JP7469856B2 - Adhesive sheet - Google Patents

Description

The present invention relates to an adhesive sheet. More specifically, it relates to an adhesive sheet suitable for fastening components constituting a mobile device.

Generally, adhesives (also called pressure-sensitive adhesives; the same applies below) are in a soft solid (viscoelastic) state at temperatures around room temperature, and have the property of easily adhering to an adherend when pressure is applied. Taking advantage of these properties, adhesives, typically in the form of adhesive sheets, are widely used for purposes such as joining, fixing, and protecting components in mobile phones and other portable devices. Patent documents 1 to 5 are technical documents related to adhesive tapes used in portable electronic devices. Patent document 5 is a technical document related to an adhesive sheet with a foam substrate.

JP 2009-215355 A JP 2013-100485 A Patent No. 6153635 Patent No. 6113889 JP 2017-002292 A

Portable devices are used while being carried, and therefore are prone to adhesion of secretions such as sebum and fingerprints, chemicals such as cosmetics, hair products, moisturizing creams, and sunscreens, or oils and fats contained in foods. In particular, touch panel type portable devices, which have become increasingly popular in recent years, are equipped with a display unit/input unit in which the display unit also functions as an input unit, and the surface of the display unit/input unit is operated by the user directly touching it with his/her fingertips, so there are many opportunities for oils and fats such as sebum to adhere to the device through the fingertips. In addition, some so-called wearable devices are used while being worn in contact with the skin, and in such a usage form, there are many opportunities for exposure to oily components such as sebum and chemicals applied to the skin. For example, in such applications, if the above-mentioned oily components (sebum, cosmetics, etc.) come into contact with the adhesive layer of the adhesive sheet, the adhesive absorbs the oily components and softens, which may cause inconveniences such as expansion, deformation, and reduced cohesive force. In relation to this point, the present inventors have studied in Patent Document 3 an adhesive sheet in which the adhesive strength is not easily reduced even when oil penetrates and the adhesive does not protrude. In addition, Patent Document 4 proposes an adhesive sheet that combines the ability to keep components fixed (retention performance) with oil resistance.

However, in Patent Document 4, for example, the performance (retention performance and oil resistance) is ensured in part by the layer shape using an epoxy-based crosslinking agent, and there is a limit to maintaining or improving adhesive properties based on the flexibility of the adhesive, such as adhesive strength and drop impact resistance. The present invention was created in consideration of the above circumstances, and its purpose is to provide an adhesive sheet that does not impair adhesive properties such as adhesive strength and drop impact resistance, or that can improve these properties, and that can also exhibit good oil and grease resistance.

According to the present specification, a pressure-sensitive adhesive sheet is provided that includes a foam substrate and a pressure-sensitive adhesive layer provided on at least one surface of the foam substrate. The pressure-sensitive adhesive layer includes an acrylic polymer as a base polymer and a tackifier resin. The tackifier resin includes a tackifier resin having a hydroxyl value of 70 mgKOH/g or more.

According to the above-mentioned configuration, good oil and grease resistance can be obtained by using a tackifier resin with a high hydroxyl value. In addition, the use of the above-mentioned high hydroxyl value resin increases the design freedom of the adhesive, which has been limited so far for ensuring oil resistance, and based on other adhesive constituents (e.g., crosslinking agents), it is possible to maintain or improve adhesive properties other than oil resistance (e.g., adhesive strength and drop impact resistance). In addition, by using a foam substrate, not only is drop impact resistance improved, but superior oil and grease resistance can be obtained compared to a substrate-less or resin film substrate. In short, according to the adhesive sheet of the above-mentioned configuration, adhesive properties such as adhesive strength and drop impact resistance are not impaired or can be improved, and good oil and grease resistance can be exhibited.
In this specification, the term "oils and fats" is used to include sebum, and oils and fats contained in chemical products such as cosmetics, foods, and the like.

In a preferred embodiment of the pressure-sensitive adhesive sheet disclosed herein, the monomer component constituting the acrylic polymer contains more than 50% by weight of an alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms at the ester terminal. The pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer does not easily absorb oily components because it contains, as a base polymer, an acrylic polymer having a _C1-6 alkyl (meth)acrylate as the main monomer component.

In a preferred embodiment of the pressure-sensitive adhesive sheet disclosed herein, the monomer component constituting the acrylic polymer contains a carboxyl group-containing monomer. This improves the cohesive strength of the pressure-sensitive adhesive layer. The inclusion of a carboxyl group-containing monomer in the monomer component can also advantageously contribute to improving the adhesion between the pressure-sensitive adhesive layer and the adherend. From the standpoint of compatibility with other components, etc., it is usually appropriate for the content of the carboxyl group-containing monomer in the monomer component to be approximately 1% by weight to 10% by weight.

In a preferred embodiment of the adhesive sheet disclosed herein, the adhesive composition for forming the adhesive layer contains an isocyanate-based crosslinking agent. The use of an isocyanate-based crosslinking agent tends to improve the adhesive layer's cohesive strength while also improving oil and grease resistance based on adhesion to the adherend. In addition, adhesives containing an isocyanate-based crosslinking agent tend to provide better drop impact resistance than other crosslinking systems.

In a preferred embodiment of the pressure-sensitive adhesive sheet disclosed herein, the tackifier resin includes a phenol-based tackifier resin. Phenol-based tackifier resins have excellent compatibility with acrylic polymers. By using a phenol-based tackifier resin as the tackifier resin, the effect of improving adhesion to the adherend is preferably achieved.

In a preferred embodiment of the adhesive sheet disclosed herein, the content of the tackifier resin in the adhesive layer is 10 parts by weight or more and 60 parts by weight or less per 100 parts by weight of the base polymer of the adhesive layer. By using an amount of the tackifier resin of 10 parts by weight or more per 100 parts by weight of the base polymer, good adhesive strength is easily obtained. Furthermore, by using an amount of the tackifier resin of 60 parts by weight or less, good compatibility with the base polymer is achieved, making it easy to obtain good adhesive properties.

The adhesive layer constituting the adhesive sheet disclosed herein has a glass transition temperature calculated from the peak temperature of tan δ in the range of -25°C to 25°C. By using an adhesive with a Tg in the above range, the effects of the technology disclosed herein are preferably realized. Drop impact resistance also tends to be improved.

In a preferred embodiment of the pressure-sensitive adhesive sheet disclosed herein, the foam substrate is a polyolefin-based foam substrate. The polyolefin-based foam substrate can retain oil and fat components well, and therefore can effectively reduce the penetration of oil and fat components into the adhesive interface.

The adhesive sheet disclosed herein can be preferably used, for example, to bond components of portable electronic devices. As mentioned above, portable electronic devices often come into contact with oily components, so it is particularly meaningful to apply the technology disclosed herein to improve oil resistance.

FIG. 2 is a cross-sectional view illustrating a schematic configuration example of a pressure-sensitive adhesive sheet. 2A and 2B are schematic diagrams showing an evaluation sample used when measuring the pressing adhesive force, in which (a) is a top view and (b) is a cross-sectional view taken along line A-A' in FIG. 2A. FIG. 2 is an explanatory diagram showing a method for measuring a pressure adhesive force. FIG. 1 is a cross-sectional view for illustrating a method of applying artificial sebum in an evaluation of pressure adhesive strength after application of artificial sebum. FIG. 2 is an explanatory diagram showing an evaluation sample used in a drop impact resistance evaluation test.

The following describes preferred embodiments of the present invention. Note that matters other than those specifically mentioned in this specification that are necessary for the implementation of the present invention can be understood by a person skilled in the art based on the teachings on the implementation of the invention described in this specification and the common general technical knowledge at the time of filing. The present invention can be implemented based on the contents disclosed in this specification and the common general technical knowledge in the field. In addition, in the following drawings, components and parts that perform the same function may be described with the same reference numerals, and duplicated descriptions may be omitted or simplified. In addition, the embodiments described in the drawings are schematic in order to clearly explain the present invention, and do not necessarily accurately represent the size or scale of the pressure-sensitive adhesive sheet of the present invention that is actually provided as a product.

As used herein, the term "adhesive" refers to a material that exhibits a soft solid (viscoelastic) state in a temperature range near room temperature and that has the property of easily adhering to an adherend by pressure, as described above. The adhesive referred to here may generally be a material that has a complex tensile modulus E ^* (1Hz)< ¹⁰⁷ dyne/ ^cm2 (typically, a material that has the above property at 25°C), as defined in "CA Dahlquist, "Adhesion: Fundamental and Practice", McLaren & Sons, (1966) P. 143".

<Configuration of Adhesive Sheet>
The adhesive sheet disclosed herein (which may be in the form of a long piece such as a tape) comprises a foam substrate and an adhesive layer provided on at least one side of the foam substrate. Such an adhesive sheet may be in the form of a single-sided adhesive sheet having an adhesive layer only on one side of the foam substrate, with only the one side being an adhesive surface (adhesive surface). Such a single-sided adhesive sheet may be used for joining or fixing parts, for example, by fixing the side not having the adhesive layer to an adherend by a method other than adhesion (for example, a method using an adhesive, a method of heat fusion, etc.). The adhesive sheet disclosed herein is typically preferably implemented in the form of a double-sided adhesive sheet (double-sided adhesive sheet with foam substrate) having adhesive layers on both sides of the foam substrate. Such a double-sided adhesive sheet is advantageous, for example, from the viewpoint of the ease of joining operations of parts and the stability of joining quality. The concept of adhesive sheet here may include those called adhesive tape, adhesive label, adhesive film, etc. In addition, the adhesive sheet disclosed herein may be in the form of a roll or a sheet. Alternatively, the pressure-sensitive adhesive sheet may be further processed into various shapes.

The adhesive sheet disclosed herein may have, for example, a cross-sectional structure as shown in FIG. 1. The adhesive sheet 1 comprises a foam substrate 10 and a first adhesive layer 21 and a second adhesive layer 22 supported on a first surface 10A and a second surface 10B of the foam substrate 10, respectively. The first surface 10A and the second surface 10B are both non-peeling surfaces (non-peeling surfaces). The adhesive sheet 1 is used by attaching the surface (first adhesive surface) 21A of the first adhesive layer 21 and the surface (second adhesive surface) 22A of the second adhesive layer 22 to an adherend, respectively. The adhesive sheet 1 before use has a configuration in which the first adhesive surface 21A and the second adhesive surface 22A are protected by release liners 31 and 32, at least the adhesive surface side of which is a surface (peeling surface) having releasability. Alternatively, the release liner 32 may be omitted, and a release liner 31 with release surfaces on both sides may be used, and the adhesive sheet 1 may be rolled up so that the second adhesive surface 22A is abutted against the back surface of the release liner 31, thereby forming a configuration in which the second adhesive surface 22A is also protected by the release liner 31.

<Adhesive Layer>
(Base polymer)
The pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer disclosed herein is an acrylic pressure-sensitive adhesive containing an acrylic polymer as a base polymer.
The base polymer refers to the main component of the rubber-like polymer (polymer that exhibits rubber elasticity in a temperature range around room temperature) contained in the adhesive layer. In addition, in this specification, the "main component" refers to a component that is contained in an amount of more than 50% by weight, unless otherwise specified. In addition, in this specification, the "acrylic adhesive" refers to an adhesive having an acrylic polymer as the base polymer (the main component of the polymer component, i.e., a component that occupies 50% by weight or more). In addition, the "acrylic polymer" refers to a polymer containing a monomer unit derived from a monomer having at least one (meth)acryloyl group in one molecule as a monomer unit constituting the polymer. Hereinafter, a monomer having at least one (meth)acryloyl group in one molecule is also referred to as an "acrylic monomer". Therefore, the acrylic polymer in this specification is defined as a polymer containing a monomer unit derived from an acrylic monomer. In addition, in this specification, the "(meth)acryloyl" means acryloyl and methacryloyl in a comprehensive sense. Similarly, the term "(meth)acrylate" comprehensively refers to acrylate and methacrylate, and the term "(meth)acrylic" comprehensively refers to acrylic and methacrylic, respectively.

(Acrylic polymer)
The acrylic polymer is preferably a polymer of a monomer raw material that contains, for example, an alkyl (meth)acrylate as a main monomer and may further contain a sub-monomer copolymerizable with the main monomer, where the main monomer refers to a component that accounts for more than 50% by weight of the monomer composition in the monomer raw material.

As the alkyl (meth)acrylate, for example, a compound represented by the following formula (1) can be suitably used.
_CH2 =C( ^R1 ) ^COOR2 (1)
Here, R ¹ in the above formula (1) is a hydrogen atom or a methyl group. R ² is a chain alkyl group having 1 to 20 carbon atoms. Hereinafter, this range of carbon atoms may be expressed as "C _1-20 ". From the viewpoint of the storage modulus of the adhesive, it is appropriate to use an alkyl (meth)acrylate in which R ² is a chain alkyl group of C _1-14 (e.g., C _2-10 or C _4-8 ) as the main monomer. From the viewpoint of adhesive properties, it is preferable to use an alkyl acrylate in which R ¹ is a hydrogen atom and R ² is a chain alkyl group of C _4-8 (hereinafter, also simply referred to as C _4-8 alkyl acrylate) as the main monomer.

Examples of alkyl(meth)acrylates in which R ² is a C _1-20 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, butyl ... Examples of the alkyl (meth)acrylate include octyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate. These alkyl (meth)acrylates may be used alone or in combination of two or more. Examples of preferred alkyl (meth)acrylates include n-butyl acrylate (BA) and 2-ethylhexyl acrylate (2EHA).

The proportion of alkyl (meth)acrylate in all monomer components used in the synthesis of the acrylic polymer is preferably 70% by weight or more, more preferably 85% by weight or more, and even more preferably 90% by weight or more. There is no particular upper limit to the proportion of alkyl (meth)acrylate, but it is usually appropriate to set it to 99.5% by weight or less (e.g., 99% by weight or less), and from the viewpoint of optimally expressing the action of secondary monomers such as carboxyl group-containing monomers, it is preferable to set it to about 98% by weight or less (e.g., 97% by weight or less).

The acrylic polymer according to a preferred embodiment is a polymer of monomer components that contain a C _1-6 alkyl(meth)acrylate as a main monomer and may further contain a sub-monomer copolymerizable with the main monomer. The C _1-6 alkyl(meth)acrylate may be used alone or in combination of two or more.

An acrylic polymer having a _C1-6 alkyl(meth)acrylate as a main monomer generally has a lower affinity for oily components than an acrylic polymer having an alkyl(meth)acrylate having an alkyl group with a larger number of carbon atoms at the ester end as a main monomer, and therefore the pressure-sensitive adhesive layer containing such an acrylic polymer as a base polymer tends not to absorb oily components into the pressure-sensitive adhesive layer.

From the viewpoint of further reducing the affinity of the pressure-sensitive adhesive layer for oil and fat components, the main monomer of the acrylic polymer is preferably a C _1-5 alkyl(meth)acrylate, more preferably a C _1-4 alkyl(meth)acrylate. From the viewpoint of adhesion to an adherend, etc., the main monomer of the acrylic polymer according to a preferred embodiment is a C _2-6 alkyl(meth)acrylate, more preferably a C _4-6 alkyl(meth)acrylate. From the viewpoint of improving the adhesion, the main monomer of the acrylic polymer according to another preferred embodiment is a C _1-6 alkyl acrylate, more preferably a C _1-4 alkyl acrylate (e.g., a C _2-4 alkyl acrylate).

