JP5616833B2 - Magnetic recording medium and binder composition for magnetic recording medium - Google Patents

Description

The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having high surface smoothness.
Furthermore, the present invention relates to a binder composition for a magnetic recording medium suitable for producing a coating type magnetic recording medium having high surface smoothness.

  In the coating type magnetic recording medium, the binder plays an important role in various characteristics such as electromagnetic conversion characteristics. For this reason, various studies have been conducted on resins that can be used as binders for magnetic recording media (see, for example, Patent Documents 1 to 3).

JP-A-8-67855 JP 2004-295926 A JP-A-6-111277

In order to achieve high density recording in the magnetic recording field, it is effective to use fine magnetic particles and to disperse the fine magnetic materials to a high degree to improve the smoothness of the magnetic layer surface. Therefore, conventionally, in order to improve the dispersibility of the magnetic material by increasing the adsorptivity of the binder to the magnetic material, an adsorptive functional group (polar group) such as a sulfonic acid group is introduced into the binder for magnetic recording media. Has been widely performed (see Patent Documents 1 to 3).
However, in recent years, with the remarkable development of information transmission means, the demand for high density recording on the magnetic recording medium has further increased. Under such circumstances, it has become difficult to cope with higher density recording by simply introducing a polar group into the binder.

  Accordingly, an object of the present invention is to provide a magnetic recording medium having excellent surface smoothness that can cope with higher density recording.

As a result of intensive studies to achieve the above object, the present inventor has obtained the following new knowledge.
As described in Patent Document 1 and the like, by using an acrylic monomer having a sulfonic acid group (—SO ₃ H), an acrylic polymer having a polar group (hereinafter referred to as “proton type polymer” or “proton type”). Called "acrylic polymer"). On the other hand, as a result of the study by the present inventors, an acrylic polymer (hereinafter referred to as “anti-salt type polymer” or “anti-salt type acrylic” in which a sulfonic acid group exists in a state of forming a salt with a methyl group-containing 6-membered aromatic amine. Is referred to as a “polymer”) in combination with a carboxylic acid compound, the magnetic material is compared with the above-mentioned proton-type polymer and a salt-type polymer in which a sulfonic acid group forms a salt with another amine. It was found that it can be highly dispersed.
As a result of further studies based on the above findings, the present inventor has completed the present invention.

That is, the above object has been achieved by the following means.
[1] A magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a nonmagnetic support,
The binder is represented by the general formula (I):
[In the general formula (I), R ¹ represents a hydrogen atom or a methyl group, X ¹ represents an oxygen atom, a sulfur atom or a —N (R ¹¹ ) — group, R ¹¹ represents a hydrogen atom or a substituent, L ¹ represents an alkylene group which may have a substituent, and Y ¹ represents a salt formed from a sulfonic acid group and a methyl group-containing 6-membered aromatic amine. ]
A polymer having a structural unit represented by:
The magnetic recording medium, wherein the magnetic layer further contains a carboxylic acid compound.
[2] The magnetic recording medium according to [1], wherein the methyl group-containing 6-membered aromatic amine includes a pyridine ring as an aromatic ring.
[3] The magnetic recording medium according to [1] or [2], wherein the methyl group-containing 6-membered aromatic amine is selected from the group consisting of α-methylpyridine, β-methylpyridine, and γ-methylpyridine. .
[4] The magnetic recording medium according to any one of [1] to [3], wherein the carboxylic acid compound is selected from the group consisting of fatty acids and aromatic carboxylic acids.
[5] The magnetic recording medium according to any one of [1] to [4], wherein the carboxylic acid compound is selected from the group consisting of cinnamic acid, benzoic acid, and oleic acid.
[6] The magnetic recording medium according to any one of [1] to [5], wherein the polymer contains 10 to 1000 μeq / g of a salt represented by Y ¹ .
[7] The polymer has the general formula (II):
[In General Formula (II), R ² represents a hydrogen atom or a methyl group, and L ² represents a single bond or a divalent linking group. ]
The magnetic recording medium according to any one of [1] to [6], further comprising a structural unit represented by:
[8] The polymer has the general formula (III):
[In general formula (III), R ³ represents a hydrogen atom or a methyl group, L ³ represents a single bond or a divalent linking group, and Z ³ represents a substituent having a cyclic structure. ]
The magnetic recording medium according to any one of [1] to [7], further comprising a structural unit represented by:
[9] General formula (I):
[In the general formula (I), R ¹ represents a hydrogen atom or a methyl group, X ¹ represents an oxygen atom, a sulfur atom or a —N (R ¹¹ ) — group, R ¹¹ represents a hydrogen atom or a substituent, L ¹ represents an alkylene group which may have a substituent, and Y ¹ represents a salt formed from a sulfonic acid group and a methyl group-containing 6-membered aromatic amine. ]
A polymer having a structural unit represented by:
Carboxylic acid compounds,
A binder composition for magnetic recording media, comprising:
[10] The binder composition for magnetic recording media according to [9], wherein the methyl group-containing 6-membered aromatic amine contains a pyridine ring as an aromatic ring.
[11] The magnetic recording medium according to [9] or [10], wherein the methyl group-containing 6-membered aromatic amine is selected from the group consisting of α-methylpyridine, β-methylpyridine, and γ-methylpyridine. Binder composition.
[12] The binder composition for magnetic recording media according to any one of [9] to [11], wherein the carboxylic acid compound is selected from the group consisting of fatty acids and aromatic carboxylic acids.
[13] The binder composition for magnetic recording media according to any one of [9] to [12], wherein the carboxylic acid compound is selected from the group consisting of cinnamic acid, benzoic acid, and oleic acid.
[14] The binder composition for magnetic recording media according to any one of [9] to [13], further comprising a ketone solvent.
[15] The binder composition for magnetic recording media according to any one of [9] to [14], wherein the polymer contains 10 to 1000 μeq / g of a salt represented by Y ¹ .
[16] The polymer has the general formula (II):
[In General Formula (II), R ² represents a hydrogen atom or a methyl group, and L ² represents a single bond or a divalent linking group. ]
The binder composition for magnetic recording media according to any one of [9] to [15], further comprising a structural unit represented by:
[17] The polymer has the general formula (III):
[In general formula (III), R ³ represents a hydrogen atom or a methyl group, L ³ represents a single bond or a divalent linking group, and Z ³ represents a substituent having a cyclic structure. ]
The binder composition for magnetic recording media according to any one of [9] to [16], further comprising a structural unit represented by:

  According to the present invention, a magnetic recording medium having excellent surface smoothness can be provided.

The present invention
A magnetic recording medium having a magnetic layer comprising a ferromagnetic powder and a binder on a nonmagnetic support, wherein the binder is represented by the general formula (I):
[In the general formula (I), R ¹ represents a hydrogen atom or a methyl group, X ¹ represents an oxygen atom, a sulfur atom or a —N (R ¹¹ ) — group, R ¹¹ represents a hydrogen atom or a substituent, L ¹ represents an alkylene group which may have a substituent, and Y ¹ represents a salt formed from a sulfonic acid group and a methyl group-containing 6-membered aromatic amine. ]
A polymer having a structural unit represented by:
The magnetic layer further includes a carboxylic acid-based compound.
The magnetic recording medium of the present invention comprises a binder comprising, as a constituent, a polymer having a structural unit containing a salt of a sulfonic acid group and a methyl group-containing 6-membered aromatic amine represented by the general formula (I). By including it in the magnetic layer together with the carboxylic acid-based compound, the ferromagnetic powder can be highly dispersed, thereby realizing excellent surface smoothness.
Hereinafter, the magnetic recording medium of the present invention will be described in more detail.

In the magnetic recording medium of the present invention, the binder of the magnetic layer contains a polymer having a structural unit represented by the above general formula (I) as a constituent component. Here, “including as a constituent component” includes not only the mode in which the polymer itself is present in the magnetic layer but also the mode in which the polymer is present as a reaction product with other components. For example, as described later, a polymer containing a hydroxyl group can be present in the magnetic layer in a state of reacting with polyisocyanate to form a crosslinked structure, and such an embodiment is also included in the present invention.
Hereinafter, the polymer having the structural unit represented by the general formula (I) will be described in more detail.

