Adhesive sheet

Abstract

Provided is a heat-peelable adhesive sheet that achieves both the maintenance of heat peelability and the reduction of particles during peeling by heating. Provided is an adhesive sheet comprising a substrate and an adhesive layer provided on at least one surface of the substrate. The surface of the adhesive layer is composed of an adhesive formed of a water-dispersible adhesive composition containing thermally expandable microspheres. Moreover, in a stringiness evaluation conducted by a nanoindenter in an environment of 25ºC, the stringiness of the surface of the adhesive layer is at most 4,000 nm. Furthermore, the strength of adhesion to a polyethylene terephthalate film at 23ºC is 2.0 N / 20 mm to 13 N / 20 mm.

Description

adhesive sheet

[0001] The present invention relates to a pressure-sensitive adhesive sheet. This application claims priority based on Japanese Patent Application No. 2024-37415 filed on March 11, 2024, and Japanese Patent Application No. 2024-122604 filed on July 29, 2024, the entire contents of which are incorporated herein by reference.

[0002] Generally, adhesives (also referred to as pressure-sensitive adhesives; the same applies hereinafter) exhibit a soft solid (viscoelastic) state at temperatures near room temperature and have the property of adhering to an adherend under pressure. Taking advantage of this property, adhesives, typically in the form of adhesive sheets having an adhesive layer, are widely used in various industrial fields. One example of the use of adhesive sheets is as a so-called processing material, where the adhesive sheet is temporarily attached to an adherend during the manufacture, processing, transportation, etc. of various adherends, and is removed from the adherend after achieving its purpose. Such adhesive sheets are used, for example, in the processing of electronic components. For example, in the manufacture of semiconductor components including semiconductor chips, in a resin encapsulation process in which the semiconductor chip is encapsulated with resin to prevent scratches on the semiconductor chip, expand metal wiring, etc., the semiconductor chip may be encapsulated on an adhesive sheet from the standpoint of workability, etc. In such a usage mode, the pressure-sensitive adhesive sheet can be used, for example, by placing multiple semiconductor chips on the pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet as a temporary fixing material, sealing the semiconductor chips together on the pressure-sensitive adhesive layer, and then, in a specified post-process, peeling the pressure-sensitive adhesive sheet from the structure including the sealing resin and the semiconductor chips.

[0003] Known pressure-sensitive adhesive sheets used as processing materials include heat-peelable pressure-sensitive adhesive sheets containing heat-expandable microspheres in the adhesive. While such pressure-sensitive adhesive sheets adhere well to adherends, heating them at a desired timing causes the components in the heat-expandable microspheres in the adhesive to expand and foam (hereinafter sometimes referred to as "expansion, etc." or simply "expansion"), reducing the adhesive strength and allowing the sheets to be easily peeled from the adherend. Patent Documents 1 and 2 disclose this type of prior art.

[0004] Japanese Patent No. 4849993 Japanese Patent Application Publication No. 2018-193563

[0005] In heat-peelable pressure-sensitive adhesive sheets containing heat-expandable microspheres, when the pressure-sensitive adhesive sheet is peeled from an adherend by heating, fine adhesive residues may adhere to the adherend surface after peeling. If such fine adhesive residues or deposits (hereinafter also referred to as "particles") are generated on the adherend surface after peeling, the adherend surface may be contaminated after heat peeling. For example, when the adherend is a semiconductor chip, there is a risk of causing connection problems with electronic components. Furthermore, to avoid such problems, multiple cleaning processes may be required for the particle-generated adherend, which tends to complicate the process. Therefore, it would be advantageous to provide a heat-peelable pressure-sensitive adhesive sheet that suppresses particle generation during heat peeling.

[0006] The generation of particles during heat-peelability is thought to be due to factors such as the expansion of heat-expandable microspheres, but the exact reason for this generation has not been elucidated. According to the inventors' studies, particularly when a pressure-sensitive adhesive formed from an aqueous-dispersible pressure-sensitive adhesive composition containing heat-expandable microspheres is used as a pressure-sensitive adhesive, conventional techniques based on technical concepts used in other types of pressure-sensitive adhesives, such as solvent-based ones, sometimes fail to sufficiently suppress particle generation or reduce the heat-peelability of the pressure-sensitive adhesive sheet, making it difficult to suppress particle generation while maintaining favorable heat-peelability. Therefore, the inventors conducted further intensive studies and found that by focusing on the stringiness of a pressure-sensitive adhesive layer containing heat-expandable microspheres, it is possible to favorably achieve both the maintenance of heat-peelability and the suppression of particle generation in a heat-peelable pressure-sensitive adhesive sheet, thereby completing the present invention. That is, an object of the present invention is to provide a heat-peelable pressure-sensitive adhesive sheet that maintains heat-peelability and reduces particle generation during heat-peelability.

[0007] According to this specification, a pressure-sensitive adhesive sheet is provided that has a substrate and a pressure-sensitive adhesive layer on at least one surface of the substrate. The surface of the pressure-sensitive adhesive layer is composed of a pressure-sensitive adhesive formed from an aqueous-dispersible pressure-sensitive adhesive composition containing heat-expandable microspheres. Here, when stringiness is evaluated using a nanoindenter in an environment of 25°C, the stringiness of the surface of the pressure-sensitive adhesive layer is 4000 nm or less. Furthermore, the pressure-sensitive adhesive sheet has an adhesive strength to a polyethylene terephthalate film (hereinafter also referred to as "initial adhesive strength to PET") at 23°C of 2.0 N / 20 mm or more and 13 N / 20 mm or less. A pressure-sensitive adhesive sheet having a surface stringiness of 4000 nm or less and an initial adhesive strength to PET of 2.0 N / 20 mm or more and 13 N / 20 mm or less is likely to achieve both maintenance of heat-peelability and reduction of particles during heat-peel. The stringiness of the surface of the adhesive layer in this specification can be determined as the displacement of the unloading curve in a load-unloading curve obtained by measurement in a 25°C environment based on the nanoindentation method.

[0008] In this specification, the fine adhesive residues or deposits, i.e., particles, are clearly distinguishable from the deposits known as "adhesive residues" that remain on the surface of an adherend when a conventional adhesive sheet (typically an adhesive sheet that does not have heat-peelability) is peeled off from the adherend, based on differences in appearance, size, shape, pattern, number, etc.

[0009] In some embodiments, the weight average molecular weight of the tetrahydrofuran-soluble component of the adhesive constituting the surface of the adhesive layer is 8.0 × 10 ４ Above 50 x 10 ４ When the weight average molecular weight of the tetrahydrofuran soluble portion of the pressure-sensitive adhesive falls within the above range, it is easy to achieve both maintenance of heat peelability and reduction of particles during heat peeling.

[0010] In some preferred embodiments, the aqueous dispersion PSA composition contains an acrylic polymer as a base polymer, which makes it easier to obtain a PSA sheet with good heat-peelability.

[0011] In some embodiments, the acrylic polymer is a polymer of a monomer component that contains 50% by weight or more of a (meth)acrylic acid alkyl ester having a chain alkyl group having 2 to 12 carbon atoms, and further contains (meth)acrylic acid. Use of such an acrylic polymer makes it easy to obtain a pressure-sensitive adhesive sheet that exhibits good heat-peelability while maintaining good adhesive properties.

[0012] In some preferred embodiments, the aqueous PSA composition further contains a crosslinking agent. The crosslinking agent includes at least one selected from the group consisting of epoxy-based crosslinking agents, oxazoline-based crosslinking agents, carbodiimide-based crosslinking agents, isocyanate-based crosslinking agents, hydrazide-based crosslinking agents, and aziridine-based crosslinking agents. By using one or more appropriate crosslinking agents from among the above crosslinking agents, a PSA sheet with excellent heat-peelability can be preferably obtained. More preferably, the content of the crosslinking agent is 0.1 to 10 parts by weight per 100 parts by weight of the base polymer.

[0013] In some preferred embodiments, the aqueous PSA composition further comprises a urethane-based compound. Use of the urethane-based compound improves the coatability of the aqueous PSA composition, making it easier to obtain a PSA sheet that exhibits good heat-peelability.

[0014] As the urethane-based compound, a urethane-based compound containing a polyoxyethylene unit is preferably used. The urethane-based compound containing a polyoxyethylene unit preferably realizes the effects of the technology disclosed herein.

[0015] In some preferred embodiments, the urethane compound is a polyether-containing urethane compound. When a polyether-containing urethane compound is used, the effects of the technology disclosed herein are more preferably realized.

[0016] In some embodiments, the thickness of the adhesive layer is 1 μm or more and 200 μm or less. The technology disclosed herein is preferably implemented in an embodiment including an adhesive layer having the above thickness.

[0017] In some embodiments, the PSA sheet is configured as a double-sided PSA sheet comprising the substrate, a first PSA layer provided on one side of the substrate as the PSA layer, and a second PSA layer provided on the other side of the substrate. Such a substrate-attached double-sided PSA sheet is preferable because it has good handleability and processability.

[0018] The pressure-sensitive adhesive sheet disclosed herein adheres well to an adherend while being easily peeled off from the adherend by heating, and therefore can be preferably used as a processing material (also called a temporary fixing material) that is peeled off and removed from the adherend after processing of an adherend (object to be processed) fixed by the pressure-sensitive adhesive layer, for example, in processing electronic components.

[0019] Fig. 1 is an example of a load-unload curve measured by nanoindentation. Fig. 2 is a cross-sectional view schematically showing one configuration example of a pressure-sensitive adhesive sheet. Fig. 3 is a cross-sectional view schematically showing another configuration example of a pressure-sensitive adhesive sheet. Fig. 4 is a cross-sectional view schematically showing another configuration example of a pressure-sensitive adhesive sheet.

[0020] Preferred embodiments of the present invention are 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 for 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. Furthermore, in the following drawings, components and parts that perform the same function may be denoted by the same reference numerals, and redundant explanations may be omitted or simplified. Furthermore, the embodiments shown in the drawings are schematic for the purpose of clearly explaining the present invention, and do not necessarily accurately represent the size or scale of the actual product provided.

[0021] In this specification, the term "adhesive" refers to a material that is in a soft solid (viscoelastic) state at temperatures around room temperature and has the property of adhering to an adherend under pressure. The adhesive referred to here is generally a material having a complex tensile modulus E ＊ (1Hz) <10 ７ dyne / cm ２ (typically, a material having the above properties at 25°C) The PSA in the technology disclosed herein can also be understood as the solid content (non-volatile content) of the PSA composition or a constituent of the PSA layer.

[0022] In this specification, the term "water-dispersed PSA composition" refers to a composition in which at least a portion of the PSA-forming components are dispersed in water. The term "water-dispersed" also includes suspended and emulsified states. The concept of the water-dispersed PSA composition includes what is called an emulsion-type PSA composition. In this specification, a PSA (layer) formed from a water-dispersed PSA composition may be referred to as a water-dispersed PSA (layer).

[0023] In this specification, the term "acrylic polymer" refers to a polymer containing more than 50% by weight of monomer units derived from an acrylic monomer as the monomer units constituting the polymer. The acrylic monomer refers to a monomer having at least one (meth)acryloyl group in one molecule.

[0024] In this specification, "(meth)acryloyl" refers to acryloyl and methacryloyl in a comprehensive sense. Similarly, "(meth)acrylate" refers to acrylate and methacrylate in a comprehensive sense, and "(meth)acrylic" refers to acrylic and methacrylic in a comprehensive sense.

[0025] <Stringiness> The pressure-sensitive adhesive sheet disclosed herein comprises an adhesive layer (at least the first adhesive layer in an embodiment in which a first and second adhesive layer are provided on each surface of a substrate). The pressure-sensitive adhesive layer disclosed herein is one in which the adhesive constituting the surface of the adhesive layer has specific stringiness. In this specification, the stringiness of the surface of an adhesive layer can be determined as the displacement of the unloading curve in a load (indentation)-unloading (withdrawal) curve obtained by pressing a microindenter from the surface (adhesive surface) of the adhesive layer to a predetermined depth in an environment of 25°C and then withdrawing the microindenter, and plotting the change in the load (vertical axis) applied to the indenter against the displacement (horizontal axis) of the indenter relative to the adhesive surface. Here, an example of a load-unloading curve obtained by the nanoindentation method is shown in Figure 1. In the load-unloading curve shown in Figure 1, the displacement of the unloading curve refers to the difference in displacement from the reference point where the displacement (horizontal axis) of the indenter is zero [nm] when the indenter is pressed in and then pulled out to the point where the load (vertical axis) becomes zero [μN], i.e., the displacement a. The stringiness of the surface of the adhesive layer is specifically measured by the method described in the Examples below.

[0026] The adhesive layer disclosed herein has a stringiness of 4000 nm or less on the surface of the adhesive layer. When the adhesive layer has a property in which the stretchability is limited to such an extent that the stringiness is 4000 nm or less, particle generation tends to be reduced. From the viewpoint of suppressing particle generation, in some embodiments, the stringiness is preferably 3800 nm or less, more preferably 3500 nm or less, and even more preferably less than 3000 nm. It may be 2800 nm or less, 2500 nm or less, 2200 nm or less, 2000 nm or less, or 1900 nm or less. The lower limit of the stringiness is not particularly limited. From the viewpoint of exerting suitable adhesive strength, the stringiness of the surface of the adhesive layer is preferably 500 nm or more, more preferably 1000 nm or more, even more preferably 1200 nm or more, and may be 1400 nm or more, 1600 nm or more, or 1700 nm or more.

[0027] <Initial Adhesion Strength to PET> The adhesive layer disclosed herein (at least the first adhesive layer in an embodiment including a first adhesive layer and a second adhesive layer) has an adhesive strength to a polyethylene terephthalate (PET) film (initial adhesion strength to PET) of 2.0 N / 20 mm or more, measured under an environment of 23°C and 50% RH. When the initial adhesion strength to PET is 2.0 N / 20 mm or more, the effects of the technology disclosed herein are preferably exhibited, and particle generation tends to be reduced. From the viewpoint of favorable fixation of an adherend, the initial adhesion strength to PET is more preferably 2.5 N / 20 mm or more. In some preferred embodiments, the initial adhesion strength to PET is 3.0 N / 20 mm or more, more preferably 4.0 N / 20 mm or more, and even more preferably 5.0 N / 20 mm or more (e.g., greater than 5.0 N / 20 mm). Furthermore, from the viewpoint of heat peelability, the initial adhesion strength to PET is preferably 13 N / 20 mm or less. The initial adhesive strength to PET may be approximately 10 N / 20 mm or less, 9.0 N / 20 mm or less, 8.0 N / 20 mm or less, or 7.0 N / 20 mm or less. When the initial adhesive strength to PET is within the range between the above lower and upper limits, there is a tendency for a good balance to be achieved between maintaining heat peelability and reducing particles. Specifically, the initial adhesive strength to PET is measured by the method described in the Examples below.

