JP7653825B2 - Copolymer, production method thereof, and aqueous dispersion - Google Patents

Description

The present invention relates to a copolymer, a method for producing the copolymer, and an aqueous dispersion. More specifically, the present invention relates to an aqueous crosslinking agent having, for example, stability of the aqueous dispersion and good reactivity at low temperatures.

It has been known that polymers having structural units derived from oxazoline or epoxy group-containing monomers react with polymers having carboxyl, hydroxyl, or thiol groups to form crosslinked structures, resulting in films with excellent mechanical and chemical properties. For this reason, polymers having structural units derived from oxazoline or epoxy group-containing monomers are widely used in a variety of applications, such as paints for automobiles, building materials (inorganic, metallic, etc.), metals, glass, plastics, woodworking, and various fibers (metal, inorganic, glass, synthetic polymers, etc.), surface treatment agents, coating agents, binders containing sizing agents and treatment agents, pressure sensitive adhesives, adhesives, sealants, films, molded products, and cast products.

For example, Patent Document 1 describes a reactive emulsion in which acrylic emulsion particles serve as a core portion and an oxazoline group-containing polymer layer is formed as a shell portion around the core portion.

Patent Document 2 describes an aqueous resin composition that contains a polymer (A0) obtained by polymerizing monomer components essentially consisting of an oxazoline group-containing monomer (a) and a reactive group-containing monomer (b), and that is characterized in that the polymer (A0) is composed of two or more types of polymers with different Tg's.

JP2008-536951 Patent Publication 2013-057081

Coatings, pressure sensitive adhesives, and other materials that use polymers containing structural units derived from oxazoline or epoxy group-containing monomers to form crosslinking reactions with other polymers containing substituents that react with oxazoline or epoxy groups contribute to improved physical properties such as water resistance and blocking resistance, but have the problem of poor reactivity at low temperatures.

In addition, from the standpoint of environmental protection and occupational safety and health, there is a demand for aqueous dispersions, and there is a demand for improved stability of aqueous dispersions containing polymers having structural units derived from oxazoline or epoxy group-containing monomers. However, it has been found that increasing the content of structural units derived from oxazoline or epoxy group-containing monomers reduces the stability of the aqueous dispersion.

The object of the present invention is to provide a copolymer that has excellent low-temperature reactivity and coating film physical properties when used as a crosslinking agent, and has excellent stability when made into an aqueous dispersion.

The inventors conducted research in consideration of the above problems and discovered that a copolymer having a structural unit derived from an oxazoline group or epoxy group-containing monomer, a structural unit derived from an anionic surfactant, and a structural unit derived from a nonionic surfactant and having a glass transition temperature of less than 10°C has excellent low-temperature reactivity and physical properties when used as a crosslinking agent, and has excellent stability when made into an aqueous dispersion, thereby completing the present invention.

The present invention provides a copolymer that has excellent low-temperature reactivity and coating film physical properties when used as a crosslinking agent, and has excellent stability when made into an aqueous dispersion.

The present invention is described in detail below.

In the following explanation, unless otherwise specified, "%" means "% by mass", "parts" means "parts by mass", and the range "A-B" means A or more and B or less. In addition, in the present invention, "(meth)acrylate" means "acrylate" or "methacrylate", and "(meth)acrylic" means "acrylic" or "methacrylic".

In this specification, the term "resin" refers to a broader concept than polymer. A resin may contain one or more polymers, and may further contain materials other than polymers as necessary.

[Copolymer of the present disclosure]
The copolymer of the present disclosure is preferably a copolymer having a structural unit derived from an oxazoline group- or epoxy group-containing monomer, a structural unit derived from an anionic surfactant, and a structural unit derived from a nonionic surfactant.

In this disclosure, a structural unit derived from an oxazoline group and/or an epoxy group-containing monomer usually refers to a structural unit (hereinafter also referred to as structural unit A) having a structure in which at least one of the unsaturated bonds contained in the oxazoline group and/or the epoxy group-containing monomer is replaced with a saturated bond such as a carbon-carbon single bond. The structural unit A is not limited to a structural unit formed by polymerization of an oxazoline group and/or an epoxy group-containing monomer, and may be a structural unit formed by another method such as a synthetic method as long as it has the same structure as the structural unit A. Similarly, the structural unit of the other monomers described later refers to a structural unit having a structure in which at least one of the unsaturated bonds contained in the respective monomers is replaced with a saturated bond such as a carbon-carbon single bond.

In the present disclosure, examples of oxazoline group- and/or epoxy group-containing monomers include oxazoline group-containing monomers and epoxy group-containing monomers.

In the present disclosure, an oxazoline group-containing monomer is a compound having at least one oxazoline group and at least one polymerizable unsaturated bond. Examples of the oxazoline group-containing monomer of the present disclosure include vinyl oxazolines {e.g., vinyl oxazoline (e.g., 2-vinyl-2-oxazoline), vinyl oxazolines having a substituent [e.g., alkyl-vinyl oxazolines (e.g., 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-4,4-dimethyl-2-oxazoline, 2-vinyl-4-ethyl-2-oxazoline, 2-vinyl-4-propyl-2-oxazoline, 2-vinyl-4-butyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-vinyl-5-ethyl-2-oxazoline, 2-vinyl-5-propyl-2-oxazoline, 2-vinyl-5-butyl-2-oxazoline, and other C _1-20 alkyl-vinyl oxazolines, preferably C _1-10 alkyl-vinyl oxazolines, and more preferably mono- or di-C _1-4 alkyl-vinyl oxazoline) and the like], isopropenyl oxazolines {for example, isopropenyl oxazoline (2-isopropenyl-2-oxazoline, etc.), isopropenyl oxazolines having a substituent [for example, alkyl-isopropenyl oxazolines (for example, C 1-20 alkyl-isopropenyl oxazolines such as 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-4,4-dimethyl-2-oxazoline, 2-isopropenyl-4-ethyl-2-oxazoline, 2-isopropenyl-4-propyl-2-oxazoline, 2-isopropenyl-4-butyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline, 2-isopropenyl-5-propyl-2-oxazoline, and 2 _- isopropenyl-5-butyl-2-oxazoline, preferably C _1-10 alkyl-isopropenyloxazoline, more preferably mono- or di-C _1-4 alkyl-isopropenyloxazoline), etc.}, and the corresponding allyloxazolines.

Among these, isopropenyl oxazolines are preferred, and 2-isopropenyl-2-oxazoline is particularly preferred.

The oxazoline group-containing monomers may be used alone or in combination of two or more.

The epoxy group-containing monomer of the present disclosure is a monomer having an epoxy group and a copolymerizable unsaturated bond. However, in the present disclosure, a monomer containing both an epoxy group and an oxazoline group is included in the oxazoline group-containing monomer. From the viewpoint of forming an adhesive layer having excellent adhesion and heat resistance while having excellent bending properties, the epoxy group-containing monomer is preferably a monofunctional epoxy group-containing monomer having one epoxy group in one molecule. Examples of the epoxy group-containing monomer include unsaturated carboxylic acid glycidyl ester and vinyl group-containing glycidyl ether, but the present invention is not limited to only these examples. These epoxy group-containing monomers may be used alone or in combination of two or more types.

Examples of unsaturated carboxylic acid glycidyl esters include glycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate [(meth)acrylic acid 3,4-epoxycyclohexylmethyl], monoglycidyl itaconate [itaconic acid monoglycidyl ester], monoglycidyl butene tricarbonate [butene tricarboxylic acid monoglycidyl ester], etc., but the present invention is not limited to these examples. These unsaturated carboxylic acid glycidyl esters may be used alone or in combination of two or more. Examples of vinyl group-containing glycidyl ethers include vinyl glycidyl ether, allyl glycidyl ether, glycidyloxyethyl vinyl ether, etc., but the present invention is not limited to these examples. These vinyl group-containing glycidyl ethers may be used alone or in combination of two or more. Among epoxy group-containing monomers, glycidyl (meth)acrylate is preferred from the viewpoint of improving adhesion, heat resistance, and bending properties.

The content of structural units derived from oxazoline group-containing monomers in the copolymer of the present disclosure is preferably 1% by mass or more (e.g., 3% by mass or more) in the copolymer from the viewpoint of improving low-temperature reactivity and physical properties of the coating film when used as a crosslinking agent, more preferably 5% by mass or more, even more preferably 10% by mass or more, and particularly preferably 15% by mass or more, and from the viewpoint of polymerization stability, is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less, and particularly preferably 25% by mass or less.

