JP7797963B2 - Heat-peelable pressure-sensitive adhesive composition and pressure-sensitive adhesive sheet - Google Patents

Description

The present invention relates to a heat-peelable pressure-sensitive adhesive composition that has sufficient peel strength within the range of normal use and whose peel strength decreases only upon heat treatment, and to a pressure-sensitive adhesive sheet having an adhesive layer formed by crosslinking the pressure-sensitive adhesive composition.

In recent years, there has been a growing trend to improve the recycling rate of home appliances, as well as various plastic and metal products. Since these products are typically manufactured by joining or gluing together numerous parts, in order to improve recycling rates, it is important to separate and disassemble each joined or glued part and separate them by material when recycling.

Adhesive sheets are often used to bond these parts together, simplifying the bonding process. These adhesive sheets are typically highly adhesive in order to ensure reliable bonding of parts. However, this means that a strong force must be applied during disassembly, which can lead to a variety of undesirable behaviors, such as damage to the parts rather than the adhesive bond, or the adhesive remaining on the parts, hindering their recycling.

From this perspective, proposals have been made for adhesives that become easily peelable when a specific trigger is activated during recycling. For example, Patent Document 1 proposes an adhesive that uses a photoresponsive compound to become easily peelable when treated with UV light, while Patent Document 2 proposes a resin that is peelable by heat using a compound with a structure that can be decomposed and recombined. Patent Document 3 also proposes a resin that is peelable by heat, combining a resin containing an amino group with a compound containing an ethylenically unsaturated group. Furthermore, Patent Document 4 proposes using a resin that is peelable by heat in an adhesive sheet for protecting chemical solutions, after crosslinking a composition of an acrylic resin and a urethane (meth)acrylate compound with an isocyanate crosslinking agent.

Japanese Patent Application Laid-Open No. 2007-70558 Special Publication No. 2013-519764 Japanese Patent Application Laid-Open No. 2019-156919 Japanese Patent Application Laid-Open No. 2016-204617

However, the adhesive in Patent Document 1 is not suitable for bonding opaque components such as colored or metallic materials, and is therefore limited in the types of substrates it can be used with. Furthermore, the adhesive using the resin in Patent Document 2 is, in principle, prone to leaving adhesive residue due to decomposition products, posing issues with recyclability. Furthermore, the resin composition in Patent Document 3 has limitations, such as the need for increased adhesion during normal use and difficulty in adjusting the heating temperature and heating time during disassembly, leaving further improvements in sight.

The inventors then conducted a detailed study of the adhesive composition of Patent Document 4 and found that there were issues with the adhesive's holding power, and that further improvements were required for use in more general-purpose applications.

Against this background, the objective of the present invention is to provide a heat-peelable adhesive composition that exhibits excellent adhesive properties during use and changes to weak adhesive properties upon heating, and an adhesive sheet comprising an adhesive layer formed by crosslinking the adhesive composition.

The present inventors conducted extensive research to solve the above problems, and as a result, discovered that the above problems can be achieved by using an adhesive composition having a specific composition, thereby completing the present invention. That is, the gist of the present invention is as follows: [1] to [7]
[1] A heat-peelable pressure-sensitive adhesive composition containing an acrylic resin (A), a compound having an ethylenically unsaturated group (B), an epoxy-based crosslinking agent (C), and a thermal polymerization initiator (D).
[2] The heat-peelable pressure-sensitive adhesive composition according to [1], wherein the acrylic resin (A) contains a carboxyl group-containing monomer as a copolymerization component.
[3] The heat-peelable pressure-sensitive adhesive composition according to [2], wherein the carboxy group-containing monomer is an acrylic acid monomer.
[4] The heat-peelable pressure-sensitive adhesive composition according to any one of [1] to [3], wherein the compound (B) having an ethylenically unsaturated group is a urethane (meth)acrylate compound.
[5] The heat-peelable pressure-sensitive adhesive composition according to any one of [1] to [4], wherein the thermal polymerization initiator (D) has a 10-hour half-life temperature of 80 to 130°C.
[6] A pressure-sensitive adhesive sheet comprising a pressure-sensitive adhesive layer formed by crosslinking the heat-peelable pressure-sensitive adhesive composition according to any one of [1] to [5].
[7] The pressure-sensitive adhesive sheet according to [6], wherein the 180° peel strength, measured using an SUS-BA plate as an adherend according to the method described in JIS Z0237:2009, is 10 N/25 mm or more after treatment at 60°C and 90% RH for 24 hours, and 2 N/25 mm or less after treatment at 150°C for 2 hours.

The heat-peelable adhesive composition of the present invention can produce an adhesive layer that exhibits excellent adhesive strength and holding power during use and can be easily peeled off by heating. This adhesive layer can be used to provide products made by joining and adhering multiple parts together, and when the product is collected and recycled, it can be easily separated and disassembled into individual parts using specified heating conditions. This can therefore significantly contribute to improving the recycling rates of various plastic and metal products.

The adhesive sheet of the present invention has the advantage that the adhesive layer can be used to efficiently join and bond components together.

The following describes in detail the embodiments of the present invention, but the present invention is not limited to these specific descriptions.

In the present invention, "(meth)acrylic" means acrylic or methacrylic, "(meth)acryloyl" means acryloyl or methacryloyl, and "(meth)acrylate" means acrylate or methacrylate, respectively.
In the present invention, the term "sheet" is not particularly distinguished from "film" or "tape," but is used to include these terms.
In the present invention, when an expression "X to Y" (X and Y are arbitrary numbers) is used, it means "X or more and Y or less" unless otherwise specified, and also includes the meaning of "preferably larger than X" or "preferably smaller than Y."

