WO2024070789A1

WO2024070789A1 - Adhesive and adhesive sheet

Info

Publication number: WO2024070789A1
Application number: PCT/JP2023/033830
Authority: WO
Inventors: 慎太郎野依; 普史形見; 真也山本; 拓也永田
Original assignee: 日東電工株式会社
Priority date: 2022-09-29
Filing date: 2023-09-19
Publication date: 2024-04-04
Also published as: JP2024050320A

Abstract

The present invention provides an adhesive which has low refractive index and high flexibility. Provided is an adhesive that has a refractive index of 1.450 or less and a storage elastic modulus (G'(-20°C)) of 2.0 × 10⁶ Pa or less at -20°C.

Description

Adhesives and adhesive sheets

The present invention relates to an adhesive and an adhesive sheet having the adhesive. This application claims priority to Japanese Patent Application No. 2022-157123, filed on September 29, 2022, the entire contents of which are incorporated herein by reference.

Generally, adhesives (also called pressure-sensitive adhesives; the same applies below) are in a soft solid (viscoelastic) state at temperatures around room temperature, and have the property of easily adhering to an adherend when pressure is applied. Taking advantage of these properties, adhesives are widely used for purposes such as joining, fixing, and protection in a variety of industrial fields, from home appliances to automobiles, various machines, electrical equipment, and electronic devices. One example of the use of adhesives is the joining of polarizing films, retardation films, cover window components, and various other light-transmitting components to other components in displays such as liquid crystal displays and organic EL displays. Technical documents related to adhesives include Patent Documents 1 to 4.

Japanese Patent Application Publication No. 2019-210343 Japanese Patent Application Publication No. 2020-111740 Japanese Patent No. 6014781 Japanese Patent Application Publication No. 2018-193553

Patent Documents 1 to 4 propose adhesives that contain (meth)acrylate copolymers that contain structural units derived from fluorine-containing acrylic monomers. All of these adhesives focus on sebum resistance and/or chemical resistance, and do not take into account the refractive index. Adhesives with low refractive indexes can be useful, for example, for exerting a function of controlling the behavior of light (reflection, waveguiding, diffraction) by utilizing the relationship between the refractive indices of adjacent materials.

Incidentally, adhesives having good flexibility can be preferably used depending on the application location and the mode of use. For example, in recent years, foldable displays and rollable displays have been put to practical use as displays such as organic EL display devices used in electronic devices such as smartphones, and adhesives used for the above applications also need to have the flexibility to conform to the adherend that is repeatedly folded. Adhesives with excellent flexibility easily conform to and adhere to curved surfaces such as three-dimensional shapes, and are suitable for use in electronic devices with curved shapes. If the flexibility of adhesives with a low refractive index can be increased, they can also be used in applications requiring the above-mentioned flexibility, and are useful.

The present invention therefore aims to provide an adhesive that has a low refractive index and high flexibility. Another related object is to provide an adhesive sheet that includes an adhesive layer made of such an adhesive.

According to this specification, a pressure-sensitive adhesive is provided that has a refractive index of 1.450 or less and a storage modulus at -20°C (G'(-20°C)) of 2.0 x 10 ⁶ Pa or less. The pressure-sensitive adhesive has a low refractive index while the storage modulus (G'(-20°C) is controlled to a low range, so that it can achieve both a low refractive index and flexibility. Such a pressure-sensitive adhesive is suitable for bonding, fixing, protection, etc. in applications such as foldable displays, where a low refractive index is desirable and flexibility that can withstand repeated folding operations is required.

In some embodiments, the pressure-sensitive adhesive preferably has a ratio (G'(-20°C)/E _B ) of the storage modulus (G'(-20°C)) [Pa] to the elongation at break (E _B ) [%] of 20 to 2000. The pressure-sensitive adhesive has a low refractive index and, since the ratio (G'(-20°C)/E _B ) is in the above range, can exhibit good flexibility.

In some embodiments, the adhesive is an adhesive formed from an active energy ray (e.g., ultraviolet ray) curable adhesive composition. Such adhesives are preferred because they tend to achieve high surface smoothness on the adhesive surface and are therefore likely to provide good optical properties.

The adhesive disclosed herein may contain an acrylic polymer (F). The monomer component constituting the acrylic polymer (F) includes a fluorine-containing acrylic monomer (Mf). The acrylic polymer (F) containing the fluorine-containing acrylic monomer (Mf) as a monomer component can effectively contribute to lowering the refractive index of the adhesive.

In some embodiments, the monomer components constituting the acrylic polymer (F) further include an acid-free hydrophilic monomer (Mh). The acid-free hydrophilic monomer (Mh) can be useful for achieving a good balance between the flexibility of the adhesive and other properties (e.g., at least one of the adhesive properties, optical properties, etc.).

As the hydrophilic monomer (Mh), for example, a monomer having at least one group selected from the group consisting of a hydroxyl group, an amide group, and a (poly)oxy C _1-2 alkylene group can be preferably used. The technology disclosed herein can be preferably carried out in an embodiment using such a hydrophilic monomer (Mh).

The technology disclosed in this specification may include an adhesive and an adhesive composition used to form the adhesive, an adhesive sheet containing the adhesive (for example, an adhesive sheet having an adhesive layer made of the above-mentioned adhesive), an adhesive sheet with a release liner in which the adhesive surface of the adhesive sheet is protected by a release liner, an optical member containing the above-mentioned adhesive layer, and the like.

In some embodiments, the content of the hydrophilic monomer (Mh) in the monomer component is preferably greater than 5.0% by weight. By using an acid-free hydrophilic monomer (Mh) in the above content, it is possible to improve the humidity and heat whitening resistance of the adhesive (the property of being less likely to whiten even when exposed to a humid and hot environment) while suppressing a decrease in flexibility.

In some embodiments, the hydrophilic monomer (Mh) preferably contains a low Tg hydrophilic monomer (Mh _L ) having a homopolymer glass transition temperature of 40° C. or less. It is advantageous from the viewpoint of suppressing an increase in storage modulus accompanying the use of the hydrophilic monomer (Mh) that at least a part of the hydrophilic monomer (Mh) is the low Tg hydrophilic monomer (Mh _L ). The content of the low Tg hydrophilic monomer (Mh _L ) in the monomer component is preferably more than 2.0% by weight from the viewpoint of easily exerting the effect of use.

In some embodiments, the monomer component constituting the acrylic polymer (F) is represented by the following formula (1):
_CH2 = ^CR1COOR2 ( ¹ )
(wherein R ¹ is a hydrogen atom or a methyl group, and R ² is a chain alkyl group having 4 to 18 carbon atoms);
The alkyl (meth)acrylate may be useful for adjusting the properties of the adhesive.

In some embodiments, the monomer components constituting the acrylic polymer (F) preferably have an acidic functional group-containing monomer content of less than 2.0% by weight. Limiting the amount of acidic functional group-containing monomer used can be advantageous from the standpoint of improving the flexibility of the adhesive (particularly flexibility at low temperatures).

According to this specification, there is provided an adhesive sheet including an adhesive layer made of any of the adhesives disclosed herein (which may be an adhesive formed from any of the adhesive compositions disclosed herein). The adhesives disclosed herein, in the form of the adhesive sheet, may be preferably used in applications requiring resistance to moist heat whitening at a low refractive index, such as optical applications.

In some embodiments, the adhesive layer has a haze of less than 3.0% after a wet heat test in which the adhesive layer is held in a wet heat environment of 85°C and 85% RH for 240 hours. Adhesive sheets that exhibit such wet heat whitening resistance can be preferably used in applications requiring wet heat whitening resistance at a low refractive index, such as optical applications.

In some embodiments, the adhesive sheet has a peel strength against a glass plate (tensile speed 300 mm/min, peel angle 180 degrees) of 0.1 N/25 mm or more. Adhesive sheets exhibiting such peel strength can be preferably used for bonding, fixing, protection, etc. in applications requiring resistance to moist heat whitening at a low refractive index, such as optical applications.

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

1 is a cross-sectional view illustrating a schematic configuration of a pressure-sensitive adhesive sheet according to an embodiment. FIG. 4 is a cross-sectional view illustrating a schematic configuration of a pressure-sensitive adhesive sheet according to another embodiment. FIG. 4 is a cross-sectional view illustrating a schematic configuration of a pressure-sensitive adhesive sheet according to another embodiment.

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

In this specification, "monomer components constituting a polymer" refers to monomers that constitute the repeating units of the polymer in the adhesive formed from the adhesive composition, regardless of whether the monomers are contained in the adhesive composition in the form of a preformed polymer (which may be an oligomer) or in the form of an unpolymerized monomer. In other words, the monomer components constituting the polymer may be contained in the adhesive composition in the form of a polymer, an unpolymer, or a partially polymerized product.

In this specification, the "base polymer" of an adhesive refers to the main component of the rubber-like polymer contained in the adhesive, and is not to be interpreted in any other restrictive manner. The rubber-like polymer refers to a polymer that exhibits rubber elasticity in a temperature range around room temperature. In addition, in this specification, the "main component" refers to a component that is contained in an amount of more than 50% by weight, unless otherwise specified.

In this specification, "acrylic polymer" refers to a polymer that contains, as a monomer unit constituting the polymer, a monomer unit derived from a monomer having at least one (meth)acryloyl group in one molecule. Hereinafter, a monomer having at least one (meth)acryloyl group in one molecule is also referred to as an "acrylic monomer". Therefore, in this specification, an acrylic polymer is defined as a polymer that contains a monomer unit derived from an acrylic monomer. A typical example of an acrylic polymer is a polymer in which the proportion of acrylic monomers in all monomers used in the synthesis of the acrylic polymer is more than 50% by weight (preferably more than 70% by weight, for example more than 80% by weight or more than 90% by weight).

In this specification, "(meth)acryloyl" refers collectively to acryloyl and methacryloyl. Similarly, "(meth)acrylate" refers collectively to acrylate and methacrylate, and "(meth)acrylic" refers collectively to acrylic and methacrylic. Therefore, the concept of acrylic monomers here can include both monomers having an acryloyl group (acrylic monomers) and monomers having a methacryloyl group (methacrylic monomers).

<Characteristics of adhesive>
(Refractive Index)
The refractive index of the adhesive disclosed herein is 1.450 or less, for example, 1.300 or more and 1.450 or less. According to the technology disclosed herein, an adhesive having such a refractive index, an adhesive composition capable of forming the adhesive, and an adhesive sheet including the adhesive can be provided. In some embodiments, the refractive index of the adhesive is preferably, for example, 1.440 or less, more preferably 1.430 or less, and may be 1.425 or less or 1.424 or less. In some embodiments, the refractive index of the adhesive may be, for example, 1.420 or less, 1.410 or less, 1.400 or less, or less than 1.400 (for example, less than 1.390). In addition, in terms of the availability of materials and the ease of compatibility with other properties (e.g., resistance to wet heat whitening and flexibility), in some embodiments, the refractive index of the adhesive may be, for example, 1.320 or more, 1.350 or more, 1.360 or more, 1.370 or more, 1.380 or more, 1.400 or more, 1.410 or more, 1.420 or more, or 1.430 or more. The technology disclosed herein can be preferably implemented in an embodiment in which the refractive index of the adhesive is, for example, 1.350 or more and 1.450 or less, or 1.380 or more and 1.450 or less, or 1.400 or more and 1.440 or less. The refractive index of the adhesive can be adjusted, for example, by the composition of the adhesive (e.g., the composition ratio of the monomer components constituting the base polymer of the adhesive).

In this specification, the refractive index of an adhesive refers to the refractive index of the surface (adhesive surface) of the adhesive. The refractive index of an adhesive can be measured using a prism coupler at a measurement temperature of 25°C and a measurement wavelength of 594 nm. As the prism coupler, a commercially available measuring device can be used, for example, the model "2010M" manufactured by Metricon or an equivalent product. As the measurement sample, an adhesive layer made of the adhesive to be evaluated can be used. Specifically, the refractive index of an adhesive can be measured by the method described in the Examples below.

(Storage Modulus)
The adhesive disclosed herein has a storage modulus at -20°C (G'(-20°C)) of 2.0 x 10 ⁶ Pa or less. An adhesive with a limited G'(-20°C) in this way can be, for example, a low refractive index adhesive having flexibility suitable for repeated folding operations in a wide temperature range including a low temperature range. From the viewpoint of obtaining higher flexibility, in some embodiments, the G'(-20°C) of the adhesive is preferably 1.5 x 10 ⁶ Pa or less, more preferably 1.0 x 10 ⁶ Pa or less, may be 8.0 x 10 ⁵ Pa or less, may be 7.0 x 10 ⁵ Pa or less, may be 6.0 x 10 ⁵ Pa or less, may be 5.5 x 10 ⁵ Pa or less, or may be 5.0 x 10 ⁵ Pa or less. The lower limit of the G'(-20°C) of the adhesive is not particularly limited, and may be, for example, 5.0×10 ³ Pa or more, 1.0×10 ⁴ Pa or more, 5.0×10 ⁴ Pa or more, 1.0×10 ⁵ Pa or more, 3.0×10 ⁵ Pa or more, or 5.0×10 ⁵ Pa or more. An adhesive having the above G'(-20°C) can have a moderate cohesive force while having flexibility. In addition, an adhesive having the above G'(-20°C) tends to easily achieve both a low refractive index and flexibility.

The storage modulus at 25°C (G'(25°C)) of the adhesive disclosed herein is not particularly limited and may be, for example, 1.0 x ¹⁰⁶ Pa or less. In some preferred embodiments, the G'(25°C) of the adhesive may be, for example, 5.0 x ¹⁰⁵ Pa or less, 1.0 x ¹⁰⁵ Pa or less, 5.0 x ¹⁰⁴ Pa or less, or 4.5 x ¹⁰⁴ Pa or less. It is preferable that the G'(25°C) of the adhesive disclosed herein is low from the viewpoint of achieving both a low refractive index and flexibility (for example, flexibility suitable for repeated folding operations). The lower limit of the G'(25°C) of the adhesive is not particularly limited and may be, for example, 1.0 x ¹⁰³ Pa or more, or 5.0 x ¹⁰³ Pa or more. In some embodiments, from the viewpoint of easily realizing an adhesive exhibiting suitable cohesiveness and durability (e.g., durability suitable for repeated folding operations), the G'(25°C) of the adhesive is suitably 1.0 x 10 ⁴ Pa or more, or may be 2.0 x 10 ⁴ Pa or more, 3.0 x 10 ⁴ Pa or more, or may be 4.0 x 10 ⁴ Pa or more.

The storage modulus at 60°C (G'(60°C)) of the adhesive disclosed herein is not particularly limited and may be, for example, 5.0x105 Pa or less, ^2.0x105 Pa or less, ^1.0x105 Pa or less, ^8.0x104 Pa or less, ^6.0x104 Pa or less, ^4.0x104 Pa ^or less, ^3.5x104 Pa or less, or ^3.0x104 Pa or less. An adhesive having a limited G'(60°C) as described above tends to provide good flexibility in the room temperature range. The lower limit of G'(60°C) is not particularly limited, and may be, for example, 1.0×10 ³ Pa or more, preferably 5.0×10 ³ Pa or more, more preferably 1.0×10 ⁴ Pa or more, and may be 2.0×10 ⁴ Pa or more, 2.5×10 ⁴ Pa or more, 3.0×10 ⁴ Pa or more, or 3.5×10 ⁴ Pa or more. Pressure-sensitive adhesives having the above G'(60°C) tend to have appropriate cohesive strength even in a high temperature range and have excellent heat resistance, and are therefore preferred.