[Polymer having structural unit represented by general formula (I)]
In the general formula (I), R ¹ represents a hydrogen atom or a methyl group, and preferably represents a methyl group.
X ¹ represents an oxygen atom, a sulfur atom or a —N (R ¹¹ ) — group. Here, R ¹¹ represents a hydrogen atom or a substituent. Examples of the substituent include an alkyl group (for example, an alkyl group having 1 to 6 carbon atoms), a hydroxyl group, an alkoxyl group (for example, an alkoxyl group having 1 to 6 carbon atoms), a halogen atom (for example, a fluorine atom, a chlorine atom, a bromine atom), A cyano group, an amino group, a nitro group, an acyl group, a carboxyl group, etc. can be mentioned. Hereinafter, unless otherwise specified, for the “substituent” in the present invention, the description of the substituent can be referred to. R ¹¹ is preferably a hydrogen atom or an alkyl group, more preferably a hydrogen atom from the viewpoint of improving dispersibility.
In the present invention, the “carbon number” of a group having a substituent means the carbon number of a portion not containing the substituent. Further, in the present invention, “to” indicates a range including numerical values described before and after that as a minimum value and a maximum value, respectively.

In general formula (I), L ¹ represents an alkylene group which may have a substituent. The number of carbon atoms of the alkylene group represented by L ¹ is preferably in the range of 1 to 10, more preferably in the range of 1 to 6, even more preferably in the range of 1 to 3, from the viewpoint of improving dispersibility. It is.

In the general formula (I), Y ¹ represents a salt formed from a sulfonic acid group and a methyl group-containing 6-membered aromatic amine. The polymer contains a sulfonic acid group in the form of a salt with a methyl group-containing 6-membered aromatic amine, so that the polymer has higher dispersibility with respect to the ferromagnetic powder than an acrylic polymer containing the sulfonic acid group itself. An improvement effect can be shown.

  In the present invention, the methyl group-containing 6-membered aromatic amine has one or more methyl groups substituted directly or indirectly via a linking group on a 6-membered aromatic ring in which at least one of the constituent atoms is a nitrogen atom. An amine having a structure. The nitrogen-containing 6-membered aromatic ring is preferably a pyridine ring from the viewpoint of further improving dispersibility. The number of methyl groups contained is 1 or more, and may be 2, 3 or 4 or more. From the viewpoint of further improving dispersibility, it is preferable that the aromatic amine contains one methyl group. The methyl group may be substituted directly or indirectly through a linking group on the nitrogen-containing 6-membered aromatic ring. Examples of the linking group include an alkylene group which may have a substituent, an alkenyl group, or a divalent group in which these are directly or indirectly linked. The methyl group is preferably directly substituted with a nitrogen-containing 6-membered aromatic ring from the viewpoint of further improving dispersibility. From the above viewpoints, particularly preferable methyl group-containing 6-membered aromatic amines include α-, β-, and γ-methylpyridines.

  The polymer can be obtained by mixing a proton-type acrylic polymer with a methyl group-containing 6-membered aromatic amine and neutralizing a sulfonic acid group contained in the proton-type acrylic polymer with the amine. It can be obtained by using an acrylic monomer capable of introducing the structural unit represented by the formula (I) as one kind of raw material monomers. Such a raw material monomer can be obtained by reacting an acrylic monomer having a sulfonic acid group represented by the following general formula (X) with a methyl group-containing 6-membered aromatic amine in a solvent.

[In General Formula (X), Z ¹ represents a sulfonic acid group, and R ¹ , X ¹ and L ¹ have the same meanings as those in General Formula (I). ]

In addition, in the acrylic monomer represented by the general formula (X), a raw material for synthesizing a binder for a magnetic recording medium conventionally containing a sulfonic acid group such as 2-acrylamido-2-methylpropanesulfonic acid. Some have been widely used as monomers. However, the acrylic monomer represented by the general formula (X) is poor in solubility in a ketone solvent widely used in a coating solution for forming a magnetic recording medium. Therefore, when a binder for a magnetic recording medium containing a sulfonic acid group is synthesized using such an acrylic monomer, a non-ketone solvent capable of dissolving the acrylic monomer has to be used.
In contrast, the inventors of the present invention have newly found that the acrylic monomer represented by the general formula (X) has a higher solubility in ketone solvents when it forms a salt with a methyl group-containing 6-membered aromatic amine. It was issued. Therefore, the acrylic monomer represented by the general formula (X) is reacted with a methyl group-containing 6-membered aromatic amine in a ketone solvent, and Z ¹ part (sulfonic acid group) in the general formula (X) contains a methyl group. Since the salt-type acrylic monomer represented by the following general formula (X-1) that forms a salt with a 6-membered aromatic amine can be dissolved in a ketone solvent, It is possible to continue the polymerization reaction in a ketone solvent. Since the binder synthesized in the ketone solvent can be used as it is for the preparation of a coating solution for forming a magnetic recording medium without removing the reaction solvent, it is extremely advantageous from the viewpoint of simplifying the production process. is there.

[In General Formula (X-1), R ¹ , X ¹ , L ¹ and Y ¹ have the same meaning as in General Formula (I). ]

In order to further improve the dispersibility, the amount of the salt represented by Y ¹ introduced into the polymer is preferably 10 to 1000 μeq / g. For this purpose, the acrylic monomer used for introducing Y ¹ is preferably used in an amount of 0.1 to 10 mol%, and 0.5 to 6 mol%, based on the total amount of raw material monomers. preferable.

The polymer can contain other structural units in addition to the structural unit represented by the general formula (I). As such a structural unit, general formula (II):
[In general formula (II), R ² represents a hydrogen atom, a halogen atom or a methyl group, and L ² represents a single bond or a divalent linking group. ]
The structural unit represented by these can be mentioned. The polymer containing the structural unit represented by the general formula (II) can contribute to further improvement in dispersibility by containing a hydroxyl group, and in a system using a polyisocyanate, the polyisocyanate and the hydroxyl group are crosslinked. This is preferable because it contributes to improving the strength of the coating film.

  Details of the general formula (II) will be described below.

In the general formula (II), R ² represents a hydrogen atom, a halogen atom or a methyl group. Examples of the halogen atom include a chlorine atom, a bromine atom and an iodine atom.
R ² preferably represents a hydrogen atom or a methyl group, more preferably a methyl group.

In the general formula (II), L ² represents a single bond or a divalent linking group, and is preferably a linking group containing any of an oxygen atom, a nitrogen atom or a sulfur atom. As the divalent linking group represented by L ² , those bonded to the main chain carbon atom through a —C (O) — group are preferred, and —C ( A divalent linking group represented by O) X ² R ²¹ — is more preferred.

  From the viewpoint of improving dispersibility, the structural unit (II) is preferably a structural unit represented by the following general formula (II-1).

In general formula (II-1), R ² represents a hydrogen atom or a methyl group.

X ² represents a divalent linking group represented by —O—, —S—, or —N (R ²² ) —, and is preferably —O—.

R ²² represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms which may have a substituent. The number of carbon atoms of the alkyl group represented by R ²² is preferably 1 or more and 4 or less. When the carbon number of the alkyl group represented by R ²² is in the above range, the dispersibility can be further improved while maintaining the solubility.

R ²¹ represents an optionally substituted alkylene group having 2 to 8 carbon atoms or a divalent group in which a plurality of alkylene groups are linked via a linking group. When the number of carbon atoms of the alkylene group contained in the group represented by R ²¹ is in the above range, dispersibility can be further improved while maintaining solubility. The linking group for linking the alkylene group is preferably an ester bond from the viewpoint of solubility. The number of alkylene groups having 2 to 8 carbon atoms contained in the group represented by R ²¹ is preferably 1 to 3 inclusive.

  The structural unit represented by the general formula (II) described above can be derived from a vinyl monomer represented by the following general formula (II ′), and the structural unit represented by the general formula (II-1) is It can be derived from an acrylic monomer represented by the general formula (II-1 ′).

