Adhesive composition and adhesive sheet

Abstract

To provide an adhesive composition capable of being used for an adhesive layer of an adhesive film which has excellent adhesive properties regardless of the polarity of an adherend, can be easily peeled off from the surface of the adherend, hardly causes stain when peeled off and can maintain adhesive strength over a long period of time.SOLUTION: There is provided an adhesive composition comprising an acrylic copolymer (A) and a curing agent (B), wherein the content of a (meth)acrylic acid alkyl ester monomer (a1) having an alkyl group having a carbon number of 1 to 3 in 100 mass% of a monomer constituting the acrylic copolymer (A) is 20 mass% or less, the content of a (meth)acrylic acid ester (a2) having an alkyl group having a carbon number of 12 to 14 without branching is 70 mass% or more and 95 mass% or less and the content of a hydroxy group-containing monomer (a3) is 0.01 mass% or more and 0.8 mass% or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a pressure-sensitive adhesive composition and a pressure-sensitive adhesive sheet. [Background technology]

[0002] There are various resin-based adhesives, and among them, acrylic adhesives are widely used because their basic properties such as adhesive strength and tack can be controlled by the combination of monomer composition and curing agent. Adhesive sheets having an adhesive layer formed from an adhesive are easy to handle and are used in a wide range of fields as labels and adhesive applications. Adhesive sheets have traditionally been required to have adhesion to highly polar adherends, but in recent years, they are required to have higher performance, such as adhesion to low polar adherends and not contaminating the adherend when peeled off. Patent Document 1 discloses a pressure-sensitive adhesive composition and a pressure-sensitive adhesive sheet, which contain an isocyanate-based curing agent and a (meth)acrylic copolymer, which is a copolymer of copolymer components containing an alkyl (meth)acrylate having a linear alkyl group with 8 to 12 carbon atoms and a functional group-containing monomer having a functional group. The pressure-sensitive adhesive sheet can be used as a temporary fixing pressure-sensitive adhesive film used for fixing to temporarily hold and reinforce products during the manufacturing process. However, the invention of Patent Document 1 has a problem that the adhesive strength to the adherend is insufficient, and furthermore, the adhesive strength cannot be maintained when the adhesive is attached to the adherend for a long period of time, both in high-polarity and low-polarity adherends. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2023-024312 A Summary of the Invention [Problem to be solved by the invention]

[0004] The problem that the present invention aims to solve is to provide a pressure-sensitive adhesive composition that has excellent adhesion (adhesive strength) regardless of the polarity of the adherend, can be easily peeled off from the surface of the adherend, is unlikely to cause contamination when peeled off (removable property), and can maintain adhesive strength over a long period of time (retentive strength), and a pressure-sensitive adhesive sheet using the same. [Means for solving the problem]

[0005] As a result of extensive investigations, the present inventors have found that the problems of the present invention can be solved in the following aspect, and have thus completed the present invention.

[0006] That is, the present invention is [1] The present invention relates to a pressure-sensitive adhesive composition comprising an acrylic copolymer (A) and a curing agent (B), wherein, relative to 100% by mass of monomers constituting the acrylic copolymer (A), the content of alkyl (meth)acrylate monomer (a1) having an alkyl group with 1 to 3 carbon atoms is 20% by mass or less, the content of unbranched alkyl (meth)acrylate monomer (a2) having an alkyl group with 12 to 14 carbon atoms is 70% by mass or more and 95% by mass or less, and the content of hydroxy group-containing monomer (a3) ​​is 0.01% by mass or more and 0.8% by mass or less. [2] The pressure-sensitive adhesive composition further comprises 1% by mass or more and 8% by mass or less of a carboxyl group-containing monomer (a4) based on 100% by mass of the monomers constituting the acrylic copolymer (A). [3] The present invention relates to the pressure-sensitive adhesive composition, wherein the curing agent (B) is an isocyanate-based curing agent. [4] The pressure-sensitive adhesive composition relates to the above, wherein the unbranched alkyl (meth)acrylate monomer (a2) having an alkyl group having 12 to 14 carbon atoms contains normal dodecyl acrylate and normal dodecyl methacrylate simultaneously. [5] The pressure-sensitive adhesive composition has a gel fraction of 50 to 95% by mass. [6] The present invention relates to a pressure-sensitive adhesive layer having a thickness of 3 to 150 μm and made of the pressure-sensitive adhesive composition. [7] The present invention relates to a pressure-sensitive adhesive sheet comprising the pressure-sensitive adhesive layer on one or both sides of a resin film. Effect of the Invention

[0007] The present invention can provide a pressure-sensitive adhesive composition and a pressure-sensitive adhesive sheet that have excellent adhesion regardless of the polarity of the adherend, can be easily peeled off from the surface of the adherend, is unlikely to cause contamination when peeled off, and can maintain adhesive strength for a long period of time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The pressure-sensitive adhesive composition and pressure-sensitive adhesive sheet according to the present invention will be described below in order. In this specification, the term "(meth)acrylate" refers to either acrylate or methacrylate, and the term "(meth)acryl" is equivalent to this. In this specification, the term "copolymer containing a monomer" means that the copolymer contains the monomer as a polymerization component, and has a structural unit derived from the monomer. The proportion of the monomer in the copolymer is based on the total amount of monomers constituting the copolymer (100% by mass). In this specification, the term "cured product of a pressure-sensitive adhesive composition" means that at least a portion of the components in the pressure-sensitive adhesive composition have undergone a crosslinking reaction, and for example, a gel-like product is also included in the cured product. Furthermore, the terms "film" and "sheet" are not differentiated by thickness. In other words, the term "sheet" in this specification includes thin film-like objects, and the term "film" in this specification includes thick sheet-like objects. Furthermore, the adherend refers to a counterpart to which the pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet is attached.