<Heat-peelable pressure-sensitive adhesive composition>
First, the heat-peelable pressure-sensitive adhesive composition of the present invention (hereinafter, sometimes simply referred to as "the pressure-sensitive adhesive composition") will be described.
The present pressure-sensitive adhesive composition contains an acrylic resin (A), a compound having an ethylenically unsaturated group (B), an epoxy-based crosslinking agent (C), and a thermal polymerization initiator (D), and each component will be described below.

[Acrylic resin (A)]
The acrylic resin (A) used in the present pressure-sensitive adhesive composition contains a (meth)acrylic acid alkyl ester monomer (a1) as a polymerization component as a main component, and preferably further contains a functional group-containing monomer (a2) as a copolymerization component. If necessary, it may also contain a carboxylic acid vinyl ester monomer (a3) or other copolymerizable monomer (a4) as a copolymerization component. When the acrylic resin (A) uses a functional group-containing monomer (a2) as a copolymerization component, this serves as a crosslinking point for the acrylic resin (A), further improving adhesion to the substrate or adherend. Furthermore, when a carboxylic acid vinyl ester monomer (a3) is used, this is preferred in terms of improving adhesion to the adherend and adhesive strength.

The (meth)acrylic acid alkyl ester monomer (a1) preferably has an alkyl group having a carbon number of usually 1 to 20, preferably 1 to 12, more preferably 1 to 8, and even more preferably 4 to 8. If the number of carbon atoms is too large, the adherend tends to be easily stained.
Specific examples include aliphatic alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, n-propyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, iso-octyl acrylate, isodecyl (meth)acrylate, lauryl (meth)acrylate, cetyl (meth)acrylate, stearyl (meth)acrylate, and i-stearyl (meth)acrylate;
Alicyclic (meth)acrylic acid esters such as cyclohexyl (meth)acrylate and isobornyl (meth)acrylate:
Examples include:
These may be used alone or in combination of two or more.

Among the above (meth)acrylic acid alkyl ester monomers (a1), n-butyl (meth)acrylate and 2-ethylhexyl (meth)acrylate are preferred in terms of copolymerizability, adhesive properties, ease of handling, and availability of raw materials.

The content of the (meth)acrylic acid alkyl ester monomer (a1) in the copolymerization components is preferably 30 to 99% by mass, particularly preferably 40 to 98% by mass, and even more preferably 50 to 95% by mass. If the content of the (meth)acrylic acid alkyl ester monomer (a1) is too low, the adhesive strength tends to decrease, while if it is too high, the adhesive strength tends to become too high.

The functional group-containing monomer (a2) may be any monomer containing a functional group that can become a crosslinking point upon reaction with the epoxy-based crosslinking agent (C) described below. Examples include hydroxyl group-containing monomers, carboxyl group-containing monomers, amino group-containing monomers, acetoacetyl group-containing monomers, isocyanate group-containing monomers, and glycidyl group-containing monomers. Among these, hydroxyl group-containing monomers and carboxyl group-containing monomers are preferred because they enable efficient crosslinking reactions and, in the present invention, reduce acid-resistant chemicals and adhesive residue. In particular, carboxyl group-containing monomers are more preferred in terms of effectiveness.

Examples of the hydroxyl group-containing monomer include hydroxyalkyl acrylates such as 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 5-hydroxypentyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, and 8-hydroxyoctyl (meth)acrylate; caprolactone-modified monomers such as caprolactone-modified 2-hydroxyethyl (meth)acrylate; oxyalkylene-modified monomers such as diethylene glycol (meth)acrylate and polyethylene glycol (meth)acrylate; and primary hydroxyl group-containing monomers such as 2-acryloyloxyethyl-2-hydroxyethyl phthalate.
hydroxyalkyl acrylate esters such as 2-hydroxypropyl (meth)acrylate and 2-hydroxybutyl (meth)acrylate, and secondary hydroxyl group-containing monomers such as 3-chloro 2-hydroxypropyl (meth)acrylate;
Tertiary hydroxyl group-containing monomers such as 2,2-dimethyl 2-hydroxyethyl (meth)acrylate:
Examples include:

Of the above hydroxyl group-containing monomers, 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate are preferably used because of their excellent reactivity with crosslinking agents.

When using a hydroxyl group-containing monomer in the present invention, it is preferable to use one that contains the impurity di(meth)acrylate in an amount of 0.5% by mass or less, more preferably 0.2% by mass or less, and even more preferably 0.1% by mass or less. Specifically, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and 4-hydroxybutyl acrylate are preferred.

Examples of the carboxyl group-containing monomer include (meth)acrylic acid, acrylic acid dimer, crotonic acid, maleic acid, maleic anhydride, fumaric acid, citraconic acid, glutaconic acid, itaconic acid, acrylamido-N-glycolic acid, and cinnamic acid, with (meth)acrylic acid being preferred.

Examples of the amino group-containing monomer include t-butylaminoethyl (meth)acrylate, ethylaminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, and diethylaminoethyl (meth)acrylate.

Examples of the acetoacetyl group-containing monomer include 2-(acetoacetoxy)ethyl (meth)acrylate and allyl acetoacetate.

Examples of the isocyanate group-containing monomer include 2-acryloyloxyethyl isocyanate, 2-methacryloyloxyethyl isocyanate, and alkylene oxide adducts thereof.