As the C _{1-6 alkyl(meth)acrylate, from the viewpoint of reducing the affinity of the pressure-sensitive adhesive layer for oil and fat components and improving adhesion to adherends and substrates, a C 1-6} alkyl(meth)acrylate having a homopolymer glass transition temperature (Tg) of about 20° C. or lower (typically about 10° C. or lower, preferably about 0° C. or lower, more preferably about ₋₁₀ ° C. or lower, and even more preferably about −15° C. or lower) can be preferably used. The technology disclosed herein can be preferably carried out, for example, in an embodiment in which the main monomer of the acrylic polymer is BA.

From the viewpoint of reducing the affinity of the PSA layer for oil and fat components, the proportion of C _1-6 alkyl (meth)acrylate (typically C _1-6 alkyl acrylate, for example BA) among the monomer components constituting the acrylic polymer is preferably about 60% by weight or more, more preferably about 75% by weight or more, and more preferably about 85% by weight or more. The technology disclosed herein can be preferably carried out, for example, in an embodiment in which about 70% by weight or more (more preferably about 80% by weight or more, even more preferably about 85% by weight or more, and may be about 90% by weight or more or about 95% by weight or more) of the monomer components is BA.

The acrylic polymer in the technology disclosed herein may be copolymerized with monomers other than those mentioned above (other monomers) as necessary, as long as the effects of the present invention are not significantly impaired. The other monomers can be used, for example, for the purpose of adjusting the Tg of the acrylic polymer, improving the cohesive strength, adjusting the initial adhesion, etc. For example, examples of monomers that can improve the cohesive strength and heat resistance of the adhesive include sulfonic acid group-containing monomers, phosphoric acid group-containing monomers, cyano group-containing monomers, vinyl esters, aromatic vinyl compounds, etc. Among these, vinyl esters are preferred. Specific examples of vinyl esters include vinyl acetate (VAc), vinyl propionate, vinyl laurate, etc. Among these, VAc is preferred.

Other monomers that can introduce functional groups that can serve as crosslinking base points into acrylic polymers or contribute to improving peel strength include hydroxyl group-containing monomers, carboxy group-containing monomers, acid anhydride group-containing monomers, amide group-containing monomers, amino group-containing monomers, imide group-containing monomers, epoxy group-containing monomers, (meth)acryloylmorpholine, vinyl ethers, etc.

A suitable example of the acrylic polymer in the technology disclosed herein is an acrylic polymer in which a carboxyl group-containing monomer is copolymerized as the other monomer. This tends to make it easier to obtain an adhesive layer with high cohesive strength. The inclusion of a carboxyl group-containing monomer in the monomer component can also advantageously contribute to improving the adhesion between the adhesive layer and the adherend or substrate. Examples of carboxyl group-containing monomers include acrylic acid (AA), methacrylic acid (MAA), carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, isocrotonic acid, etc. Among these, AA and MAA are preferred as carboxyl group-containing monomers. AA is particularly preferred.

Other suitable examples of acrylic polymers in the technology disclosed herein include acrylic polymers in which a hydroxyl-containing monomer is copolymerized as the other monomer. The hydroxyl-containing monomer may be copolymerized with a carboxyl-containing monomer. Examples of hydroxyl-containing monomers include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; polypropylene glycol mono(meth)acrylate; and N-hydroxyethyl (meth)acrylamide. Among these, preferred hydroxyl-containing monomers include hydroxyalkyl (meth)acrylates having a hydroxyl group at the end of a linear alkyl group having about 2 to 4 carbon atoms, such as 2-hydroxyethyl acrylate (HEA) and 4-hydroxybutyl acrylate.

The above "other monomers" can be used alone or in combination of two or more. The total content of the other monomers can be, for example, less than about 50% by weight (typically about 0.001 to 40% by weight) of the total monomer components, and is usually about 25% by weight or less (typically about 0.01 to 25% by weight, for example about 0.1 to 20% by weight).

When a carboxyl group-containing monomer is used as the other monomer, the content is usually about 0.1% by weight or more of the total monomer components, preferably about 0.2% by weight or more, more preferably about 0.5% by weight or more, even more preferably about 1% by weight or more, and particularly preferably about 2% by weight or more (e.g. about 3% by weight or more). When the content of the carboxyl group-containing monomer increases, the cohesive force of the adhesive layer generally tends to improve. In addition, the amount of the carboxyl group-containing monomer is preferably about 20% by weight or less of the total monomer components, preferably about 15% by weight or less, more preferably about 12% by weight or less, even more preferably about 10% by weight or less, and particularly preferably about 8% by weight or less (e.g. about 7% by weight or less). By setting the amount of the carboxyl group-containing monomer to be used within the above range, for example, when a tackifier resin described later is blended, the blending effect is appropriately exerted, and an adhesive layer that exhibits good adhesion to the adherend or substrate can be suitably realized.

When a hydroxyl-containing monomer is used as the other monomer, the content is usually about 0.001% by weight or more of the total monomer components, and preferably about 0.01% by weight or more (typically about 0.02% by weight or more). The content of the hydroxyl-containing monomer is usually about 10% by weight or less of the total monomer components, and preferably about 5% by weight or less, and more preferably about 2% by weight or less.

The copolymer composition of the acrylic polymer is suitably designed so that the Tg of the polymer is about -15°C or less (typically about -70°C or more and -15°C or less). Here, the Tg of the acrylic polymer refers to the Tg calculated by the Fox formula based on the composition of the monomer components used in the synthesis of the polymer. The Fox formula, as shown below, is a relational expression between the Tg of the copolymer and the glass transition temperature Tgi of a homopolymer obtained by homopolymerizing each of the monomers constituting the copolymer.
1/Tg=Σ(Wi/Tgi)
In the above Fox formula, Tg represents the glass transition temperature (unit: K) of the copolymer, Wi represents the weight fraction of monomer i in the copolymer (copolymerization ratio on a weight basis), and Tgi represents the glass transition temperature (unit: K) of a homopolymer of monomer i.

The glass transition temperature of the homopolymer used in calculating Tg is a value described in a publicly known document. 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℃
n-Butyl acrylate -55℃
Ethyl acrylate -22℃
Methyl acrylate 8℃
Methyl methacrylate 105℃
2-Hydroxyethyl acrylate -15℃
4-Hydroxybutyl acrylate -40℃
Vinyl acetate 32℃
Styrene 100℃
Acrylic acid 106℃
Methacrylic acid 228℃

For the glass transition temperatures of homopolymers of monomers other than those listed above, the values given in "Polymer Handbook" (3rd Edition, John Wiley & Sons, Inc., 1989) shall be used. For monomers for which multiple values are given in this document, the highest value shall be used.

For monomers for which the glass transition temperature of the homopolymer is not described in the above literature, the value obtained by the following measurement method is used.
Specifically, 100 parts by weight of monomer, 0.2 parts by weight of 2,2'-azobisisobutyronitrile, and 200 parts by weight of ethyl acetate as a polymerization solvent are charged into a reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser tube, and stirred for 1 hour while passing nitrogen gas through. After removing oxygen from the polymerization system in this way, the temperature is raised to 63°C and reacted for 10 hours. Then, the mixture is cooled to room temperature to obtain a homopolymer solution with a solid content concentration of 33% by weight. Then, the homopolymer solution is cast and applied onto a release liner, and dried to prepare a test sample (sheet-shaped homopolymer) with a thickness of about 2 mm. This test sample was punched out into a disk shape with a diameter of 7.9 mm, sandwiched between parallel plates, and the viscoelasticity was measured in a shear mode at a temperature range of −70° C. to 150° C. and a heating rate of 5° C./min using a viscoelasticity tester (manufactured by TA Instruments Japan, model name "ARES") while applying a shear strain of 1 Hz. The peak top temperature of tan δ (loss tangent) was taken as the Tg of the homopolymer.

From the viewpoint of adhesion to the adherend or substrate, the Tg of the acrylic polymer is advantageously about -25°C or less, preferably about -35°C or less, and more preferably about -40°C or less, although not particularly limited thereto. From the viewpoint of the cohesive strength of the adhesive layer, the Tg of the acrylic polymer is advantageously about -65°C or more, preferably about -60°C or more, and more preferably about -55°C or more. The technology disclosed herein can be preferably implemented in an embodiment in which the Tg of the acrylic polymer is about -65°C or more and -35°C or less (e.g., about -55°C or more and -40°C or less). The Tg of the acrylic polymer can be adjusted by appropriately changing the monomer composition (i.e., the type and amount ratio of the monomers used in the synthesis of the polymer).

The method for obtaining the acrylic polymer is not particularly limited, and various polymerization methods known as methods for synthesizing acrylic polymers, such as solution polymerization, emulsion polymerization, bulk polymerization, suspension polymerization, and photopolymerization, can be appropriately adopted. For example, the solution polymerization method can be preferably adopted. As a monomer supply method when performing solution polymerization, a lump-sum charging method in which all monomer raw materials are supplied at once, a continuous supply (dropping) method, a divided supply (dropping) method, etc. can be appropriately adopted. The polymerization temperature can be appropriately selected depending on the type of monomer and solvent used, the type of polymerization initiator, etc., and can be, for example, about 20°C to 170°C (typically about 40°C to 140°C). In a preferred embodiment, the polymerization temperature can be about 75°C or less (more preferably about 65°C or less, for example about 45°C to 65°C).

The solvent (polymerization solvent) used in the solution polymerization can be appropriately selected from conventionally known organic solvents. For example, any one of the following solvents or a mixture of two or more solvents can be used: aromatic compounds such as toluene (typically aromatic hydrocarbons); acetate esters such as ethyl acetate; aliphatic or alicyclic hydrocarbons such as hexane and cyclohexane; halogenated alkanes such as 1,2-dichloroethane; lower alcohols such as isopropyl alcohol (for example, monohydric alcohols having 1 to 4 carbon atoms); ethers such as tert-butyl methyl ether; ketones such as methyl ethyl ketone; etc.

The initiator used for polymerization can be appropriately selected from conventionally known polymerization initiators depending on the type of polymerization method. For example, one or more azo-based polymerization initiators such as 2,2'-azobisisobutyronitrile (AIBN) can be preferably used. Other examples of polymerization initiators include persulfates such as potassium persulfate; peroxide-based initiators such as benzoyl peroxide (BPO) and hydrogen peroxide; substituted ethane-based initiators such as phenyl-substituted ethane; aromatic carbonyl compounds; and the like. Further examples of polymerization initiators include redox-based initiators formed by combining peroxides with reducing agents. Such polymerization initiators can be used alone or in combination of two or more. The amount of polymerization initiator used may be a normal amount, and can be selected from the range of, for example, about 0.005 to 1 part by weight (typically about 0.01 to 1 part by weight) per 100 parts by weight of the total monomer components.

According to the solution polymerization, a polymerization reaction liquid in the form of an acrylic polymer dissolved in an organic solvent is obtained. The adhesive layer in the technology disclosed herein may be formed from an adhesive composition containing the polymerization reaction liquid or an acrylic polymer solution obtained by subjecting the reaction liquid to an appropriate post-treatment. The acrylic polymer solution may be prepared by adjusting the polymerization reaction liquid to an appropriate viscosity (concentration) as necessary. Alternatively, an acrylic polymer solution may be prepared by synthesizing an acrylic polymer by a polymerization method other than solution polymerization (e.g., emulsion polymerization, photopolymerization, bulk polymerization, etc.) and dissolving the acrylic polymer in an organic solvent.

The weight average molecular weight (Mw) of the base polymer (preferably an acrylic polymer) in the technology disclosed herein is not particularly limited, and may be, for example, in the range of about 10×10 ⁴ to 500×10 ^4. From the viewpoint of adhesive performance, the Mw of the base polymer is preferably about 30×10 ⁴ or more, more preferably about 45×10 ⁴ or more (typically about 65×10 ⁴ or more), and preferably about 200×10 ⁴ or less, more preferably about 150×10 ⁴ or less, and even more preferably about 130×10 ⁴ or less). Here, Mw refers to a value calculated in terms of standard polystyrene obtained by GPC (gel permeation chromatography). As the GPC device, for example, the model name "HLC-8320GPC" (column: TSKgelGMH-H (S), manufactured by Tosoh Corporation) can be used.

(Tackifier resin)
The adhesive layer disclosed herein preferably contains a tackifier resin in addition to the base polymer. As the tackifier resin, one or more selected from various known tackifier resins such as rosin-based resins, terpene resins, modified terpene resins, phenol-based resins, styrene-based resins, hydrocarbon-based tackifier resins, epoxy-based tackifier resins, polyamide-based tackifier resins, elastomer-based tackifier resins, and ketone-based resins can be used. Among these, phenol-based tackifier resins are preferred.

Examples of phenolic tackifying resins include terpene phenolic resins, hydrogenated terpene phenolic resins, alkyl phenolic resins, and rosin phenolic resins.
Terpene phenolic resin refers to a polymer containing a terpene residue and a phenol residue, and is a concept that includes both a copolymer of a terpene with a phenolic compound (terpene-phenol copolymer resin) and a homopolymer or copolymer of a terpene modified with phenol (phenol-modified terpene resin). Suitable examples of terpenes constituting such terpene phenolic resins include monoterpenes such as α-pinene, β-pinene, and limonene (including d-, l-, and d/l-forms (dipentene)). Hydrogenated terpene phenolic resin refers to a hydrogenated terpene phenolic resin having a structure obtained by hydrogenating such a terpene phenolic resin. It is also called hydrogenated terpene phenolic resin.
Alkylphenol resins are resins (oil-based phenolic resins) obtained from alkylphenols and formaldehyde. Examples of alkylphenol resins include novolac and resol types.
Rosin phenolic resins are typically phenol-modified products of rosins or the above-mentioned various rosin derivatives (including rosin esters, unsaturated fatty acid modified rosins, and unsaturated fatty acid modified rosin esters). Examples of rosin phenolic resins include rosin phenolic resins obtained by adding phenol to rosins or the above-mentioned various rosin derivatives using an acid catalyst and then thermally polymerizing the resulting mixture.

The concept of rosin resin (rosin tackifier resin) used here includes both rosins and rosin derivative resins. However, those that fall under the category of rosin phenolic resins described below are treated as belonging to the phenolic resin category rather than to rosin resins.
Examples of rosins include unmodified rosins (raw rosins) such as gum rosin, wood rosin, and tall oil rosin; and modified rosins obtained by modifying these unmodified rosins through hydrogenation, disproportionation, polymerization, or the like (hydrogenated rosin, disproportionated rosin, polymerized rosin, other chemically modified rosins, etc.).

The rosin derivative resin is typically a derivative of the above-mentioned rosins. The concept of rosin-based resin as used herein includes derivatives of unmodified rosin and derivatives of modified rosin (including hydrogenated rosin, disproportionated rosin, and polymerized rosin).
Examples of rosin derivative resins include rosin esters such as unmodified rosin esters, which are esters of unmodified rosin and alcohols, and modified rosin esters, which are esters of modified rosin and alcohols; unsaturated fatty acid modified rosins, for example, obtained by modifying rosins with unsaturated fatty acids; unsaturated fatty acid modified rosin esters, for example, obtained by modifying rosin esters with unsaturated fatty acids; rosin alcohols, for example, obtained by reducing the carboxyl groups of rosins or the various rosin derivatives mentioned above (including rosin esters, unsaturated fatty acid modified rosins, and unsaturated fatty acid modified rosin esters); and metal salts of rosins or the various rosin derivatives mentioned above.

