WO2024162267A1

WO2024162267A1 - Adhesive sheet

Info

Publication number: WO2024162267A1
Application number: PCT/JP2024/002669
Authority: WO
Inventors: 健太熊倉; 哲士本田; 雅永井田
Original assignee: 日東電工株式会社
Priority date: 2023-01-30
Filing date: 2024-01-29
Publication date: 2024-08-08

Abstract

Provided is an adhesive sheet comprising an adhesive layer that includes a polymer, a monomer, and a thermal polymerization initiator. The polymer includes an ethylenically unsaturated group, and the monomer also includes an ethylenically unsaturated group. The thermal polymerization initiator includes a peroxide-based polymerization initiator.

Description

Adhesive sheet

The present invention relates to a pressure-sensitive adhesive sheet.
This application claims priority based on Japanese Patent Application No. 2023-011970 filed on January 30, 2023, and Japanese Patent Application No. 2023-122021 filed on July 26, 2023, the entire contents of which are incorporated herein by reference.

Generally, adhesives (also called pressure-sensitive adhesives; the same applies below) are in a soft solid (viscoelastic) state at temperatures around room temperature, and have the property of easily adhering to an adherend when pressure is applied. Adhesives are widely used in various fields in the form of a supported adhesive sheet having an adhesive layer on a support, or in the form of a support-less adhesive sheet without a support, due to the ease of application to an adherend. Some such adhesives are used by adhering to an adherend, and are removed from the adherend after their adhesive purpose has been fulfilled. Prior art documents disclosing this type of conventional technology include Patent Documents 1 to 4. Patent Documents 1 to 4 disclose thermosetting adhesives.

Japanese Patent Application Publication No. 2015-29105 Japanese Patent Application Publication No. 2016-204617 Japanese Patent Application Publication No. 2019-56101 Japanese Patent Application Publication No. 1998-209087

　Adhesives used in applications where the adhesive is peeled off from an adherend are required to have good adhesion while attached to the adherend, and to have the ability to be easily peeled off from the adherend after the adhesive has completed its purpose. For example, it is desirable for an adhesive applied to an adherend that is to be heat-treated to have easy peelability, so that it can be easily peeled off from the adherend after being heated while attached to the adherend. However, when an adherend such as glass, metal, or resin is heated at high temperatures while the adhesive is attached to it, the adhesive adheres to the surface of the adherend, increasing the peeling force (heavy peeling), which may cause problems such as reduced peelability and the generation of adhesive residue. In this regard, reference is made to Non-Patent Documents 1 and 2, which describe that various polymers have an adhesive force when heated. For example, in the case of thermosetting adhesives such as those described in Patent Documents 1 to 4, it is difficult to cause the adhesive to harden during high-temperature heating before the adhesive adheres to the adherend, and the adhesive is not able to reduce the peeling force or suppress the increase in peeling force due to the adhesive hardening, and it is not possible to have stable easy peelability after high-temperature heating.

In light of this background, the present inventors have focused on thermosetting adhesives containing a thermal polymerization initiator in the adhesive, and as a result of their research and development, they have succeeded in obtaining an adhesive that has easy peelability (heat-resistant peelability) even after being attached to an adherend and heated at high temperatures. Such heat-resistant peelability can also be called heat-resistant peelability, since it remains easy to peel even after heat treatment, which usually increases the peel strength. It is expected that adhesive sheets with such heat-resistant peelability and heat-resistant peelability will be applied to various applications and various adherends due to their usefulness. However, as a result of the inventors' investigations, it has been found that, depending on the adherend material, the peel strength does not decrease when heated, and heat-resistant peelability cannot be obtained in some cases. Specifically, it has been confirmed that even if an adhesive exhibits good heat-resistant peelability when applied to a glass adherend, for example, peeling itself becomes difficult after heating when applied to an adherend whose surface is made of an organic material such as epoxy resin. As a result of intensive research, we have succeeded in creating an adhesive that can exhibit heat-peelability even on organic material surfaces and can exhibit heat-peelability regardless of the adherend material, leading to the completion of the present invention. In other words, the present invention provides an adhesive sheet that can exhibit heat-peelability on various adherends, including adherends containing organic materials.

According to this specification, there is provided an adhesive sheet having an adhesive layer containing a polymer, a monomer, and a thermal polymerization initiator. The polymer contains an ethylenically unsaturated group, and the monomer also contains an ethylenically unsaturated group. The thermal polymerization initiator contains a peroxide-based polymerization initiator. According to the above configuration, both the polymer and the monomer contained in the adhesive layer have an ethylenically unsaturated group, and by using a peroxide-based polymerization initiator as the thermal polymerization initiator, the curing reaction of the adhesive layer proceeds quickly when heated, so that it is possible to exhibit easy peelability by heating even on the surface of an organic material, for example, and it is possible to reliably exhibit easy peelability by heating. According to the above adhesive sheet, it is possible to exhibit easy peelability by heating on various adherends, such as adherends containing organic materials. Such an adhesive sheet has a wide range of applications and is useful.

In some preferred embodiments, the adhesive layer contains a polyfunctional acrylate monomer as the monomer. The effects of the technology disclosed herein are preferably achieved in embodiments using a polyfunctional acrylate monomer as the monomer.

In some preferred embodiments, the content of the monomer in the adhesive layer is less than 50 parts by weight per 100 parts by weight of the polymer. According to the technology disclosed herein, the desired heat peelability can be preferably achieved by setting the amount of monomer in the adhesive layer within the above range.

In some embodiments, the adhesive layer has a gel fraction of 50% or more before heating. The technology disclosed herein is preferably implemented in a configuration having an adhesive layer with a gel fraction of a predetermined value or more.

In some preferred embodiments, the Young's modulus Y1 [MPa] of the pressure-sensitive adhesive layer after heat treatment at 180°C for 30 minutes is 200 times or more the Young's modulus Y0 [MPa] before heating, i.e., the Young's modulus change ratio after heating (Y1/Y0) is 200 or more. The higher the Young's modulus change ratio after heating, the higher the degree of hardening of the pressure-sensitive adhesive layer when heated, and the easier it tends to be to obtain excellent peelability upon heating.

The adhesive sheet disclosed herein can exhibit heat-sensitive peelability with respect to various adherends, including adherends containing organic materials. Therefore, it can be preferably used, for example, in a form in which it is attached to the surface of a substrate whose surface is made of an organic material, and then peeled from the surface. The adhesive sheet disclosed herein has excellent heat-sensitive peelability (heat-resistant peelability), and can therefore be easily peeled from the surface of the organic material, even after being exposed to heat of, for example, over 150°C.

FIG. 1 is a cross-sectional view showing a schematic example of an adhesive sheet.

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

In this specification, the "base polymer" of an adhesive refers to the main component of the rubber-like polymer contained in the adhesive. The rubber-like polymer refers to a polymer that exhibits rubber elasticity in a temperature range around room temperature. In addition, in this specification, the "main component" refers to a component that is contained in an amount of more than 50% by weight, unless otherwise specified.

In this specification, "acrylic polymer" refers to a polymer that contains, as a monomer unit constituting the polymer, a monomer unit derived from a monomer having at least one (meth)acryloyl group in one molecule. In this specification, an acrylic polymer is defined as a polymer that contains a monomer unit derived from an acrylic monomer.

In addition, in this specification, "acrylic monomer" refers to a monomer having at least one (meth)acryloyl group in one molecule. Here, "(meth)acryloyl group" refers collectively to acryloyl groups and methacryloyl groups. Therefore, the concept of acrylic monomer here can include both monomers having an acryloyl group (acrylic monomers) and monomers having a methacryloyl group (methacrylic monomers). Similarly, in this specification, "(meth)acrylic acid" refers collectively to acrylic acid and methacrylic acid, and "(meth)acrylate" refers collectively to acrylate and methacrylate. The same applies to other similar terms.

Furthermore, in this specification, "weight" may be read as "mass." For example, "% by weight" may be read as "% by mass," and "parts by weight" may be read as "parts by mass."

<Adhesive sheet>
The adhesive sheet disclosed herein is configured to include an adhesive layer. The adhesive sheet may be a substrate-attached adhesive sheet having the above-mentioned adhesive layer on one or both sides of a non-releasable substrate (support substrate), or may be a substrate-less adhesive sheet (i.e., an adhesive sheet without a non-releasable substrate. Typically, the adhesive sheet is made of an adhesive layer) having the above-mentioned adhesive layer held by a release liner. The concept of adhesive sheet here may include those called adhesive tape, adhesive label, adhesive film, etc. The adhesive sheet disclosed herein may be in the form of a roll or a sheet. Alternatively, the adhesive sheet may be in the form of an adhesive sheet further processed into various shapes.

The cross-sectional structure of the adhesive sheet is shown in FIG. 1. As shown in FIG. 1, the adhesive sheet 1 has an adhesive surface 1A, and is in the form of a one-sided adhesive sheet in which an adhesive layer 20 is provided on one surface 10A of a sheet-like base layer (support base) 10. The adhesive sheet 1 is used by attaching the surface 20A of the adhesive layer 20, which is the adhesive surface 1A, to an adherend. The back surface 10B of the base layer 10 (the surface opposite to the one surface 10A) is also the back surface 1B of the adhesive sheet 1, and constitutes the outer surface of the adhesive sheet 1. The adhesive sheet 1 before use (i.e., before being attached to an adherend) may be in the form of an adhesive sheet 50 with a release liner, in which the adhesive surface 1A is protected by a release liner 30, at least the adhesive layer 20 side of which is the release surface. Alternatively, the adhesive sheet may have a configuration in which the other surface (back surface) 10B of the base layer 10 is the release surface, and the adhesive layer 20 is in contact with the back surface by rolling up the adhesive sheet 1 into a roll, thereby protecting the surface (adhesive surface 1A).

<Adhesive Layer>
(polymer)
In the technology disclosed herein, the type of adhesive is not particularly limited. The adhesive layer may contain one or more of various rubber-like polymers such as acrylic polymers, rubber polymers (e.g., natural rubber, synthetic rubber, mixtures thereof, etc.), polyester polymers, urethane polymers, polyether polymers, silicone polymers, polyamide polymers, and fluorine polymers that can be used in the field of adhesives. The above polymers may be used as base polymers in adhesives and function as structural polymers that form the adhesive. From the viewpoint of adhesive performance, cost, etc., an adhesive containing an acrylic polymer or a rubber polymer as a base polymer may be preferably adopted. Among them, an adhesive (acrylic adhesive) having an acrylic polymer with excellent heat resistance as a base polymer is preferable.

　The following mainly describes acrylic adhesives and adhesive layers made of such adhesives, i.e., adhesive sheets having acrylic adhesive layers, but it is not intended to limit the adhesive layers disclosed herein to acrylic adhesive layers.

(Acrylic polymer)
In some embodiments, the acrylic polymer is an acrylic polymer in which more than 50% by weight of the monomer components constituting the polymer is an acrylic monomer. The proportion of the acrylic monomer in the monomer components is suitably 60% by weight or more, preferably 70% by weight or more, more preferably 80% by weight or more, and even more preferably 85% by weight or more, and may be, for example, 90% by weight or more. The upper limit of the proportion of the acrylic monomer in the monomer components constituting the acrylic polymer is 100% by weight, and the proportion of the acrylic monomer may be, for example, 98% by weight or less, 95% by weight or less, or 92% by weight or less, from the viewpoint of obtaining the effect of using a non-acrylic monomer. The acrylic monomer may be used alone or in combination of two or more kinds.

In some preferred embodiments, the monomer component includes an alkoxy group-containing (meth)acrylate. Acrylic polymers that include an alkoxy group-containing (meth)acrylate as a monomer component tend to provide good adhesion and are compatible with, for example, the monomer contained in the adhesive layer described below (hereinafter, sometimes referred to as "combined monomer" to distinguish it from the monomer component used in the synthesis of the polymer). The alkoxy group-containing (meth)acrylate can be used alone or in combination of two or more.

Examples of alkoxy group-containing (meth)acrylates include alkoxyalkyl (meth)acrylates such as methoxyethyl (meth)acrylate, 3-methoxypropyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, propoxyethyl (meth)acrylate, butoxyethyl (meth)acrylate, and ethoxypropyl (meth)acrylate; alkoxy(poly)alkylene glycol (meth)acrylates such as methoxydiethylene glycol (meth)acrylate, methoxydipropylene glycol (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, ethoxydipropylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, methoxypolypropylene glycol (meth)acrylate, ethoxypolyethylene glycol (meth)acrylate, and ethoxypolypropylene glycol (meth)acrylate; and the like. Of these, alkoxyalkyl (meth)acrylates are preferred, and among these, alkoxyalkyl (meth)acrylates having an alkoxy group with 1 to 4 carbon atoms (e.g., 1, 2 or 3 carbon atoms) are more preferred, with methoxyethyl (meth)acrylate being particularly preferred.

The content of the alkoxy group-containing (meth)acrylate in the monomer component constituting the acrylic polymer is not particularly limited. From the viewpoint of effectively obtaining the effect of using the alkoxy group-containing (meth)acrylate, the content of the alkoxy group-containing (meth)acrylate in the monomer component is usually about 1% by weight or more, for example, 10% by weight or more, or 30% by weight or more. In some embodiments, the content of the alkoxy group-containing (meth)acrylate in the monomer component is, for example, more than 30% by weight, preferably 40% by weight or more, more preferably 50% by weight or more (for example, more than 50% by weight), and even more preferably 55% by weight or more, from the viewpoint of adhesive properties such as adhesive strength and compatibility with the blended monomer. The upper limit of the content of the alkoxy group-containing (meth)acrylate in the above monomer component is, in some embodiments, approximately 99% by weight or less, may be 90% by weight or less, preferably 80% by weight or less, more preferably 70% by weight or less, even more preferably 65% by weight or less, and may be 60% by weight or less, from the viewpoint of introducing an ethylenically unsaturated group into the polymer and obtaining the effects of other copolymerizable monomers such as functional group-containing monomers.