The content of structural units derived from epoxy group-containing monomers in the copolymer of the present disclosure is preferably 1% by mass or more (e.g., 3% by mass or more) in the copolymer from the viewpoint of improving low-temperature reactivity and physical properties of the coating film when used as a crosslinking agent, more preferably 5% by mass or more, even more preferably 10% by mass or more, and particularly preferably 15% by mass or more, and from the viewpoint of polymerization stability, is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less, and particularly preferably 25% by mass or less.

In the present disclosure, it is preferable that the structural units derived from the oxazoline group- and/or epoxy group-containing monomers essentially contain structural units derived from the oxazoline group-containing monomers. By essentially containing structural units derived from the oxazoline group-containing monomers, it is expected that the low-temperature reactivity and the physical properties of the coating film formed by the crosslinking reaction will be good, and it is also expected that the storage stability will be good when used as an aqueous dispersion and a coating composition.

The copolymer of the present disclosure preferably has structural units derived from an anionic surfactant and structural units derived from a nonionic surfactant from the viewpoint of improving low-temperature reactivity, stability during emulsion polymerization, and stability when made into an aqueous dispersion.

The structural units derived from anionic surfactants and structural units derived from nonionic surfactants of the present disclosure refer to structural units having a structure in which at least one of the unsaturated bonds contained in an anionic surfactant having a reactive group and a nonionic surfactant having a reactive group is replaced with a saturated bond such as a carbon-carbon single bond. Having a reactive group means a surfactant having an unsaturated double bond in the monomer and capable of polymerizing with other monomers, specifically, having a radically polymerizable double bond such as a vinyl group, (meth)acryloyl group, allyl group, or propenyl group in the molecule. Hereinafter, the structural unit derived from a surfactant having a reactive group is also referred to as structural unit B, the structural unit derived from an anionic surfactant is also referred to as structural unit b1, and the structural unit derived from a nonionic surfactant is also referred to as structural unit b2.

Preferred examples of the anionic surfactant having a reactive group according to the present disclosure include an anionic surfactant represented by the following formula (1) and an anionic surfactant represented by the following formula (2).

Among these, the anionic surfactant having a reactive group is preferably an anionic surfactant represented by the following formula (1) from the viewpoint of, for example, industrial productivity, more specifically, the ability to maintain a low viscosity of the (meth)acrylic resin emulsion and excellent handleability.

In formula (1), R ^1a represents an aliphatic alkyl group having 6 to 24 carbon atoms, and X ^1a represents --SO ₃ NH ₄ or --SO ₃ Na.

In addition, in formula (1), n1 represents the average number of moles of oxyethylene groups added (also referred to as the "average number of repeating oxyethylene units"). n1 is an integer from 5 to 50, and preferably an integer from 10 to 30.

Examples of commercially available anionic surfactants represented by formula (1) include Aqualon (registered trademark) KH-10 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether ammonium sulfate [average number of moles of oxyethylene groups added: 10], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], Aqualon (registered trademark) KH-05 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether ammonium sulfate [average number of moles of oxyethylene groups added: 5], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], and Adeka Reasoap (registered trademark) SR-20 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether ammonium sulfate [average number of moles of oxyethylene groups added: 20], amount of active ingredient: 100% by mass, ADEKA Co., Ltd.].

In formula (2), R ^2a represents a substituent represented by the following formula (A) or an aliphatic alkyl group having 6 to 24 carbon atoms, and R ^3a represents a methyl group.

In addition, in formula (2), X ^2a represents -SO ₃ M, -COOM, or PO ₃ M.

M represents an alkali metal atom, an alkaline earth metal atom, or an ammonium group.

Specific examples of alkali metal atoms include sodium atoms and potassium atoms.

Specific examples of alkaline earth metal atoms include calcium atoms and barium atoms.

X ^2a is, for example, preferably --SO ₃ NH ₄ .

In formula (2), n2 represents the average number of moles of oxyethylene groups added (also referred to as the "average number of repeating oxyethylene units"). n2 is an integer from 5 to 50, and preferably an integer from 10 to 30.

In formula (A), m represents an integer of 1 to 3, and from the viewpoint of surface activity, for example, 2 is preferred.

Examples of commercially available anionic surfactants represented by formula (2) include Aqualon (registered trademark) HS-10 [active ingredient: polyoxyethylene nonylpropenyl phenyl ether ammonium sulfate [average number of moles of oxyethylene groups added: 10], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], Aqualon (registered trademark) BC-10 [active ingredient: polyoxyethylene nonylpropenyl phenyl ether ammonium sulfate [average number of moles of oxyethylene groups added: 10], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], Aqualon (registered trademark) BC-20 [active ingredient: polyoxyethylene nonylpropenyl phenyl ether ammonium ether sulfate [average number of moles of oxyethylene groups added: 20], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], Aqualon (registered trademark) AR-10 [active ingredient: polyoxyethylene styrenated propenyl phenyl ether sulfate ammonium [average number of moles of oxyethylene groups added: 10], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], Aqualon (registered trademark) AR-20 [active ingredient: polyoxyethylene styrenated propenyl phenyl ether sulfate ammonium [average number of moles of oxyethylene groups added: 20], amount of active ingredient: 97% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], etc.

As an anionic surfactant having a reactive group, from the viewpoint of the water resistance and polymerization stability of the coating film, the ADEKA REASOAP SR series (ADEKA REASOAP SR-10, SR-20, etc.) is preferred, and ADEKA REASOAP (registered trademark) SR-10 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether ammonium sulfate [average number of moles added of oxyethylene groups: 10], active ingredient amount: 100% by mass, ADEKA Corporation] is more preferred.

In the copolymer of the present disclosure, the content of structural units derived from an anionic surfactant is preferably 0.1% by mass or more, and more preferably 0.5% by mass or more, relative to 100% by mass of the copolymer, from the viewpoint of improving polymerization stability, and is preferably 5% by mass or less, and more preferably 2% by mass or less, from the viewpoint of improving water resistance.

Preferred examples of the nonionic surfactant having a reactive group according to the present disclosure include a nonionic surfactant represented by the following formula (3) and a nonionic surfactant represented by the following formula (4).

Among these, as the nonionic surfactant having a reactive group, for example, the nonionic surfactant represented by the following formula (3) is more preferred from the viewpoint of excellent copolymerizability with monomers.

In formula (3), R ^1b represents an aliphatic alkyl group having 6 to 24 carbon atoms, and X ^1b represents a hydrogen atom.

In addition, in formula (3), n3 represents the average number of moles of oxyethylene groups added (also referred to as the "average number of repeating oxyethylene units"). n3 is an integer from 5 to 40, and preferably an integer from 10 to 30.

In formula (4), R ^2b represents an aliphatic alkyl group having 6 to 24 carbon atoms, R ^3b represents a methyl group, and X ^2b represents a hydrogen atom.

In addition, in formula (4), n4 represents the average number of moles of oxyethylene groups added (also referred to as the "average number of repeating oxyethylene units"). n4 is an integer from 10 to 50, and preferably an integer from 10 to 30.

Examples of commercially available nonionic surfactants represented by formula (3) include ADEKA REASOAP (registered trademark) ER-10 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether [average number of moles of oxyethylene groups added: 10], amount of active ingredient: 100% by mass, ADEKA Corporation], ADEKA REASOAP (registered trademark) ER-30 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether [average number of moles of oxyethylene groups added: 30], amount of active ingredient: 65% by mass, ADEKA Corporation], and ADEKA REASOAP (registered trademark) ER-40 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether [average number of moles of oxyethylene groups added: 40], amount of active ingredient: 60% by mass, ADEKA Corporation].

From the viewpoint of the water resistance and polymerization stability of the coating film, the Adeka Reasoap ER series is preferred as a nonionic surfactant having a reactive group, and an example of a commercially available product of a nonionic surfactant having a reactive group is Adeka Reasoap (registered trademark) ER-20 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether [average number of moles of oxyethylene groups added: 20], active ingredient amount: 100% by mass, ADEKA Corporation].

In the copolymer of the present disclosure, the content of structural units derived from a nonionic surfactant is preferably 1% by mass or more, more preferably 2% by mass or more, relative to 100 parts by mass of the copolymer, from the viewpoint of improving polymerization stability, and is preferably 6% by mass or less, more preferably 5% by mass or less, from the viewpoint of improving water resistance.

In the copolymer of the present disclosure, it is preferable that the content ratio of structural units derived from a nonionic surfactant is higher than that of structural units derived from an anionic surfactant, and the mass ratio of the content ratio of structural units derived from an anionic surfactant to the content ratio of structural units derived from a nonionic surfactant is preferably 5/95 to 40/60, more preferably 10/90 to 30/70, even more preferably 15/85 to 25/75, and particularly preferably 17/83 to 23/77, from the viewpoint of improving the stability of the aqueous dispersion containing the copolymer.