[0009] Unless otherwise noted, each of the various components appearing in this specification may be used independently, either alone or in combination of two or more. In addition, in this specification, a numerical range specified using "~" includes the numerical values ​​before and after "~" as the lower and upper limit values.

[0010] In addition, in this specification, the alkyl(meth)acrylate monomer (a1) having an alkyl group with 1 to 3 carbon atoms may be abbreviated as monomer (a1), the unbranched alkyl(meth)acrylate monomer (a2) having an alkyl group with 12 to 14 carbon atoms may be abbreviated as monomer (a2), the hydroxyl group-containing monomer (a3) ​​may be abbreviated as monomer (a3), the carboxyl group-containing monomer (a4) may be abbreviated as monomer (a4), and the other monomer (a5) may be abbreviated as monomer (a5).

[0011] <Adhesive composition> The pressure-sensitive adhesive composition according to the present invention (hereinafter also referred to as the present pressure-sensitive adhesive composition) is a pressure-sensitive adhesive composition containing an acrylic copolymer (A) and a curing agent (B) described below.

[0012] <Acrylic copolymer (A)> The acrylic copolymer is a copolymer of a monomer mixture, and in 100% by mass of the monomers constituting the acrylic copolymer (A) (same as in 100% by mass of the monomer mixture), the content of alkyl (meth)acrylate monomer (a1) having an alkyl group with 1 to 3 carbon atoms is 20% by mass or less, the content of unbranched alkyl (meth)acrylate monomer (a2) having an alkyl group with 12 to 14 carbon atoms is 70% by mass or more and 95% by mass or less, and the content of hydroxy group-containing monomer (a3) ​​is 0.01% by mass or more and 0.8% by mass or less. It is preferable that the monomer mixture further contains 1% by mass or more and 8% by mass or less of a carboxyl group-containing monomer (a4) based on 100% by mass of the monomer mixture.

[0013] [Alkyl (meth)acrylate monomer (a1) having an alkyl group having 1 to 3 carbon atoms] The content of the alkyl (meth)acrylate monomer (a1) having an alkyl group with 1 to 3 carbon atoms is 20% by mass or less in 100% by mass of the monomer mixture. By making the content of the monomer (a1) 20% by mass or less, it is possible to improve the adhesive strength to a highly polar adherend while maintaining the adhesive strength to a low polarity adherend. The content of the monomer (a1) may be 0% by mass, is preferably 3% by mass or more, and is more preferably 5% by mass or more and 15% by mass or less.

[0014] Examples of the monomer (a1) include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, etc. Among them, in terms of adhesive strength, methyl (meth)acrylate and ethyl (meth)acrylate are preferred, and methyl (meth)acrylate is more preferred.

[0015] [Alkyl (meth)acrylate monomer (a2) having an unbranched alkyl group having 12 to 14 carbon atoms] The content of the alkyl (meth)acrylate monomer (a2) having an unbranched alkyl group having 12 to 14 carbon atoms in 100% by mass of the monomer mixture is 70% by mass or more and 95% by mass or less, preferably 75% by mass or more and 95% by mass or less, and more preferably 80% by mass or more and 95% by mass or less. When the content of the monomer (a2) is 80% by mass or more and 95% by mass or less, the adhesive strength to a low-polarity adherend can be improved.

[0016] Examples of the monomer (a2) include normal dodecyl (meth)acrylate, normal tridecyl (meth)acrylate, and normal tetradecyl (meth)acrylate. An alkyl (meth)acrylate monomer having an unbranched alkyl group with 12 to 14 carbon atoms has better adhesion to a substrate than an alkyl (meth)acrylate monomer (a2) having a branched alkyl group with 12 to 14 carbon atoms. Although the reason for this is merely a hypothesis, it is believed that an alkyl (meth)acrylate monomer having an unbranched alkyl group with 12 to 14 carbon atoms has higher substrate permeability and a stronger anchoring effect on the substrate. In terms of improving adhesion to low polarity adherends and improving removability by imparting cohesive force to the acrylic copolymer, it is preferable to contain both normal dodecyl acrylate and normal dodecyl methacrylate. In terms of adhesion to high polarity adherends, it is more preferable that the content of normal dodecyl acrylate is greater than the content of normal dodecyl methacrylate.

[0017] [Hydroxy group-containing monomer (a3)] The content of the hydroxyl group-containing monomer (a3) ​​is 0.01 to 0.8% by mass, preferably 0.03 to 0.7% by mass, and more preferably 0.05 to 0.5% by mass, based on 100% by mass of the monomer mixture. By making the content of the hydroxyl group-containing monomer (a3) ​​0.01% by mass or more, the cohesive strength due to crosslinking can be ensured, and by making it 0.8% by mass or less, the decrease in adhesive strength after application over time can be reduced.