[0028] <Heat-Expandable Microsphere-Containing Water-Dispersed Pressure-Sensitive Adhesive Composition (a)> In the pressure-sensitive adhesive sheet disclosed herein, the pressure-sensitive adhesive constituting the surface of the pressure-sensitive adhesive layer (at least the first pressure-sensitive adhesive layer in an embodiment in which a substrate has a first and a second pressure-sensitive adhesive layer on each side) is formed from a water-dispersed pressure-sensitive adhesive composition containing heat-expandable microspheres. Hereinafter, the heat-expandable microsphere-containing water-dispersed pressure-sensitive adhesive composition may be referred to as the "water-dispersed pressure-sensitive adhesive composition (a)" or simply as the "pressure-sensitive adhesive composition (a)." In the technology disclosed herein, a pressure-sensitive adhesive (water-dispersed pressure-sensitive adhesive) formed from the water-dispersed pressure-sensitive adhesive composition is used as the heat-expandable microsphere-containing pressure-sensitive adhesive. The use of a water-dispersed pressure-sensitive adhesive composition is beneficial in terms of eliminating organic solvents and being environmentally friendly. With conventional water-dispersed pressure-sensitive adhesives, innovations based on conventional technical concepts for other types of pressure-sensitive adhesives, such as solvent-based ones, have sometimes failed to sufficiently suppress particle generation or reduced the heat-separability of the pressure-sensitive adhesive sheet. By applying the technology disclosed herein, the effects of maintaining heat-separability and reducing particles can be effectively achieved even with water-dispersed pressure-sensitive adhesives containing heat-expandable microspheres.

[0029] (Acrylic Polymer) In some preferred embodiments, the water-dispersed PSA composition (a) is an acrylic PSA composition containing an acrylic polymer as a base polymer. Here, "base polymer" refers to the main component among the polymer components contained in the PSA composition (which may also be a PSA). Furthermore, in this specification, "main component" refers to a component contained in an amount of more than 50 wt %, unless otherwise specified. The acrylic PSA composition is typically an acrylic emulsion PSA composition containing a water-dispersed acrylic polymer. The water-dispersed acrylic polymer has an emulsion form in which the acrylic polymer is dispersed in water.

[0030] The acrylic polymer is preferably, for example, a polymer of a monomer raw material (monomer component) that contains an alkyl (meth)acrylate and may further contain another monomer copolymerizable with the alkyl (meth)acrylate.

[0031] As the (meth)acrylic acid alkyl ester, for example, a compound represented by the following formula (1) can be suitably used: CH ２ = C(R １ ) COOR ２ (1) Here, R in the above formula (1) １ is a hydrogen atom or a methyl group. ２ is a chain alkyl group having 1 to 20 carbon atoms (hereinafter, this range of carbon atoms is referred to as "C １－２０ From the viewpoint of the storage modulus of the adhesive, R ２ is C １－１４ (Meth)acrylic acid alkyl esters are preferred, and R ２ is C １－１２ More preferred is a (meth)acrylic acid alkyl ester, where R ２ is C １－１０ More preferred is a (meth)acrylic acid alkyl ester in which R ２ Particularly preferred are (meth)acrylic acid alkyl esters in which is a butyl group or a 2-ethylhexyl group.

[0032] The above R ２ is C １－２０Examples of alkyl (meth)acrylate esters having a chain alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isopropyl (meth)acrylate, ... isopropyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, isopropyl (meth)acrylate, iso Examples of the (meth)acrylic acid alkyl ester include octyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate. The above-mentioned (meth)acrylic acid alkyl esters can be used alone or in combination of two or more.

[0033] The technology disclosed herein is ２ is C １－２０ In some preferred embodiments, the above-mentioned R alkyl (meth)acrylate is preferably used as the monomer component of the acrylic polymer. ２ is C １－２０ The proportion of the (meth)acrylic acid alkyl ester, which is a chain alkyl group of the formula R 1 in the monomer component, may be about 75% by weight or more, about 90% by weight or more, or about 95% by weight or more. ２ is C １－２０ In an embodiment in which other monomers are used, the upper limit of the proportion of the alkyl (meth)acrylate ester, which is a chain alkyl group, may be, for example, 99.9% by weight or less, 99.5% by weight or less, or 99% by weight or less (e.g., 98.5% by weight or less).

[0034] In some preferred embodiments, the acrylic polymer contains, as a monomer component, the above R ２ is C ２－１２ The acrylic polymer having such a monomer composition is likely to have good adhesive properties (for example, adhesive strength). ２ is C ２－１２ As the alkyl (meth)acrylate having a chain alkyl group of R ２ is C ４－１２ More preferred is a (meth)acrylic acid alkyl ester, where R ２ is C ４－１０ More preferred are alkyl (meth)acrylates in which R ２ is C ４－９ More preferred is a (meth)acrylic acid alkyl ester in which R ２ is C ４－８ Particularly preferred is a (meth)acrylic acid alkyl ester in which the chain alkyl group R ２ is C ２－１２ The alkyl (meth)acrylate esters having a chain alkyl group of R may be used alone or in combination of two or more. ２ is C ２－１２ Suitable examples of the alkyl (meth)acrylate ester, which is a chain alkyl group, include one selected from n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and octyl (meth)acrylate, used alone or in combination.

[0035] Although not particularly limited, in some embodiments, the R ２ is C ２－１２ (Meth)acrylic acid alkyl ester (for example, R ２ is C ４－１２The proportion of the chain alkyl group ((meth)acrylic acid alkyl ester) in the monomer component is suitably about 50% by weight or more, preferably about 70% by weight or more, more preferably about 80% by weight or more, and may be about 90% by weight or more, or may be about 95% by weight or more. ２ is C ２－１２ In an embodiment in which other monomers are used, the upper limit of the proportion of the alkyl (meth)acrylate ester, which is a chain alkyl group, may be, for example, 99.9% by weight or less, 99.5% by weight or less, or 99% by weight or less (e.g., 98.5% by weight or less).

[0036] In some embodiments, the monomer component of the acrylic polymer is ２ is C １－２０ The alkyl (meth)acrylate ester, which is a chain alkyl group, includes an alkyl methacrylate ester. The acrylic polymer containing the alkyl methacrylate ester as a monomer component can have good adhesive properties while moderately increasing the modulus of elasticity. Such a pressure-sensitive adhesive can be suitable for applications requiring removability, for example. In some embodiments, the alkyl methacrylate ester includes an alkyl methacrylate ester of R ２ is C １－１２ Preferably, alkyl methacrylate esters having a chain alkyl group represented by the formula: R ２ is C ２－１２ and R ２ is C ２－８ and R ２ is C ２－６ and R ２ is C ４－６ The alkyl methacrylate esters may be used singly or in combination of two or more.

[0037] In embodiments using an acrylic polymer containing a methacrylic acid alkyl ester as a monomer component, the proportion of the methacrylic acid alkyl ester in the monomer component is not particularly limited. In some embodiments, the proportion of the methacrylic acid alkyl ester in the monomer component may be approximately 1 wt% or more, preferably approximately 10 wt% or more, more preferably approximately 20 wt% or more, even more preferably approximately 30 wt% or more, and may be approximately 40 wt% or more, or may be approximately 45 wt% or more. This allows the effects of containing the methacrylic acid alkyl ester to be effectively exerted. Furthermore, from the viewpoint of adhesive properties such as adhesive strength, in some embodiments, the proportion of the methacrylic acid alkyl ester in the monomer component may be approximately 70 wt% or less, approximately 60 wt% or less, or approximately 55 wt% or less.

[0038] In some embodiments, the monomer component of the acrylic polymer is ２ is C １－２０ The (meth)acrylic acid alkyl ester, which is a chain alkyl group, includes an acrylic acid alkyl ester and a methacrylic acid alkyl ester. An acrylic polymer having such a monomer composition can have good adhesive properties while moderately increasing the modulus of elasticity. Such a pressure-sensitive adhesive can be suitable for applications requiring removability, for example. As the methacrylic acid alkyl ester, the above-mentioned ones can be preferably used. The above-mentioned acrylic acid alkyl ester is not particularly limited, but includes R ２ is C ４－１２ The alkyl acrylate ester is preferably a chain alkyl group represented by R ２ is C ７－１０ More preferred is an alkyl acrylate ester having a chain alkyl group represented by R ２ is C ７－９ More preferred are alkyl acrylates having a chain alkyl group of the formula: The alkyl acrylates may be used alone or in combination of two or more.

[0039] In the embodiment in which the alkyl acrylate and the alkyl methacrylate are used in combination, the weight C of the alkyl acrylateＡＡ and the weight C of the alkyl methacrylate ester ＡＭ The ratio of (C ＡＡ / C ＡＭ ) is not particularly limited. In order to effectively obtain the effect of using the alkyl acrylate ester, in some embodiments, the above ratio (C ＡＡ / C ＡＭ ) may be about 30 / 70 or more, or may be 40 / 60 or more. In addition, from the viewpoint of effectively obtaining the effect of using the alkyl methacrylate ester, in some embodiments, the above ratio (C ＡＡ / C ＡＭ ) may be approximately 99 / 1 or less, approximately 90 / 10 or less, approximately 80 / 20 or less, approximately 70 / 30 or less, or approximately 60 / 40 or less.

[0040] Furthermore, the monomer component of the acrylic polymer preferably contains a carboxy group-containing monomer. The carboxy group-containing monomer can be useful for introducing crosslinking points and increasing the cohesive strength of the adhesive. By using a carboxy group-containing monomer as a monomer component, polymer particles having carboxy groups can be obtained. The carboxy groups of these polymer particles can serve as crosslinking points. Examples of carboxy group-containing monomers include ethylenically unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, and crotonic acid; ethylenically unsaturated dicarboxylic acids such as maleic acid, itaconic acid, and citraconic acid, and their anhydrides (maleic anhydride, itaconic anhydride, etc.); and the like. The carboxy group-containing monomers can be used alone or in combination of two or more.

[0041] As the carboxy group-containing monomer, acrylic acid and methacrylic acid are preferably used. Among them, acrylic acid is particularly preferred. By using (meth)acrylic acid as a monomer component, the carboxy group of (meth)acrylic acid becomes a crosslinking point (particularly a crosslinking point between polymer particles), and a PSA with a uniform elastic modulus distribution is easily obtained. Furthermore, polymer particles obtained using a carboxy group-containing monomer (typically acrylic acid) tend to have good dispersion stability. In some embodiments, acrylic acid and methacrylic acid may be used in combination. A PSA containing an acrylic polymer synthesized using a combination of acrylic acid and methacrylic acid can have high-performance adhesive properties (e.g., excellent repulsion resistance).

[0042] When the monomer component of the acrylic polymer contains a carboxyl group-containing monomer, the proportion of the carboxyl group-containing monomer (preferably (meth)acrylic acid) in the monomer component is not particularly limited. From the viewpoints of improving cohesive strength and elastic modulus, and thus controlling thermal expansion and thermal peelability, in some embodiments, the proportion of the carboxyl group-containing monomer is, for example, preferably about 0.1 wt% or more, more preferably about 0.5 wt% or more, even more preferably about 1.0 wt% or more, and particularly preferably about 1.5 wt% or more. Furthermore, from the viewpoints of adhesive strength and thermal peelability, in some preferred embodiments, the proportion of the carboxyl group-containing monomer (preferably (meth)acrylic acid) is suitably about 20 wt% or less, and may be about 15 wt% or less, may be about 10 wt% or less, may be about 5.0 wt% or less, or may be 3.0 wt% or less.

[0043] Furthermore, the monomer component of the acrylic polymer may contain other copolymerizable monomers other than the carboxy group-containing monomer for the purpose of introducing crosslinking points, improving cohesive strength, adjusting the glass transition temperature, etc. Non-limiting examples of the other copolymerizable monomers include various functional group-containing monomers such as hydroxyl group-containing monomers (e.g., hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate), amide group-containing monomers (e.g., (meth)acrylamide and N,N-dimethyl (meth)acrylamide), amino group-containing monomers (e.g., aminoethyl (meth)acrylate and N,N-dimethylaminoethyl (meth)acrylate), monomers having a nitrogen atom-containing ring (e.g., N-vinyl-2-pyrrolidone and N-(meth)acryloylmorpholine), monomers containing a sulfonic acid group or a phosphoric acid group, epoxy group-containing monomers, cyano group-containing monomers, keto group-containing monomers, and alkoxysilyl group-containing monomers (e.g., 3-(meth)acryloxypropyltrimethoxysilane). Other copolymerizable monomers that may be used include vinyl esters, vinyl ethers, aromatic vinyl compounds, (meth)acrylic acid esters having an alicyclic hydrocarbon group, (meth)acrylic acid esters having an aromatic hydrocarbon group, olefin-based monomers, halogen atom-containing monomers such as vinyl chloride and fluorine atom-containing (meth)acrylates, isocyanate group-containing monomers, and alkoxy group-containing monomers. Examples of other copolymerizable monomers that may be used include monomers having multiple functional groups in one molecule (polyfunctional monomers).

[0044] When the acrylic polymer is copolymerized with the other copolymerizable monomer, the proportion of the other copolymerizable monomer in the monomer components of the acrylic polymer may be, for example, about 0.1% by weight or more, for example, about 0.5% by weight or more, or about 1% by weight or more. Also, taking into consideration the adhesive effect of (meth)acrylic acid alkyl esters and the like, in some embodiments, the proportion of the other copolymerizable monomer is suitably about 40% by weight or less, for example, about 30% by weight or less, about 20% by weight or less, 10% by weight or less, 3% by weight or less, or less than 1% by weight (for example, less than 0.1% by weight).

[0045] Although not particularly limited, it is suitable for the acrylic polymer to be designed so that the glass transition temperature (Tg) of the polymer is −15° C. or lower (e.g., −75° C. or higher and −15° C. or lower). From the viewpoint of improving adhesive strength, in some embodiments, the Tg of the acrylic polymer is preferably −20° C. or lower, more preferably −25° C. or lower, and may be −30° C. or lower. Furthermore, in some embodiments, the Tg of the acrylic polymer is −70° C. or higher and may be −65° C. or higher. From the viewpoint of the cohesive strength, elastic modulus, etc. of the PSA, in some preferred embodiments, the Tg of the acrylic polymer is −55° C. or higher, more preferably −50° C. or higher, even more preferably −45° C. or higher, and particularly preferably −40° C. or higher and may be −35° C. or higher. By setting the Tg of the acrylic polymer within the above range, the resulting PSA has appropriate cohesive strength and can be suitable for applications such as removability. The Tg of the acrylic polymer can be adjusted by the type and amount ratio of monomers used in synthesizing the polymer.