[R ² and L ^{2 in} General Formula (II ′) are the same as those in General Formula (II), respectively. ]

[R ² , X ² and R ²¹ in the general formula (II-1 ′) have the same meanings as those in the general formula (II-1), respectively. ]

Specific examples of the monomer represented by the general formula (II ′) or the general formula (II-1 ′) include the following monomers. However, the present invention is not limited to the following specific examples.
Hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, polyethylene glycol mono (meth) acrylate, polypropylene glycol mono (meth) acrylate, polyethylene glycol polypropylene glycol mono (meth) acrylate, glycerol mono (meth) acrylate, 3-chloro 2-hydroxyalkyl (meth) acrylates such as hydroxypropyl (meth) acrylate, vinyl ethers such as hydroxyethyl vinyl ether, hydroxypropyl vinyl ether, hydroxybutyl vinyl ether, hydroxyethyl mono (meth) allyl ether, hydroxypropyl mono (meth) Allyl ether, hydroxybutyl mono (meth) allyl ether, diethylene glycol mono (me ) Allyl ether, dipropylene glycol mono (meth) allyl ether, glycerol mono (meth) allyl ether, 3-chloro-2-hydroxypropyl (meth) allyl ether (meth) allyl ethers, (meth) allyl alcohol.

  From the viewpoint of further improving dispersibility, the structural unit preferably contained in the polymer includes a structural unit represented by the following general formula (III) and a structural unit represented by the following general formula (IV): Can also be mentioned. Details of each will be described below.

In the general formula (III), R ³ represents a hydrogen atom, a halogen atom, or a methyl group. The details are as described for R ² in the general formula (II).

L ³ represents a single bond or a divalent linking group. The divalent linking group represented by L ³ preferably contains a hetero atom, and preferably binds to the alicyclic cyclic group represented by Z ³ on the hetero atom. Examples of the hetero atom include an oxygen atom, a nitrogen atom, and a sulfur atom. L ³ is preferably a single bond or a divalent linking group bonded to a main chain carbon atom via a —C (O) — group, and —C (O) in the general formula (III-1) described later. A divalent linking group represented by X ³ — is more preferred.

In general formula (III), Z ³ represents a substituent having a cyclic structure. The substituent represented by Z ³ preferably represents an alicyclic cyclic group from the viewpoint of improving dispersibility. The alicyclic ring group may be saturated or unsaturated, and may be any of a monocyclic ring, a polycyclic ring, and a condensed ring. Moreover, you may have a substituent. The substituent is as described above for R ^{11 in the} general formula (I).

The monocyclic alicyclic group represented by Z ³ is preferably a 5- to 6-membered group, and specific examples thereof include a cyclohexyl group and a cyclopentyl group. The cyclic cyclic group is preferably a 7- to 10-membered group, and specific examples include a bicycloalkyl group, an adamantyl group, a norbornyl group, and an isobornyl group. From the viewpoint of improving dispersibility, Z ³ is preferably an alicyclic fused ring group. As the alicyclic condensed ring, an alicyclic condensed ring represented by Y ³¹ in the general formula (III-1) described later is preferable.

  From the viewpoint of improving dispersibility, the structural unit represented by the general formula (III) is preferably a structural unit represented by the following general formula (III-1).

In general formula (III-1), R ³ represents a hydrogen atom or a methyl group, and is preferably a methyl group.

X ³ represents a divalent linking group represented by —O—, —S—, or —N (R ³¹ ) —, and is preferably —O—.

R ³¹ represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms which may have a substituent. The number of carbon atoms of the alkyl group represented by R ³¹ is preferably 1 or more and 4 or less. When the carbon number of the alkyl group represented by R ³¹ is in the above range, dispersibility can be further improved while maintaining solubility.

Z ³¹ represents an alicyclic fused ring group. The alicyclic fused ring group represented by Z ³¹ is preferably 7 to 10 membered. Specific examples of the preferable condensed ring group include an adamantyl group, a norbornyl group, and a dicyclopentanyl group.

  The structural unit represented by the general formula (III) described above can be derived from a vinyl monomer represented by the following general formula (III ′), and the structural unit represented by the general formula (III-1) is These can be derived from acrylic monomers represented by the following general formula (III-1 ′).

[R ³ , L ³ and Z ³ in the general formula (III ′) are as defined in the general formula (III). ]

[R ³ , X ³ and Z ³¹ in the general formula (III-1 ′) have the same meanings as those in the general formula (III-1). ]

  Specific examples of the monomer represented by formula (III ′) or formula (III-1 ′) are shown below. However, the present invention is not limited to the following specific examples.

In the general formula (IV), R ⁴ represents a hydrogen atom, a halogen atom, or a methyl group. The details are as described for R ² in the general formula (II).

In the general formula (IV), L ⁴ represents a single bond or a divalent linking group, and is preferably a linking group containing any of an oxygen atom, a nitrogen atom or a sulfur atom. L ⁴ is preferably a single bond or a divalent linking group bonded to the main chain carbon atom via a —C (O) — group, and —C (O) in the general formula (IV-1) described later. A divalent linking group represented by X ⁴ — is more preferred.

  In general formula (IV), Y represents a hydrocarbon group having 8 to 50 carbon atoms. The hydrocarbon group is a substituted or unsubstituted linear, branched, or cyclic saturated or unsaturated hydrocarbon group, and is preferably a linear or branched hydrocarbon group. When the number of carbon atoms is 8 or more, it can contribute to improvement in dispersibility, and when the number of carbon atoms is 50 or less, solubility can be ensured. The number of carbon atoms of the hydrocarbon group is preferably 12 or more and 30 or less from the viewpoint of dispersibility and solubility. The hydrocarbon group represented by Y is more preferably an alkyl group having 12 to 30 carbon atoms, and still more preferably an alkyl group having 12 to 18 carbon atoms.

  From the viewpoint of improving dispersibility, the structural unit represented by the general formula (IV) is preferably a structural unit represented by the following general formula (IV-1).

In general formula (IV-1), R ⁴ represents a hydrogen atom or a methyl group, and is preferably a methyl group.

  n represents an integer of 12 or more and 30 or less, and is more preferably an integer of 12 or more and 18 or less.

X ⁴ represents a divalent linking group represented by — (O) m ¹ —, — (S) m ² — or — {N (R ⁴¹ )} m ³ —, and — (O) m ¹ — It is preferable to represent the represented bivalent coupling group. Here, m ¹ , m ² , and m ³ each independently represent an integer of 1 or more.

R ⁴¹ represents an alkyl group having 1 to 8 carbon atoms which may have a substituent. The number of carbon atoms of the alkyl group represented by R ⁴¹ is preferably 1 or more and 4 or less. When the carbon number of the alkyl group represented by R ⁴¹ is in the above range, dispersibility can be further improved while maintaining solubility.
In addition, m ¹ , m ² , and m ³ are preferably integers of 5 or less from the viewpoint of maintaining solubility.

  The structural unit represented by the general formula (IV) described above can be derived from a vinyl monomer represented by the following general formula (IV ′), and the structural unit represented by the general formula (IV-1) is It can be derived from an acrylic monomer represented by the following general formula, general formula (IV ′).

[R ⁴ , L ⁴ and Y in the general formula (IV ′) have the same meanings as those in the general formula (IV). ]

[R ⁴ , X ⁴ and n in the general formula (IV-1 ′) have the same meanings as those in the general formula (IV-1), respectively. ]

  Specific examples of the monomer represented by the general formula (IV ′) or the general formula (IV-1 ′) are shown below. However, the present invention is not limited to the following specific examples.

From the viewpoint of achieving both excellent dispersibility and high coating film strength, the polymer contains 5 structural units represented by the general formula (II) based on all the polymer units constituting the copolymer. It is preferably contained in an amount of from mol% to 80 mol%, more preferably from 15 mol% to 70 mol%, still more preferably from 30 mol% to 60 mol%.
From the viewpoint of obtaining even better dispersibility, the polymer preferably contains 5 mol% or more and 75 mol% or less of the structural unit represented by the general formula (III), and contains 15 mol% or more and 60 mol% or less. More preferably, it is more preferably 30 mol% or more and 50 mol% or less, and more preferably 5 mol% or more and 75 mol% or less of the structural unit represented by the general formula (IV). More preferably, it is contained in an amount of 10 mol% or less and more preferably 30 mol% or less.
Further, as described above, the acrylic monomer used for introducing Y ¹ in the general formula (I) is preferably used in an amount of 0.1 to 10 mol% based on the total amount of raw material monomers, It is preferable to use 0.5 to 6 mol%.
Therefore, the blending ratio of each monomer during the polymerization reaction is preferably set so as to obtain a polymer having the above preferred composition. In addition to the above-mentioned monomers, ethylenically unsaturated carboxylic acid ester monomers, aromatic vinyl monomers, ethylenically unsaturated nitrile monomers, ethylenically unsaturated acid monomers, alkyl vinyl ether monomers, vinyl ester monomers, ethylene Known polymerizable components such as a polymerizable unsaturated polycarboxylic acid anhydride can also be used.