(Storage modulus ratio)
The ratio of G'(-20°C) to G'(25°C) of the adhesive, i.e., the storage modulus ratio (G'(-20°C)/G'(25°C)), may be, for example, 1500 or less, 1200 or less, 1000 or less, or 900 or less. An adhesive having the ratio (G'(-20°C)/G'(25°C)) limited to a predetermined value or less is preferable because the change in the modulus of elasticity is suppressed in the temperature range from the low temperature range to the room temperature range, and therefore the adhesive is likely to exhibit stable characteristics (flexibility, etc.) against temperature changes. In some embodiments, the (G'(-20°C)/G'(25°C)) is suitably 800 or less, advantageously 600 or less, and preferably 500 or less, 400 or less, or 300 or less, and may be 200 or less, 100 or less, or may be 75 or less, 60 or less, 45 or less, 30 or less, 20 or less, or 15 or less. The lower limit of the ratio (G'(-20°C)/G'(25°C)) is typically more than 1.0, and may be, for example, 1.1 or more. In consideration of a balance with other properties, in some embodiments, the ratio (G'(-20°C)/G'(25°C)) may be 2.0 or more, 5.0 or more, 10 or more, 15 or more, or 20 or more.

The ratio of G'(25°C) to G'(60°C) of the adhesive, i.e., the storage modulus ratio (G'(25°C)/G'(60°C)), may be, for example, 100 or less or 70 or less, advantageously 50 or less, preferably 40 or less or 30 or less, may be 20 or less, may be 10 or less, may be 8.0 or less, 6.0 or less, 4.0 or less, 3.0 or less, or 2.0 or less, may be 1.8 or less, may be 1.6 or less, may be 1.5 or less, or may be 1.4 or less. An adhesive having the ratio (G'(25°C)/G'(60°C)) limited to a predetermined value or less has a suppressed change in the modulus of elasticity in the temperature range from room temperature to a high temperature range, and therefore is likely to exhibit stable characteristics (flexibility, etc.) against temperature changes. The lower limit of the ratio (G'(25°C)/G'(60°C)) is typically more than 1.0, and may be, for example, 1.1 or more. Taking into account the balance with other properties, in some embodiments, the ratio (G'(25°C)/G'(60°C)) may be 1.2 or more, 1.3 or more, or 1.4 or more.

The storage modulus of the adhesive at each temperature can be measured by the method described in the Examples below, and the above-mentioned storage modulus ratios can be calculated from the results. The storage modulus and storage modulus ratio of the adhesive can be adjusted, for example, by selecting the composition of the monomer components that make up the base polymer of the adhesive, selecting the type and amount of crosslinking agent used, etc.

(Glass-transition temperature)
The glass transition temperature (Tg) of the adhesive disclosed herein is not particularly limited, but is suitably 5°C or less (e.g., -60°C or more and 5°C or less) from the viewpoint of easily obtaining good flexibility in the low temperature range. In some embodiments, the Tg of the adhesive is advantageously 0°C or less (e.g., -5°C or less), preferably -10°C or less (e.g., -15°C or less), more preferably -20°C or less, and may be -22°C or less, or may be -24°C or less. The lower the Tg of the adhesive, the more excellent the adhesive properties such as adhesion to the adherend tend to be. The lower limit of the Tg of the adhesive may be, for example, -50°C or more, -40°C or more, or -30°C or more. An adhesive having the above Tg tends to easily obtain a moderate cohesive force. In addition, an adhesive that is compatible with a low refractive index and a low elastic modulus tends to be easily formed. The Tg of the adhesive can be measured by the method described in the Examples below. The Tg of the pressure-sensitive adhesive can be adjusted, for example, by selecting the composition of the monomer components constituting the base polymer of the pressure-sensitive adhesive, selecting the type and amount of the crosslinking agent used, and the like.

(Elongation at break)
The elongation at break ( _EB ) of the pressure-sensitive adhesive is not particularly limited, and may be, for example, in the range of 200% or more and 10,000% or less. From the viewpoint of flexibility and elongation deformability, in some embodiments, the _EB of the pressure-sensitive adhesive may be, for example, 300% or more, advantageously 400% or more, preferably 500% or more, more preferably 750% or more, may be 800% or more, may be 900% or more, may be 1000% or more, or may be 1200% or more. In addition, from the viewpoint of compatibility with other properties, and processability and handleability of the pressure-sensitive adhesive or a pressure-sensitive adhesive sheet having the pressure-sensitive adhesive, in some embodiments, the _EB of the pressure-sensitive adhesive is suitably 9,000% or less, advantageously 8,000% or less, preferably 7,000% or less, more preferably 6,000% or less, may be 5,000% or less, may be 4,000% or less, may be 3,000% or less, 2,000% or less, or 1,500% or less. The _EB of the pressure-sensitive adhesive can be measured by the method described in the Examples below. The _EB of the pressure-sensitive adhesive can be adjusted, for example, by selecting the composition of the monomer components constituting the base polymer of the pressure-sensitive adhesive, selecting the type and amount of the crosslinking agent, etc.

(Ratio (G'(25°C)/ _EB ))
In some embodiments, the ratio of the storage modulus at 25°C (G'(25°C)) to the elongation at break (E _B ), i.e., the ratio (G'(25°C)/E _B ), may be, for example, 1.0x10 ⁵ or less, 5.0x10 ⁴ or less, 1.0x10 ⁴ or less, 5.0x10 ³ or less, 1.0x10 ³ or less, 500 or less, 300 or less, or 150 or less. The ratio (G'(25°C)/E _B ) is a dimensionless number calculated from the numerical portion of the G'(25°C) of the pressure-sensitive adhesive expressed in units of "Pa" and the numerical portion of the E _B of the pressure-sensitive adhesive expressed in units of "%". A higher (harder) G'(25°C) and a smaller E _B of the adhesive are both factors that increase the value of the ratio (G'(25°C)/E _B ). For example, a brittle material or a weak (low cohesive strength) material is more likely to tear when stretched and has a smaller E _B value. On the other hand, lowering G'(25°C) and increasing E _B both have an effect on the ratio (G'(25°C)/E _B ) in the direction of decreasing the value of the ratio. An adhesive in which the ratio (G'(25°C)/E _B ) is limited to a predetermined value or less exhibits good flexibility due to moderate flexibility and moderate stretch deformation (resistance to tearing). Therefore, an adhesive having a refractive index of a predetermined value or less and having the ratio (G'(25°C)/E _B ) in the above range is preferable from the viewpoint of achieving both a low refractive index and flexibility. The pressure-sensitive adhesive having the above flexibility may be suitable for applications in which repeated folding is expected, for example. From the viewpoint of realizing better flexibility, in some preferred embodiments, the ratio (G'(25°C)/ _EB ) is suitably less than 100 (e.g., 95 or less), and may be 80 or less, 70 or less, 60 or less, or 50 or less. The lower limit of the ratio (G'(25°C)/ _EB ) is not particularly limited. In some embodiments, taking into consideration the balance with other properties, the ratio (G'(25°C)/ _EB ) may be, for example, 5.0 or more, 10 or more, 15 or more, 20 or more, 25 or more, 30 or more, 40 or more, or 45 or more.

(Ratio (G'(-20°C)/ _EB ))
In some embodiments, the ratio of the storage modulus at -20°C (G'(-20°C)) to the elongation at break (E _B ), i.e., the ratio (G'(-20°C)/E _B ), may be, for example, 5.0 x 10 ⁶ or less, 5.0 x 10 ⁵ or less, 5.0 x 10 ⁴ or less, 1.0 x 10 ⁴ or less, or 5.0 x 10 ³ or less. The ratio (G'(-20°C)/E _B ) is a dimensionless number calculated from the numerical part when the G'(-20°C) of the pressure-sensitive adhesive is expressed in units of "Pa" and the numerical part when the E _B of the pressure-sensitive adhesive is expressed in units of "%". A pressure-sensitive adhesive having a refractive index of a predetermined value or less and having the ratio (G'(-20°C)/E _B ) in the above range is preferred from the viewpoint of achieving both a low refractive index and relatively rapid followability to deformation. From the viewpoint of realizing better flexibility, in some preferred embodiments, the ratio (G'(-20°C)/ _EB ) is suitably 2500 or less, advantageously 2000 or less, preferably 1500 or less (e.g. 1300 or less), more preferably 1200 or less (e.g. 1000 or less), and may be 800 or less, 600 or less, 500 or less, 450 or less, or 400 or less. There is no particular lower limit for the ratio (G'(-20°C)/ _EB ). In some embodiments, taking into consideration the balance with other properties, the ratio (G'(-20°C)/ _EB ) may be, for example, 20 or more, 50 or more, 100 or more, 150 or more, 200 or more, 250 or more, 300 or more, 350 or more, or 400 or more.

(Young's modulus)
The Young's modulus of the adhesive disclosed herein may be, for example, in the range of about 0.01 MPa to 50 MPa. In some embodiments, from the viewpoint of initial adhesion, etc., the Young's modulus of the adhesive is suitably 30 MPa or less (e.g., 20 MPa or less or 10 MPa or less), and is advantageously 8.0 MPa or less (e.g., 6.0 MPa or less, 4.5 MPa or less, 3.0 MPa or less, or 2.0 MPa or less), and may be 1.0 MPa or less, 0.80 MPa or less, or less than 0.80 MPa. In some embodiments, from the viewpoint of improving flexibility, the Young's modulus of the adhesive may be, for example, less than 0.60 MPa, less than 0.50 MPa, less than 0.20 MPa, less than 0.10 MPa, or less than 0.08 MPa. In addition, in some embodiments, from the viewpoint of processability, handleability, etc. of the adhesive or the adhesive sheet having the adhesive, the Young's modulus of the adhesive may be, for example, 0.03 MPa or more, or 0.05 MPa or more. In some embodiments, the Young's modulus of the adhesive may be 0.06 MPa or more, or 0.07 MPa or more, taking into consideration the balance with other properties. The Young's modulus of the adhesive can be measured by the method described in the Examples below. The Young's modulus of the adhesive can be adjusted, for example, by selecting the composition of the monomer components constituting the base polymer, selecting the type and amount of the crosslinking agent, etc.

(Breaking Stress)
The rupture stress of the adhesive disclosed herein may be, for example, within the range of about 0.10 MPa to 30 MPa. In some embodiments, from the viewpoint of the processability and handling of the adhesive or the adhesive sheet having the adhesive, the rupture stress of the adhesive is, for example, suitably 0.2 MPa or more or 0.3 MPa or more, and may be 0.4 MPa or more. In some embodiments, from the viewpoint of easily increasing the flexibility of the adhesive, the rupture stress of the adhesive is suitably 15 MPa or less, preferably 12 MPa or less, may be 10 MPa or less, may be 8.0 MPa or less, may be 4.0 MPa or less, may be 2.0 MPa or less, may be 1.0 MPa or less, may be 0.8 MPa or less, or may be 0.6 MPa or less. The rupture stress of the adhesive can be measured by the method described in the Examples below. The rupture stress of the adhesive can be adjusted, for example, by selecting the composition of the monomer components constituting the base polymer, selecting the type and amount of the crosslinking agent, etc.

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

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

(Acrylic Polymer (F))
In some preferred embodiments, the pressure-sensitive adhesive disclosed herein contains an acrylic polymer (F). The monomer component constituting the acrylic polymer (F) includes a fluorine-containing acrylic monomer. In other words, the acrylic polymer (F) includes the fluorine-containing acrylic monomer as a monomer unit. The pressure-sensitive adhesive disclosed herein is preferably an acrylic pressure-sensitive adhesive including the acrylic polymer (F) as a base polymer. Hereinafter, the acrylic polymer (F) may be abbreviated as "polymer (F)".

(Fluorine-containing acrylic monomer)
The polymer (F) may be a polymer of monomer components including at least a fluorine-containing acrylic monomer and may further include another monomer (copolymerizable monomer) having copolymerizability with the monomer. The fluorine-containing acrylic monomer is not particularly limited as long as it is an acrylic monomer having at least one fluorine atom in the molecule. For example, a fluorine-containing (meth)acrylic acid ester can be suitably used. A suitable example of the fluorine-containing (meth)acrylic acid ester is one having a fluorinated hydrocarbon group at the ester end. Examples of the fluorinated hydrocarbon group include a fluorinated aliphatic hydrocarbon group, a fluorinated alicyclic hydrocarbon group, and a fluorinated aromatic hydrocarbon group. A suitable example of the fluorinated hydrocarbon group is a fluorinated aliphatic hydrocarbon group. Examples of the fluorinated aliphatic hydrocarbon group include a fluorinated alkyl group. In the fluorinated aliphatic hydrocarbon group, the aliphatic hydrocarbon portion may be linear or branched. In the fluorinated aliphatic hydrocarbon group, the fluorine atom may be bonded to any carbon atom in the aliphatic hydrocarbon group portion. The fluorine atom bonded to one carbon atom may be singular or plural. The number of carbon atoms to which fluorine atoms are bonded is not particularly limited.

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

Fluorinated alicyclic hydrocarbon groups include fluorinated cycloalkyl groups. As with the above fluorinated aliphatic hydrocarbon groups, in fluorinated alicyclic hydrocarbon groups, the fluorine atom may be bonded to any carbon atom of the alicyclic hydrocarbon group, and the number of fluorine atoms bonded to one carbon atom may be either single or multiple. Furthermore, there is no particular limit to the number of carbon atoms to which fluorine atoms are bonded. Fluorinated alicyclic hydrocarbon groups include, for example, cyclohexyl groups having one fluorine atom, such as 2-fluorocyclohexyl, 3-fluorocyclohexyl, and 4-fluorocyclohexyl groups; cyclohexyl groups having two fluorine atoms, such as 2,4-difluorocyclohexyl and 2,6-difluorocyclohexyl groups; and cyclohexyl groups having three fluorine atoms, such as 2,4,6-trifluorocyclohexyl groups.

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

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

As the fluorine atom-containing (meth)acrylic acid ester, fluorinated alkyl (meth)acrylates (particularly fluorinated alkyl acrylates) are suitable. Examples of fluorinated alkyl (meth)acrylates include 2-(perfluorohexyl)ethyl acrylate (e.g., "C6SFA Monomer" manufactured by Daikin Industries, Ltd.), 2,2,2-trifluoroethyl acrylate (e.g., "Viscoat 3F" manufactured by Osaka Organic Chemical Industry Co., Ltd.), 2,2,3,3-tetrafluoropropyl acrylate (e.g., "Viscoat 4F" manufactured by Osaka Organic Chemical Industry Co., Ltd.), 1H,1H,5H-octafluoropentyl acrylate (e.g., "Viscoat 8F" manufactured by Osaka Organic Chemical Industry Co., Ltd.), 1H,1H,5H-octafluoropentyl methacrylate (e.g., "Viscoat 8FM" manufactured by Osaka Organic Chemical Industry Co., Ltd.), and 2-(heptadecafluorononyl)ethyl acrylate (e.g., "FA-108" manufactured by Kyoeisha Chemical Co., Ltd.).