In some other embodiments, the monomer component constituting the acrylic polymer may contain a chain alkyl (meth)acrylate having a linear or branched alkyl group having 1 to 20 carbon atoms at the ester end. Hereinafter, a chain alkyl (meth)acrylate having an alkyl group having X to Y carbon atoms at the ester end may be referred to as a "C _X-Y alkyl (meth)acrylate". In this specification, "chain" is used to mean both linear and branched. The chain alkyl (meth)acrylates may be used alone or in combination of two or more.

Non-limiting specific examples of C _1-20 alkyl (meth)acrylates include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, and 2-ethylhexyl. (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, and the like.

In an embodiment in which a C _1-20 alkyl (meth)acrylate is used as a monomer component constituting an acrylic polymer, it is preferable to use at least a C _4-20 alkyl (meth)acrylate as the C _1-20 alkyl (meth)acrylate, and it is more preferable to use at least a C _4-18 alkyl (meth)acrylate. In some embodiments, it is preferable to use a C _4-8 alkyl (meth)acrylate as the C _1-20 alkyl (meth)acrylate. Of these, it is more preferable to use a C _4-8 alkyl acrylate. The C _4-8 alkyl (meth)acrylate may be used alone or in combination of two or more. The use of a C _4-8 alkyl (meth)acrylate tends to make it easier to obtain good adhesive properties (adhesive strength, etc.). For example, an acrylic polymer containing one or both of n-butyl acrylate (BA) and 2-ethylhexyl acrylate (2EHA) as the monomer component is preferable, and an acrylic polymer containing at least 2EHA is particularly preferable. In some other embodiments in which a C _1-20 alkyl (meth)acrylate is used, a C _7-12 alkyl (meth)acrylate may be preferably used. The C _{7-12 alkyl (meth)acrylate may be used alone or in combination of two or more. As the C 7-12} _alkyl (meth)acrylate, a C _7-10 alkyl acrylate is preferred, a C _7-9 alkyl acrylate is more preferred, and a C ₈ alkyl acrylate is even more preferred.

In an embodiment in which a C _1-20 alkyl (meth)acrylate is used as a monomer component constituting an acrylic polymer, the content of the C _1-20 alkyl (meth)acrylate in the monomer component is not particularly limited. From the viewpoint of effectively obtaining the effect of using the C _1-20 alkyl (meth)acrylate, in some embodiments, the content of the C _1-20 alkyl (meth)acrylate in the monomer component is usually about 1% by weight or more, for example, 10% by weight or more, 30% by weight or more, or 50% by weight or more (for example, more than 50% by weight). In some embodiments, from the viewpoint of introducing an ethylenically unsaturated group into the polymer or obtaining the effects of other copolymerizable monomers, the content of the C _1-20 alkyl (meth)acrylate is about 99% by weight or less, may be 90% by weight or less, may be about 70% by weight or less, may be 50% by weight or less (for example, less than 50% by weight), may be 30% by weight or less, may be 10% by weight or less, may be 1% by weight or less, or may be 0.1% by weight or less. The monomer component may be substantially free of C _1-20 alkyl (meth)acrylate.

In some embodiments, the monomer components constituting the acrylic polymer preferably contain other monomers other than the above alkoxyalkyl (meth)acrylate and linear alkyl (meth)acrylate. Such other monomers may be monomers (copolymerizable monomers) that are copolymerizable with the alkoxyalkyl (meth)acrylate and linear alkyl (meth)acrylate. The other monomers may be used, for example, to introduce an ethylenically unsaturated group into the polymer. As the other monomers, monomers having a polar group (e.g., a carboxy group, a hydroxyl group, a nitrogen atom-containing ring, etc.) may be preferably used. The monomers having a polar group may be useful for introducing crosslinking points into the acrylic polymer or for increasing the cohesive strength of the adhesive. The other monomers may be used alone or in combination of two or more.

Other monomers that can be used include, for example, carboxyl group-containing monomers, acid anhydride group-containing monomers, hydroxyl group-containing monomers, amide group-containing monomers, amino group-containing monomers, monomers having a nitrogen atom-containing ring, monomers containing a sulfonic acid group or a phosphoric acid group, epoxy group-containing monomers, cyano group-containing monomers, isocyanate group-containing monomers, monomers having a succinimide skeleton, maleimides, itaconimides, aminoalkyl (meth)acrylates, alkoxysilyl group-containing monomers, vinyl esters, vinyl ethers, aromatic vinyl compounds, olefins, (meth)acrylic acid esters having an alicyclic hydrocarbon group, (meth)acrylic acid esters having an aromatic hydrocarbon group, and other heterocyclic ring-containing (meth)acrylates such as tetrahydrofurfuryl (meth)acrylate, halogen atom-containing (meth)acrylates such as vinyl chloride and fluorine atom-containing (meth)acrylates, silicon atom-containing (meth)acrylates such as silicone (meth)acrylates, and (meth)acrylic acid esters obtained from alcohols derived from terpene compounds.

When using other monomers such as those mentioned above, the amount used is not particularly limited, but it is appropriate that it is 1 weight % or more of the total monomer components. From the viewpoint of better exerting the effect of using the other monomers, the amount of the other monomers used may be 10 weight % or more of the total monomer components, 20 weight % or more, or 30 weight % or more. Also, from the viewpoint of making it easier to balance the adhesive properties, it is appropriate that the amount of the other monomers used is 60 weight % or less of the total monomer components, and it is preferably 50 weight % or less (for example, less than 50 weight %), and may be 45 weight % or less.

In some embodiments, the monomer component constituting the acrylic polymer includes a monomer having a nitrogen atom. The use of a monomer having a nitrogen atom can increase the cohesive strength of the adhesive and favorably improve the adhesive strength. As the monomer having a nitrogen atom, for example, an amide group-containing monomer, an amino group-containing monomer, or a monomer having a nitrogen atom-containing ring can be used. The monomer having a nitrogen atom can be used alone or in combination of two or more types.

Non-limiting examples of monomers having a nitrogen atom include the following:
Amide group-containing monomers: for example, (meth)acrylamide; N,N-dialkyl(meth)acrylamides such as N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-dipropyl(meth)acrylamide, N,N-diisopropyl(meth)acrylamide, N,N-di(n-butyl)(meth)acrylamide, and N,N-di(t-butyl)(meth)acrylamide; N-monoalkyl(meth)acrylamides such as N-ethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N-butyl(meth)acrylamide, and N-n-butyl(meth)acrylamide; N-vinyl carboxylic acid amides such as N-vinylacetamide; monomers having a hydroxyl group and an amide group, for example, N-(2-hydroxyethyl)(meth)acrylamide, N-(2-hydroxyethyl)(meth)acrylamide, N-hydroxyalkyl(meth)acrylamides such as N-(1-hydroxypropyl)(meth)acrylamide, N-(3-hydroxypropyl)(meth)acrylamide, N-(2-hydroxybutyl)(meth)acrylamide, N-(3-hydroxybutyl)(meth)acrylamide, and N-(4-hydroxybutyl)(meth)acrylamide; monomers having an alkoxy group and an amide group, for example, N-alkoxyalkyl(meth)acrylamide such as N-methoxymethyl(meth)acrylamide, N-methoxyethyl(meth)acrylamide, and N-butoxymethyl(meth)acrylamide; and others such as N,N-dimethylaminopropyl(meth)acrylamide, alkoxydiacetone(meth)acrylamide, vinylformamide, and vinylacetamide.
Amino group-containing monomers: for example, aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, t-butylaminoethyl (meth)acrylate.
Monomers having a nitrogen atom-containing ring: for example, N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine, N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperazine, N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, N-vinyloxazole, N-(meth)acryloyl-2-pyrrolidone, N-(meth)acryloylpiperidine, N-(meth)acryloylpyrrolidone N-vinyl morpholine, N-(meth)acryloylmorpholine, N-vinyl morpholine, N-vinyl-3-morpholinone, N-vinyl-2-caprolactam, N-vinyl-1,3-oxazin-2-one, N-vinyl-3,5-morpholinedione, N-vinyl pyrazole, N-vinyl isoxazole, N-vinyl thiazole, N-vinyl isothiazole, N-vinyl pyridazine, and the like (for example, lactams such as N-vinyl-2-caprolactam).

A suitable example of a monomer having a nitrogen atom is a monomer having a nitrogen atom-containing ring. Among these, N-vinyl-2-pyrrolidone (NVP) and N-acryloylmorpholine (ACMO) are preferred.

The amount of the monomer having a nitrogen atom (preferably a monomer having a nitrogen atom-containing ring) used is not particularly limited. In some embodiments, the amount of the monomer having a nitrogen atom used in the monomer component may be 1% by weight or more, or may be 3% by weight or more. In some preferred embodiments, the amount of the monomer having a nitrogen atom used in the monomer component is 5% by weight or more, more preferably 7% by weight or more, even more preferably 9% by weight or more, or may be 10% by weight or more, or may be 12% by weight or more, or may be 14% by weight or more. The more the amount of the monomer having a nitrogen atom used, the more the cohesive strength of the adhesive tends to improve. In some embodiments, the amount of the monomer having a nitrogen atom used is suitably, for example, 40% by weight or less of the entire monomer component, and may be 35% by weight or less. In some preferred embodiments, the amount of the monomer having a nitrogen atom used in the monomer component is 30% by weight or less, more preferably 25% by weight or less, even more preferably 20% by weight or less, or may be 18% by weight or less.

In some embodiments, the monomer component includes a hydroxyl group-containing monomer. The use of a hydroxyl group-containing monomer can adjust the cohesive strength and crosslink density of the adhesive, improving the adhesive strength. In addition, a hydroxyl group-containing monomer is also preferably used as a means of introducing an ethylenically unsaturated group into a polymer. Examples of hydroxyl group-containing monomers that can be used include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, and (4-hydroxymethylcyclohexyl)methyl (meth)acrylate. For example, 2-hydroxyethyl acrylate (HEA) and 4-hydroxybutyl acrylate (4HBA) can be preferably used. The hydroxyl group-containing monomers can be used alone or in combination of two or more.

When a hydroxyl-containing monomer is used, the amount is not particularly limited, and may be, for example, 0.01% by weight or more, 0.1% by weight or more, or 0.5% by weight or more of the total monomer components. In some embodiments, the amount of the hydroxyl-containing monomer used is 1% by weight or more, more preferably 2% by weight or more, or 3% by weight or more of the total monomer components. In some preferred embodiments, the amount of the hydroxyl-containing monomer used is 5% by weight or more, more preferably 7% by weight or more, even more preferably 10% by weight or more, and particularly preferably 12% by weight or more of the total monomer components. Such an amount of the hydroxyl-containing monomer is suitable when the hydroxyl-containing monomer is used as a means for introducing an ethylenically unsaturated group into the polymer. In some embodiments, the amount of the hydroxyl-containing monomer used is, for example, 40% by weight or less of the total monomer components, and is preferably 30% by weight or less, more preferably 20% by weight or less, and even more preferably 15% by weight or less.

In some preferred embodiments, the monomer component of the acrylic polymer is a monomer having a polar group (polar group-containing monomer) that is a combination of a monomer having a nitrogen atom (e.g., an amide group-containing monomer such as (meth)acrylamide, a monomer having a nitrogen atom-containing ring such as NVP or ACMO) and a hydroxyl group-containing monomer (e.g., HEA, 4HBA). This allows for a good balance between adhesive strength and cohesive strength. In an embodiment in which a monomer having a nitrogen atom and a hydroxyl group-containing monomer are used in combination, the weight ratio (A _N /A _OH ) of the amount of the monomer having a nitrogen atom A _N to the amount of the hydroxyl group-containing monomer A _OH is not particularly limited, and may be, for example, 0.1 or more, 0.5 or more, 1.0 or more, 1.2 or more, 1.5 or more, or 1.8 or more. The weight ratio (A _N /A _OH ) may be, for example, 10 or less, 5 or less, 3 or less, or 2.5 or less.

In some embodiments, the monomer component may include a carboxy group-containing monomer. Non-limiting examples of carboxy 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, and the like. Preferred examples include AA and MAA. The carboxy group-containing monomers may be used alone or in combination of two or more. For example, AA and MAA may be used in combination.

The amount of the carboxyl group-containing monomer used may be, for example, 0.01% by weight or more, 0.1% by weight or more, 1% by weight or more, 3% by weight or more, 6% by weight or more, or 8% by weight or more of the total monomer components. The greater the amount of the carboxyl group-containing monomer used, the greater the tendency for the adhesive to have improved cohesive strength. The proportion of the carboxyl group-containing monomer may be, for example, 20% by weight or less, 10% by weight or less, 3% by weight or less, 1% by weight or less (for example, less than 1% by weight), or 0.1% by weight or less. The monomer components may be substantially free of the carboxyl group-containing monomer.

Furthermore, as the other monomer, it is preferable to use a monomer having a functional group (functional group A) that can react with a functional group (functional group B) of a compound having an ethylenically unsaturated group, which will be described later. In this embodiment, the type of the other monomer is determined by the above-mentioned compound type. As the other monomer having functional group A, for example, a carboxy group-containing monomer, an epoxy group-containing monomer, a hydroxyl group-containing monomer, and an isocyanate group-containing monomer are preferable, and a hydroxyl group-containing monomer is particularly preferable. By using a hydroxyl group-containing monomer as the other monomer, the acrylic polymer has a hydroxyl group. On the other hand, by using, for example, an isocyanate group-containing monomer as the compound having an ethylenically unsaturated group, the hydroxyl group of the acrylic polymer reacts with the isocyanate group of the compound, and the ethylenically unsaturated group derived from the compound is introduced into the acrylic polymer.