As described above, the reaction for obtaining the polymer is performed by reacting an acrylic monomer represented by the general formula (X) with a methyl group-containing 6-membered aromatic amine in a ketone solvent, and then reacting the general formula (X Z ¹ part (sulfonic acid group) in) formed a salt with a methyl group-containing 6-membered aromatic amine, and after obtaining an anti-salt acrylic monomer represented by the general formula (X-1), etc. It is preferable to add the polymerizable components simultaneously or sequentially and then carry out the polymerization reaction in a ketone solvent.

  The concentration of the acrylic monomer represented by the general formula (X) added to the reaction solvent is preferably 1% by mass or more and 25% by mass or less from the viewpoint of reaction efficiency. The amount of the methyl group-containing 6-membered aromatic amine to be reacted with the acrylic monomer represented by the general formula (X) is preferably an excess amount relative to the acrylic monomer from the viewpoint of reaction efficiency, and the reaction solvent (preferably The amount is preferably 1.0 equivalent or more with respect to the acrylic monomer represented by the general formula (X) added to the ketone solvent. Further, from the viewpoint of keeping the amount of unreacted methyl group-containing 6-membered aromatic amine remaining after the reaction low, the amount of methyl group-containing 6-membered aromatic amine relative to the acrylic monomer represented by the general formula (X) is 5. It is preferably 0 equivalent or less.

  As said ketone solvent, acetone, methyl ethyl ketone, cyclohexanone, etc. can be used individually or in mixture in arbitrary ratios. It is preferable to perform the reaction between the acrylic monomer represented by the general formula (X) and the methyl group-containing 6-membered aromatic amine in methyl ethyl ketone because the reaction rate of salt formation is high and the reaction can be completed in a short time. Further, after the reaction, when subjecting the obtained salt-type acrylic monomer represented by the general formula (X-1) to a polymerization reaction with another polymerizable component, the decomposition temperature of the polymerization initiator is determined. It is also desirable that the boiling point of the ketone solvent is high. In this respect, preferred ketone solvents are methyl ethyl ketone and cyclohexanone.

  The reaction between the acrylic monomer represented by the general formula (X) and the methyl group-containing 6-membered aromatic amine is preferably carried out at 0 ° C. or higher from the viewpoint of reaction efficiency. The reaction solvent is represented by the general formula (X). It is desirable to carry out at a temperature below the boiling point of the acrylic monomer and methyl group-containing 6-membered aromatic amine. When the reaction is performed at a temperature exceeding the boiling point, it is preferable to appropriately perform an operation that does not volatilize low-boiling substances, such as using a cooling tube or sealing the tube. The above reaction may be performed under reduced pressure, but may proceed sufficiently even under atmospheric pressure. The reaction time may be appropriately set within a range in which the reaction for obtaining the salt-type acrylic monomer represented by the general formula (X-1) sufficiently proceeds, and may be, for example, about 30 minutes to 16 hours. .

  Next, to the reaction system in which the above reaction for obtaining the salt-type acrylic monomer represented by the general formula (X-1) is performed, other polymerizable components are added simultaneously or sequentially to perform a polymerization reaction, The polymer can be obtained. It is also possible to add a ketone solvent as necessary. In order to increase the reaction efficiency, it is preferable to completely dissolve the polymerizable monomer after adding it to the reaction solvent.

The polymerization reaction can be performed in the presence of a known polymerization initiator, chain transfer agent or the like. The polymerization conditions vary depending on the polymerizable compound used, the polymerization initiator, the type of chain transfer agent, and the like, but in general, the liquid temperature is preferably about 50 to 80 ° C., and the reaction time is preferably about 1 to 30 hours.
The polymerization reaction may be performed in the air or in an atmosphere of a gas inert to the reaction such as nitrogen or argon. Moreover, you may carry out under reduced pressure and you may carry out under atmospheric pressure. In the polymerization, in addition to the above-described components, other components generally added to the polymerization reaction may be added to the polymerization reaction system.

  From the viewpoint of obtaining a high-strength coating film, the polymer preferably has a mass average molecular weight of 1,000 or more, and from the viewpoint of keeping the viscosity of the paint at a predetermined concentration in an appropriate range in order to maintain good workability. It is preferable that it is 200,000 or less. Moreover, it is preferable that molecular weight distribution (Mw / Mn) which is ratio of the mass average molecular weight Mw and the number average molecular weight is 1.00-5.50. More preferably, it is 1.01 to 5.40. A molecular weight distribution of 5.50 or less is preferable because the composition distribution is small and good dispersibility can be obtained. The average molecular weight in the present invention refers to a value determined in terms of standard polystyrene. The molecular weight of the polymer can be controlled by the raw material composition, reaction conditions, and the like.

[Carboxylic acid compound]
The magnetic recording medium of the present invention contains a carboxylic acid compound in the magnetic layer together with the polymer. Thereby, the ferromagnetic powder can be highly dispersed and excellent surface smoothness can be realized. As the carboxylic acid compound, only one kind may be used, or two or more kinds may be used in combination.

In the present invention, the carboxylic acid compound means a compound containing a carboxyl group (—COOH). The carboxyl group contained is at least one, and may be 2, 3 or 4 or more. From the viewpoint of further improving the dispersibility, it is preferable that the carboxylic acid compound has one carboxyl group. The carboxylic acid compound is preferably a fatty acid or an aromatic carboxylic acid from the viewpoint of further improving dispersibility. The fatty acid may be a saturated fatty acid or an unsaturated fatty acid, and is preferably an unsaturated fatty acid from the viewpoint of improving dispersibility. The carbon number of the fatty acid is preferably 14 to 24, more preferably 16 to 22. Specific examples of the fatty acid include oleic acid, elaidic acid, erucic acid and the like. Among these, oleic acid is preferable from the viewpoint of further improving dispersibility.
The aromatic ring contained in the aromatic carboxylic acid is preferably a 5-membered ring, a 6-membered ring or a 7-membered ring or a condensed ring thereof, more preferably a 5-membered ring or a 6-membered ring, More preferably, it is a 6-membered ring. Specific examples include a benzene ring, a naphthalene ring, an anthracene ring, and a phenanthrene ring. Among them, a benzene ring is preferable. The carboxyl group may be substituted directly on the aromatic ring or indirectly through a linking group. Examples of the linking group include an alkylene group which may have a substituent, an alkenyl group, or a divalent group in which these are directly or indirectly linked. Carbon number of a coupling group becomes like this. Preferably it is 2-10, More preferably, it is 2-5. Preferred examples of the aromatic carboxylic acid include cinnamic acid, benzoic acid, 1-naphthalene carboxylic acid, 2-naphthalene carboxylic acid, etc. Among these, cinnamic acid and benzoic acid are preferred from the viewpoint of further improving dispersibility. preferable.

  In the magnetic recording medium of the present invention, the polymer having the structural unit represented by the general formula (I) is used in the range of 5 to 50 parts by mass with respect to 100 parts by mass of the ferromagnetic powder from the viewpoint of improving dispersibility. It is preferable to use, and it is more preferable to use in 7-45 mass parts. Moreover, it is preferable to use 0.1-10 mass parts with respect to 100 mass parts of ferromagnetic powder from a viewpoint of a dispersibility improvement, and, as for a carboxylic acid type compound, it is more preferable to use 0.5-8 mass parts. .

The magnetic recording medium of the present invention can also contain other resin components such as known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof in addition to the polymer as a binder for the magnetic layer. However, even if not used together, according to the present invention, a magnetic recording medium having a magnetic layer having high surface smoothness can be realized by using the polymer together with a carboxylic acid compound.
When using the said polymer and another resin component together, it is preferable that the usage-amount of another resin component shall be 1-100 mass parts with respect to 100 mass parts of said polymers, and shall be 10-100 mass parts. It is more preferable.

