic organic compounds (hereinafter referred to as heterocyclic organic compounds) that can be used as additives ( _HRO ) include thioepoxy compounds and compounds containing tris(indole) rings. Examples of thioepoxy compounds include bis(2,3-cyclothiopropyl) disulfide and its polymer (refractive index 1.74) described in Japanese Patent No. 3712653. Examples of tris(indole) ring compounds include compounds containing at least one (e.g., 3 to 40, preferably 5 to 20) tris(indole) ring per molecule. Furthermore, since a tricyclic ring has aromatic properties, compounds having a tricyclic ring are also included in the concept of the aforementioned aromatic ring-containing compound, and compounds having multiple tricyclic rings are also included in the concept of the aforementioned multiple aromatic ring-containing compound.

In several aspects, a compound without ethylenically unsaturated groups can be suitably used as the additive (H _RO ). This can inhibit thermal or light-induced degradation of the adhesive composition (progression of gelation or increase in viscosity leading to reduced leveling properties), thereby improving storage stability. The use of an additive without ethylenically unsaturated groups (H _RO ) is also advantageous from the perspective of suppressing dimensional changes or deformations (warping, undulation, etc.), as well as the generation of optical strain, caused by reactions with ethylenically unsaturated groups in adhesive layers containing the additive (H _RO ) or laminates (e.g., laminated sheets) containing the adhesive layer.

In the case of using an oligomer as an additive (H _RO ), the oligomer can be obtained by polymerizing the corresponding monomer components using known methods. When producing the oligomer using free radical polymerization, a polymerization initiator, chain transfer agent, emulsifier, etc. suitable for free radical polymerization can be appropriately added to the monomer components to carry out polymerization. The polymerization initiator, chain transfer agent, emulsifier, etc. suitable for free radical polymerization are not particularly limited and 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 and the reaction conditions, and their amount can be appropriately adjusted according to their type. Examples of the chain transfer agent include lauryl mercaptan, glycidyl mercaptan, hydroxyacetic acid, 2-hydroxyethanol, α-thioglycerol, thioglycolic acid, 2-ethylhexyl thioglycolate, and 2,3-dihydroxy-1-propanol. A single chain transfer agent may be used alone, or two or more may be mixed. The amount of the chain transfer agent used can be adjusted to obtain an oligomer having a desired weight-average molecular weight, depending on the composition of the monomer components used in the synthesis of the oligomer or the type of chain transfer agent. In some embodiments, the amount of the chain transfer agent used is suitably about 15 parts by weight or less, 10 parts by weight or less, or about 5 parts by weight or less, based on 100 parts by weight of the total amount of monomers used in the synthesis of the oligomer. The lower limit of the amount of the chain transfer agent used relative to 100 parts by weight of the total amount of monomers that can be used to synthesize the oligomer is not particularly limited, and can be, for example, 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.

In embodiments where an additive (H _RO ) is used as a refractive index enhancer, the amount of the additive (H _RO ) used per 100 parts by weight of the base polymer (or the total amount of such compounds when multiple compounds are used) is not particularly limited and can be set according to the intended purpose. From the perspective of increasing the refractive index of the adhesive, the amount of the additive (H _RO ) used per 100 parts by weight of the base polymer can be, for example, 1 part by weight or more, 3 parts by weight or more is advantageous, 5 parts by weight or more is preferred, 7 parts by weight or more, 10 parts by weight or more, 15 parts by weight or more, and even 20 parts by weight or more. In several aspects, the amount of the additive (H _RO ) used relative to 100 parts by weight of the base polymer can be set to, for example, 80 parts by weight or less. From the perspective of balancing the high refractive index of the adhesive with suppressing a decrease in adhesion or optical properties, it is advantageous to set the amount to 60 parts by weight or less, and preferably 45 parts by weight or less. In several aspects where adhesion or optical properties are more important, the amount of the additive (H _RO ) used relative to 100 parts by weight of 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 even 10 parts by weight or less.

(Plasticizer) In some aspects, the high-refractive-index adhesive layer may include a plasticizer with a lower molecular weight than the base polymer of the adhesive layer. The use of a plasticizer can enhance the flexibility of the high-refractive-index adhesive layer, thereby improving adhesion to the adherend, as well as the overall flexibility and adaptability to deformation. Based on compatibility and transparency within the adhesive layer, organic materials are suitable plasticizers. The plasticizer can also be a material that can be used as the aforementioned refractive index enhancer (e.g., the additive ( _HRO )).

The molecular weight of the plasticizing material is not particularly limited as long as it is lower than that of the base polymer. In several aspects, from the perspective of easily exhibiting the plasticizing effect, the molecular weight of the plasticizing material may be 30,000 or less, 25,000 or less, less than 10,000, preferably less than 5,000, more preferably less than 3,000 (e.g., less than 1,000), less than 800, less than 600, less than 500, or less than 400. A moderately high molecular weight of the plasticizing material is advantageous from the perspective of improving compatibility within the adhesive layer. Furthermore, in several aspects, from the perspective of easily exerting a sufficient plasticizing effect, a molecular weight of the plasticizing material of 130 or more is appropriate, and is preferably 150 or more, more preferably 170 or more, and may be 200 or more, 250 or more, or even 300 or more. In several aspects, the molecular weight of the plasticizing material may be greater than 500, greater than 1000, or greater than 2000. A molecular weight of the plasticizing material that is not too low is also preferred from the perspective of improving the heat resistance of the adhesive layer or preventing contamination by the adherend.

Non-limiting examples of compounds that can be used as plasticizers include: compounds that can be used as monomers (m1) (e.g., aromatic ring (meth)acrylates having benzyl, phenoxy, or naphthyl groups, monomers having a fluorene structure, monomers having a dinaphthothiophene structure, monomers having a dibenzothiophene structure, etc.); oligomers containing compounds that can be used as monomers (m1) as monomer units; compounds having a structure in which a portion having an ethylenically unsaturated group is removed from a compound that can be used as monomer (m1) and replaced with a hydrogen atom or a group without an ethylenically unsaturated group (e.g., 3-phenoxybenzyl alcohol), etc. From the perspective of improving softness, low-Tg monomers such as n-butyl acrylate or 2-ethylhexyl acrylate can also be copolymerized in oligomers containing compounds that can be used as monomers (m1) as monomer units. As the plasticizing material, one or more known plasticizers (such as phthalate-based plasticizers, terephthalate-based plasticizers, adipate-based plasticizers, adipic acid-based polyesters, and ethylene glycol benzoate) may be used.

In several aspects, the plasticizing material may suitably use an organic material having a refractive index of about 1.50 or more (preferably 1.53 or more). Specific examples of compounds that can be selected as plasticizing materials include: diethylene glycol dibenzoate (refractive index 1.55), dipropylene glycol dibenzoate (refractive index 1.54), 3-phenoxytoluene (refractive index 1.57), 3-ethylbiphenyl (refractive index 1.59), 3-methoxybiphenyl (refractive index 1.61), 4-methoxybiphenyl (refractive index 1.57), polyethylene glycol dibenzoate, 3-phenoxybenzyl alcohol (refractive index 1.59), triphenyl phosphate (refractive index 1.56), benzyl benzoate (refractive index 1.57), 4-(tertiary butyl) Examples of the plasticizer include, but are not limited to, phenyldiphenyl phosphate (refractive index 1.56), trimethylphenyl phosphate (refractive index 1.55), butylbenzyl phthalate (refractive index 1.54), methyl rosin (refractive index 1.53), alkylbenzyl phthalate (refractive index 1.53), butyl(phenylsulfonyl)amine (refractive index 1.53), trimethyl trimellitate (refractive index 1.52), benzyl phthalate (refractive index 1.52), 2-ethylhexyldiphenyl phosphate (refractive index 1.51), and tris(2,4-di-tert-butylphenyl) phosphite. From the perspective of refractive index and compatibility, diethylene glycol dibenzoate is preferably used. The upper limit of the refractive index of the plasticizer is not particularly limited, but may be, for example, 3.00 or less. In several aspects, from the perspectives of ease of preparation of the adhesive composition and compatibility within the adhesive, a refractive index of the plasticizer of 2.50 or less is suitable, 2.00 or less is advantageous, and 1.90 or less, 1.80 or less, or even 1.70 or less is possible. The refractive index of the plasticizer is measured, similar to the refractive index of the monomer, using an Abbe refractometer at a wavelength of 589 nm and a temperature of 25°C. If a nominal value of the refractive index at 25°C is provided by the manufacturer, that nominal value may be used.

In the embodiment where a plasticizing material is used, the amount of the plasticizing material used relative to 100 parts by weight of the base polymer is not particularly limited and can be set according to the intended purpose. From the perspective of enhancing the plasticizing effect, the amount of the plasticizing material used relative to 100 parts by weight of the base polymer can be, for example, 0.1 parts by weight or more, or 0.5 parts by weight or more. From the perspective of achieving a higher plasticizing effect, the amount is preferably 1 part by weight or more, preferably 3 parts by weight or more, and can be 5 parts by weight or more, 7 parts by weight or more, 10 parts by weight or more, 15 parts by weight or more, or even 20 parts by weight or more. Furthermore, from the perspective of balancing the adhesive's high refractive index with its transparency and plasticizing effect, the amount of plasticizer used relative to 100 parts by weight of the base polymer is appropriately set at approximately 100 parts by weight or less, preferably 80 parts by weight or less, more preferably 60 parts by weight or less, and can be 45 parts by weight or less, 35 parts by weight or less, or even 25 parts by weight or less. In certain aspects where adhesive or optical properties are more important, the amount of plasticizer used relative to 100 parts by weight of the base polymer can be, for example, 15 parts by weight or less, 10 parts by weight or less, or even 5 parts by weight or less.

(Leveling Agent) In some embodiments, the adhesive composition used to form the adhesive layer may contain a leveling agent, if desired, to improve the appearance of the resulting adhesive layer (e.g., to improve thickness uniformity) or to enhance the applicability of the adhesive composition. Non-limiting examples of leveling agents include acrylic leveling agents, fluorine leveling agents, and silicone leveling agents. The leveling agent can be selected from commercially available leveling agents and applied using conventional methods.

In several aspects, the leveling agent may preferably include a polymer (hereinafter referred to as "polymer (B)") comprising a monomer having a polyorganosiloxane skeleton (hereinafter referred to as "monomer S1") and a monomer raw material of an acrylic monomer (hereinafter referred to as "monomer raw material B"). Polymer (B) may be a copolymer of monomer S1 and an acrylic monomer. Polymer (B) may be used alone or in combination of two or more.

Monomer S1 is not particularly limited, and any monomer containing a polyorganosiloxane skeleton can be used. Monomer S1 can be suitably used in a structure having a polymerizable reactive group at one end. Among them, monomer S1 having a structure having a polymerizable reactive group at one end and not having a functional group at the other end that produces a cross-linking reaction with the base polymer (e.g., an acrylic polymer) of the adhesive composition mixed with the leveling agent can be suitably used. Commercially available products include, for example, one-terminal reactive polysiloxane oils manufactured by Shin-Etsu Chemical Co., Ltd. (e.g., models such as X-22-174ASX, X-22-2426, X-22-2475, and KF-2012). Monomer S1 can be used alone or in combination of two or more.

The functional group equivalent weight of monomer S1 can be, for example, approximately 100 g/mol to 30,000 g/mol. In several ideal embodiments, the functional group equivalent weight is, for example, 500 g/mol or greater, 800 g/mol or greater, 1,500 g/mol or greater, or 2,000 g/mol or greater. Alternatively, the functional group equivalent weight can be, for example, 20,000 g/mol or less, less than 10,000 g/mol, 7,000 g/mol or less, or 5,500 g/mol or less. A functional group equivalent weight of monomer S1 within this range is likely to achieve a good leveling effect. When two or more monomers with different functional group equivalent weights are used as monomer S1, the functional group equivalent weight of monomer S1 can be calculated as the sum of the products of the functional group equivalent weights of each monomer and the weight fraction of each monomer.

Here, "functional group equivalent" means the weight of the main skeleton (e.g., polydimethylsiloxane) bonded to each functional group. The relevant notation unit g/mol is converted to 1 mol of functional groups. The functional group equivalent of monomer S1 can be calculated, for example, from the spectral intensity of ^1H -NMR (proton NMR) based on 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 carried out according to the general structural analysis method of ^1H -NMR spectrum analysis, and if necessary, refer to the description of Japanese Patent No. 5951153. In the functional group equivalent of monomer S1, the above-mentioned functional group means a polymerizable functional group (e.g., ethylenically unsaturated groups such as (meth)acryloyl, vinyl, and allyl).

The content of monomer S1 in monomer raw material B can be any appropriate value within the range that allows the desired effect of monomer S1, and is not limited to a specific range. In several aspects, 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.

In addition to monomer S1, monomer raw material B also includes an acrylic monomer that can be copolymerized with monomer S1. This can improve the compatibility of polymer (B) in the adhesive layer. Examples of acrylic monomers that can be used in monomer raw material B include alkyl acrylates. The "alkyl" mentioned here refers to a chain (including straight chain and branched chain) alkyl group, and does not include _alicyclic hydrocarbon groups described below. In several aspects, monomer raw material B can contain at least one _C4-12 alkyl (meth)acrylate (preferably _C4-10 alkyl (meth)acrylate, such as _C6-10 alkyl (meth)acrylate). In several other aspects, monomer raw material B can contain at least one C1-18 alkyl methacrylate (preferably _C1-14 alkyl methacrylate, such as _C1-10 alkyl (meth)acrylate). The monomer raw material B may contain, for example, one or more selected from methyl methacrylate (MMA), n-butyl methacrylate (BMA), and 2-ethylhexyl methacrylate (2EHMA) as acrylic monomers.