The copolymer of the present disclosure may have a structural unit derived from another monomer (also referred to as structural unit C) other than the structural unit derived from the oxazoline group or epoxy group-containing monomer, the structural unit derived from the anionic surfactant, and the structural unit derived from the nonionic surfactant. The structural unit derived from the other monomer is a structural unit formed by polymerizing the other monomers described below.

Examples of other monomers include alkyl group-containing (meth)acrylic acid ester monomers, hydroxyl group-containing (meth)acrylic acid ester monomers, carboxyl group-containing monomers, aromatic vinyl monomers, maleimide group-containing monomers, amide group-containing monomers, N-substituted (meth)acrylamide monomers, olefin-based monomers, and halogen-containing monomers. Of these, alkyl group-containing (meth)acrylic acid ester monomers, aromatic vinyl monomers, maleimide group-containing monomers, and vinyl cyanide monomers are preferred, and aromatic vinyl monomers are more preferred from the viewpoint of heat resistance, and alkyl group-containing (meth)acrylic acid ester monomers are preferred from the viewpoint of improving the physical properties of the coating film when used as a crosslinking agent.

Examples of aromatic vinyl monomers include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o-ethylstyrene, m-ethylstyrene, p-ethylstyrene, dimethylstyrene, tert-methylstyrene, α-chlorostyrene, β-chlorostyrene, and divinylbenzene, but the present invention is not limited to these examples. These aromatic vinyl monomers may be used alone or in combination of two or more. The aromatic vinyl monomer may have a functional group such as an alkyl group such as a methyl group or a tert-butyl group, a nitro group, a nitrile group, an alkoxyl group, an acyl group, a sulfone group, or a hydroxyl group on the benzene ring. Among aromatic vinyl monomers, styrene is preferred from the viewpoint of heat resistance and low dielectric properties. The aromatic vinyl monomer may contain an oligomer of the aromatic vinyl monomer within a range that does not impede the object of the present invention.

Examples of alkyl group-containing (meth)acrylic acid ester monomers include linear or branched alkyl group-containing (meth)acrylic acid ester monomers having 1 to 20 carbon atoms.

Examples of alkyl group-containing (meth)acrylic acid ester monomers include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and ethyl (meth)acrylate. Examples of the alkyl group-containing (meth)acrylic acid ester monomer include isooctyl acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, and cyclohexyl (meth)acrylate. The alkyl group-containing (meth)acrylic acid ester monomer may be used alone or in combination of two or more.

As the alkyl group-containing (meth)acrylic acid ester monomer, a (meth)acrylic acid alkyl ester having an alkyl group with 1 to 14 carbon atoms is preferred, a (meth)acrylic acid alkyl ester having an alkyl group with 2 to 10 carbon atoms is more preferred, and a (meth)acrylic acid alkyl ester having an alkyl group with 4 to 8 carbon atoms is even more preferred.

In addition, as the alkyl group-containing (meth)acrylic acid ester monomer of the present disclosure, an alkyl acrylate ester having an alkyl group with 1 to 14 carbon atoms is preferred, an alkyl acrylate ester having an alkyl group with 2 to 10 carbon atoms is more preferred, and an alkyl acrylate ester having an alkyl group with 4 to 8 carbon atoms is even more preferred.

Examples of maleimide group-containing monomers include maleimide, N-methylmaleimide, N-ethylmaleimide, N-propylmaleimide, N-isopropylmaleimide, N-butylmaleimide, N-sec-butylmaleimide, N-tert-butylmaleimide, N-amylmaleimide, N-hexylmaleimide, N-heptylmaleimide, N-octylmaleimide, N-nonylmaleimide, N-decylmaleimide, N-decylmaleimide, N-undecylmaleimide, N-dodecylmaleimide (N-laurylmaleimide), N-tridecylmaleimide, N-tetradecylmaleimide, N-pentadecylmaleimide, N-hexadecylmaleimide, N-heptadecadecylmaleimide, N-hex ... Examples of the maleimide-containing monomer include N-cyclohexylmaleimide, N-octadecylmaleimide (N-stearylmaleimide), N-nonadecylmaleimide, N-allylmaleimide, N-cyclohexylmaleimide, N-phenylmaleimide, N-hydroxyphenylmaleimide, N-methylphenylmaleimide, N-dimethylphenylmaleimide, N-ethylphenylmaleimide, N-diethylphenylmaleimide, N-phenylmethylmaleimide, N-methoxyphenylmaleimide, N-chlorophenylmaleimide, N-dichlorophenylmaleimide, N-trichlorophenylmaleimide, and N-tribromophenylmaleimide, but the present invention is not limited to these examples. These maleimide group-containing monomers may be used alone or in combination of two or more types.

Examples of hydroxyl group-containing (meth)acrylic acid ester monomers include hydroxyalkyl (meth)acrylates (e.g., 2-hydroxyethyl (meth)acrylate, etc.).

Examples of carboxyl group-containing monomers include (meth)acrylic acid, maleic acid, fumaric acid, crotonic acid, itaconic acid, citraconic acid, maleic anhydride, maleic acid monomethyl ester, maleic acid monobutyl ester, itaconic acid monomethyl ester, itaconic acid monobutyl ester, and vinylbenzoic acid, but the present invention is not limited to these examples.

Nitrogen atom-containing monomers include N-vinylpyrrolidone, dimethylaminoethyl (meth)acrylate, acrylonitrile, diacetone acrylamide, etc., but the present invention is not limited to these examples.

The content of the structural units derived from the aromatic vinyl monomer of the present disclosure is preferably 0.1% by mass or more, and more preferably 0.3% by mass or more, relative to 100 parts by mass of the copolymer from the viewpoint of improving heat resistance, and is preferably 40% by mass or less, more preferably 10% by mass or less, and more preferably 5% by mass or less from the viewpoint of low-temperature reactivity.

The content ratio of the structural units derived from the aromatic vinyl monomer of the present disclosure is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more, per 100 parts by mass of the structural units derived from other monomers, from the viewpoint of improving heat resistance, and is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less, from the viewpoint of low-temperature reactivity.

From the viewpoint of low-temperature reactivity, the content of structural units derived from the alkyl group-containing (meth)acrylic acid ester monomer of the present disclosure is preferably 40% by mass or more, more preferably 50% by mass or more, 60% by mass or more, 70% by mass or more, and 75% by mass or more, based on 100 parts by mass of the copolymer, and is preferably 95% by mass or less, more preferably 90% by mass or less, and 85% by mass.

From the viewpoint of low-temperature reactivity, the content of the structural units derived from the alkyl group-containing (meth)acrylic acid ester monomer of the present disclosure is preferably 50% by mass or more, more preferably 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, and 95% by mass or more, in that order, relative to 100 parts by mass of the structural units derived from other monomers, and is preferably less than 100% by mass, and more preferably 99.5% by mass or less.

The glass transition temperature (Tg) of the copolymer (non-volatile content) of the present disclosure is not particularly limited, but from the viewpoint of low-temperature reactivity, it is preferably less than 10°C, more preferably 0°C or less, even more preferably -10°C or less, and particularly preferably -20°C or less, and -30°C or less, in that order; from the viewpoint of the blocking resistance of the coating film, it is preferably -100°C or more, more preferably -80°C or more, and even more preferably -70°C or more, 60°C or more, and -50°C or more, in that order.

The glass transition temperature can be measured by differential scanning calorimetry, but it may also be estimated using the Fox equation below.

In formula (1), Tg' is the Tg (absolute temperature) of the polymer. _W1 ', _W2 ', ..., _Wn ' are the mass fractions of each monomer with respect to the total monomer components. _T1 , _T2 , ..., _Tn are the glass transition temperatures (absolute temperatures) of homopolymers (single polymers) composed of each monomer component.

The Tg values of representative homopolymers used to calculate the glass transition temperature (Tg) of the polymerizable monomer component by the above formula (1) are shown below.
2-Isopropenyl-2-oxazoline (IPO): 100°C
Styrene (St): 100 ° C.
Butyl acrylate (BA): -54°C
The copolymer of the present disclosure may be dissolved in a solution or may be dispersed in water in the form of an aqueous dispersion, but is preferably an aqueous dispersion from the viewpoints of environmental protection and occupational safety and hygiene.