[0046] In this specification, the Tg of an acrylic polymer refers to the value calculated from the Fox equation based on the Tg of a homopolymer of each monomer constituting the acrylic polymer and the weight fraction (copolymerization ratio on a weight basis) of the monomer. The Fox equation, as shown below, is a relationship 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 Fox equation, Tg represents the glass transition temperature (unit: K) of the copolymer, Wi represents the weight fraction (copolymerization ratio on a weight basis) of monomer i in the copolymer, and Tgi represents the glass transition temperature (unit: K) of a homopolymer of monomer i.

[0047] The glass transition temperatures of the homopolymers used to calculate Tg are values ​​listed in publicly available documents. For example, for the monomers listed below, the following values ​​are used as the glass transition temperatures of the homopolymers of the monomers: 2-ethylhexyl acrylate -70°C n-butyl acrylate -55°C n-butyl methacrylate 20°C ethyl acrylate -20°C methyl methacrylate 105°C methyl acrylate 8°C acrylic acid 106°C methacrylic acid 228°C 2-hydroxyethyl acrylate -15°C

[0048] For the glass transition temperatures of homopolymers of monomers other than those listed above, the values ​​described in "Polymer Handbook" (3rd edition, John Wiley & Sons, Inc., 1989) shall be used. If multiple values ​​for a single monomer are described in this document, the highest value shall be used. If the Tg of the homopolymer is not described in the Polymer Handbook, the value obtained by the measurement method described in Japanese Patent Application Publication No. 2007-51271 shall be used.

[0049] Acrylic polymers can be synthesized by emulsion polymerization. The emulsion polymerization method is not particularly limited, and can be carried out by appropriately adopting various monomer supply methods, polymerization conditions, materials used, etc., similar to those of conventionally known general emulsion polymerizations. For example, the monomer supply method can be appropriately selected from a batch feed method in which all monomer raw materials are supplied at once, a continuous feed (dropping) method, a divided feed (dropping) method, etc. It is preferable that a part or all of the monomer raw materials be supplied as a dispersion (emulsion) emulsified in water. For example, when the monomer raw materials are supplied by a dropwise method, they are preferably added dropwise in the form of an emulsion. The polymerization temperature can be, for example, about 20°C or higher (usually 40°C or higher), and is suitably about 100°C or lower (usually 80°C or lower).

[0050] The initiator used for polymerization can be appropriately selected from conventionally known polymerization initiators. For example, azo initiators, peroxide initiators, redox initiators formed by combining peroxides with reducing agents, substituted ethane initiators, etc. can be used. In emulsion polymerization, it is preferable to use a water-soluble polymerization initiator. The polymerization initiator can be used alone or in combination of two or more. The amount of polymerization initiator used is not particularly limited as long as it is a normal amount. For example, it can be selected from the range of about 0.005 parts by weight or more (preferably 0.01 parts by weight or more) and about 1 part by weight or less (preferably 0.8 parts by weight or less) per 100 parts by weight of the monomer component.

[0051] Emulsion polymerization of the monomer raw material is usually carried out in the presence of an emulsifier. The emulsifier used in emulsion polymerization is not particularly limited, and surfactants such as anionic surfactants and nonionic surfactants can be used. In some embodiments, from the viewpoint of improving adhesive performance and water resistance, it is preferable to use a surfactant having a radical polymerizable functional group (reactive emulsifier). The emulsifier can be used alone or in combination of two or more.

[0052] As the reactive emulsifier, for example, one having a structure in which a radically polymerizable functional group is introduced into an anionic surfactant or a nonionic surfactant is used. From the viewpoint of polymerization stability during emulsion polymerization, emulsifying properties, etc., it is preferable to use an anionic reactive emulsifier. Examples of the radically polymerizable functional group include a propenyl group, an isopropenyl group, an acryloyl group, a methacryloyl group, a vinyl group, a vinyl ether group (vinyloxy group), an allyl ether group (allyloxy group), etc. The concept of a propenyl group here includes a 1-propenyl group (CH ３ -CH=CH-) and 2-propenyl group (CH ２ =CH-CH ２ -; sometimes referred to as an allyl group.

[0053] Examples of anionic reactive emulsifiers include polyoxyethylene (allyloxymethyl) alkyl ether sulfates (e.g., ammonium salts), polyoxyethylene nonylpropenyl phenyl ether sulfates (e.g., ammonium salts), alkyl allyl sulfosuccinates (e.g., sodium salts), methacryloxy polyoxypropylene sulfates (e.g., sodium salts), and polyoxyalkylene alkenyl ether sulfates (e.g., ammonium salts in which the alkenyl group terminates in an isopropenyl group).When anionic reactive surfactants form salts, the salts may be, for example, metal salts such as sodium salts, or non-metal salts such as ammonium salts or amine salts.Examples of nonionic reactive surfactants include polyoxyethylene nonylpropenyl phenyl ether.

[0054] Commercially available reactive emulsifiers include those manufactured by Daiichi Kogyo Seiyaku Co., Ltd. under the trade names "Aqualon HS-05," "Aqualon HS-10," "Aqualon HS-1025," "Aqualon HS-20," "Aqualon KH-10," "Aqualon KH-1025," "Aqualon KH-05," "Aqualon BC-0515," "Aqualon BC-10," "Aqualon BC-1025," "Aqualon BC-20," "Aqualon BC-2020," "Aqualon RN-20," "Aqualon RN-30," "Aqualon RN-50," and "Aqualon AR-10." ", "Aqualon AR-20", "Aqualon AR-1025", "Aqualon AR-2020", ADEKA Corporation products under the trade names "ADEKA REASOAP SE-10N", "ADEKA REASOAP SR-1025", and "ADEKA REASOAP SR-2090", Kao Corporation products under the trade names "LATEMUL PD-104", "LATEMUL PD-420", "LATEMUL PD-430", and "LATEMUL PD-450", Sanyo Chemical Industries, Ltd. products under the trade names "ELEMINOL JS-20" and "ELEMINOL RS-3000", and Nippon Nyukazai Co., Ltd. product under the trade name "ANTOX MS-60".

[0055] The amount of emulsifier used in emulsion polymerization is not particularly limited, and an appropriate amount is used taking into consideration polymerization stability, dispersion stability of the polymerization reaction product, low contamination of the adherend, etc. From this perspective, the amount of emulsifier used is usually 0.1 parts by weight or more, preferably 0.5 parts by weight or more, per 100 parts by weight of the monomer components. From the perspective of obtaining higher stability, it may be 1.0 parts by weight or more, or even 1.5 parts by weight or more. Furthermore, in some embodiments, the amount of emulsifier used is, for example, 10 parts by weight or less, preferably 5 parts by weight or less, or may be 4 parts by weight or less, 3 parts by weight or less, or 2.5 parts by weight or less, per 100 parts by weight of the monomer components.

[0056] In the polymerization, a chain transfer agent (which may also be understood as a molecular weight regulator or polymerization degree regulator) may be used as needed. Examples of the chain transfer agent that can be used include mercaptans such as dodecyl mercaptan and 2-ethylhexyl thioglycolate. Alternatively, a chain transfer agent that does not contain a sulfur atom (non-sulfur chain transfer agent) may be used. One type of chain transfer agent may be used alone, or two or more types may be used in combination. Although not particularly limited, when a chain transfer agent is used, the amount used may typically be, for example, about 0.001 to 2 parts by weight per 100 parts by weight of the monomer components.

[0057] According to the emulsion polymerization, it is possible to prepare a polymerization solution in the form of an emulsion in which an acrylic polymer is dispersed in water (also referred to as an aqueous dispersion of an acrylic polymer or an acrylic polymer emulsion). Usually, from the viewpoint of dispersion stability, etc., a pH adjuster such as aqueous ammonia can be added to the polymerization solution to adjust the pH to an appropriate range (for example, a range of about pH 6 to 9).

[0058] (Heat-Expandable Microspheres) The aqueous-dispersible pressure-sensitive adhesive composition (a) contains heat-expandable microspheres. A pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer surface containing heat-expandable microspheres formed from the aqueous-dispersible pressure-sensitive adhesive composition (a) can be heated at a desired timing to expand the heat-expandable microspheres, thereby easily releasing the bond between the pressure-sensitive adhesive layer and the adherend. The heat-expandable microspheres can be used singly or in combination of two or more types.

[0059] The heat-expandable microspheres can be any suitable type as long as they are capable of expanding or foaming upon heating. For example, microspheres having an elastic shell encapsulating a substance that expands easily upon heating can be used. Such heat-expandable microspheres can be produced by any suitable method, such as coacervation or interfacial polymerization.

[0060] Examples of substances that expand easily when heated include low-boiling liquids such as propane, propylene, butene, normal butane, isobutane, isopentane, neopentane, normal pentane, normal hexane, isohexane, heptane, octane, petroleum ether, methane halides, and tetraalkylsilanes; and azodicarbonamide, which gasifies by thermal decomposition.

[0061] Examples of materials constituting the shell include polymers composed of nitrile monomers such as acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethoxyacrylonitrile, and fumaronitrile; carboxylic acid monomers such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid; vinylidene chloride; vinyl acetate; (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate, and β-carboxyethyl acrylate; styrene monomers such as styrene, α-methylstyrene, and chlorostyrene; and amide monomers such as acrylamide, substituted acrylamide, methacrylamide, and substituted methacrylamide. Polymers composed of these monomers may be homopolymers or copolymers. Examples of such copolymers include vinylidene chloride-methyl methacrylate-acrylonitrile copolymer, methyl methacrylate-acrylonitrile-methacrylonitrile copolymer, methyl methacrylate-acrylonitrile copolymer, and acrylonitrile-methacrylonitrile-itaconic acid copolymer.

[0062] The heat-expandable microspheres may be formed using inorganic or organic blowing agents, such as ammonium carbonate, ammonium bicarbonate, sodium bicarbonate, ammonium nitrite, sodium boron hydroxide, and various azides. Examples of organic blowing agents include fluorinated alkane compounds such as trichloromonofluoromethane and dichloromonofluoromethane; azo compounds such as azobisisobutyronitrile, azodicarbonamide and barium azodicarboxylate; hydrazine compounds such as paratoluenesulfonylhydrazide, diphenylsulfone-3,3'-disulfonylhydrazide, 4,4'-oxybis(benzenesulfonylhydrazide) and allylbis(sulfonylhydrazide); semicarbazide compounds such as p-toluylenesulfonylsemicarbazide and 4,4'-oxybis(benzenesulfonylsemicarbazide); triazole compounds such as 5-morpholyl-1,2,3,4-thiatriazole; and N-nitroso compounds such as N,N'-dinitrosopentamethylenetetramine and N,N'-dimethyl-N,N'-dinitrosoterephthalamide.

[0063] Commercially available heat-expandable microspheres may be used. Specific examples of commercially available heat-expandable microspheres include those manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd. under the trade name "Matsumoto Microsphere" (grades: F-30, F-30D, F-36, F-36D, F-36LV, F-50, F-50D, F-65, F-65D, FN-100SS, FN-100SSD, FN-180SS, FN-180SSD, F-180, F-190D, F-260D, F-2800D), and those manufactured by Nippon Phillite Co., Ltd. under the trade name "Expancel" (grades: F-30, F-30D, F-36, F-36D, F-36LV, F-50, F-50D, F-65, F-65D, FN-100SS, FN-100SSD, FN-180SS, FN-180SSD, F-180, F-190D, F-260D, F-2800D). Examples thereof include "Daiform" manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd. (grades: H750, H850, H1100, S2320D, S2640D, M330, M430, M520), and "Advancell" manufactured by Sekisui Chemical Co., Ltd. (grades: EML101, EMH204, EHM301, EHM302, EHM303, EM304, EHM401, EM403, EM501).

[0064] The average particle size of the heat-expandable microspheres before heating may be, for example, about 0.5 μm to 80 μm, about 1 μm to 50 μm, about 5 μm to 45 μm, or about 10 μm to 35 μm. The manufacturer's nominal value listed in the product catalog can be used as the average particle size. If the manufacturer's nominal value is unknown, a value determined by particle size distribution measurement using a laser scattering method can be used.

[0065] The heat-expandable microspheres preferably have an appropriate strength so that they do not burst until their volumetric expansion rate reaches at least 5 times, more preferably at least 7 times, and even more preferably at least 10 times. When such heat-expandable microspheres are used, their adhesive strength can be efficiently reduced by heat treatment.

[0066] The content of heat-expandable microspheres in the aqueous PSA composition (a), and in the PSA formed from the aqueous PSA composition (a), can be appropriately set depending on the desired adhesive strength reduction, etc. From the viewpoint of heat-peelability, in some embodiments, the content of heat-expandable microspheres is, for example, about 1 part by weight or more, preferably about 10 parts by weight or more, more preferably about 20 parts by weight or more, and may be about 25 parts by weight or more, per 100 parts by weight of the acrylic polymer. Furthermore, from the viewpoint of maintaining adhesive properties, PSA layer formability, etc., in some embodiments, the content of heat-expandable microspheres is, for example, about 200 parts by weight or less, preferably about 150 parts by weight or less, more preferably about 100 parts by weight or less, even more preferably about 70 parts by weight or less, particularly preferably about 50 parts by weight or less (e.g., less than 50 parts by weight), and may be about 40 parts by weight or less, per 100 parts by weight of the acrylic polymer.

[0067] (Crosslinking Agent) In some embodiments, the aqueous-dispersible PSA composition (a) preferably contains a crosslinking agent. The use of a crosslinking agent allows the resulting PSA to have adequate cohesive strength, resulting in a PSA sheet with excellent heat-peelability. Furthermore, increasing the degree of crosslinking in the PSA layer improves the elastic modulus of the PSA layer. This can be utilized to enhance the precision of the thermal expansion of the heat-expandable microspheres. More specifically, in PSAs formed from the aqueous-dispersible PSA composition (a), crosslinked structures are formed within and between acrylic polymer particles to improve cohesive strength and elastic modulus. Since interparticle crosslinking is considered effective for thermal peeling due to the thermal expansion of heat-expandable microspheres, the use of an external crosslinking agent capable of forming interparticle crosslinks is preferred. In this specification, the term "external crosslinking agent" refers to a crosslinking agent added after the synthesis of the acrylic polymer (post-addition). Aqueous crosslinking agents are preferably used as the crosslinking agent. The use of aqueous crosslinking agents is also beneficial in terms of eliminating organic solvents and being environmentally friendly. The term "aqueous crosslinking agent" encompasses both aqueous-dispersible crosslinking agents and water-soluble crosslinking agents.