[0018] The monomer (a3) ​​having a hydroxy group is not limited as long as it is a monomer having a hydroxy group in the molecule, and specific examples thereof include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. Of these, 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate are preferred from the viewpoint of adhesive strength and cohesive strength.

[0019] [Carboxyl group-containing monomer (a4)] The carboxyl group-containing monomer (a4) is not limited as long as it is a monomer having one or more carboxyl groups in the molecule. The monomer having a carboxyl group imparts high adhesive strength and cohesive strength to a highly polar adherend, and when combined with a curing agent, it also serves as a crosslinking point with the curing agent. The number of carboxyl groups in one molecule of the carboxyl group-containing monomer is preferably 1 to 3, more preferably 1 or 2, and even more preferably 1, from the standpoint of the adhesiveness of the adhesive layer.

[0020] Examples of the monomer (a4) include carboxyalkyl (meth)acrylates such as 2-carboxyethyl (meth)acrylate; (meth)acrylic acid, fumaric acid, maleic acid, maleic anhydride, maleic acid monoalkyl ester, vinyl benzoic acid, etc., among which (meth)acrylic acid is preferred. The monomer having a carboxyl group may be used alone or in combination of two or more. When the monomer (a4) is used, the content of the monomer (a4) in 100% by mass of the monomer mixture is preferably 0.01 to 20% by mass, more preferably 0.1 to 10% by mass, and even more preferably 1 to 8% by mass. By setting the content within the above range, it is possible to impart high adhesive strength and cohesive strength to adherends with high polarity while reducing the difference in adhesive strength to adherends with low polarity.

[0021] [Other monomers (a5)] The acrylic copolymer may further contain another monomer (a5) that is not classified into the above monomers (a1) to (a4). Examples of the monomer (a5) include alkoxyalkyl (meth)acrylates such as methoxymethyl (meth)acrylate, 2-methoxyethyl (meth)acrylate, 3-methoxypropyl (meth)acrylate, 3-ethoxypropyl (meth)acrylate, 4-methoxybutyl (meth)acrylate, and 4-ethoxybutyl (meth)acrylate; Aminoalkyl (meth)acrylates such as aminomethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, and N,N-dimethylaminopropyl (meth)acrylate; Monomers having an amide group, such as (meth)acrylamide, N-methylol (meth)acrylamide, N,N-dimethyl (meth)acrylamide, diacetone (meth)acrylamide, N-vinylacetamide, N-vinylpyrrolidone, and 4-acryloylmorpholine; Alkyl (meth)acrylates having an alkyl group having 4 to 11 carbon atoms, such as normal butyl (meth)acrylate, normal pentyl (meth)acrylate, normal hexyl (meth)acrylate, normal pentyl (meth)acrylate, normal octyl (meth)acrylate, normal nonyl (meth)acrylate, normal decyl (meth)acrylate, and normal undecyl (meth)acrylate; Alkyl (meth)acrylates having a branched alkyl group having 12 to 14 carbon atoms, such as isododecyl (meth)acrylate, isotridecyl (meth)acrylate, and isotetradecyl (meth)acrylate; Alkyl (meth)acrylates having an alkyl group having 15 or more carbon atoms, such as normal pentadecyl (meth)acrylate, normal hexadecyl (meth)acrylate, normal heptadecyl (meth)acrylate, and normal octadecyl (meth)acrylate; Examples of the monomer include vinyl acetate, acrylonitrile, styrene, glycidyl (meth)acrylate, etc. These monomers may be used alone or in combination of two or more.

[0022] (Production of Acrylic Copolymer (A)) The acrylic copolymer (A) can be produced by polymerizing a monomer mixture containing the above-mentioned monomer (a1), monomer (a2), and monomer (a3), and, if necessary, monomer (a4) and / or monomer (a5). The polymerization can be carried out by known polymerization methods such as solution polymerization, bulk polymerization, emulsion polymerization, and suspension polymerization, but solution polymerization is preferred. The solvent used in the solution polymerization is preferably, for example, acetone, methyl acetate, ethyl acetate, toluene, xylene, anisole, methyl ethyl ketone, cyclohexanone, etc. The polymerization temperature is preferably a boiling point reaction at 60 to 120°C. The polymerization time is preferably about 5 to 12 hours.

[0023] The polymerization initiator used in the polymerization is preferably a radical polymerization initiator, and conventionally known substances such as peroxides and azo compounds can be appropriately used as the radical polymerization initiator. Examples of the peroxide include dialkyl peroxides such as di-t-butyl peroxide, dicumyl peroxide, t-butylcumyl peroxide, α,α'-bis(t-butylperoxy-m-isopropyl)benzene, and 2,5-di(t-butylperoxy)hexyne-3; Peroxyesters such as t-butyl peroxybenzoate, t-butyl peroxyacetate, and 2,5-dimethyl-2,5-di(benzoylperoxy)hexane; ketone peroxides such as cyclohexanone peroxide, 3,3,5-trimethylcyclohexanone peroxide, and methylcyclohexanone peroxide; Peroxyketals such as 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane, 1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, and n-butyl-4,4-bis(t-butylperoxy)valerate; Hydroperoxides such as cumene hydroperoxide, diisopropylbenzene hydroperoxide, and 2,5-dimethylcyclohexane-2,5-dihydroperoxide; Diacyl peroxides such as benzoyl peroxide, decanoyl peroxide, lauroyl peroxide, and 2,4-dichlorobenzoyl peroxide; Examples include peroxydicarbonates such as bis(t-butylcyclohexyl) peroxydicarbonate.