(Monomer (Mf))
In some embodiments, the fluorine-containing acrylic monomer is represented by the following formula (2):
CH ₂ ═CR ¹ COO(CH ₂ ) _n -Rf (2)
(wherein R ¹ is a hydrogen atom or a methyl group, n is 1 or 2, and Rf is a linear fluorinated alkyl group having 3 to 6 carbon atoms); fluorinated alkyl (meth)acrylates (Mf) (hereinafter sometimes abbreviated as "monomer (Mf)") represented by the formula (2) may be preferably used. The monomer component constituting the polymer (F) may contain any one of the fluorinated alkyl (meth)acrylates represented by the formula (2) alone or in combination of two or more. The proportion of the monomer (Mf) in the fluorine-containing acrylic monomers contained in the monomer component constituting the polymer (F) may be, for example, 25% by weight or more, 50% by weight or more, 75% by weight or more, or 100% by weight.

The number of carbon atoms of the chain fluorinated alkyl group represented by Rf in the above formula (2) is preferably 4 or more, more preferably 5 or more (for example, 6) from the viewpoint of the effect of reducing the refractive index and flexibility. The above chain fluorinated alkyl group may be linear or branched, but is preferably linear from the viewpoint of flexibility. The above chain fluorinated alkyl group may be a perfluoroalkyl group or a partially fluorinated alkyl group (for example, a partially fluorinated alkyl group having a structure in which one or two fluorine atoms bonded to the terminal carbon atom in the perfluoroalkyl group are replaced with hydrogen atoms). From the viewpoint of the effect of reducing the refractive index, the above chain fluorinated alkyl group is preferably a chain (preferably linear) perfluoroalkyl group. In addition, n in the above formula (2) is preferably 2 from the viewpoint of the flexibility of the adhesive. R ¹ in the above formula (2) is preferably a hydrogen atom from the viewpoint of the flexibility of the adhesive and the polymerization reactivity of the monomer (Mf).

Among the fluorinated alkyl (meth)acrylates represented by the above formula (2), 2-(perfluorohexyl)ethyl acrylate is an example of a preferred monomer (Mf) from the viewpoint of achieving a good balance between a low refractive index and flexibility. 2-(perfluorohexyl)ethyl acrylate may be used alone or in combination with other fluorine-containing acrylic monomers. The proportion of 2-(perfluorohexyl)ethyl acrylate in the fluorine-containing acrylic monomers contained in the monomer components constituting the polymer (F) may be, for example, 25% by weight or more, 50% by weight or more, 75% by weight or more, or 100% by weight.

The content of the fluorine-containing acrylic monomer in the monomer components constituting the polymer (F) (which may be the content of the monomer (Mf) or the content of 2-(perfluorohexyl)ethyl acrylate) may be, for example, 15% by weight or more, and from the viewpoint of achieving a low refractive index, it is advantageous to have it be 20% by weight or more, preferably 25% by weight or more, and more preferably 30% by weight or more or 35% by weight or more. From the viewpoint of making it easier to realize an adhesive with a lower refractive index, in some embodiments, the above content may be, for example, 40% by weight or more, 43% by weight or more or more than 43% by weight, 45% by weight or more or more than 45% by weight, or 47% by weight or more or more than 47% by weight. In addition, the upper limit of the content of the fluorine-containing acrylic monomer in the monomer component constituting the polymer (F) (which may be the content of the monomer (Mf) or the content of 2-(perfluorohexyl)ethyl acrylate) is set so that the total with the content of other monomers does not exceed 100% by weight, and may be, for example, less than 98% by weight, less than 95% by weight, or 90% by weight or less. In some embodiments, the content is suitably 80% by weight or less from the viewpoint of the flexibility of the adhesive, advantageously 70% by weight or less, may be 60% by weight or less, may be 55% by weight or less, or may be 50% by weight or less. The content of the fluorine-containing acrylic monomer may also be applied to the content of the monomer (Mf) in the monomer component constituting the polymer (F) or the content of 2-(perfluorohexyl)ethyl acrylate in the monomer component constituting the polymer (F).

(Acid-Free Hydrophilic Monomer (Mh))
The monomer components constituting the polymer (F) may further include an acid-free hydrophilic monomer (Mh) (hereinafter, sometimes abbreviated as "hydrophilic monomer (Mh)" or "monomer (Mh)"). The monomer (Mh) can be useful, for example, for imparting an appropriate cohesive force to the adhesive, improving peel strength, and suppressing a decrease in transparency due to moisture (for example, suppressing an increase in haze value), etc. As the acid-free hydrophilic monomer (Mh), a monomer having an ethylenically unsaturated group and a hydrophilic group in the molecule and having no acidic functional group is used. The concept of the acidic functional group includes a carboxy group, a sulfo group, and a phosphate group. Therefore, the acid-free hydrophilic monomer (Mh) is a monomer having an ethylenically unsaturated group and a hydrophilic group in the molecule and having no acidic functional group of any of a carboxy group, a sulfo group, and a phosphate group. The fact that the monomer (Mh) is acid-free is advantageous from the viewpoint of suppressing a decrease in the flexibility of the adhesive (particularly an increase in the storage modulus in the low temperature range) associated with the use of the monomer (Mh). The acid-free hydrophilic monomer (Mh) can be used alone or in combination of two or more kinds.

Examples of ethylenically unsaturated groups contained in the monomer (Mh) include (meth)acryloyl, vinyl, and (meth)allyl groups. From the viewpoint of flexibility, preferred ethylenically unsaturated groups include acryloyl, vinyl, and allyl groups. From the viewpoint of polymerization reactivity, preferred ethylenically unsaturated groups include acryloyl and methacryloyl groups (more preferably acryloyl groups). From the viewpoint of suppressing a decrease in the flexibility of the adhesive, a compound having one ethylenically unsaturated group in one molecule (i.e., a monofunctional monomer) is preferably used as the monomer (Mh).

The hydrophilic group of the monomer (Mh) may be, for example, a hydroxyl group, an amide group, an amino group, a nitrogen atom-containing ring, a (poly)oxy C _1-2 alkylene group, etc. An acid-free monomer having at least one such hydrophilic group in the molecule can be used as the monomer (Mh). In some embodiments, one or more monomers selected from at least one of a hydroxyl group-containing monomer, an amide group-containing monomer, an amino group-containing monomer, a monomer having a nitrogen atom-containing ring, and a (poly)oxy C _1-2 alkylene group-containing monomer are used as the monomer (Mh).

Examples of the hydroxyl group-containing monomer include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 3-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. The hydroxyl group-containing monomers may be used alone or in combination of two or more. As the monomer (Mh), one or more hydroxyl group-containing monomers may be used in combination with one or more other monomers (e.g., amide group-containing monomers). The proportion of the hydroxyl group-containing monomer in the monomer (Mh) can be, for example, 10% by weight or more, 20% by weight or more, 25% by weight or more, 33% by weight or more, 50% by weight or more, 65% by weight or more, 80% by weight or more, or 90% by weight or more, and can be 100% by weight or less, 85% by weight or less, 70% by weight or less, 55% by weight or less, 40% by weight or less, 30% by weight or less, 25% by weight or less, 20% by weight or less, or 15% by weight or less.

Suitable examples of hydroxyl group-containing monomers include 4-hydroxybutyl acrylate (4HBA) (Tg of homopolymer: -40°C) and 2-hydroxyethyl acrylate (HEA) (Tg of homopolymer: -15°C). From the viewpoint of improving the flexibility of the adhesive (particularly improving flexibility in the low temperature range), 4HBA, which has a lower homopolymer Tg, is more preferable. In some preferred embodiments, from the viewpoint of achieving a good balance between the wet heat whitening resistance and flexibility of the adhesive, 50% by weight or more (e.g., more than 50% by weight, more than 70% by weight, or more than 85% by weight) and 100% by weight or less of the hydroxyl group-containing monomers used as monomers (Mh) may be 4HBA.

Examples of the amide group-containing monomer include (meth)acrylamide; N,N-dialkyl (meth)acrylamides such as N,N-dimethyl (meth)acrylamide and N,N-diethyl (meth)acrylamide; N-monoalkyl (meth)acrylamides such as N-ethyl (meth)acrylamide and N-isopropyl (meth)acrylamide; N-vinyl carboxylic acid amides such as N-vinyl formamide and N-vinyl acetamide; (meth)acrylamides having a hydroxyl group such as N-(2-hydroxyethyl)acrylamide and N-methylolacrylamide; N-vinyl cyclic amides such as N-vinyl pyrrolidone (NVP), N-vinyl piperidone, N-vinyl caprolactam and N-vinyl-3-morpholinone; cyclic amides having an N-(meth)acryloyl group such as 1-(meth)acryloyl-2-pyrrolidone and 1-(meth)acryloyl piperidin-2-one; and the like. The amide group-containing monomers can be used alone or in combination of two or more. As the monomer (Mh), one or more amide group-containing monomers may be used in combination with one or more other monomers (e.g., hydroxyl group-containing monomers). The proportion of the amide group-containing monomer in the monomer (Mh) can be, for example, 10% by weight or more, 20% by weight or more, 25% by weight or more, 33% by weight or more, 50% by weight or more, 65% by weight or more, 80% by weight or more, or 90% by weight or more, and can be 100% by weight or less, 85% by weight or less, 70% by weight or less, 55% by weight or less, 40% by weight or less, 30% by weight or less, 25% by weight or less, 20% by weight or less, or 15% by weight or less. One suitable example of the amide group-containing monomer is N-vinyl cyclic amide. Among them, NVP is preferable.

Some amide group-containing monomers, such as N-(2-hydroxyethyl)acrylamide, are also hydroxyl group-containing monomers, and some, such as N-vinyl cyclic amide, are also monomers that have a nitrogen atom-containing ring.

Examples of the amino group-containing monomer include dimethylaminoethyl (meth)acrylate and t-butylaminoethyl (meth)acrylate. The amino group-containing monomer may be used alone or in combination of two or more. As the monomer (Mh), one or more amino group-containing monomers may be used in combination with one or more other monomers. The proportion of the amino group-containing monomer in the monomer (Mh) may be, for example, 10% by weight or more, 20% by weight or more, 25% by weight or more, 33% by weight or more, 50% by weight or more, 65% by weight or more, 80% by weight or more, or 90% by weight or more, and may be 100% by weight or less, 85% by weight or less, 70% by weight or less, 55% by weight or less, 40% by weight or less, 30% by weight or less, 25% by weight or less, 20% by weight or less, or 15% by weight or less.

Examples of the monomer having the nitrogen atom-containing ring include vinylpyridine, vinylpyrimidine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinylisoxazole, (meth)acryloylmorpholine, (meth)acryloylpiperidine, (meth)acryloylpyrrolidine, etc. The monomer having a nitrogen atom-containing ring may be used alone or in combination of two or more. As the monomer (Mh), one or more monomers having a nitrogen atom-containing ring may be used in combination with one or more other monomers. The proportion of the monomer having a nitrogen atom-containing ring in the monomer (Mh) can be, for example, 10% by weight or more, 20% by weight or more, 25% by weight or more, 33% by weight or more, 50% by weight or more, 65% by weight or more, 80% by weight or more, or 90% by weight or more, and can be 100% by weight or less, 85% by weight or less, 70% by weight or less, 55% by weight or less, 40% by weight or less, 30% by weight or less, 25% by weight or less, 20% by weight or less, or 15% by weight or less.

The oxy _C1-2 alkylene group in the (poly)oxy _C1-2 alkylene group-containing monomer refers to an oxyalkylene group having 1 to 2 carbon atoms, i.e., an -O( _CH2 ) _q- group (where q is 1 or 2). The (poly)oxy _C1-2 alkylene group refers collectively to an oxy _C1-2 alkylene group and a polyoxy _C1-2 alkylene group, and can be expressed as an -(O( _CH2 ) _q ) _r- group. Here, q in the above formula is 1 or 2, and it is preferable that q is 2, i.e., the -(O( _CH2 ) _q ) _r- group is a (poly)oxyethylene group. In the above formula, r may be, for example, 1 or more or 2 or more, and may be, for example, 30 or less, 20 or less, 15 or less, 10 or less, 5 or less, or 3 or less. Examples of the (poly)oxy C _1-2 alkylene group-containing monomer include polyoxyethylene (meth)acrylates such as methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylate, ethoxyethoxyethyl (meth)acrylate, methoxypolyoxyethylene (meth)acrylate, and ethoxypolyoxyethylene (meth)acrylate. As the polyoxyethylene (meth)acrylate, for example, one having a polyoxyethylene group with a lower limit of r of 3 or more, 4 or more, or 5 or more in the above formula can be used. The (poly)oxy C _1-2 alkylene group-containing monomer can be used alone or in combination of two or more. As the monomer (Mh), one or two or more (poly)oxy C _1-2 alkylene group-containing monomers may be used in combination with one or two or more other monomers. The proportion of the (poly)oxy C _1-2 alkylene group-containing monomer in the monomer (Mh) can be, for example, 10% by weight or more, 20% by weight or more, 25% by weight or more, 33% by weight or more, 50% by weight or more, 65% by weight or more, 80% by weight or more, or 90% by weight or more, and can be 100% by weight or less, 85% by weight or less, 70% by weight or less, 55% by weight or less, 40% by weight or less, 30% by weight or less, 25% by weight or less, 20% by weight or less, or 15% by weight or less. Examples of (poly)oxy C _1-2 alkylene group-containing monomers that can be preferably used from the viewpoint of flexibility of the adhesive include methoxyethyl acrylate (MEA, homopolymer Tg: -50°C), ethoxyethoxyethyl acrylate (also known as ethyl carbitol acrylate (CBA), homopolymer Tg: -67°C), and the like.

The content of the acid-free hydrophilic monomer (Mh) in the monomer components constituting the polymer (F) is set so that the total content of the other monomers does not exceed 100% by weight, and may be, for example, more than 0.5% by weight or more than 1.0% by weight. From the viewpoint of easily obtaining a higher usage effect, it may be more than 2.0% by weight or more than 3.0% by weight. In some embodiments, the content of the monomer (Mh) in the monomer components is advantageously more than 5.0% by weight from the viewpoint of suppressing the phenomenon in which the adhesive whitens due to exposure to high temperature and high humidity conditions (humid heat whitening), and may be, for example, more than 5.5% by weight, more than 6.0% by weight, 8.0% by weight or more, 10% by weight or more, 15% by weight or more, 20% by weight or more, or 25% by weight or more. The content of the monomer (Mh) in the monomer component may be, for example, 50% by weight or less. From the viewpoint of reducing the refractive index of the adhesive, it is advantageous to have it be 40% by weight or less, and it is preferably 35% by weight or less, and may be 30% by weight or less, 25% by weight or less, 20% by weight or less, or 15% by weight or less. In some embodiments, the content of the monomer (Mh) in the monomer component may be 10% by weight or less, 5.0% by weight or less, 1.0% by weight or less, or 0.5% by weight or less. The monomer (Mh) may not be used.