[0068] The type of crosslinking agent is not particularly limited, and can be appropriately selected from, for example, isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, melamine-based crosslinking agents, peroxide-based crosslinking agents, urea-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, carbodiimide-based crosslinking agents, hydrazide-based crosslinking agents, amine-based crosslinking agents, etc. The crosslinking agents can be used alone or in combination of two or more.

[0069] In some preferred embodiments, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, carbodiimide-based crosslinking agents, isocyanate-based crosslinking agents, hydrazide-based crosslinking agents, and aziridine-based crosslinking agents are used. For example, in embodiments in which the aqueous PSA composition (a) contains an acrylic polymer having a carboxy group, the crosslinking agent can form a crosslinked structure with the carboxy group of the acrylic polymer. Among the crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, carbodiimide-based crosslinking agents, hydrazide-based crosslinking agents, and aziridine-based crosslinking agents are more preferred, epoxy-based crosslinking agents and oxazoline-based crosslinking agents are even more preferred, and epoxy-based crosslinking agents are particularly preferred. The use of a small amount of an epoxy-based crosslinking agent can effectively increase the degree of crosslinking, thereby limiting the impact of increasing the amount of crosslinking agent.

[0070] As the epoxy-based crosslinking agent, a compound having two or more epoxy groups per molecule can be used. Epoxy-based crosslinking agents having 3 to 5 epoxy groups per molecule are preferred. As the epoxy-based crosslinking agent, either water-soluble or water-dispersible types can be used, but water-soluble epoxy-based crosslinking agents are preferred. Specific examples of epoxy-based crosslinking agents include N,N,N',N'-tetraglycidyl-m-xylylenediamine, 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, 1,6-hexanediol diglycidyl ether, (poly)ethylene glycol diglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, and the like. Examples of commercially available epoxy crosslinking agents include those manufactured by DIC Corporation under the trade name "Epiclon CR-5L" and those manufactured by Nagase ChemteX Corporation under the trade names "Denacol EX-313," "Denacol EX-512," "Denacol EX-810," "Denacol EX-821," "Denacol EX-830," and "Denacol EX-850."

[0071] As the oxazoline-based crosslinking agent, a compound having one or more oxazoline groups per molecule can be used. The oxazoline group may be any of a 2-oxazoline group, a 3-oxazoline group, and a 4-oxazoline group. An oxazoline-based crosslinking agent having a 2-oxazoline group can be preferably used. For example, a water-soluble copolymer or a water-dispersible copolymer obtained by copolymerizing an addition-polymerizable oxazoline such as 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, or 2-isopropenyl-5-ethyl-2-oxazoline with another monomer can be used as the oxazoline-based crosslinking agent. Both water-soluble and water-dispersible oxazoline-based crosslinking agents can be used, but water-soluble oxazoline-based crosslinking agents are preferred. Commercially available oxazoline crosslinking agents include, for example, trade names "Epocross WS-500," "Epocross WS-700," "Epocross K-2010E," "Epocross K-2020E," and "Epocross K-2035E," manufactured by Nippon Shokubai Co., Ltd.

[0072] The carbodiimide crosslinking agent may be a low molecular weight compound or a high molecular weight compound having two or more carbodiimide groups. Commercially available carbodiimide crosslinking agents include the Carbodilite V series (aqueous solution type) such as "Carbodilite V-02," "Carbodilite V-02-L2," and "Carbodilite V-04," manufactured by Nisshinbo Chemical Inc., and the Carbodilite E series (aqueous dispersion type) such as "Carbodilite E-01," "Carbodilite E-02," and "Carbodilite E-04."

[0073] Examples of isocyanate-based crosslinking agents that can be used include polyfunctional isocyanate compounds having two or more functionalities. Examples include aromatic isocyanates such as tolylene diisocyanate, xylene diisocyanate, polymethylene polyphenyl diisocyanate, tris(p-isocyanatophenyl)thiophosphate, and diphenylmethane diisocyanate; alicyclic isocyanates such as isophorone diisocyanate; and aliphatic isocyanates such as hexamethylene diisocyanate. Both water-soluble and water-dispersible isocyanate-based crosslinking agents can be used. As the isocyanate-based crosslinking agent, so-called blocked isocyanate-type isocyanate-based crosslinking agents in which the isocyanate group is blocked may also be used. Commercially available products include the "Burnoc DNW" series manufactured by DIC Corporation, the "Aquanate" series manufactured by Tosoh Corporation, the "Takenate WD" series manufactured by Mitsui Chemicals, Inc., and the "Elastron BN" series manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.

[0074] The hydrazide-based crosslinking agent contains a hydrazino group (H ２ A hydrazino group-containing compound having two or more N-NH- groups can be used. Examples include polycarboxylic acid polyhydrazides such as oxalic acid dihydrazide, malonic acid dihydrazide, glutaric acid dihydrazide, succinic acid dihydrazide, and adipic acid dihydrazide, and hydantoins such as 1,3-bis(hydrazinocarbonoethyl)-5-isopropylhydantoin.

[0075] Examples of the aziridine crosslinking agent include trimethylolpropane tris[3-(1-aziridinyl)propionate] and trimethylolpropane tris[3-(1-(2-methyl)aziridinylpropionate)]. Either water-soluble or water-dispersible aziridine crosslinking agents can be used. Examples of commercially available aziridine crosslinking agents include trade names "ChemiTite PZ-33" and "ChemiTite DZ-22E" manufactured by Nippon Shokubai Co., Ltd.

[0076] In embodiments in which the aqueous dispersion PSA composition (a) contains a crosslinking agent, the content of the crosslinking agent in the PSA composition (a) is not particularly limited. From the viewpoint of improving cohesive strength and elastic modulus, in some embodiments, the content of the crosslinking agent in the PSA composition (a) is approximately 0.1 parts by weight or more, preferably approximately 0.3 parts by weight or more, more preferably approximately 0.5 parts by weight or more, even more preferably approximately 0.8 parts by weight or more, particularly preferably approximately 1.0 parts by weight or more, per 100 parts by weight of the acrylic polymer. It may also be approximately 2.0 parts by weight or more, approximately 3.0 parts by weight or more, or approximately 4.0 parts by weight or more. In some embodiments, the content of the crosslinking agent may be approximately 20 parts by weight or less, or approximately 15 parts by weight or less, per 100 parts by weight of the acrylic polymer. By appropriately limiting the amount of crosslinking agent used, it is possible to achieve a favorable balance between adhesive strength and heat peelability. In some preferred embodiments, the content of the crosslinking agent relative to 100 parts by weight of the acrylic polymer may be approximately 10 parts by weight or less, approximately 8.0 parts by weight or less, more preferably approximately 5.0 parts by weight or less, even more preferably approximately 4.0 parts by weight or less, even more preferably approximately 3.0 parts by weight or less, still more preferably approximately 2.5 parts by weight or less, particularly preferably approximately 2.0 parts by weight or less (e.g., less than 2.0 parts by weight), and may be approximately 1.6 parts by weight or less.

[0077] (Tackifier) ​​In some embodiments, the aqueous dispersion PSA composition (a) preferably contains a tackifier. The use of a tackifier can improve adhesive strength and ensure good adhesion to the adherend before heat peeling. Examples of tackifiers include rosin-based tackifier resins (including rosin derivative tackifier resins), petroleum-based tackifier resins, terpene-based tackifier resins, phenol-based tackifier resins, and ketone-based tackifier resins. These can be used alone or in combination of two or more.

[0078] Suitable examples of the tackifier include rosin-based tackifier resins and terpene-based tackifier resins. By using a tackifier selected from rosin-based tackifier resins and terpene-based tackifier resins, the adhesive strength of the water-dispersible acrylic adhesive can be preferably improved. Suitable examples of the terpene-based tackifier resin include terpene phenol-based resins.

[0079] In some embodiments, the use of rosin-based tackifying resins is particularly preferred. The use of rosin-based tackifying resins can effectively increase the adhesive strength of aqueous acrylic adhesives. Examples of rosin-based tackifying resins include rosins such as gum rosin, wood rosin, and tall oil rosin, as well as stabilized rosins (e.g., stabilized rosins obtained by disproportionating or hydrogenating the rosin), polymerized rosins (e.g., rosin polymers, typically dimers), and modified rosins (e.g., unsaturated acid-modified rosins modified with unsaturated acids such as maleic acid, fumaric acid, and (meth)acrylic acid). Examples of rosin derivative tackifying resins include esters of the rosin-based resins (e.g., rosin esters such as stabilized rosin esters and polymerized rosin esters), phenol-modified rosin resins (phenol-modified rosins), and esters thereof (phenol-modified rosin esters). Suitable examples of rosin-based tackifying resins include stabilized rosin esters and polymerized rosin esters.

[0080] The softening point of the tackifier used is not particularly limited. From the viewpoint of improving cohesive strength, the softening point of the tackifier may be, for example, about 80°C or higher, preferably about 100°C or higher, more preferably about 120°C or higher, and even more preferably 130°C or higher. In some preferred embodiments, the aqueous PSA composition (a) may contain a high-softening-point tackifier having a softening point of 140°C or higher. Such a high-softening-point tackifier tends to increase cohesive strength in high-temperature environments and improve high-temperature stability. The softening point of the high-softening-point tackifier is more preferably 145°C or higher, and particularly preferably 150°C or higher (e.g., above 150°C). There is no particular upper limit for the softening point of the tackifier, but from the viewpoint of compatibility, low-temperature properties, etc., it is usually appropriate that the softening point be 200°C or lower, preferably 180°C or lower, and may be 175°C or lower.

[0081] The softening point of the tackifier is defined as the value measured based on the softening point test method (ring and ball method) specified in JIS K5902 and JIS K2207. Specifically, the sample is melted as quickly as possible at the lowest possible temperature and carefully filled into a ring placed on a flat metal plate, avoiding the formation of bubbles. After cooling, any raised portion of the ring, including the top edge, is cut off with a slightly heated knife. Next, a holder (ring stand) is placed in a glass container (heating bath) with a diameter of at least 85 mm and a height of at least 127 mm, and glycerin is poured into it to a depth of at least 90 mm. Next, a steel ball (diameter 9.5 mm, weight 3.5 g) and the ring filled with the sample are immersed in the glycerin without touching each other, and the glycerin temperature is maintained at 20°C ± 5°C for 15 minutes. Next, the steel ball is placed in the center of the surface of the sample in the ring and placed in its fixed position on the holder. Next, keeping the distance from the top of the ring to the glycerin surface at 50 mm, place a thermometer, align the center of the thermometer's mercury bulb with the center of the ring, and heat the container. The flame of the Bunsen burner used for heating should be aimed midway between the center of the bottom and the edge of the container, ensuring even heating. After heating begins and reaching 40°C, the rate of increase in bath temperature must be 5.0 ± 0.5°C per minute. The sample gradually softens, flows down the ring, and finally touches the bottom plate, at which point the temperature is read and considered the softening point. Two or more samples should be measured at the same time, and the average value should be used.

[0082] The tackifier is typically added to an aqueous emulsion of an acrylic polymer. The manner in which the tackifier is added is not particularly limited, but the tackifier is usually added in the form of an aqueous dispersion (tackifier emulsion) in which the tackifier is dispersed in water. The tackifier used in this manner is a tackifier that can be dispersed in water, i.e., a so-called water-dispersed tackifier.

[0083] In embodiments in which the aqueous PSA composition (a) contains a tackifier, the amount of the tackifier (preferably a high-softening point tackifier) ​​used is typically about 1 part by weight or more per 100 parts by weight of the acrylic polymer, from the viewpoint of optimally exerting the effects of its use, and in some preferred embodiments, it is about 5 parts by weight or more, more preferably about 8 parts by weight or more, even more preferably about 10 parts by weight or more, or even about 12 parts by weight or more. Furthermore, from the viewpoint of cohesive strength and the like, in some embodiments, the amount of the tackifier (preferably a high-softening point tackifier) ​​used is typically less than 50 parts by weight per 100 parts by weight of the acrylic polymer, and in some preferred embodiments, it is about 40 parts by weight or less, or may be about 30 parts by weight or less, or may be about 25 parts by weight or less, or may be about 20 parts by weight or less.

[0084] (Thickener) In some embodiments, the aqueous PSA composition (a) preferably contains a thickener. By incorporating a thickener into the aqueous PSA composition (a), the coatability of the aqueous PSA composition (a) can be improved, facilitating the formation of a PSA layer with good quality. In this specification, the term "thickener" refers to a component that, when added to an aqueous dispersion in which at least a portion of a resin component, such as a resin emulsion or latex, is dispersed in water, increases the viscosity (thickening effect) compared to when the component is not added. The thickener is not particularly limited, and examples thereof include polyacrylic acids, carboxylic acid copolymers, urethane compounds, polyvinyl alcohols, celluloses (e.g., hydroxycellulose), and polyethers (e.g., polyethylene glycol). These may be used alone or in combination of two or more.

[0085] In some preferred embodiments, the aqueous PSA composition (a) contains a urethane-based compound as a thickener. The urethane-based compound can improve the coatability of the PSA composition without impairing its adhesive properties or heat peelability (including heat peel control). In some preferred embodiments, a compound with associative thickening properties (also referred to as a urethane associative thickener) is used as the urethane-based compound. Such urethane-based compounds have a hydrophobic portion and a hydrophilic portion, and typically have a molecular weight of approximately 1,000 or more but less than 100,000. They can exert a thickening effect based on a structure and function different from that of so-called polymeric thickeners, which exert a thickening effect based on a high molecular weight of several hundred thousand or more. The urethane-based compounds can be used alone or in combination of two or more.

[0086] As the urethane-based compound, any material capable of thickening the aqueous PSA composition can be used without particular limitation. Examples of the urethane-based compound include ester-based polyurethanes and ether-based polyurethanes. In the aqueous PSA composition (a) disclosed herein, a polyether-containing urethane-based compound, which is a reaction product of a polyether and an isocyanate, is preferably used. Such a polyether-containing urethane-based compound typically contains a polyoxyalkylene unit, and the polyoxyalkylene unit preferably contains a polyoxyethylene unit.

[0087] In embodiments using a urethane compound having polyoxyalkylene units, the proportion of polyoxyalkylene units (typically polyoxyethylene units) in the urethane compound is not particularly limited and may be, for example, within the range of 1 to 99.9 mol%. From the viewpoint of effectively exerting the effects of the polyoxyalkylene units, the proportion of the polyoxyalkylene units is preferably approximately 30 mol% or more, more preferably approximately 50 mol% or more, and may be approximately 70 mol% or more, or may be approximately 90 mol% or more. Furthermore, from the viewpoint of effectively exerting the effects of the non-polyoxyalkylene portions (typically hydrophobic portions), the proportion of the polyoxyalkylene units may be 99.5 mol% or less, or may be 99 mol% or less.