[0024] Examples of the azo compound include 2,2'-azobisbutyronitrile such as 2,2'-azobisisobutyronitrile (abbreviation: AIBN) and 2,2'-azobis(2-methylbutyronitrile); 2,2'-azobisvaleronitrile such as 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) and 2,2'-azobis(2,4-dimethylvaleronitrile); 2,2'-azobispropionitriles such as 2,2'-azobis(2-hydroxymethylpropionitrile); Examples include 1,1'-azobis-1-alkanenitriles such as 1,1'-azobis(cyclohexane-1-carbonitrile).

[0025] The polymerization initiator is preferably used in an amount of 0.01 to 10 parts by mass, more preferably 0.1 to 2 parts by mass, based on 100 parts by mass of the monomer mixture.

[0026] (Weight average molecular weight (Mw)) The weight average molecular weight of the acrylic copolymer (A) is preferably 500,000 to 1,500,000, more preferably 600,000 to 1,200,000. By setting the weight average molecular weight within the above range, contamination of the adherend upon peeling can be suppressed without impairing the adhesive strength to the high polarity adherend and the low polarity adherend. The weight average molecular weight is a value calculated in terms of polystyrene measured by gel permeation chromatography (GPC).

[0027] <Hardening agent (B)> The adhesive composition contains a curing agent (B). In the adhesive composition, the curing agent (B) is mainly used to crosslink the acrylic copolymer (A). Crosslinking by the curing agent improves the heat resistance, chemical stability, and adhesive strength of the adhesive composition. Examples of the curing agent include isocyanate-based curing agents, epoxy-based curing agents, aziridine-based curing agents, and chelate-based curing agents, and these can be used alone or in combination of two or more. As the curing agent, an isocyanate-based curing agent is preferred, since it gives a composition having good adhesive strength with a small difference between adhesive strength to a high polarity adherend and adhesive strength to a low polarity adherend, and also has excellent adhesion to substrates.

[0028] The isocyanate-based curing agent is an isocyanate having two or more isocyanate groups. For example, the isocyanate is preferably an aromatic polyisocyanate, an aliphatic polyisocyanate, an araliphatic polyisocyanate, an alicyclic polyisocyanate, and their biuret, nurate, and adduct, and more preferably an aliphatic polyisocyanate, an alicyclic polyisocyanate, and their biuret, nurate, and adduct from the viewpoint of yellowing resistance.

[0029] Examples of aromatic polyisocyanates include 1,3-phenylene diisocyanate, 4,4'-diphenyl diisocyanate, 1,4-phenylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4'-toluidine diisocyanate, 2,4,6-triisocyanate toluene, 1,3,5-triisocyanate benzene, dianisidine diisocyanate, 4,4'-diphenyl ether diisocyanate, and 4,4',4"-triphenylmethane triisocyanate.

[0030] Examples of aliphatic polyisocyanates include trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate (also known as HMDI), pentamethylene diisocyanate, 1,2-propylene diisocyanate, 2,3-butylene diisocyanate, 1,3-butylene diisocyanate, dodecamethylene diisocyanate, and 2,4,4-trimethylhexamethylene diisocyanate.

[0031] Examples of the aromatic aliphatic polyisocyanate include ω,ω'-diisocyanate-1,3-dimethylbenzene, ω,ω'-diisocyanate-1,4-dimethylbenzene, ω,ω'-diisocyanate-1,4-diethylbenzene, 1,4-tetramethylxylylene diisocyanate, and 1,3-tetramethylxylylene diisocyanate.

[0032] Examples of alicyclic polyisocyanates include 3-isocyanatemethyl-3,5,5-trimethylcyclohexyl isocyanate (also known as IPDI, isophorone diisocyanate), 1,3-cyclopentane diisocyanate, 1,3-cyclohexane diisocyanate, 1,4-cyclohexane diisocyanate, methyl-2,4-cyclohexane diisocyanate, methyl-2,6-cyclohexane diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), and 1,4-bis(isocyanatemethyl)cyclohexane.

[0033] The biuret form is a self-condensation product having a biuret bond formed by self-condensation of an isocyanate monomer, such as a biuret form of hexamethylene diisocyanate.

[0034] The nurate form is a trimer of an isocyanate monomer, such as a trimer of hexamethylene diisocyanate, a trimer of isophorone diisocyanate, or a trimer of tolylene diisocyanate.

[0035] The adduct is a bifunctional or higher isocyanate compound obtained by reacting an isocyanate monomer with a bifunctional or higher low-molecular active hydrogen-containing compound. Examples of the adduct include a compound obtained by reacting trimethylolpropane with hexamethylene diisocyanate, a compound obtained by reacting trimethylolpropane with tolylene diisocyanate, a compound obtained by reacting trimethylolpropane with xylylene diisocyanate, a compound obtained by reacting trimethylolpropane with isophorone diisocyanate, and a compound obtained by reacting 1,6-hexanediol with hexamethylene diisocyanate.