Examples of the glycidyl group-containing monomer include glycidyl (meth)acrylate and allyl glycidyl (meth)acrylate.

These functional group-containing monomers (a2) may be used alone or in combination of two or more.

The content of the functional group-containing monomer (a2) in the copolymerization components is preferably 0.1 to 30% by mass, particularly preferably 0.5 to 20% by mass, and even more preferably 1 to 15% by mass. If the content of functional group-containing monomer (a2) is too high, the viscosity of the acrylic resin (A) tends to be too high, and the adhesive strength immediately after application tends to be too high. If the content is too low, the degree of crosslinking tends to decrease, and the adherend tends to be easily contaminated.

Examples of the vinyl carboxylic acid ester monomer (a3) include vinyl acetate, vinyl propionate, vinyl stearate, and vinyl benzoate. Among these, vinyl acetate is preferred in terms of adhesion and cohesive strength to the adherend. These vinyl carboxylic acid ester monomers (a3) may be used alone or in combination of two or more.

The content of the carboxylic acid vinyl ester monomer (a3) is preferably 0 to 20% by mass, more preferably 0 to 10% by mass, and particularly preferably 0 to 7% by mass, based on the total copolymerization components. If the content of the carboxylic acid vinyl ester monomer (a3) is too high, it tends to dissolve when immersed in hydrofluoric acid. Therefore, for applications requiring chemical resistance, it is preferable not to include the carboxylic acid vinyl ester monomer (a3).

Examples of the other copolymerizable monomer (a4) include:
Monomers containing one aromatic ring, such as phenyl (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, phenyldiethylene glycol (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, styrene, and α-methylstyrene;
(meth)acrylic acid ester monomers containing a biphenyloxy structure, such as biphenyloxyethyl (meth)acrylate;
(meth)acrylamide-based monomers such as ethoxymethyl(meth)acrylamide, n-butoxymethyl(meth)acrylamide, (meth)acryloylmorpholine, dimethyl(meth)acrylamide, diethyl(meth)acrylamide, dimethylaminopropylacrylamide, and (meth)acrylamide N-methylol(meth)acrylamide;
Monomers containing an alkoxy group or an oxyalkylene group, such as 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, methoxydiethylene glycol (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, and polypropylene glycol mono(meth)acrylate;
Acrylonitrile, methacrylonitrile, vinyl chloride, vinylidene chloride, alkyl vinyl ether, vinyl toluene, vinyl pyridine, vinyl pyrrolidone, itaconic acid dialkyl ester, fumaric acid dialkyl ester, allyl alcohol, acrylic chloride, methyl vinyl ketone, allyl trimethyl ammonium chloride, dimethyl allyl vinyl ketone, etc.:
Examples include:

The content of the other copolymerizable monomer (a4) in the copolymerization components is preferably 0 to 40% by mass, particularly preferably 0 to 30% by mass, and even more preferably 0 to 25% by mass. If the amount of other copolymerizable monomer (a4) is too high, the adhesive properties tend to be reduced.

In the present invention, the (meth)acrylic resin (A) is produced by polymerizing the above-mentioned (meth)acrylic acid alkyl ester monomer (a1), preferably the functional group-containing monomer (a2), and, if necessary, other copolymerizable monomers (a4), etc., as copolymerization components. This polymerization can be carried out by a conventionally known method such as solution radical polymerization, suspension polymerization, bulk polymerization, or emulsion polymerization, and can be selected as appropriate. Among these, solution radical polymerization is preferred because it allows for the safe and stable production of (meth)acrylic resin (A) with any monomer composition.

In the solution radical polymerization, for example, monomer components such as (meth)acrylic acid alkyl ester monomer (a1), functional group-containing monomer (a2), and other copolymerizable monomers (a4), as well as a polymerization initiator, are mixed or added dropwise to an organic solvent, and polymerization is carried out under reflux or typically at 50 to 98°C for approximately 0.1 to 20 hours.

Examples of organic solvents used in the polymerization reaction include aromatic hydrocarbons such as toluene and xylene, aliphatic hydrocarbons such as hexane, esters such as ethyl acetate and butyl acetate, aliphatic alcohols such as n-propyl alcohol and isopropyl alcohol, and ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. These organic solvents can be used alone or in combination.

Among these organic solvents, esters such as ethyl acetate and butyl acetate, and ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone are preferred from the standpoints of ease of polymerization reaction, chain transfer effect, ease of drying when applying the adhesive composition, and safety, and ethyl acetate is particularly preferred.

In addition, examples of polymerization initiators used in such solution radical polymerization include azo initiators such as 2,2'-azobisisobutyronitrile, 2,2'-azobis-2-methylbutyronitrile, 4,4'-azobis(4-cyanovaleric acid), and 2,2'-azobis(methylpropionic acid), which are common radical polymerization initiators, and organic peroxides such as benzoyl peroxide, lauroyl peroxide, di-t-butyl peroxide, and cumene hydroperoxide. These polymerization initiators can be selected appropriately depending on the monomers used. These polymerization initiators can be used alone or in combination of two or more.

In this way, the acrylic resin (A) used in the present invention can be obtained.

The weight-average molecular weight (Mw) of the acrylic resin (A) is preferably 100,000 to 3,000,000, particularly preferably 150,000 to 1,000,000, and even more preferably 200,000 to 800,000. If the weight-average molecular weight is too small, the toughness and cohesive strength of the resulting adhesive layer tend to decrease, and the transferability, holding power, and shear strength after curing tend to decrease. Furthermore, if the weight-average molecular weight is too large, the viscosity tends to become too high, resulting in increased scaling during polymerization, reduced compatibility with other components, and reduced handleability.