The content of the hydroxyl group-containing monomer in the monomer component constituting the polymer (F) may be, for example, more than 0.5% by weight or more than 1.0% by weight, or more than 2.0% by weight or more than 3.0% by weight. In some embodiments, the content of the hydroxyl group-containing monomer is advantageously more than 4.0% by weight or more than 5.0% by weight from the viewpoint of favorably achieving both flexibility and wet heat whitening resistance at a low refractive index, and may be, for example, more than 5.5% by weight, more than 6.0% by weight, 8.0% by weight or more, 10% by weight or more, or 15% by weight or more. The content of the hydroxyl group-containing monomer in the monomer component constituting the polymer (F) may be, for example, 50% by weight or less, and from the viewpoint of reducing the refractive index of the adhesive, it is preferably 40% by weight or less or 35% by weight or less, and may be 30% by weight or less, 25% by weight or less, 20% by weight or less, or 15% by weight or less, and may be 10% by weight or less, 5.0% by weight or less, 1.0% by weight or less, or 0.5% by weight or less. The hydroxyl group-containing monomer may not be used.
The above description regarding the content of the hydroxyl group-containing monomer in the monomer component constituting the polymer (F) can also be applied to the content of the monomer containing a (poly)oxy C _1-2 alkylene group in the monomer component constituting the polymer (F).

The content of the amide group-containing monomer in the monomer component constituting the polymer (F) may be, for example, more than 0.5% by weight or more than 1.0% by weight, and from the viewpoint of easily obtaining a higher use effect, it may be more than 2.0% by weight or more than 3.0% by weight, more than 4.0% by weight, more than 5.0% by weight, more than 5.5% by weight, more than 6.0% by weight, or more than 8.0% by weight. The content of the amide group-containing monomer in the monomer component constituting the polymer (F) is preferably 40% by weight or less or 30% by weight or less from the viewpoint of lowering the refractive index of the adhesive, and is suitably 20% by weight or less from the viewpoint of the flexibility of the adhesive (particularly, flexibility in the low temperature range), preferably 15% by weight or less, more preferably 10% by weight or less, and may be 5.0% by weight or less, 1.0% by weight or less, or 0.5% by weight or less. It is not necessary to use an amide group-containing monomer.
The above description regarding the content of the amide group-containing monomer in the monomer component constituting the polymer (F) can also be applied to the content of the monomer having a nitrogen atom-containing ring and the content of the amino group-containing monomer in the monomer component constituting the polymer (F).

In some embodiments of the technology disclosed herein, a hydrophilic monomer having a homopolymer glass transition temperature (Tg) of 40° C. or less (preferably 25° C. or less, more preferably 0° C. or less, more preferably −10° C. or less, for example −20° C. or less, −25° C. or less, or −30° C. or less) (hereinafter also referred to as “low Tg hydrophilic monomer (Mh _L )” or “monomer (Mh _L )”) is used as at least a part of the monomer (Mh). By using the low Tg hydrophilic monomer (Mh _L ), it is possible to enjoy the effects of using the hydrophilic monomer (Mh) while suppressing an increase in the storage modulus G′. The lower limit of the Tg of the homopolymer of the monomer (Mh _L ) is not particularly limited. The Tg of the homopolymer of the monomer (Mh _L ) can be, for example, −80° C. or more, −70° C. or more, −60° C. or more, or −50° C. or more. The monomer (Mh _L ) can be used alone or in combination of two or more.

As the monomer (Mh _L ), a compound having a corresponding Tg can be appropriately selected from among the compounds encompassed by the concept of the acid-free hydrophilic monomer (Mh) disclosed herein (for example, the monomers (Mh) exemplified above). Examples of the acid-free hydrophilic monomer (Mh) that can be used as the monomer (Mh _L ) include 2-hydroxyethyl acrylate, 4-hydroxybutyl acrylate, methoxyethyl acrylate, ethoxyethoxyethyl acrylate, etc.

The ratio of the low Tg hydrophilic monomer (Mh _L ) in the monomer (Mh) is not particularly limited. From the viewpoint of enhancing the effect of using the monomer (Mh _L ), in some embodiments, the ratio of the monomer (Mh _L ) in the monomer (Mh) is, for example, suitably 5% by weight or more, advantageously 10% by weight or more, preferably 20% by weight or more, may be 25% by weight or more, may be 33% by weight or more, 50% by weight or more, 65% by weight or more, 80% by weight or more, or 90% by weight or more. The above ratio may be 100% by weight. That is, only one or two or more low Tg hydrophilic monomers (Mh _L ) may be used as the monomer (Mh). In addition, in some embodiments, the ratio of the monomer (Mh _L ) in the monomer (Mh) may be, for example, 90% by weight or less, 75% by weight or less, 50% by weight or less, 30% by weight or less, 20% by weight or less, or 10% by weight or less.

The content of the low Tg hydrophilic monomer (Mh _L ) in the monomer component constituting the polymer (F) is not particularly limited, and may be, for example, more than 0.5 wt%, more than 1.0 wt%, more than 2.0 wt%, more than 3.0 wt%, or more than 4.0 wt%. From the viewpoint of enhancing the effect of using the monomer (Mh _L ), in some embodiments, the content of the monomer (Mh _L ) in the monomer component constituting the polymer (F) may be, for example, more than 5.0 wt%, more than 5.5 wt%, more than 6.0 wt%, 8.0 wt% or more, 10 wt% or more, 15 wt% or more, 20 wt% or more, or 25 wt% or more. In addition, the upper limit of the content of the monomer (Mh _L ) in the above monomer component is set so that the total with the contents of other monomers does not exceed 100% by weight, and may be, for example, 50% by weight or less. From the viewpoint of reducing the refractive index of the adhesive, it is advantageous to have it be 40% by weight or less, and it is preferably 35% by weight or less, or it may be 30% by weight or less, 25% by weight or less, 20% by weight or less, 15% by weight or less, 10% by weight or less, or 5% by weight or less.

In some embodiments, the acid-free hydrophilic monomer (Mh) contained in the monomer component constituting the polymer (F) advantageously has a glass transition temperature Tg _h based on the composition of the monomer (Mh) of 100° C. or less, preferably 80° C. or less, and more preferably 60° C. or less. By using the monomer (Mh) so that Tg _h is equal to or less than the above temperature, it is possible to enjoy the effects of using the hydrophilic monomer (Mh) while suppressing an increase in the storage modulus G'. In some embodiments where flexibility is more important, Tg _h is suitably 50° C. or less, advantageously 40° C. or less, preferably 30° C. or less or 15° C. or less, and may be 0° C. or less, -10° C. or less, -20° C. or less, -25° C. or less, or -30° C. or less. The lower limit of Tg _h is not particularly limited, and may be, for example, -80° C. or more, -70° C. or more, -60° C. or more, or -50° C. or more.

Here, the glass transition temperature _Tgh based on the composition of the monomer (Mh) refers to the glass transition temperature calculated by the Fox formula based on the composition of only the acid-free hydrophilic monomer (Mh) among the monomer components constituting the polymer (F). The Fox formula, as shown below, is a relational expression between the Tg of a copolymer and the glass transition temperature Tgi of a homopolymer obtained by homopolymerizing each of the monomers constituting the copolymer.
1/Tg=Σ(Wi/Tgi)
In the above Fox formula, Tg represents the glass transition temperature (unit: K) of the copolymer, Wi represents the weight fraction (copolymerization ratio by weight) 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 to calculate Tg is the value described in publicly available documents such as "Polymer Handbook" (3rd Edition, John Wiley & Sons, Inc., 1989) and manufacturer catalogs. For monomers for which multiple values are described in the Polymer Handbook, the highest value is adopted. If the Tg of the homopolymer is not described in publicly available documents, the value obtained by the measurement method described in Japanese Patent Application Publication No. 2007-51271 is used.

The glass transition temperature _Tgh can be calculated from the glass transition temperature of a homopolymer of each acid-free hydrophilic monomer used as the monomer (Mh) and the weight fraction of each acid-free hydrophilic monomer in the total amount of the monomer (Mh) by applying the Fox formula to only the monomer (Mh) among the monomer components constituting the polymer (F). In an embodiment in which only one type of acid-free hydrophilic monomer is used as the monomer (Mh), the Tg of the homopolymer of the monomer and the glass transition temperature _Tgh are the same.

( _C4-18 Chain Alkyl (Meth)acrylate)
The monomer components constituting the polymer (F) include a fluorine-containing acrylic monomer, an acid-free hydrophilic monomer (Mh), and a monomer represented by the following formula (1):
_CH2 = ^CR1COOR2 ( ¹ );
In the above formula (1), R ¹ is a hydrogen atom or a methyl group, and R ² is a linear alkyl group having 4 to 18 carbon atoms (hereinafter, this range of carbon atoms may be expressed as "C _4-18 "). The monomer (M1) represented by the above formula (1) can be rephrased as a C _4-18 linear alkyl (meth)acrylate. The monomer (M1) can be useful for adjusting the storage modulus G' of the pressure-sensitive adhesive and improving the elongation. The monomer (M1) can be used alone or in combination of two or more kinds.

Specific examples of _C4-18 chain alkyl (meth)acrylates include n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, and nonyl (meth)acrylate. acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, and the like, but are not limited to these.

In some embodiments, the monomer component constituting the polymer (F) advantageously contains, as the monomer (M1), an alkyl (meth)acrylate in which R ² in the above formula (1) is a C _4-12 chain alkyl group, i.e., a C _4-12 chain alkyl (meth)acrylate, from the viewpoint of the storage modulus of the adhesive, and preferably contains a C _4-10 chain alkyl (meth)acrylate, more preferably a C _4-10 chain alkyl acrylate (e.g., a C _5-9 chain alkyl acrylate), from the viewpoint of the low temperature properties of the adhesive. Specific examples of C _4-12 chain alkyl (meth)acrylate include n-butyl acrylate, 2-ethylhexyl acrylate, n-heptyl acrylate, n-octyl acrylate, isooctyl acrylate, isononyl acrylate, isodecyl acrylate, lauryl acrylate, lauryl methacrylate, etc. These can be used alone or in combination of two or more.

In an embodiment in which the monomer component constituting the polymer (F) contains the monomer (M1), the content of the monomer (M1) in the monomer component can be set so that the effect of use is appropriately exhibited. In some embodiments, the content of the monomer (M1) may be, for example, 1% by weight or more, 5% by weight or more, or 8% by weight or more. In some embodiments, the content of the monomer (M1) may be 10% by weight or more, 15% by weight or more, 20% by weight or more, 30% by weight or more, 35% by weight or more, 40% by weight or more, or 45% by weight or more. The upper limit of the content of the monomer (M1) in the monomer component is set so that the total with the content of other monomers does not exceed 100% by weight, and may be, for example, less than 80% by weight. From the viewpoint of easily balancing the low refractive index and other properties, in some embodiments, the content of the monomer (M1) in the monomer component constituting the polymer (F) is suitably 75% by weight or less, may be 65% by weight or less, 55% by weight or less, or may be 50% by weight or less. The technology disclosed herein can also be implemented in embodiments in which the content of the monomer (M1) is 45% by weight or less, 35% by weight or less, 30% by weight or less, 25% by weight or less, 10% by weight or less, 5% by weight or less, or 0% by weight.

In some embodiments, as at least a part of the monomer (M1), a linear C4-18 alkyl (meth)acrylate ( _low Tg alkyl (meth)acrylate) having a homopolymer Tg of 0°C or less (more preferably -10°C or less, even more preferably -25°C or less, for example -35°C or less, -45°C or less, -50°C or less, -55°C or less, or -60°C or less) may be preferably used. Such a low Tg alkyl (meth)acrylate may be useful for improving the flexibility of the adhesive. The lower limit of the Tg of the alkyl (meth)acrylate is not particularly limited, and may be, for example, -85°C or more, -80°C or more, or -75°C or more. Specific examples of the low Tg alkyl(meth)acrylate include n-butyl acrylate (BA), 2-ethylhexyl acrylate (2EHA), hexyl acrylate (HxA), n-octyl acrylate (NOAA), isononyl acrylate, isodecyl acrylate, lauryl acrylate, lauryl methacrylate, isostearyl acrylate, etc. From the viewpoint of flexibility in the low temperature range, _C4-10 chain alkyl(meth)acrylate is preferred, and _C4-10 chain alkyl acrylate (for example, _C5-9 chain alkyl acrylate) is more preferred.

The proportion of the low Tg alkyl (meth)acrylate in the monomer (M1) is not particularly limited. From the viewpoint of enhancing the effect of using the low Tg alkyl (meth)acrylate, in some embodiments, the proportion of the low Tg alkyl (meth)acrylate in the monomer (M1) is, for example, suitably 5% by weight or more, advantageously 10% by weight or more, preferably 20% by weight or more, may be 25% by weight or more, may be 33% by weight or more, may be 50% by weight or more, 65% by weight or more, 80% by weight or more, or may be 90% by weight or more. The above proportion may be 100% by weight. That is, only one or more kinds of low Tg alkyl (meth)acrylate may be used as the monomer (M1). In addition, in some embodiments, the proportion of the low Tg alkyl (meth)acrylate in the monomer (M1) may be, for example, 90% by weight or less, 75% by weight or less, 50% by weight or less, 30% by weight or less, 20% by weight or less, or 10% by weight or less.

The content of the low Tg alkyl (meth)acrylate in the monomer component constituting the polymer (F) may be, for example, 1% by weight or more, 5% by weight or more, or 8% by weight or more. In some embodiments, the content of the low Tg alkyl (meth)acrylate may be 10% by weight or more, 15% by weight or more, 20% by weight or more, 30% by weight or more, 35% by weight or more, 40% by weight or more, or 45% by weight or more. The upper limit of the content of the low Tg alkyl (meth)acrylate in the monomer component is set so that the total with the content of other monomers does not exceed 100% by weight, and may be, for example, less than 80% by weight. From the viewpoint of easily balancing the low refractive index with other properties, in some embodiments, the content of the low Tg alkyl (meth)acrylate is suitably 75% by weight or less, and may be 65% by weight or less, 55% by weight or less, or 50% by weight or less. The technology disclosed herein can also be implemented in an embodiment in which the content of the low Tg alkyl (meth)acrylate in the monomer components constituting the polymer (F) is 45% by weight or less, 35% by weight or less, 30% by weight or less, 25% by weight or less, 10% by weight or less, 5% by weight or less, or 0% by weight.

(Other Monomers)
The monomer component constituting the polymer (F) may contain monomers (hereinafter also referred to as "other monomers") other than the above-mentioned fluorine-containing acrylic monomer, monomer (Mh) and monomer (M1) as necessary.

