Aqueous pressure-sensitive adhesive and pressure-

Abstract

To provide an aqueous pressure-sensitive adhesive capable of forming a pressure-sensitive adhesive layer having moderate pressure-sensitive adhesive force and peelability, having good wettability to a resin film substrate when coated with the pressure-sensitive adhesive, having a uniform coated surface of the pressure-sensitive adhesive, capable of exhibiting high pressure-sensitive adhesive force, and also having peelability.SOLUTION: The acrylic polymer particles (A) have a D10 of 1 μm or more and 7 μm or less, a D50 of 10 μm or more and less than 30 μ m, and a D90 of 20 μm or more and 40 μm or less, where the D10, the D50, and the D90 are particle size at a cumulative frequency of 10%, 50%, and 90%, respectively, from the small particle size side in a volume-based cumulative particle size distribution. The acrylic polymer particles (B) have a D50 of 0.1 μm or more and 0.6 μm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Removable pressure-sensitive adhesive sheets are known as pressure-sensitive adhesive sheets. For example, widely used pressure-sensitive adhesive sheets include a pressure-sensitive adhesive layer and a substrate, the pressure-sensitive adhesive layer contains polymer particles, the pressure-sensitive adhesive layer surface of the pressure-sensitive adhesive sheet is attached to an adherend, and the pressure-sensitive adhesive sheet can be repeatedly peeled from and attached to an adherend as needed.

[0003] For example, Patent Document 1 discloses a removable adhesive sheet in which an adhesive layer is formed by applying an adhesive containing a spherical adhesive and an emulsion-type binder adhesive to one side of a base sheet, and the spherical adhesive is either spherical adhesive (A) obtained by polymerizing an ethylenically unsaturated monomer in the presence of 0.1 to 3 parts by mass of polyacrylic acid per 100 parts by mass of the ethylenically unsaturated monomer component, or spherical adhesive (B) obtained by adding 0.1 to 7 parts by mass of polyacrylic acid per 100 parts by mass of the solid content of the spherical adhesive to a spherical adhesive obtained by polymerizing an ethylenically unsaturated monomer.

[0004] Patent Document 2 also discloses an aqueous removable pressure-sensitive adhesive and a pressure-sensitive adhesive sheet using the same, which comprises acrylic particles having a D50 average particle size of 20 to 60 μm, acrylic particles (i) having a D50 average particle size of 0.1 to 0.6 μm, and a crosslinking agent, wherein the acrylic particles (i) are obtained by polymerizing a monomer composition containing a carboxyl group-containing monomer, a hydroxyl group-containing monomer, and other monomers in the presence of an alkali metal salt of an oxyacid and an alkali metal salt of polyphosphoric acid, an anionic reactive emulsifier, and a nonionic nonreactive emulsifier having 7 to 20 repeating ethylene oxide units. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2006 / 104235 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-105353 Summary of the Invention [Problem to be solved by the invention]

[0006] A removable adhesive sheet is required to have both adequate adhesive strength and releasability. However, in general, adhesive layers of adhesive sheets with strong adhesive strength tend to have poor releasability, while adhesive layers with good releasability tend to have poor adhesive strength. Generally, aqueous adhesives containing polymer particles with a D50 of 1 μm or more have excellent releasability because the adhesive layer has physical irregularities derived from the polymer particles. However, when the substrate to which the adhesive is applied is a resin film, the adhesive has low wettability to the substrate compared to when the substrate is paper, and the polymer particles aggregate in the adhesive layer, preventing the formation of a uniform adhesive coating surface and resulting in low adhesive strength. Therefore, it has been difficult to achieve both adequate adhesive strength and releasability.

[0007] Therefore, the problem to be solved by the present invention is to provide a water-based pressure-sensitive adhesive that can form a pressure-sensitive adhesive layer having appropriate adhesive strength and releasability, that has good wettability to a resin film substrate when coated with the pressure-sensitive adhesive, that can produce a uniformly coated pressure-sensitive adhesive surface that exhibits high adhesive strength, and that also has releasability, and further to provide a pressure-sensitive adhesive sheet having appropriate adhesive strength and releasability. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to solve the above problems and have arrived at the present invention. Specifically, the present invention relates to an aqueous pressure-sensitive adhesive containing at least two types of acrylic polymer particles (A) and acrylic polymer particles (B), wherein, in a volume-based cumulative particle size distribution, D10, D50, and D90 are the particle sizes at which the cumulative frequency from the small particle size side becomes 10%, 50%, and 90%, respectively, for the acrylic polymer particles (A) D10 is 1 μm or more and 7 μm or less, D50 is 10 μm or more and 30 μm or less, and D90 is 20 μm or more and 40 μm or less, and for the acrylic polymer particles (B) D50 is 0.1 μm or more and 0.6 μm or less.

[0009] The present invention also relates to the above aqueous pressure-sensitive adhesive, wherein the mass ratio (A / B) of the acrylic polymer particles (A) to the acrylic polymer particles (B) is 80 / 20 to 50 / 50.

[0010] The present invention also relates to the above aqueous pressure-sensitive adhesive, wherein the acrylic polymer particles (A) comprise a suspension polymer of a monomer composition containing a (meth)acrylic acid alkyl ester monomer and a carboxy group-containing monomer.

[0011] The present invention also relates to the above aqueous pressure-sensitive adhesive, wherein the acrylic polymer particles (B) comprise an emulsion polymer of a monomer composition containing a carboxy group-containing monomer and a hydroxy group-containing monomer, and the monomer composition (100% by mass) contains 1% by mass or more and 5% by mass or less of the carboxy group-containing monomer and 2% by mass or more and 10% by mass or less of the hydroxy group-containing monomer.

[0012] The present invention also relates to the above aqueous pressure-sensitive adhesive for use on a resin film substrate.

[0013] The present invention also relates to a pressure-sensitive adhesive sheet comprising a substrate and a pressure-sensitive adhesive layer formed from the above-mentioned water-based pressure-sensitive adhesive on at least one surface thereof.

[0014] The present invention also relates to a method for producing an aqueous pressure-sensitive adhesive containing at least two types of acrylic polymer particles (A) and acrylic polymer particles (B), comprising the steps of: producing acrylic polymer particles (A) by suspension polymerization of a monomer composition in the presence of an anionic emulsifier having an oxyethylene group; and producing acrylic polymer particles (B) by emulsion polymerization in the presence of an anionic emulsifier having a polyoxyethylene group. In the cumulative particle size distribution on a volume basis, when D10, D50, and D90 are the particle sizes at which the cumulative frequency from the small particle size side is 10%, 50%, and 90%, respectively, the acrylic polymer particles (A) have a D10 of 1 μm or more and 7 μm or less, a D50 of 10 μm or more and 30 μm or less, and a D90 of 20 μm or more and 40 μm or less, and the acrylic polymer particles (B) have a D50 of 0.1 μm or more and 0.6 μm or less. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a water-based pressure-sensitive adhesive that can form a pressure-sensitive adhesive layer having appropriate adhesive strength and releasability, that has good wettability to a resin film substrate when coated with the pressure-sensitive adhesive, that can produce a uniform pressure-sensitive adhesive coated surface that exhibits high adhesive strength and releasability, and that also has appropriate adhesive strength and releasability.Furthermore, it is possible to provide a pressure-sensitive adhesive sheet having appropriate adhesive strength and releasability. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 shows the particle size distribution of acrylic polymer particles A-1 and AX-1. DETAILED DESCRIPTION OF THE INVENTION

[0017] An embodiment of the present invention will be described. The present invention is not limited to the following embodiment. First, the terms and symbols used in this specification will be described. D10, D50, and D90 refer to particle sizes at which the cumulative frequency from the small particle size side in the volume-based cumulative particle size distribution is 10%, 50%, and 90%, respectively. A numerical range specified using "to" includes the numerical values ​​before and after "to" as the lower and upper limits. Unless otherwise specified, the terms "(meth)acrylic," "(meth)acrylo," "(meth)acrylic acid," "(meth)acrylate," and "(meth)acryloyloxy" represent "acrylic or methacrylic," "acrylo or methacrylo," "acrylic acid or methacrylic acid," "acrylate or methacrylate," and "acryloyloxy or methacryloyloxy," respectively. "Monomer" and "monomer" are synonymous. "Monomer composition" refers to a mixture consisting of only monomers.