[0036] The isocyanate-based curing agent is preferably a trifunctional isocyanate compound from the viewpoint of forming a sufficient crosslinked structure. The isocyanate compound is more preferably an adduct or nurate, which is a reaction product between an isocyanate monomer and a trifunctional low-molecular active hydrogen-containing compound. The isocyanate compound is preferably a trimethylolpropane adduct of hexamethylene diisocyanate, a nurate of hexamethylene diisocyanate, a trimethylolpropane adduct of tolylene diisocyanate, a nurate of tolylene diisocyanate, a trimethylolpropane adduct of isophorone diisocyanate, or a nurate of isophorone diisocyanate, and more preferably a trimethylolpropane adduct of hexamethylene diisocyanate, a trimethylolpropane adduct of tolylene diisocyanate, or a trimethylolpropane adduct of isophorone diisocyanate.

[0037] Examples of epoxy curing agents include glycerin diglycidyl ether, 1,6-hexanediol diglycidyl ether, N,N,N',N'-tetraglycidyl-m-xylylenediamine, 1,3-bis(N,N'-diglycidylaminomethyl)cyclohexane, and N,N,N',N'-tetraglycidylaminophenylmethane.

[0038] Examples of the aziridine-based curing agent include N,N'-diphenylmethane-4,4'-bis(1-aziridinecarboxite), tris-2,4,6-(1-aziridinyl)-1,3,5-triazine, and 4,4'-bis(ethyleneiminocarbonylamino)diphenylmethane.

[0039] The metal chelate curing agent is preferably a coordination compound of a polyvalent metal such as aluminum, iron, copper, zinc, tin, titanium, nickel, antimony, magnesium, vanadium, chromium, or zirconium with acetylacetone or ethyl acetoacetate. Examples of the metal chelate include aluminum ethyl acetoacetate diisopropylate, aluminum trisacetylacetonate, aluminum bisethyl acetoacetate monoacetylacetonate, and aluminum alkyl acetoacetate diisopropylate.

[0040] The content of the curing agent (B) in the pressure-sensitive adhesive composition is preferably 0.005 to 15 parts by mass, more preferably 0.01 to 10 parts by mass, even more preferably 0.02 to 5 parts by mass, and particularly preferably 0.03 to 4 parts by mass, relative to 100 parts by mass of the acrylic copolymer (A), since a composition having good adhesive strength with a small difference between the adhesive strength to a high-polarity adherend and the adhesive strength to a low-polarity adherend can be obtained.

[0041] The pressure-sensitive adhesive layer preferably has a gel fraction of 50 to 95% by mass. When the gel fraction is 50% by mass or more, the cohesive strength of the pressure-sensitive adhesive composition is improved, and less adhesive is left on the adherend when peeled off. When the gel fraction is 95% by mass or less, a flexible pressure-sensitive adhesive layer is easily obtained, the adhesion to low-polarity or high-polarity adherends is further improved, and the adhesive strength can be maintained for a long period of time. The gel fraction of the pressure-sensitive adhesive layer is more preferably 53 to 93% by mass, and even more preferably 76 to 90% by mass. The method for measuring the gel fraction will be described in detail in the Examples section.

[0042] The adhesive composition may contain a solvent. The solvent can be appropriately selected from those that have low reactivity with each component constituting the adhesive composition and can dissolve or disperse each component, taking into consideration the coating property, etc. Specific examples of suitable solvents include hydrocarbon solvents such as toluene, xylene, hexane, and heptane; ester solvents such as ethyl acetate and butyl acetate; ketone solvents such as acetone and methyl ethyl ketone; halogenated hydrocarbon solvents such as dichloromethane and chloroform; ether solvents such as diethyl ether, methoxytoluene, and dioxane, and other hydrocarbon solvents. The solvents can be used alone or in combination of two or more.

[0043] <Adhesive layer> The pressure-sensitive adhesive layer according to the present invention (hereinafter also referred to as the present pressure-sensitive adhesive layer) is a layer made of the present pressure-sensitive adhesive composition, and can be obtained by drying the applied product and, if necessary, curing it.

[0044] The thickness of the adhesive layer is preferably 3 to 150 μm. When the thickness of the adhesive layer is 3 μm or more, a flexible adhesive layer is easily obtained, and adhesion to a low-polarity or high-polarity adherend is further improved. When the thickness is 150 μm or less, less adhesive is left on the adherend upon peeling. The thickness of the adhesive layer is more preferably 5 to 100 μm, and further preferably 10 to 50 μm.

[0045] <Adhesive sheet> The adhesive sheet according to the present invention (hereinafter also referred to as the present adhesive sheet) comprises an adhesive layer made of the present adhesive composition on a substrate. The adhesive sheet can be produced, for example, by applying an adhesive to a substrate, drying the adhesive, and curing it as necessary. The present adhesive sheet comprises an adhesive layer on one or both sides of the substrate.

[0046] When applying the pressure-sensitive adhesive composition, the viscosity may be adjusted by adding the above-mentioned solvent, or the viscosity of the pressure-sensitive adhesive composition may be reduced by heating.

[0047] Examples of the substrate include cellophane, plastic sheets, rubber, foams, fabrics, rubber fabrics, resin-impregnated fabrics, glass plates, and wood, and may be in the form of a plate or a film. The substrate may be used alone or may be a laminate of a plurality of substrates. The substrate may have a surface subjected to a release treatment (hereinafter referred to as a separator).