Examples of the aqueous solvent of the present disclosure include water and mixed solvents of water and a water-soluble organic solvent. Examples of the water-soluble organic solvent include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, and tert-butyl alcohol; polyhydric alcohols such as ethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, and diethylene glycol; and ketones such as acetone and methyl ethyl ketone, but the present invention is not limited to these examples. Each of these aqueous media may be used alone, or two or more types may be used in combination.

From the viewpoint of improving the dispersion stability of the emulsion particle-containing aqueous dispersion, the water content in the aqueous solvent of the present disclosure is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, and even more preferably 80% by mass or more, with the upper limit of the content being 100% by mass. Among the aqueous solvents, water is preferred. The emulsion particle-containing aqueous dispersion of the present invention may contain, for example, additives within a range that does not impair the object of the present invention. Examples of the additives include colorants such as pigments, leveling agents, ultraviolet absorbers, ultraviolet stabilizers, antioxidants, polymerization inhibitors, fillers, coupling agents, rust inhibitors, antibacterial agents, metal deactivators, wetting agents, defoamers, surfactants, reinforcing agents, plasticizers, lubricants, antifogging agents, anticorrosive agents, pigment dispersants, flow regulators, peroxide decomposers, mold decolorizers, fluorescent brighteners, organic flame retardants, inorganic flame retardants, drip prevention agents, melt flow modifiers, antistatic agents, algae inhibitors, mildew inhibitors, flame retardants, slip agents, metal chelating agents, antiblocking agents, heat stabilizers, processing stabilizers, dispersants, thickeners, rheology control agents, foaming agents, antiaging agents, preservatives, antistatic agents, silane coupling agents, antioxidants, film-forming assistants, etc., but the present invention is not limited to only these examples. These additives may be used alone or in combination of two or more. The amounts of these additives vary depending on the type of additive and cannot be determined in general terms, so it is preferable to determine them appropriately depending on the type of additive.

[Method for producing the copolymer of the present disclosure]
The method for producing the copolymer of the present disclosure includes a step of polymerizing an oxazoline group or epoxy group-containing monomer, an anionic surfactant having a reactive group, and a nonionic surfactant having a reactive group. The term "having a reactive group" refers to a surfactant having an unsaturated double bond in the monomer and capable of polymerizing with other monomers, specifically, a surfactant having a radically polymerizable double bond such as a vinyl group, a (meth)acryloyl group, an allyl group, or a propenyl group in the molecule.

In the method for producing the copolymer disclosed herein, the oxazoline group or epoxy group-containing monomer is also referred to as monomer A.

Examples of the oxazoline group-containing monomer of the present disclosure used in the method for producing the copolymer of the present disclosure include vinyl oxazolines {e.g., vinyl oxazoline (2-vinyl-2-oxazoline, etc.), vinyl oxazolines having a substituent [e.g., alkyl-vinyl oxazolines (e.g., 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-4,4-dimethyl-2-oxazoline, 2-vinyl-4-ethyl-2-oxazoline, 2-vinyl-4-propyl-2-oxazoline, 2-vinyl-4-butyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-vinyl-5-ethyl-2-oxazoline, 2-vinyl-5-propyl-2-oxazoline, 2-vinyl-5-butyl-2-oxazoline, and other C _1-20 alkyl-vinyl oxazolines, preferably C _1-10 alkyl-vinyl oxazolines, and more preferably mono- or di-C _1-4 alkyl-vinyl oxazoline) and the like], isopropenyl oxazolines {for example, isopropenyl oxazoline (2-isopropenyl-2-oxazoline, etc.), isopropenyl oxazolines having a substituent [for example, alkyl-isopropenyl oxazolines (for example, C 1-20 alkyl-isopropenyl oxazolines such as 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-4,4-dimethyl-2-oxazoline, 2-isopropenyl-4-ethyl-2-oxazoline, 2-isopropenyl-4-propyl-2-oxazoline, 2-isopropenyl-4-butyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline, 2-isopropenyl-5-propyl-2-oxazoline, and 2 _- isopropenyl-5-butyl-2-oxazoline, preferably C _1-10 alkyl-isopropenyloxazoline, more preferably mono- or di-C _1-4 alkyl-isopropenyloxazoline), etc.}, and the corresponding allyloxazolines.

Among these, isopropenyl oxazolines are preferred, and 2-isopropenyl-2-oxazoline is particularly preferred.

The oxazoline group-containing monomers may be used alone or in combination of two or more.

Methods for polymerizing the monomer components of the present disclosure include, for example, bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization, but the present invention is not limited to these examples. Among these polymerization methods, solution polymerization and emulsion polymerization are preferred, with emulsion polymerization being more preferred, because they allow easy preparation of polymers having oxazoline groups in their side chains.

Epoxy group-containing monomers used in the method for producing the copolymer of the present disclosure include, for example, unsaturated carboxylic acid glycidyl esters and vinyl group-containing glycidyl ethers, but the present invention is not limited to these examples. These epoxy group-containing monomers may be used alone or in combination of two or more types.

Examples of unsaturated carboxylic acid glycidyl esters include glycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate [(meth)acrylic acid 3,4-epoxycyclohexylmethyl], monoglycidyl itaconate [itaconic acid monoglycidyl ester], monoglycidyl butene tricarbonate [butene tricarboxylic acid monoglycidyl ester], etc., but the present invention is not limited to these examples. These unsaturated carboxylic acid glycidyl esters may be used alone or in combination of two or more. Examples of vinyl group-containing glycidyl ethers include vinyl glycidyl ether, allyl glycidyl ether, glycidyloxyethyl vinyl ether, etc., but the present invention is not limited to these examples. These vinyl group-containing glycidyl ethers may be used alone or in combination of two or more. Among epoxy group-containing monomers, glycidyl (meth)acrylate is preferred from the viewpoint of improving adhesion, heat resistance, and bending properties.

In the method for producing the copolymer of the present disclosure, the amount of the oxazoline group-containing monomer used is preferably 1% by mass or more (e.g., 3% by mass or more) relative to 100 parts by mass of the monomer used in the copolymer from the viewpoint of improving low-temperature reactivity and physical properties of the coating film when used as a crosslinking agent, more preferably 5% by mass or more, even more preferably 10% by mass or more, and particularly preferably 15% by mass or more, and from the viewpoint of polymerizability, is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less, and particularly preferably 25% by mass or less.

In the method for producing the copolymer of the present disclosure, the amount of the epoxy group-containing monomer used is preferably 1% by mass or more (e.g., 3% by mass or more) relative to 100 parts by mass of the monomer used in the copolymer from the viewpoint of improving low-temperature reactivity and physical properties of the coating film when used as a crosslinking agent, more preferably 5% by mass or more, even more preferably 10% by mass or more, and particularly preferably 15% by mass or more, and from the viewpoint of polymerizability, is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less, and particularly preferably 25% by mass or less.

In the present disclosure, it is preferable to use an oxazoline group-containing monomer as the oxazoline group- and/or epoxy group-containing monomer. By including an oxazoline group-containing monomer as an essential component, it is expected that the low-temperature reactivity and the physical properties of the coating film formed by the crosslinking reaction will be good, and it is also expected that the storage stability will be good when used as an aqueous dispersion.

In the method for producing the copolymer of the present disclosure, it is preferable to use an anionic surfactant having a reactive group and a nonionic surfactant having a reactive group from the viewpoint of improving low-temperature reactivity, stability during emulsion polymerization, and stability when made into an aqueous dispersion. Having a reactive group means a surfactant that has an unsaturated double bond in the monomer and can be polymerized with other monomers, specifically, a surfactant that has a radically polymerizable double bond such as a vinyl group, a (meth)acryloyl group, an allyl group, or a propenyl group in the molecule. Hereinafter, a surfactant having a reactive group is also referred to as monomer B, an anionic surfactant having a reactive group is also referred to as monomer b1, and a nonionic surfactant having a reactive group is also referred to as monomer b2.

Preferred examples of the anionic surfactant having a reactive group according to the present disclosure include an anionic surfactant represented by the following formula (1) and an anionic surfactant represented by the following formula (2).

Among these, the anionic surfactant having a reactive group is preferably an anionic surfactant represented by the following formula (1) from the viewpoint of, for example, industrial productivity, more specifically, the ability to maintain a low viscosity of the (meth)acrylic resin emulsion and excellent handleability.

In formula (1), R ^1a represents an aliphatic alkyl group having 6 to 24 carbon atoms, and X ^1a represents --SO ₃ NH ₄ or --SO ₃ Na.

In addition, in formula (1), n1 represents the average number of moles of oxyethylene groups added (also referred to as the "average number of repeating oxyethylene units"). n1 is an integer from 5 to 50, and preferably an integer from 10 to 30.