[0088] In an embodiment in which a polyether-containing urethane compound is used as the urethane compound, the isocyanate used to form the polyether-containing urethane compound may be a monoisocyanate or a polyisocyanate. The isocyanate preferably includes a polyisocyanate (e.g., a bifunctional polyisocyanate). The polyether used to form the polyether-containing urethane compound may be a polyether monool or a polyether polyol. The polyol preferably includes a polyether polyol (e.g., a bifunctional polyether polyol).

[0089] The isocyanate used to form the urethane compound is not particularly limited, and any of aromatic isocyanates such as tolylene diisocyanate, aralkyl isocyanates such as xylylene diisocyanate and tetramethylxylylene diisocyanate, alicyclic isocyanates such as hydrogenated xylylene diisocyanate and isophorone diisocyanate, and aliphatic isocyanates such as hexamethylene diisocyanate can be used. The above isocyanates can be used alone or in combination of two or more.

[0090] The polyether used to form the urethane compound is not particularly limited, and examples thereof include polyoxyalkylene polyols composed of oxyalkylene units having 2 to 4 carbon atoms, polytetramethylene ether polyols, etc. Among these, polyethers containing polyoxyethylene units are preferred. The above polyethers can be used alone or in combination of two or more.

[0091] The urethane compound may be a commercially available product. For example, commercially available associative urethane compounds include those manufactured by BYK under the trade names "RHEOBYK-H 3300VF," "RHEOBYK-T 1010VF," and "RHEOBYK-L 1400VF," those manufactured by ADEKA under the trade names "ADEKA NOL UH-450VF" and "ADEKA NOL UH-420," and those manufactured by San Nopco under the trade name "SN Thickener 612."

[0092] In embodiments in which the aqueous dispersion PSA composition (a) contains a thickener, the content of the thickener in the PSA composition (a) is appropriately set so as to obtain the desired viscosity characteristics. In some embodiments, the content of the thickener (e.g., a urethane-based compound) is suitably 0.01 parts by weight or more relative to 100 parts by weight of the acrylic polymer. From the viewpoint of effectively obtaining the viscosity adjusting effect, it is preferably 0.05 parts by weight or more, more preferably 0.10 parts by weight or more, and even more preferably 0.12 parts by weight or more. In other preferred embodiments, the content of the thickener (e.g., a urethane-based compound) may be 0.3 parts by weight or more, 0.5 parts by weight or more, 0.8 parts by weight or more, or 1 part by weight or more relative to 100 parts by weight of the acrylic polymer. In addition, since too much thickener may adversely affect film-forming properties, etc., in some embodiments, the content of the thickener (e.g., urethane compound) may be, for example, about 20 parts by weight or less, and preferably about 15 parts by weight or less, relative to 100 parts by weight of the acrylic polymer. In some preferred embodiments, it is about 10 parts by weight or less, more preferably about 5 parts by weight or less, even more preferably about 2 parts by weight or less, and even more preferably about 1 part by weight or less (e.g., less than 1 part by weight), and may be about 0.5 parts by weight or less (e.g., less than 0.5 parts by weight), or may be about 0.3 parts by weight or less (e.g., less than 0.3 parts by weight). By limiting the amount of thickener used as described above, the effects on adhesive properties and heat peelability can be suppressed.

[0093] (Other Additives) The aqueous dispersion pressure-sensitive adhesive composition (a) may optionally contain various additives to the extent that the effects of the present invention are not significantly impaired. Examples of such optional additives include leveling agents, crosslinking aids, release adjusters, plasticizers, softeners, fillers, colorants (pigments, dyes, etc.), conductive agents, flame retardants, rust inhibitors, preservatives, mildew inhibitors, antistatic agents, antioxidants, UV absorbers, antioxidants, light stabilizers, etc. As for such various additives, conventionally known ones can be used in the usual way, and since they do not particularly characterize the present invention, detailed explanations will be omitted.

[0094] The aqueous dispersion-type PSA composition (a) can be prepared, for example, by mixing an aqueous dispersion of an acrylic polymer with other components (such as heat-expandable microspheres.) The aqueous dispersion of the acrylic polymer can be, for example, a polymerization reaction liquid obtained by emulsion polymerization, or a polymerization reaction liquid that has been subjected to treatment such as pH adjustment (e.g., neutralization), adjustment of the nonvolatile content, or adjustment of the viscosity, as necessary.

[0095] (Dispersion medium) The water-dispersed PSA composition (a) disclosed herein contains water as a dispersion medium. Furthermore, the water-dispersed PSA composition (a) may contain a liquid other than water, if necessary. The liquid other than water may be, for example, one or more selected from various organic solvents (e.g., lower alcohols) that are uniformly miscible with water. Typically, the proportion of water in the volatile components contained in the water-dispersed PSA composition (a) is, for example, approximately 90% by weight or more, and preferably about 95 to 100% by weight.

[0096] In some embodiments, from the viewpoint of environmental considerations and eliminating organic solvents, the aqueous dispersion PSA composition (a) is preferably substantially free of organic solvents. Here, "a PSA composition substantially free of organic solvents" means that the amount of organic solvent in the PSA composition is less than 1 wt % (e.g., less than 0.1 wt %).

[0097] Although not particularly limited, the nonvolatile content (NV) of the aqueous PSA composition (a) is suitably about 20% by weight or more, preferably about 30% by weight or more, from the viewpoint of drying efficiency, etc. Furthermore, from the viewpoint of coatability, etc., the NV is suitably about 75% by weight or less, and may be about 60% by weight or less, or may be about 50% by weight or less.

[0098] (Formation of Pressure-Sensitive Adhesive (Layer)) The formation of a pressure-sensitive adhesive (layer) using the aqueous-dispersible pressure-sensitive adhesive composition (a) can be carried out in accordance with known pressure-sensitive adhesive (layer) formation methods. For example, a method (direct method) can be used in which a pressure-sensitive adhesive composition is directly applied (typically coated) to a substrate and then dried to form a pressure-sensitive adhesive layer. Alternatively, a pressure-sensitive adhesive composition can be applied to a surface (release surface) having releasability and then dried to form a pressure-sensitive adhesive layer on the surface. For pressure-sensitive adhesive sheets having a substrate, from the viewpoint of smoothness of the adhesive surface, a method (transfer method) in which a pressure-sensitive adhesive layer formed on the release surface is transferred to the substrate can be preferably used. The release surface can be the surface of a release liner, the back surface of a release-treated substrate, or the like. The pressure-sensitive adhesive layer is typically formed continuously, but may be formed in a regular or random pattern such as dots or stripes depending on the purpose and application.

[0099] The pressure-sensitive adhesive composition can be applied using a known or conventional coater, such as a gravure roll coater, reverse roll coater, kiss roll coater, comma coater, dip roll coater, die coater, bar coater, knife coater, or spray coater. Alternatively, the pressure-sensitive adhesive composition may be applied by impregnation or curtain coating. From the viewpoints of promoting the crosslinking reaction and improving production efficiency, the pressure-sensitive adhesive composition is preferably dried under heating. The drying temperature can be, for example, about 40°C to 150°C, and preferably about 50°C to 130°C (e.g., 60°C to 100°C). After drying, the pressure-sensitive adhesive composition may be further aged for the purposes of adjusting component migration within the pressure-sensitive adhesive layer, promoting the crosslinking reaction, and alleviating distortion that may exist within the substrate or pressure-sensitive adhesive layer.

[0100] <Adhesive Sheet> (Configuration Example) The adhesive sheet disclosed herein is configured to include an adhesive layer. The surface of the adhesive layer is configured with an adhesive formed from the above-mentioned aqueous dispersion adhesive composition (a). The adhesive sheet may be a substrate-attached adhesive sheet having an adhesive layer on one or both sides of a substrate (support substrate), or may be a substrate-less adhesive sheet (i.e., an adhesive sheet without a substrate; typically, an adhesive sheet consisting of an adhesive layer) in which the adhesive layer is supported on a release liner (which can also be understood as a substrate with a release surface). The concept of adhesive sheet here may include what are called adhesive tapes, adhesive labels, adhesive films, etc. Furthermore, the adhesive sheet may be in the form of a roll or a sheet. Alternatively, it may be an adhesive sheet processed into various shapes.

[0101] The pressure-sensitive adhesive sheet disclosed herein may be in the form of, for example, a double-sided pressure-sensitive adhesive sheet having the cross-sectional structure schematically shown in FIG. 2 . This pressure-sensitive adhesive sheet 1 includes a substrate 5 and a first pressure-sensitive adhesive layer 10 and a second pressure-sensitive adhesive layer 20 supported on both sides of the substrate 5, respectively. In other words, the pressure-sensitive adhesive sheet 1 includes the first pressure-sensitive adhesive layer 10, the substrate 5, and the second pressure-sensitive adhesive layer 20, in this order. More specifically, the first pressure-sensitive adhesive layer 10 and the second pressure-sensitive adhesive layer 20 are provided on the first surface 5A and the second surface 5B (both of which are non-releasable) of the substrate 5, respectively. In this pressure-sensitive adhesive sheet 1, the first pressure-sensitive adhesive layer 10 is formed from the above-described aqueous dispersion-type pressure-sensitive adhesive composition (a) and contains heat-expandable microspheres 100. The first pressure-sensitive adhesive layer 10 has a single-layer structure. Therefore, the entire first pressure-sensitive adhesive layer 10 is composed of a layer (layer A 12) that forms the surface (adhesive surface) of the first pressure-sensitive adhesive layer 10. As shown in Fig. 2 , before use (before attachment to an adherend), double-sided pressure-sensitive adhesive sheet 1 is in a form in which first adhesive surface 1A, which is the surface of first adhesive layer 10, and second adhesive surface 1B, which is the surface of second adhesive layer 20, are each protected by two independent release liners 31, 32. Specifically, in double-sided pressure-sensitive adhesive sheet 1, the surface of first adhesive layer 10 (first adhesive surface 1A) is protected by release surface 31A of release liner 31, and the surface of second adhesive layer 20 (second adhesive surface 1B) is protected by release surface 32A of release liner 32. Alternatively, first adhesive surface 1A and second adhesive surface 1B may be superimposed on a single release liner, both of which are release surfaces, and wound in a spiral shape.

[0102] The PSA sheet may have, for example, a cross-sectional structure as schematically shown in Fig. 3. The PSA sheet 2 shown in Fig. 3 differs from the PSA sheet 1 shown in Fig. 2 in that the first PSA layer 10 has a two-layer structure. Specifically, in the PSA sheet 2, the first PSA layer 10 includes an A layer 12 that constitutes the surface (adhesive surface) of the first PSA layer 10, and a B layer 14 that is disposed between the A layer 12 and the substrate 5 and adjacent to the A layer 12. The PSA sheet 2 includes a second PSA layer 20 on the second surface 5B of the substrate 5, which includes the A layer 12, the B layer 14, the substrate 5, and the second PSA layer 20, in this order. In this PSA sheet 2, the A layer 12 of the first PSA layer 10 is formed from the above-mentioned aqueous PSA composition (a) and contains heat-expandable microspheres 100, while the B layer 14 does not contain heat-expandable microspheres 100.

[0103] The pressure-sensitive adhesive sheet 3 shown in Fig. 4 is configured as a substrate-attached single-sided pressure-sensitive adhesive sheet including a substrate 5 and a pressure-sensitive adhesive layer 10 provided on a first surface 5A of the substrate 5. A second surface 5B of the substrate 5 forms the back surface of the pressure-sensitive adhesive sheet 3. The pressure-sensitive adhesive layer 10, like the first pressure-sensitive adhesive layer 10 of the pressure-sensitive adhesive sheet 2 shown in Fig. 3, includes a layer A 12 that forms the surface (adhesive surface) of the pressure-sensitive adhesive layer 10, and a layer B 14 that is disposed between the layer A 12 and the substrate 5 and adjacent to the layer A 12. The layer A 12 of the pressure-sensitive adhesive layer 10 is formed from the above-mentioned aqueous pressure-sensitive adhesive composition (a) and contains heat-expandable microspheres 100, while the layer B 14 does not contain heat-expandable microspheres 100.

[0104] The release liner is not particularly limited, and examples thereof include release liners in which the surface of a liner substrate such as a resin film or paper is release-treated, and release liners made of low-adhesion materials such as fluorine-based polymers (e.g., polytetrafluoroethylene) and polyolefin-based resins (e.g., polyethylene and polypropylene). For the release treatment, for example, a silicone-based or long-chain alkyl-based release treating agent can be used. In some embodiments, a release-treated resin film can be preferably used as the release liner.

[0105] In this specification, the adhesive layer (including the first adhesive layer and the second adhesive layer) refers to a layer having at least a surface (adhesive surface) made of an adhesive. Therefore, when the adhesive layer has a multilayer structure, the layers other than the layer that makes up the surface (adhesive surface) may be adhesive layers or non-adhesive layers (for example, non-adhesive viscoelastic layers).

[0106] <Adhesive Layer (First Adhesive Layer)> The adhesive constituting the surface of the adhesive layer disclosed herein (at least the first adhesive layer in an embodiment in which a first and second adhesive layer are provided on each side of a substrate; the same applies hereinafter unless otherwise specified) is formed from the above-mentioned water-dispersible adhesive composition (a) containing heat-expandable microspheres. Therefore, the above-mentioned adhesive is a heat-expandable microsphere-containing adhesive. By forming the surface of the adhesive layer from the above-mentioned heat-expandable microsphere-containing adhesive (e.g., Layer A described below), after the adhesive layer surface is attached to an adherend, heating at a desired timing causes the components in the heat-expandable microspheres in the heat-expandable microsphere-containing adhesive to expand, etc., thereby reducing or eliminating the adhesive strength, and the adhesive bond to the adherend can be easily released.

[0107] The thickness of the adhesive layer is not particularly limited and may be, for example, approximately 1 μm or more, or approximately 3 μm or more. From the viewpoints of the smoothness, adhesion, removability, etc. of the adhesive surface, in some embodiments, the thickness of the adhesive layer is preferably approximately 10 μm or more, more preferably approximately 20 μm or more, even more preferably approximately 30 μm or more, and particularly preferably approximately 40 μm or more. The upper limit of the thickness of the adhesive layer is not particularly limited, but is usually appropriate to be 300 μm or less, and preferably 200 μm or less. In some preferred embodiments, the thickness of the adhesive layer may be 150 μm or less, 100 μm or less, 70 μm or less, or 60 μm or less. A thickness of the adhesive layer that is not too large is advantageous in terms of productivity because it shortens the drying time, and is also desirable from the viewpoint of avoiding contamination by low molecular weight components caused by insufficient drying and cohesive failure due to insufficient crosslinking.