[0048] Examples of plastic sheets (also called plastic films) include polyvinyl alcohol films, triacetyl cellulose films, polyolefin resin films such as polypropylene, polyethylene, polycycloolefin, and ethylene-vinyl acetate copolymers, polyester resin films such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, polycarbonate resin films, polynorbornene resin films, polyarylate resin films, acrylic resin films, polyphenylene sulfide resin films, polystyrene resin films, vinyl resin films, polyamide resin films, polyimide resin films, and epoxy resin films.

[0049] In the present invention, the method of applying the adhesive composition is not particularly limited, and examples thereof include a Mayer bar, an applicator, a brush, a spray, a roller, a gravure coater, a die coater, a lip coater, a comma coater, a knife coater, a reverse coater, and a spin coater. The drying method is not particularly limited, and examples thereof include hot air drying, infrared rays, and a reduced pressure method. Drying conditions vary depending on the curing form of the adhesive composition, the film thickness, and the selected solvent, but hot air heating at about 60 to 130°C is usually sufficient.

[0050] The adhesive sheet of the present invention can be obtained by (a) coating an adhesive composition on the release-treated surface of a release-treated film, drying the composition, and laminating a substrate on the surface of the adhesive layer, or by (b) directly coating an adhesive composition on a substrate, drying the composition, and laminating the release-treated surface of a release-treated film on the surface of the adhesive layer. EXAMPLES

[0051] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples. The blending ratios are expressed in terms of solid content except for the solvent. In addition, "parts" refers to "parts by mass."

[0052] <Production Example of Acrylic Copolymer> (Acrylic Copolymer (A-1)) A reaction vessel (hereinafter simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping device, and a nitrogen inlet tube was charged with 10 parts of methyl acrylate, 28.6 parts of dodecyl acrylate, 57 parts of dodecyl methacrylate, 4.3 parts of acrylic acid, 0.1 parts of hydroxyethyl acrylate, 67 parts of ethyl acetate, and 0.04 parts of benzoyl peroxide (BPO), and the air in the reaction vessel was replaced with nitrogen gas. Next, the mixture was heated to 80°C while stirring under a nitrogen atmosphere to start the reaction. Furthermore, an equal amount of the mixture charged in the reaction vessel was charged into a dropping funnel, dropped, and a polymerization reaction was carried out at reflux temperature for 7 hours under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain a copolymer solution with a non-volatile content of 50% and a viscosity of 7000 mPa·s. The weight average molecular weight (Mw) of the obtained copolymer was measured by the following method, and the weight average molecular weight was 750,000. The resulting copolymer is designated as copolymer (A-1).

[0053] (Acrylic copolymers (A-2 to A-49, A'-1 to A'8)) Acrylic copolymers (A-2 to A-49, A'-1 to A'-8) were synthesized in the same manner as for the production of acrylic copolymer (A-1), except that the compositions and amounts (parts by mass) were changed to those shown in Tables 1 to 4. The weight average molecular weights of the obtained acrylic copolymers are shown in Tables 1 to 4.

[0054] [Table 1]

[0055] [Table 2]

[0056] [Table 3]

[0057] [Table 4]

[0058] The abbreviations are as follows: [Monomer (a1)] MA: Methyl acrylate (alkyl group carbon number 1) MMA: Methyl methacrylate (alkyl group carbon number 1) EA: Ethyl acrylate (alkyl group carbon number: 2) EMA: Ethyl methacrylate (alkyl group carbon number: 2) PA: Propyl acrylate (alkyl group carbon number: 3) [Monomer (a2)] DDA: normal dodecyl acrylate (alkyl group carbon number: 12) DDMA: normal dodecyl methacrylate (alkyl group carbon number 12) TrDA: normal tridecyl acrylate (alkyl group carbon number: 13) TrDMA: normal tridecyl methacrylate (alkyl group carbon number: 13) TeDA: normal tetradecyl acrylate (alkyl group carbon number 14) TeDMA: normal tetradecyl acrylate (alkyl group carbon number 14) [Monomer (a3)] HEA: Hydroxyethyl acrylate 4HBA: 4-Hydroxy n-butyl acrylate [Monomer (a4)] AA: Acrylic acid MAA: Methacrylic acid [Other monomers] 2EHA: 2-Ethylhexyl acrylate (alkyl group has 8 carbon atoms, branched) BA: n-butyl acrylate (alkyl group carbon number: 4) BMA: n-butyl methacrylate (alkyl group carbon number: 4) ODMA: Octadecyl methacrylate (alkyl group carbon number 18) iso-DDA: iso-dodecyl acrylate (alkyl group has 12 carbon atoms, branched) iso-DDMA: iso-dodecyl methacrylate (alkyl group has 12 carbon atoms, branched) iso-TrDA: iso-tridecyl acrylate (alkyl group has 13 carbon atoms, branched) iso-TrDMA: iso-tridecyl methacrylate (alkyl group has 13 carbon atoms, branched) iso-TeDA: iso-tetradecyl acrylate (alkyl group carbon number 14, branched) iso-TeDMA: iso-tetradecyl acrylate (alkyl group has 14 carbon atoms, branched)

[0059] The materials used in the examples and comparative examples are listed below. <Hardening agent (B)> B-1: Isocyanate-based hardener (TDI / TMP: "Coronate L", Tosoh Corporation, trimethylolpropane-modified toluene diisocyanate) B-2: Epoxy curing agent (N,N,N',N'-tetraglycidyl-m-xylylenediamine (TETRAD-X, manufactured by Mitsubishi Gas Chemical Co., Inc.) B-3: Chelate-based hardener (aluminum chelate A, aluminum tris(acetylacetonate), manufactured by Kawaken Fine Chemicals Co., Ltd.) B-4: Aziridine-based curing agent (Chemithite PZ-33, Nippon Shokubai Co., Ltd. trimethylolpropane tris[3-(aziridin-1-yl)propionate])

[0060] <Example 1> <Preparation of Pressure-Sensitive Adhesive Composition> 100 parts of the acrylic copolymer (A-1), 4 parts of the curing agent (B-1), and ethyl acetate as a dilution solvent were mixed and stirred to obtain a pressure-sensitive adhesive composition of Example 1.