Other examples of the acrylic monomer include (meth)acrylates having alicyclic alkyl groups. Examples include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, dicyclopentyl (meth)acrylate, and 1-adamantyl (meth)acrylate. (Meth)acrylates having no alicyclic alkyl groups may also be used.

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

Other examples of monomers that can be included in the monomer raw material B together with monomer S1 include: carboxyl group-containing monomers, acid anhydride group-containing monomers, hydroxyl group-containing monomers, epoxy group-containing monomers, cyano group-containing monomers, isocyanate group-containing monomers, amide group-containing monomers, monomers having a nitrogen atom-containing ring, (meth)acrylic acid aminoalkyl esters, vinyl esters, vinyl ethers, alkenes, (meth)acrylates having aromatic hydrocarbon groups, and (meth)acrylates containing halogen atoms, etc., which can be used in acrylic polymers.

The Mw of polymer (B) can be, for example, 5,000 or greater, preferably 10,000 or greater, and can also be 15,000 or greater. Alternatively, the Mw of polymer (B) can be, for example, 200,000 or less, preferably 100,000 or less, 50,000 or less, and can also be 30,000 or less. By setting the Mw of polymer (B) within an appropriate range, good compatibility and leveling properties can be achieved.

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), a chain transfer agent may be used as needed. Examples of such chain transfer agents include hydroxyl-containing compounds such as tertiary dodecylmercaptan, hydroxyethanol, and α-thioglycerol; thioglycolates such as thioglycolic acid and methyl thioglycolate; and α-methylstyrene dimer. The amount of chain transfer agent used is not particularly limited and can be appropriately adjusted to yield a polymer (B) having the desired molecular weight. In various embodiments, the amount of chain transfer agent used per 100 parts by weight of the monomers may be, for example, 0.1 to 5 parts by weight, 0.2 to 3 parts by weight, or 0.5 to 2 parts by weight.

The amount of polymer (B) used per 100 parts by weight of the base polymer (e.g., acrylic polymer) can be, for example, 0.001 parts by weight or more. To achieve higher efficacy, it can be 0.01 parts by weight or more, or even 0.03 parts by weight or more. Furthermore, the amount of polymer (B) used can be, for example, 3 parts by weight or less. To minimize the effect on the refractive index, 1 part by weight or less is appropriate. It can also be 0.5 parts by weight or less, or even 0.1 parts by weight or less.

(Inorganic Particles) The techniques disclosed herein can also be suitably implemented without substantially using inorganic particles as refractive index enhancers. In particular, in several embodiments, the use of inorganic particles as refractive index enhancers is permissible as long as the desired optical properties (total light transmittance, haze value) are met without significantly compromising the adhesive's properties. Examples of inorganic particles that can be used as refractive index enhancers include those composed of inorganic oxides (specifically, metal oxides) such as titanium oxide (titania, _TiO2 ), zirconium oxide (zirconia, _ZrO2 ), aluminum oxide, zinc oxide, tin oxide, copper oxide, barium _titanate , and niobium oxide ( _Nb2O5 ). The average particle size of the above-mentioned inorganic particles (referring to the 50% volume average particle size obtained by the laser scattering diffraction method) can be selected from a range of about 10nm to 100nm. In addition, the refractive index of the inorganic particles is measured for a single layer film of the material constituting the inorganic particles (using a film thickness that can be used for refractive index measurement) using a commercially available spectroscopic elliptical polarizer under the conditions of a measurement wavelength of 589nm and a measurement temperature of 23°C. The spectroscopic elliptical polarizer can be, for example, the product name "EC-400" (manufactured by JA.Woolam Co., Ltd.) or its equivalent. When using inorganic particles as a refractive index enhancer, the amount used should be less than 5 parts by weight, and more preferably less than 1 part by weight, relative to 100 parts by weight of the base polymer. In the embodiment where the additive (H _RO ) is used, the amount of the inorganic particles used is preferably set to be no more than 2 times the amount of the additive (H _RO ) used, and more preferably no more than 1 time or no more than 0.5 times the amount of the additive (H RO ) used.

(Crosslinking Agent) In the technology disclosed herein, the adhesive composition used to form the adhesive layer may contain a crosslinking agent, if necessary, to adjust the adhesive's cohesive strength. Examples of crosslinking agents well known in the adhesive field include isocyanate crosslinkers, epoxy crosslinkers, ethyleneimine crosslinkers, oxazoline crosslinkers, melamine resins, and metal chelate crosslinkers. Among these, isocyanate crosslinkers are particularly suitable. Other examples of crosslinking agents include monomers having two or more ethylenically unsaturated groups per molecule, i.e., polyfunctional monomers. Crosslinking agents may be used singly or in combination.

Isocyanate crosslinking agents can use isocyanate compounds with two or more functional groups, such as trimethylene diisocyanate, butyl diisocyanate, hexamethylene diisocyanate (HDI), diisocyanate and other aliphatic polyisocyanates; cyclopentyl diisocyanate, cyclohexyl diisocyanate, isophorone diisocyanate (IPDI), 1,3-bis(isocyanatomethyl) ... Alicyclic isocyanates such as hexane; aromatic isocyanates such as 2,4-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and stubyl diisocyanate (XDI); and modified polyisocyanates obtained by modifying isocyanate compounds with alloformate bonds, diurea bonds, isocyanurate bonds, uretdione bonds, urea bonds, carbodiimide bonds, uretonimine bonds, and tri-ketotrione bonds. Examples of commercially available products include: TAKENATE 300S, TAKENATE 500, TAKENATE 600, TAKENATE D165N, TAKENATE D178N (all manufactured by Takeda Pharmaceutical Company), Sumidur T80, Sumidur L, Desmodur N3400 (all manufactured by Bayer Urethane Co., Ltd.), Millionate MR, Millionate MT, Coronate L, Coronate HL, Coronate HX (all manufactured by Tosoh Corporation), etc. The isocyanate compound may be used alone or in combination of two or more. A bifunctional isocyanate compound may also be used in combination with a trifunctional or higher functional isocyanate compound.

Examples of epoxy crosslinking agents include bisphenol A, epichlorohydrin-type epoxy resins, ethylenyl glycidyl ether, polyethylene glycol diglycidyl ether, glycerol diglycidyl ether, glycerol triglycidyl ether, 1,6-hexanediol glycidyl ether, trihydroxymethylenepropane triglycidyl ether, diglycidyl aniline, diaminoglycidylamine, N,N,N',N'-tetraglycidyl-intermediate diamine, and 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane. These crosslinking agents may be used alone or in combination of two or more.

Examples of the multifunctional monomer include ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,1 2-Dodecanediol di(meth)acrylate, trihydroxymethylpropane tri(meth)acrylate, tetrahydroxymethylmethane tri(meth)acrylate, allyl(meth)acrylate, vinyl(meth)acrylate, divinylbenzene, bisphenoxyethanolfluorene di(meth)acrylate, bisphenol A di(meth)acrylate, epoxy acrylate, polyester acrylate, urethane acrylate, butyl glycol di(meth)acrylate, hexyl glycol di(meth)acrylate, etc. The polyfunctional monomer may be used alone or in combination of two or more.

When a crosslinking agent (which may be a multifunctional monomer) is used, the amount used is not particularly limited. For example, it can be set in the range of approximately 0.001 to 5.0 parts by weight relative to 100 parts by weight of the base polymer. From the perspective of improving the softness of the adhesive, in several aspects, the amount of the crosslinking agent used relative to 100 parts by weight of the base polymer is preferably 3.0 parts by weight or less, more preferably 2.0 parts by weight or less, and can be 1.0 parts by weight or less, 0.5 parts by weight or less, or even 0.2 parts by weight or less. Furthermore, from the perspective of properly utilizing the effect of the crosslinking agent, in several aspects, the amount of the 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, or even 0.08 parts by weight or more.

In order to make the crosslinking reaction more efficient, a crosslinking catalyst can also be used. Examples of crosslinking catalysts include: tetrabutyl titanium, tetraisopropyl titanium, iron (III) acetylacetonate, butyltin oxide, dioctyltin dilaurate and other metal crosslinking catalysts. Among them, tin-based crosslinking catalysts such as dioctyltin dilaurate are preferred. There is no particular restriction on the amount of crosslinking catalyst used. Taking into account the balance between the speed of the crosslinking reaction and the service life of the adhesive composition, the amount of the crosslinking catalyst used relative to 100 parts by weight of the base polymer can be set to a range of about 0.0001 parts by weight or more and 1 part by weight or less, and preferably set to a range of 0.001 parts by weight or more and 0.5 parts by weight or less.

The adhesive composition may contain a compound that can produce keto-enol tautomerism as a crosslinking delay agent. This can achieve the effect of extending the service life of the adhesive composition. For example, a compound that can produce keto-enol tautomerism can be appropriately used in an adhesive composition containing an isocyanate-based crosslinker. Various β-dicarbonyl compounds can be used as compounds that can produce keto-enol tautomerism. For example, β-diketones (acetylacetone, 2,4-hexanedione, etc.) or acetoacetic esters (methyl acetoacetate, ethyl acetoacetate, etc.) can be appropriately used. Compounds that can produce keto-enol tautomerism can be used alone or in combination of two or more. The amount of the compound capable of producing keto-enol tautomerism used can be, for example, 0.1 parts by weight to 20 parts by weight, 0.5 parts by weight to 10 parts by weight, or 1 part by weight to 5 parts by weight, relative to 100 parts by weight of the base polymer.

(Tackifier) The adhesive layer disclosed herein may also contain a tackifier. Examples of known tackifiers include rosin-based tackifier resins, terpene-based tackifier resins, phenol-based tackifier resins, hydrocarbon-based tackifier resins, ketone-based tackifier resins, polyamide-based tackifier resins, epoxy-based tackifier resins, and elastomer-based tackifier resins. These resins may be used singly or in combination. The amount of tackifier resin used is not particularly limited and can be adjusted to achieve appropriate adhesive properties based on the intended purpose and application. In some aspects, from the perspective of refractive index or transparency, the amount of the adhesive used is preferably 30 parts by weight or less, preferably 10 parts by weight or less, and more preferably 5 parts by weight or less, based on 100 parts by weight of the base polymer of the adhesive layer. The technology disclosed herein can be suitably implemented without the use of an adhesive.

(Other Additives) In the disclosed technology, the adhesive composition used to form the adhesive layer may also include, as needed, plasticizers, softeners, colorants, antistatic agents, anti-aging agents, UV absorbers, antioxidants, light stabilizers, preservatives, and other commonly known additives for adhesive compositions, as long as they do not significantly impair the effects of the present invention. These additives can be conventionally used and do not contribute to any particular characteristics of the present invention, so detailed descriptions are omitted.

(Peel Strength) In several aspects, the adhesive layer disclosed herein preferably has a peel strength of approximately 1.0 N/25 mm or greater (e.g., 1.5 N/25 mm or greater), preferably 2 N/25 mm or greater, more preferably 3 N/25 mm or greater, and can be 4 N/25 mm or greater, 6 N/25 mm or greater, 8 N/25 mm or greater, 10 N/25 mm or greater, or even 12 N/25 mm or greater. The upper limit of the peel strength is not particularly limited, and can be, for example, 30 N/25 mm or less, 25 N/25 mm or less, or 20 N/25 mm or less.

The peel strength is measured by pressing the adherend onto an alkaline glass plate and placing it in an environment of 23°C and 50% RH for 30 minutes. The adherend is then autoclaved in a pressurized degassing apparatus (autoclave) at 50°C and 0.5 MPa for 30 minutes. After 24 hours in an atmosphere of 23°C and 50% RH, the adherend is peeled off at a 180-degree angle and a tensile speed of 300 mm/min, and the adhesion strength is measured. During the measurement, a suitable backing material (e.g., a polyethylene terephthalate (PET) film approximately 25µm to 50µm thick) may be attached to the adherend as needed for reinforcement. More specifically, the peel strength can be measured according to the method described in the examples below. When the disclosed double-sided adhesive sheet is formed by laminating a high-refractive index adhesive layer and a low-refractive index adhesive layer to form a laminate having a first adhesive surface and a second adhesive surface, in several aspects, the peel strength described above preferably applies to at least the first adhesive surface (the adhesive surface formed by the high-refractive index adhesive layer), and more preferably to both the first and second adhesive surfaces. The peel strength of the first adhesive surface against the glass plate and the peel strength of the second adhesive surface against the glass plate may be of the same level or different.

<Low-Refractive Index Layer> In the disclosed technology, the refractive index _n2 of the low-refractive index layer (preferably a low-refractive index adhesive layer) is preferably lower than the refractive index _n1 of the high-refractive index adhesive layer. This difference in refractive index between the high-refractive index adhesive layer and the low-refractive index layer can be used to control the behavior of light passing through a laminate containing these layers. The refractive index _n2 of the low-refractive index layer can, for example, be in the range of approximately 1.35 to 1.55. In several aspects, from the perspective of increasing the refractive index difference with the refractive index _n1 of the high-refractive-index adhesive layer and thus enhancing the front brightness enhancement effect described below, the refractive index _n2 of the low-refractive-index layer is preferably, for example, 1.49 or less, and more preferably 1.47 or less (e.g., 1.46 or less or 1.45 or less). It can be 1.43 or less, 1.41 or less, or even 1.40 or less. Furthermore, from the perspective of material availability or the ease of balancing adhesion properties, in several aspects, the refractive index _n2 of the low-refractive-index layer can be, for example, 1.36 or greater, 1.38 or greater, 1.40 or greater, or 1.42 or greater.