[0018] <Water-based adhesive> An aqueous pressure-sensitive adhesive according to one embodiment of the present invention contains acrylic polymer particles (A) having a D10 of 1 μm to 7 μm, a D50 of 10 μm to 30 μm, and a D90 of 20 μm to 40 μm, and acrylic polymer particles (B) having a D50 of 0.1 μm to 0.6 μm. The aqueous pressure-sensitive adhesive of the present invention may further contain any component other than the acrylic polymer particles (A) and the acrylic polymer particles (B).

[0019] [Acrylic polymer particles (A)] The acrylic polymer particles (A) have a D10 of 1 μm or more, a D50 of 10 μm or more, and a D90 of 20 μm or more, preferably a D10 of 2 μm or more, a D50 of 15 μm or more, and a D90 of 25 μm or more, more preferably a D10 of 2.5 μm or more, a D50 of 18 μm or more, and a D90 of 28 μm or more. The acrylic polymer particles (A) also have a D10 of 7 μm or less, a D50 of 30 μm or less, and a D90 of 40 μm or less, preferably a D10 of 6 μm or less, a D50 of 28 μm or less, and a D90 of 35 μm or less, more preferably a D10 of 5 μm or less, a D50 of 25 μm or less, and a D90 of 33 μm or less. Within the above numerical ranges, a pressure-sensitive adhesive sheet exhibiting the desired appropriate adhesive strength can be obtained.

[0020] The particle size distribution of the acrylic polymer particles (A) can be measured using a laser diffraction / scattering particle size analyzer (e.g., Microtrac MT-3300 manufactured by Microtrac-Bell Corporation). For the measurement, samples can be prepared by diluting the suspension polymer or aqueous adhesive with water or a dispersing medium as needed to achieve a concentration suitable for measurement. The D10, D50, and D90 can be calculated from the cumulative particle size distribution of the obtained particle size distribution. Suspension polymers will be discussed later.

[0021] The acrylic polymer particles (A) preferably contain a suspension polymer of a monomer composition containing a (meth)acrylic acid alkyl ester monomer and a carboxy group-containing monomer.

[0022] (Meth)acrylic acid alkyl ester monomer In this specification, the (meth)acrylic acid alkyl ester monomer refers to a monomer having one or more (meth)acryloyloxy groups and one alkyl group in the molecule.

[0023] The alkyl group in the (meth)acrylic acid alkyl ester monomer may be linear or branched. The use of a linear or branched alkyl group further enhances polymerization stability during suspension polymerization. The number of carbon atoms in the alkyl group is preferably 1 to 20, more preferably 1 to 15, and even more preferably 1 to 10.

[0024] Examples of the (meth)acrylic acid alkyl ester monomer include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, stearyl (meth)acrylate, cyclohexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, dodecyl (meth)acrylate, tetradecyl (meth)acrylate, hexadecyl (meth)acrylate, octadecyl (meth)acrylate, lauryl (meth)acrylate, etc. The (meth)acrylic acid alkyl ester monomer (A) can be used alone or in combination of two or more. The (meth)acrylic acid alkyl ester monomer preferably contains an acrylic acid alkyl ester monomer, and more preferably contains at least one selected from butyl acrylate and 2-ethylhexyl acrylate. By containing a (meth)acrylic acid alkyl ester monomer, the flexibility of the obtained PSA sheet is further increased.

[0025] Carboxylic acid-containing monomers The carboxyl group-containing monomer is a monomer having at least one polymerizable unsaturated group and at least one carboxyl group in the molecule. The polymerizable unsaturated group is preferably a polymerizable carbon-carbon double bond-containing group, more preferably a group selected from the group consisting of a CH═CH— group (vinyl group), a CH═C(CH)— group, a (meth)acryloyl group, and a (meth)acryloyloxy group.

[0026] Examples of carboxy group-containing monomers include (meth)acrylic acid, monohydroxyethyl phthalate (meth)acrylate, p-carboxybenzyl (meth)acrylate, monohydroxyethyl succinate (meth)acrylate, β-carboxyethyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, and crotonic acid. The carboxy group-containing monomer (B) can be used alone or in combination of two or more. The carboxy group-containing monomer (B) preferably contains (meth)acrylic acid, and more preferably contains acrylic acid. The inclusion of (meth)acrylic acid further improves the polymerization stability during suspension polymerization and the dispersion stability of the PSA.

[0027] The monomer composition constituting the acrylic polymer particles (A) may contain other monomers other than the (meth)acrylic acid alkyl ester monomer and the carboxyl group-containing monomer. Examples of other monomers include polyfunctional (meth)acrylic acid alkyl ester monomers such as alkyl di(meth)acrylate and alkyl tri(meth)acrylate; aromatic ring-containing (meth)acrylic acid ester monomers such as phenyl (meth)acrylate, phenoxyethyl (meth)acrylate, and benzyl (meth)acrylate; styrene-based monomers such as styrene and α-methylstyrene; vinyl ether-based monomers such as ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, and isobutyl vinyl ether; fatty acid vinyl-based monomers such as vinyl acetate and vinyl propionate; and functional group-containing monomers other than carboxyl groups, such as hydroxyl groups, amino groups, mercapto groups, and oxetanyl groups.

[0028] The content of the (meth)acrylic acid alkyl ester monomer is preferably 90% by mass or more, more preferably 92% by mass or more, and even more preferably 95% by mass or more, based on 100% by mass of the monomer composition. When the content is 90% by mass or more, the polymerization rate during suspension polymerization increases, and the polymerization stability and dispersion stability of the PSA are further improved. In consideration of the content of the carboxy group-containing monomer, the content of the (meth)acrylic acid alkyl ester monomer is preferably 99.5% by mass or less, more preferably 99% by mass or less, and even more preferably 98% by mass or less, based on the total mass of the monomers.

[0029] From the viewpoint of dispersion stability of the monomer composition during polymerization, the content of the carboxy group-containing monomer is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more, relative to 100% by mass of the monomer composition. A content of 0.5% by mass or more further enhances polymerization stability during suspension polymerization and dispersion stability of the PSA. The content of the carboxy group-containing monomer is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less, relative to 100% by mass of the monomer composition. When the content is 20% by mass or less, the viscosity of the suspension polymer obtained by suspension polymerization can be reduced.

[0030] The content of the other monomer is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, based on 100% by mass of the monomer composition. When the content is 20% by mass or less, the polymerization rate can be increased.

[0031] Anionic emulsifier The monomer composition for producing the acrylic polymer particles (A) preferably contains an anionic emulsifier containing an oxyethylene group in the molecule. The anionic emulsifier containing an oxyethylene group in the molecule may be an anionic emulsifier containing a polyoxyethylene group in the molecule. The anionic emulsifier contains an oxyethylene group in the molecule and further has at least one anionic functional group and at least one lipophilic group as hydrophilic groups. The anionic emulsifiers can be used alone or in combination of two or more.