  The magnetic recording medium of the present invention can also contain polyisocyanate as a magnetic layer component. In particular, if a polymer having a structural unit (II) containing a hydroxyl group and a polyisocyanate are used in combination, a hydroxyl layer and an isocyanate group form a crosslinked structure, thereby forming a magnetic layer having high coating strength. it can. As the polyisocyanate, it is preferable to use a tri- or higher functional polyisocyanate from the viewpoint of further improving the coating film strength. Specific examples of the trifunctional or higher functional polyisocyanate include a compound obtained by adding 3 mol of TDI (tolylene diisocyanate) to trimethylolpropane (TMP), a compound obtained by adding 3 mol of HDI (hexamethylene diisocyanate) to TMP, and IPDI to TMP. (Isophorone diisocyanate) 3 mol addition compound, TMP 3 mol addition XDI (xylylene diisocyanate) 3 adduct type polyisocyanate compound, TDI condensation isocyanurate type trimer, TDI condensation isocyanurate 5 amount , Condensed isocyanurate heptamers of TDI, and mixtures thereof, isocyanurate type condensates of HDI, isocyanurate type condensates of IPDI, and crude MDI. The amount of polyisocyanate used can be, for example, 0 to 80 parts by mass with respect to 100 parts by mass of the binder resin, and is preferably 50 to 80 parts by mass from the viewpoint of improving the coating film strength.

Furthermore, the present invention provides
A polymer having a structural unit represented by the general formula (I); and
Carboxylic acid compounds,
And a binder composition for magnetic recording media (hereinafter also referred to as “the composition of the present invention”).
As described above, the polymer represented by the general formula (I) is a salt-type polymer, so that it can highly disperse ferromagnetic powder as compared with a proton-type polymer. By using in combination with a compound, a further excellent dispersibility improvement effect can be obtained. Therefore, the composition of the present invention is preferably used as a coating solution for forming a magnetic layer by mixing with a constituent component of the magnetic layer such as a ferromagnetic powder. As described above, the polymer represented by the general formula (I) can be synthesized in a ketone solvent widely used for a magnetic recording medium forming coating solution, so without removing the reaction solvent after synthesis, Mixing with a carboxylic acid compound and other components is extremely advantageous from the viewpoint of simplifying the process in that a coating solution for forming a magnetic layer can be prepared.

  The polymer and the carboxylic acid compound contained in the composition of the present invention are as described above, and the various components that can be arbitrarily mixed are described in the following specific embodiments of the magnetic recording medium of the present invention. Can be referred to.

  Next, specific embodiments of the magnetic recording medium of the present invention will be described in more detail.

Magnetic layer
(i) Ferromagnetic powder Examples of the ferromagnetic powder contained in the magnetic layer include acicular ferromagnets, tabular magnetic bodies, and spherical or elliptical magnetic bodies. From the viewpoint of high density recording, the average long axis length of the acicular ferromagnet is preferably 20 nm or more and 50 nm or less, and more preferably 20 nm or more and 45 nm or less. The average plate diameter of the tabular magnetic body is preferably 10 nm or more and 50 nm or less in terms of a hexagonal plate diameter. When reproducing with a magnetoresistive head, it is necessary to reduce the noise, and the plate diameter is preferably 40 nm or less. If the plate diameter is in the above range, stable magnetization can be expected without thermal fluctuation. In addition, since noise is reduced, it is suitable for high-density magnetic recording. The spherical or elliptical magnetic body preferably has an average diameter of 10 nm to 50 nm from the viewpoint of high density recording.

The average particle size of the ferromagnetic powder can be measured by the following method.
The ferromagnetic powder is photographed with a transmission electron microscope H-9000, manufactured by Hitachi, at a photographing magnification of 100,000, and printed on a photographic paper so that the total magnification is 500,000 times to obtain a particle photograph. The target magnetic material is selected from the particle photograph, the outline of the powder is traced with a digitizer, and the particle size is measured with the image analysis software KS-400 manufactured by Carl Zeiss. Measure the size of 500 particles. The average particle size measured by the above method is defined as the average particle size of the ferromagnetic powder.

In the present invention, the size of the powder such as a magnetic material (hereinafter referred to as “powder size”) is (1) the shape of the powder is needle-like, spindle-like, or column-like (however, the height is the bottom surface). In the case of larger than the maximum major axis, etc., it is represented by the length of the major axis constituting the powder, that is, the major axis length, and (2) the shape of the powder is plate-shaped or columnar (however, the thickness or height is (Smaller than the maximum major axis of the plate surface or the bottom surface), and (3) the shape of the powder is spherical, polyhedral, unspecified, etc. When the major axis constituting the body cannot be specified, it is represented by the equivalent circle diameter. The equivalent circle diameter is a value obtained by a circle projection method.
The average powder size of the powder is an arithmetic average of the above powder sizes, and is obtained by carrying out the measurement as described above for 500 primary particles. Primary particles refer to an independent powder without aggregation.

The average acicular ratio of the powder is determined by measuring the short axis length of the powder in the above measurement, that is, the short axis length, and calculating the arithmetic average of the values of (long axis length / short axis length) of each powder. Point to. Here, the minor axis length is defined by the above-mentioned powder size in the case of (1), the length of the minor axis constituting the powder, and in the case of (2), the thickness or height, respectively. In the case of (3), since there is no distinction between the major axis and the minor axis, (major axis length / minor axis length) is regarded as 1 for convenience.
When the shape of the powder is specific, for example, in the case of the definition (1) of the powder size, the average powder size is referred to as the average major axis length, and in the case of the definition (2), the average powder size. Is called the average plate diameter, and the arithmetic average of (maximum major axis / thickness to height) is called the average plate ratio. In the case of definition (3), the average powder size is referred to as an average diameter (also referred to as an average particle diameter or an average particle diameter).

  Each magnetic material described above is described in detail in paragraphs [0097] to [0110] of JP-A-2009-96798.

(Ii) Additive The magnetic recording medium of the present invention contains a carboxylic acid compound in the magnetic layer, and other additives can be added to the magnetic layer as needed along with the carboxylic acid compound. According to the present invention, by including a polymer having a structural unit represented by the general formula (I) together with a carboxylic acid compound in the magnetic layer, even when other additives coexist, the magnetic substance is highly enhanced. Can be dispersed. Examples of the additive include an abrasive, a lubricant, a dispersant / dispersion aid, an antifungal agent, an antistatic agent, an antioxidant, and a solvent. For details of specific examples and the like of the above additives, reference can be made to, for example, paragraphs [0111] to [0115] of JP2009-96798A.

Carbon black can be added to the magnetic layer as necessary. Examples of carbon black that can be used in the magnetic layer include rubber furnace, rubber thermal, color black, and acetylene black. The specific surface area of the carbon black is preferably 100 to 500 m ² / g, more preferably 150~400m ² / g, DBP oil absorption preferably 20 to 400 ml / 100 g, more preferably 30 to 200 ml / 100 g. The particle size of carbon black is preferably 5 to 80 nm, more preferably 10 to 50 nm, and still more preferably 10 to 40 nm. The pH of carbon black is preferably 2 to 10, the water content is preferably 0.1 to 10%, and the tap density is preferably 0.1 to 1 g / ml. Regarding carbon black that can be used in the magnetic layer, for example, “Carbon Black Handbook” edited by Carbon Black Association can be referred to. They are available as commercial products.

  These additives used in the present invention can be properly used in the magnetic layer and further in the nonmagnetic layer described later as needed. All or a part of the additives used in the present invention may be added in any step during the production of the coating liquid for the magnetic layer or nonmagnetic layer. For example, when mixing with a ferromagnetic powder before the kneading step, when adding at a kneading step with a ferromagnetic powder, a binder and a solvent, when adding at a dispersing step, when adding after dispersing, when adding just before coating, etc. There is.

Nonmagnetic Layer The magnetic recording medium of the present invention can have a magnetic layer directly on a nonmagnetic support, and a nonmagnetic layer containing a nonmagnetic powder and a binder is provided between the nonmagnetic support and the magnetic layer. Can also have. As the resin component constituting the binder of the nonmagnetic layer, a polymer having the structural unit represented by the general formula (I) described above may be used, a known thermoplastic resin, thermosetting resin, Other resin components such as reactive resins and mixtures thereof may be used.

  The nonmagnetic powder may be an inorganic substance or an organic substance. Carbon black or the like can also be used. Examples of the inorganic substance include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. These nonmagnetic powders are available as commercial products, and can also be produced by a known method.

Specifically, titanium oxide such as titanium dioxide, cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO ₂ , SiO ₂ , Cr ₂ O ₃ , α-alumina, β-alumina having an α conversion of 90 to 100%, γ-alumina, α-iron oxide, goethite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide, copper oxide, MgCO ₃ , CaCO ₃ , BaCO ₃ , SrCO ₃ , BaSO ₄ , silicon carbide Titanium carbide or the like is used alone or in combination of two or more. Preferred are α-iron oxide and titanium oxide.