In some aspects, the ratio (n ₁ /n ₂ ) of the refractive index n ₁ of the high-refractive-index adhesive layer to the refractive index n ₂ of the low-refractive-index layer can be, for example, greater than 1.00, and can be approximately 1.01 or greater, preferably approximately 1.02 or greater, or approximately 1.03 or greater. In some aspects, the ratio (n ₁ /n ₂ ) is advantageously approximately 1.05 or greater, preferably approximately 1.07 or greater, more preferably approximately 1.10 or greater, and can be approximately 1.11 or greater. The upper limit of the ratio (n ₁ /n ₂ ) is not particularly limited. In some aspects, from the viewpoint of adhesion properties or transparency, the ratio (n ₁ /n ₂ ) may be, for example, approximately 1.20 or less, approximately 1.18 or less, approximately 1.16 or less, approximately 1.14 or less, or approximately 1.12 or less.

In several aspects, the difference between the refractive index _n1 of the high-refractive-index adhesive layer and the refractive index _n2 of the low-refractive-index layer, i.e., the refractive index difference ( _n1 - _n2 ), can be, for example, greater than 0.00, 0.01 or greater, 0.02 or greater, 0.03 or greater, 0.05 or greater, 0.10 or greater, 0.15 or greater, 0.20 or greater, or 0.25 or greater. The upper limit of the refractive index difference ( _n1 - _n2 ) is not particularly limited. In several aspects, from the perspectives of adhesive properties and transparency, the refractive index ( _n1 - _n2 ) can be, for example, 0.30 or less, 0.26 or less, 0.21 or less, 0.18 or less, or 0.16 or less.

In several ideal embodiments, the storage modulus G' of the low refractive index layer at 25°C (hereinafter referred to as "storage modulus G' _V2 (25)") should be lower than the storage modulus G' of the high refractive index adhesive layer at 25°C (storage modulus G' _V1 (25)). In other words, G' _V2 (25) <G' _V1 (25). According to the above structure, by laminating the low refractive index layer with the high refractive index adhesive layer, adhesion or flexibility can be imparted, and the tracking of height differences or curved surfaces can be improved, thereby realizing a laminated sheet (adhesive sheet) that can be suitably applied to various device design applications.

The storage elastic modulus G' _V2 (25) is not particularly limited and may be, for example, in the range of 1.0 kPa to 500 kPa. From the perspective of improving the softness imparted by the low refractive index layer or enhancing the ability to track deformation, in several aspects, the storage elastic modulus G' _V2 (25) is appropriately 400 kPa or less, preferably 300 kPa or less, more preferably 200 kPa or less (e.g., 180 kPa or less or 150 kPa or less), and may be 120 kPa or less, 90 kPa or less, or even 70 kPa or less. Furthermore, from the perspective of imparting appropriate cohesion to the low refractive index layer, in several embodiments, the storage modulus G' _V2 (25) is appropriately 5.0 kPa or higher, and preferably 10 kPa or higher, and can be 15 kPa or higher, 25 kPa or higher, 35 kPa or higher, 60 kPa or higher, or 80 kPa or higher. From the perspective of easily achieving higher cohesion or adhesion properties, in several embodiments, the storage modulus G' _V2 (25) can be 95 kPa or higher, 110 kPa or higher, or 140 kPa or higher.

In the embodiment where the low refractive index layer is an adhesive layer, the type of adhesive constituting the adhesive layer is not particularly limited. The adhesive constituting the low refractive index adhesive layer can be one or more of various rubbery polymers such as acrylic polymers, rubber polymers (such as natural rubber, synthetic rubber, and mixtures thereof), polyester polymers, urethane polymers, polyether polymers, polysilicone polymers, polyamide polymers, and fluoropolymers that can be used in the adhesive field as a base polymer. From the perspective of adhesive performance or cost, an adhesive comprising an acrylic polymer or a rubber polymer as a base polymer can be appropriately used. Among them, an adhesive having an acrylic polymer as a base polymer (acrylic adhesive) is preferred. In the case where the high-refractive-index adhesive layer is an acrylic adhesive layer, from the perspective of adhesion between the high-refractive-index adhesive layer and the low-refractive-index adhesive layer, it is appropriate to adopt a low-refractive-index adhesive layer as the acrylic adhesive layer.

In several aspects, the acrylic polymer is preferably a polymer of, for example, the following monomer raw materials: containing an alkyl (meth)acrylate, and further containing other monomers copolymerizable with the alkyl (meth)acrylate (copolymerizable monomers). In the monomer raw material, the content of the alkyl (meth)acrylate can be, for example, 10% by weight or more, 25% by weight or more, 35% by weight or more, or 45% by weight or more. The acrylic polymer can also be a polymer containing an alkyl (meth)acrylate as a main monomer and further containing the copolymerizable monomer as a secondary monomer. Here, the main monomer refers to a component whose monomer composition accounts for more than 50% by weight in the monomer raw material. More than 55% by weight or more than 60% by weight of the monomer composition can also be an alkyl (meth)acrylate.

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

The copolymerizable monomers mentioned above are useful for introducing crosslinking points into acrylic polymers or improving the cohesive force of acrylic polymers. The copolymerizable monomers mentioned above may be one or more functional monomers such as carboxyl group-containing monomers, hydroxyl group-containing monomers, acid anhydride group-containing monomers, amide group-containing monomers, amino group-containing monomers, monomers having nitrogen atom rings, sulfonic acid group-containing monomers, and phosphoric acid group-containing monomers. Other examples of copolymerizable monomers include vinyl ester monomers such as vinyl acetate, aromatic vinyl compounds such as styrene, (meth)acrylates containing non-aromatic rings, and alkoxy group-containing monomers. Specific examples include the monomers described above as the base polymer for the high refractive index adhesive layer, but are not limited thereto. For example, from the perspective of enhancing cohesive strength, the copolymerizable monomer is preferably an acrylic polymer obtained by copolymerizing a carboxyl group-containing monomer and/or a hydroxyl group-containing monomer. Suitable examples of carboxyl group-containing monomers include acrylic acid and methacrylic acid. Suitable examples of hydroxyl group-containing monomers include 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate.

In several aspects, to lower the refractive index n ₂ of the low-refractive-index layer, a fluorinated monomer can be used as the copolymerizable monomer. The content of the fluorinated monomer in the monomer raw material can be, for example, 10% by weight or greater, 25% by weight or greater, or 35% by weight or greater. To facilitate achieving a low-refractive-index layer with a lower refractive index, the content of the fluorinated monomer is preferably 40% by weight or greater, preferably 45% by weight or greater, and more preferably 55% by weight or greater. It can be 60% by weight or greater, 75% by weight or greater, 85% by weight or greater, 90% by weight or greater, or even 95% by weight or greater. The upper limit of the fluorinated monomer content in the monomer raw material is not particularly limited and can be 100% by weight. In several aspects, from the perspective of the cohesion of the low refractive index layer, the content of the fluorinated monomer is suitably 99.9% by weight or less, preferably 99.5% or less, 99% by weight or less, 97% by weight or less, or even 92% by weight or less. Fluorinated monomers may be used alone or in combination of two or more.

Fluorine-containing monomers can suitably be fluorine-containing acrylic monomers. Fluorine-containing acrylic monomers are not particularly limited as long as they are acrylic monomers having at least one fluorine atom in the molecule. For example, fluorine-containing (meth)acrylates can suitably be used. Suitable examples of fluorine-containing (meth)acrylates include those having a fluorinated alkyl group at the end of the ester. Examples of fluorinated alkyl groups include fluorinated aliphatic alkyl groups, fluorinated alicyclic alkyl groups, fluorinated aromatic alkyl groups, and the like. Fluorinated alkyl groups are preferably fluorinated aliphatic alkyl groups. Examples of fluorinated aliphatic alkyl groups include fluorinated alkyl groups, and the like. In fluorinated aliphatic alkyl groups, the aliphatic alkyl portion can be a straight chain or a branched chain. In addition, in fluorinated aliphatic alkyl groups, the fluorine atom can be bonded to any carbon atom of the aliphatic alkyl portion. The number of fluorine atoms bonded to a single carbon atom may be singular or plural. There is no particular limitation on the number of carbon atoms bonded to fluorine atoms.

In fluorinated aliphatic alkyl groups (including fluorinated alkyl groups), the number of carbon atoms in the alkyl portion is not particularly limited. In several aspects, considering compatibility with other copolymerizable monomers, fluorinated aliphatic alkyl groups having a carbon number of, for example, 1 to 18 (preferably 1 to 12) are preferred. Specific examples of fluorinated aliphatic alkyl groups include fluorinated methyl groups such as trifluoromethyl, difluoromethyl, and monofluoromethyl; and fluorinated ethyl groups such as pentafluoroethyl, 1,1,2,2-tetrafluoroethyl, 1,2,2,2-tetrafluoroethyl, 1,1,2-trifluoroethyl, 1,2,2-trifluoroethyl, 2,2,2-trifluoroethyl, 1,1-difluoroethyl, 1,2-difluoroethyl, 2,2-difluoroethyl, 1-monofluoroethyl, and 2-monofluoroethyl. Examples of the fluorinated alkyl group having 3 or more carbon atoms include various fluorinated alkyl groups in which a single or multiple fluorine atoms are bonded to at least one of the carbon atoms in the alkyl portion, similarly to the fluorinated methyl group or the fluorinated ethyl group exemplified above.

Examples of the fluorinated alicyclic alkyl group include fluorinated cycloalkyl groups. Similar to the above-mentioned fluorinated aliphatic alkyl groups, in the fluorinated alicyclic alkyl group, the fluorine atom may be bonded to any carbon atom of the alicyclic alkyl group, and the number of fluorine atoms bonded to one carbon atom may be singular or plural. Furthermore, there is no particular limitation on the number of carbon atoms bonded to fluorine atoms. Fluorinated alicyclic alkyl groups include, for example, cyclohexyl groups having one fluorine atom, such as 2-fluorocyclohexyl, 3-fluorocyclohexyl, and 4-fluorocyclohexyl; cyclohexyl groups having two fluorine atoms, such as 2,4-difluorocyclohexyl and 2,6-difluorocyclohexyl; and cyclohexyl groups having three fluorine atoms, such as 2,4,6-trifluorocyclohexyl.

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

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

The (meth)acrylate containing a fluorine atom is preferably a fluorinated alkyl (meth)acrylate (especially a fluorinated alkyl acrylate). Examples of the fluorinated alkyl (meth)acrylate include 2,2,2-trifluoroethyl acrylate (trade name "Viscoat 3F" manufactured by Osaka Organic Chemical Industry Co., Ltd., etc.), 2,2,3,3-tetrafluoropropyl acrylate (trade name "Viscoat 4F" manufactured by Osaka Organic Chemical Industry Co., Ltd., etc.), 1H,1H,5H-octafluoropentyl acrylate (trade name "Viscoat 8F" manufactured by Osaka Organic Chemical Industry Co., Ltd., etc.), 1H,1H,5H-octafluoropentyl methacrylate (trade name "Viscoat 8FM" manufactured by Osaka Organic Chemical Industry Co., Ltd., etc.), 2-(heptadecafluorononyl)ethyl acrylate (trade name "FA-108" manufactured by Kyoeisha Chemical Co., Ltd., etc.), 1H,1H,2H,2H-tridecafluorooctyl acrylate (trade name "Viscoat 13F" etc.)

From the perspectives of lowering the refractive index and achieving softness, the fluorinated alkyl group in the fluorinated alkyl (meth)acrylate advantageously has 3 or more carbon atoms, preferably 4 or more, preferably 5 or more, more preferably 6 or more or 7 or more, and particularly preferably 8 or more. From the perspectives of adhesion, the fluorinated alkyl group advantageously has 18 or less carbon atoms, preferably 14 or less, preferably 12 or less, and can be 10 or less, or can be 9 or less. In some aspects, the fluorinated alkyl group can have 7 or less carbon atoms, or can be 5 or less. Furthermore, in some aspects, the fluorine-containing (meth)acrylate is preferably a fluorinated alkyl (meth)acrylate in which the carbon at the 1-position of the alkyl group is not bonded to a fluorine. For example, a fluorinated alkyl (meth)acrylate in which either the carbon at the 1-position or the carbon at the 2-position of the alkyl group is not bonded to a fluorine can be suitably used, such as 1H,1H,2H,2H-tridecafluorooctyl acrylate.