Specific examples of rosin esters include, but are not limited to, methyl esters of unmodified rosin or modified rosin (hydrogenated rosin, disproportionated rosin, polymerized rosin, etc.), triethylene glycol esters, glycerin esters, pentaerythritol esters, etc.

Examples of terpene resins (terpene-based tackifying resins) include polymers of terpenes (typically monoterpenes) such as α-pinene, β-pinene, d-limonene, l-limonene, and dipentene. The polymers may be homopolymers of one type of terpene, or copolymers of two or more types of terpenes. Examples of homopolymers of one type of terpene include α-pinene polymers, β-pinene polymers, and dipentene polymers.
Examples of modified terpene resins include those obtained by modifying the above-mentioned terpene resins.Specific examples include styrene-modified terpene resins, hydrogenated terpene resins, etc.However, those that fall under the terpene phenol resins or hydrogenated terpene phenol resins described below are treated as belonging to phenolic resins rather than modified terpene resins.

Examples of hydrocarbon-based tackifying resins include various hydrocarbon resins such as aliphatic hydrocarbon resins, aromatic hydrocarbon resins, aliphatic cyclic hydrocarbon resins, aliphatic/aromatic petroleum resins (styrene-olefin copolymers, etc.), aliphatic/alicyclic petroleum resins, hydrogenated hydrocarbon resins, coumarone resins, and coumarone-indene resins.

In one preferred embodiment, the tackifier resin includes one or more phenol-based tackifier resins (typically terpene phenol resins). The use of a phenol-based tackifier resin can improve the adhesion of the adhesive layer to the adherend and effectively suppress the penetration of oil and fat components from the interface with the adherend. In addition, phenol-based tackifier resins tend to have a lower affinity for oil and fat components than, for example, rosin-based tackifier resins. Therefore, the inclusion of a phenol-based tackifier resin can also help suppress the penetration (oil absorption) of oil and fat components into the adhesive layer. In addition, phenol-based tackifier resins tend to have excellent compatibility in embodiments using acrylic polymers as the base polymer, and have the advantage of easily exhibiting the desired adhesive properties.

The technology disclosed herein may be preferably implemented, for example, in an embodiment in which approximately 25% by weight or more (more preferably approximately 30% by weight or more) of the total amount of tackifier resin is terpene phenol resin. Approximately 50% by weight or more of the total amount of tackifier resin may be terpene phenol resin, or approximately 80% by weight or more (e.g. approximately 90% by weight or more) may be terpene phenol resin. Substantially all of the tackifier resin (e.g. approximately 95 to 100% by weight, or even approximately 99 to 100% by weight) may be terpene phenol resin.

The content of the phenolic tackifier resin (e.g., terpene phenolic resin) is not particularly limited, and is suitably about 5 parts by weight or more per 100 parts by weight of the base polymer. From the viewpoint of adhesion and oil and grease resistance, it is preferably about 10 parts by weight or more, more preferably about 15 parts by weight or more, and even more preferably about 20 parts by weight or more (e.g., about 25 parts by weight or more). The content of the phenolic tackifier resin (e.g., terpene phenolic resin) is suitably about 80 parts by weight or less, and from the viewpoint of compatibility with the base polymer, adhesive strength, drop impact resistance, and other adhesive properties, it is preferably less than 70 parts by weight, more preferably about 60 parts by weight or less, even more preferably about 55 parts by weight or less, and particularly preferably about 45 parts by weight or less (e.g., about 40 parts by weight or less).

The adhesive layer disclosed herein is characterized by containing a tackifier resin having a hydroxyl value of 70 mgKOH/g or more. By using such a high hydroxyl value resin, good oil and grease resistance can be obtained without impairing adhesive properties such as adhesive strength. The hydroxyl value of the high hydroxyl value resin is suitably about 80 mgKOH/g or more (e.g., about 100 mgKOH/g or more). A preferred embodiment of the adhesive layer contains a tackifier resin having a hydroxyl value of about 120 mgKOH/g or more (e.g., about 130 mgKOH/g or more). By using such a high hydroxyl value resin, it is possible to further improve oil and grease resistance while ensuring adhesive strength. The hydroxyl value of the high hydroxyl value resin is preferably about 140 mgKOH/g or more, more preferably about 150 mgKOH/g or more, even more preferably about 170 mgKOH/g or more, and particularly preferably about 190 mgKOH/g or more (e.g., about 200 mgKOH/g or more). There is no particular upper limit to the hydroxyl value of the high hydroxyl value resin. From the viewpoint of compatibility with the base polymer, the hydroxyl value of the high hydroxyl value resin is usually about 350 mg KOH/g or less, and preferably about 300 mg KOH/g or less (e.g., about 250 mg KOH/g or less).

Here, the hydroxyl value can be a value measured by potentiometric titration as specified in JIS K0070: 1992. A specific measurement method is as follows.
[Method for measuring hydroxyl value]
1. Reagent (1) As the acetylation reagent, take about 12.5 g (about 11.8 mL) of acetic anhydride, add pyridine to make the total volume 50 mL, and stir thoroughly. Alternatively, take about 25 g (about 23.5 mL) of acetic anhydride, add pyridine to make the total volume 100 mL, and stir thoroughly.
(2) A 0.5 mol/L potassium hydroxide ethanol solution is used as the measurement reagent.
(3) In addition, toluene, pyridine, ethanol and distilled water are prepared.
2. Procedure (1) Accurately weigh out about 2 g of a sample into a flat-bottom flask, add 5 mL of an acetylation reagent and 10 mL of pyridine, and attach an air condenser.
(2) The flask is heated in a 100°C bath for 70 minutes, then allowed to cool, and 35 mL of toluene is added from the top of the cooling tube as a solvent and stirred, followed by addition of 1 mL of distilled water and stirring to decompose the acetic anhydride. To complete the decomposition, the flask is heated again in the bath for 10 minutes and allowed to cool.
(3) Wash the cooling tube with 5 mL of ethanol and remove it. Next, add 50 mL of pyridine as a solvent and stir.
(4) Add 25 mL of 0.5 mol/L potassium hydroxide ethanol solution using a volumetric pipette.
(5) Potentiometric titration is performed using a 0.5 mol/L potassium hydroxide ethanol solution. The inflection point of the obtained titration curve is taken as the end point.
(6) A blank test is carried out by carrying out steps (1) to (5) above without adding any sample.
3. Calculation Calculate the hydroxyl value using the following formula.
Hydroxyl value (mgKOH/g) = [(B-C) x f x 28.05] / S + D
here,
B: Amount (mL) of 0.5 mol/L potassium hydroxide ethanol solution used in the blank test,
C: Amount (mL) of 0.5 mol/L potassium hydroxide ethanol solution used in the sample,
f: factor of 0.5 mol/L potassium hydroxide ethanol solution,
S: weight of sample (g),
D: acid value,
28.05: 1/2 of the molecular weight of potassium hydroxide, 56.11,
It is.

As the high hydroxyl value resin, any of the above-mentioned tackifier resins having a hydroxyl value equal to or greater than a predetermined value can be used. The high hydroxyl value resin can be used alone or in combination of two or more. For example, a phenol-based tackifier resin having a hydroxyl value of 70 mg KOH/g or more can be preferably used as the high hydroxyl value resin. In a preferred embodiment, a terpene phenol resin having a hydroxyl value of 70 mg KOH/g or more is used as the tackifier resin. Terpene phenol resins are advantageous because the hydroxyl value can be controlled as desired by the copolymerization ratio of phenol.

Although not particularly limited, the proportion of high hydroxyl value resin (e.g., terpene phenol resin) in the total tackifier resin contained in the adhesive layer can be, for example, approximately 25% by weight or more, preferably approximately 30% by weight or more, and more preferably approximately 50% by weight or more (e.g., approximately 80% by weight or more, typically approximately 90% by weight or more). Substantially all of the tackifier resin (e.g., approximately 95 to 100% by weight, or even approximately 99 to 100% by weight) may be a high hydroxyl value resin. Thus, the adhesive layer disclosed herein may contain a tackifier resin that does not fall under the category of a high hydroxyl value resin (specifically, a tackifier resin with a hydroxyl value of less than 70 mg KOH/g) to the extent that the effect of the invention is not impaired.

The content of the high hydroxyl value resin is preferably about 5 parts by weight or more (e.g., 10 parts by weight or more) per 100 parts by weight of the base polymer. This allows a pressure-sensitive adhesive sheet that exhibits adhesion to the adherend and excellent oil and grease resistance to be preferably realized. From the viewpoint of obtaining a better effect, the content of the high hydroxyl value resin per 100 parts by weight of the base polymer is preferably about 15 parts by weight or more, more preferably about 20 parts by weight or more, even more preferably about 25 parts by weight or more, and particularly preferably about 30 parts by weight or more (e.g., about 35 parts by weight or more). The upper limit of the content of the high hydroxyl value resin is not particularly limited, and from the viewpoint of compatibility with the base polymer and initial adhesion, in one embodiment, it is usually about 80 parts by weight or less per 100 parts by weight of the base polymer, and from the viewpoint of adhesive properties such as adhesive strength and drop impact resistance, it is preferably less than 70 parts by weight, more preferably about 60 parts by weight or less, even more preferably about 55 parts by weight or less, and particularly preferably about 50 parts by weight or less (e.g., about 45 parts by weight or less).

The softening point of the tackifier resin is not particularly limited. From the viewpoint of improving the cohesive force, in one embodiment, a tackifier resin having a softening point (softening temperature) of about 80° C. or more (preferably about 100° C. or more) can be preferably used. The softening point is more preferably about 110° C. or more (for example, about 120° C. or more). The technology disclosed herein can be preferably implemented in an embodiment in which the tackifier resin having the above softening point is more than 50% by weight (more preferably more than 70% by weight, for example more than 90% by weight) of the total tackifier resin contained in the adhesive layer. For example, a phenol-based tackifier resin (such as a terpene phenol resin) having such a softening point can be preferably used. The upper limit of the softening point of the tackifier resin is not particularly limited. From the viewpoint of adhesion to the adherend or substrate, in one embodiment, a tackifier resin having a softening point of about 200° C. or less (more preferably about 180° C. or less) can be preferably used. The softening point may be, for example, about 150° C. or less, or about 140° C. or less. The softening point of the tackifier resin can be measured based on the softening point test method (ring and ball method) specified in JIS K2207.

The content of the tackifier resin is not particularly limited, and is suitably about 5 parts by weight or more (e.g., 10 parts by weight or more) per 100 parts by weight of the base polymer. This allows the effect of improving adhesion to the adherend to be suitably exhibited. From the viewpoint of obtaining higher adhesion, the content of the tackifier resin per 100 parts by weight of the base polymer is preferably about 15 parts by weight or more, more preferably about 20 parts by weight or more, even more preferably about 25 parts by weight or more, and particularly preferably about 30 parts by weight or more (e.g., about 35 parts by weight or more). The upper limit of the content of the tackifier resin is not particularly limited. From the viewpoint of compatibility with the base polymer and initial adhesion, in one embodiment, it is usually suitable to set the content to about 80 parts by weight or less per 100 parts by weight of the base polymer, preferably about 60 parts by weight or less, more preferably about 55 parts by weight or less, and even more preferably about 50 parts by weight or less (e.g., about 45 parts by weight or less).

(Crosslinking Agent)
The adhesive composition used to form the adhesive preferably contains a crosslinking agent. By including a crosslinking agent in the adhesive composition, a crosslinked structure is introduced into the adhesive. 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, amine-based crosslinking agents, and the like. The crosslinking agent can be used alone or in combination of two or more types. From the viewpoint of adhesion to the adherend and drop impact resistance, an isocyanate-based crosslinking agent is preferred, and from the viewpoint of adhesion state retention performance (including layer shape retention), etc., an epoxy-based crosslinking agent is preferred. According to the technology disclosed herein, a higher performance adhesive sheet can be provided without using an epoxy-based crosslinking agent or while reducing the amount of the epoxy-based crosslinking agent used. For example, by using an isocyanate-based crosslinking agent as the main crosslinking agent component, it is possible to enhance the prevention of penetration of oil and grease components from the adhesive interface, and to achieve a high level of both oil and grease resistance and drop impact resistance.

As the epoxy-based crosslinking agent, any compound having two or more epoxy groups in one molecule can be used without particular restrictions. Epoxy-based crosslinking agents having 3 to 5 epoxy groups in one molecule are preferred. Epoxy-based crosslinking agents can be used alone or in combination of two or more types.

Specific examples of epoxy crosslinking agents include, but are not limited to, N,N,N',N'-tetraglycidyl-m-xylylenediamine, 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, 1,6-hexanediol diglycidyl ether, polyethylene glycol diglycidyl ether, polyglycerol polyglycidyl ether, etc. Commercially available epoxy crosslinking agents include "TETRAD-C" and "TETRAD-X" manufactured by Mitsubishi Gas Chemical Company, Inc., "Epicron CR-5L" manufactured by DIC Corporation, "Denacol EX-512" manufactured by Nagase ChemteX Corporation, and "TEPIC-G" manufactured by Nissan Chemical Industries, Ltd.

When an epoxy-based crosslinking agent is used, the amount is not particularly limited, and can be, for example, 3 parts by weight or less per 100 parts by weight of the base polymer. From the viewpoint of improving the adhesive strength and anchoring strength to the adherend or substrate, it is preferable to set the amount of epoxy-based crosslinking agent to 100 parts by weight of the base polymer to 1 part by weight or less, and more preferably to set it to 0.5 parts by weight or less (typically 0.2 parts by weight or less, for example 0.1 parts by weight or less, and even more preferably 0.05 parts by weight or less). By reducing the amount of epoxy-based crosslinking agent used, there is a tendency for drop impact resistance to be improved. In addition, from the viewpoint of optimally exerting the effect of improving the cohesive strength, the amount of epoxy-based crosslinking agent to 100 parts by weight of the base polymer can be set to 0.001 parts by weight or more (for example 0.005 parts by weight or more).

As the isocyanate-based crosslinking agent, a polyfunctional isocyanate (a compound having an average of two or more isocyanate groups per molecule, including those having an isocyanurate structure) can be preferably used. The isocyanate-based crosslinking agent can be used alone or in combination of two or more types.

Examples of the polyfunctional isocyanate include aliphatic polyisocyanates, alicyclic polyisocyanates, and aromatic polyisocyanates.
Specific examples of aliphatic polyisocyanates include 1,2-ethylene diisocyanate; tetramethylene diisocyanates such as 1,2-tetramethylene diisocyanate, 1,3-tetramethylene diisocyanate, and 1,4-tetramethylene diisocyanate; hexamethylene diisocyanates such as 1,2-hexamethylene diisocyanate, 1,3-hexamethylene diisocyanate, 1,4-hexamethylene diisocyanate, 1,5-hexamethylene diisocyanate, 1,6-hexamethylene diisocyanate, and 2,5-hexamethylene diisocyanate; 2-methyl-1,5-pentane diisocyanate, 3-methyl-1,5-pentane diisocyanate, and lysine diisocyanate.