[0032] Examples of anionic functional groups include a carboxyl group, a carboxylato group, a sulfo group, a sulfoxylato group, a phospho group, and a phosphoxylato group. Counter cations of the carboxylato group, sulfoxylato group, and phosphoxylato group may be, for example, sodium ions, potassium ions, and ammonium ions. The lipophilic group may be a monovalent group containing carbon atoms, and examples of the lipophilic group include alkyl groups, alkenyl groups, aryl groups, heteroaryl groups, arylalkyl groups, and alkylaryl groups. The lipophilic group is preferably a monovalent group having 6 or more carbon atoms. The use of a monovalent group having 6 or more carbon atoms increases emulsifying power, and further improves stability during suspension polymerization and dispersion stability of the PSA.

[0033] The oxyethylene groups contained in the anionic emulsifier may be a polyoxyethylene structure in which oxyethylene groups are bonded to each other. The anionic emulsifier contains, for example, an anionic emulsifier containing a polyoxyethylene structure having two or more oxyethylene groups. The number of oxyethylene groups contained in the polyoxyethylene structure is preferably 5 or more, more preferably 7 or more, and even more preferably 10 or more. When the anionic emulsifier contains a polyoxyethylene structure, the hydrophilicity of the emulsifier tends to be increased, and stability during suspension polymerization tends to be further improved. Furthermore, the number of oxyethylene groups contained in the polyoxyethylene structure is, for example, 100 or less, preferably 80 or less, and more preferably 60 or less. When the number of oxyethylene groups is 100 or less, a good balance between hydrophilicity and lipophilicity is achieved, and stability during suspension polymerization tends to be further improved.

[0034] The weight-average molecular weight of the anionic emulsifier is preferably 200 or more, more preferably 300 or more, and even more preferably 500 or more. When the weight-average molecular weight is 200 or more, the hydrophilicity of the emulsifier increases, and stability during suspension polymerization tends to be further improved. The weight-average molecular weight of the anionic emulsifier is preferably 6,000 or less, more preferably 5,000 or less, and even more preferably 4,000 or less. When the weight-average molecular weight is 6,000 or less, a good balance between hydrophilicity and lipophilicity is achieved, and stability during suspension polymerization tends to be further improved.

[0035] In this specification, the weight average molecular weight refers to the weight average molecular weight calculated in terms of standard polystyrene by gel permeation chromatography (GPC). A specific measurement method is shown in the Examples.

[0036] The anionic emulsifier may be, for example, a compound represented by the following formula (I) or a compound represented by the following formula (II).

[0037] Formula (I) [ka]

[0038] In formula (I), R represents a monovalent group containing a carbon atom, X represents an anionic functional group, and n represents an integer of 1 or greater.

[0039] Formula (II) [ka]

[0040] In formula (II), R represents a monovalent group containing a carbon atom, R' represents a divalent group containing a carbon atom, X represents an anionic functional group, and n represents an integer of 1 or more.

[0041] Examples of the monovalent group containing a carbon atom represented by R include an alkyl group, an alkenyl group, an aryl group, a heteroaryl group, an arylalkyl group, and an alkylaryl group. The alkyl group may be linear or branched. The number of carbon atoms in the alkyl group is preferably 6 to 20, more preferably 8 to 18, and even more preferably 8 to 16. The alkenyl group may be linear or branched. The number of carbon atoms in the alkenyl group is preferably 8 to 20, more preferably 8 to 18, and even more preferably 8 to 16. The aryl group may be a monocyclic group (such as a phenyl group, a tolyl group, or a xylyl group), a condensed ring group (such as a naphthyl group or anthryl group), or a ring assembly group (such as a biphenylyl group or a terphenylyl group). The number of carbon atoms in the aryl group is preferably 6 to 20, more preferably 6 to 18, and even more preferably 6 to 16. The heteroaryl group may be a single ring (such as a thienyl group or a pyridyl group), a condensed ring (such as a benzothienyl group or a benzopyridyl group), or a ring assembly group (such as a bithienyl group or a bipyridyl group). The number of carbon atoms in the heteroaryl group is preferably 2 to 20, more preferably 2 to 18, and even more preferably 2 to 16.

[0042] The monovalent group containing carbon atoms is preferably an alkyl group or an aryl group. Preferred examples of the alkyl group include a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, and an icosyl group. Preferred examples of the aryl group include a phenyl group, a naphthyl group, an anthryl group, a phenanthryl group, a pyrenyl group, a fluorenyl group, and a biphenyl group.

[0043] Examples of R' include an alkylene group and an alkenylene group. The alkylene group may be linear or branched. The alkylene group preferably has 1 to 10 carbon atoms, more preferably 1 to 8 carbon atoms, and even more preferably 1 to 5 carbon atoms. The alkenylene group may be linear or branched. The alkenylene group preferably has 1 to 10 carbon atoms, more preferably 1 to 8 carbon atoms, and even more preferably 1 to 5 carbon atoms.

[0044] X represents an anionic functional group, and examples of X include -COO - Y + , -SO3 - Y + , -PO(O - Y + )2(where Y + represents a cation). + As a specific example, H + , Na + , K. + , NH4 + etc.

[0045] n represents an integer of 1 or more, and preferably 2 or more. n is preferably 10 or more, more preferably 20 or more or 30 or more, and even more preferably 50 or more. n is preferably 100 or less, more preferably 90 or less, and even more preferably 80 or less.

[0046] Examples of the anionic emulsifier include carboxylic acid type anionic emulsifiers, sulfonic acid type anionic emulsifiers, sulfate ester type anionic emulsifiers, phosphate ester type anionic emulsifiers, etc. From the viewpoint of stability during suspension polymerization, sulfate ester type anionic emulsifiers are preferred.

[0047] Commercially available anionic emulsifiers include, for example, "Newcol(R) 1000-SN," "Newcol(R) 1105-SN," "Newcol(R) 1305-SN," "Newcol(R) 1703-SFD," "Newcol(R) 707-SF," "Newcol(R) 707-SFC," "Newcol(R) 707-SN," "Newcol(R) 723-SF," and "Newcol(R) 740-SF" (a compound represented by formula (I) (where R is an aryl group and X is -SO3 - Y + , Y is NH4 + )), "Latemul E-118B" and "Latemul E-150" manufactured by Kao Corporation (a compound represented by formula (I) (where R is an alkyl group and X is -SO3 - Y + , Y is Na + )), "Sannoru TD-3130" manufactured by Lion Corporation (a compound represented by formula (I) (R is an alkyl group, X is -SO3 - Y + , Y is Na + )) etc.

[0048] The content of the anionic emulsifier is 0.5 to 5 parts by mass relative to 100 parts by mass of the monomer composition containing the (meth)acrylic acid alkyl ester monomer (A) and the carboxy group-containing monomer (B). When the content of the anionic emulsifier is 0.5 parts by mass or more, stability during suspension polymerization tends to be excellent. The content of the anionic emulsifier is preferably 0.6 parts by mass or more, more preferably 0.7 parts by mass or more. When the content of the anionic emulsifier is 5 parts by mass or less, the adhesive strength of the resulting pressure-sensitive adhesive sheet tends to be increased. The content of the anionic emulsifier is preferably 4 parts by mass or less, more preferably 3 parts by mass or less.

[0049] Alkali metal hydroxides The monomer composition for producing the acrylic polymer particles (A) preferably contains an alkali metal hydroxide, such as sodium hydroxide or potassium hydroxide.