In several aspects, the low-refractive-index layer is an acrylic adhesive layer, and the acrylic polymer serving as the base polymer of the adhesive can be a polymer of the following monomer raw materials: comprising at least a fluorinated acrylic monomer (e.g., a fluorinated alkyl (meth)acrylate) as described above, and can further comprise other monomers copolymerizable with the fluorinated acrylic monomer (copolymerizable monomer). The monomer raw material may or may not contain an alkyl (meth)acrylate. The content of the fluorinated acrylic monomer in the monomer raw material can be, for example, 10% by weight or greater, 25% by weight or greater, or 35% by weight or greater. From the perspective of easily achieving a low-refractive-index layer with a lower refractive index, the content of the fluorinated acrylic monomer is preferably 40% by weight or more, preferably 45% by weight or more, and more preferably 55% by weight or more. It can be 60% by weight or more, 75% by weight or more, 85% by weight or more, 90% by weight or more, or even 95% by weight or more. The upper limit of the content of the fluorinated acrylic monomer in the monomer raw material is not particularly limited and can be 100% by weight. In several aspects, from the perspective of the cohesiveness of the low-refractive-index layer, the content of the fluorinated acrylic monomer is suitably 99.9% by weight or less, and is preferably 99.5% or less, can be 99% by weight or less, can be 97% by weight or less, or can be 92% by weight or less. The fluorinated acrylic monomers can be used alone or in combination of two or more.

The monomer raw materials used to prepare the base polymer of the low refractive index layer can be composed of a copolymerizable monomer in addition to a fluorine-containing acrylic monomer (e.g., a fluorinated alkyl (meth)acrylate). The copolymerizable monomers can include one or more functional monomers such as carboxyl-containing monomers, hydroxyl-containing monomers, acid anhydride-containing monomers, amide-containing monomers, amino-containing monomers, monomers having a nitrogen-containing ring (e.g., N-vinyl cyclic amides such as N-vinyl-2-pyrrolidone), sulfonic acid-containing monomers, and phosphoric acid-containing monomers. Other examples of copolymerizable monomers include vinyl ester monomers such as vinyl acetate, aromatic vinyl compounds such as styrene, (meth)acrylates containing non-aromatic rings such as cycloalkyl (meth)acrylates or isoborneol (meth)acrylates, and alkoxy-containing monomers. Specific examples include, but are not limited to, the monomers described above as the base polymer for the high-refractive index adhesive layer. For example, from the perspective of enhancing cohesive strength, the copolymerizable monomer is preferably an acrylic polymer copolymerized with a carboxyl group-containing monomer and/or a hydroxyl group-containing monomer.

In several ideal aspects, the monomer raw material used to prepare the base polymer of the low refractive index layer may be a composition comprising a fluorine-containing monomer (e.g., a fluorine-containing acrylic monomer such as fluorinated alkyl (meth)acrylate) and further comprising a hydroxyl-containing monomer. The hydroxyl-containing monomer helps to enhance cohesion or introduce crosslinking points, etc. Suitable examples of hydroxyl-containing monomers include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl acrylate or 4-hydroxybutyl acrylate. From the perspective of enhancing flexibility at room temperature, 4-hydroxybutyl acrylate can be more preferably used. The content of the hydroxyl-containing monomer in the monomer raw material is not particularly limited, and for example, it can be 0.01% by weight or more (preferably 0.1% by weight or more, more preferably 0.5% by weight or more). In several aspects, the content of the hydroxyl-containing monomer in the monomer raw material may be 0.7% or more by weight, 0.9% or more by weight, or 1.5% or more by weight. The upper limit of the content of the hydroxyl-containing monomer is not particularly limited, and may be, for example, 15% or less by weight or 10% or less by weight. In several aspects, from the perspective of lowering the refractive index, the content of the hydroxyl-containing monomer in the monomer raw material is preferably less than 10% by weight, and is preferably less than 5% by weight, less than 3% by weight, less than 2.5% by weight, or less than 1.5% by weight.

In several aspects, to prevent coloration or discoloration (e.g., yellowing) of the low-refractive index layer, the content of carboxyl-containing monomers in the monomer raw materials used to prepare the base polymer of the low-refractive index layer is preferably limited. The content of carboxyl-containing monomers in the monomer raw materials can be, for example, less than 1% by weight, preferably less than 0.5% by weight, more preferably less than 0.3% by weight, and even more preferably less than 0.1% by weight (e.g., less than 0.05% by weight). Limiting the content of carboxyl-containing monomers is also beneficial from the perspective of preventing corrosion of metal materials that may come into contact with or be located adjacent to the low-refractive index layer (e.g., metal wiring or metal films that may be present on an adherend). The technology disclosed herein can be preferably implemented in embodiments where the monomer raw materials do not contain carboxyl-containing monomers. For similar reasons, in several aspects, the content of monomers containing acidic functional groups (including, in addition to carboxyl groups, sulfonic acid groups, phosphoric acid groups, etc.) in the monomer raw materials used to prepare the base polymer of the low refractive index layer is preferably limited. The content of acidic functional monomers in the monomer raw materials in these aspects can be applied to the ideal content of carboxyl-containing monomers described above. The disclosed technology can also be suitably implemented in embodiments in which the monomer raw materials do not contain acidic monomers (i.e., when the base polymer of the low refractive index layer is acid-free).

The base polymer of the low-refractive index layer and the base polymer of the high-refractive index adhesive layer can be prepared using conventional polymerization methods. The weight-average molecular weight (Mw) of the base polymer is not particularly limited and can be, for example, in the range of approximately 10×10 ⁴ to 500×10 ⁴ , or approximately 20×10 ⁴ to 200×10 ^4. In several aspects, from the perspective of adhesion to the high-refractive index adhesive layer, the Mw of the base polymer of the low-refractive index adhesive layer is suitably 150×10 ⁴ or less, preferably 120×10 ⁴ or less (e.g., 95×10 ⁴ or less), 75×10 ⁴ or less, 68×10 ⁴ or less, or even 60×10 ⁴ or less. In some aspects, the base polymer may have an Mw of, for example, 30 × 10 ⁴ or greater, 40 × 10 ⁴ or greater, or even 50 × 10 ⁴ or greater, from the perspective of the cohesiveness of the low-refractive-index adhesive layer. To adjust the Mw, a conventionally known chain transfer agent may be used as needed.

While not particularly limited, from the perspective of adhesion, the Tg of the base polymer of the low refractive index layer (e.g., acrylic polymer) is advantageously below approximately 0°C, and preferably below approximately -5°C (e.g., below approximately -15°C or below -25°C). Furthermore, from the perspective of cohesive strength of the adhesive layer, the Tg of the base polymer of the low refractive index layer is above approximately -75°C, and preferably above approximately -70°C (e.g., above -50°C, further preferably above -30°C). The Tg of the acrylic polymer can be adjusted by appropriately varying the monomer composition (i.e., the types or amounts of monomers used in synthesizing the polymer).

A known crosslinking agent can be used in the low-refractive index layer. Furthermore, the low-refractive index layer may contain a binder or other additive. The crosslinking agent or binder can be appropriately selected from the same materials used in the high-refractive index adhesive layer and used in an appropriate amount.

In embodiments where the adhesive composition used to form the low-refractive index adhesive layer includes a crosslinker, the crosslinker may suitably be an isocyanate-based crosslinker, for example. In several embodiments, based on the viewpoint of adhesion to the high-refractive index adhesive layer, the amount of the isocyanate-based crosslinker used may be, for example, less than 0.5 parts by weight, less than 0.3 parts by weight, less than 0.2 parts by weight, or less than 0.15 parts by weight, relative to 100 parts by weight of the base polymer of the adhesive composition. Furthermore, from the perspective of properly utilizing the crosslinking agent, in several aspects, the amount of the isocyanate crosslinking agent used 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, or 0.08 parts by weight or more, relative to 100 parts by weight of the base polymer.

<Adhesive Layer Preparation> In the disclosed technology, the adhesive constituting the adhesive layer (which may be a high-refractive index adhesive layer and/or a low-refractive index layer; the same applies hereinafter) can be an adhesive formed by curing a solvent-based, active energy ray-curable, water-dispersible, or hot-melt adhesive composition through drying, crosslinking, polymerization, cooling, or the like. This means, it can be a cured product of any of these adhesive compositions. The adhesive composition can be cured by a single curing method (e.g., drying, crosslinking, polymerization, cooling, etc.), or by using two or more curing methods simultaneously or in multiple stages. For solvent-based adhesive compositions, the adhesive is typically formed by drying the composition (preferably followed by crosslinking). Active energy ray-curable adhesive compositions are typically formed by irradiating the adhesive with active energy rays to cause polymerization and/or crosslinking reactions to proceed. If drying of the active energy ray-curable adhesive composition is necessary, irradiation with active energy rays may be performed after drying.

The adhesive layer of the adhesive sheet disclosed herein can be formed by applying (e.g., coating) an adhesive composition to a suitable surface and then curing the composition. Application of the adhesive composition can be performed using conventional coating machines such as gravure roll coaters, reverse roll coaters, contact roll coaters, dip roll coaters, rod coaters, doctor blade coaters, and spray coaters.

The adhesive layer disclosed herein may be a post-curing adhesive layer or a non-post-curing adhesive layer. Here, a post-curing adhesive layer refers to an adhesive layer that can be further cured by irradiation with heat or active energy rays (e.g., ultraviolet rays). Examples of post-curing adhesive layers include adhesive layers having unreacted ethylenically unsaturated groups in the side chains of the base polymer, or adhesive layers containing unreacted multifunctional monomers. In several embodiments, the adhesive layer preferably does not have post-curing properties. Adhesive layers without post-curing properties do not undergo the dimensional changes associated with the subsequent curing reaction (i.e., they exhibit excellent dimensional stability). This helps suppress warping of the adhesive layer or the adherend to which it is applied. The absence of dimensional changes associated with post-curing (such as shrinkage) is also beneficial from the perspective of suppressing optical strain in the adhesive layer.

The thickness of the adhesive layer is not particularly limited, and can be set to, for example, 3µm or more, and preferably 5µm or more. With an adhesive layer having a thickness of 5µm or more, good adhesive properties can be easily obtained. In addition, the adhesive layer of the aforementioned thickness absorbs the unevenness that may exist on the surface of the adherend and is easily bonded to the adherend with good adhesion. From the perspective of preventing coloring or color unevenness caused by light interference, it is also preferable that the thickness of the adhesive layer (for example, the thickness of a high refractive index adhesive layer) is 5µm or more. In several aspects, the thickness of the adhesive layer can be 10µm or more, 20µm or more, 30µm or more, 50µm or more, 70µm or more, or 85µm or more. Furthermore, in several embodiments, the thickness of the adhesive layer can be, for example, less than 300µm, less than 250µm, less than 200µm, less than 150µm, or less than 120µm. The fact that the thickness of the adhesive layer is not too large is advantageous from the perspective of thinning the laminate or light-emitting device containing the adhesive layer. The technology disclosed herein is suitable for implementation in an embodiment in which the thickness of the adhesive layer is within the range of 3µm to 200µm (preferably 5µm to 100µm). In several embodiments, the thickness of the adhesive layer can be applied to at least the thickness _T1 of the high-refractive-index adhesive layer. The thickness _T2 of the low-refractive-index adhesive layer can also be selected from the same range. Regardless of whether or not it is an adhesive layer, the thickness of the adhesive layer described above also applies to the thickness _T2 of the low-refractive index layer. The thickness _T1 of the high-refractive index adhesive layer and the thickness _T2 of the low-refractive index layer may be the same or different. The ratio ( _T1 / _T2 ) of the thickness _T1 of the high-refractive index adhesive layer to the thickness _T2 of the low-refractive index layer may be, for example, 0.1 or greater, 0.3 or greater, 0.5 or greater, 0.8 or greater, 1.2 or greater, or 1.5 or greater. Furthermore, the ratio ( _T1 / _T2 ) may be, for example, 20 or less, 10 or less, 5 or less, or 3 or less. In several aspects, the ratio ( _T1 / _T2 ) may be less than 2, less than 1.5, or less than 1.

Methods for obtaining a structure (laminated sheet) formed by laminating a high-refractive index adhesive layer and a low-refractive index layer (typically a low-refractive index adhesive layer) include, for example, forming a high-refractive index adhesive layer and a low-refractive index layer on a releasable surface (e.g., the release surface of a release pad) and laminating them together; applying a composition for forming the low-refractive index layer on the high-refractive index adhesive layer and curing it, or conversely applying an adhesive composition for forming the high-refractive index adhesive layer on the low-refractive index layer and curing it, etc., but are not limited to these methods. When laminating the pre-formed high-refractive index adhesive layer and the low-refractive index layer, a treatment to promote adhesion between the layers may be performed as needed. For example, autoclave treatment or roll press treatment may be performed, but the present invention is not limited to these.

<Supporting Substrate> The high-refractive index adhesive layer and the low-refractive index layer can be laminated on one surface of the supporting substrate in this order or in reverse order. This configuration of the high-refractive index adhesive layer and the low-refractive index layer laminated on the supporting substrate can also be considered an adhesive sheet with a substrate attached. Therefore, the present disclosure provides an adhesive sheet with a substrate attached (adhesive product), comprising: a laminated sheet comprising a high-refractive index adhesive layer and a low-refractive index layer (preferably a low-refractive index adhesive layer); and a supporting substrate supporting the laminated sheet.