Examples of the other monomers include C _1-3 alkyl (meth)acrylates such as methyl (meth)acrylate and ethyl (meth)acrylate; chain alkyl (meth)acrylates in which the chain alkyl group has 19 or more carbon atoms (for example, about 19 to 24 carbon atoms), such as nonadecyl (meth)acrylate and eicosyl (meth)acrylate; non-aromatic ring-containing monomers such as cyclohexyl (meth)acrylate and isobornyl (meth)acrylate; aromatic ring-containing monomers such as styrene, α-methylstyrene and vinyl toluene; vinyl ester monomers such as vinyl acetate; olefin monomers such as ethylene, butadiene and isobutylene; chlorine-containing monomers such as vinyl chloride; vinyl ether monomers such as methyl vinyl ether; cyano group-containing monomers such as acrylonitrile and methacrylonitrile; and acidic functional group-containing monomers.

When the other monomers are used, the amount used is not particularly limited and can be appropriately set within a range in which the total amount of the monomer components does not exceed 100% by weight. In some embodiments, the content of the other monomers in the monomer components constituting the polymer (F) can be, for example, about 35% by weight or less, and it is appropriate to set it to about 25% by weight or less (e.g., 0 to 25% by weight), and it may be about 20% by weight or less (e.g., 0 to 20% by weight), about 10% by weight or less, about 5% by weight or less, for example, about 1% by weight or less, 0.5% by weight or less, 0.3% by weight or less, 0.1% by weight or less, or 0.05% by weight or less. The other monomers may not be used.

When an acidic functional group-containing monomer is used as the other monomer, the acidic functional group-containing monomer may be one or more selected from carboxy group-containing monomers such as (meth)acrylic acid, (meth)acrylate carboxyethyl, 2-(meth)acryloyloxyethyl succinic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, isocrotonic acid, etc.; sulfonic acid group-containing monomers; and phosphoric acid group-containing monomers. When an acidic functional group-containing monomer is used, the content of the acidic functional group-containing monomer in the monomer component constituting polymer (F) may be, for example, 0.01% by weight or more. The upper limit of the content of the carboxy group-containing monomer in the monomer component is set so that the total amount of the carboxy group-containing monomer and the amount of the other monomers used does not exceed 100% by weight. In some embodiments, the content of the acidic functional group-containing monomer in the monomer component constituting the polymer (F) is suitably less than 7.0 wt%, preferably less than 5.0 wt%, more preferably less than 3.0 wt%, and may be less than 2.0 wt%, less than 1.0 wt%, less than 0.5 wt%, less than 0.3 wt%, less than 0.1 wt%, or less than 0.05 wt%. The content of the acidic functional group-containing monomer (e.g., carboxyl group-containing monomer) is limited in this manner, which is preferable from the viewpoint of suppressing coloring or discoloration (e.g., yellowing) of the adhesive, and is also advantageous from the viewpoint of improving the flexibility of the adhesive (particularly, flexibility in the low temperature range). In addition, the content of the acidic functional group-containing monomer is limited, which is also preferable from the viewpoint of suppressing corrosion of metal materials (e.g., metal wiring, metal films, etc. that may be present on the adherend) that may be placed in contact with or in close proximity to the adhesive disclosed herein. The technology disclosed herein can be preferably implemented in an embodiment in which the monomer component constituting the polymer (F) does not contain an acidic functional group-containing monomer (i.e., an embodiment in which the polymer (F) is acid-free).

<Adhesive Composition>
The adhesive disclosed herein can be formed using an adhesive composition containing the monomer components of the above-mentioned composition in the form of a polymer, an unpolymerized product (i.e., a form in which the polymerizable functional group is unreacted), or a mixture thereof. The adhesive composition can be in various forms, such as a composition containing an adhesive (adhesive component) in an organic solvent (solvent-type adhesive composition), a composition in which an adhesive is dispersed in an aqueous solvent (water-dispersed adhesive composition), a composition prepared to be cured by active energy rays such as ultraviolet rays or radiation to form an adhesive (active energy ray-curable adhesive composition), or a hot melt-type adhesive composition that is applied in a heated and molten state and forms an adhesive when cooled to around room temperature.

In this specification, "active energy rays" refers to energy rays that have the energy to cause chemical reactions such as polymerization reactions, crosslinking reactions, and decomposition of initiators. Examples of active energy rays include light such as ultraviolet rays, visible light, and infrared rays, as well as radiation such as alpha rays, beta rays, gamma rays, electron beams, neutron beams, and X-rays.

In some preferred embodiments, the pressure-sensitive adhesive composition contains at least a part of the monomer components constituting the polymer (F) (which may be a part of the type of monomer or a part of the amount) in the form of a polymer. The polymerization method for forming the polymer is not particularly limited, and various conventionally known polymerization methods can be appropriately adopted. For example, thermal polymerization such as solution polymerization, emulsion polymerization, and bulk polymerization (typically performed in the presence of a thermal polymerization initiator); photopolymerization performed by irradiating with light such as ultraviolet rays (typically performed in the presence of a photopolymerization initiator); radiation polymerization performed by irradiating with radiation such as β rays and γ rays; and the like can be appropriately adopted. Among these, photopolymerization is preferable. In these polymerization methods, the mode of polymerization is not particularly limited, and the polymerization can be performed by appropriately selecting a conventionally known monomer supply method, polymerization conditions (temperature, time, pressure, light exposure amount, radiation exposure amount, etc.), and materials used other than the monomer (polymerization initiator, surfactant, etc.).

For polymerization, a known or commonly used photopolymerization initiator or thermal polymerization initiator may be used depending on the polymerization method, polymerization mode, etc. Such polymerization initiators may be used alone or in appropriate combination of two or more types.

The photopolymerization initiator is not particularly limited, but examples that can be used include ketal-based photopolymerization initiators, acetophenone-based photopolymerization initiators, benzoin ether-based photopolymerization initiators, acylphosphine oxide-based photopolymerization initiators, α-ketol-based photopolymerization initiators, aromatic sulfonyl chloride-based photopolymerization initiators, photoactive oxime-based photopolymerization initiators, benzoin-based photopolymerization initiators, benzyl-based photopolymerization initiators, benzophenone-based photopolymerization initiators, and thioxanthone-based photopolymerization initiators.

Specific examples of ketal-based photopolymerization initiators include 2,2-dimethoxy-1,2-diphenylethan-1-one (eg, Omnirad 651, a product name of IGM Resins).
Specific examples of acetophenone-based photopolymerization initiators include 1-hydroxycyclohexyl-phenyl-ketone (e.g., trade name "Omnirad 184" manufactured by IGM Resins), 4-phenoxydichloroacetophenone, 4-t-butyl-dichloroacetophenone, 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one, 2-hydroxy-2-methyl-1-phenyl-propan-1-one, methoxyacetophenone, and the like.
Specific examples of the benzoin ether-based photopolymerization initiator include benzoin ethers such as benzoin methyl ether, benzoin ethyl ether, benzoin propyl ether, benzoin isopropyl ether, and benzoin isobutyl ether, and substituted benzoin ethers such as anisole methyl ether.
Specific examples of the acylphosphine oxide photopolymerization initiator include bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-di-n-butoxyphenylphosphine oxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, and the like.
Specific examples of α-ketol-based photopolymerization initiators include 2-methyl-2-hydroxypropiophenone, 1-[4-(2-hydroxyethyl)phenyl]-2-methylpropan-1-one, etc. Specific examples of aromatic sulfonyl chloride-based photopolymerization initiators include 2-naphthalenesulfonyl chloride, etc. Specific examples of photoactive oxime-based photopolymerization initiators include 1-phenyl-1,1-propanedione-2-(o-ethoxycarbonyl)-oxime, etc. Specific examples of benzoin-based photopolymerization initiators include benzoin, etc. Specific examples of benzyl-based photopolymerization initiators include benzyl, etc.
Specific examples of the benzophenone-based photopolymerization initiator include benzophenone, benzoylbenzoic acid, 3,3'-dimethyl-4-methoxybenzophenone, polyvinylbenzophenone, α-hydroxycyclohexyl phenyl ketone, and the like.
Specific examples of the thioxanthone-based photopolymerization initiator include thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone, 2,4-dimethylthioxanthone, isopropylthioxanthone, 2,4-dichlorothioxanthone, 2,4-diethylthioxanthone, isopropylthioxanthone, 2,4-diisopropylthioxanthone, dodecylthioxanthone, and the like.

Thermal polymerization initiators are not particularly limited, but examples of initiators that can be used include azo-based polymerization initiators, peroxide-based initiators, redox-based initiators that are a combination of peroxides and reducing agents, and substituted ethane-based initiators. More specifically, examples of initiators that can be used include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylpropionamidine) disulfate, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis(N,N'-dimethyleneisobutylamidine), 2,2'-azobis[N-(2-carboxyethyl)-2-methyl Examples include, but are not limited to, azo-based initiators such as propionamidine hydrate; persulfates such as potassium persulfate and ammonium persulfate; peroxide-based initiators such as benzoyl peroxide, t-butyl hydroperoxide, and hydrogen peroxide; substituted ethane-based initiators such as phenyl-substituted ethane; redox-based initiators such as a combination of a persulfate and sodium hydrogen sulfite, or a combination of a peroxide and sodium ascorbate; and the like. Thermal polymerization can be preferably carried out at a temperature of, for example, about 20 to 100°C (typically 40 to 80°C).

The amount of such a thermal polymerization initiator or photopolymerization initiator used can be a normal amount depending on the polymerization method, polymerization mode, etc., and is not particularly limited. For example, about 0.001 to 5 parts by weight (typically about 0.01 to 2 parts by weight, for example about 0.01 to 1 part by weight) of polymerization initiator can be used per 100 parts by weight of the monomer to be polymerized.

(Adhesive composition containing polymerized and unpolymerized monomer components)
The pressure-sensitive adhesive composition according to some embodiments includes a polymerization reaction product of a monomer mixture containing at least a part of a monomer component (raw material monomer). Typically, the monomer component is included in the form of a polymer, and the remaining part is included in the form of an unpolymerized product (unreacted monomer). The pressure-sensitive adhesive composition including the polymerized product and the unpolymerized product of the monomer component can be preferably used as, for example, an active energy ray curable pressure-sensitive adhesive composition. The polymerization reaction product of the monomer mixture can be prepared by at least partially polymerizing the monomer mixture.
The polymerization reaction product is preferably a partial polymer of the monomer mixture. Such a partial polymer is a mixture of a polymer derived from the monomer mixture and an unreacted monomer, and typically has a syrup-like appearance (viscous liquid). Hereinafter, a partial polymer having such properties may be referred to as a "monomer syrup" or simply as a "syrup."

The polymerization method for obtaining the above-mentioned polymerization reaction product is not particularly limited, and various polymerization methods as described above can be appropriately selected and used. From the viewpoints of efficiency and simplicity, a photopolymerization method can be preferably used. With photopolymerization, the polymerization conversion rate of the above-mentioned monomer mixture can be easily controlled by the polymerization conditions such as the amount of light irradiation (light amount).

The polymerization conversion rate (monomer conversion) of the monomer mixture in the partial polymer is not particularly limited. The polymerization conversion rate can be, for example, about 70% by weight or less, and is preferably about 60% by weight or less. From the viewpoint of ease of preparation and coatability of the adhesive composition containing the partial polymer, the polymerization conversion rate is usually appropriately about 50% by weight or less, and preferably about 40% by weight or less (for example, about 35% by weight or less). The lower limit of the polymerization conversion rate is not particularly limited, but is typically about 1% by weight or more, and usually about 5% by weight or more is appropriate.

The adhesive composition containing the partial polymer of the monomer mixture can be obtained, for example, by partially polymerizing the monomer mixture containing all of the raw material monomers by an appropriate polymerization method (for example, photopolymerization method). The adhesive composition containing the partial polymer can be blended with other components (for example, a photopolymerization initiator, a crosslinking agent (which may be a polyfunctional monomer), etc.) that are used as needed. There are no particular limitations on the method of blending such other components, and for example, they may be contained in the monomer mixture in advance, or they may be added to the partial polymer.

The adhesive composition disclosed herein may be in a form in which a partial polymer or a complete polymer of a monomer mixture containing some types of monomers among the monomer components (raw material monomers) is dissolved in the remaining types of monomers or their partial polymers. Adhesive compositions in such a form are also included in examples of adhesive compositions containing polymerized and unpolymerized monomer components. In this specification, the term "completely polymerized" refers to a polymerization conversion rate of more than 95% by weight.

As such, a photopolymerization method can be preferably used as a curing method (polymerization method) when forming an adhesive from an adhesive composition containing a polymerized product and an unpolymerized product of a monomer component. In an adhesive composition containing a polymerization reactant prepared by a photopolymerization method, it is particularly preferable to use a photopolymerization method as a curing method. Since the polymerization reactant obtained by the photopolymerization method already contains a photopolymerization initiator, when the adhesive composition containing this polymerization reactant is further cured to form an adhesive, it can be photocured without adding a new photopolymerization initiator. Alternatively, the adhesive composition may be a composition in which a photopolymerization initiator is added as necessary to the polymerization reactant prepared by the photopolymerization method. The added photopolymerization initiator may be the same as or different from the photopolymerization initiator used to prepare the polymerization reactant. An adhesive composition prepared by a method other than photopolymerization can be made photocurable by adding a photopolymerization initiator. A photocurable adhesive composition has the advantage that even a thick adhesive layer can be easily formed. In a preferred embodiment, photopolymerization when forming an adhesive from the adhesive composition can be performed by ultraviolet irradiation. For ultraviolet irradiation, known high pressure mercury lamps, low pressure mercury lamps, metal halide lamps, etc. can be used.

(Adhesive composition containing monomer component in the form of a completely polymerized product)
The pressure-sensitive adhesive composition according to some other embodiments contains the monomer component in the form of a complete polymer. Such a pressure-sensitive adhesive composition may be in the form of, for example, a solvent-based pressure-sensitive adhesive composition containing a polymer (F) which is a complete polymer of the monomer component (for example, a complete polymer by solution polymerization or emulsion polymerization) in an organic solvent, or an aqueous dispersion-based pressure-sensitive adhesive composition in which the polymer (F) is dispersed in an aqueous solvent. The solvent (polymerization solvent) used in the solution polymerization of the monomer component can be appropriately selected from conventionally known organic solvents. For example, any one solvent selected from aromatic compounds such as toluene (typically aromatic hydrocarbons); acetate esters such as ethyl acetate; aliphatic or alicyclic hydrocarbons such as hexane and cyclohexane; halogenated alkanes such as 1,2-dichloroethane; lower alcohols such as isopropyl alcohol (for example, monohydric alcohols having 1 to 4 carbon atoms); ethers such as tert-butyl methyl ether; ketones such as methyl ethyl ketone; or the like, or a mixed solvent of two or more solvents can be used.

(Crosslinking Agent)
The adhesive composition may contain a crosslinking agent as necessary for the purpose of adjusting the cohesive strength of the adhesive. As the crosslinking agent, a crosslinking agent known in the field of adhesives, such as an isocyanate-based crosslinking agent, an epoxy-based crosslinking agent, an aziridine-based crosslinking agent, an oxazoline-based crosslinking agent, a melamine-based resin, or a metal chelate-based crosslinking agent, may be used. Among these, preferred examples include an isocyanate-based crosslinking agent and an epoxy-based crosslinking agent. Other examples of the crosslinking agent include a monomer having two or more ethylenically unsaturated groups in one molecule, that is, a polyfunctional monomer having two or more functions. The crosslinking agent may be used alone or in combination of two or more types.