[0108] The pressure-sensitive adhesive layer may have a single-layer structure or a multi-layer structure. In an embodiment in which the pressure-sensitive adhesive layer has a single-layer structure, the entire pressure-sensitive adhesive layer is formed from the water-dispersible pressure-sensitive adhesive composition (a). In an embodiment in which the pressure-sensitive adhesive layer has a multi-layer structure of two or more layers, at least the layer constituting the surface of the pressure-sensitive adhesive layer (surface layer) is formed from the water-dispersible pressure-sensitive adhesive composition (a). In this specification, whether the pressure-sensitive adhesive layer has a single-layer structure or a multi-layer structure (in other words, regardless of the presence or absence of layer B), the layer constituting the surface of the pressure-sensitive adhesive layer is referred to as layer A. Furthermore, the pressure-sensitive adhesive and the pressure-sensitive adhesive layer constituting the surface of the pressure-sensitive adhesive layer can be referred to as the pressure-sensitive adhesive constituting layer A or layer A, and vice versa.

[0109] <Layer A> Layer A, which constitutes the surface of the pressure-sensitive adhesive layer, is formed from the above-mentioned water-dispersible pressure-sensitive adhesive composition (a). Therefore, Layer A is a heat-expandable microsphere-containing pressure-sensitive adhesive layer. The composition of Layer A (the components contained and the content ratio of each component) is the same as that described for the above-mentioned water-dispersible pressure-sensitive adhesive composition (a), except for volatile components such as water, and therefore a redundant description will be omitted.

[0110] (Weight-average molecular weight) In some preferred embodiments, the weight-average molecular weight (Mw) of the sol component of the adhesive (adhesive constituting layer A) constituting the surface of the adhesive layer is 8.0 × 10 ４ When the Mw of the sol component of the pressure-sensitive adhesive constituting the A layer is within the above range, the cohesive strength of the pressure-sensitive adhesive is improved, and particle generation during heat peeling is likely to be suppressed. From the viewpoint of achieving a favorable balance between adhesive strength and cohesive strength, the Mw of the sol component of the pressure-sensitive adhesive constituting the A layer is 9.0 × 10 ４ It is preferable that the ratio is 9.5×10 or more. ４ or more (e.g., 10 x 10 ４ More preferably, it is 12 × 10 ４ or more, 15 × 10 ４ or more, 17 × 10 ４ or more, and may be 19 × 10 ４ It may be 20 × 10 or more. ４ The upper limit of the Mw of the sol component of the pressure-sensitive adhesive constituting the layer A is not particularly limited, but from the viewpoint of adhesive strength, it is preferably 50×10４ It is preferably equal to or less than 45×10 ４ is less than or equal to 40×10 ４ Less than 35 x 10 ４ Less than 30 x 10 ４ Less than 25 x 10 ４ The Mw of the sol component of the PSA constituting Layer A can be adjusted, for example, by the type and amount of polymerization initiator used when preparing the base polymer, the polymerization temperature, the type and amount of emulsifier used, whether or not a chain transfer agent is used and the type and amount of chain transfer agent used if used, the composition of the monomer raw materials, the type and degree of crosslinking (gel fraction), etc.

[0111] Here, the Mw of the sol component of the PSA constituting Layer A can be measured by the following method. Specifically, a sample of the PSA to be measured is immersed in tetrahydrofuran (THF) at room temperature (e.g., 23°C) for 7 days to elute the THF-soluble component. The THF-insoluble component is removed by filtration, and the resulting filtrate is concentrated or diluted as necessary to prepare a THF solution containing a THF-soluble component at a concentration of approximately 0.1 to 0.3 wt%. This THF solution is subjected to gel permeation chromatography (GPC) to determine the weight-average molecular weight (Mw) based on standard polystyrene. More specifically, the Mw of the sol component can be measured according to the method described in the Examples below. The Mw of the sol component of the PSA constituting Layer A determined by the above method can be rephrased as the Mw of the tetrahydrofuran-soluble component of the PSA constituting Layer A.

[0112] The thickness of Layer A is not particularly limited, and may be, for example, approximately 1 μm or more, or approximately 3 μm or more. From the viewpoints of the smoothness, adhesiveness, removability, etc. of the adhesive surface, in some embodiments, the thickness of Layer A is usually appropriate to be approximately 7 μm or more, preferably approximately 10 μm or more, more preferably approximately 15 μm or more, even more preferably approximately 25 μm or more, and particularly preferably approximately 30 μm or more. The upper limit of the thickness of Layer A is not particularly limited, but is usually appropriate to be 300 μm or less, and may be 200 μm or less, 150 μm or less, 100 μm or less, or 70 μm or less. Having a thickness of Layer A that is not too large is advantageous in terms of productivity, since it shortens the drying time, and is also desirable from the viewpoint of avoiding contamination by low-molecular-weight components caused by insufficient drying and cohesive failure due to insufficient crosslinking. In some embodiments, the thickness of Layer A may be 60 μm or less, 50 μm or less, or 45 μm or less.

[0113] Although not particularly limited, the thickness ratio of the A layer to the entire thickness of the adhesive layer may be approximately 1% or more, 10% or more, or 30% or more. In some preferred embodiments, the A layer thickness ratio may be 50% or more (e.g., more than 50%), 60% or more, or 70% or more. In other embodiments, the A layer thickness ratio may be 80% or more, 90% or more, 95% or more, or 100%. In addition, in embodiments in which the adhesive layer has a multilayer structure, such as including an A layer and a B layer, the A layer thickness ratio may be 99% or less, 95% or less, 90% or less, 85% or less, or 80% or less, from the viewpoint of achieving the effect of disposing other layers such as the B layer.

[0114] <Layer B> In embodiments in which the PSA sheet is a substrate-attached PSA sheet comprising a substrate, the PSA layer may have, in addition to Layer A constituting the surface (adhesive surface), Layer B between Layer A and the substrate. Layer B is typically disposed adjacent to Layer A. Layer B may be a PSA layer or a non-adhesive layer (e.g., a non-adhesive viscoelastic or elastic layer). From the viewpoint of adhesion between Layer A and the substrate, Layer B is preferably a PSA layer. Layer B may be a porous layer or a non-porous layer in which air bubbles are substantially absent within the layer. Examples of porous layers include foamed layers containing air bubbles. Note that "substantially absent" within the layer refers to an air bubble content of less than 3% by volume. In some embodiments, the air bubble content in Layer B is preferably less than 1% by volume. A non-porous Layer B is preferably employed in the PSA sheets disclosed herein.

[0115] In some embodiments, Layer B is preferably a layer that does not contain heat-expandable microspheres or has a lower content of heat-expandable microspheres than Layer A. By disposing such Layer B between Layer A and the substrate, Layer A is held to the substrate via Layer B, and the bond between the adherend and Layer A can be released by thermal peeling. Although not particularly limited, in some preferred embodiments, the content of heat-expandable microspheres in Layer B is suitably less than 10 wt%, and may be less than 3 wt%, less than 1 wt%, or less than 0.1 wt%. It is particularly preferred that Layer B is substantially free of heat-expandable microspheres.

[0116] In an embodiment in which the adhesive layer comprises an A layer and a B layer, the differences between the A layer and the B layer may be, for example, differences in the type of layer-forming composition (water-dispersible, solvent-based, etc.), differences in the base polymer (for example, differences in the composition of the monomer components constituting the base polymer, differences in weight-average molecular weight, differences in the structure of the polymer chain, etc.), differences in additives (different types and amounts used of tackifiers, crosslinkers, and thickeners, or whether or not additives are included), etc.

[0117] Layer B may be formed from any of a water-dispersed composition, a solvent-based composition, an active energy ray-curable composition, etc. In some embodiments, Layer B is preferably formed from a water-dispersed composition. This allows the entire pressure-sensitive adhesive layer including Layer A and Layer B to be formed from a water-dispersed composition. This is significant in terms of eliminating organic solvents and being environmentally friendly. Layer B is preferably a pressure-sensitive adhesive layer formed from a water-dispersed pressure-sensitive adhesive composition. In such embodiments, the water-dispersed pressure-sensitive adhesive composition forming Layer B can be prepared by applying the same details as those described for the water-dispersed pressure-sensitive adhesive composition (a) above, except for the content of heat-expandable microspheres.

[0118] In some preferred embodiments, Layer B contains the same or similar components (e.g., an acrylic polymer, a crosslinking agent, a thickener, etc.) as Layer A. This improves interlayer adhesion between Layer A and Layer B, allowing the pressure-sensitive adhesive layer to anchor well to the substrate while retaining Layer A. Such Layer B can function as an undercoat layer (also referred to as an anchor layer) for Layer A. For example, Layer B preferably contains an acrylic polymer as the base polymer. In such embodiments, Layer B is an acrylic viscoelastic layer, preferably an acrylic pressure-sensitive adhesive layer. Furthermore, Layer B preferably contains the same type of crosslinking agent and / or thickener as the crosslinking agent and / or thickener contained in Layer A. For example, if Layer A contains an epoxy-based crosslinking agent, Layer B preferably also contains an epoxy-based crosslinking agent. The components that Layer B may contain, the content ratios of these components, and the method for forming Layer B, except for the content of heat-expandable microspheres, can be set within the ranges described for Layer A, and therefore a redundant description will be omitted.

[0119] Furthermore, Layer B may contain a tackifier or may not contain a tackifier. In some embodiments, Layer B preferably does not contain a tackifier or has a limited amount of tackifier. Eliminating the use of a tackifier or limiting the amount used is advantageous in terms of cohesion, etc. In some embodiments, the amount of tackifier in Layer B may be, for example, less than 10 parts by weight, less than 3 parts by weight, less than 1 part by weight, or less than 0.5 parts by weight, per 100 parts by weight of the base polymer.

[0120] In an embodiment in which the adhesive layer has an A layer and a B layer, the relative thickness of the A layer and the B layer is not particularly limited. The B layer may be thicker than the A layer, may be approximately the same as the A layer, or may be thinner than the A layer. In some preferred embodiments, the B layer is thinner than the A layer. Such a B layer is preferable as an undercoat layer for the A layer.

[0121] The thickness of the B layer is not particularly limited and may be, for example, approximately 0.5 μm or more, or approximately 1 μm or more. From the viewpoint of improving adhesion with the A layer, in some embodiments, the thickness of the B layer is suitably approximately 3 μm or more, preferably approximately 5 μm or more, and may be approximately 8 μm or more, approximately 10 μm or more, or approximately 12 μm or more. The upper limit of the thickness of the B layer is not particularly limited and may be, for example, approximately 1000 μm or less. In some embodiments, the thickness of the B layer is suitably approximately 300 μm or less, may be approximately 200 μm or less, or may be approximately 100 μm or less. In some preferred embodiments, the thickness of the B layer is less than 100 μm, more preferably 50 μm or less (e.g., less than 50 μm), even more preferably 30 μm or less (e.g., less than 50 μm), and particularly preferably 20 μm or less. A B layer configured to be thin in this manner is preferred as an undercoat layer for the A layer.

[0122] The method for forming a multilayer adhesive layer comprising an A layer and a B layer is not particularly limited, and can be formed using various known or commonly used methods.For example, a B layer-forming material (e.g., a pressure-sensitive adhesive composition) can be directly applied to a substrate, or can be applied to any suitable release processing material (e.g., a release liner) to form a B layer on the release processing material, and then transferred to a substrate to form a B layer on the substrate, and then a pressure-sensitive adhesive composition for forming an A layer can be directly applied to the B layer provided on the substrate to form a pressure-sensitive adhesive layer; or a pressure-sensitive adhesive composition can be applied to any suitable release processing material (e.g., a release liner) to form an A layer on the release processing material, and then the A layer can be transferred to the B layer on the substrate to form a pressure-sensitive adhesive layer.

[0123] In addition to the above-mentioned layer A and layer B, the adhesive layer may have another layer (such as an easy-adhesion layer) between layer B and the substrate.

[0124] <Second Adhesive Layer> In some embodiments, the PSA sheet may be in the form of a substrate-attached double-sided PSA sheet. Such a substrate-attached double-sided PSA sheet comprises a substrate, a first adhesive layer provided on one side of the substrate, and a second adhesive layer provided on the other side of the substrate. The second adhesive layer is not particularly limited and may be, for example, a PSA layer comprising one or more PSAs selected from various known PSAs, such as acrylic PSAs, rubber PSAs (natural rubber, synthetic rubber, mixtures thereof, etc.), silicone PSAs, polyester PSAs, urethane PSAs, polyether PSAs, polyamide PSAs, and fluorine PSAs. The configuration (type, composition, thickness, etc.) of the second adhesive layer is not particularly limited and may be designed according to the type of adherend, usage mode, required properties, etc. The second adhesive layer can be formed from a water-dispersed adhesive composition, a solvent-based adhesive composition containing an adhesive in an organic solvent, an active energy ray-curable adhesive composition prepared to form an adhesive upon curing with active energy rays such as ultraviolet light or radiation, or a hot-melt adhesive composition that is applied in a heated and molten state and forms an adhesive upon cooling to around room temperature. Such an adhesive layer can be formed using known or conventional adhesive layer forming means (drying, curing, coating). The thickness of the second adhesive layer is preferably within the thickness range of the adhesive layer (first adhesive layer).

[0125] <Substrate> The substrate of the pressure-sensitive adhesive sheet disclosed herein may be, for example, a resin sheet, a nonwoven fabric, paper, metal foil, a woven fabric, a rubber sheet, a foam sheet, a laminate thereof (particularly a laminate including a resin sheet), or the like. Examples of resins constituting the resin sheet include polyester resins such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polybutylene terephthalate (PBT); polyolefin resins such as polyethylene (PE), polypropylene (PP), and ethylene-propylene copolymers; ethylene-vinyl acetate copolymers (EVA); polyamides (nylons); wholly aromatic polyamides (aramids); polyimides (PI); polyvinyl chloride (PVC); polyphenylene sulfide (PPS); fluorine-based resins; polyether ether ketone (PEEK); and the like. Examples of nonwoven fabrics include nonwoven fabrics made from heat-resistant natural fibers, such as nonwoven fabrics containing Manila hemp; and synthetic resin nonwoven fabrics, such as polypropylene resin nonwoven fabrics, polyethylene resin nonwoven fabrics, and ester resin nonwoven fabrics. Examples of the metal foil include copper foil, stainless steel foil, aluminum foil, etc. Examples of the paper include Japanese paper, kraft paper, etc.