The dispersity of the acrylic resin (A) [weight average molecular weight (Mw)/number average molecular weight (Mn)] is preferably 10 or less, particularly preferably 7 or less, and even more preferably 5.5 or less. If the dispersity is too high, the cohesive strength tends to decrease. The lower limit of the dispersity is usually 1.

The weight average molecular weight of the acrylic resin (A) is a weight average molecular weight converted to standard polystyrene molecular weight, and is measured using a high performance liquid chromatograph (manufactured by Waters Japan, "Waters 2695 (main body)" and "Waters 2414 (detector)") equipped with three columns: Shodex GPC KF-806L (exclusion limit molecular weight: 2 x 10 ⁷ , separation range: 100 to 2 x 10 ⁷ , number of theoretical plates: 10,000 plates/column, filler material: styrene-divinylbenzene copolymer, filler particle size: 10 μm) connected in series; the number average molecular weight can also be measured in a similar manner. The dispersity can be determined from the measured values of the weight average molecular weight and number average molecular weight.

The glass transition temperature (Tg) of the acrylic resin (A) is preferably -70 to 10°C, more preferably -60 to 0°C, and particularly preferably -50 to -10°C. If the glass transition temperature of the acrylic resin (A) is too low, the adhesive strength after heating tends not to decrease sufficiently, and if it is too low, the adherend tends to be more susceptible to contamination.

The glass transition temperature (Tg) is a value calculated by applying the glass transition temperature and weight fraction of each of the monomers constituting the acrylic resin (A) to the following Fox's formula:
Here, the glass transition temperature when the monomers constituting the acrylic resin (A) are made into a homopolymer is usually measured by a differential scanning calorimeter (DSC), and can be measured by a method in accordance with JIS K 7121-1987 or JIS K 6240.

[Compound (B) having an ethylenically unsaturated group]
The compound (B) having an ethylenically unsaturated group (hereinafter sometimes simply referred to as "ethylenically unsaturated compound") used in the present pressure-sensitive adhesive composition is not particularly limited as long as it has an ethylenically unsaturated group, but in order to obtain a composition with excellent peeling properties after heating, the number of ethylenically unsaturated groups per molecule is usually preferably 2 to 15, more preferably 3 to 12, and particularly preferably 4 to 10. If the number of ethylenically unsaturated groups is too high, the crosslinking density after heating will tend to be too high, making the pressure-sensitive adhesive layer more susceptible to cracking, whereas if the number is too low, sufficient crosslinking density will not be obtained, making peeling after heating more difficult.

The ethylenically unsaturated compound (B) is preferably a (meth)acrylate compound in terms of compatibility with the acrylic resin (A), and is particularly preferably a urethane (meth)acrylate compound (B1).

The urethane (meth)acrylate compound (B1) may be a urethane (meth)acrylate compound (b1*2) which is a reaction product of a hydroxyl group-containing (meth)acrylate compound (b1) and a polyvalent isocyanate compound (b2), or a urethane (meth)acrylate compound (b1*2*3) which is a reaction product of a hydroxyl group-containing (meth)acrylate compound (b1), a polyvalent isocyanate compound (b2), and a polyol compound (b3). Among these, in the present invention, it is preferable to use a urethane (meth)acrylate compound (b1*2) in terms of releasability after heating.

Examples of the ethylenically unsaturated compound (B2) other than the urethane (meth)acrylate compound (B1) include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, butylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, ethylene oxide-modified bisphenol A di(meth)acrylate, propylene oxide-modified bisphenol A di(meth)acrylate, cyclohexane dimethacrylate, methyl ... compounds having two ethylenically unsaturated groups, such as ethanol di(meth)acrylate, ethoxylated cyclohexanedimethanol di(meth)acrylate, dimethyloldicyclopentane di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, glycerin di(meth)acrylate, pentaerythritol di(meth)acrylate, ethylene glycol diglycidyl ether di(meth)acrylate, diethylene glycol diglycidyl ether di(meth)acrylate, phthalic acid diglycidyl ester di(meth)acrylate, hydroxypivalic acid-modified neopentyl glycol di(meth)acrylate, and isocyanuric acid ethylene oxide-modified diacrylate;
compounds having three ethylenically unsaturated groups, such as trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, tri(meth)acryloyloxyethoxytrimethylolpropane, ethylene oxide isocyanuric acid modified triacrylate, caprolactone modified pentaerythritol tri(meth)acrylate, ethylene oxide modified pentaerythritol tri(meth)acrylate, and ethoxylated glycerin triacrylate;
compounds having four or more ethylenically unsaturated groups, such as pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, glycerin polyglycidyl ether poly(meth)acrylate, caprolactone-modified dipentaerythritol penta(meth)acrylate, caprolactone-modified dipentaerythritol hexa(meth)acrylate, caprolactone-modified pentaerythritol tetra(meth)acrylate, ethylene oxide-modified dipentaerythritol penta(meth)acrylate, ethylene oxide-modified dipentaerythritol hexa(meth)acrylate, and ethylene oxide-modified pentaerythritol tetra(meth)acrylate;
etc.

These ethylenically unsaturated compounds (B) may be used alone or in combination of two or more. The content of the ethylenically unsaturated compound (B) is typically 5 to 100 parts by mass, preferably 10 to 80 parts by mass, and particularly preferably 20 to 60 parts by mass, per 100 parts by mass of the acrylic resin (A). If the content of the ethylenically unsaturated compound (B) is too low, peeling after heating tends to be difficult, while if it is too high, the stain resistance of the adherend after peeling tends to decrease.