Specific examples of alicyclic polyisocyanates include isophorone diisocyanate; cyclohexyl diisocyanates such as 1,2-cyclohexyl diisocyanate, 1,3-cyclohexyl diisocyanate, and 1,4-cyclohexyl diisocyanate; cyclopentyl diisocyanates such as 1,2-cyclopentyl diisocyanate and 1,3-cyclopentyl diisocyanate; hydrogenated xylylene diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated diphenylmethane diisocyanate, hydrogenated tetramethylxylene diisocyanate, and 4,4'-dicyclohexylmethane diisocyanate.

Specific examples of aromatic polyisocyanates include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 2,2'-diphenylmethane diisocyanate, 4,4'-diphenylether diisocyanate, 2-nitrodiphenyl-4,4'-diisocyanate, 2,2'-diphenylpropane-4,4'-diisocyanate, 3 , 3'-dimethyldiphenylmethane-4,4'-diisocyanate, 4,4'-diphenylpropane diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, naphthylene-1,4-diisocyanate, naphthylene-1,5-diisocyanate, 3,3'-dimethoxydiphenyl-4,4'-diisocyanate, xylylene-1,4-diisocyanate, xylylene-1,3-diisocyanate, etc.

A preferred example of a polyfunctional isocyanate is a polyfunctional isocyanate having an average of three or more isocyanate groups per molecule. Such a trifunctional or higher isocyanate may be a multimer (typically a dimer or trimer) of a bifunctional or trifunctional or higher isocyanate, a derivative (e.g., an addition reaction product of a polyhydric alcohol and two or more molecules of a polyfunctional isocyanate), a polymer, or the like. Examples of polyfunctional isocyanates include a dimer or trimer of diphenylmethane diisocyanate, an isocyanurate of hexamethylene diisocyanate (a trimer adduct of an isocyanurate structure), a reaction product of trimethylolpropane and tolylene diisocyanate, a reaction product of trimethylolpropane and hexamethylene diisocyanate, polymethylene polyphenyl isocyanate, polyether polyisocyanate, polyester polyisocyanate, and the like. Commercially available products of such polyfunctional isocyanates include "Duranate TPA-100" manufactured by Asahi Kasei Chemicals Corporation, and "Coronate L," "Coronate HL," "Coronate HK," "Coronate HX," and "Coronate 2096" manufactured by Nippon Polyurethane Industry Co., Ltd.

When an isocyanate-based crosslinking agent is used, the amount used is not particularly limited, and can be, for example, more than 0 parts by weight and about 10 parts by weight or less (typically 0.01 to 10 parts by weight) per 100 parts by weight of the base polymer. From the viewpoint of achieving both cohesive strength and adhesion, between cohesive strength and prevention of penetration of oil and fat components, drop impact resistance, etc., the amount of the isocyanate-based crosslinking agent per 100 parts by weight of the base polymer is preferably about 0.5 parts by weight or more, more preferably about 1 part by weight or more, and even more preferably about 1.5 parts by weight or more (for example, about 2.5 parts by weight or more, and even more preferably about 3.5 parts by weight or more). From the same viewpoint, the amount of the isocyanate-based crosslinking agent per 100 parts by weight of the base polymer is preferably about 8 parts by weight or less, more preferably about 6 parts by weight or less, even more preferably about 5 parts by weight or less, and particularly preferably about 4 parts by weight or less.

The technology disclosed herein is preferably implemented in an embodiment in which an epoxy-based crosslinking agent and an isocyanate-based crosslinking agent are used in combination. In such an embodiment, the relationship between the content of the epoxy-based crosslinking agent and the content of the isocyanate-based crosslinking agent is not particularly limited. The content of the epoxy-based crosslinking agent can be, for example, approximately 1/50 or less of the content of the isocyanate-based crosslinking agent. From the viewpoint of more suitably achieving both adhesion and cohesive strength to the adherend or substrate, it is appropriate that the content of the epoxy-based crosslinking agent is approximately 1/75 or less of the content of the isocyanate-based crosslinking agent, and preferably approximately 1/100 or less (for example, 1/150 or less). In addition, from the viewpoint of suitably exerting the effect of using the epoxy-based crosslinking agent in combination with the isocyanate-based crosslinking agent, it is usually appropriate that the content of the epoxy-based crosslinking agent is approximately 1/1000 or more of the content of the isocyanate-based crosslinking agent, for example, approximately 1/500 or more.

The total amount of crosslinking agent used is not particularly limited, and can be selected, for example, from a range of about 0.005 parts by weight or more (e.g., 0.01 parts by weight or more, typically 0.1 parts by weight or more) to about 10 parts by weight or less (e.g., about 8 parts by weight or less, preferably about 5 parts by weight or less) per 100 parts by weight of the base polymer.

(Other additives)
In addition to the above-mentioned components, the adhesive composition may contain various additives, as necessary, that are common in the field of adhesives, such as a leveling agent, a crosslinking assistant, a plasticizer, a softener, an antistatic agent, an antiaging agent, an ultraviolet absorber, an antioxidant, a light stabilizer, etc. As for such various additives, conventionally known ones can be used in the usual manner, and they do not particularly characterize the present invention, so detailed explanations will be omitted.

The adhesive layer (layer made of adhesive) disclosed herein may be an adhesive layer formed from an aqueous adhesive composition, a solvent-based adhesive composition, a hot melt-type adhesive composition, or an active energy ray curable adhesive composition. The aqueous adhesive composition refers to an adhesive composition in a form containing an adhesive (adhesive layer forming component) in a solvent (aqueous solvent) mainly composed of water, and typically includes those called aqueous dispersion type adhesive compositions (compositions in which at least a part of the adhesive is dispersed in water). The solvent-based adhesive composition refers to an adhesive composition in a form containing an adhesive in an organic solvent. The technology disclosed herein may be preferably implemented in an embodiment having an adhesive layer formed from a solvent-based adhesive composition from the viewpoint of adhesive properties, etc.

The adhesive layer disclosed herein can be formed by a conventional method. For example, a method can be adopted in which an adhesive composition is applied to a surface having releasability (release surface) or a non-release surface and then dried to form an adhesive layer. In an adhesive sheet having a substrate, for example, a method (direct method) can be adopted in which an adhesive composition is directly applied (typically coated) to the substrate and then dried to form an adhesive layer. In addition, a method (transfer method) can be adopted in which an adhesive composition is applied to a surface having releasability (release surface) and then dried to form an adhesive layer on the surface, and the adhesive layer is then transferred to a substrate. From the viewpoint of productivity, the transfer method is preferred. As the release surface, the surface of a release liner, the back surface of a substrate that has been subjected to a release treatment, etc. can be used. The adhesive layer disclosed herein is typically formed continuously, but is not limited to such a form, and may be an adhesive layer formed in a regular or random pattern such as a dotted or striped pattern.

The pressure-sensitive adhesive composition can be applied using a conventionally known coater such as a gravure roll coater, a die coater, a bar coater, etc. Alternatively, the pressure-sensitive adhesive composition may be applied by impregnation or curtain coating.
From the viewpoints of promoting the crosslinking reaction, improving production efficiency, etc., the pressure-sensitive adhesive composition is preferably dried under heating. The drying temperature can be, for example, about 40 to 150° C., and is usually preferably about 60 to 130° C. After drying the pressure-sensitive adhesive composition, aging may be further performed for the purpose of adjusting component migration in the pressure-sensitive adhesive layer, advancing the crosslinking reaction, relaxing distortion that may exist in the pressure-sensitive adhesive layer, etc.

Although not particularly limited, it is appropriate that the Tg of the adhesive layer constituting the adhesive sheet disclosed herein is controlled to approximately 25°C or less from the viewpoint of adhesion to the adherend and prevention of oil and fat component penetration based on the adhesion. An adhesive layer having the above Tg also tends to have excellent drop impact resistance. The Tg of the adhesive layer is preferably approximately 20°C or less (typically approximately 15°C or less, for example approximately 10°C or less). In addition, from the viewpoint of processability, etc., the Tg of the adhesive layer is appropriate to be approximately -25°C or more, preferably approximately -15°C or more, more preferably approximately -10°C or more (for example approximately -5°C or more), and may be approximately 0°C or more (for example approximately 5°C or more). According to the technology disclosed herein, an adhesive having the above Tg and capable of exhibiting desired adhesive properties (for example, adhesive strength and drop impact resistance) can realize improved oil and fat resistance (typically oil and fat resistant adhesion reliability). In an embodiment in which oil and grease resistance is improved by selecting a tackifier resin type (typically a high hydroxyl value resin), the Tg in the above range can favorably exhibit the oil and grease resistance improving effect based on the viscoelastic properties of the adhesive and the chemical properties of the tackifier resin. In this specification, the Tg of the adhesive layer refers to the glass transition temperature determined from the peak temperature of tan δ in dynamic viscoelasticity measurement. The Tg of the adhesive layer can be adjusted by the adhesive composition (e.g., the Tg of the base polymer, the softening point of the tackifier resin, the type of crosslinker, and the content ratio of these components) and the manufacturing method (polymer polymerization conditions, etc.).

The value (peak strength) of the tan δ peak of the adhesive layer is typically 1.0 or more, preferably about 1.5 or more, more preferably about 1.8 or more, and even more preferably about 2.0 or more. In adhesives having a tan δ peak in a relatively low temperature range (typically in the range of -25°C to 25°C), those having a peak strength of a predetermined value or more can have excellent drop impact resistance. In addition, the above-mentioned tan δ peak strength is usually about 3.0 or less, preferably about 2.5 or less, and may be less than about 2.2 (e.g., less than 2.0). The technology disclosed herein can realize improved oil and grease resistance (typically oil and grease resistant adhesion reliability) in an adhesive that has the above-mentioned tan δ peak strength and can exhibit desired adhesive properties (e.g., drop impact resistance). The tan δ peak strength of the adhesive layer can be adjusted by the adhesive composition (e.g., Tg of the base polymer, softening point of the tackifier resin, type of crosslinker, and content ratio of these components) and the manufacturing method (polymer polymerization conditions, etc.).

The 25°C storage modulus of the adhesive layer disclosed herein is not particularly limited, and may be, for example, approximately 1 MPa or less. The 25°C storage modulus of the adhesive layer may be approximately 0.5 MPa or less, and is preferably approximately 0.3 MPa or less (for example, approximately 0.25 MPa or less). When the 25°C storage modulus of the adhesive layer is low, the flexibility of the adhesive layer increases in the normal temperature range, making it easier to bring the adhesive surface into close contact with the adherend surface. This is significant in preventing the penetration of oil and fat components into the interface with the adherend. With an adhesive whose storage modulus is limited to a predetermined value or less, good drop impact resistance is easily obtained. In addition, the 25°C storage modulus may be approximately 0.01 MPa or more. With an adhesive whose 25°C storage modulus is a predetermined value or more, it has appropriate cohesiveness in the normal temperature range, making it easy to increase the adhesive strength. It may also be advantageous from the viewpoint of improving processability when processing the adhesive sheet into a narrow width. From this viewpoint, the 25°C storage modulus is suitably about 0.02 MPa or more, preferably about 0.05 MPa or more, more preferably about 0.1 MPa or more, even more preferably about 0.14 MPa or more, and particularly preferably about 0.18 MPa or more (e.g., about 0.2 MPa or more). According to the technology disclosed herein, an adhesive that has the above-mentioned 25°C storage modulus and can exhibit desired adhesive properties (e.g., adhesive strength and drop impact resistance) can achieve improved oil and grease resistance (typically oil and grease-resistant adhesion reliability). The 25°C storage modulus of the adhesive layer can be adjusted by the composition of the adhesive layer, the manufacturing method, etc.

The 25°C loss modulus of the adhesive layer disclosed herein is not particularly limited, and may be, for example, approximately 0.01 MPa or more. The 25°C loss modulus of the adhesive layer is suitably approximately 0.02 MPa or more, preferably 0.05 MPa or more, more preferably approximately 0.1 MPa or more, even more preferably approximately 0.15 MPa or more, and particularly preferably approximately 0.17 MPa or more (for example, approximately 0.2 MPa or more). According to an adhesive exhibiting a 25°C loss modulus of a predetermined value or more, adhesion to an adherend is improved based on its viscosity term (loss modulus). When the 25°C loss modulus of the adhesive layer is high, the drop impact resistance also tends to be improved. In addition, the 25°C loss modulus of the adhesive layer may be, for example, approximately 1 MPa or less. From the viewpoint of cohesiveness, processability, etc., the 25°C loss modulus of the adhesive layer may be approximately 0.5 MPa or less, preferably approximately 0.3 MPa or less (for example, approximately 0.25 MPa or less). According to the technology disclosed herein, it is possible to realize improved oil and grease resistance (typically oil and grease resistant adhesion reliability) in an adhesive that has the above-mentioned 25°C loss modulus and can exhibit desired adhesive properties (e.g., adhesive strength and drop impact resistance). The 25°C loss modulus of the adhesive layer can be adjusted by the composition of the adhesive layer, the manufacturing method, etc.

The Tg (peak temperature of tan δ), the peak strength at the tan δ (G"/G') peak, the 25°C storage modulus (G'(25°C)), and the 25°C loss modulus (G"(25°C)) of the adhesive layer are obtained by dynamic viscoelasticity measurement. A specific measuring device that can be used is the "ARES" model manufactured by TA Instruments Co., Ltd. or an equivalent product. The specific measurement operation and measurement conditions can be set according to the measurement conditions described in the examples below, or to obtain results that are equivalent to or correspond to those obtained when the measurement conditions are followed.

The thickness of the adhesive layer is not particularly limited. In order to avoid the adhesive sheet from becoming excessively thick, the thickness of the adhesive layer is usually about 100 μm or less, preferably about 70 μm or less, more preferably about 60 μm or less, and even more preferably about 50 μm or less. In general, as the thickness of the adhesive layer becomes smaller, the adhesion to the adherend decreases, and there is a tendency for oil and fat components to easily penetrate from the interface with the adherend. Therefore, it is particularly meaningful to apply the technology disclosed herein to improve the oil and fat resistance. There is no particular limit to the lower limit of the thickness of the adhesive layer, but from the viewpoint of adhesion to the adherend, it is advantageous to set it to about 3 μm or more, preferably about 10 μm or more, and more preferably about 20 μm or more (for example, about 30 μm or more). When the adhesive sheet disclosed herein is configured as a double-sided adhesive sheet, it may be an adhesive sheet having an adhesive layer of the above thickness on both sides of the substrate. In addition, in a double-sided adhesive sheet with a substrate having a first adhesive layer and a second adhesive layer on each side of the substrate, the first adhesive layer and the second adhesive layer may be of the same thickness or may be of different thicknesses.