Examples of commercially available anionic surfactants represented by formula (1) include Aqualon (registered trademark) KH-10 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether ammonium sulfate [average number of moles of oxyethylene groups added: 10], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], Aqualon (registered trademark) KH-05 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether ammonium sulfate [average number of moles of oxyethylene groups added: 5], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], and Adeka Reasoap (registered trademark) SR-20 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether ammonium sulfate [average number of moles of oxyethylene groups added: 20], amount of active ingredient: 100% by mass, ADEKA Co., Ltd.].

In formula (2), R ^2a represents a substituent represented by the following formula (A) or an aliphatic alkyl group having 6 to 24 carbon atoms, and R ^3a represents a methyl group.

In addition, in formula (2), X ^2a represents -SO ₃ M, -COOM, or PO ₃ M.

M represents an alkali metal atom, an alkaline earth metal atom, or an ammonium group.

Specific examples of alkali metal atoms include sodium atoms and potassium atoms.

Specific examples of alkaline earth metal atoms include calcium atoms and barium atoms.

X ^2a is, for example, preferably --SO ₃ NH ₄ .

In formula (2), n2 represents the average number of moles of oxyethylene groups added (also referred to as the "average number of repeating oxyethylene units"). n2 is an integer from 5 to 50, and preferably an integer from 10 to 30.

In formula (A), m represents an integer of 1 to 3, and from the viewpoint of surface activity, for example, 2 is preferred.

Examples of commercially available anionic surfactants represented by formula (2) include Aqualon (registered trademark) HS-10 [active ingredient: polyoxyethylene nonylpropenyl phenyl ether ammonium sulfate [average number of moles of oxyethylene groups added: 10], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], Aqualon (registered trademark) BC-10 [active ingredient: polyoxyethylene nonylpropenyl phenyl ether ammonium sulfate [average number of moles of oxyethylene groups added: 10], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], Aqualon (registered trademark) BC-20 [active ingredient: polyoxyethylene nonylpropenyl phenyl ether ammonium ether sulfate [average number of moles of oxyethylene groups added: 20], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], Aqualon (registered trademark) AR-10 [active ingredient: polyoxyethylene styrenated propenyl phenyl ether sulfate ammonium [average number of moles of oxyethylene groups added: 10], amount of active ingredient: 99% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], Aqualon (registered trademark) AR-20 [active ingredient: polyoxyethylene styrenated propenyl phenyl ether sulfate ammonium [average number of moles of oxyethylene groups added: 20], amount of active ingredient: 97% by mass, Dai-ichi Kogyo Seiyaku Co., Ltd.], etc.

As an anionic surfactant having a reactive group, from the viewpoint of the water resistance and polymerization stability of the coating film, the ADEKA REASOAP SR series (ADEKA REASOAP SR-10, SR-20, etc.) is preferred, and ADEKA REASOAP (registered trademark) SR-10 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether ammonium sulfate [average number of moles added of oxyethylene groups: 10], active ingredient amount: 100% by mass, ADEKA Corporation] is more preferred.

In the method for producing the copolymer of the present disclosure, the amount of the anionic surfactant having a reactive group used is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, relative to 100% by mass of the copolymer, from the viewpoint of improving polymerization stability, and is preferably 5% by mass or less, more preferably 2% by mass or less, from the viewpoint of improving water resistance.

Preferred examples of the nonionic surfactant having a reactive group according to the present disclosure include a nonionic surfactant represented by the following formula (3) and a nonionic surfactant represented by the following formula (4).

Among these, as the nonionic surfactant having a reactive group, for example, the nonionic surfactant represented by the following formula (3) is more preferred from the viewpoint of excellent copolymerizability with monomers.

In formula (3), R ^1b represents an aliphatic alkyl group having 6 to 24 carbon atoms, and X ^1b represents a hydrogen atom.

In addition, in formula (3), n3 represents the average number of moles of oxyethylene groups added (also referred to as the "average number of repeating oxyethylene units"). n3 is an integer from 5 to 40, and preferably an integer from 10 to 30.

In formula (4), R ^2b represents an aliphatic alkyl group having 6 to 24 carbon atoms, R ^3b represents a methyl group, and X ^2b represents a hydrogen atom.

In addition, in formula (4), n4 represents the average number of moles of oxyethylene groups added (also referred to as the "average number of repeating oxyethylene units"). n4 is an integer from 10 to 50, and preferably an integer from 10 to 30.

Examples of commercially available nonionic surfactants represented by formula (3) include ADEKA REASOAP (registered trademark) ER-10 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether [average number of moles of oxyethylene groups added: 10], amount of active ingredient: 100% by mass, ADEKA Corporation], ADEKA REASOAP (registered trademark) ER-30 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether [average number of moles of oxyethylene groups added: 30], amount of active ingredient: 65% by mass, ADEKA Corporation], and ADEKA REASOAP (registered trademark) ER-40 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether [average number of moles of oxyethylene groups added: 40], amount of active ingredient: 60% by mass, ADEKA Corporation].

From the viewpoint of the water resistance and polymerization stability of the coating film, the Adeka Reasoap ER series (Adeka Reasoap ER-10, ER-20, etc.) is preferred as a nonionic surfactant having a reactive group, and an example of a commercially available product of a nonionic surfactant having a reactive group is Adeka Reasoap (registered trademark) ER-20 [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether [average number of moles added of oxyethylene groups: 20], active ingredient amount: 100% by mass, ADEKA Corporation] is more preferred.

In the method for producing the copolymer of the present disclosure, it is preferable that the content ratio of the nonionic surfactant is higher than the amount of the anionic surfactant, and the amount of the nonionic surfactant having a reactive group used is preferably 1% by mass or more, more preferably 2% by mass or more, relative to 100 parts by mass of the copolymer from the viewpoint of improving polymerization stability, and is preferably 6% by mass or less, more preferably 5% by mass or less, from the viewpoint of improving water resistance.

In the method for producing the copolymer of the present disclosure, it is preferable that the content ratio of structural units derived from a nonionic surfactant is higher than that of structural units derived from an anionic surfactant, and the mass ratio of the content ratio of the anionic surfactant having a reactive group to the amount of the nonionic surfactant having a reactive group used is preferably 5/95 to 40/60, more preferably 10/90 to 30/70, even more preferably 15/85 to 25/75, and particularly preferably 17/83 to 23/77, from the viewpoint of improving the stability of the aqueous dispersion containing the copolymer.

In the method for producing the copolymer of the present disclosure, a monomer other than the oxazoline group- or epoxy group-containing monomer, the reactive anionic surfactant, and the reactive nonionic surfactant (also referred to as monomer C) may be used.

Examples of other monomers include alkyl group-containing (meth)acrylic acid ester monomers, hydroxyl group-containing (meth)acrylic acid ester monomers, carboxyl group-containing monomers, aromatic vinyl monomers, maleimide group-containing monomers, amide group-containing monomers, N-substituted (meth)acrylamide monomers, olefin-based monomers, and halogen-containing monomers. Of these, alkyl group-containing (meth)acrylic acid ester monomers, aromatic vinyl monomers, maleimide group-containing monomers, and vinyl cyanide monomers are preferred, and aromatic vinyl monomers are more preferred from the viewpoint of heat resistance, and alkyl group-containing (meth)acrylic acid ester monomers are preferred from the viewpoint of improving the physical properties of the coating film when used as a crosslinking agent.

Examples of aromatic vinyl monomers include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o-ethylstyrene, m-ethylstyrene, p-ethylstyrene, dimethylstyrene, tert-methylstyrene, α-chlorostyrene, β-chlorostyrene, and divinylbenzene, but the present invention is not limited to these examples. These aromatic vinyl monomers may be used alone or in combination of two or more. The aromatic vinyl monomer may have a functional group such as an alkyl group such as a methyl group or a tert-butyl group, a nitro group, a nitrile group, an alkoxyl group, an acyl group, a sulfone group, or a hydroxyl group on the benzene ring. Among aromatic vinyl monomers, styrene is preferred from the viewpoint of heat resistance and low dielectric properties. The aromatic vinyl monomer may contain an oligomer of the aromatic vinyl monomer within a range that does not impede the object of the present invention.

Examples of alkyl group-containing (meth)acrylic acid ester monomers include linear or branched alkyl group-containing (meth)acrylic acid ester monomers having 1 to 20 carbon atoms.