[Epoxy-based crosslinking agent (C)]
The epoxy-based crosslinking agent (C) used in the present pressure-sensitive adhesive composition reacts with the functional groups in the acrylic resin (A) to form a crosslinked structure.

Examples of the epoxy-based crosslinking agent (C) include bisphenol A-epichlorohydrin epoxy resins, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerin diglycidyl ether, glycerin triglycidyl ether, 1,6-hexanediol diglycidyl ether, trimethylolpropane triglycidyl ether, sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, pentaerythritol polyglycidyl erythritol, diglycerol polyglycidyl ether, N,N,O-triglycidyl-p-aminophenol, 1,3'-bis(N,N-diglycidylaminomethyl)cyclohexane, and N,N,N',N'-tetraglycidyl-m-xylylenediamine.

Among these, epoxy-based crosslinking agents containing nitrogen atoms, such as N,N,O-triglycidyl-p-aminophenol, 1,3'-bis(N,N-diglycidylaminomethyl)cyclohexane, and N,N,N',N'-tetraglycidyl-m-xylenediamine, are preferred because they facilitate the smooth formation of crosslinked structures, with 1,3'-bis(N,N-diglycidylaminomethyl)cyclohexane and N,N,N',N'-tetraglycidyl-m-xylenediamine being particularly preferred.

The epoxy-based crosslinking agent (C) may be used alone or in combination of two or more types. Its content is typically preferably 0.001 to 30 parts by mass, more preferably 0.01 to 20 parts by mass, and particularly preferably 0.03 to 15 parts by mass, per 100 parts by mass of the acrylic resin (A) and the ethylenically unsaturated compound (B) combined. If the amount of epoxy-based crosslinking agent (C) is too small, the cohesive strength when forming an adhesive layer from the adhesive composition will decrease, which tends to cause adhesive residue. If the amount is too large, the flexibility and adhesive strength of the formed adhesive layer will decrease, and it will tend to separate from the adherend.

[Thermal polymerization initiator (D)]
The thermal polymerization initiator (D) used in the present pressure-sensitive adhesive composition may be any one that initiates a curing reaction by heating, and examples thereof include methyl ethyl ketone peroxide, cyclohexanone peroxide, methylcyclohexanone peroxide, methylacetoacetate peroxide, acetylacetate peroxide, 1,1-bis(t-hexylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-hexylperoxy)-cyclohexane, 1,1-bis(t-butylperoxide), 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)-2-methylcyclohexane, 1,1-bis(t-butylperoxy)-cyclohexane, 1,1-bis(t-butylperoxy)cyclododecane, 1,1-bis(t-butylperoxy)butane, 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane, p-menthane hydroperoxide, diisopropylbenzene hydroperoxide, 1,1,3,3-tetramethylbutane Tyl hydroperoxide, cumene hydroperoxide, t-hexyl hydroperoxide, t-butyl hydroperoxide, t-butyl hydroperoxymalonic acid, α,α'-bis(t-butylperoxy)diisopropylbenzene, dicumyl peroxide, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane, t-butylcumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexyne-3 , isobutyryl peroxide, 3,5,5-trimethylhexanoyl peroxide, octanoyl peroxide, lauroyl peroxide, stearoyl peroxide, succinic acid peroxide, m-toluoylbenzoyl peroxide, benzoyl peroxide, di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, bis(4-t-butylcyclohexyl) peroxydicarbonate, di-2-ethoxyethyl peroxydicarbonate , di-2-ethoxyhexyl peroxydicarbonate, di-3-methoxybutyl peroxydicarbonate, di-s-butyl peroxydicarbonate, di(3-methyl-3-methoxybutyl)peroxydicarbonate, α,α'-bis(neodecanoylperoxy)diisopropylbenzene, cumyl peroxyneodecanoate, 1,1,3,3-tetramethylbutyl peroxyneodecanoate, 1-cyclohexyl-1-methylethyl peroxyneodecanoate, t-hexyl peroxyneodecanoate peroxyneodecanoate, t-butylperoxyneodecanoate, t-hexylperoxypivalate, t-butylperoxypivalate, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, 2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexanoate, 1-cyclohexyl-1-methylethylperoxy-2-ethylhexanoate, t-hexylperoxy-2-ethylhexanoate, t-butylperoxy-2-ethylhexanoate xanoate, t-hexylperoxyisopropyl monocarbonate, t-butylperoxyisobutyrate, t-butylperoxymalate, t-butylperoxy-3,5,5-trimethylhexanoate, t-butylperoxylaurate, t-butylperoxyisopropyl monocarbonate, t-butylperoxy-2-ethylhexyl monocarbonate, t-butylperoxyacetate, t-butylperoxy-m-toluylbenzoate, t-butylperoxybenzoate organic peroxide initiators such as t-butylperoxy, bis(t-butylperoxy)isophthalate, 2,5-dimethyl-2,5-bis(m-toluylperoxy)hexane, t-hexylperoxybenzoate, 2,5-dimethyl-2,5-bis(benzoylperoxy)hexane, t-butylperoxyallyl monocarbonate, t-butyltrimethylsilyl peroxide, 3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, and 2,3-dimethyl-2,3-diphenylbutane;
2-Phenylazo-4-methoxy-2,4-dimethylvaleronitrile, 1-[(1-cyano-1-methylethyl)azo]formamide, 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis(2-methyl-N-phenylpropionamidine) dihydrochloride, 2,2'-azobis[N-(4-chlorophenyl)-2-methylpropionamidine]dihydridochloride , 2,2'-azobis[N-(4-hydrophenyl)-2-methylpropionamidine]dihydrochloride, 2,2'-azobis[2-methyl-N-(phenylmethyl)propionamidine]dihydrochloride, 2,2'-azobis[2-methyl-N-(2-propenyl)propionamidine]dihydrochloride, 2,2'-azobis[N-(2-hydroxyethyl)-2-methylpropionamidine]dihydrochloride, 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2' -Azobis[2-(4,5,6,7-tetrahydro-1H-1,3-diazepin-2-yl)propane]dihydrochloride, 2,2'-azobis[2-(3,4,5,6-tetrahydropyrimidin-2-yl)propane]dihydrochloride, 2,2'-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidin-2-yl)propane]dihydrochloride, 2,2'-azobis[2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane]dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane], 2,2'-azobis[2-methyl-N-[1,1-bis( azo initiators such as 2,2'-azobis[2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide], 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide], 2,2'-azobis(2-methylpropionamide), 2,2'-azobis(2,4,4-trimethylpentane), 2,2'-azobis(2-methylpropane), dimethyl-2,2-azobis(2-methylpropionate), 4,4'-azobis(4-cyanopentanoic acid), and 2,2'-azobis[2-(hydroxymethyl)propionitrile];
etc.