<Foam Base Material>
The adhesive sheet disclosed herein comprises a foam substrate. Specifically, the adhesive sheet is configured as an adhesive sheet having an adhesive layer on at least one surface of the foam substrate. In the technology disclosed herein, the foam substrate is a substrate having a portion with bubbles (cell structure), typically a substrate including at least one layer of laminar foam (foam layer). The foam substrate may be a substrate composed of one or more foam layers. The foam substrate may be, for example, a substrate substantially composed of only one or more foam layers. Although not particularly limited, a suitable example of a foam substrate in the technology disclosed herein is a foam substrate composed of a single layer (one layer) of foam layer. By using a foam substrate, superior oil and grease resistance can be obtained compared to a substrate-less structure or a structure using a resin film substrate. The reason for this, although not particularly limited, is thought to be the improvement of adhesion to the adherend interface by using a foam substrate, the sealing property of the foam substrate itself, and the reduction of penetration of oil and grease components into the adhesive interface as a bulk.

The density D (referring to apparent density; hereinafter, the same applies unless otherwise specified) of the foam substrate is not particularly limited, and may be, for example, about 0.1 to 0.9 g/cm ^3. From the viewpoint of absorbing oil and fat components and reducing the amount of oil and fat components penetrating into the adhesive interface, the density D of the foam substrate is suitably about 0.8 g/cm ³ or less, and preferably about 0.7 g/cm ³ or less (e.g., about 0.6 g/cm ³ or less). In one embodiment, the density D of the foam substrate may be less than 0.5 g/cm ³ , or may be less than 0.4 g/cm ^3. In addition, from the viewpoint of increasing the oil and fat resistance based on the sealing property of the foam substrate itself, the density D of the foam substrate is preferably about 0.12 g/cm ³ or more, more preferably about 0.15 g/cm ³ or more, and even more preferably about 0.2 g/cm ³ or more (e.g., about 0.3 g/cm ³ or more). In one embodiment, the density D of the foam substrate may be about 0.4 g/cm ³ or more, about 0.5 g/cm ³ or more (e.g., more than 0.5 g/cm ³ ), or even 0.55 g/cm ³ or more. A foam substrate having a density in the above range also tends to have excellent drop impact resistance. The density D (apparent density) of the foam substrate can be measured in accordance with JIS K 6767.

The average bubble diameter of the foam substrate is not particularly limited, but from the viewpoint of suppressing the deterioration of performance due to narrowing, it is preferably about 300 μm or less, more preferably about 200 μm or less, and even more preferably about 150 μm or less. From the viewpoint of exhibiting higher performance of oil and grease resistance, waterproofness, and dust resistance, the average bubble diameter of the foam substrate is preferably about 120 μm or less, and more preferably about 100 μm or less (typically about 90 μm or less, for example about 80 μm or less, and even more preferably about 70 μm or less). The lower limit of the average bubble diameter is not particularly limited, but from the viewpoint of absorbing oil and grease components and reducing the amount of oil and grease components penetrating into the adhesive interface, it is usually appropriate to be about 10 μm or more, preferably about 20 μm or more, more preferably about 30 μm or more, and even more preferably about 40 μm or more (for example, about 50 μm or more). In one embodiment, the average bubble diameter may be 55 μm or more, or may be 60 μm or more. Increasing the average cell diameter also tends to improve drop impact resistance. Note that the average cell diameter here refers to the average cell diameter in terms of spheres, obtained by observing the cross section of the foam substrate with an electron microscope.

The bubbles contained in the foam substrate are preferably in a shape relatively close to a circle in plan view of the foam substrate. That is, it is preferable that the average bubble diameter in the flow direction (hereinafter also referred to as "MD") of the foam substrate is not too different from the average bubble diameter in the width direction (hereinafter also referred to as "CD"). The degree of deviation of the shape of the bubbles from a circular shape can be grasped using as an index the ratio of the average bubble diameter in the MD (MD average bubble diameter) to the average bubble diameter in the CD of the foam substrate (CD average bubble diameter), that is, the "aspect ratio (MD/CD)" represented by the following formula. If this aspect ratio (MD/CD) is closer to 1, it can be said that the shape of the bubbles contained in the foam substrate in plan view is closer to a circle.
Aspect ratio (MD/CD) = MD average cell diameter/CD average cell diameter

In one embodiment of the technology disclosed herein, the aspect ratio (MD/CD) of the bubbles contained in the foam substrate is preferably 0.7 or more, more preferably 0.75 or more, and even more preferably 0.8 or more, and may be, for example, 0.85 or more. In one embodiment, the aspect ratio may be 0.9 or more, and may be 0.95 or more (for example, approximately 1.0 or more). The aspect ratio (MD/CD) is preferably 1.3 or less, more preferably 1.25 or less, and even more preferably 1.2 or less, and may be, for example, 1.15 or less. When the aspect ratio (MD/CD) is not too small than 1, the handleability of the adhesive sheet using the foam substrate can be improved. When the aspect ratio (MD/CD) is not too large than 1, the oil and grease resistance, waterproofness, and dust resistance of the adhesive sheet using the foam substrate can be improved. As described below, for a foam substrate that constitutes an adhesive sheet that can be used in a form having a narrow portion (particularly in the form of an annular member having a narrow portion), it is particularly significant that the aspect ratio (MD/CD) is close to 1.

Here, the MD of the foam substrate refers to the extrusion direction in the manufacturing process of the foam substrate. Although not particularly limited, the MD of a long foam substrate such as a tape usually coincides with the long direction. The CD of the foam substrate refers to a direction perpendicular to the MD of the foam substrate and along the surface of the foam substrate. The thickness direction of the foam substrate (hereinafter also referred to as "VD") is a direction perpendicular to both the MD and the CD.

The MD average cell diameter of the foam substrate is measured as follows. That is, the foam substrate is cut at approximately the center in the CD along a plane parallel to the MD and VD (i.e., a plane whose perpendicular line coincides with the CD), and the center of the cut surface is photographed with a scanning electron microscope (SEM). The photographed image is printed on an A4 size paper, and a straight line of 60 mm in length parallel to the MD is drawn on the image. At this time, the magnification of the SEM is adjusted so that about 10 to 20 bubbles are present on the 60 mm straight line. The number of bubbles present on the straight line is visually counted, and the MD average cell diameter is calculated according to the following formula.
MD average bubble diameter (μm)=60 (mm)×10 ³ /(bubble number (pieces)×magnification ratio)

The CD average bubble diameter of the foam substrate is measured as follows. That is, the foam substrate is cut along a plane parallel to the CD and VD (i.e., a plane whose perpendicular line coincides with the MD), and the center of the cut surface is photographed with an SEM. The photographed image is printed on an A4 size paper, and a straight line of 60 mm in length parallel to the CD is drawn on the image. At this time, the magnification of the SEM is adjusted so that about 10 to 20 bubbles are present on the 60 mm straight line. The number of bubbles present on the straight line is visually counted, and the CD average bubble diameter is calculated according to the following formula.
CD average bubble diameter (μm)=60 (mm)×10 ³ /(bubble number (pieces)×magnification ratio)

When drawing a straight line, try to make sure that the line passes through the air bubble without touching it at a point. If some of the air bubbles come into contact with the line at a point, count these bubbles as one. Furthermore, if both ends of the line are located inside the air bubble without penetrating it, count these bubbles as 0.5.

The average cell diameter in each direction of the foam substrate can be controlled, for example, by adjusting the composition of the foam substrate (e.g., the amount of foaming agent used) and the manufacturing conditions (conditions in the foaming process, stretching process, etc.).

As the foam substrate in the technology disclosed herein, one in which the relationship between the 10% compression strength C ₁₀ [kPa] and the 30% compression strength C ₃₀ [kPa] satisfies the following formula: (C ₃₀ /C ₁₀ )≦5.0 can be preferably used. Here, the 10% compression strength of the foam substrate refers to the load (load at a compression rate of 10%) when a measurement sample, which is a 30 mm square cut of the foam substrate and stacked to a thickness of about 2 mm, is sandwiched between a pair of flat plates and compressed by a thickness equivalent to 10% of the original thickness. In other words, it refers to the load when the measurement sample is compressed to a thickness equivalent to 90% of the original thickness. Similarly, the 30% compression strength C ₃₀ [kPa] and the 25% compression strength C ₂₅ [kPa] described later refer to the load when the measurement sample is compressed by a thickness equivalent to 30% or 25% of the original thickness.
The compressive strength of the foam substrate at any compression ratio is measured in accordance with JIS K 6767. As a specific measurement procedure, the measurement sample is set at the center of the pair of flat plates, and the gap between the flat plates is narrowed to continuously compress the sample to any compression ratio, and the flat plates are stopped at that point and the load is measured after 10 seconds have elapsed. The compressive strength of the foam substrate can be controlled, for example, by the degree of crosslinking or density of the material constituting the foam substrate, the size or shape of the bubbles, etc.

A small compression strength ratio ( _C30 / _C10 ) means that the difference in the degree of compression has a small effect on the compression strength. For example, when there are unevenness such as steps or scratches on the bonding surface of the adhesive sheet, when the width of the adhesive sheet is partially different, or when a part of the bonding part of the adhesive sheet is subjected to a larger stress than the other parts, a part of the adhesive sheet may be compressed more than the other parts. When the width of the adhesive sheet is narrowed, the difference in the degree of compression due to the steps or partial width difference tends to become more significant. If the difference in compression strength due to the difference in the degree of compression is too large, distortion will concentrate on the part where the degree of compression changes, and this part may become the starting point of peeling of the adhesive sheet or damage to the foam substrate. In an adhesive sheet using a foam substrate with a small ( _C30 / _C10 ), the difference in compression strength due to the difference in the degree of compression is small, so that the peeling and damage to the foam substrate are less likely to occur. This can be advantageous from the viewpoint of improving drop impact resistance. From the viewpoint of obtaining a better effect, ( _C30 / _C10 ) is more preferably 4.5 or less, and even more preferably 4.0 or less. ( _C30 / _C10 ) may be 3.5 or less. There is no particular lower limit to ( _C30 / _C10 ), but it is suitably 2.5 or more, and may be 3.0 or more.

The 25% compressive strength C ₂₅ of the foam substrate is not particularly limited, and may be, for example, 20 kPa or more (typically 40 kPa or more). C ₂₅ is usually appropriate to be 250 kPa or more, and preferably 300 kPa or more (for example, 400 kPa or more). A pressure-sensitive adhesive sheet having such a foam substrate can exhibit good durability against impact such as dropping even if it is narrow. For example, the pressure-sensitive adhesive sheet can be better prevented from being torn off by impact. The upper limit of C ₂₅ is not particularly limited, but is usually appropriate to be 1300 kPa or less (for example, 1200 kPa or less). In one embodiment, C ₂₅ may be 1000 kPa or less, 800 kPa or less, or even 600 kPa or less (for example, 500 kPa or less). Better results can be achieved with a PSA sheet having a foam substrate in which the relationship between _C25 [kPa] and apparent density D [g/ ^cm3 ] satisfies the following formula: ₁₅₀ ≦C25×D≦400 (e.g., 200≦ _C25 ×D≦350, preferably 240≦ _C25 ×D≦300).

In another preferred embodiment, the C ₂₅ of the foam substrate can be 20 kPa to 200 kPa (typically 30 kPa to 150 kPa, for example 40 kPa to 120 kPa). A pressure-sensitive adhesive sheet comprising such a foam substrate has low compressive strength relative to its density, and therefore can have excellent cushioning properties even when it is narrow. For example, the foam substrate can absorb the impact of a drop, thereby better preventing peeling of the pressure-sensitive adhesive sheet. A pressure-sensitive adhesive sheet comprising a foam substrate in which the relationship between C ₂₅ [kPa] and apparent density D [g/cm ³ ] satisfies the following formula: 100≦C ₂₅ /D≦400 (for example 150≦C ₂₅ /D≦350, preferably 200≦C ₂₅ /D≦300) can achieve better results.

The tensile elongation of the foam substrate is not particularly limited. For example, a foam substrate having a tensile elongation in the machine direction (MD) of 200% to 800% (more preferably 400% to 600%) can be suitably used. Also, a foam substrate having a tensile elongation in the transverse direction (TD) of 50% to 800% (more preferably 200% to 500%) is preferred. The elongation of the foam substrate is measured in accordance with JIS K 6767. The elongation of the foam substrate can be controlled, for example, by the degree of crosslinking, apparent density (expansion ratio), etc.

The tensile strength of the foam substrate is not particularly limited. For example, a foam substrate having a tensile strength in the machine direction (MD) of 5 MPa to 35 MPa (preferably 10 MPa to 30 MPa) can be suitably used. A foam substrate having a tensile strength in the transverse direction (TD) of 1 MPa to 25 MPa (more preferably 5 MPa to 20 MPa) is preferred. The tensile strength of the foam substrate is measured in accordance with JIS K 6767. The tensile strength of the foam substrate can be controlled, for example, by the degree of crosslinking, apparent density (expansion ratio), etc.

The material of the foam substrate is not particularly limited. Usually, a foam substrate including a foam layer formed from a foam of a plastic material (plastic foam) is preferred. The plastic material (meaning including rubber materials) for forming the plastic foam is not particularly limited and can be appropriately selected from among known plastic materials. The plastic material can be used alone or in an appropriate combination of two or more types.

Specific examples of plastic foams include polyolefin-based resin foams such as polyethylene foams and polypropylene foams; polyester-based resin foams such as polyethylene terephthalate foams, polyethylene naphthalate foams, and polybutylene terephthalate foams; polyvinyl chloride-based resin foams such as polyvinyl chloride foams; vinyl acetate-based resin foams; polyphenylene sulfide resin foams; amide-based resin foams such as aliphatic polyamide (nylon) resin foams and fully aromatic polyamide (aramid) resin foams; polyimide-based resin foams; polyether ether ketone (PEEK) foams; styrene-based resin foams such as polystyrene foams; urethane-based resin foams such as polyurethane resin foams; and the like. In addition, rubber-based resin foams such as polychloroprene rubber foams may be used as plastic foams.

A preferred example of the foam is a polyolefin-based resin foam (hereinafter also referred to as "polyolefin-based foam"). The polyolefin-based foam base material has a high affinity with oil and fat components and can retain the oil and fat components well, so that the penetration of the oil and fat components into the adhesive interface can be effectively reduced. As the plastic material (i.e., polyolefin-based resin) constituting the polyolefin-based foam, various known or commonly used polyolefin-based resins can be used without particular limitation. For example, polyethylene such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, etc. can be mentioned. Examples of LLDPE include Ziegler-Natta catalyst-based linear low-density polyethylene, metallocene catalyst-based linear low-density polyethylene, etc. Such polyolefin-based resins can be used alone or in appropriate combination of two or more types.

Suitable examples of the foam substrate in the technology disclosed herein include polyolefin foam substrates such as polyethylene foam substrates substantially composed of a polyethylene resin foam and polypropylene foam substrates substantially composed of a polypropylene resin foam, from the viewpoints of oil component absorption, waterproofness, dust resistance, drop impact resistance, etc. Here, polyethylene resin refers to a resin in which ethylene is the main monomer (i.e., the main component of the monomers), and may include HDPE, LDPE, LLDPE, etc., as well as ethylene-propylene copolymers and ethylene-vinyl acetate copolymers in which the copolymerization ratio of ethylene exceeds 50% by weight. Similarly, polypropylene resin refers to a resin in which propylene is the main monomer. As the foam substrate in the technology disclosed herein, a polyethylene foam substrate may be preferably used.