[0050] The content of alkali metal hydroxide is 20 to 90 mol% based on the molar amount of carboxy groups contained in the carboxy group-containing monomer. When the content of alkali metal hydroxide is 20 mol% or more, stability during suspension polymerization tends to be excellent. The content of alkali metal hydroxide is preferably 25 mol% or more, more preferably 30 mol% or more. When the content of alkali metal hydroxide is 90 mol% or less, stability during suspension polymerization tends to be excellent. The content of alkali metal hydroxide is preferably 85 mol% or less, more preferably 80 mol% or less.

[0051] Optional ingredients When producing the acrylic polymer particles (A), optional components other than the monomer composition may be contained. Examples of optional components include a solvent. The solvent preferably contains water. The solvent may be water alone or a mixed solvent of water and a hydrophilic solvent such as alcohol. Examples of other optional components include a polymerization initiator, a chain transfer agent, a thickener, a nonionic emulsifier, and an emulsifier other than the above-mentioned anionic emulsifier.

[0052] The polymerization initiator is preferably an oil-soluble polymerization initiator. Examples of oil-soluble polymerization initiators include benzoyl peroxide, lauroyl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxypivalate, t-butyl peroxybenzoate, t-butyl peroxy-2-ethylhexanoate, 2,2'-azobisisobutyronitrile, 2,2'-azobis-2-methylbutyronitrile, 2,2'-azobis-2,4-dimethylvaleronitrile, 2,2'-azobis-4-methoxy-2,4-dimethylvaleronitrile, and 2,2'-azobismethylisobutyrate. The polymerization initiators can be used alone or in combination of two or more. From the viewpoint of reaction rate during suspension polymerization, organic peroxides are preferred, and benzoyl peroxide is more preferred.

[0053] The content of the polymerization initiator is preferably 0.01 to 1 part by mass, more preferably 0.05 to 0.8 parts by mass, and even more preferably 0.1 to 0.5 parts by mass, relative to 100 parts by mass of the monomer composition. When the content is 0.01 part by mass or more, the polymerization reaction during suspension polymerization tends to proceed quickly. When the content is 1 part by mass or less, the polymerization reaction during suspension polymerization does not occur suddenly, and the reaction heat can be controlled.

[0054] The chain transfer agent may be, for example, a compound having a sulfhydryl group or a hydroxyl group. Examples of compounds having a sulfhydryl group include thiols such as lauryl mercaptan, 2-mercaptoethyl alcohol, dodecyl mercaptan, and mercaptosuccinic acid; alkyl mercaptopropionates such as n-butyl mercaptopropionate and octyl mercaptopropionate; and alkoxyalkyl mercaptopropionates such as methoxybutyl mercaptopropionate. Examples of compounds having a hydroxyl group include alcohols such as methyl alcohol, n-propyl alcohol, isopropyl alcohol, t-butyl alcohol, and benzyl alcohol. The chain transfer agents can be used alone or in combination of two or more.

[0055] The content of the chain transfer agent is preferably 0 to 0.5 parts by mass, more preferably 0 to 0.4 parts by mass, and even more preferably 0 to 0.3 parts by mass, relative to 100 parts by mass of the monomer composition. When the content is 0.5 parts by mass or less, the gel fraction of the acrylic particles obtained by suspension polymerization increases, and the removability of the pressure-sensitive adhesive sheet obtained from the acrylic polymer particles (A) improves.

[0056] (Method for producing acrylic polymer particles (A)) A method for producing acrylic polymer particles (A) according to one embodiment of the present invention includes preparing a monomer composition containing specific amounts of a (meth)acrylic acid alkyl ester monomer and a carboxyl group-containing monomer; and suspension polymerizing the monomers. In this specification, preparing the monomer composition may be referred to as a "preparation step," and suspension polymerizing the monomers containing the monomers may be referred to as a "polymerization step." The method for producing acrylic polymer particles (A) may further include any optional step.

[0057] (Preparation process) In the preparation step, a monomer composition is prepared that contains at least 0.5 to 5 parts by mass of an anionic emulsifier containing an oxyethylene group in the molecule, and 20 to 80 mol % of an alkali metal hydroxide relative to the molar amount of carboxy groups contained in the carboxy group-containing monomer, relative to a total of 100 parts by mass of a (meth)acrylic acid alkyl ester monomer and a monomer containing a carboxy group. The monomer composition may further contain optional components. The mixing method is not particularly limited, and may be a method in which all components are added in order to a container and stirred, or a method in which some components are added in order to one container and stirred to obtain a composition, the remaining components are added in order to another container and stirred to obtain a composition, and then the two compositions are mixed and stirred.

[0058] (Polymerization process) In the polymerization step, a (meth)acrylic acid alkyl ester monomer and a carboxy group-containing monomer are suspension polymerized. From the viewpoint of increasing the polymerization rate, the polymerization reaction temperature is preferably 70°C or higher, more preferably 80°C or higher. From the viewpoint of safety, the polymerization reaction temperature is preferably 96°C or lower, more preferably 90°C or lower. From the viewpoint of safety, the polymerization reaction time is preferably 4 hours or longer, more preferably 5 hours or longer. From the viewpoint of productivity, the polymerization reaction time is preferably 12 hours or shorter, more preferably 10 hours or shorter.

[0059] A suspension polymer containing acrylic polymer particles (A) is obtained by suspension polymerization of a monomer using a monomer composition. The acrylic polymer particles (A) are particles obtained by suspension polymerization of a monomer. The suspension polymer may further contain components that remain unreacted, reaction by-products, etc. The suspension polymer has a milky white emulsion appearance. Since the D50 is 10 μm or more and 30 μm or less, the acrylic polymer particles (A) are easily separated from the water solvent. Therefore, it is preferable to adjust the viscosity using a thickener after the polymerization step. Polyacrylic acid-based, urethane-based, and cellulose-based thickeners can be used as the thickener. From the viewpoint of affinity with the acrylic polymer particles (A), polyacrylic acid-based thickeners are preferred.

[0060] (Optional process) The method for producing acrylic polymer particles (A) may further include, as optional steps, a step of neutralizing the acrylic polymer particles (A), a step of separating the acrylic polymer particles (A) from the suspension polymer, a step of washing the acrylic polymer particles (A), etc. A basic compound can be used for the neutralization. Examples of the basic compound include ammonia; amines such as monoethylamine, dimethylethanolamine, and methylpropanolamine, etc.

[0061] According to the manufacturing method of the embodiment of the present invention, acrylic polymer particles (A) having a desired particle size distribution can be produced. The monomer composition used in the manufacturing method preferably contains an anionic emulsifier containing an oxyethylene group in the molecule. This is thought to improve the dispersion stability of the monomer composition during suspension polymerization, allowing the monomer to be dispersed in the monomer composition at a suitable size and high concentration. Furthermore, it is thought to prevent the monomer composition from separating into a layer containing the monomer and a layer containing the solvent. As a result, it is possible to obtain acrylic polymer particles (A) at a high concentration and with a desired particle size distribution.

[0062] The particle size distribution, D10, D50 and D90, of the acrylic polymer particles (A) can be adjusted by changing the monomer concentration of the monomer dispersion, the contents of components such as anionic emulsifiers and alkali metal hydroxides contained in the monomer composition, etc.

[0063] For example, the particle size distribution of the acrylic polymer particles (A) tends to narrow as the content of the anionic emulsifier increases, and the particle size distribution of the acrylic polymer particles (A) tends to widen as the content of the anionic emulsifier decreases. Also, the particle size distribution of the acrylic polymer particles (A) tends to narrow as the content of the alkali metal hydroxide increases, and the particle size distribution of the acrylic polymer particles (A) tends to widen as the content of the alkali metal hydroxide decreases.