The shape of the nonmagnetic powder may be any of acicular, spherical, polyhedral and plate shapes.
The crystallite size of the nonmagnetic powder is preferably 4 nm to 1 μm, and more preferably 40 to 100 nm. A crystallite size in the range of 4 nm to 1 μm is preferred because it does not become difficult to disperse and has a suitable surface roughness.
The average particle size of these nonmagnetic powders is preferably 5 nm to 2 μm. The range of 5 nm to 2 μm is preferable because the dispersion is good and the surface roughness is suitable. However, if necessary, nonmagnetic powders having different average particle diameters can be combined, or even a single nonmagnetic powder can have the same effect by widening the particle size distribution. The average particle size of the particularly preferred nonmagnetic powder is 10 to 200 nm. Paragraphs [0123] to [0132] of JP-A-2009-96798 can be referred to for details of the nonmagnetic powder that can be used in the magnetic recording medium of the present invention.

Carbon black can be mixed in the nonmagnetic layer together with nonmagnetic powder to lower the surface electrical resistance, reduce the light transmittance, and obtain the desired μ Vickers hardness. Μ Vickers hardness of the nonmagnetic layer is normally 25 to 60 kg / mm ^2, preferably in order to adjust the head contact, and a 30 to 50 kg / mm ^2, a thin film hardness meter (NEC Corp. HMA-400) Can be measured using a diamond triangular pyramid needle with a ridge angle of 80 degrees and a tip radius of 0.1 μm at the tip of the indenter. It is standardized that the light transmittance is generally 3% or less for absorption of infrared rays having a wavelength of about 900 nm, for example, 0.8% or less for a VHS magnetic tape. For this purpose, rubber furnace, rubber thermal, color black, acetylene black and the like can be used.

The specific surface area of carbon black employed in the nonmagnetic layer is preferably from 100 to 500 m ² / g, more preferably 150 to 400 m ² / g, DBP oil absorption preferably 20 to 400 ml / 100 g, more preferably 30~200ml / 100g. The particle size of carbon black is preferably 5 to 80 nm, more preferably 10 to 50 nm, and still more preferably 10 to 40 nm. The pH of carbon black is preferably 2 to 10, the water content is preferably 0.1 to 10%, and the tap density is preferably 0.1 to 1 g / ml. For carbon black that can be used in the nonmagnetic layer, for example, “Carbon Black Handbook” edited by Carbon Black Association can be referred to. They are available as commercial products.

  Further, an organic powder can be added to the nonmagnetic layer according to the purpose. Examples of such organic powder include acrylic styrene resin powder, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment, but polyolefin resin powder, polyester resin powder, polyamide resin powder, polyimide resin, and the like. Resin powder and polyfluorinated ethylene resin can also be used. As the production method, those described in JP-A Nos. 62-18564 and 60-255827 can be used.

  Known techniques relating to the magnetic layer can be applied to the binder resin, lubricant, dispersant, additive, solvent, dispersion method, etc. of the nonmagnetic layer. In particular, with respect to the binder resin amount, type, additive, and dispersant addition amount and type, known techniques relating to the magnetic layer can be applied.

Non-magnetic support Examples of the non-magnetic support include known ones such as biaxially stretched polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamideimide, and aromatic polyamide. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferable.

Backcoat layer In general, a magnetic tape for recording computer data is strongly required to have repeated running characteristics as compared with a video tape and an audio tape. In order to maintain such high storage stability, a backcoat layer can be provided on the surface of the nonmagnetic support opposite to the surface on which the magnetic layer is provided. The backcoat layer coating solution can be formed by dispersing particle components such as an abrasive and an antistatic agent and a binder in an organic solvent. Various inorganic pigments and carbon black can be used as the particulate component. Moreover, as a binder, resin, such as a nitrocellulose, a phenoxy resin, a vinyl chloride resin, a polyurethane, can be used individually or in mixture, for example. In order to form the back coat layer, it is possible to use the aforementioned polymer.

Layer Structure In the magnetic recording medium of the present invention, the preferred thickness of the nonmagnetic support is 3 to 80 μm. Moreover, when providing the said smoothing layer between a nonmagnetic support body and a nonmagnetic layer or a magnetic layer, the thickness of a smoothing layer is 0.01-0.8 micrometer, Preferably it is 0.02-0.6 micrometer. is there. Moreover, the thickness of the said backcoat layer is 0.1-1.0 micrometer, for example, Preferably it is 0.2-0.8 micrometer.

  The thickness of the magnetic layer is optimized depending on the saturation magnetization amount, head gap length, and recording signal band of the magnetic head to be used, and is preferably 10 nm to 100 nm, more preferably from the viewpoint of increasing the capacity. Is 20 nm to 80 nm. There may be at least one magnetic layer, and the magnetic layer may be separated into two or more layers having different magnetic characteristics, and a configuration related to a known multilayer magnetic layer can be applied.

  The thickness of the nonmagnetic layer is preferably 0.2 to 3.0 μm, more preferably 0.3 to 2.5 μm, and further preferably 0.4 to 2.0 μm. In the case where the magnetic recording medium of the present invention has a nonmagnetic layer, the nonmagnetic layer exhibits its effect as long as it is substantially nonmagnetic. For example, as an impurity or intentionally a small amount of magnetic material Even if it contains, the effect of this invention is shown and it can be considered as a structure substantially the same as the magnetic recording medium of this invention. “Substantially the same” means that the residual magnetic flux density of the nonmagnetic layer is 10 mT (100 G) or less or the coercive force is 7.96 kA / m (100 Oe) or less, preferably the residual magnetic flux density and the coercive force. It means not having.

Manufacturing Method The process of manufacturing a coating solution for forming each layer such as a magnetic layer and a nonmagnetic layer may comprise at least a kneading process, a dispersing process, and a mixing process provided before and after these processes as necessary. preferable. Each process may be divided into two or more stages. All raw materials such as ferromagnetic powder, non-magnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant, and solvent used in the present invention may be added at the beginning or middle of any step. In addition, individual raw materials may be added in two or more steps. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion.
Moreover, a coating liquid can also be manufactured by adding the said raw material simultaneously or sequentially to the binder composition for magnetic recording media of this invention. For example, after pulverizing powder components such as ferromagnetic powder and nonmagnetic powder with a kneader, the binder resin composition for magnetic recording media of the present invention (and other binder components optionally used in combination) is added and kneaded. A coating liquid can be prepared by performing a process, adding various additives to this kneaded material, and performing a dispersion | distribution process.

  In order to prepare the coating liquid for forming each layer, a conventional known manufacturing technique can be used as a partial process. In the kneading step, it is preferable to use a kneading force such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. In the case of using a kneader, the kneading treatment may be performed using 15 to 500 parts by mass of a binder (preferably 30% by mass or more of the total binder) with respect to 100 parts by mass of the ferromagnetic powder or nonmagnetic powder. preferable. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Further, glass beads can be used to disperse the magnetic layer coating liquid and the nonmagnetic layer coating liquid. In addition to glass beads, zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media, are suitable. The particle diameter and filling rate of these dispersion media are optimized. A well-known thing can be used for a disperser.

  In the method of manufacturing a magnetic recording medium, for example, a magnetic layer coating solution is formed on the surface of a nonmagnetic support under running so as to have a predetermined film thickness. Here, a plurality of magnetic layer coating solutions may be applied sequentially or simultaneously, or a nonmagnetic layer coating solution and a magnetic layer coating solution may be applied sequentially or simultaneously. The coating machines that apply the coating liquid for forming each layer include air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure coat, kiss coat, and cast coat. Spray coating, spin coating, etc. can be used. As for these, for example, “Latest Coating Technology” (May 31, 1983) issued by General Technology Center Co., Ltd. can be referred to. For details of the coating process, paragraphs [0067] and [0068] of JP-A No. 2004-295926 can also be referred to.

  The medium after the coating process can be subjected to various post-treatments such as drying treatment, magnetic layer orientation treatment, and surface smoothing treatment (calendar treatment). For details of these processes, reference can be made, for example, to paragraphs [0070] to [0073] of Japanese Patent Application Laid-Open No. 2004-295926.