When using other monomers for the purpose of reacting with a compound having an ethylenically unsaturated group, the amount of the other monomers (preferably hydroxyl group-containing monomers) is appropriately set to about 1% by weight or more of the total monomer components from the viewpoint of the adhesive properties such as thermosetting property and cohesive strength, and is preferably about 5% by weight or more, more preferably about 10% by weight or more, and may be about 12% by weight or more. Also, from the viewpoint of maintaining good adhesive properties such as adhesive strength, the amount of the other monomers is appropriately set to about 40% by weight or less of the total monomer components, and is preferably about 30% by weight or less, more preferably about 25% by weight or less, and may be about 20% by weight or less (e.g., 15% by weight or less).

The acrylic polymer may contain, as another monomer component, a polyfunctional monomer having at least two ethylenically unsaturated groups, such as a (meth)acryloyl group or a vinyl group. By using a polyfunctional monomer as a monomer component, the cohesive strength of the adhesive can be increased. The polyfunctional monomer can be used as a crosslinking agent. There are no particular limitations on the polyfunctional monomer, and for example, one suitable one from among those exemplified as blended monomers contained in the adhesive layer described below can be used alone or in combination of two or more.

The amount of polyfunctional monomer used is not particularly limited, and can be appropriately set so that the purpose of using the polyfunctional monomer is achieved. The amount of polyfunctional monomer used can be about 3% by weight or less of the monomer component, preferably about 2% by weight or less, and more preferably about 1% by weight or less (e.g., about 0.5% by weight or less). When using a polyfunctional monomer, the lower limit of the amount used is not particularly limited as long as it is greater than 0% by weight. Usually, the effect of using the polyfunctional monomer can be appropriately achieved by setting the amount of polyfunctional monomer used to about 0.001% by weight or more of the monomer component (e.g., about 0.01% by weight or more).

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, solution polymerization 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, and the like 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, and the like, and can be, for example, about 20°C to 170°C (typically about 40°C to 140°C).

The solvent (polymerization solvent) used in 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, but not limited to, azo-based polymerization initiators, peroxide-based polymerization initiators, redox-based polymerization initiators formed by combining peroxides with reducing agents, substituted ethane-based polymerization initiators, etc. can be used. As the polymerization initiator, for example, one or more types can be selected from those exemplified as the thermal polymerization initiators added to the adhesive layer described below.

The amount of the polymerization initiator used is not particularly limited and may be a normal amount depending on the polymerization method and polymerization mode. For example, about 0.001 to 5 parts by weight (typically about 0.01 to 2 parts by weight, e.g., about 0.01 to 1 part by weight) of the polymerization initiator can be used per 100 parts by weight of the total monomer components to be polymerized.

(Polymer having an ethylenically unsaturated group)
The adhesive layer disclosed herein contains a polymer having an ethylenically unsaturated group such as an acryloyl group, a methacryloyl group, a vinyl group, or an allyl group. According to an adhesive containing a polymer having an ethylenically unsaturated group, the ethylenically unsaturated group of the polymer reacts upon heating, and the adhesive can be thermally cured to a high degree of hardening, and excellent heat peelability can be obtained. In addition, according to an adhesive containing a polymer having an ethylenically unsaturated group, sufficient heat peelability, heat resistance, and even a peel force reduction rate after heating can be achieved while limiting the amount of blended monomer used.

In some embodiments, a polymer having an ethylenically unsaturated group in a side chain is used as the polymer having an ethylenically unsaturated group. As the monomer component of the polymer having an ethylenically unsaturated group, one or more of the monomer components exemplified for the above polymer can be used in the above content range.

The amount of ethylenically unsaturated groups in a polymer having ethylenically unsaturated groups is not particularly limited, and from the viewpoint of thermosetting properties, etc., it is appropriate to make it 0.01 mmol per 1 g of polymer (hereinafter also referred to as mmol/g) or more, and it may be 0.1 mmol/g or more, or 0.5 mmol/g or more. In addition, the amount of ethylenically unsaturated groups in the above polymer is appropriate to be 10.0 mmol/g or less, and may be 5.0 mmol/g or less, 3.0 mmol/g or less, 2.5 mmol/g or less, or 2.0 mmol/g or less.

The amount of ethylenically unsaturated groups in a polymer is measured by the following method, for example, when the ethylenically unsaturated groups are (meth)acryloyl groups.
First, 0.25 mg of the polymer to be measured is dissolved in 50 mL of THF (tetrahydrofuran), and 15 mL of methanol is added to obtain a solution. Next, 10 mL of 4N aqueous sodium hydroxide solution is added to the above solution to obtain a mixed solution. Next, the above mixed solution is stirred at a liquid temperature of 40° C. for 2 hours. Furthermore, 10.2 mL of 4N methanesulfonic acid solution is added to the above mixed solution and stirred. 5 mL of demineralized water is added to this, followed by 2 mL of methanol to prepare a measurement solution.
The content of (meth)acrylic acid in the measurement solution is measured by HPLC (High Performance Liquid Chromatography) (absolute calibration curve method), and the content of ethylenically unsaturated groups is calculated.
(HPLC measurement conditions)
Column: Phenomenex Synergi 4μ Polar-RP 80A (4.6 mm x 250 mm)
Column temperature: 40°C
Flow rate: 1.0mL/min
Detector wavelength: 210 nm
Eluent: THF (for HPLC) 55/buffer water (containing 0.2% phosphoric acid and 0.2% triethylamine) 45
Aqueous solution injection volume: 5 μL

An example of a method for measuring the content of ethylenically unsaturated groups other than (meth)acryloyl groups is a method for measuring the bromine number in accordance with JIS K2605: 1996. In this measurement method, the content of ethylenically unsaturated groups other than (meth)acryloyl groups is determined by converting the number of grams of bromine ( _Br2 ) added to 100 g of the polymer to be measured into the number of moles of bromine ( _Br2 ) added to 1 g of the polymer.

The method of introducing an ethylenically unsaturated group into a polymer is not particularly limited, and an appropriate method can be selected from among methods known to those skilled in the art. From the viewpoint of molecular design, etc., a method of introducing an ethylenically unsaturated group into a side chain of a polymer is preferable. For example, a method of reacting (typically condensation, addition reaction) a compound having an ethylenically unsaturated group and a functional group (functional group B) that can react with a functional group (functional group A) introduced into an acrylic polymer by copolymerization, so that the ethylenically unsaturated group does not disappear, can be preferably adopted. Examples of combinations of functional group A and functional group B include a combination of a carboxy group and an epoxy group, a combination of a carboxy group and an aziridyl group, and a combination of a hydroxyl group and an isocyanate group. Among them, a combination of a hydroxyl group and an isocyanate group is preferable from the viewpoint of reaction traceability. From the viewpoint of polymer design, etc., a combination in which the acrylic polymer has a hydroxyl group and the above compound has an isocyanate group is particularly preferable.

The compound having an ethylenically unsaturated group may have a functional group B capable of reacting with functional group A, as described above. Suitable examples of such compounds include isocyanate group-containing monomers (isocyanate group-containing compounds) such as 2-(meth)acryloyloxyethyl isocyanate. Of these, 2-(meth)acryloyloxyethyl isocyanate is more preferred. An acrylic polymer having an ethylenically unsaturated group can be obtained by reacting the isocyanate group of the isocyanate group-containing compound having an ethylenically unsaturated group with the hydroxyl group of the acrylic polymer to form a bond (specifically a urethane bond).

The amount of the compound having an ethylenically unsaturated group (e.g., an isocyanate group-containing monomer) added is not particularly limited, but from the viewpoint of reactivity with the functional group A (e.g., a hydroxyl group) in the polymer, the molar ratio (M _A /M _B ) of the moles of the functional group A (M _A ) to the moles of the functional group B (isocyanate group) (M _B ) may be set in the range of about 0.5 to 2 (e.g., 1 to 1.5).

The adhesive layer may contain a polymer that is substantially free of ethylenically unsaturated groups (a polymer in which the amount of ethylenically unsaturated groups is less than 0.01 mmol/g) to the extent that the effect of the technology disclosed herein is not impaired. The amount of such polymer that is substantially free of ethylenically unsaturated groups used is suitably less than 50% by weight of the entire polymer (specifically, base polymer) contained in the adhesive layer, and may be less than 30% by weight, less than 10% by weight, less than 3% by weight, or less than 1% by weight. The adhesive layer may be substantially free of the above-mentioned polymer that is substantially free of ethylenically unsaturated groups.

The molecular weight of the polymer (e.g., acrylic polymer) is not particularly limited and can be set in an appropriate range according to the required performance. The weight average molecular weight (Mw) of the polymer is suitably about 1×10 ⁴ or more, for example, about 10×10 ⁴ or more. By using a polymer having a Mw of a predetermined value or more, the cohesive force and the adhesive force can be well balanced. In some embodiments, the Mw may be 20×10 ⁴ or more, 30×10 4 or more, about 40×10 ⁴ or more, about 50×10 ⁴ or more, for example, about 55×10 ⁴ or more, from the viewpoint of obtaining heat resistance and good adhesiveness. The upper limit of the Mw of the polymer is not particularly limited, and may be, for example, about 1000×10 ⁴ ^or less, or about 100×10 ⁴ or less. Here, Mw refers to a value calculated in terms of standard polystyrene obtained by gel permeation chromatography (GPC). As the GPC device, for example, a model named "HLC-8320GPC" (column: TSKgelGMH-H(S), manufactured by Tosoh Corporation) may be used.

(monomer)
The adhesive layer contains a monomer (a blended monomer) in addition to the polymer. The monomer has an ethylenically unsaturated group. The ethylenically unsaturated group of the monomer functions as a polymerizable functional group (typically a radically polymerizable functional group). By including the monomer in the adhesive layer, the monomer is included in the adhesive layer in a pre-reacted (unreacted) state. As a result, after the adhesive layer is formed, the monomer included in the adhesive layer reacts with the ethylenically unsaturated group of the polymer during heat treatment under predetermined conditions, and the adhesive is heat-cured with a high degree of hardening, and can exhibit excellent heat-peelability. By including the monomer, a thermosetting adhesive having heat-resistant peelability even after heat treatment can be formed. More specifically, when the adhesive is heated, for example, at a high temperature while attached to the adherend, it usually adsorbs to the surface of the adherend. Therefore, the adhesive's adhesive strength to the adherend is strengthened, resulting in heavy peeling. According to the technology disclosed herein, by including a polymer and a monomer having an ethylenically unsaturated group in the adhesive together with a thermal polymerization initiator (specifically, a peroxide-based polymerization initiator), the reaction (radical polymerization reaction) between the monomer and the thermal polymerization initiator proceeds rapidly when heated, and the adhesive can be cured prior to the adhesion of the adhesive to the adherend. This can reduce the adhesive strength to the adherend. Furthermore, even if heating is continued thereafter, the adhesive strength to the adherend does not increase and is maintained within a predetermined range, so that the adhesive can exhibit excellent heat peelability. Note that the technology disclosed herein is not limited to the above considerations. The above monomers can be used alone or in combination of two or more.

Examples of ethylenically unsaturated groups contained in the above monomers include, but are not limited to, acryloyl groups, methacryloyl groups, vinyl groups, and allyl groups. Suitable examples of ethylenically unsaturated groups include acryloyl groups and methacryloyl groups. Of these, acryloyl groups are preferred. Hereinafter, compounds having acryloyl groups and/or methacryloyl groups may be referred to as acrylic monomers. Compounds having vinyl groups may be referred to as vinyl monomers.

Although not particularly limited, it is appropriate to use a monomer having a molecular weight of 100 or more as the above monomer. In some preferred embodiments, the molecular weight of the above monomer may be, for example, 150 or more, 250 or more, 300 or more, 350 or more, 400 or more, 450 or more, or 500 or more. The molecular weight of the above monomer is usually about 100,000 or less, for example, about 10,000 or less (e.g., less than 10,000) is appropriate, 5,000 or less (e.g., less than 5,000) is preferable, 1,500 or less, 1,000 or less (e.g., less than 1,000), 800 or less, or 600 or less. The use of the above monomer having a molecular weight in the above range can be advantageous, for example, in terms of the preparation and coatability of the adhesive composition. The above molecular weight is a molecular weight calculated from the manufacturer's nominal value or molecular structure. For the above monomers having a molecular weight equal to or greater than a certain value, the weight average molecular weight (Mw) calculated using standard polystyrene standards obtained by GPC may be used.

In some preferred embodiments, the monomer is a monomer having a weight loss rate of 1% or less (specifically 1.0% or less) when reaching 180°C in TGA (thermogravimetric analysis) under a temperature rise condition of 10°C/min. By using a monomer having heat resistance with a weight loss rate of 1% or less at 180°C (hereinafter also referred to as "heat-resistant monomer"), the adhesive layer has easy peelability due to the inclusion of the monomer, while suppressing outgas generation during heating. By using the heat-resistant monomer, it is possible to achieve both easy peelability due to heating and reduced outgassing. From the viewpoint of reducing outgassing, in some preferred embodiments, the weight loss rate of the heat-resistant monomer at 180°C is 0.9% or less, more preferably 0.8% or less, even more preferably 0.7% or less, particularly preferably 0.6% or less, and may be 0.5% or less. The lower limit of the weight loss rate at 180°C of the heat-resistant monomer is theoretically 0%, and may be 0.1% or more, 0.2% or more, or 0.3% or more in practice. As heat-resistant monomers, trimethylolpropane triacrylate (TMPTA, weight loss rate at 180°C is 1%) and dipentaerythritol hexaacrylate (DPHA, weight loss rate at 180°C is 0.5%) are preferably used. Heat-resistant monomers can be used alone or in combination of two or more.