Of these, organic peroxide initiators are preferred because they generate less gas when heated. These thermal polymerization initiators (D) may be used alone or in combination of two or more.

The reaction temperature and reaction time during curing by heating vary depending on the type of thermal polymerization initiator (D). Therefore, it is generally preferable to use one with a 10-hour half-life temperature of 70 to 150°C, and it is even more preferable to use one with a 10-hour half-life temperature of 80 to 130°C. In other words, one with a 10-hour half-life temperature that is too low tends to be less stable in actual use, while one with a 10-hour half-life temperature that is too high tends to make it difficult to facilitate peeling by heating. Therefore, for example, 1,1'-di-(t-hexylperoxy)cyclohexane (87°C), 1,1-di(t-butylperoxy)cyclohexane (91°C), t-hexylperoxyisopropyl monocarbonate (95°C), t-butylperoxymalonate (96°C), t-butylperoxybenzoate (99°C), t-butylperoxyacetate (102°C), t-butylbenzoyl peroxide (104°C), dicumyl peroxide (120°C), di-t-butyl peroxide (128°C), diisopropylbenzene hydroperoxide (145°C), etc. are suitable. Note that the temperature in parentheses indicates the 10-hour half-life temperature.

The content of the thermal polymerization initiator (D) is typically preferably 0.01 to 20 parts by mass, more preferably 0.1 to 15 parts by mass, and particularly preferably 0.2 to 10 parts by mass, per 100 parts by mass of the ethylenically unsaturated compound (B). If the amount of the thermal polymerization initiator (D) is too small, curing tends to be insufficient and the physical properties tend to become unstable, while if the amount is too large, no additional effect tends to be obtained.

[Other ingredients]
The present pressure-sensitive adhesive composition may further contain, within the scope of the effects of the present invention, additives such as a resin other than the acrylic resin (A), a photopolymerization initiator, an antistatic agent, an antioxidant, a plasticizer, a filler, a pigment, a diluent, an antioxidant, an ultraviolet absorber, an ultraviolet stabilizer, and a tackifier. These additives may be used alone or in combination of two or more. Antioxidants are particularly effective in maintaining the stability of the adhesive layer. When an antioxidant is added, its content is not particularly limited, but is preferably 0.01 to 5% by mass. In addition to the additives, the present pressure-sensitive adhesive composition may also contain small amounts of impurities contained in the raw materials used to produce the components of the pressure-sensitive adhesive composition.

In this way, the present pressure-sensitive adhesive composition can be obtained by mixing an acrylic resin (A), an ethylenically unsaturated compound (B), an epoxy-based crosslinking agent (C), a thermal polymerization initiator (D), and, if necessary, appropriate optional components.

The adhesive sheet of the present invention (hereinafter sometimes referred to as "the adhesive sheet") can be obtained by crosslinking the adhesive composition to form an adhesive layer. The adhesive sheet is useful as an adhesive sheet that is intended to be used by bonding various adhesion target members (adherends) together, such as metal plates, plastic plates, and semiconductor wafers, and then peeled off after use. It is also useful as a protective sheet that temporarily covers and protects the surface of specific members, and as a temporary adhesive sheet for temporarily bonding (adhering) specific parts together during the manufacturing process. The adhesive sheet is described below.

The present pressure-sensitive adhesive sheet is generally composed of a substrate sheet, a pressure-sensitive adhesive layer made of the present pressure-sensitive adhesive composition, and a release film.
The present pressure-sensitive adhesive sheet can be obtained, for example, as follows. First, the present pressure-sensitive adhesive composition is applied directly onto a release film or a substrate sheet, either as is or after adjusting the concentration with an appropriate organic solvent to increase fluidity. The composition is then dried, for example, by heat treatment at 80 to 105°C for 0.5 to 10 minutes, and integrated with the substrate sheet or release film as an adhesive layer to obtain a pressure-sensitive adhesive sheet. Furthermore, in order to balance the adhesive properties, further aging may be performed after drying.

The aging conditions are typically room temperature (23°C) to 70°C, and the time is typically 1 to 30 days. Specifically, the aging can be performed under conditions such as 1 to 20 days at 23°C, 3 to 10 days at 23°C, or 1 to 7 days at 40°C.