As described above, when polyisocyanate is used for improving the coating strength, a crosslinked structure can be formed even in the heating in the drying treatment or the calendering treatment. In addition to the above, it is preferable to perform heat treatment. In the heat treatment in this case, for example, the heating temperature is 35 to 100 ° C, preferably 50 to 80 ° C. The heat treatment time is, for example, 12 to 72 hours, preferably 24 to 48 hours.
The obtained magnetic recording medium can be cut into a desired size using a cutting machine, a punching machine, or the like.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the embodiment shown in the examples. The “parts” and “%” shown below indicate parts by mass and mass% unless otherwise specified.

1. Reference example for acrylic polymer synthesis

  In the following synthesis examples 1 to 7, synthesis examples 1 to 3 are synthesis examples of an anti-salt acrylic polymer having a structural unit represented by the general formula (I), and synthesis examples 4 and 5 are general formulas. It is a synthesis example (comparative synthesis example) of an anti-salt acrylic polymer in which a methyl group-containing 6-membered aromatic amine in the structural unit represented by (I) is substituted with an aliphatic tertiary amine. In a synthesis example (comparative synthesis example) of an anti-salt acrylic polymer in which a methyl group-containing 6-membered aromatic amine in the structural unit represented by the general formula (I) is substituted with a methyl group-containing 5-membered aromatic amine Yes, Synthesis Example 7 is a synthesis example (comparative synthesis example) of a proton-type acrylic polymer.

[Synthesis Example 1]
To a 500 ml flask, add 1.0 part of 2-acrylamido-2-methylpropanesulfonic acid, 0.45 part of α-methylpyridine, and 8.0 part of methyl ethyl ketone, and stir at 25 ° C. for 1 hour or more under light-shielding conditions until complete dissolution. It was. Since free α-methylpyridine was measured by the method described later, it was not detected, and thus a salt-type acrylic monomer in which a sulfonic acid group and α-methylpyridine formed a salt was formed in the above solution. confirmed. Note that 2-acrylamido-2-methylpropanesulfonic acid hardly dissolves in methyl ethyl ketone in a state where it does not form a counter salt with α-methylpyridine. When 2-acrylamido-2-methylpropanesulfonic acid and α-methylpyridine were added to methyl ethyl ketone by the above operation, an undissolved substance was not confirmed and a completely dissolved solution was obtained. It can be confirmed that the total amount of 2-methylpropanesulfonic acid formed a counter salt with α-methylpyridine.
In the completely dissolved solution, 10 parts of 2-hydroxyethyl methacrylate, 14.0 parts of isobornyl methacrylate, 5.0 parts of stearyl methacrylate, 0.60 part of dimethyl-2,2′-azobis (2-methylpropionate) Then, 18.0 parts of methyl ethyl ketone was added, and the mixture was completely dissolved by stirring at 25 ° C. for 1 hour or more under light shielding conditions.
After adding 77 parts of methyl ethyl ketone to a 500 ml flask and raising the internal temperature to 70 ° C., the mixed solution was added dropwise over 3 hours while adjusting the external temperature so that the internal temperature was in the range of 65 to 72 ° C. . After completion of dropping, the reaction was further continued for 5 hours to obtain a solution of acrylic polymer A.
When the mass average molecular weight (Mw) and the number average molecular weight (Mn) of the obtained resin were determined by the methods described later, Mw = 37,000 and Mn = 23,000. When free α-methylpyridine was measured by the method described later, it was not detected. Since this result was proved from the comparison between Synthesis Examples 8 to 10 and Synthesis Examples 11 to 13 described later that the amine is bonded to the sulfonic acid group by acid-base interaction, the presence of the amine before and after the polymerization reaction. The state did not change, and it was confirmed that the sulfonic acid group formed a counter salt with α-methylpyridine in the obtained resin.
When the salt concentration of the sulfonic acid group and α-methylpyridine in the obtained resin was measured by the method described later, it was 150 μeq / g.

Measurement method (1) Measurement of average molecular weight Using a DMF solvent containing 0.3% lithium bromide, GPC (gel permeation chromatography) was used, and the average molecular weight was calculated in terms of standard polystyrene.
(2) Salt concentration in the resin The amount of sulfur element is quantified from the peak area of sulfur (S) element by fluorescent X-ray analysis, converted to the amount of sulfur element per kg of resin, and the sulfonic acid group in the resin and the specified organic The salt concentration with the amine was determined.
(3) Measurement of free organic amine The obtained reaction solution was diluted with methyl ethyl ketone so that the solid content concentration was 5% by mass, and the free organic amine was analyzed using gas chromatography under the following conditions.
Apparatus: Shimadzu GC-17A
Column: DB-1
Column temperature: 50 ° C
Inlet temperature: 250 ° C
Detector temperature: 250 ° C
Column heating program: 50 ° C / 5 minutes → 10 ° C / 1 minute to 250 ° C → 250 ° C / 10 minutes

[Synthesis Example 2]
A solution of acrylic polymer B was obtained according to Synthesis 1 except that 0.45 part of α-methylpyridine was changed to 0.45 part of β-methylpyridine. When free β-methylpyridine was measured by the above-mentioned method, it was not detected.
When the mass average molecular weight (Mw) and the number average molecular weight (Mn) of the obtained resin were determined by the above-described methods, Mw = 38,000 and Mn = 23,000. It was 150 microeq / g when the salt concentration of the sulfonic acid group in the obtained resin and (beta) -methylpyridine was measured by the above-mentioned method. Further, when free β-methylpyridine was measured by the above-mentioned method, it was not detected.

[Synthesis Example 3]
A solution of acrylic polymer C was obtained according to Synthesis Example 1, except that 0.45 part of α-methylpyridine was changed to 0.45 part of γ-methylpyridine. When free γ-methylpyridine was measured by the method described above, it was not detected.
When the mass average molecular weight (Mw) and the number average molecular weight (Mn) of the obtained resin were determined by the above-described methods, Mw = 37,000 and Mn = 23,000. It was 150 microeq / g when the salt concentration of the sulfonic acid group in the obtained resin and (gamma) -methylpyridine was measured by the above-mentioned method. Further, when free γ-methylpyridine was measured by the above-mentioned method, it was not detected.

[Synthesis Example 4 (Comparative Synthesis Example)]
A solution of acrylic polymer D was obtained according to Synthesis Example 1 except that 0.45 part of α-methylpyridine was changed to 0.62 part of diisopropylethylamine. When free diisopropylethylamine was measured by the above-mentioned method, it was not detected.
When the mass average molecular weight (Mw) and the number average molecular weight (Mn) of the obtained resin were determined by the above-described methods, Mw = 46,000 and Mn = 24,000. The salt concentration of the sulfonic acid group and diisopropylethylamine in the obtained resin was measured by the method described above, and found to be 150 μeq / g. Further, when free diisopropylethylamine was measured by the above-mentioned method, it was not detected.

[Synthesis Example 5 (Comparative Synthesis Example)]
A solution of acrylic polymer E was obtained according to Synthesis Example 1 except that 0.45 part of α-methylpyridine was changed to 0.73 part of diazabicycloundecene. When free diazabicycloundecene was measured by the above-mentioned method, it was not detected.
When the mass average molecular weight (Mw) and the number average molecular weight (Mn) of the obtained resin were determined by the above-described methods, Mw = 38,000 and Mn = 23,000. The salt concentration of the sulfonic acid group and diazabicycloundecene in the obtained resin was measured by the above-mentioned method and found to be 150 μeq / g. Moreover, when free diazabicycloundecene was measured by the above-mentioned method, it was not detected.

[Synthesis Example 6 (Comparative Synthesis Example)]
A solution of acrylic polymer F was obtained according to Synthesis Example 1 except that 0.45 part of α-methylpyridine was changed to 0.40 part of N-methylimidazole. When free N-methylimidazole was measured by the above-mentioned method, it was not detected.
When the mass average molecular weight (Mw) and number average molecular weight (Mn) of the obtained resin were determined by the above-described methods, Mw = 57,000 and Mn = 27,000. It was 150 microeq / g when the salt concentration of the sulfonic acid group in the obtained resin and N-methylimidazole was measured by the above-mentioned method. Moreover, when free N-methylimidazole was measured by the above-mentioned method, it was not detected.