Specifically, the weight loss rate of the above monomer when heated to 180°C can be measured using a differential thermal analyzer (manufactured by TA Instruments, product name "Discovery TGA") under measurement conditions of a temperature rise of 10°C/min, in an air atmosphere, and at a flow rate of 25 mL/min.

In some preferred embodiments, a polyfunctional monomer is used as the monomer. In this specification, a polyfunctional monomer refers to a polymerizable compound having two or more ethylenically unsaturated groups in one molecule, and also includes those called oligomers. Hereinafter, a compound having two or more acryloyl groups and/or methacryloyl groups may be referred to as a polyfunctional acrylic monomer. Also, a compound having two or more vinyl groups may be referred to as a polyfunctional vinyl monomer.

In some preferred embodiments, the number of ethylenically unsaturated groups in one molecule of the polyfunctional monomer may be 3 or more, preferably 4 or more, more preferably 5 or more, and may be 6 or more. The more ethylenically unsaturated groups in the polyfunctional monomer, the better the curing property when heated, and the easier it is to obtain easy peeling by heating. In addition, a polyfunctional monomer having a larger number of ethylenically unsaturated groups (functional groups) can obtain easy peeling by heating with a relatively small amount of use. This is advantageous because it also leads to a reduction in the amount of outgassing derived from the polyfunctional monomer. The upper limit of the number of ethylenically unsaturated groups in one molecule of the polyfunctional monomer is not limited to a specific range, and may be, for example, 50 or less, 40 or less, 30 or less, 20 or less, or 15 or less. In some embodiments, the number of ethylenically unsaturated groups in one molecule of the polyfunctional monomer may be, for example, 10 or less, 8 or less, or 6 or less. A polyfunctional monomer having the above number of ethylenically unsaturated groups is likely to achieve both good adhesion and easy peeling by heating, and also tends to have excellent storage stability.

As the polyfunctional monomer, various polyfunctional acrylate monomers having two or more ethylenically unsaturated groups or polyfunctional vinyl monomers can be used. Among them, polyfunctional acrylate monomers can be preferably used. Although not particularly limited, polyfunctional acrylate monomers tend to be compatible and easily exhibit desired properties when used in combination with acrylic polymers. The polyfunctional acrylate monomers and polyfunctional vinyl monomers can each be used alone or in combination of two or more.

　Multifunctional monomers include 1,6-hexanediol di(meth)acrylate, 1,12-dodecanediol di(meth)acrylate, ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, allyl (meth)acrylate, alkylene oxide modified bisphenol A di(meth)acrylate, alkylene oxide modified neopentyl glycol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, dimethylol dicyclopentadi(meth)acrylate, vinyl (meth)acrylate, divinylbenzene, etc. difunctional monomers such as trimethylolpropane tri(meth)acrylate, trimethylolpropane ethoxy tri(meth)acrylate, glycerin propoxy triacrylate, tetramethylolmethane tri(meth)acrylate, and pentaerythritol tri(meth)acrylate; tetrafunctional monomers such as pentaerythritol alkoxy tetra(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, and pentaerythritol tetra(meth)acrylate. pentafunctional monomers such as sorbitol penta(meth)acrylate and dipentaerythritol penta(meth)acrylate; hexafunctional monomers such as dipentaerythritol hexa(meth)acrylate, sorbitol hexa(meth)acrylate, alkylene oxide modified hexa(meth)acrylate, and caprolactone modified dipentaerythritol hexa(meth)acrylate; and di- or higher functional epoxy acrylates, polyester acrylates, and urethane acrylates. Among these, preferred examples include 1,6-hexanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, and dipentaerythritol hexa(meth)acrylate. Among these, dipentaerythritol hexa(meth)acrylate is particularly preferred.

The content of the polyfunctional monomer in the adhesive layer is not particularly limited. In some embodiments, the content of the polyfunctional monomer may be approximately 1 part by weight or more, or 3 parts by weight or more, relative to 100 parts by weight of the polymer (specifically, the base polymer, preferably an acrylic polymer) contained in the adhesive layer. The appropriate amount of the polyfunctional monomer may vary depending on its molecular weight, the number of functional groups, etc., but in some preferred embodiments, the content of the polyfunctional monomer is 5 parts by weight or more, 7 parts by weight or more, 8 parts by weight or more, or 9 parts by weight or more, more preferably 10 parts by weight or more (e.g., more than 10 parts by weight), more preferably 15 parts by weight or more, even more preferably 20 parts by weight or more, and even more preferably 25 parts by weight or more, relative to 100 parts by weight of the polymer contained in the adhesive layer, from the viewpoint of improving the heat peelability. By including a sufficient amount of the polyfunctional monomer in the adhesive layer, the polyfunctional monomer contained in the adhesive layer reacts quickly when heated, and the adhesive layer is thermally cured, thereby realizing the heat peelability. The upper limit of the content of the polyfunctional monomer in the adhesive layer is not particularly limited, and can be set to achieve the desired adhesive properties. In some embodiments, from the viewpoint of compatibility with the polymer (specifically, the base polymer, for example, an acrylic polymer), the amount of the polyfunctional monomer relative to 100 parts by weight of the polymer is about 200 parts by weight or less, preferably 160 parts by weight or less, more preferably 150 parts by weight or less, even more preferably 140 parts by weight or less, and may be 120 parts by weight or less, or may be 90 parts by weight or less. In some preferred embodiments, the amount of the polyfunctional monomer relative to 100 parts by weight of the polymer may be 70 parts by weight or less, 50 parts by weight or less (e.g., less than 50 parts by weight), 45 parts by weight or less (e.g., less than 45 parts by weight), 40 parts by weight or less, 35 parts by weight or less, 30 parts by weight or less, 25 parts by weight or less, 20 parts by weight or less (e.g., less than 20 parts by weight), 18 parts by weight or less, 15 parts by weight or less, or 12 parts by weight or less. According to the technology disclosed herein, the desired heat peelability can be preferably achieved with a composition in which the amount of the polyfunctional monomer in the pressure-sensitive adhesive layer is limited as described above. In addition, by limiting the amount of the polyfunctional monomer used, the generation of low molecular weight components derived from the polyfunctional monomer after heating is suppressed, and contamination of the adherend surface caused by such low molecular weight components can be prevented.

In embodiments in which a polyfunctional monomer is used as the monomer, the amount of the polyfunctional monomer in the total monomer is not particularly limited. In some embodiments, from the viewpoint of effectively exerting the effect of containing the polyfunctional monomer, the amount of the polyfunctional monomer is appropriately about 10% by weight or more of the total monomer, preferably 30% by weight or more, more preferably about 50% by weight or more (e.g., more than 50% by weight), even more preferably 70% by weight or more, even more preferably 90% by weight or more, particularly preferably 95% by weight or more, and may be 99 to 100% by weight. In some embodiments, the monomer contained in the adhesive composition may essentially consist of the polyfunctional monomer.

Furthermore, one or more types of monofunctional monomers containing one ethylenically unsaturated group in one molecule may be used as the above monomer. As the monofunctional monomer, known monofunctional acrylate monomers or vinyl monomers may be used. For example, one or more types of acrylate monomers (alkoxy group-containing (meth)acrylates, chain alkyl (meth)acrylates, etc.) exemplified as monomer components of the above polymer may be used.

The content of the monomer in the adhesive layer is not particularly limited. In some embodiments, the content of the monomer may be about 1 part by weight or more, or may be 3 parts by weight or more, relative to 100 parts by weight of the polymer (specifically, the base polymer, preferably an acrylic polymer) contained in the adhesive layer. The appropriate amount of the monomer may vary depending on its molecular weight, the number of functional groups, etc., but in some preferred embodiments, the content of the monomer is 5 parts by weight or more, 7 parts by weight or more, 8 parts by weight or more, or 9 parts by weight or more, more preferably 10 parts by weight or more (e.g., more than 10 parts by weight), more preferably 15 parts by weight or more, even more preferably 20 parts by weight or more, and even more preferably 25 parts by weight or more, relative to 100 parts by weight of the polymer contained in the adhesive layer, from the viewpoint of improving the heat peelability. By including a sufficient amount of the monomer in the adhesive layer, the monomer contained in the adhesive layer reacts quickly when heated, and the adhesive layer is thermally cured, thereby realizing the heat peelability. The upper limit of the content of the monomer in the adhesive layer is not particularly limited, and can be set to achieve the desired adhesive properties. In some embodiments, from the viewpoint of compatibility with the polymer (specifically, the base polymer, for example, an acrylic polymer), the amount of the monomer relative to 100 parts by weight of the polymer is suitably about 200 parts by weight or less, preferably 160 parts by weight or less, more preferably 150 parts by weight or less, and even more preferably 140 parts by weight or less, and may be 120 parts by weight or less, or may be 90 parts by weight or less. In some preferred embodiments, the amount of the monomer relative to 100 parts by weight of the polymer may be 70 parts by weight or less, 50 parts by weight or less (for example, less than 50 parts by weight), 45 parts by weight or less (for example, less than 45 parts by weight), 40 parts by weight or less, 35 parts by weight or less, 30 parts by weight or less, 25 parts by weight or less, 20 parts by weight or less (for example, less than 20 parts by weight), 18 parts by weight or less, 15 parts by weight or less, or 12 parts by weight or less. According to the technology disclosed herein, the desired heat peelability can be preferably achieved with a composition in which the amount of monomer in the adhesive layer is limited as described above. In addition, by limiting the amount of the monomer used, the generation of low molecular weight components derived from the monomer after heating can be suppressed, and contamination of the adherend surface caused by such low molecular weight components can be prevented.

(Thermal Polymerization Initiator)
The adhesive layer contains a thermal polymerization initiator in addition to the polymer and the monomer. Here, the thermal polymerization initiator refers to a polymerization initiator that generates radicals by heating. The thermal polymerization initiator includes at least a peroxide-based polymerization initiator. By including a peroxide-based polymerization initiator in the adhesive layer as a thermal polymerization initiator, the reaction of the ethylenically unsaturated group in the adhesive layer during heating, i.e., the curing reaction of the adhesive layer, proceeds rapidly, and the adhesive layer can reliably exhibit heat-sensitive peelability for various adherends including materials (typically organic materials) that are easily attached to the adhesive layer by heating. One of the reasons for this is thought to be the high initiation efficiency of the peroxide-based polymerization initiator (particularly, the organic peroxide-based polymerization initiator). In addition, the peroxide-based polymerization initiator generates a radical (-O.) by cleaving -O-O- contained in the compound, but since this cleavage reaction is reversible, it is thought that recombination of -O-O- occurs when the radical does not collide with the ethylenically unsaturated group of the blended monomer or polymer. The recombined initiator can undergo a cleavage reaction again during a specified heating time, collide with the compounded monomer, etc., and react. Therefore, with the peroxide-based polymerization initiator, the thermal curing of the adhesive layer proceeds quickly at a reaction speed that is significantly faster than that of other initiators (e.g., azo-based initiators). And, since the thermal curing speed is faster than the speed at which the adhesive layer and the adherend are firmly attached to each other due to heating, the peeling force after heating is reliably reduced, and it is considered that heat-peelability and heat-resistant peelability can be obtained. Note that the technology disclosed herein is not limited to the above considerations.

As the peroxide-based polymerization initiator, for example, organic peroxides such as diacyl peroxides, peroxy esters, peroxy dicarbonates, monoperoxy carbonates, peroxy ketals, dialkyl peroxides, hydroperoxides, and ketone peroxides are preferably used. Suitable examples of peroxide-based polymerization initiators include benzoyl peroxide compounds (typically dibenzoyl peroxide (BPO)) having a benzoyl group that may have a substituent. The peroxide-based polymerization initiators can be used alone or in combination of two or more.

Specific examples of peroxide polymerization initiators include BPO, 1,1-di(t-hexylperoxy)cyclohexane, cyclohexanone peroxide, 3,3,5-trimethylcyclohexanone peroxide, methylcyclohexanone peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, n-butyl-4,4-bis(t-butylperoxy)valerate, cumene hydroperoxide, 2,5-dimethylhexane-2,5-dihydroperoxide, and 1,3-bis(t-butylperoxy)-m-isopropanol. Examples include diphenylbenzene, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3, diisopropylbenzene hydroperoxide, t-butylcumyl peroxide, didecanoyl peroxide, dilauroyl peroxide, 2,4-dichlorobenzoyl peroxide, di(4-t-butylcyclohexyl)peroxydicarbonate, t-butylperoxybenzoate, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, t-butyl hydroperoxide, and di-t-butyl peroxide.

The content of the peroxide-based polymerization initiator in the adhesive layer is not particularly limited. In some embodiments, the content of the peroxide-based polymerization initiator in the adhesive layer is suitably 0.1 parts by weight or more relative to 100 parts by weight of the polymer (specifically, the base polymer, e.g., an acrylic polymer) contained in the adhesive layer, and is preferably 0.2 parts by weight or more, more preferably 0.3 parts by weight or more, even more preferably 0.4 parts by weight or more, particularly preferably 0.5 parts by weight or more, may be 0.6 parts by weight or more, or may be 0.7 parts by weight or more. The higher the content of the peroxide-based polymerization initiator, the higher the frequency of collisions between the peroxide-based polymerization initiator and the ethylenically unsaturated group in the adhesive layer, and the easier it is for the curing reaction to proceed. In addition, in some embodiments, the amount of the peroxide-based polymerization initiator relative to 100 parts by weight of the polymer may be, for example, about 10 parts by weight or less, or about 5 parts by weight or less. In some preferred embodiments, the amount of the peroxide-based polymerization initiator relative to 100 parts by weight of the polymer is suitably 3 parts by weight or less (less than 3 parts by weight), preferably 2.5 parts by weight or less, more preferably 2.0 parts by weight or less, even more preferably 1.5 parts by weight or less, particularly preferably less than 1.2 parts by weight (e.g., 1.1 parts by weight or less), and may be 1.0 parts by weight or less (e.g., less than 1.0 parts by weight), 0.9 parts by weight or less, 0.8 parts by weight or less, 0.7 parts by weight or less, or 0.6 parts by weight or less. By setting the content of the peroxide-based polymerization initiator within a predetermined range, it is possible to preferably realize a pressure-sensitive adhesive having efficient thermosetting and easy peeling by heating while obtaining adhesive properties such as adhesive strength and storage stability.