The method for producing the plastic foam (typically a polyolefin foam) is not particularly limited, and various known methods can be appropriately adopted. For example, the plastic foam can be produced by a method including a molding step, a crosslinking step, and a foaming step of the plastic material or the plastic foam. In addition, a stretching step can be included as necessary.
Examples of methods for crosslinking the plastic foam include a chemical crosslinking method using an organic peroxide or an ionizing radiation crosslinking method using ionizing radiation, and these methods can be used in combination. Examples of the ionizing radiation include electron beams, α-rays, β-rays, and γ-rays. The dose of the ionizing radiation is not particularly limited, and can be set to an appropriate dose in consideration of the target physical properties (e.g., degree of crosslinking) of the foam base material.

The foam base material may contain various additives such as fillers (inorganic fillers, organic fillers, etc.), antioxidants, antioxidants, UV absorbers, antistatic agents, lubricants, plasticizers, flame retardants, surfactants, etc., as necessary.

The foam substrate in the technology disclosed herein may be colored in order to impart desired design properties and optical properties (e.g., light blocking properties, light reflectivity, etc.) to the adhesive sheet comprising the foam substrate. For this coloring, known organic or inorganic colorants can be used alone or in appropriate combination of two or more.

For example, when the adhesive sheet disclosed herein is used for light blocking purposes, the visible light transmittance of the foam substrate is not particularly limited, but is preferably 0% to 15%, and more preferably 0% to 10%, similar to the visible light transmittance of the adhesive sheet described below. Furthermore, when the adhesive sheet disclosed herein is used for light reflection purposes, the visible light reflectance of the foam substrate is preferably 20% to 100%, and more preferably 25% to 100%, similar to the visible light reflectance of the adhesive sheet.

The visible light transmittance of the foam substrate can be determined by using a spectrophotometer (e.g., a spectrophotometer model "U-4100" manufactured by Hitachi High-Technologies Corporation) to measure the intensity of light at a wavelength of 550 nm irradiated from one side of the foam substrate and transmitted to the other side. The visible light reflectance of the foam substrate can be determined by using the above spectrophotometer to measure the intensity of light at a wavelength of 550 nm irradiated from one side of the foam substrate and reflected. The visible light transmittance and visible light reflectance of the pressure-sensitive adhesive sheet can also be determined by a similar method.

When the pressure-sensitive adhesive sheet disclosed herein is used for light-shielding purposes, the foam substrate is preferably colored black. The black color is preferably 35 or less (e.g., 0 to 35) in L* (lightness) as defined by the L*a*b* color system, more preferably 30 or less (e.g., 0 to 30). Note that a* and b* as defined by the L*a*b* color system can each be appropriately selected according to the value of L*. There are no particular limitations on a* and b*, but both are preferably in the range of -10 to 10 (more preferably -5 to 5, and even more preferably -2.5 to 2.5). For example, it is preferable that both a* and b* are 0 or approximately 0.

In this specification, the L*, a*, and b* defined in the L*a*b* color system are determined by measurement using a colorimeter (for example, a colorimeter manufactured by Minolta, product name "CR-200"). The L*a*b* color system is a color space recommended by the International Commission on Illumination (CIE) in 1976, and refers to the color space known as the CIE1976 (L*a*b*) color system. The L*a*b* color system is also defined in JIS Z 8729 in the Japanese Industrial Standards.

Examples of black colorants that can be used when coloring the foam substrate black include carbon black (furnace black, channel black, acetylene black, thermal black, lamp black, etc.), graphite, copper oxide, manganese dioxide, aniline black, perylene black, titanium black, cyanine black, activated carbon, ferrite (non-magnetic ferrite, magnetic ferrite, etc.), magnetite, chromium oxide, iron oxide, molybdenum disulfide, chromium complexes, composite oxide-based black pigments, anthraquinone-based organic black pigments, etc. From the standpoint of cost and availability, carbon black is an example of a black colorant that is preferred. The amount of black colorant used is not particularly limited, and can be adjusted appropriately to impart the desired optical properties.

When the pressure-sensitive adhesive sheet disclosed herein is used for light reflection applications, the foam substrate is preferably colored white. The white color is preferably an L* (brightness) of 87 or more (e.g., 87 to 100) as defined by the L*a*b* color system, and more preferably 90 or more (e.g., 90 to 100). The a* and b* defined by the L*a*b* color system can each be appropriately selected according to the value of L*. For example, it is preferable that a* and b* are both in the range of -10 to 10 (more preferably -5 to 5, and even more preferably -2.5 to 2.5). For example, it is preferable that a* and b* are both 0 or approximately 0.

Examples of white colorants used to color the foam substrate white include titanium oxide (rutile titanium dioxide, anatase titanium dioxide, and other titanium dioxides), zinc oxide, aluminum oxide, silicon oxide, zirconium oxide, magnesium oxide, calcium oxide, tin oxide, barium oxide, cesium oxide, yttrium oxide, magnesium carbonate, calcium carbonate (light calcium carbonate, heavy calcium carbonate, and other), barium carbonate, zinc carbonate, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, aluminum silicate, and magnesium silicate. Examples of white colorants include inorganic white colorants such as calcium silicate, barium sulfate, calcium sulfate, barium stearate, zinc oxide, zinc sulfide, talc, silica, alumina, clay, kaolin, titanium phosphate, mica, gypsum, white carbon, diatomaceous earth, bentonite, lithopone, zeolite, sericite, and hydrated halloysite, and organic white colorants such as acrylic resin particles, polystyrene resin particles, polyurethane resin particles, amide resin particles, polycarbonate resin particles, silicone resin particles, urea-formaldehyde resin particles, and melamine resin particles. The amount of white colorant used is not particularly limited, and can be adjusted appropriately to impart the desired optical properties.

The surface of the foam substrate may be subjected to an appropriate surface treatment as necessary. This surface treatment may be, for example, a chemical or physical treatment to enhance adhesion to an adjacent material (e.g., an adhesive layer). Examples of such surface treatments include corona discharge treatment, chromate treatment, ozone exposure, flame exposure, ultraviolet irradiation treatment, plasma treatment, application of a primer, etc.

The thickness of the foam substrate is not particularly limited and can be appropriately set depending on the strength and flexibility of the adhesive sheet, the purpose of use, etc. From the viewpoint of making the joint thinner, the thickness of the foam substrate is usually about 0.70 mm or less, preferably about 0.40 mm or less, and more preferably about 0.30 mm or less. The technology disclosed herein can be preferably implemented in an embodiment in which the thickness of the foam substrate is about 0.20 mm or less (typically 0.18 mm or less, for example 0.16 mm or less) from the viewpoint of processability when processing the adhesive sheet into a narrow width. In addition, from the viewpoint of reducing the amount of oil and fat components penetrating into the adhesive interface, the thickness of the foam substrate is about 0.05 mm or more, preferably about 0.06 mm or more, and more preferably about 0.07 mm or more (for example, about 0.08 mm or more). The technology disclosed herein can be preferably implemented in an embodiment in which the foam substrate has a thickness of approximately 0.10 mm or more (typically greater than 0.10 mm, preferably 0.12 mm or more, e.g., 0.13 mm or more). As the foam substrate becomes thicker, the drop impact resistance also improves, and the desired drop impact resistance tends to be exhibited even in a narrower configuration.

<Release Liner>
In the technology disclosed herein, a release liner can be used during the formation of the adhesive layer, the preparation of the adhesive sheet, the storage, distribution, and shaping of the adhesive sheet before use. The release liner is not particularly limited, and for example, a release liner having a release treatment layer on the surface of a liner substrate such as a resin film or paper, or a release liner made of a low-adhesion material such as a fluorine-based polymer (polytetrafluoroethylene, etc.) or a polyolefin-based resin (polyethylene, polypropylene, etc.) can be used. The release treatment layer can be formed by surface-treating the liner substrate with a release treatment agent such as a silicone-based, long-chain alkyl-based, fluorine-based, or molybdenum sulfide.

<Total thickness of adhesive sheet>
The total thickness of the adhesive sheet disclosed herein (including the adhesive layer and possibly further including a substrate layer, but not including a release liner) is not particularly limited. The total thickness of the adhesive sheet can be, for example, about 800 μm or less, and from the viewpoint of making the portable device thinner, it is usually appropriate to have a total thickness of about 500 μm or less, and preferably about 350 μm or less (e.g., about 300 μm or less). The technology disclosed herein can also be implemented in the form of an adhesive sheet (typically a double-sided adhesive sheet) having a total thickness of about 150 μm or less (more preferably about 100 μm or less, even more preferably less than about 60 μm, for example about 50 μm or less). The lower limit of the thickness of the adhesive sheet is not particularly limited, but it is usually appropriate to have a thickness of about 60 μm or more, preferably about 100 μm or more, more preferably about 150 μm or more, even more preferably about 180 μm or more, and particularly preferably about 200 μm or more (e.g., about 220 μm or more).

<Characteristics of adhesive sheet>
The adhesive sheet (typically a double-sided adhesive sheet) disclosed herein is not particularly limited, but it is appropriate that it exhibits an initial pressing adhesive strength of 0.5 MPa or more. An adhesive sheet with such a high initial pressing adhesive strength has excellent adhesion reliability. For example, it can exhibit sufficient adhesive strength in fixing or joining members. Even in an embodiment in which the adhesive sheet with a narrow width is bonded to an adherend, peeling due to internal stress is unlikely to occur. The initial pressing adhesive strength is preferably about 0.7 MPa or more, more preferably about 0.9 MPa or more, even more preferably about 1.0 MPa or more, and particularly preferably about 1.2 MPa or more (for example, 1.3 MPa or more). The initial pressing adhesive strength is measured by the method described in the Examples below.

In addition, the adhesive sheet (typically a double-sided adhesive sheet) disclosed herein is not particularly limited, but it is appropriate that the adhesive force retention rate, which is the ratio of the pressing adhesive force S2 24 hours after the application of artificial sebum to the pressing adhesive force S1 before the application of artificial sebum, is 25% or more. An adhesive sheet that satisfies this characteristic has a predetermined or higher adhesive force even after the application of artificial sebum, and therefore exhibits good adhesive reliability even when in contact with oil and fat components. The adhesive force retention rate after the application of artificial sebum is preferably about 40% or more, more preferably about 50% or more, even more preferably about 60% or more, and particularly preferably about 70% or more (for example, about 75% or more, or even about 80% or more). The adhesive force retention rate [%] after the application of artificial sebum is calculated from the formula: S2/S1×100;. The pressing adhesive force S1 before the application of artificial sebum is the above-mentioned initial pressing adhesive force [MPa], and the pressing adhesive force S2 24 hours after the application of artificial sebum is measured by the method described in the Examples below.

The adhesive sheet disclosed herein is suitable for having a pressing adhesive strength S2 (adhesive strength after application of artificial sebum) of approximately 0.3 MPa or more (e.g., approximately 0.4 MPa or more) 24 hours after application of artificial sebum. An adhesive sheet that satisfies this characteristic has an adhesive strength of a predetermined level or more even after application of artificial sebum, and can therefore exhibit good adhesive strength even when used in an environment exposed to oil and fat components. The adhesive strength after application of artificial sebum is preferably approximately 0.5 MPa or more (e.g., approximately 0.6 MPa or more), more preferably approximately 0.7 MPa or more, even more preferably approximately 0.8 MPa or more, and particularly preferably approximately 1 MPa or more (e.g., approximately 1.1 MPa or more).

A preferred embodiment of the adhesive sheet has a 180-degree peel strength of approximately 10 N/20 mm or more. An adhesive sheet exhibiting such adhesive strength has high adhesion to the adherend, and therefore can have excellent performance in preventing the penetration of oil and fat components from the interface between the adhesive layer and the adherend. The 180-degree peel strength is more preferably approximately 15 N/20 mm or more, and even more preferably approximately 17 N/20 mm or more (e.g., approximately 18 N/25 mm or more). From the viewpoint that the higher the adhesion to the adherend, the better, the upper limit of the 180-degree peel strength is not particularly limited, but is usually approximately 65 N/20 mm or less (typically approximately 55 N/20 mm or less, for example approximately 40 N/20 mm or less). The above 180-degree peel strength is measured by the method described in the Examples below.

<Applications>
The adhesive sheet disclosed herein does not impair adhesive properties such as adhesive strength and drop impact resistance, or can improve them, and can exhibit good oil and grease resistance. Taking advantage of these characteristics, the adhesive sheet can be preferably used to fix members in various portable devices (portable devices). For example, it is suitable for fixing members (including various wiring) in portable electronic devices. Non-limiting examples of the portable electronic devices include mobile phones, smartphones, tablet computers, notebook computers, various wearable devices (for example, wristwear type worn on the wrist like a wristwatch, modular type worn on a part of the body with a clip or strap, eyewear type including glasses (monocular type and binocular type, including head-mounted type), clothing type attached to shirts, socks, hats, etc. in the form of accessories, earwear type attached to the ears like earphones, etc.), digital cameras, digital video cameras, audio equipment (portable music players, IC recorders, etc.), calculators (calculators, etc.), portable game devices, electronic dictionaries, electronic organizers, electronic books, in-vehicle information equipment, portable radios, portable televisions, portable printers, portable scanners, portable modems, etc. Non-limiting examples of portable devices other than portable electronic devices include mechanical wristwatches and pocket watches, flashlights, hand mirrors, commuter pass holders, etc. In this specification, "portable" does not mean that it is sufficient to simply be portable, but rather that it has a level of portability that allows an individual (average adult) to carry it relatively easily.

A particularly preferred embodiment of the adhesive sheet is used for joining and fixing components of a portable electronic device with a touch panel. The portable electronic device is equipped with a display unit/input unit (typically a touch panel) in which the display unit also functions as an input unit, and is operated by the user directly touching the surface of the display unit/input unit with the fingertips. Therefore, secretions such as sebum and fingerprints, chemical products such as cosmetics, hair products, moisturizing creams, and sunscreens, and oil and fat components contained in foods, etc. are likely to adhere to the portable electronic device. The oil and fat resistance of the adhesive sheet disclosed herein can be preferably exhibited for portable electronic devices that frequently come into contact with such oil and fat components.

The adhesive sheet (typically a double-sided adhesive sheet) disclosed herein can be used in the form of a bonding material processed into various shapes to fasten components constituting a portable device. Particularly preferred applications include applications for fastening components constituting a portable electronic device. In particular, it can be preferably used in portable electronic devices having a liquid crystal display device. For example, it is suitable for applications such as joining a display unit (which may be the display unit of a liquid crystal display device) or a display unit protection member to a housing in such portable electronic devices.

A preferred embodiment of such a bonding material is one having a narrow portion with a width of 4.0 mm or less (e.g., 2.0 mm or less, typically less than 2.0 mm). The adhesive sheet disclosed herein can be used as a bonding material having a shape (e.g., frame-shaped) that includes such a narrow portion, and can effectively fix members. In one embodiment, the width of the narrow portion may be 1.5 mm or less, 1.0 mm or less, or about 0.5 mm or less. There is no particular lower limit to the width of the narrow portion, but from the viewpoint of the handleability of the adhesive sheet, a width of 0.1 mm or more (typically 0.2 mm or more) is usually appropriate.