The material of the supporting substrate is not particularly limited and can be appropriately selected according to the purpose of use or the state of use. Non-limiting examples of the substrate that can be used 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), polyethylene naphthalate (PEN) as the main component, polyvinyl chloride films with polyvinyl chloride as the main component, and other plastic films; Foam sheets made of foams such as polyurethane foam, polyethylene (PE) foam, and polychloroprene foam; woven and non-woven fabrics made from various fibrous materials (which may be natural fibers such as linen and cotton, synthetic fibers such as polyester and vinyl, and semi-synthetic fibers such as acetate) either alone or in blends; paper such as Japanese paper, wood-free paper, kraft paper, and wrinkled paper; and metal foils such as aluminum foil and copper foil. Composite substrates of these materials are also possible. Examples of composite substrates include substrates formed by laminating metal foil with the aforementioned plastic film, and plastic substrates reinforced with inorganic fibers such as glass cloth.

In several aspects, various film substrates can be suitably used. The film substrate can be a porous substrate such as a foam film or a non-woven sheet, a non-porous substrate, or a substrate having a structure formed by laminating porous layers and non-porous layers. In several aspects, the film substrate can suitably use a resin film that is independent and can maintain its shape (self-supporting or non-dependent) as a base film. Here, "resin film" refers to a non-porous structure and is typically a resin film that does not substantially contain bubbles (no holes). Therefore, the resin film is a concept that can be distinguished from foam film or non-woven fabric. The resin film that is independent and can maintain its shape (self-supporting or non-dependent) can suitably be used. The resin film may be a single-layer structure or a multi-layer structure of two or more layers (e.g., a three-layer structure).

Materials constituting the resin film include, for example, polyester resins based on polyesters such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN), polyolefin resins based on polyolefins such as polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, and ethylene-butylene copolymer, cellulose resins such as cellulose triacetate, acetate resins, polysulfone resins, polyethersulfone resins, polycarbonate resins, polyamide (PA) resins such as nylon 6, nylon 66, and partially aromatic polyamides, and polyimide (PI) resins. Cyclic polyolefin resins such as polyols, transparent polyimide resins, polyamide imide (PAI), polyetheretherketone (PEEK), polyether sulphite (PES), and norbornene resins, (meth)acrylic resins, polyvinyl chloride resins, polyvinylidene chloride resins, polystyrene resins, polyvinyl alcohol resins, ethylene-vinyl acetate copolymer resins, ethylene-vinyl alcohol copolymer resins, polyarylate resins, polyphenylene sulfide (PPS) resins, polyurethane (PU), ethylene-vinyl acetate copolymer (EVA), polytetrafluoroethylene (PTFE), and fluorinated polyimide resins, etc.

The resin film may be formed using a resin material containing only one of the resins mentioned above, or may be formed using a resin material blended with two or more resins. The resin film may be unstretched or stretched (e.g., uniaxially stretched or biaxially stretched). For example, 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, etc. may be suitably used. From the perspective of strength or dimensional stability, examples of ideal resin films include PET film, PEN film, PPS film, and PEEK film. From the perspective of ease of acquisition, PET film and PPS film are particularly suitable, with PET film being the most preferred.

The resin film may be blended with known additives such as light stabilizers, antioxidants, antistatic agents, colorants (dyes, pigments, etc.), fillers, slipping agents, and anti-adhesive agents, as needed, within a range that does not significantly impair the effects of the present invention. The amount of additives blended is not particularly limited and can be appropriately determined based on the intended use of the adhesive sheet.

The method for producing the resin film is not particularly limited. For example, conventionally known resin film forming methods such as extrusion molding, inflation molding, T-die casting, and calender roll molding can be appropriately adopted.

The substrate may be substantially composed of the base film. Alternatively, the substrate may include auxiliary layers in addition to the base film. Examples of such auxiliary layers include optical property adjustment layers (e.g., coloring layers, antireflection layers), printed layers or laminated layers for imparting a desired appearance to the substrate, antistatic layers, primer layers, release layers, and other surface treatment layers.

In several aspects, a light-transmitting substrate (hereinafter also referred to as a light-transmitting substrate) can be suitably used as a supporting substrate. In this way, an adhesive sheet with a light-transmitting substrate can be formed. The total light transmittance of the light-transmitting substrate can be, for example, greater than 50%, or can be greater than 70%. In several ideal aspects, the total light transmittance of the supporting substrate is greater than 80%, preferably greater than 90%, or can be greater than 95% (for example, 95-100%). The above-mentioned total light transmittance is measured using a commercially available transmittance meter in accordance with JIS K 7136:2000. The transmittance meter can use the product name "HAZEMETER HM-150" manufactured by Murakami Color Technology Laboratory or its equivalent. A suitable example of the above-mentioned light-transmitting substrate is a light-transmitting resin film. The above-mentioned light-transmitting substrate can also be an optical film.

The thickness of the substrate is not particularly limited and can be selected based on the intended use and usage. For example, the substrate thickness can be 500µm or less. However, from the perspective of handling and processability, it is preferably 300µm or less, and can be 150µm or less, 100µm or less, 50µm or less, 25µm or less, or even 10µm or less. A smaller substrate thickness tends to improve its ability to conform to the surface shape of the adherend. Furthermore, from the perspective of handling and processability, the substrate thickness can be, for example, 2µm or more, 10µm or more, or 25µm or more.

The surface of the substrate on which the adhesive layer is to be deposited may also be subjected to conventional surface treatments such as coma discharge treatment, plasma treatment, UV irradiation treatment, acid treatment, alkaline treatment, or application of a primer to form a base coat, as needed. Such surface treatments may be intended to enhance the adhesive layer's anchoring properties on 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 between 0.01µm and 1µm, preferably between 0.1µm and 1µm. Other treatments that may be applied to the substrate as needed include antistatic layer formation, coloring layer formation, printing, etc. These treatments may be applied individually or in combination.

In the disclosed technology, when a high-refractive index adhesive layer and a low-refractive index layer form an adhesive sheet attached to a substrate, the thickness of the adhesive sheet can be, for example, 1000µm or less, 350µm or less, 200µm or less, 120µm or less, 75µm or less, or even 50µm or less. Furthermore, from the perspective of handling properties, the thickness of the adhesive sheet can be, for example, 10µm or more, 25µm or more, 80µm or more, or even 130µm or more. The thickness of the adhesive sheet refers to the thickness of the portion attached to the adherend. For example, in the double-sided adhesive sheet 2 without a substrate as shown in FIG2 , the thickness refers to the thickness from the first surface (first adhesive surface) 10A of the adhesive layer to the second surface (second adhesive surface) 10B, and does not include the thickness of the release pads 31 and 32.

<Laminated Sheet with Peel Backing> Before incorporating the high-refractive-index adhesive layer and the low-refractive-index layer into a light-emitting device, the disclosed laminate can be formed into an adhesive product (laminated sheet with peel backing) in which the adhesive surface of the laminate, comprising the high-refractive-index adhesive layer and the low-refractive-index layer, abuts the release surface of a release backing. Therefore, according to the present specification, a laminated sheet (adhesive product) with a peel-off liner is provided, which includes a laminated sheet having a high-refractive-index adhesive layer and a low-refractive-index layer, and a peel-off liner having a peel-off surface abutting the adhesive surface of the laminated sheet.

The release liner is not particularly limited. For example, a release liner substrate such as a resin film or paper (which may be a paper laminated with a resin such as polyethylene) having a release treatment layer may be used. Alternatively, a release liner may be formed from a resin film formed from a low-adhesion material such as a fluoropolymer (such as polytetrafluoroethylene) or a polyolefin resin (such as polyethylene or polypropylene). The release treatment layer may be formed by surface-treating the release liner substrate with a release treatment agent. The peeling agent can be a known peeling agent such as a silicone peeling agent, a long-chain alkyl peeling agent, a fluorine peeling agent, or molybdenum (IV) sulfide. In some embodiments, a peeling pad having a peeling layer obtained using a silicone peeling agent can be used. The thickness and formation method of the peeling layer are not particularly limited and can be set to provide adequate peeling properties on the adhesive surface of the peeling pad.

In some aspects, from the perspective of smoothness of the adhesive surface, a release liner (hereinafter referred to as a release film) having a release-treated layer on a resin film (hereinafter referred to as a release film substrate) can be preferably used as the release liner substrate. Various plastic films can be used as the release film substrate. In this specification, the term "plastic film" is typically a non-porous sheet material, distinguishable from (i.e., excluding) nonwoven fabrics, for example.

The material of the above-mentioned plastic film can be, for example, polyester resins such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polyolefin resins such as polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, ethylene-butene copolymer, cellulose resins such as cellulose triacetate, acetate resins, polyester resins, polyester Examples of the release film substrate include cyclic polyolefin resins such as ether resins, polycarbonate resins, polyamide resins, polyimide resins, and norbornene resins, (meth)acrylic resins, polyvinyl chloride resins, polyvinylidene chloride resins, polystyrene resins, polyvinyl alcohol resins, ethylene-vinyl acetate copolymer resins, ethylene-vinyl alcohol copolymer resins, polyarylate resins, and polyphenylene sulfide resins. Release film substrates formed from any one or a mixture of two or more of these resins may also be used. Preferred release film substrates include polyester resin films (e.g., PET films) formed from polyester resins.

The plastic film used as the release film substrate can be any of unoriented, uniaxially oriented, and biaxially oriented films. Furthermore, the plastic film can be a single-layer structure or a multilayer structure comprising two or more sublayers. The plastic film can also be blended with known additives used in release film substrates for adhesive sheets, such as antioxidants, anti-aging agents, heat stabilizers, light stabilizers, UV absorbers, colorants such as pigments or dyes, lubricants, fillers, antistatic agents, and nucleating agents. In a multi-layer plastic film, each additive can be mixed into all sub-layers or only into a portion of the sub-layers.

In several ideal embodiments, the release film substrate (typically a plastic film) described above can be used in a layer on the release surface that contains a limited amount of inorganic particles (e.g., pigments, lubricants, fillers, etc.), or that substantially contains no such particles. "Substantially free" here means that the amount of particles (e.g., inorganic particles) in the layer is less than 1% by weight, and preferably less than 0.1% by weight (e.g., 0-0.01% by weight). Release films having such a release film substrate tend to have a low arithmetic mean roughness Ra or maximum height Rz of the release surface. When the release film substrate (typically a plastic film) has a multi-layer structure, the particle content in the layer on the release side can be less than 1/10 (e.g., less than 1/50) of the particle content in the layers other than the release side layer.

In a laminated sheet having a release liner on each of the first and second adhesive surfaces, the release liner disposed on one of the adhesive surfaces (hereinafter referred to as one release liner) and the release liner disposed on the other adhesive surface (hereinafter referred to as the other release liner) may be of the same material and structure, or may be of different materials and structures.

The thickness of the release liner (preferably a release film) is not particularly limited and can be, for example, approximately 10µm to 500µm. From the perspective of the release liner's strength and dimensional stability, a release liner thickness of 20µm or greater is appropriate, preferably 30µm or greater, preferably 35µm or greater, 40µm or greater, and even 45µm or greater. Furthermore, from the perspective of the release liner's handling properties (e.g., ease of winding), a release liner thickness of 300µm or less is appropriate, preferably 250µm or less, and can be 200µm or less, 150µm or less, or even 130µm or less. In several ideal embodiments, the thickness of the peel liner is approximately 125µm or less, and may be approximately 115µm or less, approximately 105µm or less, approximately 90µm or less, or approximately 70µm or less. By setting the peel liner thickness below a predetermined value, the peel liner is less likely to form curling marks when rolled, allowing for smooth removal from the adhesive sheet. This results in a smoother adhesive surface after the peel liner is removed.

In a laminated sheet with a release liner having one release liner and another release liner, the release liners may have the same or different thicknesses. In some embodiments, from the perspective of peeling workability, one release liner and the other release liner preferably have different thicknesses. For example, the thickness of the thicker release liner is preferably at least about 1.1 times the thickness of the thinner release liner (e.g., at least about 1.25 times; the upper limit is not particularly limited, e.g., 5 times or less).

(Arithmetic Average Roughness Ra of the Adhesive Side) In some aspects, to achieve a highly smooth adhesive surface, the arithmetic average roughness Ra of the adhesive side surface of a release liner (preferably a release film) is preferably limited to a predetermined value (e.g., less than approximately 100 nm, more preferably less than 50 nm). In some aspects, the arithmetic average roughness Ra of the adhesive side surface of the release liner is, for example, preferably less than approximately 30 nm, more preferably less than approximately 25 nm, and may be less than approximately 20 nm, or even less than approximately 18 nm. Furthermore, from the perspective of ease of manufacture and handling of the release liner, in several aspects, the arithmetic average roughness Ra can be, for example, approximately 5 nm or greater, approximately 10 nm or greater, or approximately 15 nm or greater. In a laminated sheet having release liners disposed on both the first and second adhesive surfaces, the adhesive-side surfaces of both release liners preferably meet any of the aforementioned arithmetic average roughness Ra. The arithmetic average roughness Ra of the adhesive-side surfaces of the two release liners can be the same or different.

(Maximum Height Rz of the Adhesive Side) In some aspects, to achieve a highly smooth adhesive surface, the peel liner (preferably a peel film) preferably has a maximum height Rz of 700 nm or less. In some aspects, the maximum height Rz of the adhesive side of the peel liner is preferably approximately 600 nm or less, and may be approximately 500 nm or less, approximately 400 nm or less, or approximately 300 nm or less. Furthermore, from the perspective of ease of manufacturing and handling of the release liner, in several aspects, the maximum height Rz can be, for example, greater than approximately 50 nm, greater than approximately 80 nm, greater than approximately 100 nm, greater than approximately 200 nm, or greater than approximately 300 nm. In a laminated sheet with release liners disposed on both the first and second adhesive surfaces, the adhesive-side surfaces of both release liners preferably meet any of the aforementioned maximum heights Rz. The maximum heights Rz of the adhesive-side surfaces of the two release liners can be the same or different.