The content of the peroxide-based polymerization initiator in the adhesive layer can also be specified by the relative relationship with the compounded monomer in the adhesive layer. In some embodiments, the amount of the peroxide-based polymerization initiator relative to 100 parts by weight of the monomer is preferably 0.1 parts by weight or more, more preferably 0.5 parts by weight or more, more preferably 0.8 parts by weight or more, even more preferably 1.0 parts by weight or more, even more preferably 1.2 parts by weight or more, particularly preferably 1.5 parts by weight or more, and may be 2.0 parts by weight or more, or may be 2.5 parts by weight or more. In some embodiments, the amount of the peroxide-based polymerization initiator relative to 100 parts by weight of the monomer is preferably 3 parts by weight or more, more preferably 5 parts by weight or more, and may be 7 parts by weight or more. The amount of the peroxide-based polymerization initiator used can be preferably adopted, for example, in a composition in which the monomer content is limited. In some embodiments, the amount of the peroxide-based polymerization initiator relative to 100 parts by weight of the monomer may be, for example, about 20 parts by weight or less, 15 parts by weight or less, 12 parts by weight or less, or 10 parts by weight or less. In some embodiments, the amount of the peroxide-based polymerization initiator relative to 100 parts by weight of the monomer may be, for example, 7 parts by weight or less, 5 parts by weight or less, or 3 parts by weight or less.

In some embodiments, the adhesive layer may contain, as a thermal polymerization initiator, a thermal polymerization initiator (non-peroxide polymerization initiator) different from the peroxide polymerization initiator, in addition to the peroxide polymerization initiator. Examples of non-peroxide polymerization initiators that can be used together with the peroxide polymerization initiator include azo polymerization initiators, redox polymerization initiators formed by a combination of a peroxide and a reducing agent, and substituted ethane polymerization initiators. Specific examples include persulfates such as potassium persulfate and ammonium persulfate; azo compounds such as 2,2'-azobisisobutyronitrile (AIBN), 2,2'-azobis(N-butyl-2-methylpropionamide), and 2,2'-azobis(2,4,4-trimethylpentane); substituted ethane initiators such as phenyl-substituted ethane; redox initiators formed by a combination of a peroxide and a reducing agent, such as a combination of a persulfate and sodium hydrogen sulfite, or a combination of a peroxide and sodium ascorbate; and the like. These non-peroxide polymerization initiators can be used alone or in combination of two or more.

Although not particularly limited, in order to effectively exert the effects of the peroxide-based polymerization initiator, in some embodiments, the proportion of the peroxide-based polymerization initiator in the total thermal polymerization initiator contained in the adhesive layer is suitably approximately 30% by weight or more, preferably 50% by weight or more, more preferably 70% by weight or more, even more preferably 90% by weight or more, and particularly preferably 95 to 100% by weight. The thermal polymerization initiator contained in the adhesive layer may be composed of a peroxide-based polymerization initiator.

In some embodiments, it is preferable to use a thermal polymerization initiator whose self-decomposition acceleration temperature (SADT) [°C] satisfies the formula: SADT+10≧60;. Here, the SADT of the thermal polymerization initiator is defined as the minimum temperature at which heat generation or self-accelerating decomposition of 6°C or more occurs within 7 days in a certain amount of container. SADT indicates the environmental temperature at the boundary of whether the thermal polymerization initiator causes decomposition. Based on the fact that the maximum temperature to which the adhesive can be exposed during storage is 60°C, the inventors have experimentally confirmed that if the thermal polymerization initiator has a SADT that is -10°C of the maximum storage temperature or higher, the self-decomposition of the thermal polymerization initiator in the adhesive is suppressed, and storage stability that can maintain good heat-peelability after storage is obtained. This is thought to be because heat is relatively less transmitted in the adhesive (solid) than in the case of the thermal polymerization initiator alone. Based on this discovery, an adhesive designed with a thermal polymerization initiator having a SADT that satisfies the above formula (hereinafter also referred to as a high SADT initiator) can suppress decomposition of the thermal polymerization initiator in the adhesive, even when the adhesive is exposed to a temperature of approximately 60°C before use, and the adhesive can maintain the desired heat-peelability. An adhesive containing a thermal polymerization initiator made of a high SADT initiator has good storage stability and can maintain good heat-peelability after storage, even when stored for a long period of time or when there is a temperature change during storage. In this specification, the nominal value listed in the manufacturer's catalog, etc., is used as the SADT of the thermal polymerization initiator.

The amount of the thermal polymerization initiator contained in the adhesive layer is not particularly limited. In some embodiments, the content of the thermal polymerization initiator in the adhesive layer is suitably 0.1 parts by weight or more relative to 100 parts by weight of the polymer (specifically, the base polymer, e.g., an acrylic polymer) contained in the adhesive layer, and is preferably 0.2 parts by weight or more, more preferably 0.3 parts by weight or more, even more preferably 0.4 parts by weight or more, particularly preferably 0.5 parts by weight or more, may be 0.6 parts by weight or more, or may be 0.7 parts by weight or more. The higher the content of the thermal polymerization initiator, the higher the frequency of collisions between the thermal polymerization initiator and the ethylenically unsaturated group in the adhesive layer, and the easier it is for the curing reaction to proceed. In addition, in some embodiments, the amount of the thermal polymerization initiator relative to 100 parts by weight of the polymer may be, for example, about 10 parts by weight or less, or may be about 5 parts by weight or less. In some preferred embodiments, the amount of the thermal polymerization initiator relative to 100 parts by weight of the polymer is 3 parts by weight or less (less than 3 parts by weight), preferably 2.5 parts by weight or less, more preferably 2.0 parts by weight or less, even more preferably 1.5 parts by weight or less, particularly preferably less than 1.2 parts by weight (e.g., 1.1 parts by weight or less), and may be 1.0 parts by weight or less (e.g., less than 1.0 parts by weight), 0.9 parts by weight or less, 0.8 parts by weight or less, 0.7 parts by weight or less, or 0.6 parts by weight or less. By keeping the total amount of the thermal polymerization initiator within a predetermined range, it is possible to preferably realize a pressure-sensitive adhesive having efficient thermosetting and easy peeling by heating while obtaining adhesive properties such as adhesive strength and storage stability.

Although not particularly limited, in some preferred embodiments, the total proportion of the above-mentioned polymer (specifically, base polymer, for example, acrylic polymer), the above-mentioned monomer (for example, polyfunctional acrylic monomer) and thermal polymerization initiator (peroxide-based polymerization initiator, etc.) in the entire adhesive layer is suitably 50% by weight or more (for example, more than 50% by weight and less than 100% by weight) from the viewpoint of effectively achieving a decrease in peel strength when heated and realizing a desired rate of decrease in peel strength after heating, and is preferably 70% by weight or more, more preferably 80% by weight or more, and even more preferably 90% by weight or more, and may be 95% by weight or more, 98% by weight or more, or 99% by weight or more (for example, 99 to 100% by weight).

(Crosslinking Agent)
The adhesive composition used to form the adhesive layer may contain a crosslinking agent as necessary, mainly for the purpose of crosslinking within the adhesive layer or between the adhesive layer and its adjacent surface. The crosslinking agent is typically contained in the adhesive layer in a form after crosslinking reaction. The use of the crosslinking agent allows the cohesive strength of the adhesive layer to be appropriately adjusted.

The type of crosslinking agent is not particularly limited, and can be selected from conventionally known crosslinking agents so that the crosslinking agent exerts an appropriate crosslinking function within the adhesive layer, for example, depending on the composition of the adhesive. Examples of crosslinking agents that can be used include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, carbodiimide-based crosslinking agents, melamine-based crosslinking agents, urea-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, hydrazine-based crosslinking agents, and amine-based crosslinking agents. These can be used alone or in combination of two or more. From the viewpoint of achieving a good balance between adhesiveness and cohesive strength, isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, and carbodiimide-based crosslinking agents are preferred, and isocyanate-based crosslinking agents are particularly preferred.

As an isocyanate-based crosslinking agent, a polyfunctional isocyanate compound having two or more functionalities can be used. Examples include aromatic isocyanates such as tolylene diisocyanate, xylene diisocyanate, polymethylene polyphenyl diisocyanate, tris(p-isocyanatophenyl)thiophosphate, and diphenylmethane diisocyanate; alicyclic isocyanates such as isophorone diisocyanate; and aliphatic isocyanates such as hexamethylene diisocyanate. Examples of commercially available products include isocyanate adducts such as trimethylolpropane/tolylene diisocyanate trimer adduct (manufactured by Tosoh Corporation, product name "Coronate L"), trimethylolpropane/hexamethylene diisocyanate trimer adduct (manufactured by Tosoh Corporation, product name "Coronate HL"), hexamethylene diisocyanate isocyanurate (manufactured by Tosoh Corporation, product name "Coronate HX"), and trimethylolpropane/xylylene diisocyanate adduct (manufactured by Mitsui Chemicals, Inc., product name "Takenate D-110N"), etc.

As the epoxy crosslinking agent, those having two or more epoxy groups in one molecule can be used without any particular restrictions. Epoxy crosslinking agents having 3 to 5 epoxy groups in one molecule are preferred. Specific examples of epoxy crosslinking agents include 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 Mitsubishi Gas Chemical Company's product names "TETRAD-X" and "TETRAD-C", DIC Corporation's product name "Epicron CR-5L", Nagase ChemteX Corporation's product name "Denacol EX-512", Nissan Chemical Industries' product name "TEPIC-G", etc.

As the oxazoline-based crosslinking agent, any agent having one or more oxazoline groups in one molecule can be used without any particular limitation.
Examples of the aziridine crosslinking agent include trimethylolpropane tris[3-(1-aziridinyl)propionate], trimethylolpropane tris[3-(1-(2-methyl)aziridinylpropionate)], and the like.
As the carbodiimide crosslinking agent, a low molecular weight compound or a high molecular weight compound having two or more carbodiimide groups can be used.

In some embodiments, an isocyanate-based crosslinking agent is used as the crosslinking agent. The isocyanate-based crosslinking agent can easily form an adhesive having good heat peelability while exhibiting a good balance of adhesive properties such as adhesive strength and cohesive strength. The isocyanate-based crosslinking agent can be used alone or in combination of two or more. Although not particularly limited, the amount of isocyanate-based crosslinking agent used is preferably less than 3 parts by weight per 100 parts by weight of the polymer (specifically, the base polymer, for example, an acrylic polymer) contained in the adhesive layer. By limiting the amount of isocyanate-based crosslinking agent used, the crosslinking density is appropriately suppressed, and at such a crosslinking density, the compounded monomers and the thermal polymerization initiator frequently collide with each other in the adhesive layer during heat treatment, heat curing progresses, and the peel force reduction rate after heating can be increased, and it is believed that the desired heat peelability and heat resistance peelability are expressed. Note that the technology disclosed herein is not limited to the above considerations. From this viewpoint, in some preferred embodiments, the amount of the isocyanate-based crosslinking agent used relative to 100 parts by weight of the polymer is 2 parts by weight or less, more preferably 1.5 parts by weight or less, even more preferably 1.0 parts by weight or less, even more preferably 0.8 parts by weight or less, and particularly preferably 0.6 parts by weight or less. By limiting the amount of the isocyanate-based crosslinking agent used, there is a tendency that sufficient adhesive strength is easily obtained. In addition, the amount of the isocyanate-based crosslinking agent used relative to 100 parts by weight of the polymer may be, for example, 0.01 parts by weight or more, and in some preferred embodiments, it may be 0.05 parts by weight or more, 0.1 parts by weight or more, 0.3 parts by weight or more, or 0.5 parts by weight or more. By appropriately setting the amount of the isocyanate-based crosslinking agent used within the above range, it is possible to preferably obtain an adhesive that exhibits adhesive properties such as adhesive strength and cohesive strength in a well-balanced manner while preferably achieving the effects of the technology disclosed herein.

When a crosslinking agent is used, the amount of the crosslinking agent used may be more than 0 parts by weight relative to 100 parts by weight of the polymer (specifically, the base polymer, for example, an acrylic polymer) contained in the adhesive layer, from the viewpoint of realizing an adhesive that exhibits adhesive properties such as adhesive strength and cohesive strength in a well-balanced manner, and may be, for example, 0.001 parts by weight or more, or may be 0.01 parts by weight or more. In some preferred embodiments, the amount of the crosslinking agent used relative to 100 parts by weight of the polymer may be 0.05 parts by weight or more, 0.1 parts by weight or more, 0.3 parts by weight or more, or 0.5 parts by weight or more. In addition, the upper limit of the amount of the crosslinking agent used may vary depending on the type of crosslinking agent used, so it is not limited to a specific range, but it is preferably limited to a predetermined amount or less. By limiting the amount of the crosslinking agent used, the crosslinking density is appropriately suppressed, and at such a crosslinking density, the blended monomers, etc. in the adhesive layer and the thermal polymerization initiator frequently collide during heat treatment, causing thermal curing to proceed, and the peel force reduction rate after heating can be increased, and it is thought that the desired heat peelability and heat resistance peelability are expressed. However, the technology disclosed herein is not limited to the above considerations. For example, the amount of the crosslinking agent used is suitably less than 10 parts by weight relative to 100 parts by weight of the polymer, and in some embodiments, it is preferably less than 5 parts by weight, and may be less than 3 parts by weight. In some embodiments, the amount of the crosslinking agent used is suitably less than 1 part by weight relative to 100 parts by weight of the polymer, and is preferably 0.9 parts by weight or less, may be 0.8 parts by weight or less, may be 0.7 parts by weight or less, may be 0.6 parts by weight or less, or may be 0.5 parts by weight or less. By limiting the amount of the crosslinking agent used, sufficient adhesive strength tends to be obtained.