The narrow portion is typically linear. Here, the term "linear" refers to a concept that includes straight lines, curved lines, folded lines (e.g., L-shaped), annular shapes such as frame shapes and circles, and composite or intermediate shapes of these. The annular shape is not limited to those that are formed by curves, but includes annular shapes that are partially or entirely linear, such as a shape that follows the periphery of a rectangle (frame shape) or a shape that follows the periphery of a sector. The length of the narrow portion is not particularly limited. For example, the effect of applying the technology disclosed herein can be suitably exhibited in a form in which the length of the narrow portion is 10 mm or more (typically 20 mm or more, e.g., 30 mm or more).

The matters disclosed by this specification include the following:
(1) A portable electronic device,
The display unit is equipped with a touch panel that also functions as an input unit,
the touch panel can be operated by direct touch with a fingertip;
The members (plural) constituting the portable electronic device are joined via an adhesive sheet,
The pressure-sensitive adhesive sheet includes a foam substrate and a pressure-sensitive adhesive layer provided on at least one surface of the foam substrate,
The pressure-sensitive adhesive layer contains an acrylic polymer as a base polymer and a tackifier resin,
The portable electronic device comprises a tackifier resin having a hydroxyl value of 70 mgKOH/g or more.
(2) The portable electronic device according to (1) above, which is a mobile phone.
(3) The portable electronic device according to (1) above, which is a smartphone.
(4) The portable electronic device according to (1) above, which is a tablet computer.
(5) The portable electronic device according to (1) above, which is a wearable device.
(6) The portable electronic device according to (1) above, which is a digital camera.
(7) The portable electronic device according to (1) above, which is a portable music player.
(8) The portable electronic device according to (1) above, which is a portable game device.
(9) The portable electronic device according to (1) above, which is an electronic dictionary.
(10) The portable electronic device according to (1) above, which is an electronic book.

(11) A pressure-sensitive adhesive sheet comprising a foam substrate and a pressure-sensitive adhesive layer provided on at least one surface of the foam substrate,
The pressure-sensitive adhesive layer contains an acrylic polymer as a base polymer and a tackifier resin,
The pressure-sensitive adhesive sheet comprises a tackifier resin having a hydroxyl value of 70 mgKOH/g or more.
(12) The pressure-sensitive adhesive sheet according to (11) above, wherein the monomer components constituting the acrylic polymer contain more than 50% by weight of an alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms at the ester terminal.
(13) The pressure-sensitive adhesive sheet according to (11) or (12) above, wherein the monomer component constituting the acrylic polymer includes a carboxy group-containing monomer.
(14) The pressure-sensitive adhesive sheet according to (13) above, wherein the amount of the carboxy group-containing monomer in the monomer component is 1 to 10% by weight.
(15) The pressure-sensitive adhesive sheet according to any one of (11) to (14) above, wherein the pressure-sensitive adhesive composition for forming the pressure-sensitive adhesive layer contains an isocyanate-based crosslinking agent.
(16) The pressure-sensitive adhesive sheet according to any one of (11) to (15) above, wherein the tackifier resin comprises a phenol-based tackifier resin.
(17) The pressure-sensitive adhesive sheet according to any one of (11) to (16) above, wherein the content of the tackifier resin in the pressure-sensitive adhesive layer is 10 parts by weight or more and 60 parts by weight or less per 100 parts by weight of the base polymer.
(18) The pressure-sensitive adhesive sheet according to any one of (11) to (17), wherein the pressure-sensitive adhesive layer has a glass transition temperature, determined from a peak temperature of tan δ, in the range of −25° C. or more and 25° C. or less.
(19) The pressure-sensitive adhesive sheet according to any one of (11) to (18) above, wherein the foam substrate is a polyolefin-based foam substrate.

(20) The pressure-sensitive adhesive sheet according to any one of (11) to (19) above, having an initial press-through adhesive strength of 0.5 MPa or more.
(21) The pressure-sensitive adhesive sheet according to any one of (11) to (20) above, having an adhesive strength retention rate, expressed as the ratio of the press-on adhesive strength 24 hours after application of the artificial sebum to the press-on adhesive strength before application of the artificial sebum, of 25% or more.
(22) The pressure-sensitive adhesive sheet according to any one of (11) to (21) above, having an initial press-through adhesive strength of 1 MPa or more, and an adhesive strength retention rate, expressed as the ratio of the press-through adhesive strength 24 hours after application of the artificial sebum to the press-through adhesive strength before application of the artificial sebum, of 50% or more.
(23) The pressure-sensitive adhesive sheet according to any one of (11) to (22) above, wherein the pressure-sensitive adhesive layer contains a tackifier resin having a hydroxyl value of 120 mg KOH/g or more.
(24) The pressure-sensitive adhesive sheet according to any one of (11) to (23) above, which has a 180 degree peel strength of 15 N/20 mm or more.

(25) The pressure-sensitive adhesive sheet according to any one of (11) to (24) above, wherein the foam substrate has a density of 0.1 to 0.9 g/ ^cm3 .
(26) The pressure-sensitive adhesive sheet according to any one of (11) to (25) above, wherein the foam base has an average cell diameter of 10 to 200 μm.
(27) The pressure-sensitive adhesive sheet according to any one of (11) to (26) above, wherein the foam substrate has a thickness of 0.05 to 0.70 mm.

(28) The pressure-sensitive adhesive sheet according to any one of (11) to (27) above, which is used for bonding components of a portable electronic device.
(29) A mobile device comprising the adhesive sheet according to any one of (11) to (27) above and a component joined by the adhesive sheet.

Several examples of the present invention are 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.

Evaluation method
[Initial pressing adhesive strength]
(1) Preparation of Evaluation Sample The double-sided pressure-sensitive adhesive sheet is cut into a window frame shape (picture frame shape) of 20 mm wide, 20 mm long, and 2 mm thick as shown in Fig. 2 to obtain a window frame-shaped double-sided pressure-sensitive adhesive sheet 102. Using this window frame-shaped double-sided pressure-sensitive adhesive sheet 102, an acrylic plate (PMMA plate) 103 of 40 mm wide, 50 mm long, and 2 mm thick, and an aluminum plate (50 mm wide, 60 mm long, and 2 mm thick) 101 having a through hole 104 with a diameter of 12 mm in the center are bonded together by pressure bonding with a predetermined load (5 kg x 10 seconds), to obtain an evaluation sample 100.
(2) Measurement of Press-Adhesive Strength The press-adhesive strength of the evaluation sample obtained above is measured by the following method. That is, as shown in FIG. 3, the evaluation sample 100, which is a laminate in which an aluminum plate 101, a window frame-shaped double-sided adhesive sheet 102, and an acrylic plate 103 are bonded together, is fixed to a support 125, and set in a universal tensile compression tester (apparatus name "tensile compression tester, TG-1kN", manufactured by Minebea Co., Ltd.). Then, a round bar 120 (contact surface diameter 10 mm) is passed through the through hole 104 of the aluminum plate 101 of the evaluation sample 100, and the acrylic plate 103 is pressed in a direction away from the aluminum plate 101 by lowering the round bar 120 at a speed of 10 mm/min. Then, the maximum stress [N] observed until the aluminum plate 101 and the acrylic plate 103 are separated is measured, and the press-adhesive strength [N/mm ² ] per unit adhesive area [mm ² ] is obtained. This is defined as the initial press-adhesive strength [MPa]. The measurement is carried out in an environment of 23° C. and 50% RH.

[Pressing adhesive strength after applying artificial sebum]
(1) Preparation of Evaluation Samples Evaluation samples are prepared in the same manner as in the measurement of the initial press-through adhesive strength.
(2) Application of Artificial Sebum As shown in Fig. 4, the evaluation sample 100 is placed with the aluminum plate 101 side facing up, and 0.01 to 0.03 mL of artificial sebum 140 is dropped onto the adhesive region of the window frame-shaped double-sided adhesive sheet 102 from the through hole 104 of the aluminum plate 101 using an injector 130. The evaluation sample 100 to which the artificial sebum 140 has been applied in this manner is left in an environment of 55°C and 95% RH for 24 hours. As the artificial sebum, a composition consisting of 33.3% triolein, 20.0% oleic acid, 13.3% squalene, and 33.4% myristyl octadecylate is used.
(3) Measurement of Compression Adhesive Strength After 24 hours from the application of the artificial sebum, the compression adhesive strength [MPa] after the application of the artificial sebum is measured for the evaluation sample 100 to which the artificial sebum 140 has been applied, in the same manner as in the initial compression adhesive strength.

[180 degree peel strength]
Under a measurement environment of 23°C and 50% RH, a polyethylene terephthalate (PET) film having a thickness of 50 μm is attached to one adhesive surface of the double-sided pressure-sensitive adhesive sheet to provide a backing, and the sheet is cut to a size of 20 mm wide and 100 mm long to prepare a measurement sample. Under an environment of 23°C and 50% RH, the other adhesive surface of the measurement sample is pressed against the surface of a stainless steel plate (SUS304BA plate) by rolling a 2 kg roller back and forth once. After leaving the sample in the same environment for 30 minutes, a universal tension and compression tester is used to measure the peel strength [N/20 mm] in accordance with JIS Z 0237:2000 under the conditions of a tension speed of 300 mm/min and a peel angle of 180 degrees. As the universal tension and compression tester, a "tension and compression tester, TG-1kN" manufactured by Minebea Co., Ltd. or an equivalent product is used. In addition, in the case of a single-sided pressure-sensitive adhesive sheet, the backing of the PET film is not necessary.

[Drop impact resistance evaluation test]
The double-sided pressure-sensitive adhesive sheet is cut into a window frame shape (picture frame shape) of 59 mm wide, 113 mm long, and 1.0 mm thick, as shown in Figures 5(a) and 5(b), to obtain a window frame-shaped double-sided pressure-sensitive adhesive sheet. This window frame-shaped double-sided pressure-sensitive adhesive sheet is used to bond a polycarbonate plate (70 mm wide, 130 mm long, and 2 mm thick) and a glass plate (59 mm wide, 113 mm long, and 0.5 mm thick) together by pressure bonding with a load of 50 N for 10 seconds, to obtain an evaluation sample.
5(a) and (b) are schematic diagrams of the evaluation sample, where (a) is a top view and (b) is a cross-sectional view taken along the line B-B' of the evaluation sample. In Fig. 5, reference numeral 203 denotes a window frame-shaped double-sided pressure-sensitive adhesive sheet, reference numeral 231 denotes a polycarbonate plate, and reference numeral 232 denotes a glass plate.
A 160 g weight is attached to the back surface of the polycarbonate plate of the evaluation sample (the surface opposite to the surface bonded to the glass plate). The evaluation sample with the weight is subjected to a drop test in which the sample is dropped from a height of 1.2 m onto a concrete plate six times at room temperature (approximately 23° C.). The direction of the drop is adjusted so that the six surfaces of the evaluation sample are successively downward. In other words, one cycle of the drop pattern is performed once for each of the six surfaces.
After each drop, the polycarbonate plate and the glass plate were visually inspected to see whether they were still bonded to each other. If the polycarbonate plate and the glass plate peeled off within six drops, the result was rated as "X", and if no peeling was observed after six drops, the result was rated as "O".

[Dynamic viscoelasticity measurement]
A pressure-sensitive adhesive composition is applied to the release surface of a PET film having a thickness of 38 μm, one side of which is treated with a silicone-based release agent, and dried at 100° C. for 2 minutes to form a pressure-sensitive adhesive layer having a thickness of 50 μm on the release surface. The pressure-sensitive adhesive layer having a thickness of 50 μm is stacked to prepare a laminated pressure-sensitive adhesive sample having a thickness of about 2 mm. The laminated pressure-sensitive adhesive sample is punched into a disk shape having a diameter of 7.9 mm, and the sample is sandwiched and fixed between parallel plates. Dynamic viscoelasticity measurement is performed under the following conditions using a viscoelasticity tester (manufactured by TA Instruments, model name "ARES") to obtain the Tg (peak top temperature of tan δ) [° C.] of the pressure-sensitive adhesive layer, the peak strength at the peak of tan δ, the 25° C. storage modulus [MPa] and the 25° C. loss modulus [MPa].
[Measurement condition]
・Measurement mode: Shear mode ・Temperature range: -70℃ to 150℃
Heating rate: 5°C/min
Measurement frequency: 1Hz

<Preparation of acrylic polymer>
<Preparation Example 1>
In a reaction vessel equipped with a stirrer, a thermometer, a nitrogen gas inlet tube, a reflux condenser and a dropping funnel, 95 parts of BA and 5 parts of AA as monomer components and 233 parts of ethyl acetate as a polymerization solvent were charged, and the mixture was stirred for 2 hours while introducing nitrogen gas. After removing oxygen from the polymerization system in this way, 0.2 parts of AIBN were added as a polymerization initiator, and the mixture was solution polymerized at 60°C for 8 hours to obtain a solution of acrylic polymer A1. The Mw of this acrylic polymer A1 was about 70 x ¹⁰⁴ .

<Preparation Example 2>
A solution of acrylic polymer A2 was obtained in the same manner as in Preparation Example 1, except that the monomer composition was changed to 100 parts of BA and 5 parts of AA, BPO was used as the polymerization initiator, and toluene was used as the polymerization solvent. The Mw of this acrylic polymer A2 was in the range of about 50×10 ⁴ to 60×10 ⁴ .

<Preparation Example 3>
A solution of acrylic polymer A3 was obtained in the same manner as in Preparation Example 1, except that the monomer composition was changed to 100 parts of BA, 5 parts of VAc, 3 parts of AA, and 0.1 parts of HEA, and toluene was used as the polymerization solvent. The Mw of this acrylic polymer A3 was about 50 × ¹⁰⁴ .

<Preparation Example 4>
A solution of acrylic polymer A4 was obtained in the same manner as in Preparation Example 1, except that the monomer composition was changed to 90 parts of 2EHA and 10 parts of AA, and BPO was used as the polymerization initiator. The Mw of this acrylic polymer A4 was about 120 × ¹⁰⁴ .

<Experiment 1>
<Example 1-1>
A pressure-sensitive adhesive composition according to this example was prepared by mixing and stirring 100 parts of acrylic polymer A1, 15 parts of tackifier resin B1 (product name "Haritac SE10", manufactured by Harima Chemicals, hydrogenated rosin glycerin ester, softening point 75 to 85°C, hydroxyl value 25 to 40 mgKOH/g), and 2 parts of an isocyanate-based crosslinking agent (product name "Coronate L", 75% ethyl acetate solution of trimethylolpropane/tolylene diisocyanate trimer adduct, manufactured by Nippon Polyurethane Industry Co., Ltd.).
The obtained adhesive composition was applied to the release surface of a 38 μm thick polyester release film (trade name "Diafoil MRF", manufactured by Mitsubishi Polyester Co., Ltd.) and dried at 100 ° C. for 2 minutes to form an adhesive layer having a thickness of 40 μm. The release surface of a 25 μm thick polyester release film (trade name "Diafoil MRF", thickness 25 μm, manufactured by Mitsubishi Polyester Co., Ltd.) was attached to this adhesive layer. In this way, a substrateless double-sided adhesive sheet having a thickness of 40 μm, both sides of which were protected by the above two polyester release films, was obtained.