Examples of the material for the base sheet include polyester resins such as polyethylene naphthate, polyethylene terephthalate, polybutylene terephthalate, and polyethylene terephthalate/isophthalate copolymers;
Polyolefin resins such as polyethylene, polypropylene, and polymethylpentene;
Polyfluoroethylene resins such as polyvinyl fluoride, polyvinylidene fluoride, and polyethylene fluoride;
Polyamides such as nylon 6 and nylon 6,6;
vinyl polymers such as polyvinyl chloride, polyvinyl chloride/vinyl acetate copolymer, ethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, polyvinyl alcohol, and vinylon;
Cellulose-based resins such as cellulose triacetate and cellophane;
acrylic resins such as polymethyl methacrylate, polyethyl methacrylate, polyethyl acrylate, and polybutyl acrylate;
Polystyrene, polycarbonate, polyarylate, polyimide:
Further examples include metal foils such as aluminum, copper, and iron, paper such as fine paper and glassine paper, and woven and nonwoven fabrics made of glass fiber, natural fiber, synthetic fiber, and the like.

These substrate sheets can be used as a single layer or as a multi-layer structure in which two or more types are laminated. Among these, synthetic resin sheets are preferred from the standpoint of lightweight properties, etc.

Furthermore, the release film may be, for example, any of the various synthetic resin sheets, paper, woven fabrics, nonwoven fabrics, etc. exemplified above as the base sheet, which have been subjected to a release treatment.

The method for applying the pressure-sensitive adhesive composition is not particularly limited as long as it is a common application method, and examples include roll coating, die coating, gravure coating, comma coating, and screen printing.

The thickness of the adhesive layer of this adhesive sheet is generally preferably 10 to 200 μm, and more preferably 15 to 100 μm.

This pressure-sensitive adhesive sheet exhibits releasability as it hardens upon heating. The heating conditions are typically a heating temperature of 100 to 250°C, and preferably 130 to 200°C. If the heating temperature is too low, releasability tends to decrease, while if the heating temperature is too high, no difference in releasability tends to be observed, which is undesirable from the perspective of energy costs. Furthermore, the heating time is typically 0.25 to 10 hours, and preferably 0.5 to 5 hours. If the heating time is too short, releasability tends to decrease, while if the heating time is too long, no difference in releasability tends to be observed, which is undesirable from the perspective of energy costs.

The adhesive strength of the pressure-sensitive adhesive sheet varies depending on the type of base sheet, the type of adherend, etc., but in order to achieve heat-peelability, it is preferable that the 180° peel strength, measured using an SUS-BA plate as the adherend according to the method described in JIS Z0237:2009, be 10 N/25 mm or more after 24 hours of treatment at 60°C and 90% RH, and 2 N/25 mm or less after 2 hours of treatment at 150°C. In particular, it is more effective for the 180° peel strength to be 20 N/25 mm or more after 24 hours of treatment at 60°C and 90% RH, and 1 N/25 mm or less after 2 hours of treatment at 150°C.

This adhesive sheet has an adhesive layer formed by crosslinking the adhesive composition, which has excellent heat-peelability. Therefore, after laminating this adhesive sheet to an adherend to temporarily protect the surface of the adherend or to another member, heating it as needed hardens the adhesive layer, reducing its adhesive strength and allowing it to be easily peeled off from the adherend.

In particular, if this adhesive sheet is used to bond parts of a product that combines and joins multiple parts made of different materials together with the intention of recycling, when recycling this product, the individual parts can be easily separated, disassembled, and sorted by material simply by heating the bonded areas, making it extremely useful for improving recycling rates.

The following provides a detailed explanation of examples of the present invention, along with comparative examples. However, the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. Note that in the examples, "%" and "parts" are based on mass.

First, the ingredients were prepared as follows:

[Preparation of acrylic resin (A-1)]
A reactor equipped with a temperature controller, thermometer, stirrer, dropping funnel, and reflux condenser was charged with 68 parts of ethyl acetate, and the temperature was raised while stirring. When the internal temperature stabilized at 78 ° C., a mixture of 70 parts of n-butyl acrylate as copolymerization components, 20 parts of methyl methacrylate, 0.1 parts of 2-hydroxyethyl methacrylate, 9.9 parts of acrylic acid, and 0.035 parts of azobisisobutyronitrile (AIBN) as a polymerization catalyst was added dropwise over 2 hours and allowed to react under reflux. Next, 3 hours after the start of the reaction, a solution of 23 parts of ethyl acetate and 0.034 parts of AIBN was added, and 5 hours after the start of the reaction, 7.6 parts of ethyl acetate was added to terminate the reaction, and an acrylic resin (A-1) solution [glass transition temperature -24.2 ° C., resin content 35.0%, viscosity 8,000 mPa s (25 ° C.)] was obtained.

[Ethylenically unsaturated compound (B-1)]
Urethane acrylate compound: reaction product of isophorone diisocyanate and dipentaerythritol pentaacrylate [ethylenically unsaturated groups: 10, weight average molecular weight: 2,300]

[Epoxy-based crosslinking agent (C-1)]
Tetrad C (Mitsubishi Gas Chemical Company, Inc.)