(Back Surface Properties) The arithmetic average roughness Ra or maximum height Rz of the back surface (opposite to the adhesive layer) of the release liner (preferably a release film) is not particularly limited. From the perspective of productivity, the arithmetic average roughness Ra of the back surface of the release liner may be, for example, greater than 30 nm (e.g., greater than 35 nm, or even greater than approximately 50 nm). From the perspective of productivity, the maximum height Rz of the back surface of the release liner may be, for example, greater than 400 nm (e.g., greater than approximately 500 nm) or greater than 800 nm (e.g., greater than 1000 nm).

The arithmetic average roughness Ra and maximum height Rz of the release film surface can be adjusted by selecting the film material, forming method, and surface treatments such as release treatment. For example, the smoothness of the layers that constitute the release surface (anti-adhesion layer, hard coat layer, oligomer prevention layer, etc.) can be adjusted, the amount of filler particles in the surface layer or the release film substrate can be reduced or eliminated (de-particleization), and other adjustments to the stretching conditions can be made.

The arithmetic average roughness Ra and maximum height Rz of the surface of a release pad (preferably a release film) are measured using a non-contact surface roughness measuring device. The non-contact surface roughness measuring device can be an optical interferometer, such as a 3D optical profiler (trade name "NewView7300", manufactured by ZYGO Corporation) or its equivalent. For example, a glass plate (sodium calcium glass plate, 1.3 mm thick, manufactured by MATSUNAMI Corporation) can be attached to the surface of the release pad opposite to the measurement surface using an adhesive and fixed. The surface shape can then be measured using a 3D optical profiler (trade name "NewView7300", manufactured by ZYGO Corporation) in an environment of 23°C and 50% RH.

<Application> In the technology disclosed herein, the high refractive index adhesive layer can be used by being adhered to various adherends constituting a light-emitting device. The constituent material of the above-mentioned adherend (adherend material) is not particularly limited, and examples thereof include: copper, silver, gold, iron, tin, palladium, aluminum, nickel, titanium, chromium, indium, zinc, etc. or metal materials such as alloys containing two or more of these, or resins such as polyimide resins, acrylic resins, polyether nitrile resins, polyether sulfone resins, polyester resins (PET resins, polyethylene naphthalate resins, etc.), polyvinyl chloride resins, etc. Various resin materials (typically plastic materials) such as olefin resins, polyphenylene sulfide resins, polyetheretherketone resins, polyamide resins (so-called arylamide resins, etc.), polyarylate resins, fluorine resins, polycarbonate resins, cellulose polymers such as cellulose diacetate or cellulose triacetate, vinyl butyral polymers, liquid crystal polymers, carbon materials such as graphene, etc., aluminum oxide, zirconium oxide, titanium oxide, SiO _2. Metal oxides such as ITO (indium tin oxide) and ATO (antimony-doped tin oxide), and mixtures thereof; nitrides such as aluminum nitride, silicon nitride, titanium nitride, gallium nitride, and indium nitride, and their complexes; and inorganic materials such as alkaline glass, alkali-free glass, quartz glass, borosilicate glass, and sapphire glass carbon. The high-refractive-index adhesive layer disclosed herein can be attached to a component (e.g., an optical component) having at least one surface composed of the above materials. Furthermore, the low-refractive-index layer (preferably a low-refractive-index adhesive layer) disclosed herein can be laminated (e.g., bonded) to the various adherends described above.

The high-refractive-index adhesive layer disclosed herein can be used in the following application configuration: after being applied to an adherend, it is not necessary to heat the adhesive to a temperature higher than room temperature (e.g., 20°C to 35°C). Alternatively, if permitted by the type of adherend, a heat treatment may be performed at least at any point after, during, or before the adherend is applied. Heat treatment may be performed to enhance the adhesive's adhesion to the adherend or to promote bonding. The heat treatment temperature can be appropriately set to obtain the desired effect within the range allowed by the constituent material of the adhesive sheet or the type of adherend, taking into account the surface condition of the adherend, etc., for example, it can be around 100°C or below, below 80°C, below 60°C, or below 50°C.

The component or material to which the adhesive layer is attached may be light-transmissive. For the adherend, the advantage of high transparency of the high refractive index adhesive layer disclosed herein can be easily obtained. The total light transmittance of the adherend may be, for example, greater than 50%, or may be greater than 70%. In several ideal embodiments, the total light transmittance of the adherend is greater than 80%, preferably greater than 90%, and more preferably greater than 95% (for example, 95-100%). The high refractive index adhesive layer disclosed herein is suitable for use in a state of being attached to an adherend (for example, an optical component) having a total light transmittance greater than a predetermined value. The total light transmittance is measured using a commercially available transmittance meter in accordance with JIS K 7136:2000. The transmittance meter may be "HAZEMETER HM-150" manufactured by Murakami Color Research Laboratory or an equivalent.

The refractive index of the adherend and the adhesive layer (high-refractive-index adhesive layer or low-refractive-index layer) placed in contact with the adherend can be of the same or different refractive index. For example, by increasing the refractive index of the adhesive layer relative to the refractive index of the adherend, light incident on the adhesive layer from the adherend side at an angle below the critical angle can be refracted toward the front side, thereby increasing frontal brightness. In this case, the refractive index of the adherend can be, for example, 1.55 or less, 1.50 or less, 1.48 or less, 1.45 or less, or less than 1.45. Alternatively, it can be 1.10 or greater, 1.20 or greater, 1.30 or greater, or 1.35 or greater. Furthermore, by having an adherend with a relatively high refractive index relative to the adhesive layer, light incident on the adherend from the adhesive layer side can be refracted toward the front side, thereby increasing the front brightness. In this case, the refractive index of the adherend can be, for example, greater than 1.60, greater than 1.65, or greater than 1.70, and can be, for example, less than 3.00, less than 2.50, or less than 2.00. On the other hand, by reducing the refractive index difference between the adhesive layer and the adherend, light reflection at the interface can be suppressed. In this case, the refractive index of the adherend can be approximately 1.55-1.80, 1.55-1.75, or 1.60-1.70. The refractive index of the adherend can be measured using the same method as the refractive index of the adhesive.

In several ideal embodiments, the adherend may have any of the aforementioned refractive indices and any of the aforementioned total light transmittances. The effects of the disclosed technology are particularly well-suited for light-emitting devices in which the adherend is attached to or laminated with a high-refractive-index adhesive layer and/or a low-refractive-index layer.

The high-refractive-index adhesive layer and low-refractive-index layer disclosed herein can be attached to various adherends, such as those described above, in the form of laminated sheets. Optical applications are a preferred example. More specifically, the laminated sheets disclosed herein can be suitably used as optical adhesive sheets for laminating optical components (optical component lamination) or for manufacturing products incorporating such optical components (optical products). Laminated sheets used in this manner can also be considered spacers placed between layers of an optical laminate.

The above-mentioned optical components refer to components having optical properties (such as polarization, light refraction, light scattering, light reflection, light transmission, light absorption, light diffraction, optical rotation, and visibility). The above-mentioned optical components are not particularly limited as long as they are components with optical properties. Examples include components constituting display devices (image display devices), input devices, and other devices (optical devices) or components used in such devices. Examples include polarizing plates, wavelength plates, phase difference plates, optical compensation films, brightness enhancement films, light guide plates, reflective films, anti-reflective films, hard coating (HC) films, impact absorbing 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 further components formed by layering such components (these are sometimes collectively referred to as "functional films"), etc. Furthermore, the terms "plate" and "film" are intended to include forms such as plate, film, and sheet. For example, "polarizing film" includes "polarizing plate" or "polarizer," and "light guide plate" includes "light guide film" or "light guide sheet." Furthermore, the term "polarizing plate" includes a circularly polarizing plate.

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

The optical component is not particularly limited and may be, for example, a component made of glass, acrylic resin, polycarbonate, polyethylene terephthalate, or metal film (e.g., a sheet, film, or plate-like component). Furthermore, the term "optical component" as used herein also includes components that maintain the visibility of a display device or input device while also performing a decorative or protective function (e.g., design films, decorative films, or surface protection films).

The high-refractive-index adhesive layer disclosed herein (which may be in the form of a laminate with a low-refractive layer) can be used, for example, between an optical film, such as a film having one or more functions of light transmission, reflection, diffusion, waveguiding, focusing, or diffraction, or a fluorescent film, and another optical component (which may be another optical film). Furthermore, it can be suitably used to bond these optical films to these other optical components. In bonding optical films having at least one of the functions of light waveguiding, focusing, or diffraction, the entire bonding layer preferably has a high refractive index, making it an ideal application for the disclosed technology.

The high-refractive-index adhesive layer disclosed herein can be suitably used for bonding optical films such as light-guiding films, diffusion films, fluorescent films, color-adjusting films, prism films, lenticular films, and microlens array films. In these applications, the trend toward miniaturization of optical components or the pursuit of higher performance requires thinning or improved light extraction efficiency. The high-refractive-index adhesive layer disclosed herein can be suitably utilized as an adhesive layer that meets these requirements. More specifically, when bonding light-guiding films or diffusion films, adjusting the refractive index of the adhesive layer (e.g., increasing its refractive index) as a bonding layer can contribute to thinning. When bonding fluorescent films, light extraction efficiency (also known as luminescence efficiency) can be improved by appropriately adjusting the refractive index difference between the fluorescent light emitter and the adhesive. When bonding tinting films, by appropriately adjusting the adhesive's refractive index to minimize the difference with the tinting pigment, scattering components can be reduced, helping to improve light transmittance. When bonding prisms, lenticular films, microlens array films, etc., controlling light diffraction by appropriately adjusting the adhesive's refractive index can help improve brightness and/or viewing angle.

The high-refractive-index adhesive layer disclosed herein (which may be in the form of a laminate with a low-refractive layer) can be suitably used when attached to a high-refractive-index adherend (which may be a high-refractive-index layer or component, etc.), thereby suppressing interfacial reflection with the adherend. The high-refractive-index adhesive layer used in this manner preferably has a small refractive index difference with the high-refractive-index adherend, as described above, and exhibits high adhesion at the interface with the adherend. Furthermore, from the perspective of improving the uniformity of the appearance, the thickness of the adhesive layer should be highly uniform, for example, the surface smoothness of the adhesive surface should be high. When the thickness of the high-refractive-index adherend is small (e.g., 5µm or less, 4µm or less, or 2µm or less), suppressing reflection at the interface is particularly important from the perspective of suppressing coloration or color unevenness caused by interference of reflected light. An example of such a use is the following: in a polarizing plate with a retardation layer comprising a polarizer, a first retardation layer, and a second retardation layer in this order, the polarizer can be used to bond the first retardation layer and/or the first retardation layer to the second retardation layer.

Furthermore, the high-refractive index adhesive layer disclosed herein can be suitably used when attached to a light-emitting layer (e.g., a high-refractive index light-emitting layer primarily composed of inorganic materials) of an optical semiconductor or the like. By reducing the refractive index difference between the light-emitting layer and the high-refractive index adhesive layer, reflection at these interfaces can be suppressed, thereby improving light extraction efficiency. Furthermore, from the perspective of preventing moisture-induced degradation of the self-luminous element, the water absorption rate of the high-refractive index adhesive layer is preferably low. From the perspective of improving brightness, the high-refractive index adhesive layer is preferably low in coloration. This is also advantageous from the perspective of suppressing unintentional coloration caused by the high-refractive index adhesive layer.

The high-refractive-index adhesive layer disclosed herein can be suitably used as a coating layer covering the lens surface, a bonding layer for a component facing the lens surface (e.g., a component having a surface shape corresponding to the lens surface), or a filling layer between the lens surface and the component in microlenses and other lens components (e.g., microlenses constituting a microlens array film or lens components such as camera microlenses) used as components of cameras or light-emitting devices. The high-refractive-index adhesive layer disclosed herein can reduce the refractive index difference with a high-refractive-index lens (e.g., a lens made of a high-refractive-index resin or a lens having a surface layer made of a high-refractive-index resin) even when placed in contact with the lens. This is advantageous from the perspective of reducing the thickness of the lens and products incorporating the lens, and can also help suppress aberrations and increase the Abbe number. In the disclosed technology, the adhesive (a viscoelastic material) that constitutes the high-refractive-index adhesive layer can also be used as the lens resin itself, for example by filling a recess or void in a suitable transparent component.

The manner in which the adhesive layer disclosed herein (which is a high refractive index adhesive layer and/or a low refractive index adhesive layer, and preferably a high refractive index adhesive layer that can also be laminated on a low refractive index layer) is used to bond optical components is not particularly limited. For example, the manner may be as follows: (1) bonding optical components to each other using the adhesive layer disclosed herein; or (2) bonding optical components to components other than optical components using the adhesive layer disclosed herein; or (3) bonding the adhesive layer disclosed herein to an adhesive sheet containing optical components, and bonding the adhesive sheet to an optical component or a component other than an optical component. In the embodiment (3) above, the adhesive sheet having the form of an optical component can be, for example, an adhesive sheet whose support is an optical component (e.g., an optical film). As described above, the adhesive sheet having the form of an optical component as a support can also be considered an adhesive-type optical component (e.g., an adhesive-type optical film). Furthermore, when the adhesive layer disclosed herein is configured as an adhesive sheet having a support and the functional film described above is used as the support, the adhesive sheet can also be considered an "adhesive-type functional film" having the adhesive layer disclosed herein on at least one side of the functional film.