Examples of alkyl group-containing (meth)acrylic acid ester monomers include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and ethyl (meth)acrylate. Examples of the alkyl group-containing (meth)acrylic acid ester monomer include isooctyl acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, and cyclohexyl (meth)acrylate. The alkyl group-containing (meth)acrylic acid ester monomer may be used alone or in combination of two or more.

As the alkyl group-containing (meth)acrylic acid ester monomer, a (meth)acrylic acid alkyl ester having an alkyl group with 1 to 14 carbon atoms is preferred, a (meth)acrylic acid alkyl ester having an alkyl group with 2 to 10 carbon atoms is more preferred, and a (meth)acrylic acid alkyl ester having an alkyl group with 4 to 8 carbon atoms is even more preferred.

In addition, as the alkyl group-containing (meth)acrylic acid ester monomer of the present disclosure, an alkyl acrylate ester having an alkyl group with 1 to 14 carbon atoms is preferred, an alkyl acrylate ester having an alkyl group with 2 to 10 carbon atoms is more preferred, and an alkyl acrylate ester having an alkyl group with 4 to 8 carbon atoms is even more preferred.

Examples of maleimide group-containing monomers include maleimide, N-methylmaleimide, N-ethylmaleimide, N-propylmaleimide, N-isopropylmaleimide, N-butylmaleimide, N-sec-butylmaleimide, N-tert-butylmaleimide, N-amylmaleimide, N-hexylmaleimide, N-heptylmaleimide, N-octylmaleimide, N-nonylmaleimide, N-decylmaleimide, N-decylmaleimide, N-undecylmaleimide, N-dodecylmaleimide (N-laurylmaleimide), N-tridecylmaleimide, N-tetradecylmaleimide, N-pentadecylmaleimide, N-hexadecylmaleimide, N-heptadecadecylmaleimide, N-hex ... Examples of the maleimide-containing monomer include N-cyclohexylmaleimide, N-octadecylmaleimide (N-stearylmaleimide), N-nonadecylmaleimide, N-allylmaleimide, N-cyclohexylmaleimide, N-phenylmaleimide, N-hydroxyphenylmaleimide, N-methylphenylmaleimide, N-dimethylphenylmaleimide, N-ethylphenylmaleimide, N-diethylphenylmaleimide, N-phenylmethylmaleimide, N-methoxyphenylmaleimide, N-chlorophenylmaleimide, N-dichlorophenylmaleimide, N-trichlorophenylmaleimide, and N-tribromophenylmaleimide, but the present invention is not limited to these examples. These maleimide group-containing monomers may be used alone or in combination of two or more types.

Examples of hydroxyl group-containing (meth)acrylic acid ester monomers include hydroxyalkyl (meth)acrylates (e.g., 2-hydroxyethyl (meth)acrylate, etc.).

Examples of carboxyl group-containing monomers include (meth)acrylic acid, maleic acid, fumaric acid, crotonic acid, itaconic acid, citraconic acid, maleic anhydride, maleic acid monomethyl ester, maleic acid monobutyl ester, itaconic acid monomethyl ester, itaconic acid monobutyl ester, and vinylbenzoic acid, but the present invention is not limited to these examples.

Nitrogen atom-containing monomers include N-vinylpyrrolidone, dimethylaminoethyl (meth)acrylate, acrylonitrile, diacetone acrylamide, etc., but the present invention is not limited to these examples.

In the method for producing the copolymer of the present disclosure, the amount of aromatic vinyl monomer used is preferably 0.1% by mass or more, and more preferably 0.3% by mass or more, based on 100 parts by mass of the monomer used in the copolymer, from the viewpoint of improving heat resistance, and is preferably 40% by mass or less, more preferably 10% by mass or less, and more preferably 5% by mass or less, from the viewpoint of low-temperature reactivity.

In the method for producing the copolymer of the present disclosure, the amount of aromatic vinyl monomer used is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more, based on 100 parts by mass of other monomers, from the viewpoint of improving heat resistance, and is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less, from the viewpoint of low-temperature reactivity.

In the method for producing the copolymer of the present disclosure, the amount of the alkyl group-containing (meth)acrylic acid ester monomer used is, from the viewpoint of low-temperature reactivity, preferably 40% by mass or more, more preferably 50% by mass or more, 60% by mass or more, 70% by mass or more, and 75% by mass or more, based on 100 parts by mass of the copolymer, and is preferably 95% by mass or less, more preferably 90% by mass or less, and 85% by mass or less.

In the method for producing the copolymer of the present disclosure, the amount of the alkyl group-containing (meth)acrylic acid ester monomer used is, from the viewpoint of low-temperature reactivity, preferably 50% by mass or more, more preferably 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, and 95% by mass or more, in that order, based on 100 parts by mass of structural units derived from other monomers, and is preferably less than 100% by mass, and more preferably 99.5% by mass or less.

The polymerization method is not particularly limited, and known polymerization methods such as suspension polymerization, emulsion polymerization, and dispersion polymerization can be used. Among these, emulsion polymerization is preferred.

In the method for producing the copolymer of the present disclosure, either a solvent-based solvent or an aqueous solvent may be used, but from the viewpoints of environmental protection and occupational safety and health, it is preferable to use an aqueous solvent.

Examples of the aqueous solvent of the present disclosure include water and mixed solvents of water and a water-soluble organic solvent. Examples of the water-soluble organic solvent include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, and tert-butyl alcohol; polyhydric alcohols such as ethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, and diethylene glycol; and ketones such as acetone and methyl ethyl ketone, but the present invention is not limited to these examples. Each of these aqueous media may be used alone, or two or more types may be used in combination.

From the viewpoint of improving the dispersion stability of the emulsion particle-containing aqueous dispersion, the water content in the aqueous solvent of the present disclosure is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, and even more preferably 80% by mass or more, with the upper limit of the water content being 100 parts by mass. Among aqueous solvents, water is preferred. In the method for producing a copolymer of the present disclosure, the polymerization initiator is not particularly limited, and examples thereof include azo compounds such as 2,2-azobis(2-diaminopropane) hydrochloride, persulfates such as potassium persulfate, peroxides such as hydrogen peroxide, and the like. Specific examples of the polymerization initiator include azo-based oily compounds (e.g., azobisisobutyronitrile, 2,2-azobis(2-methylbutyronitrile), 2,2-azobis(2,4-dimethylvaleronitrile), etc.), aqueous compounds (e.g., anionic 4,4-azobis(4-cyanovaleric acid), cationic 2,2-azobis(2-methylpropionamidine)); redox-based oily peroxides (e.g., benzoyl peroxide, parachlorobenzoyl peroxide, lauroyl peroxide, t-butyl perbenzoate, etc.), aqueous peroxides (e.g., potassium persulfate, ammonium peroxide, etc.), etc. It is to be noted that only one type of polymerization initiator may be used, or two or more types may be used in combination.

The amount of the polymerization initiator used is not particularly limited, but is preferably 0.05 to 1 mass % relative to 100 parts by mass of the total monomer components, and more preferably 0.1 to 0.5 mass %.

The above-mentioned addition method is not particularly limited, and may be, for example, any method such as batch addition, divided addition, or continuous dropwise addition. Furthermore, in order to hasten the completion of the polymerization, a portion of the polymerization initiator may be added before or after the end of the dropwise addition of the polymerizable monomer component in the final stage.

In the method for producing the copolymer of the present disclosure, for example, a reducing agent such as sodium hydrogen sulfite or a transition metal salt such as ferrous sulfate may be added to promote decomposition of the polymerization initiator. In the emulsion polymerization step, known additives such as a pH buffer, a chelating agent, a chain transfer agent, and a film-forming aid may be added as necessary. Examples of chain transfer agents include compounds having a thiol group such as t-dodecyl mercaptan. Examples of film-forming aids include 2,2,4-trimethyl-1,3-pentanediol isobutyrate. The amount of the additives used is not particularly limited, but is preferably 0.01 to 20% by mass, more preferably 0.1 to 10% by mass, based on 100% by mass of the resin emulsion.

In the method for producing the copolymer of the present disclosure, surfactants other than anionic surfactants having reactive groups and nonionic surfactants having reactive groups may be used.

Surfactants other than anionic surfactants having a reactive group and nonionic surfactants having a reactive group include cationic surfactants having a reactive group and surfactants not having a reactive group.

In the method for producing a copolymer of the present disclosure, the content of surfactants other than the anionic surfactant having a reactive group and the nonionic surfactant having a reactive group is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less, relative to 100 parts by mass of the total surfactants, and it is particularly preferable that they are not used substantially. "Substantially not used" is not limited to not being used at all, but also includes the use of a small amount. In particular, when the use of a large amount of surfactant may cause problems with physical properties, the polymerization may be performed using a small amount of surfactant that does not cause such problems. As the surfactant, a known surfactant that can be used in an emulsion polymerization method can be used.