<Example 1-2>
A pressure-sensitive adhesive composition of this example was prepared in the same manner as in Example 1-1, except that tackifier resin B1 was changed to tackifier resin B2 (product name "SUMILITE RESIN PR-12603N", manufactured by Sumitomo Bakelite Co., Ltd., terpene-modified phenolic resin, softening point 130 to 140°C, hydroxyl value 1 to 20 mgKOH/g), and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-3>
A pressure-sensitive adhesive composition of this example was prepared in the same manner as in Example 1-1, except that tackifier resin B1 was changed to tackifier resin B3 (product name "Mighty Ace G125", manufactured by Yasuhara Chemical Co., Ltd., terpene phenol resin, softening point 125°C, hydroxyl value 140 mgKOH/g), and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-4>
A pressure-sensitive adhesive composition of this example was prepared in the same manner as in Example 1-1, except that tackifier resin B1 was changed to tackifier resin B4 (product name "YS Polystar S145", manufactured by Yasuhara Chemical Co., Ltd., terpene phenol resin, softening point 145°C, hydroxyl value 70 to 110 mgKOH/g), and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-5>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-3, except that the blending amount of tackifier resin B3 was changed to 30 parts, and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-6>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-4, except that the blending amount of tackifier resin B4 was changed to 30 parts, and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-7>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-1, except that the acrylic polymer A1 was changed to the acrylic polymer A2, and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-8>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-7, except that tackifier resin B1 was changed to tackifier resin B2, and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-9>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-7, except that tackifier resin B1 was changed to tackifier resin B3, and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-10>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-7, except that tackifier resin B1 was changed to tackifier resin B4, and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-11>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-7, except that tackifier resin B1 was changed to tackifier resin B5 (product name "Tamanol 803L", manufactured by Arakawa Chemical Industries, Ltd., terpene phenol resin, softening point 145 to 160°C, hydroxyl value 1 to 20 mgKOH/g), and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-12>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-8, except that the blending amount of tackifier resin B2 was changed to 30 parts, and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-13>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-9, except that the blending amount of tackifier resin B3 was changed to 30 parts, and a substrate-less double-sided pressure-sensitive adhesive sheet was obtained.

<Example 1-14>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-1 by mixing and stirring 100 parts of the acrylic polymer A2 and 30 parts of the tackifier resin B3.
Two commercially available release liners (product name "SLB-80W3D", manufactured by Sumika Kakoshi Co., Ltd.) were prepared, and the adhesive composition was applied to one side (release surface) of each of the release liners so that the thickness after drying was 40 μm, and dried at 100° C. for 2 minutes to form an adhesive layer on each of the release surfaces of the two release liners. These adhesive layers were attached to both sides of a polyethylene foam sheet (thickness 0.15 mm, density 0.56 g/cm ³ , 10% compressive strength (C ₁₀ ) 167 kPa, 25% compressive strength (C ₂₅ ) 468 kPa, 30% compressive strength (C ₃₀ ) 627 kPa, average bubble diameter 55 μm) that had been corona discharge-treated on both sides. The release liner was left on the adhesive layer as it was and used to protect the surface (adhesive surface) of the adhesive layer. The obtained structure was passed once through a laminator (0.3 MPa, speed 0.5 m/min) at 80° C., and then cured for one day in an oven at 50° C. In this way, a pressure-sensitive adhesive sheet according to this example (double-sided pressure-sensitive adhesive sheet with a foam substrate) was obtained.

<Example 1-15>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-14, except that acrylic polymer A2 was changed to acrylic polymer A1, and a double-sided pressure-sensitive adhesive sheet with a foam substrate was obtained.

<Example 1-16>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 1-14, except that acrylic polymer A2 was changed to acrylic polymer A3, and a double-sided pressure-sensitive adhesive sheet with a foam substrate was obtained.

For each adhesive sheet, the initial compression adhesive strength (S1) [MPa] and the compression adhesive strength (S2) [MPa] 24 hours after application of the artificial sebum were measured, and the adhesive strength maintenance rate after application of the artificial sebum (S2/S1) [%] was calculated. The results are shown in Tables 1 and 2, along with an outline of the adhesive composition and sheet configuration.

As shown in Table 1, in Experiment 1, evaluations were performed by changing the type of acrylic polymer and the type of tackifier resin. Among tackifier resins B1 to B5, Examples 1-3, 1-5, 1-9, and 1-13, which used tackifier resin B3, showed relatively high oil and grease resistant adhesive reliability in compositions containing acrylic polymers A1 and A2 as base polymers. Also, as shown in Table 2, when tackifier resin B3 was used to make a double-sided PSA sheet with a foam substrate, the initial adhesive strength and the adhesive strength after the artificial sebum application were both improved compared to a substrate-less PSA sheet, and the adhesive strength retention rate after the artificial sebum application was also equal to or higher. In particular, Example 1-15, which used acrylic polymer A1 and tackifier resin B3, demonstrated a strong adhesive strength of 0.7 MPa or more both initially and after the artificial sebum application.

<Experiment 2>
<Example 2-1>
A pressure-sensitive adhesive composition according to this example was prepared using 100 parts of acrylic polymer A1 and 15 parts of tackifier resin B3 in the same manner as in Example 1-3 of Experiment 1. A pressure-sensitive adhesive sheet (foam-substrate-attached double-sided pressure-sensitive adhesive sheet) was obtained in which pressure-sensitive adhesive layers (thickness 40 μm) were provided on both sides of a polyethylene foam sheet (thickness 0.15 mm) in the same manner as in Example 1-14 of Experiment 1, except that the obtained pressure-sensitive adhesive composition was used.

<Example 2-2>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 2-1, except that the blending amount of tackifier resin B3 was changed to 30 parts, and a double-sided pressure-sensitive adhesive sheet with a foam substrate was obtained.

<Example 2-3>
A pressure-sensitive adhesive composition of this example was prepared in the same manner as in Example 2-1, except that tackifier resin B3 was changed to tackifier resin B6 (product name "YS Polystar N125", manufactured by Yasuhara Chemical Co., Ltd., terpene phenol resin, softening point 125°C, hydroxyl value 170 mgKOH/g), and a double-sided pressure-sensitive adhesive sheet with a foam substrate was obtained.

<Example 2-4>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 2-3, except that the blending amount of tackifier resin B6 was changed to 30 parts, and a double-sided pressure-sensitive adhesive sheet with a foam substrate was obtained.

<Example 2-5>
A pressure-sensitive adhesive composition of this example was prepared in the same manner as in Example 2-1, except that tackifier resin B3 was changed to tackifier resin B7 (product name "Mighty Ace K125", manufactured by Yasuhara Chemical Co., Ltd., terpene phenol resin, softening point 125°C, hydroxyl value 200 mg KOH/g), and a double-sided pressure-sensitive adhesive sheet with a foam substrate was obtained.

<Example 2-6>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 2-5, except that the blending amount of tackifier resin B7 was changed to 30 parts, and a double-sided pressure-sensitive adhesive sheet with a foam substrate was obtained.

For the adhesive sheets of each example, the initial pressing adhesive strength (S1) [MPa] and the pressing adhesive strength (S2) [MPa] 24 hours after the application of artificial sebum were measured, and the adhesive strength retention rate after the application of artificial sebum (S2/S1) [%] was calculated. In addition, for the adhesive layers of Examples 2-1, 2-2, and 2-6, the Tg [°C], peak strength of tan δ (G"/G'), 25°C storage modulus (G'(25°C)) [MPa], and 25°C loss modulus (G"(25°C)) [MPa] were measured. The results are shown in Table 3, along with an outline of the adhesive composition and sheet configuration.

<Experiment 3>
<Example 3-1>
A pressure-sensitive adhesive composition according to this example was prepared by mixing and stirring 100 parts of acrylic polymer A4, 20 parts of tackifier resin B5, and 0.05 parts of an epoxy crosslinking agent (product name "TETRAD-C", 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, manufactured by Mitsubishi Gas Chemical Co., Ltd.).
Except for using the obtained pressure-sensitive adhesive composition, a pressure-sensitive adhesive sheet (double-sided pressure-sensitive adhesive sheet with foam substrate) having pressure-sensitive adhesive layers (thickness 40 μm) provided on both sides of a polyethylene foam sheet (thickness 0.15 mm) was obtained in the same manner as in Example 1-14 of Experiment 1.

<Example 3-2>
A pressure-sensitive adhesive composition according to this example was prepared by stirring and mixing 100 parts of acrylic polymer A1, 30 parts of tackifier resin B7, and 2 parts of an isocyanate-based crosslinking agent (product name "Coronate L", a 75% ethyl acetate solution of trimethylolpropane/tolylene diisocyanate trimer adduct, manufactured by Nippon Polyurethane Industry Co., Ltd.) A foam substrate-attached double-sided pressure-sensitive adhesive sheet was obtained in the same manner as in Example 3-1, except that the obtained pressure-sensitive adhesive composition was used.

<Example 3-3>
A double-sided PSA sheet with a foam substrate was obtained in the same manner as in Example 3-2, except that 100 parts of acrylic polymer A1 and 30 parts of tackifier resin B4 were used.

For each adhesive sheet, the initial pressure-sensitive adhesive strength (S1) [MPa] and the pressure-sensitive adhesive strength (S2) [MPa] 24 hours after application of the artificial sebum were measured, and the adhesive strength maintenance rate after application of the artificial sebum (S2/S1) [%] was calculated. The results are shown in Table 4, along with an outline of the adhesive composition and sheet configuration.

In Experiment 2, an evaluation was performed using a tackifier resin with a high hydroxyl value. As shown in Table 3, the higher the hydroxyl value, the higher the adhesive strength retention rate after the application of artificial sebum. In addition, in the embodiment using a high hydroxyl value resin, the higher the content, the higher the adhesive strength retention rate after the application of artificial sebum, as well as the initial pressing adhesive strength. Furthermore, in Experiment 3, an evaluation was performed on a foam substrate-attached adhesive sheet using tackifier resins B4, B5, or B7. As shown in Table 4, Example 3-2, which used tackifier resin B7 (hydroxyl value 200 mgKOH/g), gave the highest adhesive strength retention rate after the application of artificial sebum. This was followed by Example 3-3, which used tackifier resin B4 (hydroxyl value 70 to 110 mgKOH/g). These results show that the hydroxyl value and content of the tackifier resin contribute to improving oil and grease resistance.
Note that Experiments 1, 2, and 3, as well as Experiment 4 described below, were carried out at different times, and it was difficult to completely match the conditions of sample preparation and evaluation (such as the operators and experimental environment), so comparisons within each experiment should not be discussed in the same breath as comparisons between experiments.

<Experiment 4>
<Example 4-1>
A pressure-sensitive adhesive composition according to this example was prepared by stirring and mixing 100 parts of acrylic polymer A1, 30 parts of tackifier resin B7, and 2 parts of an isocyanate-based crosslinking agent (product name "Coronate L", a 75% ethyl acetate solution of trimethylolpropane/tolylene diisocyanate trimer adduct, manufactured by Nippon Polyurethane Industry Co., Ltd.). A pressure-sensitive adhesive sheet (foam-substrate-attached double-sided pressure-sensitive adhesive sheet) was obtained in which pressure-sensitive adhesive layers (40 μm thick) were provided on both sides of a polyethylene foam sheet (0.15 mm thick) in the same manner as in Example 1-14 of Experiment 1, except that the obtained pressure-sensitive adhesive composition was used.

<Example 4-2>
A pressure-sensitive adhesive composition according to this example was prepared in the same manner as in Example 4-1, except that the blending amount of tackifier resin B7 was changed to 40 parts, and a double-sided pressure-sensitive adhesive sheet with a foam substrate was obtained.

<Example 4-3>
A pressure-sensitive adhesive composition of this example was prepared in the same manner as in Example 4-1, except that a 12 μm thick PET film (product name "Lumirror", manufactured by Toray Industries, Inc.) was used as the substrate instead of the polyethylene foam sheet, and a double-sided pressure-sensitive adhesive sheet with a foam substrate was obtained.

For each adhesive sheet, the 180° peel strength [N/20 mm], initial pressure adhesion (S1) [MPa], and pressure adhesion 24 hours after application of artificial sebum (S2) [MPa] were measured, and the adhesion retention rate after application of artificial sebum (S2/S1) [%] was calculated. Drop impact resistance was also evaluated. The results are shown in Table 5, along with an outline of the adhesive composition and sheet configuration.

An adhesive composition in which tackifier resin B7 with a high hydroxyl value was mixed with acrylic polymer A1 was used with a foam substrate, and the results showed that, as shown in Table 5, in Examples 4-1 and 4-2, the initial pressure adhesion and adhesion retention rate after application of artificial sebum were improved. These examples also showed good results in the drop impact resistance evaluation test. On the other hand, in Example 4-3, which used a PET film as the substrate, the initial pressure adhesion and adhesion after application of artificial sebum tended to be lower than in the example using a foam substrate, resulting in poor drop impact resistance.

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

REFERENCE SIGNS LIST 1 Adhesive sheet 10 Foam substrate 10A First surface 10B Second surface 21 First adhesive layer 22 Second adhesive layer 21A First adhesive surface 22A Second adhesive surface 31, 32 Release liner

Claims (9)

A pressure-sensitive adhesive sheet comprising a foam substrate and a pressure-sensitive adhesive layer provided on at least one surface of the foam substrate,
The pressure-sensitive adhesive layer contains an acrylic polymer as a base polymer and a tackifier resin (excluding those in which the pressure-sensitive adhesive layer contains a liquid xylene-based resin) ,
The tackifier resin includes a tackifier resin having a hydroxyl value of 70 mgKOH/g or more,
The tackifier resin having a hydroxyl value of 70 mgKOH/g or more includes a tackifier resin having a hydroxyl value of 120 mgKOH/g or more,
the proportion of the tackifier resin having a hydroxyl value of 70 mgKOH/g or more in the total tackifier resin contained in the pressure-sensitive adhesive layer is 80% by weight or more ;
The pressure-sensitive adhesive layer has a glass transition temperature, determined from a peak temperature of tan δ, in the range of −25° C. to 25° C. The adhesive sheet according to claim 1, wherein the monomer components constituting the acrylic polymer contain more than 50% by weight of alkyl (meth)acrylate having an alkyl group with 1 to 6 carbon atoms at the ester end. The adhesive sheet according to claim 1 or 2, wherein the monomer component constituting the acrylic polymer includes a carboxyl group-containing monomer. The adhesive sheet according to claim 3, wherein the amount of the carboxyl group-containing monomer in the monomer component is 1 to 10% by weight. The adhesive sheet according to any one of claims 1 to 4, wherein the adhesive composition for forming the adhesive layer contains an isocyanate-based crosslinking agent. The adhesive sheet according to any one of claims 1 to 5, wherein the tackifier resin includes a phenol-based tackifier resin. The adhesive sheet according to any one of claims 1 to 6, wherein the content of the tackifier resin in the adhesive layer is 10 parts by weight or more and 60 parts by weight or less per 100 parts by weight of the base polymer. The pressure-sensitive adhesive sheet according to any one of claims 1 to 7 , wherein the foam substrate is a polyolefin foam substrate. The pressure-sensitive adhesive sheet according to any one of claims 1 to 8 , which is used for bonding components of a portable electronic device.

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