As described above, based on the technology disclosed herein, an optical laminate is provided, comprising the adhesive layer disclosed herein and a member (e.g., a resin film such as an optical film) onto which the adhesive layer is laminated, for example, by lamination. The member onto which the adhesive layer is laminated, for example, by lamination, can have the same refractive index as the material of the adherend. Furthermore, the difference between the refractive index of the adhesive layer and the member (refractive index difference) can be the refractive index difference between the adherend and the adhesive layer. The members constituting the laminate are already described with respect to the members, materials, and adherends described above, and therefore will not be repeated.

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 sheet comprising an adhesive layer; and It has an adhesive surface formed by the adhesive layer; The adhesive layer has a refractive index greater than 1.570, a total light transmittance of 86% or more, and a haze value of 3.0% or less. [2] The adhesive sheet as described in [1] above, wherein the thickness of the adhesive layer is 5µm or more. [3] The adhesive sheet as described in [1] or [2] above, wherein the peeling strength (adhesion) to the glass plate is 3N/25mm or more. [4] The adhesive sheet as described in any one of [1] to [3] above, wherein the arithmetic average roughness Ra of the adhesive surface is 100nm or less. [5] The adhesive sheet as described in any one of [1] to [4] above, wherein the water absorption rate of the adhesive layer is 1.0% or less. [6] The adhesive sheet as described in any one of [1] to [5] above, which is in the form of a laminate comprising the adhesive layer and a light-transmitting substrate. [7] The adhesive sheet as described in [6] above, wherein the light-transmitting substrate is a resin film. [8] The adhesive sheet as described in any one of [1] to [5] above, which is a double-sided adhesive sheet composed of the adhesive layer. [9] An adhesive sheet with a peel-off liner, comprising: The adhesive sheet as described in any one of [1] to [8] above; and A peel-off liner disposed on the adhesive surface of the adhesive sheet. [10] An adhesive composition for forming an adhesive layer of an adhesive sheet as described in any one of [1] to [8] above.

[11] An adhesive composition comprising: an acrylic polymer (A) comprising an aromatic ring-containing monomer (m1) as a monomer unit; and an additive (H _RO ) which is an organic material having a higher refractive index than the acrylic polymer (A). [12] The adhesive composition of [11] above, wherein the refractive index of the additive (H _RO ) is greater than 1.60. [13] The adhesive composition of [11] or [12] above, wherein the content of the additive (H _RO ) relative to 100 parts by weight of the acrylic polymer (A) is greater than 0 parts by weight and less than 60 parts by weight. [14] The adhesive composition of any one of the above [11] to [13], wherein the additive (H _RO ) comprises at least one compound selected from the group consisting of aromatic ring-containing compounds and heterocyclic ring-containing compounds. [15] The adhesive composition of any one of the above [11] to [14], wherein the additive (H _RO ) comprises a compound having two or more aromatic rings in one molecule. [16] The adhesive composition of the above [15], wherein the additive (H _RO ) comprises a compound satisfying at least one of the following as the compound having two or more aromatic rings in one molecule: (i) a structure comprising two non-condensed aromatic rings directly chemically bonded; and (ii) a structure comprising two condensed aromatic rings. [17] The adhesive composition of any one of the above [11] to [16], wherein the content of the aromatic ring-containing monomer (m1) in the monomer components constituting the acrylic polymer (A) is 50% by weight or more. [18] The adhesive composition of any one of the above [11] to [17], wherein the content of the aromatic ring-containing monomer (m1) in the monomer components constituting the acrylic polymer (A) is greater than 70% by weight and less than 100% by weight, and more than 50% by weight of the aromatic ring-containing monomer (m1) is a monomer having a glass transition temperature of 10°C or less in its homopolymer. [19] An adhesive composition as described in any one of [11] to [18] above, wherein the monomer component constituting the acrylic polymer (A) further contains a monomer (m2), wherein the monomer (m2) has at least one of a hydroxyl group and a carboxyl group. [20] An adhesive composition as described in any one of [11] to [18] above, which is used to form an adhesive layer of an adhesive sheet as described in any one of [1] to [8] above. [21] An adhesive formed from the adhesive composition as described in any one of [11] to [20] above, and having a refractive index higher than 1.570. [22] An adhesive sheet comprising an adhesive layer formed from an adhesive, wherein the adhesive is formed from the adhesive composition as described in any one of [11] to [20] above. [23] The adhesive sheet as described in [22] above, wherein the haze value of the adhesive layer is less than 1.0%.

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

[34] A light-emitting device comprising: a self-luminous element; a low refractive index layer disposed on a visually observable side of the self-luminous element; and a high refractive index adhesive layer laminated in direct contact with the low refractive index layer; the high refractive index adhesive layer having a refractive index _n1 greater than 1.570, a total light transmittance of 86% or greater, and a haze value of 3.0% or less. [35] The light-emitting device of [34] above, wherein the ratio ( _n1 / _n2 ) of the refractive index _n1 of the high refractive index adhesive layer to the refractive index _n2 of the low refractive index layer is greater than 1.05. [36] The light-emitting device of [35] or [36] above, wherein the arithmetic average roughness Ra of the surface of the high refractive index adhesive layer is less than 100 nm. [37] The light-emitting device of any one of [34] to [36] above, wherein the ratio (T ₁ /T ₂ ) of the thickness T ₁ of the high refractive index adhesive layer to the thickness T ₂ of the low refractive index layer is 0.5 to 5. [38] The light-emitting device of any one of [34] to [37] above, wherein the thickness T ₁ of the high refractive index adhesive layer is greater than 5 µm. [39] A light-emitting device as described in any one of [34] to [38] above, wherein the total light transmittance of the laminate composed of the high refractive index adhesive layer and the low refractive index layer is 86% or more and the haze value is 3.0% or less. [40] A light-emitting device as described in any one of [34] to [39] above, wherein the high refractive index adhesive layer is a layer formed by the adhesive composition as described in any one of [11] to [18] above. [41] A light-emitting device as described in any one of [34] to [40] above, wherein the high refractive index adhesive layer is an adhesive layer in the adhesive sheet as described in any one of [1] to [5] above.

[101] An adhesive comprising an acrylic polymer (F), wherein the acrylic polymer (F) comprises a fluorine-containing acrylic monomer (M1) as a monomer unit; the adhesive has a refractive index of 1.46 or less, and a storage elastic modulus G' at 25°C of 1.0 kPa or more and 400 kPa or less. [102] The adhesive as described in [101] above, wherein the content of the fluorine-containing acrylic monomer (M1) in the monomer components constituting the acrylic polymer (F) is 25 wt% or more. [103] The adhesive as described in [101] or [102] above, wherein the fluorine-containing acrylic monomer (M1) comprises a fluorine-containing alkyl (meth)acrylate. [104] The adhesive of any one of [101] to [103] above, wherein the acrylic polymer (F) contains a hydroxyl group-containing monomer as a monomer unit. [105] A laminate comprising: a low-refractive-index adhesive layer formed from the adhesive of any one of [101] to [104] above; and a high-refractive-index adhesive layer laminated on the low-refractive-index adhesive layer. [106] The laminate of [105] above, wherein the ratio (n ₁ /n ₂ ) of the refractive index n ₁ of the high-refractive-index adhesive layer to the refractive index n ₂ of the low-refractive-index adhesive layer is 1.02 or greater. [107] The laminated sheet of [105] or [106] above, wherein the refractive index _n1 of the high refractive index adhesive layer is greater than 1.570. [108] The laminated sheet of any one of [105] to [107] above, wherein the storage elastic modulus G' of the high refractive index adhesive layer at 25°C is less than 700 kPa. [109] The laminated sheet of any one of [105] to [108] above, wherein the total light transmittance is greater than 86% and the haze value is less than 3.0%. [110] A light-emitting device comprising: a self-luminous element and a laminate as described in any one of [105] to [109]; wherein the laminate is disposed on a side closer to the viewer than the self-luminous element. [111] An adhesive as described in any one of [101] to [104], which is used to form the viscoelastic layer _V2 in the interlayer as described in any one of [28] to [31]. [112] An adhesive as described in any one of [101] to [104], which is used to form the low refractive index layer in the light-emitting device as described in any one of [34] to [41].

EXAMPLES The following describes several experimental experiments related to the present invention. In the following description, "parts" and "%" representing amounts or contents are by weight unless otherwise specified.

<Preparation of acrylic adhesive composition C1> In a four-necked flask equipped with a stirring blade, a thermometer, a nitrogen inlet tube and a cooler, m-phenoxybenzyl acrylate (produced by Kyoeisha Chemical Co., Ltd., trade name "LIGHT ACRYLATE") as a monomer component was added. 95 parts of POB-A (refractive index: 1.566, homopolymer Tg: -35°C; hereinafter referred to as "POB-A"), 5 parts of 4-hydroxybutyl acrylate (4HBA), 0.2 parts of 2,2'-azobisisobutyronitrile (AIBN) as a polymerization initiator, and 100 parts of toluene as a polymerization solvent were added to the flask. The polymerization reaction was carried out for 6 hours while slowly stirring and introducing nitrogen. The temperature in the flask was maintained at approximately 60°C to prepare a 50% acrylic polymer A1 solution. The average molecular weight (Mw) of the acrylic polymer A1 was 500,000. In the acrylic polymer A1, the Tg (i.e., Tg _T ) due to the monomer composition is -35°C, and the Tg (i.e., Tg _m1 ) due to the aromatic ring-containing monomer composition is -35°C. The acrylic polymer A1 solution (50%) was diluted to 30% with ethyl acetate. To this solution (100 parts of non-volatile components), 10 parts of a 1% ethyl acetate solution of a trifunctional isocyanate compound (manufactured by Tosoh Corporation, trade name "Coronate HX") of hexamethylene diisocyanate as a crosslinking agent (0.1 parts of non-volatile components), 2 parts of acetylacetone as a crosslinking retarder, and 1 part of a 1% ethyl acetate solution of iron (III) acetylacetonate (0.01 parts of non-volatile components) as a crosslinking catalyst were added and stirred to prepare acrylic adhesive composition C1.

<Preparation of Acrylic Adhesive Composition C2> Into a four-necked flask equipped with a stirrer, thermometer, nitrogen inlet, and cooler were added 72 parts of POB-A as monomer components, 23 parts of 1-naphthyl methacrylate (Kyoeisha Chemical Co., Ltd., trade name "LIGHT ACRYLATE NMT-A," refractive index: 1.595, homopolymer Tg: 31°C; hereinafter referred to as "NMT-A"), 5 parts of 4HBA, 0.2 parts of AIBN as a polymerization initiator, and 100 parts of toluene as a polymerization solvent. While gently stirring and introducing nitrogen, the polymerization reaction was allowed to proceed for 6 hours, maintaining the liquid temperature in the flask at approximately 60°C, to prepare a 50% solution of acrylic polymer A2. The polymerization average molecular weight (Mw) of the acrylic polymer A2 was 500,000. Into a separable flask equipped with a thermometer, stirrer, reflux cooler, and nitrogen inlet were placed 20 parts of POB-A as monomer components, 80 parts of NMT-A, 0.2 parts of AIBN as a polymerization initiator, 3.5 parts of α-thioglycerol as a chain transfer agent, and 67 parts of methyl ethyl ketone. Nitrogen was then introduced and nitrogen substitution was performed while stirring for approximately one hour. The flask was then heated to 70°C and allowed to react for 12 hours, yielding an acrylic oligomer (oligomer B) with a weight average molecular weight (Mw) of 4000 and a refractive index of 1.63. The acrylic polymer A2 solution (50%) was diluted to 30% with ethyl acetate. To 334 parts of this solution (100 parts of non-volatile components), 20 parts of the prepared oligomer B, 10 parts of a 1% ethyl acetate solution of a trifunctional isocyanate compound of hexamethylene diisocyanate (manufactured by Tosoh Corporation, trade name "Coronate HX") as a crosslinking agent (0.1 parts of non-volatile components), 2 parts of acetylacetone as a crosslinking retarder, and 1 part of a 1% ethyl acetate solution of iron (III) acetylacetonate (0.01 parts of non-volatile components) as a crosslinking catalyst were added and stirred to prepare acrylic adhesive composition C2.

Preparation of Acrylic Adhesive Composition C3 Into a four-necked flask equipped with a stirring blade, thermometer, nitrogen inlet, and cooler were added 65 parts of 2-ethylhexyl acrylate, 30 parts of 1H,1H,5H-octafluoropentyl acrylate (Viscoat 8F, manufactured by Osaka Organic Chemical Industry Co., Ltd.), 3 parts of N-vinyl-2-pyrrolidone (NVP, manufactured by Nippon Catalyst), and 2 parts of 4HBA as monomer components; 0.2 parts of AIBN as a polymerization initiator; and 200 parts of ethyl acetate as a polymerization solvent. The mixture was gently stirred while nitrogen was introduced, and the temperature in the flask was maintained at approximately 60°C. A polymerization reaction was carried out for 9 hours to prepare a 33% solution of acrylic polymer A3. The acrylic polymer A3 had a polymerization average molecular weight (Mw) of 550,000. A 33% solution of the acrylic polymer A3 was diluted to 30% with ethyl acetate. A 1% ethyl acetate solution of hexamethylene diisocyanate (Coronate HX, a trifunctional isocyanate compound, manufactured by Tosoh Corporation) was added as a crosslinking agent to 100 parts of the non-volatile components (solids) and stirred to prepare acrylic adhesive composition C3.