In the method for producing the copolymer of the present disclosure, the atmosphere during polymerization is not particularly limited, but from the viewpoint of increasing the efficiency of the polymerization reaction, it is preferable to use an inert gas such as nitrogen gas.

In the method for producing the copolymer of the present disclosure, the polymerization temperature during polymerization is not particularly limited, but is usually preferably 50 to 100°C, more preferably 60 to 95°C. The polymerization temperature may be constant or may be changed during the polymerization reaction.

In the method for producing the copolymer of the present disclosure, the polymerization time is not particularly limited and may be set appropriately depending on the progress of the reaction. For example, the polymerization time from start to finish is preferably in the range of 2 to 24 hours, and more preferably in the range of 4 to 8 hours.

[Uses of the copolymer of the present disclosure]
Typical applications of the copolymer or aqueous dispersion of the present disclosure include various inks such as ink for flexographic printing and ink for inkjet printers, various paints such as paints for road markings, conductive paints, paints for plastics, paints for inorganic building materials, paints for metals, paints for leather, and paints for repair; primers for organic substrates, inorganic substrates, and poorly adhesive substrates; various fiber treatment agents such as binders for water repellency and finishing, binders for coating and impregnation of woven fabrics, and tire cord treatment agents; various lamination adhesives such as dry lamination and extrusion lamination; various adhesives such as wood adhesives and structural adhesives; antistatic agents; conductive rollers used in printing equipment; top coats; pressure sensitive adhesives; various cosmetics such as nail polish and hair setting agents; raw materials for water absorbent resins; and the like.

<Production Example 1 (Production of polymer having 2-oxazoline group)>
A flask equipped with a stirrer, a reflux condenser, a nitrogen inlet tube, a thermometer and a dropping funnel was charged with 692 parts of deionized water. A pre-emulsion consisting of 31.5 parts of a 25% aqueous solution of an anionic surfactant ("ADEKA REASOAP SR-10/ADEKA CORPORATION"), 94.4 parts of a nonionic surfactant ("ADEKA REASOAP ER-20"/ADEKA CORPORATION), 167.6 parts of deionized water, 624.8 parts of butyl acrylate, 3.9 parts of styrene, 157.4 parts of 2-isopropenyl-2-oxazoline and 6 parts of divinylbenzene was prepared in the dropping funnel, and 65 parts of the pre-emulsion, which corresponds to 6% of the total amount of all polymerizable monomer components, was added to the flask, and the pH was adjusted to 9.0 with an appropriate amount of 25% by mass ammonia water, and the temperature was raised to 70°C by slowly pouring in nitrogen gas. 152 parts of a 2.5% by mass aqueous solution of potassium persulfate was injected thereinto to initiate polymerization. The remainder of the pre-emulsion was then uniformly dropped over 180 minutes. Nitrogen gas was continuously blown in during the reaction, and after completion of the dropwise addition, the dropping funnel was washed with 125 parts of deionized water and poured into the flask, the internal temperature was raised to 80°C, and stirring was continued for 3 hours to complete the reaction. The mixture was then cooled, filtered through a 300 mesh wire screen, and the aggregates were separated to obtain an aqueous dispersion of a polymer having a 2-oxazoline group (polymer (A)) with a pH of 8.0. The nonvolatile content was measured to be 40.0% by mass. The glass transition temperature of the polymer was estimated by the Fox formula to be -34°C. In addition, the average particle size (volume average particle size) of the emulsion particles contained in the aqueous dispersion was measured using a particle size distribution measuring device (NICOMP Model 380 manufactured by Particle Sizing Systems) using a dynamic light scattering method.
The amounts of the monomers and surfactants used are shown in Table 1 as ratios (parts by mass) relative to 100 parts by weight of the total amount of all monomer components. The evaluation results of various physical properties of the obtained aqueous dispersion are shown in Table 1.

<Production Examples 2 to 12>
Aqueous dispersions were obtained in the same manner as in Production Example 1, except that the surfactants and monomer components shown in Table 1 were used for polymerization. The results of evaluating various physical properties of these dispersions are shown in Table 1. The various physical properties were evaluated as follows.
<Tg>
The calculation was made from the monomer composition used in the polymerization using the following Fox equation.

計算式（１）中、Ｔｇ’は、重合体のＴｇ（絶対温度）である。Ｗ_１’、Ｗ_２’、・・・Ｗ_ｎ’は、全単量体成分に対する各単量体の質量分率である。Ｔ_１、Ｔ_２、・・・Ｔ_ｎは、それぞれ各単量体成分からなるホモポリマー（単独重合体）のガラス転移温度（絶対温度）である。なおジビニルベンゼン（ＤＶＢ）を含む重合性単量体については該ＤＶＢを除いて算出したＴｇを該重合性単量体成分のガラス転移温度とした。

In formula (1), Tg' is the Tg (absolute temperature) of the polymer. _W1 ', _W2 ', ... _Wn ' are the mass fractions of each monomer with respect to the total monomer components. _T1 , _T2 , ... _Tn are the glass transition temperatures (absolute temperatures) of homopolymers (single polymers) composed of each monomer component. For polymerizable monomers containing divinylbenzene (DVB), the Tg calculated excluding the DVB was used as the glass transition temperature of the polymerizable monomer component.

The Tg values of the respective homopolymers are shown below.
2-Isopropenyl-2-oxazoline (IPO): 100°C
Styrene (St): 100 ° C.
Butyl acrylate (BA): -54°C

<Non-volatile content (NV)>
About 1 g of the obtained aqueous resin composition was weighed out and dried in a hot air dryer at 150° C. for 30 minutes. The dry residue was taken as non-volatile matter and the ratio to the mass before drying was calculated and expressed as mass %.
formula:
[Non-volatile content (mass%) in aqueous resin composition]
= ([Dry residue amount] ÷ [1 g of aqueous resin composition]) × 100

<pH>
The value was measured at 25° C. using a pH meter (Horiba, Ltd., "F-23").
<Viscosity>
The viscosity was measured using a BM type viscometer (manufactured by Tokyo Keiki Co., Ltd.) at 30 min-1 and 25° C. When measuring the viscosity, a rotor was selected according to the viscosity.
<Average particle size>
The volume average particle diameter was measured using a particle size distribution measuring device ("NICOMP Model 380" manufactured by Particle Sizing Systems) using a dynamic light scattering method.

<Ease of emulsification>
An aqueous solution was prepared in a flask by adding 782.4 parts by mass of deionized water and a fixed amount (3% by mass based on the total amount of deionized water and monomer mixture) of each of the surfactants used in Examples 1 to 8 and Comparative Examples 1 to 4. The ease of emulsification when a monomer mixture having the same composition as the above monomer mixture was added to the aqueous solution was evaluated according to the following criteria.
A: The monomer mixture was added all at once and emulsified by stirring at 300 rpm for 15 minutes.
B: The monomer mixture could not be emulsified when added all at once, but could be emulsified when added in 10 portions with stirring at 300 rpm for 15 minutes after each addition.
C: Unable to emulsify.

<Polymerization stability>
The mass of the aggregate separated immediately after the emulsion polymerization was measured, and the percentage of the aggregate relative to the total amount of the aqueous dispersion was calculated and evaluated according to the following criteria. The results are shown in Table 1.
A: The amount of aggregates is less than 0.1%. B: The amount of aggregates is 0.1 to 1.0%.
C: The amount of aggregates is greater than 1.0%

<Mechanical stability>
A Maron mechanical stability test was carried out on each of the obtained aqueous dispersions under the following conditions. The test machine used was a product name "No. 156 Maron mechanical stability tester" manufactured by Yasuda Seiki Seisakusho Co., Ltd. After the test, the sample was filtered through a 100-mesh wire screen, and the mass of the recovered aggregates was measured. The ratio of the mass of the generated aggregates to the amount of sample used in the test was calculated, and evaluated according to the following criteria. The results are shown in Table 1. The smaller the aggregation rate, the better the mechanical stability.
A: The amount of aggregates is less than 0.03%. B: The amount of aggregates is 0.03 to 0.1%.
C: The amount of aggregates is greater than 0.1% (test condition)
Sample size: 70g
Load: 10kg
Rotation speed: 1000 rpm
Test duration: 10 minutes

<Water whitening resistance>
Each of the obtained aqueous dispersions was applied to a glass plate (length: 15 cm, width: 7 cm) using an applicator in an atmosphere having a temperature of 23±2° C. and a relative humidity of 65±3% so that the film thickness after drying would be 150 μm. The glass plate was left in this atmosphere for 2 minutes, then heated at a temperature of 120° C. for 30 minutes, and then allowed to cool to produce a glass plate on which a coating film was formed.
The opening of a glass tube with a diameter of 30 mm and a length of 30 mm was placed on the coating film of the glass plate on which the coating film was formed and adhered, and then 10 mL of ion-exchanged water was poured into the glass tube so that the coating film inside the glass tube would come into contact with water. The glass tube was then left for 24 hours in an atmosphere with a temperature of 23±2° C. and a relative humidity of 65±3%. The state of the coating film in contact with the ion-exchanged water was then visually observed and evaluated based on the following evaluation criteria. The smaller the change in the coating film after the test compared to the coating film before the test, the better the water whitening resistance.
(Evaluation Criteria)
A: No change was observed in the coating.
B: Slight cloudiness is observed in parts of the coating film.
C: Whitening is observed throughout the entire coating film.