[0064] [Acrylic polymer particles (B)] The acrylic polymer particles (B) have a D50 of 0.10 μm or more, preferably 0.12 μm or more, and more preferably 0.15 μm or more, and a D50 of 0.6 μm or less, preferably 0.5 μm or less, and more preferably 0.4 μm or less.

[0065] The particle size distribution and D50 of the acrylic polymer particles (B) can be measured in the same manner as for the acrylic polymer particles (A). The emulsion polymer will be described later.

[0066] When D50 is 0.1 μm or more, the dispersion stability of the acrylic polymer particles (B) is improved and the viscosity of the adhesive can be kept low.When D50 is 0.6 μm or less, the dispersion stability of the adhesive after mixing with the acrylic polymer particles (A) is improved.

[0067] The acrylic polymer particles (B) preferably contain, as the monomers constituting the particles, a (meth)acrylic acid alkyl ester monomer, a carboxy group-containing monomer, and a hydroxy group-containing monomer, and more preferably contain, relative to 100% by mass of the monomer composition, 1 to 10% by mass of a monomer including a carboxy group-containing monomer and 1 to 10% by mass of a monomer including a hydroxy group-containing monomer.

[0068] (Acrylic polymer particles (B)) The acrylic polymer particles (B) are obtained by emulsion polymerization of a monomer composition containing specific amounts of a (meth)acrylic acid alkyl ester monomer, a carboxyl group-containing monomer, and a hydroxyl group-containing monomer. An anionic emulsifier is preferably used for the emulsion polymerization. The acrylic polymer particles (B) are preferably prepared by adding 2 to 10 parts by mass of the anionic emulsifier per 100 parts by mass of the monomer composition.

[0069] Examples of the (meth)acrylic acid alkyl ester monomer and the carboxy group-containing monomer include the (meth)acrylic acid alkyl ester monomer and the carboxy group-containing monomer described in the above acrylic polymer particles (A) and the monomers exemplified below. Examples of the hydroxyl group-containing monomer include the monomers in which the carboxy group in the above carboxy group-containing monomer examples is replaced with a hydroxyl group and the monomers exemplified below.

[0070] The (meth)acrylic acid alkyl ester monomer may be the (meth)acrylic acid alkyl ester monomer described above in connection with the acrylic polymer particles (A). The use of a (meth)acrylic acid alkyl ester monomer further enhances the stability during emulsion polymerization and the dispersion stability of the PSA. It is more preferable that the (meth)acrylic acid alkyl ester monomer contains at least one selected from the group consisting of 2-ethylhexyl acrylate, butyl acrylate, and methyl methacrylate.

[0071] The carboxyl group-containing monomer may be the same as that described above for the acrylic polymer particles (A). The use of a carboxyl group-containing monomer further improves the stability during emulsion polymerization and the dispersion stability of the pressure-sensitive adhesive. It is more preferable that the monomer contains (meth)acrylic acid, and even more preferable that the monomer contains acrylic acid.

[0072] The hydroxyl group-containing monomer preferably contains at least one selected from the group consisting of 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 6-hydroxyhexyl (meth)acrylate, as well as polyalkylene glycol (meth)acrylates such as polyethylene glycol (meth)acrylate and polypropylene glycol (meth)acrylate. Use of the hydroxyl group-containing monomer (c) further enhances the stability during emulsion polymerization and the dispersion stability of the PSA. More preferably, the monomer contains at least one selected from the group consisting of 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, and 4-hydroxybutyl acrylate.

[0073] The content of the (meth)acrylic acid alkyl ester monomer is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, based on 100% by mass of the monomer composition. When the content is 80% by mass or more, the polymerization rate can be increased. As a result, the cohesive strength of the acrylic polymer particles is increased, and removability is further improved. The content of the (meth)acrylic acid alkyl ester monomer may be, for example, 98% by mass or less, 97% by mass or less, or 95% by mass or less, based on the total mass of the monomers, taking into account the content of the carboxyl group-containing monomer and the hydroxyl group-containing monomer.

[0074] From the viewpoint of dispersion stability of the monomer composition during polymerization and stability when mixed with the acrylic polymer particles (A), the content of the carboxyl group-containing monomer is preferably 1% by mass or more, more preferably 1.2% by mass or more, and even more preferably 1.5% by mass or more, based on 100% by mass of the monomer composition. The content of the carboxyl group-containing monomer is preferably 10% by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less, based on the total mass of the monomers. When the content is 10% by mass or less, the viscosity of the emulsion polymer obtained by emulsion polymerization can be reduced.

[0075] From the viewpoint of dispersion stability of the monomer composition during polymerization and stability when mixed with the acrylic polymer particles (A), the content of the hydroxyl group-containing monomer is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, based on 100% by mass of the monomer composition. Excellent stability after mixing tends to result in a single peak in the particle size distribution measured by laser diffraction scattering. The content of the hydroxyl group-containing monomer is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 7% by mass or less, based on the total mass of the monomers. When the content is 10% by mass or less, the viscosity of the emulsion polymer obtained by emulsion polymerization can be reduced.

[0076] The monomer composition constituting the acrylic polymer particles (B) may further contain any monomer other than the (meth)acrylic acid alkyl ester monomer, the carboxy group-containing monomer, and the hydroxy group-containing monomer. The content of the optional monomer is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, based on the total mass of the monomers. When the content is 20% by mass or less, the polymerization rate can be increased.

[0077] The content of the monomer in the monomer composition for producing the acrylic polymer particles (B) is preferably 70% by mass or more but less than 100% by mass, based on 100% by mass of the solid content of the monomer composition excluding the solvent. When the content of the monomer is 70% by mass or more, the polymerization rate can be increased. The content of the monomer is preferably 75% by mass or more, more preferably 80% by mass or more.

[0078] Anionic emulsifier The monomer composition for producing the acrylic polymer particles (B) preferably contains 2 to 10 parts by mass of an anionic emulsifier per 100 parts by mass of the monomer composition. The anionic emulsifier may be any of the anionic emulsifiers described above for the acrylic polymer particles (A). The anionic emulsifiers may be used alone or in combination of two or more.

[0079] When the amount of anionic emulsifier added per 100 parts by mass of the monomer composition is 2 parts by mass or more, the stability during emulsion polymerization tends to be excellent. The amount of anionic emulsifier added per 100 parts by mass of the monomer composition is preferably 3 parts by mass or more, more preferably 4 parts by mass or more. When the amount of anionic emulsifier added is 10 parts by mass or less, the adhesive strength of the resulting pressure-sensitive adhesive sheet tends to be increased. The content of the anionic emulsifier is preferably 9 parts by mass or less, more preferably 7 parts by mass or less.

[0080] Optional ingredients The acrylic polymer particles may contain optional components other than the monomer composition, such as water, a solvent such as alcohol, a polymerization initiator, a chain transfer agent, a thickener, a polymer, and an emulsifier such as a nonionic emulsifier.

[0081] The polymerization initiator is preferably a water-soluble polymerization initiator. The water-soluble polymerization initiator is preferably a peroxide, and a combination of a peroxide and a reducing agent (hereinafter also referred to as a redox initiator) may be used. When a redox initiator is used, the molecular structure of the acrylic polymer particles (B) tends to become a linear structure with fewer branches, thereby further improving removability.

[0082] Examples of peroxides include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate, and tert-butyl hydroperoxide. Examples of reducing agents include sodium pyrosulfite, L-ascorbic acid, and iron(II) salts.