Examples of polyfunctional monomers include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, Bifunctional (meth)acrylates such as 1,12-dodecanediol di(meth)acrylate, glycerin di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, stearic acid modified pentaerythritol di(meth)acrylate, dicyclopentenyl di(meth)acrylate, di(meth)acryloyl isocyanurate, bisphenol A di(meth)acrylate, and alkylene oxide modified bisphenol di(meth)acrylate;
For example, trifunctional (meth)acrylates such as trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, tetramethylolmethane tri(meth)acrylate, and tris(acryloyloxyethyl)isocyanurate;
For example, polyfunctional (meth)acrylates having 4 or more functional groups, such as ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol monohydroxypenta(meth)acrylate, dipentaerythritol penta(meth)acrylate, alkyl-modified dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, epoxy acrylates having 4 or more (meth)acryloyl groups, polyester acrylates having the same, and urethane acrylates having the same;
Polyfunctional monomers (e.g., bifunctional monomers) having at least one ethylenically unsaturated group other than a (meth)acryloyl group, such as allyl (meth)acrylate, vinyl (meth)acrylate, and divinylbenzene;
The polyfunctional monomers may be used alone or in combination of two or more.

As an isocyanate-based crosslinking agent, a bifunctional or higher isocyanate compound can be used, for example, aliphatic polyisocyanates such as trimethylene diisocyanate, butylene diisocyanate, pentamethylene diisocyanate (PDI), hexamethylene diisocyanate (HDI), and dimer acid diisocyanate; alicyclic isocyanates such as cyclopentylene diisocyanate, cyclohexylene diisocyanate, isophorone diisocyanate (IPDI), and 1,3-bis(isocyanatomethyl)cyclohexane; 2,4-tolylene diisocyanate. aromatic isocyanates such as 4,4'-diphenylmethane diisocyanate and xylylene diisocyanate (XDI); modified polyisocyanates obtained by modifying the above-mentioned isocyanate compounds with an allophanate bond, a biuret bond, an isocyanurate bond, a uretdione bond, a urea bond, a carbodiimide bond, a uretonimine bond, an oxadiazinetrione bond or the like (e.g., an isocyanurate of HDI, an allophanate of HDI, etc.); polyhydric alcohol adducts of the above-mentioned isocyanate compounds (e.g., a trimethylolpropane adduct of XDI, etc.); and the like. Examples of commercially available products include trade names Takenate 300S, Takenate 500, Takenate 600, Takenate D110N, Takenate D120N, Takenate D140N, Takenate D160N, Takenate D165N, Takenate D178N, Takenate D178NL (all manufactured by Mitsui Chemicals, Inc.), Sumidur T80, Sumidur L, Desmodur N3400 (all manufactured by Sumika Bayer Urethane Co., Ltd.), Millionate MR, Millionate MT, Coronate L, Coronate HL, Coronate HX, Coronate 2770 (all manufactured by Tosoh Corporation), trade names Duranate A201H, Duranate TPA-100 (all manufactured by Asahi Kasei Corporation), etc. The isocyanate compounds can be used alone or in combination of two or more types. A bifunctional isocyanate compound may be used in combination with a trifunctional or higher isocyanate compound.

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

In some embodiments, a bifunctional crosslinking agent having two crosslinking reactive groups per molecule (e.g., an ethylenically unsaturated group, an isocyanate group, etc.) is used as at least a portion of the crosslinking agent. By using a bifunctional crosslinking agent, it is easy to form a flexible crosslinked structure. The bifunctional crosslinking agent can be used alone or in combination of two or more types. Examples of bifunctional crosslinking agents include bifunctional monomers such as bifunctional (meth)acrylates and bifunctional isocyanate compounds. The bifunctional crosslinking agent may be used in combination with a trifunctional or higher crosslinking agent.

In some embodiments, a non-cyclic crosslinking agent (also called a chain crosslinking agent) that does not have a ring structure such as an aromatic ring or an aliphatic ring is preferably used as the crosslinking agent. By using a chain crosslinking agent as the crosslinking agent, it becomes easier to form a highly flexible crosslinked structure, and therefore it becomes easier to form a highly flexible adhesive.
As the acyclic crosslinking agent, for example, among the above-mentioned polyfunctional monomers, it is preferable to use a polyfunctional monomer that does not have a ring structure, such as a chain alkylene diol di(meth)acrylate or an alkylene glycol di(meth)acrylate. Also, for example, among the above-mentioned isocyanate-based crosslinking agents, it is preferable to use an isocyanate-based compound that does not have a ring structure, such as an aromatic ring or an isocyanurate ring. Specific examples of the above-mentioned acyclic isocyanates include aliphatic isocyanate-based compounds (e.g., PDI and HDI) and modified aliphatic isocyanate-based compounds (e.g., polyisocyanate modified PDI or HDI modified by allophanate bond, biuret bond, urea bond, or carbodiimide bond). The acyclic crosslinking agent can be used alone or in combination of two or more. In some preferred embodiments, an acyclic bifunctional crosslinking agent can be used as the crosslinking agent.

When using a crosslinking agent (which may be a polyfunctional monomer), the amount used is not particularly limited and can be set so as to obtain the desired characteristics. The amount of crosslinking agent used can be, for example, in the range of about 0.001 to 5.0 parts by weight per 100 parts by weight of the monomer components constituting the polymer (F). From the viewpoint of improving the flexibility and elongation of the adhesive, in some embodiments, the amount of crosslinking agent used per 100 parts by weight of the monomer components is appropriately 3.0 parts by weight or less, and preferably 2.0 parts by weight or less, and may be, for example, 1.5 parts by weight or less, 1.0 parts by weight or less, 0.50 parts by weight or less, 0.30 parts by weight or less, 0.25 parts by weight or less, 0.20 parts by weight or less, 0.15 parts by weight or less, 0.12 parts by weight or less, 0.10 parts by weight or less, or 0.09 parts by weight or less. In addition, from the viewpoint of appropriately exerting the effect of using the crosslinking agent, in some embodiments, the amount of the crosslinking agent used relative to 100 parts by weight of the above-mentioned monomer components may be, for example, 0.005 parts by weight or more, 0.010 parts by weight or more, 0.015 parts by weight or more, 0.02 parts by weight or more, 0.04 parts by weight or more, 0.06 parts by weight or more, or 0.08 parts by weight or more, or 0.10 parts by weight or more, or 0.15 parts by weight or more.

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

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

(Silane coupling agent)
In some embodiments, the pressure-sensitive adhesive may contain a silane coupling agent. The silane coupling agent may be useful for improving the adhesive strength to the adherend. The silane coupling agent may be used alone or in combination of two or more.

Silane coupling agents include silicon compounds having an epoxy structure such as 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; amino group-containing silicon compounds such as 3-aminopropyltrimethoxysilane, N-(2-aminoethyl)3-aminopropyltrimethoxysilane, and N-(2-aminoethyl)3-aminopropylmethyldimethoxysilane; 3-chloropropyltrimethoxysilane; (meth)acrylic group-containing silane coupling agents such as acetoacetyl group-containing trimethoxysilane, 3-acryloxypropyltrimethoxysilane, and 3-methacryloxypropyltriethoxysilane; and isocyanate group-containing silane coupling agents such as 3-isocyanatepropyltriethoxysilane. Among these, preferred examples include 3-glycidoxypropyltrimethoxysilane and acetoacetyl group-containing trimethoxysilane.

The amount of the silane coupling agent used can be set so as to obtain the desired effect of use, and is not particularly limited. In some embodiments, the amount of the silane coupling agent used may be, for example, 0.001 parts by weight or more relative to 100 parts by weight of the monomer components constituting the polymer (F), and from the viewpoint of obtaining a higher effect, it may be 0.005 parts by weight or more, 0.01 parts by weight or more, 0.05 parts by weight or more, 0.1 parts by weight or more, or 0.2 parts by weight or more. In addition, from the viewpoint of storage stability of the adhesive, in some embodiments, the amount of the silane coupling agent used may be, for example, less than 3.0 parts by weight, less than 2.0 parts by weight, less than 1.5 parts by weight, less than 1.0 parts by weight, or 0.5 parts by weight or less relative to 100 parts by weight of the monomer components constituting the polymer (F). The silane coupling agent may not be used.

(Tackifier)
The adhesive layer in the technology disclosed herein may contain a tackifier. As the tackifier, known tackifier resins such as rosin-based tackifier resins, terpene-based tackifier resins, phenol-based tackifier resins, hydrocarbon-based tackifier resins, ketone-based tackifier resins, polyamide-based tackifier resins, epoxy-based tackifier resins, and elastomer-based tackifier resins can be used. These can be used alone or in combination of two or more. The amount of the tackifier resin used is not particularly limited, and can be set so that appropriate adhesive performance is exhibited depending on the purpose and application. In some embodiments, from the viewpoint of refractive index and transparency, the amount of the tackifier used is appropriately 30 parts by weight or less, preferably 10 parts by weight or less, and more preferably 5 parts by weight or less, relative to 100 parts by weight of the monomer components constituting the polymer (F). The technology disclosed herein can be preferably implemented in an embodiment in which a tackifier is not used.

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

(Gel Fraction)
The gel fraction of the adhesive disclosed herein is appropriately set according to the purpose of use, the mode of use, etc., and is not limited to a specific range. The gel fraction is, for example, about 10% or more, about 30% or more, preferably about 40% or more, more preferably about 50% or more, and may be 75% or more, 85% or more, 90% or more, 92% or more, 94% or more, 96% or more, or 98% or more, from the viewpoint of providing the adhesive with appropriate cohesiveness and appropriately expressing adhesive properties. In addition, the gel fraction of the adhesive is appropriately 99.9% or less, for example, 99.7% or less, 99.5% or less, 99% or less, 97% or less, 95% or less, or 93% or less, from the viewpoint of favorably achieving both a low refractive index and adhesive properties. A gel fraction that is not too high can also be advantageous from the viewpoint of appropriately following unevenness that may exist on the surface of the adherend and providing good adhesion. The gel fraction is measured by the following method.

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

(Active energy ray curable adhesive)
In some preferred embodiments, the adhesive disclosed herein may be an adhesive formed from an active energy ray (e.g., ultraviolet ray) curable adhesive composition, i.e., an adhesive that is a cured product of an active energy ray curable adhesive composition. Such an adhesive is preferable because it is easy to realize high surface smoothness on the adhesive surface (interface with the release surface) and easy to obtain good optical properties, since the adhesive composition can be cured while the surface of the adhesive composition is in contact with a smooth release surface (e.g., a release liner having the release surface) to form an adhesive (adhesive layer). In addition, the active energy ray curable adhesive composition does not contain a solvent that should be removed in the process of forming the adhesive from the adhesive composition, or even if it does, it contains a small amount (typically less than 10 wt%, less than 5 wt%, or less than 1 wt% of the adhesive composition), so that an adhesive (adhesive layer) with high homogeneity in the thickness direction is easily obtained. This can be advantageous from the viewpoint of optical isotropy. In addition, by forming an adhesive using an adhesive composition that does not contain a solvent or has a low solvent content, it is easy to suppress the bias of hydrophilicity in the adhesive and to improve the resistance to moist heat whitening.

(Polymerization rate)
The adhesive disclosed herein is suitable for a polymerization rate (polymerization conversion rate) of 95.0% by weight or more, preferably 97% by weight or more (e.g., 97.5% by weight or more), and more preferably 98.5% by weight or more (e.g., 99.0% by weight or more). A higher polymerization rate of the adhesive means that the adhesive contains less unreacted monomer. It is preferable to set the polymerization rate of the adhesive to a predetermined level or more in order to prevent the uneven distribution of low molecular weight substances that may be contained in the adhesive from having a disadvantage on adhesive properties and optical properties. The polymerization rate of the adhesive can be measured by the method described in the Examples below.

<Adhesive sheet>
This specification provides a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer. The pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer may be a pressure-sensitive adhesive formed from any of the pressure-sensitive adhesive compositions disclosed herein (e.g., a cured product of the pressure-sensitive adhesive composition).

The adhesive sheet may be a substrate-attached adhesive sheet having the adhesive layer on one or both sides of a non-removable substrate (support substrate), or may be a substrate-less adhesive sheet having the adhesive layer supported on a release liner (i.e., an adhesive sheet without a non-removable substrate. Typically, the adhesive sheet is made of an adhesive layer). The concept of adhesive sheet here may include those referred to as adhesive tape, adhesive label, adhesive film, etc. The adhesive sheet disclosed herein may be in the form of a roll or a sheet. Alternatively, it may be an adhesive sheet that has been further processed into various shapes.

The configuration of a double-sided adhesive type substrateless adhesive sheet (substrateless double-sided adhesive sheet) is shown in Figs. 1 and 2. The adhesive sheet 1 shown in Fig. 1 has a configuration in which both

sides

21A and 21B of the substrateless adhesive layer 21 are protected by

release liners

31 and 32, at least the adhesive layer side of which is a release surface. The adhesive sheet 2 shown in Fig. 2 has a configuration in which one surface (adhesive surface) 21A of the substrateless adhesive layer 21 is protected by a release liner 31, both sides of which are release surfaces, and when this is rolled up, the other surface (adhesive surface) 21B of the adhesive layer 21 abuts against the back surface of the release liner 31, so that the other surface 21B is also protected by the release liner 31. The technology disclosed here is preferably implemented in the form of a substrateless adhesive sheet made of an adhesive layer, for example, from the viewpoint of reducing the thickness of the adhesive sheet and increasing the transparency of the adhesive sheet. The substrateless adhesive sheet is also preferable from the viewpoint of flexibility (for example, flexibility to follow the adherend when repeatedly folded).

The adhesive sheet disclosed herein may have, for example, a cross-sectional structure as shown in FIG. 3. The adhesive sheet 3 shown in FIG. 3 includes a support substrate 10 and a first adhesive layer 21 and a second adhesive layer 22 supported on a first surface 10A and a second surface 10B of the support substrate 10, respectively. The first surface 10A and the second surface 10B are both non-peeling surfaces (non-peeling surfaces). The adhesive sheet 3 is used by attaching the surface (first adhesive surface) 21A of the first adhesive layer 21 and the surface (second adhesive surface) 22A of the second adhesive layer 22 to an adherend, respectively. That is, the adhesive sheet 3 is configured as a double-sided adhesive sheet (double-sided adhesive sheet). Before use, the adhesive sheet 3 has a configuration in which the first adhesive surface 21A and the second adhesive surface 22A are protected by

release liners

31 and 32, at least the adhesive surface side of which is a surface (peeling surface) having releasability. Alternatively, the release liner 32 may be omitted, and a release liner 31 with release surfaces on both sides may be used, and the adhesive sheet 3 may be rolled up so that the second adhesive surface 22A is in contact with the back surface of the release liner 31, thereby forming a configuration in which the second adhesive surface 22A is also protected by the release liner 31.