[0061] <Preparation of adhesive sheet> The adhesive composition of Example 1 was applied onto a 38 μm thick polyester separator [trade name "Superstic" SP-PET38, manufactured by Lintec Corporation, the same applies below] so that the thickness after drying would be 50 μm, and the adhesive layer was formed by drying in a hot air oven at 100° C. for 2 minutes. After drying, the adhesive layer was laminated onto a 25 μm thick polyethylene naphthalate (PEN) film (Teonex, manufactured by Teijin Co., Ltd.), and further aged at 23° C. and a relative humidity of 50% (hereinafter 50% RH) for 7 days to obtain the adhesive sheet of Example 1.

[0062] <Examples 2 to 62 and Comparative Examples 1 to 8> Except for changing the types and blending ratios of the acrylic copolymer and curing agent as shown in Tables 5 and 6, the pressure-sensitive adhesive compositions and pressure-sensitive adhesive sheets of Examples 2 to 62 and Comparative Examples 1 to 8 were obtained in the same manner as in Example 1.

[0063] [Measurement of gel fraction of pressure-sensitive adhesive composition] The gel fraction of the resulting pressure-sensitive adhesive composition was measured by the following method, and the results are shown in Tables 5 and 6.

[0064] <Measurement of gel fraction> (Preparation of sheet samples) The obtained adhesive composition was applied to a release-treated surface of a polyethylene terephthalate (PET) film (release film) with one side treated for release, so that the thickness after drying would be 25 μm. Then, after drying at 105° C. for 2 minutes, the coating film was attached to a commercially available polyethylene terephthalate (PET) film with a thickness of 50 μm and aged for 7 days in an atmosphere of 23° C. and 50% RH to prepare a sheet sample. (Measurement method) A test piece 3 mm wide and 100 mm long was cut out from the obtained sheet sample and its weight was measured. The release film was peeled off from the test piece, and the exposed adhesive layer was attached to a 300 mesh stainless steel wire netting. The wire netting was folded so that the test piece would not peel off, and the test piece was wrapped in the netting and immersed in ethyl acetate as an extraction solution and left to stand at 50°C for 24 hours. After immersion, the wire netting was removed, washed with a small amount of ethyl acetate, and dried at 100°C for 30 minutes, and then its weight was measured. The gel fraction was calculated by the following formula (1). (Gel fraction)={(W2-W0-W3) / (W1-W0-W3)}×100 (Equation 1) W0: Weight of wire mesh W1: Weight of wire mesh + test piece W2: Weight of wire mesh + test specimen after drying W3: Weight of the PET film of the test piece

[0065] [Evaluation of adhesive sheets] The adhesive sheets thus obtained were evaluated for adhesive strength, removability, holding power, and pot life by the methods described below. The results are shown in Tables 5 and 6.

[0066] <Adhesive strength evaluation> The pressure-sensitive adhesive sheets of each Example and Comparative Example were cut to a size of 25 mm in width and 100 mm in length. Next, in an environment of 23°C and 50% RH, the separator was peeled off from the pressure-sensitive adhesive sheet, and the exposed pressure-sensitive adhesive layer was attached to the following adhesion surfaces 1 and 2, respectively, and rolled back and forth once with a 2 kg roll to prepare a measurement sample. After storing this measurement sample in an environment of 23°C and 50% RH for 24 hours, the peel strength was determined using a tensile tester (Orientec Co., Ltd.'s "Tensilon") at a peel speed of 300 mm / min and a peel angle of 180°. (Surface to be adhered to) Adhesive surface 1: Stainless steel (SUS) plate (highly polar adherend) Adhesion surface 2: PP board (low polarity adherend)

[0067] (Adhesive strength evaluation criteria) (1) Evaluation of SUS plates ◎: Peel strength is 4N / 25mm or more and less than 8N / 25mm (excellent). ○: Peel strength is 3N / 25mm or more and less than 4N / 25mm or 8N / 25mm or more and less than 10N / 25mm (very good). △: Peel strength is 1N / 25mm or more and less than 3N / 25mm or 10N / 25mm or more and less than 15N / 25mm (good). ▲: Peel strength is 0.5N / 25mm or more and less than 1N / 25mm or 15N / 25mm or more and less than 20N / 25mm (usable). ×: Peel strength is less than 0.5N / 25mm or more than 20N / 25mm (not usable). (2) Evaluation of PP boards ◎: Peel strength is 4N / 25mm or more and less than 8N / 25mm (excellent). ○: Peel strength is 3N / 25mm or more and less than 4N / 25mm or 8N / 25mm or more and less than 10N / 25mm (very good). △: Peel strength is 1N / 25mm or more and less than 3N / 25mm or 10N / 25mm or more and less than 15N / 25mm (good). ▲: Peel strength is 0.5N / 25mm or more and less than 1N / 25mm or 15N / 25mm or more and less than 20N / 25mm (usable). ×: Peel strength is less than 0.5N / 25mm or more than 20N / 25mm (not usable).