A crosslinking catalyst may be used to promote the crosslinking reaction more effectively. Examples of crosslinking catalysts include metal-based crosslinking catalysts such as tetra-n-butyl titanate, tetraisopropyl titanate, nursem ferric, butyltin oxide, and dioctyltin dilaurate. The amount of the crosslinking catalyst used is not particularly limited. The amount of the crosslinking catalyst used may be, for example, approximately 0.0001 parts by weight or more, approximately 0.001 parts by weight or more, or approximately 0.005 parts by weight or more, relative to 100 parts by weight of the polymer (specifically, the base polymer, for example, an acrylic polymer) contained in the adhesive layer, and may be approximately 1 part by weight or less, approximately 0.1 parts by weight or less, or approximately 0.05 parts by weight or less.

The adhesive composition used to form the adhesive layer may contain a compound that generates keto-enol tautomerism as a crosslinking retarder, if desired. For example, a compound that generates keto-enol tautomerism may be preferably used in an adhesive composition that contains an isocyanate-based crosslinking agent or an adhesive composition that can be used by blending an isocyanate-based crosslinking agent. This can provide an effect of extending the pot life of the adhesive composition.
As the compound that causes keto-enol tautomerization, various β-dicarbonyl compounds can be used. Specific examples include β-diketones such as acetylacetone and 2,4-hexanedione; acetoacetates such as methyl acetoacetate and ethyl acetoacetate; propionylacetates such as ethyl propionylacetate; isobutyrylacetates such as ethyl isobutyrylacetate; malonic acid esters such as methyl malonate and ethyl malonate; and the like. Among these, preferred compounds include acetylacetone and acetoacetates. The compounds that cause keto-enol tautomerization can be used alone or in combination of two or more.
The amount of the compound that causes keto-enol tautomerization used may be, for example, 0.1 parts by weight or more and 20 parts by weight or less, and appropriately 0.5 parts by weight or more and 15 parts by weight or less, for example, 1 part by weight or more and 10 parts by weight or less, or may be 1 part by weight or more and 5 parts by weight or less, relative to 100 parts by weight of the polymer (specifically, the base polymer, e.g., an acrylic polymer) contained in the pressure-sensitive adhesive layer.

(Other ingredients)
The adhesive layer may contain, as necessary, various additives that are common in the field of adhesives, such as tackifiers, silane coupling agents, peel strength regulators (surfactants, etc.), viscosity regulators (e.g. thickeners), leveling agents, plasticizers, fillers, colorants such as pigments and dyes, stabilizers, preservatives, antiaging agents, etc. As for such various additives, those that are conventionally known can be used in the usual manner, and they do not particularly characterize the present invention, so detailed explanations will be omitted.
The technology disclosed herein can achieve desired adhesive properties such as adhesive strength without using a tackifier. In some embodiments, the content of the tackifier in the adhesive layer can be, for example, less than 10 parts by weight, or even less than 5 parts by weight, relative to 100 parts by weight of the polymer (specifically, the base polymer, for example, an acrylic polymer) contained in the adhesive layer. The content of the tackifier may be less than 1 part by weight (for example, less than 0.5 parts by weight), or less than 0.1 parts by weight (0 parts by weight or more and less than 0.1 parts by weight). The adhesive layer may be free of a tackifier.

(Form of Pressure-Sensitive Adhesive Composition)
Although not particularly limited, the adhesive layer disclosed herein can be preferably formed using a solvent-based adhesive composition. The above-mentioned solvent-based adhesive composition is an adhesive composition in a form containing an adhesive-forming component in an organic solvent. The solvent-based adhesive composition typically contains a solution polymer of a monomer component, a blended monomer, a thermal polymerization initiator (specifically, a peroxide-based polymerization initiator), and optionally other additives. The effect of the technology disclosed herein can be effectively exhibited in a form including a solvent-based adhesive (layer). The solvent contained in the solvent-based adhesive composition can be appropriately selected from conventionally known organic solvents. For example, any one solvent selected from aromatic compounds such as toluene (typically aromatic hydrocarbons); esters such as ethyl acetate and butyl 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; and the like, or a mixed solvent of two or more solvents can be used.

(Formation of Pressure-Sensitive Adhesive Layer)
The adhesive layer disclosed herein can be formed by a conventionally known method. The adhesive composition is applied (e.g., coated) to a suitable surface, and then a curing treatment is appropriately performed to form the adhesive in the form of a layer (adhesive layer). The curing means (e.g., drying, crosslinking, polymerization, cooling, etc.) of the adhesive composition may be applied by only one type, or may be applied by two or more types simultaneously or in multiple stages. In the case of a solvent-based adhesive composition, the adhesive can typically be formed by drying (preferably further crosslinking) the composition.

For example, in the case of a substrate-less double-sided adhesive sheet, a method can be adopted in which an adhesive composition is applied to a surface (release surface) having releasability, and then the adhesive composition is cured to form an adhesive layer on the surface. In the case of an adhesive sheet with a substrate, a method (direct method) can be adopted in which an adhesive composition is directly applied (typically coated) to the substrate and cured to form an adhesive layer. Alternatively, a method (transfer method) can be adopted in which an adhesive composition is applied to a surface (release surface) having releasability, cured to form an adhesive layer on the surface, and the adhesive layer is transferred to a substrate. The release surface can be the surface of a release liner, the back surface of a substrate that has been subjected to a release treatment, or the like. 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 dots or stripes.

The pressure-sensitive adhesive composition can be applied using a known or commonly used coater such as a gravure roll coater, a reverse roll coater, a kiss roll coater, a dip roll coater, a die coater, a bar coater, a knife coater, a spray coater, etc. Alternatively, the pressure-sensitive adhesive composition may be applied by impregnation, a curtain coating method, or the like.
From the viewpoint of promoting the crosslinking reaction and improving the production efficiency, the adhesive composition is preferably dried under heating. The drying temperature is not particularly limited, but can be, for example, about 40 to 100 ° C., and is usually preferably about 60 to 80 ° C. For example, drying at the above temperature (for example, drying for about 1 to 10 minutes, more specifically, drying for about 3 to 7 minutes) is performed at a low heating temperature and the solvent volatilization is in progress, so that the reaction of the monomer and the deactivation of the thermal polymerization initiator are negligible in the adhesive composition containing the monomer and the thermal polymerization initiator. In addition, after drying the adhesive composition, aging may be performed for the purpose of adjusting the component migration in the adhesive layer, progressing the crosslinking reaction, and relaxing distortion that may exist in the substrate or the adhesive layer.

(Thickness)
The thickness of the adhesive layer is not particularly limited. The thickness of the adhesive layer is usually 1 μm or more, may be 2 μm or more, or may be 3 μm or more. The larger the thickness of the adhesive layer, the more the adhesive strength to the adherend tends to improve. In some preferred embodiments, the thickness of the adhesive layer is 5 μm or more, may be 10 μm or more, may be 15 μm or more, may be 20 μm or more, may be 25 μm or more. The upper limit of the thickness of the adhesive layer is, for example, appropriately about 200 μm or less, may be 100 μm or less (for example, less than 100 μm), or may be 50 μm or less. By limiting the thickness of the adhesive layer within a predetermined range, it is possible to prevent the occurrence of glue residue due to cohesive failure, and it is easy to obtain peelability. In addition, a thin adhesive layer is advantageous in terms of thinning the adhesive sheet, and tends to be excellent in followability to the adherend. In some preferred embodiments, the thickness of the adhesive layer is 40 μm or less, and may be 30 μm or less.

(Gel Fraction)
The gel fraction of the pressure-sensitive adhesive layer is not particularly limited. In some embodiments, from the viewpoint of obtaining sufficient adhesion to the adherend and good heat peelability, the initial (before heating) gel fraction (weight basis) of the pressure-sensitive adhesive layer is, for example, 85% or less, preferably 80% or less, more preferably 75% or less, and may be 70% or less, 65% or less, or 60% or less. The lower the initial gel fraction, the higher the post-heat peel force reduction rate tends to be. In addition, from the viewpoint of obtaining pressure-sensitive adhesive layer formability, appropriate cohesiveness, and holding power, in some embodiments, the initial gel fraction of the pressure-sensitive adhesive layer is appropriately 20% or more, preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more, and may be 60% or more, or may be 70% or more. Pressure-sensitive adhesives having an appropriate initial gel fraction within the above range tend to have good heat curing reactivity.

In addition, the gel fraction of the pressure-sensitive adhesive layer after heating is preferably higher than the gel fraction before heating.
Gel fraction increase rate after heating [%] = (G1/G0-1) x 100
(In the above formula, G1 is the gel fraction [%] of the pressure-sensitive adhesive layer after heat treatment at 180° C. for 30 minutes, and G0 is the gel fraction [%] of the pressure-sensitive adhesive layer before heating.)
The gel fraction increase rate after heating calculated by is preferably 10% or more, more preferably 20% or more, even more preferably 30% or more, and may be 40% or more, 50% or more, 60% or more, or 70% or more. The pressure-sensitive adhesive having the above gel fraction increase rate after heating is easily cured by heating, and tends to easily obtain a high peel force reduction rate after heating, and thus excellent heat peelability and heat resistance peelability. In some embodiments, the upper limit of the gel fraction increase rate after heating is appropriately set according to the desired thermosetting property, and may be, for example, 90% or less, 80% or less, 70% or less, 60% or less, 50% or less, 40% or less, or 30% or less.

Although not particularly limited, in some embodiments, the gel fraction after heating (by weight) of the adhesive layer is appropriately 50% or more (e.g., more than 50%) from the viewpoint of exhibiting easy peelability upon heating, and is preferably 70% or more, more preferably 80% or more, even more preferably 85% or more, and particularly preferably 90% or more, and may be 95% or more. In some embodiments, the gel fraction after heating of the adhesive layer may be, for example, 99% or less, 95% or less, or 90% or less.

The gel fraction of the adhesive layer can be adjusted mainly by the monomer composition of the polymer, Mw, amount of monomers blended, type and amount of crosslinking agent, etc. The gel fraction after heating can be adjusted mainly by the design of the polymer (e.g., the amount of ethylenically unsaturated groups introduced), type and amount of monomers blended, and type and amount of thermal polymerization initiator. Specifically, each of the above gel fractions is measured by the method described in the Examples below.

(Young's Modulus)
In some embodiments, the Young's modulus Y1 [MPa] of the pressure-sensitive adhesive layer after 30 minutes of heat treatment at 180° C. is preferably 200 times or more of the Young's modulus Y0 [MPa] before heating, that is, the Young's modulus change ratio after heating (Y1/Y0) is preferably 200 or more. The higher the Young's modulus change ratio after heating (Y1/Y0), the higher the degree of curing of the pressure-sensitive adhesive layer during heating, and the easier it is to obtain excellent heat peelability. In some preferred embodiments, the Young's modulus change ratio after heating (Y1/Y0) may be 300 or more, 500 or more, 800 or more, 1000 or more, 1200 or more, 1500 or more, 1800 or more, or 2000 or more. In addition, from the viewpoint of providing the pressure-sensitive adhesive layer with an appropriate elastic modulus before heating, in some embodiments, the above-mentioned Young's modulus change ratio after heating (Y1/Y0) is suitably about 10,000 or less, preferably 5,000 or less, more preferably 2,500 or less, may be 1,500 or less, may be 1,000 or less, or may be 500 or less.

Although not particularly limited, in some embodiments, the Young's modulus before heating (initial Young's modulus) Y0 of the adhesive layer is, for example, suitably about 10 MPa or less, preferably 1 MPa or less, more preferably 0.5 MPa or less, even more preferably 0.3 MPa or less, and may be 0.2 MPa or less. An adhesive layer having the above pre-heating Young's modulus Y0 tends to easily obtain sufficient adhesion to the adherend and good heat peelability. In addition, from the viewpoint of obtaining adhesive layer formability, appropriate cohesiveness, and holding power, in some embodiments, the Young's modulus before heating Y0 of the adhesive layer is suitably 0.01 MPa or more, preferably 0.03 MPa or more, more preferably 0.05 MPa or more, even more preferably 0.08 MPa or more, and may be 0.10 MPa or more, 0.12 MPa or more, or 0.15 MPa or more.

Although not particularly limited, in some embodiments, the Young's modulus Y1 after heating of the adhesive layer is suitably 10 MPa or more, preferably 30 MPa or more, more preferably 50 MPa or more, and even more preferably 70 MPa or more. In some preferred embodiments, the Young's modulus Y1 after heating may be 100 MPa or more, 150 MPa or more, or 200 MPa or more. An adhesive layer having a Young's modulus after heating of a predetermined value or more tends to have a high degree of hardening after heating and to easily obtain excellent peelability upon heating. In some embodiments, the Young's modulus Y1 after heating may be, for example, approximately 500 MPa or less, 300 MPa or less, 150 MPa or less, 100 MPa or less, 70 MPa or less, or 50 MPa or less.

The Young's modulus of the adhesive layer can be adjusted mainly by the monomer composition of the polymer, Mw, amount of monomers blended, type and amount of crosslinking agent, etc. The Young's modulus after heating Y1 can be adjusted mainly by the design of the polymer (e.g., the amount of ethylenically unsaturated groups introduced), type and amount of monomers blended, and type and amount of thermal polymerization initiator. Specifically, each of the Young's moduli Y0 and Y1 is measured by the method described in the Examples below.