[0083] Specific preferred combinations of peroxide and reducing agent in the redox initiator include, for example, a combination of peroxide and ascorbic acid (such as a combination of aqueous hydrogen peroxide and ascorbic acid), a combination of peroxide and an iron (II) salt (such as a combination of aqueous hydrogen peroxide and an iron (II) salt), a combination of persulfate and sodium hydrogen sulfite, etc. From the viewpoint of further improving removability, a combination of ammonium persulfate and sodium pyrosulfite is more preferred.

[0084] The content of the polymerization initiator is preferably 0.2 to 1 part by mass, more preferably 0.2 to 0.6 parts by mass, relative to 100 parts by mass of the solid content of the monomer composition. When it is 0.2 to 1 part by mass, emulsion polymerization property and removability tend to be further improved. In addition, the redox initiator is preferably a peroxide / reducing agent (mass ratio) of about 2 / 1.

[0085] The monomer composition preferably contains a chain transfer agent.

[0086] (Method for producing acrylic polymer particles (B)) The method for producing acrylic polymer particles (B) includes preparing a monomer composition containing specific amounts of a (meth)acrylic acid alkyl ester monomer, a carboxyl group-containing monomer, and a hydroxyl group-containing monomer, and 2 to 10 mass% of an anionic emulsifier relative to 100 mass% of the monomer composition; and emulsion-polymerizing the monomers. As with the method for producing acrylic polymer particles (A), in this specification, preparing the monomer composition may be referred to as a "preparation step," and emulsion-polymerizing the monomers may be referred to as a "polymerization step." Other optional steps may also be included.

[0087] (Preparation process) In the preparation step, a monomer composition is prepared that contains at least a (meth)acrylic acid alkyl ester monomer, a carboxyl group-containing monomer, and a hydroxyl group-containing monomer, with the carboxyl group-containing monomer and the hydroxyl group-containing monomer accounting for 1 to 10 mass% of the monomer composition in 100 mass% of the monomer composition, and 2 to 10 mass parts of an anionic emulsifier per 100 mass parts of the monomer composition. The monomer composition may further contain optional components. The mixing method and the like are as described for the acrylic polymer particles (A).

[0088] (Polymerization process) In the polymerization step, the monomer composition is emulsion-polymerized. In the emulsion polymerization, the polymerization reaction temperature is preferably 70°C or higher, more preferably 80°C or higher, from the viewpoint of increasing the polymerization rate. From the viewpoint of safety, the polymerization reaction temperature is preferably 96°C or lower, more preferably 90°C or lower. From the viewpoint of safety, the polymerization reaction time is preferably 1 hour or longer, more preferably 2 hours or longer. From the viewpoint of productivity, the polymerization reaction time is preferably 12 hours or shorter, more preferably 10 hours or shorter.

[0089] An emulsion polymer containing acrylic polymer particles (B) is obtained by emulsion polymerization of monomers using a monomer composition. The acrylic polymer particles (B) are particles obtained by emulsion polymerization of monomers. The emulsion polymer may further contain unreacted remaining components, reaction by-products, etc. The emulsion polymer has a milky white emulsion appearance, similar to the suspension polymer obtained from the acrylic polymer particles (A). Compared to the acrylic polymer particles (A), the D50 is smaller, at 0.1 μm or more and 0.6 μm or less, so the acrylic polymer particles (B) are less likely to separate from the water solvent. Therefore, they can be used without adding a thickener.

[0090] (Optional process) The method for producing the acrylic polymer particles (B) may include any steps, similar to those for producing the acrylic polymer particles (A).

[0091] [Composition, manufacturing method, etc. of water-based adhesive] In a preferred embodiment, the mass ratio (A / B) of the acrylic polymer particles (A) to the acrylic polymer particles (B) in the aqueous pressure-sensitive adhesive is preferably 80 / 20 to 50 / 50. A mass ratio (A / B) of 80 / 20 to 50 / 50 tends to improve adhesive strength and removability. The mass ratio (A / B) is more preferably 75 / 25 to 52 / 48, and even more preferably 70 / 30 to 55 / 45.

[0092] The aqueous adhesive contains water as a solvent. The aqueous adhesive may contain additives commonly used in aqueous adhesives, as needed. Examples of additives include neutralizers, defoamers, preservatives, wetting agents, leveling agents, thickeners, coloring pigments, plasticizers, antioxidants, and UV absorbers.

[0093] The nonvolatile content of the aqueous pressure-sensitive adhesive is preferably 40 to 60% by mass. When the nonvolatile content is 40% by mass or more, the pressure-sensitive adhesive dries well when applied and has high adhesive strength. When the nonvolatile content is 60% by mass or less, the storage stability of the aqueous pressure-sensitive adhesive is good. The nonvolatile content is preferably 42 to 55% by mass, and more preferably 45 to 50% by mass.

[0094] The method for producing the aqueous pressure-sensitive adhesive is not particularly limited. For example, the method for producing the aqueous pressure-sensitive adhesive includes mixing a composition containing acrylic polymer particles (A) having a D10 of 1.0 μm to 7.0 μm, a D50 of 10 μm to 30 μm, and a D90 of 20 μm to 40 μm with a composition containing acrylic polymer particles (B) having a D50 of 0.10 μm to 0.60 μm. A dispersing machine such as a paint conditioner, ball mill, sand mill, attritor, pearl mill, Co-ball mill, homomixer, homogenizer, wet jet mill, or microbead mill can be used for mixing.

[0095] <Adhesive sheet> The pressure-sensitive adhesive sheet according to one embodiment of the present invention comprises a substrate and a pressure-sensitive adhesive layer in contact with at least one surface of the substrate, the pressure-sensitive adhesive layer being formed using the aqueous pressure-sensitive adhesive of the embodiment. The coating amount of the pressure-sensitive adhesive layer is, for example, 1 to 20 g / m in a dry state. 2 is.

[0096] Examples of the substrate include plastic films such as polyethylene terephthalate film and polyolefin film, and paper such as fine paper, coated paper, impregnated paper, and synthetic paper. From the viewpoint of the particle size of the acrylic polymer particles (A), it is more preferable that the pressure-sensitive adhesive sheet of the present invention uses a plastic film substrate.

[0097] The pressure-sensitive adhesive sheet may further have a release sheet on the surface of the pressure-sensitive adhesive layer opposite to the surface in contact with the substrate. Examples of the release sheet include paper or plastic film to which releasability has been imparted with silicone or the like.

[0098] The pressure-sensitive adhesive sheet can be produced, for example, by coating a release sheet with an aqueous pressure-sensitive adhesive, drying the coating, and then laminating the sheet to a substrate. Alternatively, the pressure-sensitive adhesive sheet can be produced by coating a substrate with an aqueous pressure-sensitive adhesive, drying the coating, and then laminating the sheet to a release sheet. For coating the pressure-sensitive adhesive, known coating devices can be used, such as roll coaters such as comma coaters, blade coaters, and gravure coaters, slot die coaters, lip coaters, and curtain coaters.

[0099] In a pressure-sensitive adhesive layer formed from an aqueous pressure-sensitive adhesive, the acrylic polymer particles (B) function as a binder, imparting adhesiveness to the pressure-sensitive adhesive layer. The acrylic polymer particles (A) are dispersed in the acrylic polymer particles (B) to form numerous protrusions on the surface of the pressure-sensitive adhesive layer. The protrusions (convex portions) function to reduce the tackiness of the pressure-sensitive adhesive layer. By containing the acrylic polymer particles (A) and the acrylic polymer particles (B), the aqueous pressure-sensitive adhesive can form a pressure-sensitive adhesive layer that has appropriate adhesive strength and is easy to removably. In particular, by having the particle diameters of the acrylic polymer particles (A) such that D10 is 1 μm or more and 7 μm or less, D50 is 10 μm or more and 30 μm or less, and D90 is 20 μm or more and 40 μm or less, the wettability of the pressure-sensitive adhesive to the film substrate when coated is improved, resulting in a good pressure-sensitive adhesive layer. [Example]

[0100] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples. In the examples, "parts" means "parts by mass" and "%" means "% by mass."