<Production Example 2 (Production of Acrylic Emulsion Having Acid Groups)>
A flask equipped with a stirrer, a reflux condenser, a nitrogen inlet tube, a thermometer and a dropping funnel was charged with 376.5 parts of deionized water. A pre-emulsion consisting of 70.8 parts of Hitenol N-08 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., a 15% by mass aqueous solution of polyoxyethylene nonylphenyl ether ammonium sulfate), 131 parts of 2-ethylhexyl acrylate, 205 parts of styrene and 17 parts of acrylic acid was prepared in the dropping funnel, of which 5% by mass of the total amount of all polymerizable monomer components was added to the flask, and nitrogen gas was slowly poured in to raise the temperature to 80°C. 21 parts by mass of a 5% by mass aqueous solution of potassium persulfate was injected thereto to initiate polymerization. After 10 minutes, the remaining monomer mixture was dropped over 3 hours. Nitrogen gas was continuously blown in during the reaction, and the temperature in the flask was kept at 80±1°C. After the dropwise addition, the temperature was kept for 1 hour, then cooled and the pH was adjusted to 8.5 with 9 parts by mass of 25% by mass ammonia water to obtain an acrylic emulsion (Polymer (B)) with a nonvolatile content of 44.1% by mass and a pH of 8.5.

<Evaluation of coating film properties>
A coating composition (clear coating) was obtained by mixing 5 parts by mass of the polymer having a 2-oxazoline group (polymer (A)) produced in Examples 1 to 8 and Comparative Examples 1 to 4 with 100 parts by mass of the acrylic emulsion having an acid group (polymer (B)) produced in Production Example 2. The following various properties of the resulting coating composition were tested. The results are shown in Table 1.

<Low temperature reactivity>
Each clear coating was applied to a glass plate (length: 15 cm, width: 7 cm) using an applicator in an atmosphere with a temperature of 23±2° C. and a relative humidity of 65±3% so that the film thickness after drying was 150 μm. This glass plate was dried in this atmosphere for 24 hours,
A glass plate on which a coating film was formed was prepared. The coating film was peeled off from the glass plate on which the coating film was formed with a cutter blade, and the peeled off coating film (free film) was cut into a size of 25 mm x 25 mm to prepare a test piece. The mass (W ₁ ) of the test piece was measured, and the test piece was immersed in ethyl acetate in this atmosphere for 24 hours, and the insoluble matter was removed by filtration. The insoluble matter was dried, and the mass (W ₂ ) of the insoluble matter was measured. The low-temperature reactivity was evaluated based on the following evaluation criteria, in comparison with the results of a blank that did not contain a polymer having a 2-oxazoline group. The higher the gel fraction increase rate, the better the low-temperature reactivity.
A: Gel fraction increase rate is 30% or more.
B: Gel fraction increase rate 0% to 30%
C: The gel fraction does not increase.
The gel fraction is calculated by the formula:
[Gel fraction]=[[Weight of test piece (W ₁ )]÷[Weight of insoluble matter (W ₂ )]]×100
It was calculated based on the above.

<Tensile test>
The coating film used in the low-temperature reactivity test was peeled off with a cutter blade from the glass plate on which the coating film was formed, and the peeled off coating film (free film) was used as a film for strength measurement. The film for strength measurement was stored in a constant temperature and humidity room (temperature 25°C, humidity 60% RH) for 24 hours. Thereafter, it was held in the chuck of a tensile tester (manufactured by Shimadzu Corporation, product name: Autograph AGS-100D) and a tensile test was performed in accordance with JIS P-8113.

<Film strength>
The tensile strength of a film formed from the acrylic emulsion clear coating material produced in Production Example 2 (a coating material not containing polymer (A)) in the above tensile test was used as a blank, and the tensile strength of the blank was compared with that of films formed from each of the clear coating materials containing the aqueous dispersions of Examples 1 to 8 and Comparative Examples 1 to 4. The results were evaluated according to the following criteria.
A: The strength was more than 1.5 times greater than that of the blank.
B: The strength was 1.0 to 1.5 times that of the blank.

<Film stretch>
In the above tensile test, the elongation of a film formed from the acrylic emulsion clear coating material produced in Production Example 2 (a coating material not containing polymer (A)) was used as a blank, and the elongation of the blank was compared with that of the films formed from each of the clear coating materials containing the aqueous dispersions of Examples 1 to 8 and Comparative Examples 1 to 4. The results were evaluated according to the following criteria.
A: Elongation was more than 1.5 times that of the blank.
B: Elongation was 1.0 to 1.5 times that of the blank.

The surfactants and monomers in Table 1 are as follows:
<Surfactant>
-Anionic surfactant-
ADEKA REASOAP SR-10: Ether sulfate type ammonium salt (manufactured by ADEKA Corporation)
ADEKA REASOAP SR-20: Ether sulfate type ammonium salt (manufactured by ADEKA Corporation)
Aqualon HS-10: Polyoxyethylene nonylpropenyl phenyl ether ammonium sulfate (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)
Aqualon KH-10: Polyoxyethylene-1-(allyloxymethyl) alkyl ether ammonium sulfate (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)
Hitenol LA-10: Polyoxyethylene alkyl ether sulfate (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)
Hitenol N-08: Polyoxyethylene nonylphenyl ether ammonium sulfate (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)

-Nonionic surfactant-
Adeka Reasoap ER-20: manufactured by ADEKA CORPORATION Aqualon RN-20: polyoxyethylene nonylpropenyl phenyl ether and polyoxyethylene nonyl phenyl ether (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)
Aqualon KN-20: Polyoxyethylene 1-(allyloxymethyl) alkyl ether (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)

<Monomer>
BA: Butyl acrylate St: Styrene DVB: Divinylbenzene MMA: Methyl methacrylate IPO: 2-Isopropenyl-2-oxazoline 2EHA: 2-Ethylhexyl acrylate

Claims (8)

The polymerizable composition has a structural unit derived from an oxazoline group-containing monomer, a structural unit derived from an anionic surfactant, a structural unit derived from a nonionic surfactant, and a structural unit derived from an alkyl group-containing (meth)acrylic acid ester monomer ,
the content of the structural unit derived from the oxazoline group-containing monomer is 10% by mass or more and 30% by mass or less based on 100 parts by mass of the copolymer,
a mass ratio of the structural unit derived from the anionic surfactant to the structural unit derived from the nonionic surfactant is 5/95 to 40/60;
A copolymer having a glass transition temperature of less than 10°C. The copolymer according to claim 1 , which has structural units derived from an aromatic vinyl compound. An aqueous dispersion comprising the copolymer according to claim 1 or 2 . A crosslinking agent comprising the copolymer according to claim 1 or 2 , or the aqueous dispersion according to claim 3 . A coating composition comprising the copolymer according to claim 1 or 2 , the aqueous dispersion according to claim 3 , or the crosslinking agent according to claim 4 . A coating film obtained by using the coating composition according to claim 5 . A method for producing a copolymer, comprising the steps of:
The method includes a step of polymerizing an oxazoline group-containing monomer, an anionic surfactant having a reactive group, a nonionic surfactant having a reactive group, and an alkyl group-containing (meth)acrylic acid ester monomer,
the content of the structural unit derived from the oxazoline group-containing monomer is 10% by mass or more and 30% by mass or less based on 100 parts by mass of the copolymer,
a mass ratio of the structural unit derived from the anionic surfactant to the structural unit derived from the nonionic surfactant is 5/95 to 40/60;
A method for producing a copolymer having a glass transition temperature of less than 10°C. The method for producing the copolymer according to claim 7 , wherein the polymerization is carried out in an aqueous solvent.