<Base layer>
The adhesive sheet disclosed herein may include a substrate layer. Various sheet-like substrates can be used as the substrate (layer) that supports (backs) the adhesive layer. As the substrate, a resin film, paper, cloth (woven fabric, nonwoven fabric, etc.), a rubber sheet, a foam sheet, a metal foil, a composite of these, etc. can be used. Examples of the resin film include polyolefin film; polyester film; vinyl chloride resin film; vinyl acetate resin film; polyamide resin film; fluororesin film; cellophane; and the like. Non-limiting examples of polyester films include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and the like. Other examples of the resin film include resin films formed from one or more engineering plastics (which may be super engineering plastics) such as polyphenylene sulfide resins, polysulfone resins, polyethersulfone resins, polyetheretherketone resins, polyarylate resins, polyamideimide resins, and polyimide resins. The use of engineering plastics is preferable from the viewpoint of heat resistance.

In some preferred embodiments, a resin film having a predetermined rigidity (strength) and excellent processability and handling properties is used as the substrate (layer). Among them, from the viewpoint of heat resistance, polyester film, polyamide resin film, engineering plastic film (e.g., polyimide resin film, etc.) are preferred as the resin film substrate. In this specification, the term "resin film" typically refers to a non-porous film, and typically refers to a resin film that does not substantially contain air bubbles (voidless). Therefore, the above-mentioned resin film is a concept that is distinguished from foam films and nonwoven fabrics. The density of the resin film that can be used as the substrate can be about 0.85 to 1.50 g/cm ³ (e.g., 0.90 g/cm ³ to 1.20 g/cm ³ , typically 0.92 g/cm ³ to 1.05 g/cm ³ ). The above-mentioned resin film may be a single-layer structure, or a multi-layer structure of two or more layers (e.g., a three-layer structure).

The base layer (e.g., a resin film) can contain known additives such as light stabilizers, antioxidants, antistatic agents, colorants (dyes, pigments, etc.), fillers, slip agents, and antiblocking agents, as necessary. The amount of additives to be added is not particularly limited, and can be set appropriately depending on the application, etc.

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

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

The thickness of the base layer is not particularly limited and can be appropriately selected depending on the purpose, but can generally be 1 to 500 μm. From the viewpoints of processability, handling, workability, etc., the thickness of the base layer is suitably 2 μm or more (e.g., 3 μm or more, typically 5 μm or more), and may be approximately 7 μm or more, or may be 10 μm or more. In some embodiments, the thickness of the base layer may be 20 μm or more, 30 μm or more, or may be 40 μm or more. In addition, the thickness of the base layer is suitably approximately 200 μm or less, and from the viewpoint of weight reduction and thinning, it is preferably approximately 100 μm or less, more preferably approximately 80 μm or less, and may be 60 μm or less. When the thickness of the base layer is smaller, the flexibility of the adhesive sheet and its ability to follow the surface shape of the adherend tend to improve.

The surface of the base layer facing the adhesive layer may be subjected to conventional surface treatments such as corona treatment, plasma treatment, ultraviolet irradiation treatment, acid treatment, alkali treatment, application of a primer, etc., as necessary. Such surface treatments may be treatments for improving the adhesion between the base layer and the adhesive layer, in other words, the anchoring ability of the adhesive layer to the base layer. The composition of the primer is not particularly limited, and may be appropriately selected from known ones. The thickness of the undercoat layer is not particularly limited, but is, for example, appropriately about 0.01 μm to 1 μm, and preferably about 0.1 μm to 1 μm. The back surface of the base layer may be subjected to the various surface treatments described above, antistatic treatment, etc.

<Total thickness>
The total thickness of the adhesive sheet disclosed herein (which may include an adhesive layer and a base layer, but does not include a release liner) is not particularly limited, and is suitably in the range of about 5 to 1000 μm. The total thickness of the adhesive sheet may be 10 μm or more, 30 μm or more, or 50 μm or more, from the viewpoints of adhesive properties, handling, etc. Also, from the viewpoints of weight reduction and thinning, in some embodiments, the total thickness of the adhesive sheet is 500 μm or less, or may be 300 μm or less. In some preferred embodiments, the total thickness of the adhesive sheet is 150 μm or less, may be 120 μm or less, or may be 100 μm or less (for example, less than 100 μm). Reducing the thickness of the adhesive sheet is advantageous in terms of thinning, miniaturization, weight reduction, resource saving, etc.

<Release liner>
The release liner used in the PSA sheet disclosed herein is not particularly limited, and may be, for example, a release liner in which the surface of a liner substrate such as a resin film or paper has been subjected to a release treatment, 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.). For the release treatment, for example, a silicone-based or long-chain alkyl-based release treating agent may be used. In some embodiments, a release-treated resin film may be preferably used as the release liner.

<Characteristics of adhesive sheet>
(Reduction rate of peeling force after heating)
In some embodiments, the adhesive sheet comprises a compound represented by the formula:
Peel strength reduction rate after heating A [%] = (1-F1/F0) x 100
(In the above formula, F1 is the post-heat peel strength [N/20 mm] measured in an environment of 23° C. after attaching the tape to an adherend and subjecting it to heat treatment at 180° C. for 30 minutes, and F0 is the pre-heat peel strength [N/20 mm].)
It is preferable that the post-heat peel strength reduction rate A obtained by the following formula is higher than 50%. A pressure-sensitive adhesive sheet satisfying the above characteristics can exhibit good heat peelability and heat resistance peelability when peeled off after heat treatment while adhering well to the adherend. In some preferred embodiments, the post-heat peel strength reduction rate A may be 60% or more, 70% or more, 80% or more, 85% or more, 90% or more, or 95% or more. The higher the post-heat peel strength reduction rate A, the better the heat peelability and heat resistance peelability can be exhibited. In addition, the post-heat peel strength reduction rate A is preferably less than 99.9%. According to a pressure-sensitive adhesive sheet having a post-heat peel strength reduction rate A of less than 99.9%, the pressure-sensitive adhesive sheet maintains the adhesion state with the adherend after heating and has the desired peelability from the adherend. This prevents the pressure-sensitive adhesive sheet from peeling off the adherend naturally due to heating, and prevents problems caused by this. From this viewpoint, the post-heat peel force reduction rate A may be 99.0% or less, for example, less than 95.0%.

(Pre-heat peel force F0)
Although not particularly limited, in some embodiments, the pressure-sensitive adhesive sheet is suitable for a pre-heat peeling force F0 of 1.0 N/20 mm or more, preferably 2.0 N/20 mm or more, more preferably 3.0 N/20 mm or more, even more preferably 4.0 N/20 mm or more, and particularly preferably 5.0 N/20 mm or more. A pressure-sensitive adhesive sheet exhibiting the above pre-heat peeling force F0 can exhibit good adhesion to various adherends such as organic materials. The upper limit of the pre-heating peeling force F0 is appropriately set according to the required adhesiveness, and is not limited to a specific range, and may be, for example, approximately 20 N/20 mm or less, or approximately 10 N/20 mm or less. The pre-heating peeling force F0 specifically refers to the peeling force against an FR-4 glass cloth-based epoxy resin copper-clad laminate measured under conditions of a peel angle of 180 degrees and a speed of 300 mm/min in an environment of 23°C. The pre-heating peeling force F0 is also called the initial peeling force. More specifically, the pre-heat peel force F0 is measured by the method described in the Examples section below.

(Post-heat peel strength F1)
Although not particularly limited, in some embodiments, the adhesive sheet is suitable for a peeling force (post-heat peeling force) F1 against an adherend after heat treatment at 180° C. for 30 minutes to be less than 3 N/20 mm, and preferably 1.0 N/20 mm or less. The adhesive sheet exhibiting the above-mentioned post-heat peeling force F1 has heat-peelability and can further have heat-resistant peelability after heat treatment. In some preferred embodiments, the post-heat peeling force F1 is less than 1.0 N/20 mm, more preferably 0.8 N/20 mm or less, even more preferably 0.6 N/20 mm or less, particularly preferably 0.5 N/20 mm or less, and may be 0.4 N/20 mm or less, or may be 0.3 N/20 mm or less, from the viewpoint of peelability. The lower limit value of the post-heat peeling force F1 may be 0.0 N/20 mm, or may be 0.01 N/20 mm or more (e.g., 0.1 N/20 mm or more). The post-heat peel strength F1 is specifically a peel strength measured under conditions of a peel angle of 180 degrees and a peel speed of 300 mm/min in a 23°C environment after a heat treatment of 180°C for 30 minutes while the tape is attached to an adherend. An FR-4 glass cloth-based epoxy resin copper-clad laminate is used as the adherend. More specifically, the post-heat peel strength F1 is measured by the method described in the Examples below.

<Removal method>
According to this specification, a method for peeling a pressure-sensitive adhesive sheet attached to an adherend from the adherend is provided. The peeling method includes a step of subjecting the adherend to a heat treatment at a predetermined temperature or higher, and then peeling the pressure-sensitive adhesive sheet from the adherend. The pressure-sensitive adhesive sheet disclosed herein has heat-peelability (heat-resistant peelability), and therefore can be easily peeled from the adherend even after being exposed to heat under predetermined conditions.

In some embodiments, the temperature of the heat treatment for the adherend to which the adhesive sheet is attached is suitably 120°C or higher, and may be about 130°C or higher, or may be about 150°C or higher. In some preferred embodiments, the temperature of the heat treatment for the adherend to which the adhesive sheet is attached is higher than 150°C, may be 160°C or higher, or may be 170°C or higher. The upper limit of the heat treatment temperature may vary depending on the purpose of the heat treatment, the heat resistance of the adherend, etc., but is generally about 260°C or lower, may be about 250°C or lower, may be 230°C or lower, may be 200°C or lower, or may be 180°C or lower. The time of the heat treatment is not particularly limited, and may be within 10 hours, may be within 5 hours, or may be within 3 hours. From the viewpoint of the efficiency of the heating process, etc., in some preferred embodiments, the time of the heat treatment may be within 1 hour, may be within 30 minutes, may be within 15 minutes, may be within 10 minutes, may be within 5 minutes. The adhesive sheet disclosed herein can exhibit heat peelability by the above-mentioned short-time heat treatment, with the adhesive being heat-cured. The heat treatment time may be 1 minute or more, 3 minutes or more, 5 minutes or more, 7 minutes or more, or 9 minutes or more. In some embodiments, the heat treatment time may be 10 minutes or more, 30 minutes or more, 60 minutes or more, more than 1 hour, more than 3 hours, more than 4 hours, or more than 5 hours. The adhesive sheet disclosed herein can exhibit heat curing and reduced peel strength by heat treatment at the above-mentioned heating temperature, and an increase in peel strength (heavy peeling) does not occur or can be suppressed even if the heated state continues for a long time. Therefore, it is possible to maintain heat peelability (heat-resistant peelability) even after a long-time heat treatment.

<Applications>
The use of the adhesive sheet disclosed herein is not particularly limited. The adhesive sheet disclosed herein can be used as an adhesive sheet for various applications requiring easy peeling by heating, such as applications in which the adhesive sheet is peeled off from an adherend by heating, taking advantage of the feature that the adhesive sheet can exhibit easy peeling by heating on various adherends, such as adherends containing organic materials, based on rapid heat curing. For example, the adhesive sheet can be used in applications in which the adhesive sheet is exposed to heat of more than 100°C (e.g., about 120°C to 260°C) when attached to the adherend. In addition, the adhesive sheet can be preferably used in applications in which the adhesive sheet is exposed to heat of more than 150°C (e.g., about 160°C to 260°C) when attached to the adherend.

The adhesive sheet disclosed herein can be used for, for example, masking applications requiring heat resistance in the adhesive sheet, temporary fixing applications, and protective applications. It can also be preferably used as a process material that is fixed to an adherend and peeled off in the manufacturing process of electronic devices and electronic components. A suitable application of the adhesive sheet disclosed herein is semiconductor element manufacturing applications. For example, it can be preferably used as a wafer fixing sheet that fixes the wafer to a fixing plate in semiconductor wafer processing (typically silicon wafer processing). The adhesive sheet disclosed herein can also be preferably used as a protective sheet that protects the wafer in the above-mentioned wafer processing. In particular, during the manufacturing of semiconductor elements, the wafer may be exposed to heat in the processing step, etc., so an adhesive sheet having heat resistance and easy peelability is preferably used. The adhesive sheet disclosed herein can also be applied to optical applications requiring heat resistance. More specifically, the adhesive sheet disclosed herein can be used as an optical adhesive sheet used in applications such as bonding optical members (for bonding optical members) and manufacturing applications of products using the above-mentioned optical members (optical products). The optical component refers to a component that has optical properties (e.g., polarization, light refraction, light scattering, light reflectivity, light transmission, light absorption, light diffraction, optical rotation, visibility, etc.).

The adhesive sheet disclosed herein can also exhibit good heat-peelability even on organic material surfaces, and is therefore preferably used in applications in which it is attached to the surface of a substrate (adherend) whose surface is made of an organic material, and then peeled off from the surface. Examples of such substrate materials include circuit boards (e.g., printed wiring boards (PCBs) and flexible circuit boards (FPCs)). The adhesive sheet can be preferably used as a temporary fixing sheet for such circuit boards.

The type of material to be attached (adherend material) disclosed herein is not particularly limited. The adhesive sheet disclosed herein can be used for fixing and protecting various members and materials. The adherend material can be made of an organic material, an inorganic material, or a composite of these. Examples of the adherend material include glass such as alkali glass and non-alkali glass; metal materials such as stainless steel (SUS) and aluminum; ceramic materials such as alumina and silica; resin materials such as polyester resins such as PET, acrylic resins, ABS resins, polycarbonate resins, polystyrene resins, transparent polyimide resins, epoxy resins, and phenolic resins; and the like. Suitable examples of the adherend material include, for example, circuit boards whose surfaces are made of epoxy resin (for example, glass cloth-based epoxy resin copper-clad laminates). Other suitable examples of the adherend material include, for example, glass materials such as alkali glass and semiconductor wafers. The glass material may be, for example, a glass plate having a surface partially provided with a transparent conductive film (e.g., an ITO (indium tin oxide) film) or an FPC (flexible printed circuit board), such as those used in tablet computers, mobile phones, organic LEDs (light-emitting diodes), etc.