[Thermal polymerization initiator (D-1)]
Perbutyl Z (NOF Corporation, 10-hour half-life temperature 104°C)

[Isocyanate-based crosslinking agent (E-1)]
Coronate L (manufactured by Tosoh Corporation)

Example 1
[Preparation of Pressure-Sensitive Adhesive Composition]
A pressure-sensitive adhesive composition was obtained by mixing 67 parts of the acrylic resin (A-1) calculated as solid content, 33 parts of an ethylenically unsaturated compound (B-1), 0.05 parts of an epoxy-based crosslinking agent (C-1), and 1 part of a thermal polymerization initiator (D-1), and diluting the mixture with ethyl acetate to a solid content of 40%.

[Preparation of adhesive sheet]
The obtained pressure-sensitive adhesive composition was applied to a substrate sheet (highly adhesive polyethylene terephthalate film, film thickness 125 μm, manufactured by Toyobo Co., Ltd.: A4360 Cosmoshine) using an applicator, and then dried at 100°C for 2 minutes. Thereafter, the composition was attached to a release film (manufactured by Mitsui Chemicals Tocello Inc.: SP-PET 38 01-BU) and aged at 40°C for 3 days to obtain a pressure-sensitive adhesive sheet (adhesive layer thickness 25 μm).

Example 2
A pressure-sensitive adhesive composition and a pressure-sensitive adhesive sheet were obtained in the same manner as in Example 1, except that the amount of the thermal polymerization initiator (D-1) in Example 1 was changed to 2 parts.

<Comparative Example 1>
A pressure-sensitive adhesive composition and a pressure-sensitive adhesive sheet were obtained in the same manner as in Example 1, except that the epoxy-based crosslinking agent (C1) in Example 1 was replaced with 2 parts of an isocyanate-based crosslinking agent (E1).

<Comparative Example 2>
A pressure-sensitive adhesive composition and a pressure-sensitive adhesive sheet were obtained in the same manner as in Example 1, except that the thermal polymerization initiator (D-1) was not added.

For each of the adhesive sheets obtained in this manner in Examples 1 and 2 and Comparative Examples 1 and 2, the 180° peel test strength and holding power of the adhesive layer were measured according to the following procedures, and the results are shown in Table 1 below.

[180° peel strength]
(1) Adhesion of adhesive sheet to SUS-BA plate
A 25 mm × 100 mm test piece was prepared from the obtained pressure-sensitive adhesive sheet, and after peeling off the release film, the test piece was pressed and attached to a stainless steel plate (SUS304BA plate) by moving a 2 kg rubber roller back and forth twice in an atmosphere of 23°C and 50% relative humidity, and then aged for 3 days in the same atmosphere to obtain a pressure-sensitive adhesive sheet (Q) adhered to the SUS-BA plate.
(2) Peel test: 23°C
The pressure-sensitive adhesive sheet (Q) was subjected to a 180° peel test in accordance with JIS Z0237:2009 to measure the peel strength.
(3) Peel test: 60°C treatment After aging the pressure-sensitive adhesive sheet (Q) for one day in an environment of 60°C/90% RH, the temperature was returned to 23°C and a 180° peel test was carried out to measure the peel strength.
(4) Peel test: 150°C treatment Similarly, the pressure-sensitive adhesive sheet (Q) was aged for 2 hours in an environment of 150°C, and then returned to 23°C and subjected to a 180° peel test, and the peel strength was measured.
In each peel test, it was visually confirmed whether or not there was any adhesive residue on the SUS-BA plate after peeling.

[Holding power]
The pressure-sensitive adhesive sheet (Q) was subjected to a holding power test in accordance with JIS Z0237:2009 (1 kg load, 40° C.) to measure the holding power of the pressure-sensitive adhesive layer to a SUS-BA plate.

From the above results, it can be seen that the pressure-sensitive adhesive sheets of Examples 1 and 2 both exhibit excellent heat peeling properties, and the adhesive layer in the bonded state has sufficient holding power.
On the other hand, Comparative Example 1, which does not contain the epoxy-based crosslinking agent (C), has a problem with holding power, and Comparative Example 2, which does not contain the thermal polymerization initiator (D), has a problem with lack of easy peelability after heating.

The present invention can be suitably used as a heat-peelable adhesive composition that exhibits strong adhesive properties when in use and changes to weak adhesive properties when heated, and as an adhesive sheet comprising an adhesive layer formed by crosslinking the adhesive composition.

Claims (7)

A heat-peelable pressure-sensitive adhesive composition containing an acrylic resin (A), a compound having an ethylenically unsaturated group (B), an epoxy-based crosslinking agent (C), and a thermal polymerization initiator (D). The heat-peelable pressure-sensitive adhesive composition according to claim 1, wherein the acrylic resin (A) contains a carboxyl group-containing monomer as a copolymerization component. The heat-peelable pressure-sensitive adhesive composition according to claim 2, wherein the carboxy group-containing monomer is an acrylic acid monomer. The heat-peelable pressure-sensitive adhesive composition according to any one of claims 1 to 3, wherein the compound (B) having an ethylenically unsaturated group is a urethane (meth)acrylate compound. The heat-peelable pressure-sensitive adhesive composition according to any one of claims 1 to 3, wherein the thermal polymerization initiator (D) has a 10-hour half-life temperature of 80 to 130°C. A pressure-sensitive adhesive sheet comprising an adhesive layer formed by crosslinking the heat-peelable pressure-sensitive adhesive composition according to any one of claims 1 to 3. The pressure-sensitive adhesive sheet according to claim 6, wherein the 180° peel strength, measured using an SUS-BA plate as an adherend according to the method specified in JIS Z0237:2009, is 10 N/25 mm or more after treatment at 60°C and 90% RH for 24 hours, and 2 N/25 mm or less after treatment at 150°C for 2 hours.