[0126] In some preferred embodiments, various sheets (hereinafter also referred to as substrate sheets) can be preferably used as the substrate material. The substrate sheet may be a porous sheet such as a foam sheet or a nonwoven fabric sheet, a nonporous sheet, or a sheet having a structure in which a porous layer and a nonporous layer are laminated. In some embodiments, the substrate sheet preferably includes a resin sheet that can independently maintain its shape (freestanding or independent) as a base sheet. Here, the term "resin sheet" refers to a resin sheet with a nonporous structure that is typically substantially bubble-free (void-free). Therefore, the resin sheet is a concept that is distinct from foam sheets and nonwoven fabrics. The resin sheet may have a single-layer structure or a multi-layer structure of two or more layers (e.g., a three-layer structure).

[0127] In some embodiments, a resin sheet made of a resin having a glass transition temperature (Tg) of 25°C or higher (preferably 40°C or higher, more preferably 50°C or higher) is preferably used as the substrate. The use of such a resin sheet is preferable because it allows the shape of the substrate to be well maintained even when heated. The resin constituting such a resin sheet is preferably a polymer having an aromatic ring, and specific examples include, but are not limited to, polyethylene terephthalate, polyimide, polyethylene naphthalate, etc.

[0128] The thickness of the substrate is not particularly limited. The substrate can be set to any appropriate thickness depending on the desired strength or flexibility, the purpose of use, etc. The thickness of the substrate is usually approximately 1000 μm or less, and may be approximately 500 μm or less, approximately 300 μm or less, or approximately 200 μm or less. In some preferred embodiments, the thickness of the substrate may be approximately 150 μm or less, or approximately 120 μm or less, from the viewpoint of handleability, etc. A small substrate thickness is advantageous in terms of thinning and weight reduction of the PSA sheet, etc. Furthermore, the thickness of the substrate may be 2 μm or more, 5 μm or more, or 10 μm or more. In some embodiments, a substrate having a thickness of 25 μm or more is used. Substrates having a thickness of a certain amount or more tend to be easy to handle and process. Furthermore, substrates having the above thickness are preferred because the shape of the substrate is easily maintained even in a pressurized environment. From this viewpoint, in some preferred embodiments, the thickness of the substrate may be 40 μm or more, 60 μm or more, or 80 μm or more.

[0129] The substrate may be subjected to a surface treatment, such as corona treatment, chromic acid treatment, ozone exposure, flame exposure, high-voltage shock exposure, ionizing radiation treatment, or coating with a primer.

[0130] <Surface Hardness of Adhesive Layer> The cohesiveness of the adhesive layer is not particularly limited. The surface hardness of the adhesive layer (typically the surface hardness of layer A constituting the surface of the adhesive layer; the same applies hereinafter) is not particularly limited, but in some preferred embodiments, it is approximately 0.2 MPa or more. The surface hardness of the adhesive layer may be approximately 0.25 MPa or more, approximately 0.3 MPa or more, approximately 0.35 MPa or more, approximately 0.4 MPa or more, approximately 0.45 MPa or more, approximately 0.5 MPa or more, or approximately 0.55 MPa or more. The upper limit of the surface hardness of the adhesive layer is not particularly limited, but is preferably approximately 0.8 MPa or less, approximately 0.75 MPa or less, approximately 0.7 MPa or less, or approximately 0.65 MPa or less. When the surface hardness of the adhesive layer is within the above range, the stringiness of the adhesive layer tends to be in an appropriate range, and the number of particles generated during thermal peeling tends to be reduced.

[0131] Here, the surface hardness of the adhesive layer in this specification is determined by a nanoindentation method in which a microindenter is pressed into the adhesive layer from the surface thereof to a depth of 3000 nm and then pulled out in a load-unload curve obtained by the nanoindentation method, and the maximum load (Pmax) [μN] on the load curve is divided by the contact projected area (A) of the indenter. Specifically, the surface hardness of the adhesive layer is measured by the method described in the Examples below.

[0132] In some preferred embodiments, after the pressure-sensitive adhesive sheet is applied to a silicon wafer (8-inch wafer) as an adherend and left for 1 hour, followed by heating and peeling at 130°C, the number of particles having a particle diameter of 1.5 μm or more and less than 2.5 μm on the surface of the silicon wafer to which the pressure-sensitive adhesive layer is attached (hereinafter also referred to as "particle number A") is 2,500 or less, more preferably 2,200 or less (e.g., 2,000 or less), even more preferably 1,500 or less, and particularly preferably 1,000 or less. There is no lower limit for particle number A. From the viewpoint of maintaining adhesive strength, particle number A may be, for example, approximately 5 or more, approximately 10 or more, approximately 30 or more, approximately 100 or more, or approximately 200 or more.

[0133] In some preferred embodiments, the pressure-sensitive adhesive sheet is applied to a silicon wafer (8-inch wafer) as an adherend, left for 1 hour, and then heated and peeled off at 130°C. After that, the number of particles having a particle diameter of 2.5 μm or more and less than 10 μm on the surface of the silicon wafer to which the pressure-sensitive adhesive layer is attached (hereinafter also referred to as "particle number B") is 400 or less, more preferably 350 or less (e.g., 200 or less), even more preferably 100 or less, and particularly preferably 50 or less. There is no lower limit for particle number B. From the viewpoint of maintaining adhesive strength, particle number B may be, for example, approximately 2 or more, approximately 5 or more, approximately 15 or more, approximately 20 or more, or approximately 25 or more.

[0134] The number of particles on the adherend surface after heat peeling is specifically measured by the method described in the examples below.

[0135] <Uses> The pressure-sensitive adhesive sheet disclosed herein has heat-peelability, and therefore can be used as a pressure-sensitive adhesive sheet for various applications in which it is required to apply heat to release the bond to the adherend after being attached to the adherend. For example, it can be suitably used as a processing material that is temporarily attached to the adherend during the manufacture, processing, transportation, etc. of various articles that serve as the adherend, and is removed from the adherend after achieving its purpose. For example, it can be suitably used as a processing material (e.g., a processing material in the processing step of electronic components, also known as a temporary fixing material) that fixes the adherend in the manufacturing process of electronic devices and electronic components and is peeled off after the processing step of the adherend, etc.

[0136] Although not particularly limited, a suitable application of the pressure-sensitive adhesive sheet is in the manufacture of semiconductor devices. For example, it can be preferably used as a wafer fixing sheet (temporary fixing material) for fixing the wafer to a fixing plate (e.g., a hard substrate, which may be a carrier) during semiconductor wafer processing (typically silicon wafer processing). One example of such an application is as a temporary fixing material when resin-encapsulating a semiconductor chip. In such a use mode, the pressure-sensitive adhesive sheet can be used, for example, by arranging semiconductor chips on the adhesive layer of the pressure-sensitive adhesive sheet as a temporary fixing material, encapsulating the semiconductor chips on the adhesive layer with an encapsulating resin, and then peeling the pressure-sensitive adhesive sheet from a structure including the encapsulating resin and the semiconductor chip in a predetermined post-process. The pressure-sensitive adhesive sheet disclosed herein can also be preferably used as a protective sheet for protecting the wafer during the wafer processing.

[0137] The matters disclosed in this specification include the following: [1] A pressure-sensitive adhesive sheet having a substrate and a pressure-sensitive adhesive layer on at least one surface of the substrate, wherein the surface of the pressure-sensitive adhesive layer is constituted by a pressure-sensitive adhesive formed from a water-dispersed pressure-sensitive adhesive composition containing heat-expandable microspheres, wherein the stringiness of the surface of the pressure-sensitive adhesive layer is 4000 nm or less when evaluated for stringiness using a nanoindenter in an environment of 25°C, and wherein the adhesive strength to a polyethylene terephthalate film at 23°C is 2.0 N / 20 mm or more and 13 N / 20 mm or less. [2] The weight-average molecular weight of the tetrahydrofuran-soluble component of the pressure-sensitive adhesive constituting the surface of the pressure-sensitive adhesive layer is 8.0 x 10 ４ Above 50 x 10 ４The pressure-sensitive adhesive sheet according to [1] above, wherein the pressure-sensitive adhesive composition is the following: [3] The pressure-sensitive adhesive sheet according to [1] or [2] above, wherein the water-dispersible pressure-sensitive adhesive composition comprises an acrylic polymer as a base polymer, and the acrylic polymer is a polymer of a monomer component containing 50% by weight or more of a (meth)acrylic acid alkyl ester having a chain alkyl group having 2 to 12 carbon atoms, and further containing (meth)acrylic acid. [4] The pressure-sensitive adhesive sheet according to any one of [1] to [3] above, wherein the water-dispersible pressure-sensitive adhesive composition further comprises a crosslinking agent, and the crosslinking agent comprises at least one selected from the group consisting of an epoxy-based crosslinking agent, an oxazoline-based crosslinking agent, a carbodiimide-based crosslinking agent, an isocyanate-based crosslinking agent, a hydrazide-based crosslinking agent, and an aziridine-based crosslinking agent. [5] The pressure-sensitive adhesive sheet according to [4] above, wherein the content of the crosslinking agent is 0.1 parts by weight or more and 10 parts by weight or less per 100 parts by weight of the base polymer. [6] The pressure-sensitive adhesive sheet according to any one of [1] to [5] above, wherein the aqueous dispersion-type pressure-sensitive adhesive composition further comprises a urethane-based compound. [7] The pressure-sensitive adhesive sheet according to [6] above, wherein the urethane-based compound comprises a polyoxyethylene unit. [8] The pressure-sensitive adhesive sheet according to [6] or [7] above, wherein the urethane-based compound is a polyether-containing urethane-based compound. [9] The pressure-sensitive adhesive sheet according to any one of [1] to [8] above, wherein the pressure-sensitive adhesive layer has a thickness of 1 μm or more and 200 μm or less.

[10] The pressure-sensitive adhesive sheet according to any one of [1] to [9] above, wherein the pressure-sensitive adhesive layer has a surface hardness of 0.2 MPa or more and 0.8 MPa or less, measured using a nanoindenter in a 25°C environment.

[11] The pressure-sensitive adhesive sheet according to any one of [1] to

[10] above, wherein the pressure-sensitive adhesive sheet is attached to a silicon wafer (8-inch wafer) as an adherend, left for 1 hour, and then heated and peeled off at 130°C, and the number of particles having a particle diameter of 1.5 μm or more and less than 2.5 μm on the surface of the silicon wafer to which the pressure-sensitive adhesive layer is attached is 2,500 or less and the number of particles having a particle diameter of 2.5 μm or more and less than 10 μm is 400 or less.

[12] The pressure-sensitive adhesive sheet according to any one of [1] to

[11] above, which is configured as a double-sided pressure-sensitive adhesive sheet including the substrate, a first pressure-sensitive adhesive layer provided on one surface of the substrate as the pressure-sensitive adhesive layer, and a second pressure-sensitive adhesive layer provided on the other surface of the substrate.

[13] The pressure-sensitive adhesive sheet according to any one of [1] to

[12] above, which is used in processing electronic components.

[0138] Several examples of the present invention will be described below, but it is not intended that the present invention be limited to those shown in these examples. In the following description, "parts" and "%" are by weight unless otherwise specified.

[0139] <Evaluation Method> (1) Weight-Average Molecular Weight (Mw) A sample of the pressure-sensitive adhesive to be measured (specifically, the pressure-sensitive adhesive constituting pressure-sensitive adhesive layer A (thermally expandable microsphere-containing pressure-sensitive adhesive layer)) was immersed in tetrahydrofuran (THF) at room temperature for 7 days to elute the THF-soluble components. The THF-insoluble components were then removed by filtration, and the resulting filtrate was concentrated or diluted as necessary to prepare a THF solution containing a THF-soluble component at a concentration of approximately 0.1 to 0.3 wt%. The THF solution was subjected to gel permeation chromatography (GPC) under the following conditions to determine the weight-average molecular weight (Mw) based on standard polystyrene. <GPC Conditions> Apparatus: HLC-8420GPC manufactured by TOSOH Column: TSKgel GMH-H(S), two columns connected Column temperature: 40°C Flow rate: 0.5 mL / min

[0140] (2) Load-Unloading Test Based on Nanoindentation Method The pressure-sensitive adhesive sheet was cut into 10 mm squares, and the entire back side (opposite the side on which the pressure-sensitive adhesive layer was provided) was fixed to a predetermined support via double-sided adhesive tape (manufactured by Nitto Denko Corporation, product name "No. 531") to serve as a measurement sample. The release liner covering the adhesive surface was peeled off, and a nanoindenter (Triboindenter manufactured by Hysitron Inc.) was used to perform a load-unloading test by nanoindentation on the exposed pressure-sensitive adhesive layer surface under the following measurement conditions to obtain a load-unloading curve. The displacement [nm] of the unloading curve obtained from the load-unloading curve was taken as the stringiness [nm] of the pressure-sensitive adhesive layer surface. The value obtained by dividing the maximum load of the load curve obtained from the load-unloading curve by the contact projected area of ​​the indenter was taken as the hardness [MPa] of the pressure-sensitive adhesive layer surface. The nanoindentation measurement was performed three times (N = 3), and the average value was used. Indenter used: Conical (spherical indenter, radius of curvature 10 μm) Measurement method: Single indentation measurement Measurement temperature: Room temperature (25°C) Indentation depth: 3000 nm Indentation speed: 500 nm / sec

[0141] (3) Initial Adhesion Strength to PET An adhesive sheet is cut to a width of 20 mm, and the entire back side (the side opposite to the side on which the adhesive layer is provided) is adhered to an SUS304 plate using a 2 kg hand roller via double-sided adhesive tape (manufactured by Nitto Denko Corporation, product name "No. 531"). Note that for adhesive sheets having a second adhesive layer on the back side of the substrate, if the second adhesive layer has stronger adhesive strength than the first adhesive layer, the back side of the adhesive sheet may be adhered to the SUS304 plate via the second adhesive layer instead of using the No. 531 double-sided adhesive tape. In an environment of 23°C and 50% RH, the release liner covering the adhesive surface (adhesive layer surface) of the PSA sheet whose back side was fixed to a SUS304 plate as described above was peeled off, and a polyethylene terephthalate (PET) film (manufactured by Toray Industries, Inc., trade name "Lumirror S-10", thickness 25 μm) was applied as an adherend by rolling a 2 kg roller back and forth once. After leaving this in the above environment for 30 minutes, the PET film was peeled from the PSA sheet using a tensile tester in accordance with JIS Z0237 at a peel angle of 180° and a tensile speed of 300 mm / min, and the 180° peel strength [N / 20 mm] was measured. The measurement was performed three times (i.e., N = 3), and the average value was used as the initial adhesive strength of the PSA sheet to PET. The tensile tester may be an "Autograph AG-120kN" manufactured by Shimadzu Corporation.