[0101] (Measurement of weight average molecular weight) The weight average molecular weight was measured using a GPC measuring device under the following conditions: Apparatus: Shodex GPC System-21 (manufactured by Resonac Co., Ltd.) Column: One Shodex KF-602.5 and two Shodex KF-606M columns (Showa Denko K.K.) were used in total of three columns connected together. Solvent: tetrahydrofuran Flow rate: 0.5ml / min Temperature: 40℃ Sample concentration: 0.1% Sample injection volume: 50 μl

[0102] (Measurement of particle size distribution of acrylic polymer particles and determination of D10, D50 and D90) The particle size distribution of acrylic polymer particles was measured using a laser diffraction / scattering particle size analyzer (Microtrac MT-3300, manufactured by Microtrac-Bell Corporation). Samples were prepared by diluting with a dispersion medium. The measurement conditions were a particle refractive index of 1.44 and a non-spherical particle shape. A semiconductor laser (oscillation wavelength 780 nm) was used as the measurement light source. The particle sizes at which the cumulative frequency from the smallest particle size side in the cumulative particle size distribution obtained by the measurement (corresponding to D10, D50, and D90, respectively) was calculated using the Microtrac Version 11.0.0 software provided with the analyzer. Measurements were performed three times, and the average values ​​of the D10, D50, and D90 values ​​obtained from each measurement were calculated and used as the D10, D50, and D90 values ​​for the acrylic polymer particles.

[0103] (Viscosity measurement of water-based adhesives) The viscosity of the aqueous adhesive was measured using a BL type viscometer (manufactured by TOKI SANGYO Co., Ltd.) at 25°C, rotor No. 4, and 60 rpm.

[0104] <Production of Acrylic Polymer Particles (A)> (Manufacturing example A-1) [Preparation of Monomer Dispersion (Preparation Step)] 95 parts of 2-ethylhexyl acrylate was added to a container as a (meth)acrylic acid alkyl ester monomer, and further 0.27 parts of "Niper BW" (75% powder of benzoyl peroxide, manufactured by NOF Corporation) was added as an oil-soluble polymerization initiator. The oil-soluble polymerization initiator was dissolved in 2-ethylhexyl acrylate. Then, 5 parts of acrylic acid was added to the container as a carboxyl group-containing monomer to obtain a monomer component. Next, 29 parts of deionized water was added to another container as a solvent, 3.3 parts (1.0 part as the net amount) of "Newcol 707-SF" (30% aqueous solution of ammonium polyoxyethylene polycyclic phenyl ether sulfate, weight average molecular weight 1,000, manufactured by Nippon Emulsion Co., Ltd.) was added as an anionic emulsifier, and 8 parts of a 25% aqueous solution of sodium hydroxide was added as an alkali metal hydroxide to obtain a water component. The above monomer component was added to the above water component and stirred to mix, and a monomer dispersion having a viscosity of 100 mPa·s and a pH of 5 was obtained.

[0105] [Suspension polymerization (polymerization step) of monomer (monomer)] 64.6 parts of water was added to a four-necked flask equipped with a stirrer, a cooling tube, a thermometer and a dropping funnel, the inside of the flask was replaced with nitrogen gas, and the internal temperature was heated to 80°C while stirring. Then, the above monomer dispersion was added from the dropping funnel to initiate a suspension polymerization reaction. After the monomer dispersion was dropped over 2 hours while maintaining the temperature in the flask at 80°C, the reaction was continued for 6 hours while stirring while maintaining the temperature in the flask at 80°C. Then, the composition obtained after suspension polymerization was cooled to obtain a composition (aqueous dispersion) containing acrylic polymer particles A-1. The nonvolatile content concentration was 50%, the D10 of the acrylic polymer particles A-1 was 3.0 μm, the D50 was 20 μm, and the D90 was 30 μm. The particle size distribution of the obtained acrylic polymer particles A-1 is shown in FIG. 1.

[0106] (Production Examples A-2 to A-22, AX-1 to AX-6> An acrylic polymer particle-containing composition containing acrylic polymer particles was obtained in the same manner as in Production Example A-1, except that the compounds and blending amounts shown in Table 1 were changed. In Production Example A-19, a 25% aqueous solution of potassium hydroxide was used instead of the 25% aqueous solution of sodium hydroxide. In Production Example AX-1, a monomer dispersion was not prepared in a separate container. Instead, all the raw materials were charged into a four-necked flask equipped with a stirrer, a condenser, a thermometer, and the dropping funnel. The inside of the flask was purged with nitrogen gas, and while stirring, the internal temperature was heated to 80 °C to carry out suspension polymerization. The particle size distribution of the obtained acrylic polymer particles AX-1 is shown in Fig. 1.

[0107] <Production of Acrylic Polymer Particles (B)> (Production Example B-1) [Preparation of Monomer Dispersion] Into a reaction vessel, 80 parts of 2-ethylhexyl acrylate and 13 parts of methyl methacrylate as (meth)acrylic acid alkyl ester monomers, 2 parts of acrylic acid as a carboxyl group-containing monomer, 5 parts of 2-ethylhexyl acrylate as a hydroxyl group-containing monomer, and 0.05 part of OTG (2-ethylhexyl thioglycolate) as a chain transfer agent were added. Further, 26.1 parts of water and 25 parts (5 parts as the net amount) of "Hitenol NF-08" (a 20% aqueous solution of sodium polyoxyethylene alkyl ether sulfate, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) as an anionic emulsifier were added and stirred to obtain a monomer dispersion. The content (solid content (parts)) of each component is shown in Table 2.

[0108] [Emulsion Polymerization of Monomer (Monomers)] Separately, 55.6 parts of water was added to a four-neck flask equipped with a stirrer, condenser, thermometer, and dropping funnel. The flask was purged with nitrogen gas and heated to an internal temperature of 80°C while stirring. Subsequently, 8.3 parts of 3% potassium persulfate was added to the flask, and the monomer dispersion was added dropwise from the dropping funnel over 240 minutes while maintaining the temperature at 80°C. The reaction was continued for 1 hour while maintaining the temperature at 80°C. Subsequently, 0.6 parts of a 3% aqueous solution of potassium persulfate was added to the flask twice every 30 minutes, and the reaction was continued for another 1 hour. The mixture was then cooled to obtain an aqueous dispersion containing acrylic polymer particles B-1 as an emulsion polymer. The nonvolatile content of the aqueous dispersion was 50%, and the D50 of the acrylic polymer particles B-1 was 0.2 μm.

[0109] <Manufacturing examples B-2~B-18, BX-1~BX-2> A water dispersion containing acrylic polymer particles was obtained in the same manner as in Production Example B-1, except that the compounds and contents were changed as shown in Table 2.

[0110] The abbreviations used in Tables 1 and 2 are as follows. The numerical values ​​in the tables represent "parts" unless otherwise specified. <Monomer> <(Meth)acrylic acid alkyl ester monomer> EHA: 2-ethylhexyl acrylate BA: Butyl acrylate MMA: Methyl methacrylate EA: Ethyl acrylate iOA: Isooctyl acrylate <Carboxy group-containing monomer> AA: Acrylic acid ·MAA: methacrylic acid <Hydroxyl group-containing monomer> HEMA: 2-hydroxyethyl methacrylate HEA: 2-hydroxyethyl acrylate 4HBA: 4-hydroxybutyl acrylate <Anionic emulsifier> Newcol 707-SF: Polyoxyethylene polycyclic phenyl ether sulfate ester ammonium salt, weight average molecular weight 1,000, manufactured by Nippon Nyukazai Co., Ltd. Hitenol NF-08: 20% aqueous solution of polyoxyethylene alkyl ether sulfate sodium salt, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.