<Adhesive Sheet Preparation> (Example 1) The acrylic adhesive composition C1 prepared above was applied to the silicone-treated surface of a single-sided silicone-treated polyethylene terephthalate (PET) film R1 (50µm thick). The film was heated at 130°C for 2 minutes to form a 25µm thick adhesive layer. The silicone-treated surface of a single-sided silicone-treated PET film R2 (38µm thick) was then laminated to the adhesive layer. This yielded an adhesive layer (high refractive index adhesive layer) protected on both sides by PET films (peel liners) R1 and R2. Furthermore, compared to the release liner R1, the release liner R2 peels relatively easily. The prepared acrylic adhesive composition C3 was applied to the silicone-treated surface of a single-sided silicone-treated PET film R1 (50µm thick) and heated at 130°C for 2 minutes to form a 10µm thick adhesive layer. The silicone-treated surface of a single-sided silicone-treated PET film R2 (38µm thick) was then laminated to the adhesive layer. In this manner, an adhesive layer (low-refractive index adhesive layer) is obtained, protected on both sides by PET films (peel-off liners) R1 and R2. The peel-off liner R2 is removed from the high-refractive index adhesive layer and the low-refractive index adhesive layer, and the adhesive surfaces are bonded together and pressed with a hand roller. The laminate is then autoclaved at 50°C and 0.60 MPa for 30 minutes, followed by aging at 50°C for 48 hours. In the above manner, a laminated sheet (a double-sided adhesive sheet without a substrate) having a two-layer structure of a high-refractive-index adhesive layer and a low-refractive-index adhesive layer is obtained. The surface of the adhesive sheet is protected by two peel pads R1.

(Example 2) Except for changing the adhesive composition used to form each adhesive layer and the thickness of each adhesive layer as shown in Table 1, a laminated sheet (a double-sided adhesive sheet without a substrate) having a two-layer structure of a high-refractive-index adhesive layer/a low-refractive-index adhesive layer was obtained in the same manner as in Example 1.

(Examples 3-5) Using the same method as Example 1, single-layer adhesive layers composed of acrylic adhesive compositions C1-C3 and having the thicknesses shown in Table 1 were prepared as adhesive sheets for Examples 3-5.

The obtained adhesive sheets were fully acclimatized to an environment of 23°C and 50% RH before use in the following measurements and evaluations.

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

(Storage Modulus G') Each adhesive layer was laminated to a thickness of approximately 1.5 mm to prepare a test specimen. Dynamic viscoelasticity measurements were performed using a TA Instruments ARES under the following conditions. The storage modulus G' at 25°C was calculated from the measurement results. The results are listed in Table 1. [Test Conditions] Deformation Mode: Torsion Test Frequency: 1 Hz Heating Rate: 5°C/min Shape: Parallel Plate, 7.9 mm Ø

Total Light Transmittance and Haze Values Test sheets of each example were 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 these test sheets were measured using a haze meter (trade name "HAZEMETER HM-150," manufactured by Murakami Color Research Laboratory) at 23°C. The total light transmittance and haze of the alkali-free glass were subtracted from the measured values to obtain the total light transmittance and haze values of the adhesive sheet. The results are listed in Table 1.

(Peel Strength on Glass Plate) Under a test environment of 23°C and 50% RH, the peel liner was peeled from one side of the adhesive sheet (the surface of the adhesive layer formed from adhesive composition C3 in Examples 1 and 2). A 50µm thick PET film was then laminated to form a test piece, which was 25mm wide and 100mm long. The peel liner was peeled from the other side of the test piece, and a 2kg roller was used to press the surface of an alkaline glass plate (1.35mm thick, frosted blue, manufactured by Matsunami Glass Industries) in a reciprocating motion. The samples were placed in this environment for 30 minutes, then placed in a pressurized degassing device (autoclave) and autoclaved for 30 minutes at 50°C and 0.5 MPa. After 24 hours in a 23°C, 50% RH atmosphere, the peel strength (adhesion) [N/25mm] was measured using a universal tensile-compression tester (TG-1kN, manufactured by Minebea) in accordance with JIS Z 0237:2000, at a tensile speed of 300 mm/min and a peel angle of 180 degrees.

[Table 1]

As shown in Table 1, the adhesive sheets of Examples 1-4 included an adhesive layer V1 having a refractive index n _V1 higher than 1.570 (a high-refractive-index adhesive layer) and exhibited high transparency. These adhesive sheets exhibited practical peel strength suitable for bonding optical components.

<Evaluation of Front Brightness Enhancement Effect> Each adhesive sheet was attached to a white LED light source. The light source was illuminated in a darkroom for at least 30 minutes to stabilize. The front brightness of the area where the adhesive sheet was attached was measured using a spectroradiometer SR-UL1R (manufactured by TOPCON TECHNOHOUSE Co., Ltd.). The average of these three measurements was used. Samples that increased the brightness of the light source without the adhesive sheet by 10% or more were rated G (Good), while those with an improvement of less than 10% were rated P (Poor). The results are listed in Table 2.

[Table 2]

As shown in Table 2, the adhesive sheets (laminated sheets) of Examples 1 and 2, which have a laminated structure consisting of a low-refractive-index adhesive layer (Example 5) and high-refractive-index adhesive layers (Examples 3 and 4), showed an improvement in frontal brightness of more than 10% compared to the case without the adhesive sheet. The adhesive sheets of Examples 3-5, which have a single-layer adhesive layer, did not show the same frontal brightness improvement effect as the adhesive sheets alone.

<Preparation of Acrylic Adhesive Composition C4> Into a four-necked flask equipped with a stirring blade, thermometer, nitrogen inlet, and cooler, 79 parts of POB-A (monomer components), 20 parts of n-butyl acrylate, 1 part of 4HBA, 0.2 parts of AIBN (polymerization initiator), and 100 parts of toluene (polymerization solvent) were added. While slowly stirring and introducing nitrogen, the polymerization reaction was allowed to proceed for 6 hours while maintaining the liquid temperature in the flask at approximately 60°C. This resulted in a 50% solution of acrylic polymer A4. The Mw of this acrylic polymer A4 was 520,000. The acrylic polymer A4 solution (50%) was diluted to 30% with ethyl acetate. To 334 parts of this solution (100 parts non-volatile components), 10 parts of a 1% ethyl acetate solution of Coronate HX (0.1 parts non-volatile components) as a crosslinking agent, 2 parts of acetylacetone (0.01 parts non-volatile components) as a crosslinking delay agent, and 1 part of a 1% ethyl acetate solution of iron (III) acetylacetonate (0.01 parts non-volatile components) as a crosslinking catalyst were added and stirred to prepare acrylic adhesive composition C4.

<Preparation of Acrylic Adhesive Composition C5> A 50% acrylic polymer A5 solution was prepared in the same manner as the acrylic polymer A4 solution, except that the monomer composition (weight ratio) was changed to POB-A/ethyl carbitol acrylate (CBA)/4HBA = 79/20/1. The Mw of this acrylic polymer A5 was 460,000. Acrylic adhesive composition C5 was prepared in the same manner as acrylic adhesive composition C4, except that the acrylic polymer A5 solution was used instead of the acrylic polymer A4 solution.

<Preparation of Acrylic Adhesive Composition C6> A 50% acrylic polymer A6 solution was prepared in the same manner as the acrylic polymer A4 solution, except that the monomer composition (weight ratio) was changed to P2H-A/4HBA = 99/1. "P2H-A" in the above monomer composition represents phenoxydiethylene glycol acrylate (Kyoeisha Chemical Co., Ltd., trade name "LIGHT ACRYLATE P2H-A," refractive index: 1.510, homopolymer Tg: -35°C). The Mw of this acrylic polymer A6 was 1,000,000. A solution (50%) of acrylic polymer A6 was diluted to 30% with ethyl acetate. To 334 parts of this solution (100 parts of non-volatile components), 20 parts of 6-ethylacrylate- _dinaphtho [2,1-b:1',2'-d]thiophene (6-acryloyloxyethyldinaphthothiophene, code: 6EDNTA, refractive index: 1.722, manufactured by SUGAI Chemical Industries, Ltd.) as an additive (HRO), 10 parts of a 1% ethyl acetate solution of Coronate HX as a crosslinking agent (0.1 parts of non-volatile components), 2 parts of acetylacetone as a crosslinking delay agent, and 1 part of a 1% ethyl acetate solution of iron (III) acetylacetonate as a crosslinking catalyst (0.01 parts of non-volatile components) were added and stirred to prepare acrylic adhesive composition C6.

<Preparation of Acrylic Adhesive Composition C7> A 50% acrylic polymer A7 solution was prepared in the same manner as the acrylic polymer A3 solution, except that the monomer composition (weight ratio) was changed to 2EHA/Viscoat 13F/4HBA = 49/50/1. "Viscoat 13F" in the above monomer composition represents 1H,1H,2H,2H-tridecafluorooctyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd., trade name "Viscoat 13F"). The Mw of this acrylic polymer A7 was 550,000. Acrylic adhesive composition C7 was prepared in the same manner as acrylic adhesive composition C3, except that the acrylic polymer A7 solution was used instead of the acrylic polymer A3 solution.

<Adhesive Sheet Preparation> (Examples 6-8) A laminated sheet (substrate-free double-sided adhesive sheet) with a two-layer structure of high-refractive index adhesive layer/low-refractive index adhesive layer was obtained in the same manner as in Example 1, except that the adhesive composition used to form each adhesive layer and the thickness of each adhesive layer were as shown in Table 3.

After the adhesive sheets obtained in Examples 6-8 were fully acclimated to an environment of 23°C and 50% RH, the various items were measured and evaluated in the same manner as in "Measurement and Evaluation (1)" above. The results are listed in Table 3.

[Table 3]

As shown in Table 3, the adhesive sheets of Examples 6-8 exhibited high transparency in a laminated structure consisting of an adhesive layer with a refractive index _n1 higher than 1.570 (a high-refractive-index adhesive layer) and a low-refractive-index layer. These adhesive sheets exhibited practical peel strength suitable for bonding optical components.

The above detailed description of the specific embodiments of the present invention is intended to be illustrative only and does not limit the scope of the patent application. The technology described in the patent application includes various modifications and variations of the specific embodiments described above.

2: Double-sided adhesive sheet without substrate 10: Laminated sheet (adhesive sheet) 10A: First surface (first adhesive surface) 10B: Second surface (second adhesive surface) 11: High-refractive-index adhesive layer 12: Low-refractive-index adhesive layer (low-refractive-index layer) 31, 32: Peel-off liner 70: Self-luminous element 80: Covering window member 100: Light-emitting device

Figure 1 is a cross-sectional view schematically illustrating the structure of a light-emitting device according to one embodiment. Figure 2 is a cross-sectional view schematically illustrating the structure of a laminate used in a light-emitting device according to one embodiment.

2: Double-sided adhesive sheet without substrate

10: Laminated Sheet (Adhesive Sheet)

10A: First surface (first adhesive surface)

10B: Second surface (second adhesive surface)

11: High refractive index adhesive layer

12: Low refractive index adhesive layer (low refractive index layer)

70: Self-luminous element

80: Covering window components

100: Light-emitting device

Claims (8)

A light-emitting device comprises: a self-luminous element; a low-refractive-index layer disposed on a visually observable side of the self-luminous element; and a high-refractive-index adhesive layer laminated in direct contact with the low-refractive-index layer; the high-refractive-index adhesive layer having a thickness _T1 of 5µm or greater; and the high-refractive-index adhesive layer having a refractive index _n1 greater than 1.570, a total light transmittance of 86% or greater, and a haze value of 3.0% or less. The light-emitting device of claim 1, wherein a ratio (n ₁ /n ₂ ) of a refractive index n ₁ of the high-refractive-index adhesive layer to a refractive index n ₂ of the low-refractive-index layer is greater than or equal to 1.05. The light-emitting device of claim 1 or 2, wherein the arithmetic average roughness Ra of the surface of the high refractive index adhesive layer is less than 100 nm. The light-emitting device of claim 1 or 2, wherein a ratio (T ₁ /T ₂ ) of a thickness T ₁ of the high-refractive-index adhesive layer to a thickness T ₂ of the low-refractive-index layer is 0.5-5. The light-emitting device of claim 1 or 2, wherein the laminate composed of the high-refractive-index adhesive layer and the low-refractive-index layer has a total light transmittance of 86% or more and a haze value of 3.0% or less. The light-emitting device of claim 1 or 2, wherein the high-refractive-index adhesive layer comprises an acrylic polymer as a base polymer, and the monomer components constituting the acrylic polymer comprise more than 50% by weight of aromatic ring-containing monomers. The light-emitting device of claim 6, wherein the aromatic ring-containing monomer comprises: a monomer containing multiple aromatic rings having two or more aromatic rings in one molecule. The light-emitting device of claim 6, wherein the content of the inorganic particles in the high refractive index adhesive layer is less than 5 parts by weight relative to 100 parts by weight of the base polymer.