[0068] <Removability evaluation> The pressure-sensitive adhesive sheet of each example was cut into a size of 25 mm wide and 100 mm long. Next, in an environment of 23°C and 50% RH, the separator was peeled off from the pressure-sensitive adhesive sheet, and the exposed pressure-sensitive adhesive layer was attached to the following SUS or PP, and rolled back and forth once with a 2 kg roll to prepare a measurement sample. The measurement sample was stored for 7 days in an environment of 60°C and 95% RH, and then peeled using a tensile tester (Orientec's "Tensilon") at a peel speed of 300 mm / min and a peel angle of 180°, and the condition of the adherend was observed after that. The area of ​​the adherend with adhesive remaining after peeling was visually confirmed relative to the area of ​​the adherend with the adhesive layer attached. The results are shown in Table 2. (Removability evaluation criteria) ◎: No contamination on the adherend (excellent). ◯: The area with residual glue is 3% or less (very good). △: The area with residual glue is more than 3% and less than 5% (good). ▲: The area with residual glue is more than 5% but less than 10% (usable). ×: The area with residual glue exceeds 10% (unusable).

[0069] <Retention strength evaluation> Adhesive sheets for each example were prepared measuring 25mm wide x 150mm long. The separator was removed from the adhesive sheet, and the adhesive layer was attached to a 25mm wide x 25mm long section of the bottom edge of a polished stainless steel plate measuring 30mm wide x 150mm long. The adhesive was then pressed back and forth once with a 2kg roll, after which a load of 1kg was applied in an atmosphere of 40°C or 80°C and the sheet was left for 70,000 seconds to measure the holding power. The length by which the top edge of the adhesive sheet surface shifted downward was measured for evaluation. The results are shown in Table 2.

[0070] (Retention strength evaluation criteria) ◎: The deviation length was 0 mm (excellent). ◯: The deviation length was more than 0 mm and less than 3.0 mm (very good). △: The deviation length was more than 3.0 mm and less than 10.0 mm (good). ▲: The deviation length was more than 10.0 mm but less than 15.0 mm (usable). ×: The shifted length exceeded 15.0 mm, or the adhesive sheet was completely peeled off from the stainless steel plate (unusable).

[0071] (Pot Life) The viscosity of the adhesive immediately after mixing with the curing agent in a 25°C atmosphere was measured using a Brookfield viscometer (rotor: 3, rotation speed: 12 rpm, measurement time: 1 minute, unit: mPa s). The adhesive was left in a 25°C atmosphere for 12 hours, and the viscosity was measured in the same manner as above. The rate of change in viscosity before and after leaving the adhesive was calculated, and the pot life was evaluated according to the following criteria. The rate of change in viscosity is expressed by Equation 1, where Va is the viscosity immediately after mixing the curing agent, and Vb is the viscosity after leaving it in an atmosphere at 25°C for 12 hours. (Formula 1) Viscosity change rate = (Vb - Va) / Va x 100 ◎: Viscosity change is less than 20% (Excellent) ○: Viscosity change is 20% or more but less than 40% (good) △: Viscosity change is 40% or more but less than 70% (fairly good). ▲: Viscosity change is 70% or more but less than 100% (practical). ×: Viscosity change of 100% or more. (Not practical)

[0072] [Table 5]

[0073] [Table 6]

Claims

1. A pressure-sensitive adhesive composition comprising an acrylic copolymer (A) and a curing agent (B), In 100% by mass of the monomers constituting the acrylic copolymer (A), The content of the alkyl (meth)acrylate monomer (a1) having an alkyl group having 1 to 3 carbon atoms is 20 mass% or less, and The content of the alkyl (meth)acrylate monomer (a2) having an unbranched alkyl group having 12 to 14 carbon atoms is 70% by mass or more and 95% by mass or less, A pressure-sensitive adhesive composition, comprising a hydroxy group-containing monomer (a3) ​​content of 0.01 mass % or more and 0.8 mass % or less.

2. The pressure-sensitive adhesive composition according to claim 1 , further comprising 1% by mass or more and 8% by mass or less of a carboxyl group-containing monomer (a4) based on 100% by mass of the monomers constituting the acrylic copolymer (A).

3. 2. The pressure-sensitive adhesive composition according to claim 1, wherein the curing agent (B) is an isocyanate-based curing agent.

4. The pressure-sensitive adhesive composition according to claim 1, wherein the unbranched alkyl (meth)acrylate monomer (a2) having an alkyl group having 12 to 14 carbon atoms simultaneously contains normal dodecyl acrylate and normal dodecyl methacrylate.

5. The pressure-sensitive adhesive composition according to claim 1, having a gel fraction of 50 to 95 mass %.

6. A pressure-sensitive adhesive layer comprising the pressure-sensitive adhesive composition according to any one of claims 1 to 5, having a thickness of 3 to 150 µm.

7. A pressure-sensitive adhesive sheet comprising a substrate and the pressure-sensitive adhesive layer according to claim 6 on one or both sides thereof.

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