[0111] [Table 1]

[0112] [Table 2]

[0113] Example 1 [Preparation of water-based adhesive] To 60 parts of the aqueous dispersion containing acrylic polymer particles (A) obtained in Production Example A-1, 40 parts of the aqueous dispersion containing acrylic polymer particles (B) obtained in Production Example B-1, 0.5 parts of 25% aqueous ammonia as a neutralizer, 0.3 parts of "Noptam 2364" (a mineral oil-based antifoaming agent, manufactured by San Nopco) as an antifoaming agent, 0.2 parts of "Levanax BX-150" (an isothiazolinone-based preservative, manufactured by Shoei Chemical Co., Ltd.) as a preservative, and 1 part of "Primal ASE-60" (a polyacrylic acid, manufactured by Dow Chemical Co.) as an alkaline thickener were added to obtain an aqueous pressure-sensitive adhesive. The viscosity of the aqueous pressure-sensitive adhesive was 2,000 mPa·s.

[0114] [Preparation of adhesive sheet (PET film)] The obtained aqueous pressure-sensitive adhesive was applied to the corona-treated surface of a 50 μm-thick polyethylene terephthalate (PET) film (E5100, manufactured by Toyobo Co., Ltd.) so that the thickness after drying was 8 g / m 2 The adhesive layer was formed by applying the adhesive with an applicator so that the adhesive layer was coated with a thickness of 100°C and drying for 1 minute at 100°C. Next, a release paper was attached to the adhesive layer and aged for 24 hours under conditions of a temperature of 23°C and a relative humidity of 50%, to obtain an adhesive sheet with a structure of "PET film / adhesive layer / release paper."

[0115] <Evaluation of adhesive sheets> The pressure-sensitive adhesive sheets obtained in the Examples and Comparative Examples were evaluated for adhesive strength, removability, and holding power according to the following methods.

[0116] (Adhesive strength) The adhesive sheet (PET film) was cut to a size of 100 mm long x 25 mm wide in an environment of 23°C and 50% relative humidity (hereinafter referred to as 50% RH) to prepare a sample. The sample was then left in an environment of 23°C and 50% RH for 24 hours, after which the release paper was peeled off and the sample was attached to a piece of fine paper that had not been coated with adhesive. A 2 kg roll was then rolled back and forth over the sample. Next, after leaving the sample in an environment of 23°C and 50% RH for 0 minutes and for 24 hours, the adhesive sheet (PET film) and the fine paper (medium-weight fine paper, thickness 95 μm, basis weight 76.7 g / m) were separated. 2 The adhesive strength of each sample was measured by peeling off the adhesive tape (manufactured by APP) from the tape. The adhesive strength was measured using a tensile tester (manufactured by Tensilon, RTG-1210) in a 180° direction at a peeling speed of 300 mm / min. The adhesive strength was evaluated according to the following criteria. ◎: Adhesive strength of 0.5N / 25mm or more. Extremely good. 〇: Adhesive strength 0.3N / 25mm or more, less than 0.5N / 25mm. Good. △: Adhesive strength 0.1N / 25mm or more, less than 0.3N / 25mm. Practical use possible. ×: Adhesive strength less than 0.1N / 25mm. Not practical.

[0117] (removability) The adhesive sheet (PET film) was cut to a length of 100 mm and a width of 25 mm under an environment of 23°C and 50% RH to prepare a sample. The release paper was then peeled off, and the sample was laminated to a piece of high-quality paper that had not been coated with adhesive. A 2 kg roll was then rolled back and forth over the sample. The adhesive sheet (PET film) and the copy paper were then peeled off by hand, and the sample was then laminated to the high-quality paper again. This procedure was repeated 50 times to measure removability. The removability was evaluated by calculating the ratio of the area of ​​torn paper to the total area of ​​the sample according to the following criteria. ◎: No paper tears at all. Very good. Good: Paper tears occurred in less than 5% of the area. △: Paper tear occurred over an area of ​​5% or more but less than 10%. Usable. ×: Paper tears occurred over 10% or more of the area. Unusable.

[0118] <Examples 2 to 43 and Comparative Examples 1 to 8> Aqueous pressure-sensitive adhesives and pressure-sensitive adhesive sheets were produced and evaluated in the same manner as in Example 1, except that the materials and compositions were changed to those shown in Tables 3 and 4.

[0119] [Table 3]

[0120] [Table 4]

Claims

1. An aqueous pressure-sensitive adhesive containing at least two types of acrylic polymer particles (A) and acrylic polymer particles (B), wherein, in a cumulative particle size distribution on a volume basis, when D10, D50, and D90 are the particle sizes at which the cumulative frequency from the small particle size side becomes 10%, 50%, and 90%, respectively, the acrylic polymer particles (A) have a D10 of 1 μm or more and 7 μm or less, a D50 of 10 μm or more and 30 μm or less, and a D90 of 20 μm or more and 40 μm or less, and the acrylic polymer particles (B) have a D50 of 0.1 μm or more and 0.6 μm or less.

2. 2. The aqueous pressure-sensitive adhesive according to claim 1, wherein the mass ratio (A / B) of the acrylic polymer particles (A) to the acrylic polymer particles (B) is 80 / 20 to 50 / 50.

3. 2. The aqueous pressure-sensitive adhesive according to claim 1, wherein the acrylic polymer particles (A) comprise a suspension polymer of a monomer composition containing a (meth)acrylic acid alkyl ester monomer and a carboxy group-containing monomer.

4. The aqueous pressure-sensitive adhesive according to claim 1, wherein the acrylic polymer particles (B) comprise an emulsion polymer of a monomer composition containing a carboxy group-containing monomer and a hydroxy group-containing monomer, and the acrylic polymer particles (B) contain, relative to 100% by mass of the monomer composition, 1% by mass or more and 5% by mass or less of the carboxy group-containing monomer and 2% by mass or more and 10% by mass or less of the hydroxy group-containing monomer.

5. The aqueous pressure-sensitive adhesive according to claim 1, which is for use on a resin film substrate.

6. A pressure-sensitive adhesive sheet comprising a substrate and a pressure-sensitive adhesive layer formed from the aqueous pressure-sensitive adhesive according to any one of claims 1 to 5 on at least one surface of the substrate.

7. A method for producing an aqueous pressure-sensitive adhesive containing at least two types of acrylic polymer particles (A) and acrylic polymer particles (B), the method comprising the steps of: producing acrylic polymer particles (A) by suspension polymerization of a monomer composition in the presence of an anionic emulsifier having an oxyethylene group; and producing acrylic polymer particles (B) by emulsion polymerization in the presence of an anionic emulsifier having a polyoxyethylene group. In a cumulative particle size distribution based on volume, when D10, D50, and D90 are the particle sizes at which the cumulative frequency from the small particle size side is 10%, 50%, and 90%, respectively, the acrylic polymer particles (A) have a D10 of 1 μm or more and 7 μm or less, a D50 of 10 μm or more and 30 μm or less, and a D90 of 20 μm or more and 40 μm or less, and the acrylic polymer particles (B) have a D50 of 0.1 μm or more and 0.6 μm or less.

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