The technology disclosed herein is preferably implemented in the form of a substrate-less or substrate-attached double-sided adhesive sheet as described above. Alternatively, the adhesive sheet disclosed herein may be in the form of a substrate-attached single-sided adhesive sheet having an adhesive layer on only one side of a non-peeling substrate (support substrate), although this is not specifically shown. An example of the form of a single-sided adhesive sheet is one having the configuration shown in FIG. 3 without either the first adhesive layer 21 or the second adhesive layer 22.

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

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

The thickness of the adhesive layer is not particularly limited, and may be, for example, 3 μm or more, suitably 5 μm or more, may be 10 μm or more, may be 15 μm or more, may be 20 μm or more, may be 30 μm or more, or may be 45 μm or more. The adhesive strength tends to increase with an increase in the thickness of the adhesive layer. In some embodiments, the thickness of the adhesive layer may be 50 μm or more, 70 μm or more, or 85 μm or more. The thickness of the adhesive layer may be, for example, 300 μm or less, 250 μm or less, 200 μm or less, 150 μm or less, or 120 μm or less. In some preferred embodiments, the thickness of the adhesive layer is 100 μm or less, more preferably 75 μm or less, even more preferably 70 μm or less, and may be 60 μm or less. It may be advantageous for the adhesive layer not to be too thick in terms of thinning the adhesive sheet, etc. In addition, a thin adhesive layer tends to have excellent conformability to the adherend. In some embodiments, the thickness of the adhesive layer may be 40 μm or less, or may be 30 μm or less. The technology disclosed herein can be preferably implemented, for example, in an embodiment in which the thickness of the adhesive layer is in the range of 3 μm to 200 μm (more preferably 5 μm to 100 μm, and even more preferably 5 μm to 75 μm). In the case of an adhesive sheet having a first adhesive layer and a second adhesive layer on the first surface and the second surface of the substrate, the thickness of the adhesive layer described above can be applied to at least the thickness of the first adhesive layer. The thickness of the second adhesive layer can also be selected from the same range. In the case of an adhesive sheet without a substrate, the thickness of the adhesive sheet is the same as the thickness of the adhesive layer.

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

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

(Haze value)
In some embodiments, the haze value (sometimes simply referred to as "haze") of the pressure-sensitive adhesive layer constituting the pressure-sensitive adhesive sheet may be, for example, 5.0% or less, preferably 3.0% or less, more preferably 2.0% or less, even more preferably 1.0% or less, 0.9% or less, 0.8% or less, 0.5% or less, or 0.3% or less. Such a pressure-sensitive adhesive sheet having a highly transparent pressure-sensitive adhesive layer, in a configuration with or without a substrate, can be preferably applied to applications requiring high light transmittance (e.g., optical applications) or applications requiring performance in which the adherend can be well visually recognized through the pressure-sensitive adhesive sheet. The lower limit of the haze value of the pressure-sensitive adhesive layer is not particularly limited, and from the viewpoint of improving transparency, the smaller the haze value, the more preferable. On the other hand, in some embodiments, taking into consideration the refractive index and adhesive properties, the haze value may be, for example, 0.05% or more, or 0.10% or more. These haze values for the pressure-sensitive adhesive layer can also be preferably applied to the haze values of a substrateless pressure-sensitive adhesive sheet (typically, a pressure-sensitive adhesive sheet consisting of a pressure-sensitive adhesive layer) when the technology disclosed herein is implemented in the form of the pressure-sensitive adhesive sheet.

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

In some embodiments, the haze value of the adhesive sheet may be, for example, 10.0% or less, preferably 5.0% or less, more preferably 3.0% or less, even more preferably 2.0% or less, 1.7% or less, 1.5% or less, 1.2% or less, 1.0% or less, or 0.8% or less. Such highly transparent adhesive sheets can be preferably applied to applications requiring high light transmittance (e.g., optical applications) or applications requiring good visibility of the adherend through the adhesive sheet. The lower limit of the haze value of the adhesive sheet is not particularly limited, and from the viewpoint of improving transparency, the smaller the haze value, the more preferable it is. On the other hand, in some embodiments, taking into consideration the refractive index and adhesive properties, the haze value may be, for example, 0.05% or more, 0.1% or more, 0.2% or more, or 0.3% or more. The haze value of the adhesive sheet can be measured in the same manner as in the measurement of the haze value of the adhesive layer. The above haze value of the adhesive sheet can be obtained by selecting the composition of the adhesive layer described above, and in configurations that have a substrate, by selecting the substrate type and substrate thickness.

(Peel Strength)
The peel strength of the adhesive sheet against a glass plate is not particularly limited. In some embodiments, the adhesive sheet has a peel strength against a glass plate (peel strength against a glass plate) of, for example, 0.1 N/25 mm or more, and may be 0.5 N/25 mm or more. In some preferred embodiments, the peel strength against the glass plate is 1.0 N/25 mm or more, more preferably 1.5 N/25 mm or more, even more preferably 2.0 N/25 mm or more, and may be 3.0 N/25 mm or more, 5.0 N/25 mm or more, 6.0 N/25 mm or more, 7.0 N/25 mm or more, 8.0 N/25 mm or more, 9.0 N/25 mm or more, or 10 N/25 mm or more. An adhesive sheet having a peel strength against a glass plate of a predetermined value or more is suitable for bonding or fixing, for example, glass members. The upper limit of the peel strength is not particularly limited, and may be, for example, 30 N/25 mm or less, 25 N/25 mm or less, or 20 N/25 mm or less.

The peel strength is determined by pressing the sheet against an alkaline glass plate as an adherend, leaving it in an environment of 23°C and 50% RH for 30 minutes, and then measuring the peel strength at a peel angle of 180 degrees and a tensile speed of 300 mm/min. If necessary, the pressure-sensitive adhesive sheet to be measured can be reinforced by attaching an appropriate backing material (for example, a polyethylene terephthalate (PET) film with a thickness of about 25 μm to about 50 μm) when making the measurement. More specifically, the peel strength can be measured according to the method described in the examples below.

(Wet and heat whitening resistance)
In some embodiments of the pressure-sensitive adhesive disclosed herein, the pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive may exhibit resistance to wet heat whitening, for example, a haze (post-wet heat haze) of less than about 3.0% after a wet heat test in which the pressure-sensitive adhesive layer is held in a wet heat environment of 85° C. and 85% RH for 240 hours. In some embodiments, the post-wet heat haze is preferably 2.0% or less, more preferably 1.0% or less, and may be 0.8% or less, 0.6% or less, 0.5% or less, 0.4% or less, 0.3% or less, or 0.2% or less. The post-wet heat haze of the pressure-sensitive adhesive layer is measured by the method described in the Examples below.
The increase in the haze after wet heat test relative to the initial haze (before the wet heat test), i.e., haze after wet heat test [%] - initial haze [%], may be, for example, less than 3.0%, preferably 2.0% or less or less than 2.0%, more preferably 1.0% or less or less than 1.0%, may be 0.8% or less, may be 0.6% or less, may be 0.5% or less, may be 0.4% or less, may be 0.3% or less, may be 0.2% or less, may be 0.1% or less, may be less than 0.1%, or may be 0.0%.

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

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

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

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

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

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

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

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

<Support substrate>
The pressure-sensitive adhesive sheet according to some embodiments may be in the form of a substrate-attached pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer on one or both sides of a support substrate. The material of the support substrate is not particularly limited, and can be appropriately selected according to the purpose and mode of use of the pressure-sensitive adhesive sheet. Non-limiting examples of substrates that can be used include plastic films such as polyolefin films mainly composed of polyolefins such as polypropylene (PP) and ethylene-propylene copolymers, polyester films mainly composed of polyesters such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN), and polyvinyl chloride films mainly composed of polyvinyl chloride; foam sheets made of foams such as polyurethane foam, polyethylene (PE) foam, and polychloroprene foam; woven and nonwoven fabrics made by single or mixed spinning of various fibrous materials (natural fibers such as hemp and cotton, synthetic fibers such as polyester and vinylon, semi-synthetic fibers such as acetate, etc.); papers such as Japanese paper, fine paper, craft paper, and crepe paper; metal foils such as aluminum foil and copper foil; and the like. The substrate may be a composite of these. Examples of such composite substrates include substrates having a structure in which a metal foil and the above-mentioned plastic film are laminated together, and plastic substrates reinforced with inorganic fibers such as glass cloth.

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

The resin material constituting the resin film may be, for example, polyester; polyolefin; polycycloolefin derived from a monomer having an aliphatic ring structure such as a norbornene structure; polyamide (PA) such as nylon 6, nylon 66, partially aromatic polyamide; polyimide (PI) such as transparent polyimide (CPI); polyamideimide (PAI); polyetheretherketone (PEEK); polyethersulfone (PES); polyphenylene sulfide (PPS); polycarbonate (PC); polyurethane (PU); ethylene-vinyl acetate copolymer (EVA); fluororesins such as polytetrafluoroethylene (PTFE); acrylic resin; cellulose-based polymers such as triacetyl cellulose (TAC); polyarylate; polystyrene; polyvinyl chloride; polyvinylidene chloride; and other resins.

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

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

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

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

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

The thickness of the substrate is not particularly limited and may be selected depending on the purpose and manner of use of the adhesive sheet. The thickness of the substrate may be, for example, 500 μm or less, and from the viewpoint of the handleability and processability of the adhesive sheet, it is preferably 300 μm or less, and may be 150 μm or less, 100 μm or less, 50 μm or less, 25 μm or less, or 10 μm or less. As the thickness of the substrate decreases, there is a tendency for the substrate to be able to conform to the surface shape of the adherend to improve. Also, from the viewpoint of handleability and processability, the thickness of the substrate may be, for example, 2 μm or more, 10 μm or more, or 25 μm or more.

The surface of the substrate on which the adhesive layer is laminated may be subjected to a conventionally known surface treatment, such as corona discharge treatment, plasma treatment, ultraviolet irradiation treatment, acid treatment, alkali treatment, or formation of an undercoat layer by applying an undercoat agent (primer), as necessary. Such a surface treatment may be a treatment for improving the anchoring ability of the adhesive layer to the substrate. The composition of the primer used to form the undercoat layer is not particularly limited, and can be appropriately selected from known ones. The thickness of the undercoat layer is not particularly limited, but is usually appropriate to be about 0.01 μm to 1 μm, and preferably about 0.1 μm to 1 μm. Other treatments that may be applied to the substrate as necessary include antistatic layer formation treatment, colored layer formation treatment, printing treatment, etc. These treatments may be applied alone or in combination.

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

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

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

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

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

The refractive index of the adherend and the refractive index of the adhesive layer placed in contact with the adherend may be similar or different. For example, by making the refractive index of the adhesive layer relatively low compared to the refractive index of the adherend, light incident on the adherend from the adhesive layer side can be refracted to the front side, thereby increasing the front brightness. On the other hand, by reducing the difference in refractive index between the adhesive layer and the adherend, light reflection at the interface can be suppressed. The refractive index of the adherend can be measured in the same manner as the refractive index of the adhesive.

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

An example of a preferred application is an optical application. More specifically, the adhesive sheet disclosed herein can be preferably used as an optical adhesive sheet used for bonding optical members (for bonding optical members) or for manufacturing products (optical products) using the optical members. The optical product may have a so-called polarizing plate-less configuration. For example, in an optical product equipped with a light source, the optical product may be an optical product in which the viewing side from the light source (such as an organic EL panel) is composed only of a layer with a polarization degree of 80% or less.

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

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

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

The adhesive disclosed herein (which may be in the form of an adhesive sheet containing the adhesive) may be used, for example, in a form in which it is disposed between an optical film, such as a film having one or more functions of light transmission, reflection, diffusion, waveguiding, focusing, diffraction, etc., or a fluorescent film, and another optical member (which may be another optical film), and may preferably be used to bond the optical film to the other optical member. In particular, in bonding optical films having at least one function of light guiding, focusing, and diffraction, it is desirable for the entire bulk of the bonding layer to have a low refractive index.

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

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

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

As will be understood from the above description and the following examples, the subject matter disclosed in this specification includes the following.
[1] A pressure-sensitive adhesive having a refractive index of 1.450 or less and a storage modulus at -20°C (G'(-20°C)) of 2.0 x 10 ⁶ Pa or less.
[2] The pressure-sensitive adhesive according to the above-mentioned [1], wherein the ratio (G'(-20°C)/E _B ) of the storage modulus (G'(-20°C)) [Pa] to the elongation at break (E _B ) [%] is 20 or more and 2000 or less.
[3] The pressure-sensitive adhesive according to the above-mentioned [1] or [2], which is an adhesive formed from an active energy ray-curable pressure-sensitive adhesive composition.
[4] The pressure-sensitive adhesive contains an acrylic polymer (F),
The monomer components constituting the acrylic polymer (F) include a fluorine-containing acrylic monomer (Mf) and an acid-free hydrophilic monomer (Mh),
The hydrophilic monomer (Mh) includes a low Tg hydrophilic monomer (Mh _L ) having a homopolymer glass transition temperature of 40° C. or lower;
The pressure-sensitive adhesive according to any one of the above [1] to [3], wherein the content of the low Tg hydrophilic monomer (Mh _L ) in the monomer components constituting the acrylic polymer (F) is more than 2.0 wt %.
[5] The pressure-sensitive adhesive according to [4] above, wherein the content of the hydrophilic monomer (Mh) in the monomer components constituting the acrylic polymer (F) is more than 5.0 wt %.
[6] The monomer component constituting the acrylic polymer (F) is represented by the following formula (1):
_CH2 = ^CR1COOR2 ( ¹ )
(wherein R ¹ is a hydrogen atom or a methyl group, and R ² is a chain alkyl group having 4 to 18 carbon atoms);
The pressure-sensitive adhesive according to the above-mentioned [4] or [5], further comprising a monomer (M1) represented by the following formula:
[7] The pressure-sensitive adhesive according to any one of [4] to [6] above, wherein the monomer component constituting the acrylic polymer (F) has a content of an acidic functional group-containing monomer of less than 2.0 wt %.
[8] A pressure-sensitive adhesive sheet comprising a pressure-sensitive adhesive layer made of the pressure-sensitive adhesive according to any one of [1] to [7] above.
[9] The pressure-sensitive adhesive sheet according to [8] above, wherein the pressure-sensitive adhesive layer has a haze of less than 3.0% after a wet heat test in which the pressure-sensitive adhesive layer is kept in a wet heat environment of 85° C. and 85% RH for 240 hours.
[10] The pressure-sensitive adhesive sheet according to the above [8] or [9], which has a peel strength from a glass plate (tensile speed: 300 mm/min, peel angle: 180 degrees) of 1.0 N/25 mm or more.

Below, several examples of the present invention are described, but it is not intended that the present invention be limited to those shown in these examples. In the following description, "parts" and "%" indicating the amount used and the content are based on weight unless otherwise specified.

<Example 1>
(Preparation of Pressure-Sensitive Adhesive Composition)
100 parts of a fluorine-containing acrylic monomer was charged into a four-neck flask together with 0.1 part of a photopolymerization initiator, and photopolymerized by irradiating ultraviolet light under a nitrogen atmosphere until the viscosity (BH viscometer, No. 5 rotor, 10 rpm, measurement temperature 30°C) reached about 15 Pa·s, to prepare a monomer syrup containing a partial polymer of the fluorine-containing acrylic monomer. 0.10 parts of 1,9-nonanediol diacrylate (NDDA) as a crosslinking agent and 0.3 parts of a silane coupling agent were added to 100 parts of the monomer syrup, and mixed uniformly to prepare an ultraviolet-curable pressure-sensitive adhesive composition U1.