The matters disclosed in this specification include the following.
[1] A pressure-sensitive adhesive layer including a polymer, a monomer, and a thermal polymerization initiator,
The polymer comprises ethylenically unsaturated groups,
The monomer contains an ethylenically unsaturated group,
The pressure-sensitive adhesive sheet, wherein the thermal polymerization initiator comprises a peroxide-based polymerization initiator.
[2] The pressure-sensitive adhesive sheet according to the above-mentioned [1], wherein the pressure-sensitive adhesive layer contains a polyfunctional acrylate monomer as the monomer.
[3] The pressure-sensitive adhesive sheet according to the above-mentioned [1] or [2], wherein the content of the monomer in the pressure-sensitive adhesive layer is less than 50 parts by weight per 100 parts by weight of the polymer.
[4] The pressure-sensitive adhesive sheet according to any one of [1] to [3] above, wherein the content of the thermal polymerization initiator in the pressure-sensitive adhesive layer is 0.1 parts by weight or more and 1 part by weight or less per 100 parts by weight of the polymer.
[5] The pressure-sensitive adhesive sheet according to any one of the above [1] to [4], wherein the pressure-sensitive adhesive layer has a pre-heating gel fraction of 50% or more.
[6] The pressure-sensitive adhesive layer has a Young's modulus Y1 [MPa] after heat treatment at 180° C. for 30 minutes that is 200 times or more the Young's modulus Y0 [MPa] before heating. The pressure-sensitive adhesive sheet according to any one of [1] to [5] above.
[7] The pressure-sensitive adhesive sheet according to any one of the above [1] to [6], which is attached to the surface of a substrate having a surface made of an organic material, and is then peeled off from the surface.

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

<Evaluation method>
(Pre-heat peel force F0)
The adhesive sheet is cut to a size of 20 mm wide and 100 mm long, and the adhesive surface of the adhesive sheet is pressed against the adherend by rolling a 2 kg roller back and forth once under an environment of 23°C and 50% RH. The adherend to which the adhesive sheet is attached is left for 6 hours under the same environment and used as an evaluation sample. The evaluation sample is set in a tensile tester under an environment of 23°C and 50% RH, and the peel strength (pre-heat peel force) F0 [N/20 mm width] is measured when the adhesive sheet is peeled off from the adherend under the conditions of a peel angle of 180 degrees and a speed of 300 mm/min. As the adherend, an FR-4 glass cloth-based epoxy resin copper-clad laminate (manufactured by Nikkan Kogyo Co., Ltd., product number "L6504C1") is used. As the tensile tester, a Shimadzu product name "EZ-S 500N" or an equivalent product can be used. When the evaluation object is a double-sided pressure-sensitive adhesive sheet, the measurement may be performed with the non-measurement surface backed with a PET film.

(Post-heat peel strength F1)
Using the pressure sensitive adhesive sheet, prepare an evaluation sample by the method described in the above-mentioned measurement of peel strength F0 before heating. The obtained evaluation sample is heated in an oven at 180°C for 30 minutes, and then removed from the oven and left to stand in an environment of 23°C and 50% RH for 30 minutes. Then, set the evaluation sample in the same environment in a tensile tester, and measure the peel strength (post-heat peel strength) F1 [N/20mm width] when the pressure sensitive adhesive sheet is peeled off from the adherend under the conditions of a peel angle of 180° and a speed of 300mm/min. The adherend, tensile tester, and other items are the same as those in the measurement of pre-heat peel strength F0.

(Gel Fraction)
The gel fraction (weight ratio of the matter insoluble in ethyl acetate) of the pressure-sensitive adhesive layer is measured by the following method.
About 0.1 g of the adhesive sample (weight Wg1) is wrapped in a porous polytetrafluoroethylene film (weight Wg2) with an average pore size of 0.2 μm in a purse shape, and the opening is tied with string (weight Wg3). As the porous polytetrafluoroethylene (PTFE) film, the product name "Nitoflon (registered trademark) NTF1122" (average pore size 0.2 μm, porosity 75%, thickness 85 μm) available from Nitto Denko Corporation or an equivalent product is used.
The package is immersed in 50 mL of ethyl acetate and kept at room temperature (typically 23° C.) for 7 days to elute only the sol component in the adhesive layer outside the film, and then the package is taken out and the ethyl acetate adhering to the outer surface is wiped off, and the package is dried at 130° C. for 2 hours, and the weight (Wg4) of the package is measured. The gel fraction of the adhesive layer can be calculated by substituting each value into the following formula.
Gel fraction [%] = [(Wg4 - Wg2 - Wg3) / Wg1] x 100
The gel fraction of the pressure-sensitive adhesive layer is measured initially (gel fraction before heating) and after the pressure-sensitive adhesive layer is heat-treated in an oven at 180° C. for 30 minutes, removed from the oven and allowed to stand in an environment of 23° C. and 50% RH for 30 minutes (gel fraction after heating).

(Young's Modulus)
The adhesive layer is prepared in a state where both sides are covered with release liners, and cut into a size of 80 mm wide and 30 mm long together with the release liners. One release liner is removed from the adhesive layer, and the adhesive layer is wound on the other release liner in the length direction so as not to introduce air bubbles, to prepare a rod-shaped sample of 30 mm long. The rod-shaped sample is set in a tensile tester (manufactured by ORIENTEC, product name "RTC-1150A") and pulled under conditions of a measurement temperature of 23°C, a chuck distance of 10 mm, and a pulling speed of 50 mm/min, and the initial elastic modulus is obtained from the rise of the obtained stress-strain curve (S-S curve), which is the Young's modulus [MPa] of the adhesive layer. The reason for setting the cut width of the adhesive layer to 80 mm is to set the cross-sectional area of the adhesive layer in the cross section along the width direction to within the range of ² to 2.5 mm2, and it is desirable to adjust the cut width so that the cross-sectional area is approximately the same depending on the thickness of the adhesive layer.
The Young's modulus of the pressure-sensitive adhesive layer is measured initially (Young's modulus before heating) and after the pressure-sensitive adhesive layer is heat-treated in an oven at 180° C. for 30 minutes, removed from the oven and allowed to stand in an environment of 23° C. and 50% RH for 30 minutes (Young's modulus after heating).

Example 1
(Preparation of Pressure-Sensitive Adhesive Composition)
A reaction vessel equipped with a cooling tube, a nitrogen introduction tube, a thermometer and a stirrer was charged with 100 parts of a monomer component containing methoxyethyl acrylate (MEA), acryloylmorpholine (ACMO) and hydroxyethyl acrylate (HEA) in a molar ratio of 80:20:20, and 65 parts of toluene as a polymerization solvent, and 0.2 parts of benzoyl peroxide was added as a thermal polymerization initiator to perform a polymerization reaction (solution polymerization) at 61 ° C. for 6 hours under a nitrogen atmosphere to obtain a solution containing an acrylic polymer a. To this solution of the acrylic polymer a, methacryloyloxyethyl isocyanate (MOI) in an amount equivalent to 16 moles relative to 20 moles of HEA used as a raw material for the acrylic polymer a was added, and an addition reaction treatment was performed at 50 ° C. for 48 hours in an air stream to obtain a solution of an acrylic polymer A having a methacryloyl group at the side chain end.
To the solution of the acrylic polymer A, 30 parts of dipentaerythritol hexaacrylate (DPHA) as a monomer, 0.5 parts of an isocyanate-based crosslinking agent (manufactured by Mitsui Chemicals, Inc., product name "Takenate D-101E"), and 0.5 parts of benzoyl peroxide (manufactured by NOF Corporation, product name "Niper BW", SADT: 75°C) as a thermal polymerization initiator were added per 100 parts of the acrylic polymer A, and the mixture was mixed uniformly to prepare a pressure-sensitive adhesive composition according to this example.

(Preparation of adhesive sheet)
The adhesive composition obtained above was applied to the release surface of a commercially available PET release liner and dried at 80°C for 5 minutes to form an adhesive layer with a thickness of 30 μm. A polyimide (PI) film with a thickness of 50 μm (product name "Kapton 200H", manufactured by DuPont-Toray Co., Ltd.) was laminated to this adhesive layer, and then aged at 50°C for 3 days. In this manner, an adhesive sheet (single-sided adhesive sheet with substrate) according to this example was produced. The adhesive surface of the adhesive sheet was protected by a release liner.

<Examples 2 to 12 and Comparative Examples 1 to 3>
Except for changing the type and amount of monomer, the type and amount of thermal polymerization initiator, and the amount of crosslinking agent as shown in Table 1, the adhesive composition according to each example was prepared in the same manner as in Example 1, and the obtained adhesive composition was used to prepare a substrate-attached single-sided adhesive sheet according to each example in the same manner as in Example 1. In Table 1, HDDA represents 1,6-hexanediol diacrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd.), Niper BMT represents the product name "Niper BMT" (manufactured by NOF Corporation, benzoyl peroxide type, SADT: 45°C), AIBN represents 2,2'-azobisisobutyronitrile, and VR-110 represents 2,2'-azobis(2,4,4-trimethylpentane) (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., product name "VR-110").

<Comparative Example 4>
A reaction vessel equipped with a cooling tube, a nitrogen inlet tube, a thermometer and a stirrer was charged with 85 parts of MEA, 10 parts of N-vinyl-2-pyrrolidone (NVP) and 5 parts of 4-hydroxybutyl acrylate (4HBA) as monomer components, and 300 parts of ethyl acetate as a polymerization solvent. 0.2 parts of 2,2′-azobisisobutyronitrile (AIBN) was then added as a thermal polymerization initiator, and a polymerization reaction (solution polymerization) was carried out at 61° C. for 6 hours under a nitrogen atmosphere to obtain a solution containing an acrylic polymer B.
To the solution of the acrylic polymer B, 100 parts of DPHA as a monomer, 0.5 parts of the isocyanate-based crosslinking agent, and 0.5 parts of benzoyl peroxide (manufactured by NOF Corporation, product name "Niper BW", SADT: 75°C) as a thermal polymerization initiator were added per 100 parts of the acrylic polymer B, and the mixture was mixed uniformly to prepare the pressure-sensitive adhesive composition of this example.
A single-sided PSA sheet with a substrate according to this example was produced in the same manner as in Example 1, except that the above PSA composition was used.

　An overview of each example and the evaluation results are shown in Table 1. In addition, in the measurement of peel strength after heating, if the adhesive layer was strongly adhered to the adherend due to heating and the peel strength could not be measured, it was evaluated as "X".

As shown in Table 1, in Examples 1 to 12, in which the polymer and monomer contained ethylenically unsaturated groups and the adhesive contained a peroxide-based polymerization initiator as the thermal polymerization initiator, it was confirmed that the peel force was sufficiently reduced after heating in a peel force evaluation in which the adhesive was attached to the surface of an organic material (epoxy resin in an FR-4 glass cloth-based epoxy resin copper-clad laminate). On the other hand, in Comparative Examples 1 and 2, in which an azo-based polymerization initiator rather than a peroxide-based initiator was used as the thermal polymerization initiator, Comparative Example 3, in which no monomer was used, and Comparative Example 4, in which a polymer without an ethylenically unsaturated group was used, none of the adhesive sheets could be peeled from the adherend after heating, and the peel force after heating could not be measured. All of the adhesive sheets according to Comparative Examples 1 to 4 can exhibit good heat-induced easy peelability from alkaline glass plates, but it is believed that, for the surface of the above organic material, they adhered to the adherend to such an extent that they could not be peeled off after heating due to differences in the surface smoothness of the adherend and the adhesion between the organic material and the adhesive when heated at high temperatures. For example, in Comparative Examples 1 and 2, the azo-based polymerization initiator has a slower reaction rate than the peroxide-based polymerization initiator, so it is believed that the adhesion between the adhesive and the adherend proceeded faster than the hardening during heating. In Comparative Example 4, a larger amount of monomer was added to increase the degree of hardening after heating (Young's modulus after heating), but peeling after heating was still difficult.

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

REFERENCE SIGNS LIST 1 Adhesive sheet

1A Adhesive surface

1B Back surface 10 Base layer 10A One surface 10B Other surface 20 Adhesive layer 20A Adhesive surface 30 Release liner 50 Adhesive sheet with release liner

Claims

A pressure-sensitive adhesive layer containing a polymer, a monomer, and a thermal polymerization initiator,
The polymer comprises ethylenically unsaturated groups,
The monomer contains an ethylenically unsaturated group,
The pressure-sensitive adhesive sheet, wherein the thermal polymerization initiator comprises a peroxide-based polymerization initiator.
The adhesive sheet according to claim 1, wherein the adhesive layer contains a polyfunctional acrylate monomer as the monomer.
The adhesive sheet according to claim 1 or 2, wherein the content of the monomer in the adhesive layer is less than 50 parts by weight per 100 parts by weight of the polymer.
The adhesive sheet according to claim 1 or 2, wherein the adhesive layer has a gel fraction of 50% or more before heating.
The adhesive sheet according to claim 1 or 2, wherein the Young's modulus Y1 [MPa] of the adhesive layer after heat treatment at 180°C for 30 minutes is 200 times or more the Young's modulus Y0 [MPa] before heating.
3. The pressure-sensitive adhesive sheet according to claim 1, which is attached to a surface of a substrate having a surface made of an organic material, and is then peeled off from the surface.