[0142] (4) Particle Count A test specimen is prepared by attaching an adhesive sheet to the entire surface of a bare silicon wafer (8 inches) as an adherend, and the test specimen is kept in an environment of 23°C and 50% RH for 1 hour. Thereafter, the test specimen is placed against a slide stand and placed in a hot air oven at 130°C while ensuring that no external force is applied to the attached adhesive sheet, and heated for 5 minutes. If the adhesive sheet does not naturally peel from the adherend after the above operation, the test specimen is removed from the hot air oven and tilted at an appropriate angle to allow the adhesive sheet to naturally peel from the adherend. Thereafter, the number of particles on the silicon wafer surface is measured under the following conditions. Note that, in this specification, "natural peeling" refers to natural peeling without the need for external force. <Measurement Conditions> Measuring device: Wafer inspection device Surfscan SP1 (manufactured by KLA-Tencor Corporation) Measurement particle size divisions: 1.5 μm or more and less than 2.5 μm and 2.5 μm or more and less than 10 μm

[0143] <Synthesis Example> (Synthesis of Acrylic Polymer P1) 30 parts of distilled water was added to a reaction vessel equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser, and the atmosphere was replaced with nitrogen at room temperature (25 ° C) for 1 hour while stirring. 0.1 parts of a polymerization initiator (trade name "VA-057", manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added thereto, and the temperature was raised to 60 ° C. 49 parts of 2-ethylhexyl acrylate (2EHA), 49 parts of butyl methacrylate (BMA), 2 parts of acrylic acid (AA), and 2.0 parts of a surfactant (trade name "ADEKA REASOAP SR-2090", manufactured by ADEKA Corporation) were emulsified in 80 parts of distilled water, and the mixture was added dropwise at 60 ° C. over 4 hours to polymerize the mixture, and then aged at 60 ° C. for 2 hours. The mixture was cooled to room temperature and adjusted to pH 8 using 10% aqueous ammonia to obtain an aqueous dispersion of acrylic polymer P1.

[0144] (Synthesis of acrylic polymer P2) A reactor equipped with a stirrer and a nitrogen inlet tube was charged with 49 parts of 2EHA, 49 parts of BMA, 2 parts of AA, 2.0 parts of a surfactant (trade name "Aqualon BC-2020", manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), and 100 parts of distilled water, and emulsified by nitrogen substitution at 25 ° C (room temperature) for 30 minutes. This emulsion was transferred to a reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser, and then substituted with nitrogen at 50 ° C for 2 hours, followed by polymerization by adding 0.1 parts of a polymerization initiator (VA-057), and then aging at 50 ° C for 2 hours. The mixture was cooled to room temperature and adjusted to pH 8 using 10% aqueous ammonia, to obtain an aqueous dispersion of acrylic polymer P2.

[0145] (Synthesis of Acrylic Polymer P3) 0.07 parts of a surfactant (trade name "Aqualon KH-1025", manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) and 30 parts of distilled water were placed in a reaction vessel equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser, and the atmosphere was replaced with nitrogen at room temperature (25°C) for 1 hour with stirring. 0.1 parts of a polymerization initiator (VA-057) was added thereto, and the temperature was raised to 60°C. To this mixture, an emulsion of 85 parts of 2EHA, 13 parts of methyl acrylate (MA), 1.25 parts of AA, 0.75 parts of methacrylic acid (MAA), 0.035 parts of t-dodecanethiol (chain transfer agent), 0.02 parts of 3-methacryloxypropyltrimethoxysilane (trade name "KBM-503", manufactured by Shin-Etsu Chemical Co., Ltd.), and 1.88 parts of a surfactant (KH-1025) in 70 parts of distilled water was added dropwise at 60°C over 4 hours to polymerize, and the mixture was further aged at 60°C for 2 hours. The mixture was cooled to room temperature and adjusted to pH 8 with 10% aqueous ammonia, yielding an aqueous dispersion of acrylic polymer P3.

[0146] (Synthesis of Acrylic Polymer P4) Using toluene as a polymerization solvent, a toluene solution of acrylic polymer P4 (monomer ratio: 2EHA / ethyl acrylate / methyl methacrylate / 2-hydroxyethyl acrylate=30 / 70 / 5 / 4) was prepared by solution polymerization using 0.2 parts of a polymerization initiator (benzoyl peroxide) per 100 parts of the monomer components.

[0147] Example 1 Preparation of Water-Dispersed Pressure-Sensitive Adhesive Composition To the aqueous dispersion of acrylic polymer P1 obtained above, 15 parts of a rosin-based tackifying resin (trade name "Super Ester E-865NT", manufactured by Arakawa Chemical Industries, Ltd., polymerized rosin ester, softening point 160°C), 1.4 parts of a water-soluble epoxy-based crosslinking agent (trade name "Denacol EX-313", manufactured by Nagase ChemteX Corporation), and 30 parts of heat-expandable microspheres (trade name "Matsumoto Microsphere F-50", maximum expansion temperature 125 to 135°C, manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd.) were added per 100 parts of the acrylic polymer, and 0.15 parts of a thickener (trade name "ADEKA NOL UH-450VF", manufactured by ADEKA Corporation, urethane association type, containing polyoxyethylene units) was further added to adjust the viscosity, thereby obtaining a water-dispersed Pressure-Sensitive Adhesive Composition A. A water-dispersed PSA composition B was obtained in the same manner as in the preparation of the water-dispersed PSA composition A, except that the tackifier resin and the heat-expandable microspheres were not added and the amounts of the water-soluble epoxy crosslinking agent and the thickener were changed to 0.5 parts and 0.075 parts, respectively, per 100 parts of the acrylic polymer.

[0148] (Preparation of Pressure-Sensitive Adhesive Sheet) The aqueous dispersion-type pressure-sensitive adhesive composition A obtained above was applied to the silicone-treated surface of a 38 μm-thick PET release liner (trade name "Diafoil MRF38", manufactured by Mitsubishi Chemical Corporation) to a dried thickness of 35 μm. This was then heated and dried at 85°C for 3 minutes to form a pressure-sensitive adhesive layer A (thermally expandable microsphere-containing pressure-sensitive adhesive layer) on the release liner. The aqueous dispersion-type pressure-sensitive adhesive composition B obtained above was also applied to a 100 μm-thick easy-adhesion-treated polyester film (trade name "Diafoil K400E", manufactured by Mitsubishi Chemical Corporation) as a substrate to a dried thickness of 15 μm. This was then heated and dried at 100°C for 3 minutes to form a pressure-sensitive adhesive layer B (undercoat layer) on the substrate. By bonding the adhesive layer A formed on the release liner and the adhesive layer B formed on the substrate, an adhesive sheet was obtained having an adhesive layer with a thickness of 50 μm on the substrate, the surface of which was protected by a release liner.

[0149] Examples 2 to 7 The composition of the pressure-sensitive adhesive layer A (type of acrylic polymer, type and amount of crosslinker, type of heat-expandable microspheres, amount of thickener) was changed as shown in Table 1. In Table 1, the carbodiimide crosslinker was an aqueous carbodiimide crosslinker manufactured by Nisshinbo Chemical Inc. (trade name "Carbodilite V-04"), and the oxazoline crosslinker was a water-soluble oxazoline crosslinker manufactured by Nippon Shokubai Co., Ltd. (trade name "Epocross WS-500"). In each of Examples 2 and 4 to 7, the pressure-sensitive adhesive layer B contained the same acrylic polymer, crosslinker, and thickener as the pressure-sensitive adhesive layer A, but did not contain a tackifier resin or heat-expandable microspheres. In Example 3, the pressure-sensitive adhesive layer B contained the same acrylic polymer and thickener as the pressure-sensitive adhesive layer A, but did not contain a tackifier resin or heat-expandable microspheres. The content of the crosslinking agent in the pressure-sensitive adhesive layer B was 0.5 parts of an epoxy-based crosslinking agent per 100 parts of the acrylic polymer in Example 2, 2.5 parts of a carbodiimide-based crosslinking agent per 100 parts of the acrylic polymer in Example 4, 1.2 parts of an oxazoline-based crosslinking agent per 100 parts of the acrylic polymer in Example 5, and 0.5 parts of an epoxy-based crosslinking agent per 100 parts of the acrylic polymer in Examples 6 and 7. The content of the thickener in the pressure-sensitive adhesive layer B in Examples 2 to 7 was 0.075 parts per 100 parts of the acrylic polymer, the same as in Example 1. In other respects, pressure-sensitive adhesive sheets according to each example were obtained in the same manner as in Example 1.

[0150] Comparative Example 1 To a toluene solution of the acrylic polymer P4 obtained above, 10 parts of a terpene phenol-based tackifier resin (trade name "SUMILITE PR12603N", manufactured by Sumitomo Bakelite Co., Ltd.), 1.4 parts of an isocyanate-based crosslinking agent (trade name "Takenate D-101A", manufactured by Mitsui Chemicals, Inc.), and 30 parts of heat-expandable microspheres (F-50) were added and mixed relative to 100 parts of the acrylic polymer, thereby obtaining a solvent-based pressure-sensitive adhesive composition A'. Furthermore, a solvent-based pressure-sensitive adhesive composition B' was obtained in the same manner as in the preparation of the solvent-based pressure-sensitive adhesive composition A', except that the tackifier resin and the heat-expandable microspheres were not added and the amount of the isocyanate-based crosslinking agent was changed to 1.0 part relative to 100 parts of the acrylic polymer. Using the solvent-based pressure-sensitive adhesive composition A' and the solvent-based pressure-sensitive adhesive composition B', a pressure-sensitive adhesive layer A' (thermally expandable microsphere-containing pressure-sensitive adhesive layer) and a pressure-sensitive adhesive layer B' (undercoat layer) were formed, respectively, in the same manner as in Example 1, to obtain pressure-sensitive adhesive sheets having a 50 μm-thick pressure-sensitive adhesive layer on a substrate, the surface of which was protected with a release liner.

[0151] Comparative Example 2 Pressure-sensitive adhesive sheets according to the respective examples were obtained in the same manner as in Comparative Example 1, except that the amount of crosslinking agent in the pressure-sensitive adhesive layer A′ was changed as shown in Table 1.

[0152] The outline of each example and the evaluation results are shown in Table 1.

[0153]

[0154] When measuring the particle counts of the pressure-sensitive adhesive sheets of Examples 1 to 7 and Comparative Examples 1 and 2 shown in Table 1, the pressure-sensitive adhesive sheets were attached to a silicon wafer as an adherend, held for 1 hour, and then heated for 5 minutes at 130°C, resulting in spontaneous peeling of the pressure-sensitive adhesive sheets from the adherend in all cases. This confirmed that the pressure-sensitive adhesive sheets of Examples 1 to 7 and Comparative Examples 1 and 2 all exhibit good heat-peelability.

[0155] As shown in Table 1, it was confirmed that the PSA sheets of Examples 1 to 7, which had a stringiness of 4000 nm or less and an initial adhesive strength to PET of 2.0 N / 20 mm or more and 13 N / 20 mm or less, had a significantly reduced number of particles in both the 1.5 to 2.5 μm and 2.5 to 10 μm particle size ranges, compared to the PSA sheet of Comparative Example 1, which had a stringiness of more than 4000 nm, and the PSA sheet of Comparative Example 2, which had an initial adhesive strength to PET of less than 2.0 N / 20 mm. In particular, it was confirmed that the PSA sheets of Examples 1, 2, and 4 to 7, in which the adhesive layer contained an appropriate amount of crosslinking agent, tended to have a further reduced number of particles compared to the PSA sheet of Example 3, which did not contain a crosslinking agent.

[0156] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0157] 1, 2, 3 Adhesive sheet 1A, 2A, 3A (First) Adhesive surface 1B, 2B Second adhesive surface 5 Base material 10 Adhesive layer (first adhesive layer) 12 A layer (thermally expandable microsphere-containing adhesive layer) 14 B layer 20 Second adhesive layer 31, 32 Release liner 100 Heat expandable microspheres

Claims

1. A pressure-sensitive adhesive sheet having a substrate and a pressure-sensitive adhesive layer on at least one surface of the substrate, wherein the surface of the pressure-sensitive adhesive layer is constituted by a pressure-sensitive adhesive formed from a water-dispersible pressure-sensitive adhesive composition containing heat-expandable microspheres, and wherein, when stringiness is evaluated using a nanoindenter in an environment of 25°C, the stringiness of the surface of the pressure-sensitive adhesive layer is 4000 nm or less, and the adhesive strength to a polyethylene terephthalate film at 23°C is 2.0 N / 20 mm or more and 13 N / 20 mm or less.

2. The weight average molecular weight of the tetrahydrofuran-soluble portion of the adhesive constituting the surface of the adhesive layer is 8.0 x 10 ４ Above 50 x 10 ４ The pressure-sensitive adhesive sheet according to claim 1, wherein:

3. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the water-dispersible pressure-sensitive adhesive composition comprises an acrylic polymer as a base polymer, and the acrylic polymer is a polymer of monomer components containing 50% by weight or more of a (meth)acrylic acid alkyl ester having a chain alkyl group with 2 to 12 carbon atoms, and further containing (meth)acrylic acid.

4. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the water-dispersible pressure-sensitive adhesive composition further comprises a crosslinking agent, and the crosslinking agent comprises at least one selected from the group consisting of epoxy-based crosslinking agents, oxazoline-based crosslinking agents, carbodiimide-based crosslinking agents, isocyanate-based crosslinking agents, hydrazide-based crosslinking agents, and aziridine-based crosslinking agents.

5. The pressure-sensitive adhesive sheet according to claim 4, wherein the content of the crosslinking agent is 0.1 parts by weight or more and 10 parts by weight or less per 100 parts by weight of the base polymer.

6. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the water-dispersible pressure-sensitive adhesive composition further contains a urethane compound.

7. The pressure-sensitive adhesive sheet according to claim 6, wherein the urethane compound contains a polyoxyethylene unit.

8. The pressure-sensitive adhesive sheet according to claim 6, wherein the urethane compound is a polyether-containing urethane compound.

9. The adhesive sheet according to claim 1 or 2, wherein the thickness of the adhesive layer is 1 μm or more and 200 μm or less.

10. The adhesive sheet according to claim 1 or 2, which is configured as a double-sided adhesive sheet comprising the substrate, a first adhesive layer as the adhesive layer provided on one side of the substrate, and a second adhesive layer provided on the other side of the substrate.

11. The pressure-sensitive adhesive sheet according to claim 1 or 2, which is used in processing electronic components.

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