Here, 2-(perfluorohexyl)ethyl acrylate was used as the fluorine-containing acrylic monomer (the same applies to the following examples unless otherwise specified). As the photopolymerization initiator, IGM Resins' product names "Omnirad 651" and "Omnirad 184" were used in a weight ratio of 1:1 (the same applies to the following examples unless otherwise specified). As the silane coupling agent, 3-glycidoxypropyltrimethoxysilane was used (the same applies to the following examples unless otherwise specified).

(Preparation of adhesive sheet)
The adhesive composition U1 obtained above was applied to a 38 μm thick release liner R1 (Mitsubishi Materials Corporation, MRF#38) with one side of a polyester film being the release surface, and then covered with a 38 μm thick release liner R2 (Mitsubishi Materials Corporation, MRE#38) with one side of a polyester film being the release surface to block air, and then irradiated with ultraviolet light to cure, thereby producing an adhesive layer (substrate-less double-sided adhesive sheet) with a thickness of 50 μm. The ultraviolet light irradiation was performed using a black light under the conditions of an illuminance of 2.5 mW/ ^cm2 and an accumulated light quantity of 2400 mJ/ ^cm2 .

<Examples 2 to 4>
100 parts of a monomer mixture containing a fluorine-containing acrylic monomer and 2-ethylhexyl acrylate (2EHA) in the weight ratio shown in Table 1 was charged into a four-neck flask together with 0.1 parts of a photopolymerization initiator, and photopolymerized in the same manner as in Example 1 to prepare a monomer syrup containing a partial polymer of the monomer mixture. 0.10 parts of 1,9-nonanediol diacrylate (NDDA) as a crosslinking agent and 0.3 parts of a silane coupling agent were added to 100 parts of the monomer syrup, and mixed uniformly to prepare ultraviolet-curable pressure-sensitive adhesive compositions U2 to U4 according to each example. A pressure-sensitive adhesive layer (substrate-less double-sided pressure-sensitive adhesive sheet) having a thickness of 50 μm was prepared in the same manner as in Example 1, except that the pressure-sensitive adhesive compositions U2 to U4 were used instead of the pressure-sensitive adhesive composition U1.

<Examples 5 to 13>
100 parts of a monomer mixture containing a fluorine-containing acrylic monomer, 2EHA and 4-hydroxybutyl acrylate (4HBA) in the weight ratios shown in Tables 1 and 2 was added to a four-neck flask together with 0.1 parts of a photopolymerization initiator, and photopolymerized in the same manner as in Example 1 to prepare a monomer syrup containing a partial polymer of the monomer mixture. NDDA in the amount shown in Tables 1 and 2 and 0.3 parts of a silane coupling agent were added to 100 parts of the monomer syrup, and mixed uniformly to prepare ultraviolet-curable pressure-sensitive adhesive compositions U5 to U13 according to each example. A pressure-sensitive adhesive layer (substrate-less double-sided pressure-sensitive adhesive sheet) having a thickness of 50 μm was prepared in the same manner as in Example 1, except that the pressure-sensitive adhesive compositions U5 to U13 were used instead of the pressure-sensitive adhesive composition U1.

<Examples 14 to 17>
Instead of 2EHA in Example 8, n-hexyl acrylate (HxA) was used in Example 14, n-octyl acrylate (NOAA) in Example 15, n-butyl acrylate (BA) in Example 16, and n-lauryl acrylate (LA) in Example 17. In other respects, ultraviolet-curable pressure-sensitive adhesive compositions U14 to U17 according to each example were prepared in the same manner as in Example 8. A 50 μm thick pressure-sensitive adhesive layer (substrate-less double-sided pressure-sensitive adhesive sheet) was produced in the same manner as in Example 1, except that the pressure-sensitive adhesive compositions U14 to U17 were used instead of the pressure-sensitive adhesive composition U1.

<Evaluation method>
(Gel Fraction)
About 0.1 g of an adhesive sample (weight _Wg1 ) was taken from the adhesive layer (substrate-less adhesive sheet) according to each example, wrapped in a porous polytetrafluoroethylene film (weight _Wg2 ) with an average pore size of 0.2 μm in a purse shape, and the opening was tied with string (weight _Wg3 ). As the porous polytetrafluoroethylene (PTFE) film, Nitto Denko Corporation's product name "Nitoflon (registered trademark) NTF1122" (average pore size 0.2 μm, porosity 75%, thickness 85 μm) was used.
The package was immersed in a sufficient amount of ethyl acetate and kept at 23° C. for 7 days to allow only the sol content in the adhesive to elute out of the film, after which the package was taken out and the ethyl acetate adhering to the outer surface was wiped off, the package was dried at 130° C. for 2 hours, and the weight of the package ( _Wg4 ) was measured. From the results obtained, the gel fraction of the adhesive layer was calculated using the following formula.
Gel fraction (%)=[( _Wg4 − _Wg2 − _Wg3 )/ _Wg1 ]×100

(Polymerization rate)
About 0.3 g of a pressure-sensitive adhesive sample was taken from the pressure-sensitive adhesive layer (substrate-less double-sided pressure-sensitive adhesive sheet) according to each example and heated for 2 hours at 130° C. Based on the weight of the sample before and after heating, the polymerization rate was calculated according to the following formula.
Polymerization rate [%] = (sample weight after heating/sample weight before heating) x 100

(Refractive Index)
The refractive index of the pressure-sensitive adhesive layer (substrate-less double-sided pressure-sensitive adhesive sheet) according to each example was measured using a prism coupler (manufactured by Metricon, model "2010M") at a measurement temperature of 25°C and a measurement wavelength of 594 nm.

(Total Light Transmittance and Haze)
A rectangular laminate having a structure in which the adhesive layer (substrate-less adhesive sheet) according to each example is sandwiched between two alkali-free glass plates (thickness 0.8 to 1.0 mm, total light transmittance 92%, haze 0.4%) (i.e., a three-layer structure of glass plate/adhesive layer/glass plate) and a size of 4.5 cm x 5 cm in plan view was prepared. The total light transmittance and haze of the laminate were measured using a haze meter ("HM-150" manufactured by Murakami Color Research Laboratory) under a measurement environment of 23°C and 50% RH. The measurement position was near the intersection of the diagonal lines of the rectangular laminate. The values obtained by subtracting the total light transmittance and haze of the two alkali-free glass plates from the measured values were taken as the total light transmittance (initial transmittance) [%] and haze (initial haze) [%] of the adhesive layer at the initial time. For a substrateless pressure-sensitive adhesive sheet consisting of the above pressure-sensitive adhesive layer, the total light transmittance [%] and haze [%] of the pressure-sensitive adhesive layer are the total light transmittance [%] and haze [%] of the pressure-sensitive adhesive sheet.

The laminate was kept in a moist heat environment of 85°C and 85% RH for 240 hours, and then left to stand in an environment of 23°C and 50% RH for 30 minutes, after which the haze of the laminate was measured in the same manner as above. The total light transmittance of the two alkali-free glass plates was subtracted from the measured value to determine the haze of the adhesive layer after moist heat (post-moist heat haze) [%].

(Storage Modulus G' and Glass Transition Temperature Tg)
The pressure-sensitive adhesive layers according to each example were laminated to a thickness of about 1.5 mm, and punched out into a disk shape with a diameter of 7.9 mm to prepare a measurement sample. Dynamic viscoelasticity measurement was performed under the following conditions using an "Advanced Rheometric Expansion System (ARES)" manufactured by Rheometric Scientific. From the measurement results, the storage modulus G' [Pa] of the pressure-sensitive adhesive at each temperature (-20°C, 25°C, and 60°C) was determined. In addition, the temperature corresponding to the peak top temperature of the loss tangent tan δ (loss modulus G"/storage modulus G') in the dynamic viscoelasticity measurement was determined as the glass transition temperature (Tg) [°C] of the pressure-sensitive adhesive.
[Measurement condition]
Deformation mode: Torsion Measurement frequency: 1 Hz
Temperature range: -50℃ to 150℃
Heating rate: 5°C/min. Shape: Parallel plate 7.9mmφ

(Young's Modulus, Breaking Stress and Elongation at Break E _B )
The adhesive layer (substrate-less double-sided adhesive sheet) according to each example was cut into a size of 30 mm in length and 40 mm in width while sandwiched between ^two release liners. The width of 40 mm was set according to the thickness of the adhesive layer so that the cross-sectional area of the adhesive layer in the cross section along the width direction was about 2 mm2. Next, one release liner was removed to expose one surface of the adhesive layer, and the adhesive layer was wound on the other release liner with its length direction as an axis to prepare a cylindrical sample with a length of 30 mm.

The cylindrical sample was set in a tension-compression tester (AGS-50NX, manufactured by Shimadzu Corporation) under a measurement environment of 23°C and 50% RH, and stretched in the axial direction of the cylinder until the sample broke under conditions of a chuck distance of 10 mm and a tensile speed of 300 mm/min. In the obtained S-S (Strain-Strength) curve, the tensile stresses corresponding to two tensile strains ( _ε1 = 5% and _ε2 = 10%) were defined as _σ1 and _σ2 , respectively, and the tensile modulus of elasticity (Young's modulus) of the pressure-sensitive adhesive layer was calculated from the tensile modulus _E0 = ( _σ2 - _σ1 )/( _ε2 - _ε1 ).

Here, the tensile strain ε was calculated based on the chuck distance.
ε=(L ₁ −L ₀ )/L ₀ or ε(%)=100×(L ₁ −L ₀ )/L ₀
ε: Tensile strain (dimensionless ratio or percentage)
L ₀ : Initial chuck distance (mm)
_L1 : Distance between chucks after extension (mm)
The tensile stress σ was calculated based on the cross-sectional area of the measurement sample before elongation.
σ=F/A
σ: tensile stress (MPa)
F: Measurement load (N)
A: Cross-sectional area of the measurement sample before elongation (mm ² )

The breaking stress [MPa] and breaking elongation E _B [%] of the sample were measured by the above elongation. The ratio (G'(-20°C)/E B ) and the ratio (G'(25°C)/E _B ) were calculated from the breaking elongation E _B [%] and the storage modulus G' [ _Pa ] at each temperature.

(Peel Strength)
In a measurement environment of 23°C and 50% RH, the release liner was peeled off from one side of the adhesive layer (substrate-less double-sided adhesive sheet) according to each example, and a PET film having a thickness of 50 μm was attached to the backing, and then cut to a size of 25 mm wide and 100 mm long to prepare a test piece. The release liner on the other side of the test piece was peeled off, and the test piece was pressed against the surface of an alkaline glass plate (manufactured by Matsunami Glass Industry Co., Ltd., thickness 1.35 mm, blue plate edge polished product) as an adherend by moving a 2 kg roller back and forth once. After leaving this in the same environment for 30 minutes, the peel strength (adhesive strength) [N/25 mm] was measured using a tensile compression tester (device name "AGS-50NX", manufactured by Shimadzu Corporation) in accordance with JIS Z 0237:2000 under the conditions of a tensile speed of 300 mm/min and a peel angle of 180 degrees. In addition, in the case of a single-sided adhesive sheet with a substrate, the backing of a PET film is not essential.

The results obtained are shown in Tables 1 and 2, along with the approximate composition of the adhesive for each example. In the tables, a "-" in the monomer component column indicates that the corresponding monomer was not used. Additionally, "ne" in Table 1 indicates that the corresponding item has not been evaluated.

As shown in the above table, the adhesives according to Examples 3 to 15 all had a refractive index of 1.450 or less and a storage modulus (G'(-20°C)) of 2.0 x 10 ⁶ Pa or less, and showed good flexibility even at low temperatures at low refractive indexes. In Examples 3 to 6 and Examples 8 to 15, better low-temperature flexibility was obtained. On the other hand, the adhesives according to Examples 1, 2, 16, and 17 had a refractive index of 1.450 or less, but a high storage modulus (G'(-20°C)) and low flexibility at low temperatures. In addition, an ultraviolet-curable adhesive composition was prepared in the same manner as in Example 1 except that 2EHA was used alone as the monomer component, and an adhesive prepared in the same manner as in Example 1 except that the adhesive composition was used had a refractive index of 1.464 measured by the above method, which did not satisfy the condition of a refractive index of 1.450 or less.

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

REFERENCE SIGNS

LIST

1, 2, 3 Adhesive sheet 10 Support substrate

10A First surface

10B Second surface 21 Adhesive layer, First adhesive layer 21A Adhesive surface, First adhesive surface 21B Adhesive surface 22 Second adhesive layer 22A

Second adhesive surface

31, 32 Release liner

Claims

A pressure-sensitive adhesive having a refractive index of 1.450 or less and a storage modulus at -20°C (G'(-20°C)) of 2.0 x 10 6 Pa or less.
The pressure-sensitive adhesive according to claim 1, wherein the ratio (G'(-20°C)/E B ) of the storage modulus (G'(-20°C)) [Pa] to the elongation at break (E B ) [%] is 20 or more and 2,000 or less.
The adhesive according to claim 1, wherein the adhesive is formed from an active energy ray-curable adhesive composition.
The pressure-sensitive adhesive contains an acrylic polymer (F),
The monomer components constituting the acrylic polymer (F) include a fluorine-containing acrylic monomer (Mf) and an acid-free hydrophilic monomer (Mh),
The hydrophilic monomer (Mh) includes a low Tg hydrophilic monomer (Mh L ) having a homopolymer glass transition temperature of 40° C. or lower;
The pressure-sensitive adhesive according to claim 1 , wherein the content of the low Tg hydrophilic monomer (Mh L ) in the monomer components constituting the acrylic polymer (F) is more than 2.0% by weight.
The adhesive according to claim 4, wherein the content of the hydrophilic monomer (Mh) in the monomer components constituting the acrylic polymer (F) is more than 5.0% by weight.
The monomer component constituting the acrylic polymer (F) is represented by the following formula (1):
CH2 = CR1COOR2 ( 1 )
(wherein R 1 is a hydrogen atom or a methyl group, and R 2 is a chain alkyl group having 4 to 18 carbon atoms);
The pressure-sensitive adhesive according to claim 4, further comprising a monomer (M1) represented by the following formula:
The adhesive according to claim 4, wherein the monomer components constituting the acrylic polymer (F) contain less than 2.0% by weight of an acidic functional group-containing monomer.
An adhesive sheet comprising an adhesive layer made of the adhesive according to any one of claims 1 to 7.
The adhesive sheet according to claim 8, wherein the adhesive layer has a haze of less than 3.0% after a moist heat test in which the adhesive layer is kept in a moist heat environment of 85°C and 85% RH for 240 hours.
The pressure-sensitive adhesive sheet according to claim 8, which has a peel strength from a glass plate (tensile speed: 300 mm/min, peel angle: 180 degrees) of 1.0 N/25 mm or more.