[Synthesis Example 7 (Comparative Synthesis Example)]
To a 100 ml flask, 1 part of 2-acrylamido-2-methylpropanesulfonic acid, 3.0 parts of water and 1.0 part of methanol were added, and stirred for 10 minutes or more at 25 ° C. or less under light shielding conditions to completely dissolve. To a 500 ml flask, add 37 parts of isobornyl methacrylate, 14 parts of stearyl methacrylate, 27 parts of 2-hydroxyethyl methacrylate, and 60 parts of methyl ethyl ketone and stir at 25 ° C. or less for 1 hour or more to completely dissolve 2-acrylamide. 2-Methylpropanesulfonic acid in water / methanol solution was added, 1.2 parts of dimethyl-2,2′-azobis (2-methylpropionate) was added, and the mixture was stirred at 25 ° C. for 10 minutes or more under light shielding conditions. It was completely dissolved.
After adding 30 parts of methyl ethyl ketone to a 500 ml flask and raising the internal temperature to 70 ° C., the mixed solution was added dropwise over 3 hours while adjusting the external temperature so that the internal temperature was in the range of 65 to 72 ° C. . After the completion of the dropwise addition, the mixture was reacted for 1 hour, 0.4 parts of dimethyl-2,2′-azobis (2-methylpropionate) was added, and the mixture was further reacted for 5 hours to obtain a resin solution and a white precipitate.
The white precipitate was removed by vacuum filtration to obtain a solution of acrylic polymer L.
When the mass average molecular weight (Mw) and the number average molecular weight (Mn) of the obtained resin were determined by the above-described methods, Mw = 70,000 and Mn = 26,000. When the sulfonate group concentration in the obtained resin was measured by the same method as the method for measuring the salt concentration in the resin, the sulfonic acid concentration was 54 μeq / g.

[Synthesis Example 8]
To 1.0 part of the resin solution obtained in Synthesis Example 7, 0.0031 part of α-methylpyridine was added and stirred for 1 hour. When the obtained liquid was measured under the above-mentioned gas chromatography conditions, no peak derived from α-methylpyridine was confirmed. From this result, it was confirmed that the added α-methylpyridine formed a salt with the sulfonic acid group of the acrylic polymer L.

[Synthesis Example 9]
To 31 parts of the resin solution obtained in Synthesis Example 7, 0.0031 part of β-methylpyridine was added and stirred for 1 hour. When the obtained liquid was measured under the above-mentioned gas chromatography conditions, no peak derived from diisopropylethylamine was confirmed. From this result, it was confirmed that the added β-methylpyridine formed a salt with the sulfonic acid group of the acrylic polymer L.

[Synthesis Example 10]
To 1.0 part of the resin solution obtained in Synthesis Example 7, 0.0031 part of γ-methylpyridine was added and stirred for 1 hour. When the obtained liquid was measured under the above-mentioned gas chromatography conditions, no peak derived from diisopropylethylamine was confirmed. From this result, it was confirmed that the added γ-methylpyridine formed a salt with the sulfonic acid group of the acrylic polymer L.

[Synthesis Example 11 (Comparative Synthesis Example)]
By the following method, the acrylic polymer M structurally different from the acrylic polymer L obtained in Synthesis Example 7 in that it does not include a unit derived from the acrylic monomer represented by the general formula (X) was synthesized.
In a 500 ml flask, add 37 parts of isobornyl methacrylate, 14 parts of stearyl methacrylate, 27 parts of 2-hydroxyethyl methacrylate, and 60 parts of 2-butanone and stir at 25 ° C. or less for 1 hour or more to completely dissolve. 1.2 parts of -2,2'-azobis (2-methylpropionate) was added, and the mixture was completely dissolved by stirring at 25 ° C for 10 minutes or more under light shielding conditions.
After adding 30 parts of 2-butanone to a 500 ml flask and raising the internal temperature to 70 ° C., adjusting the external temperature so that the internal temperature is in the range of 65 to 72 ° C., the mixed solution is added over 3 hours. It was dripped. After the completion of the dropwise addition, the mixture was reacted for 1 hour, 0.4 parts of dimethyl-2,2′-azobis (2-methylpropionate) was added, and the mixture was further reacted for 5 hours to obtain a resin solution and a white precipitate. Thereafter, the white precipitate was removed by filtration under reduced pressure to obtain a transparent resin solution.
When the mass average molecular weight (Mw) and the number average molecular weight (Mn) of the obtained resin were determined by the above-described method, Mw = 65,000 and Mn = 25,000.

[Synthesis Example 12 (Comparative Synthesis Example)]
To 1.0 part of the resin solution obtained in Synthesis Example 11, 0.0031 part of α-methylpyridine was added and stirred for 1 hour. When the obtained liquid was measured under the above-mentioned gas chromatography conditions, a peak derived from α-methylpyridine was confirmed. From this result, it was confirmed that the added α-methylpyridine was not introduced into the acrylic polymer M.

[Synthesis Example 13 (Comparative Synthesis Example)]
To 1.0 part of the resin solution obtained in Synthesis Example 11, 0.0031 part of β-methylpyridine was added and stirred for 1 hour. When the obtained liquid was measured under the above-mentioned gas chromatography conditions, a peak derived from β-methylpyridine was confirmed. From this result, it was confirmed that the added β-methylpyridine was not introduced into the acrylic polymer M.

[Synthesis Example 14 (Comparative Synthesis Example)]
To 1.0 part of the resin solution obtained in Synthesis Example 11, 0.0031 part of γ-methylpyridine was added and stirred for 1 hour. When the obtained liquid was measured under the above-mentioned gas chromatography conditions, a peak derived from γ-methylpyridine was confirmed. From this result, it was confirmed that the added γ-methylpyridine was not introduced into the acrylic polymer M.

  From the comparison between Synthesis Examples 8 to 10 and Synthesis Examples 11 to 13, it was confirmed that the methyl group-containing 6-membered aromatic amine was bonded to the sulfonic acid group by an acid-base interaction.

2. Examples and Comparative Examples of Binder Composition (Magnetic Paint) for Magnetic Recording Medium Formation

[Example 1]
The following ferromagnetic hexagonal ferrite powder 2.2 parts by mass, acrylic polymer A 1 part by mass, and transcinnamic acid 0.10 part by mass were suspended in a solution consisting of cyclohexanone 6.0 parts by mass and methyl ethyl ketone 9.0 parts by mass. .
Ferromagnetic hexagonal barium ferrite powder Composition excluding oxygen (molar ratio): Ba / Fe / Co / Zn = 1/9 / 0.2 / 1
Hc: 176 kA / m (2200 Oe), average plate diameter: 25 nm, average plate ratio: 3
BET specific surface area: 65 m ² / g
σs: 49 A · m ² / kg (49 emu / g)
pH: 7
To the obtained suspension, 60 parts by mass of 0.1 mmφ zirconia beads (manufactured by Nikkato) were added and dispersed for 15 hours to obtain a magnetic coating.
The dispersed particle size of the hexagonal ferrite powder in the obtained magnetic coating material was measured by the method described later to be 37 nm.

Method for Measuring Dispersion Particle Size The magnetic coating material was diluted with a mixed solution containing cyclohexanone and methyl ethyl ketone in a volume ratio of cyclohexanone 6.0: methyl ethyl ketone 9.0 to a solid content concentration of 0.2% by mass (solid content Represents the total mass of hexagonal ferrite powder, acrylic polymer, and carboxylic acid compound).
The average particle size of the hexagonal ferrite powder in the diluted solution measured using a dynamic light scattering type particle size distribution analyzer LB-500 manufactured by HORRIBA was defined as the dispersed particle size. A smaller dispersed particle size means that the hexagonal ferrite powder does not aggregate and has better dispersibility.

[Example 2]
It was 39 nm when the dispersed particle diameter was measured by the same operation as in Example 1 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer B.

[Example 3]
It was 39 nm when the dispersed particle diameter was measured by the same operation as in Example 1 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer C.

[Comparative Example 1]
The dispersed particle diameter was measured by the same operation as in Example 1 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer D, and it was 56 nm.

[Comparative Example 2]
It was 44 nm when the dispersed-particle diameter was measured by the same operation as Example 1 except the point which changed acrylic polymer A 1 mass part into acrylic polymer E 1 mass part.

[Comparative Example 3]
The dispersed particle diameter was measured by the same operation as in Example 1 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer F, and it was 50 nm.

[Comparative Example 4]
It was 52 nm when the dispersed particle diameter was measured by the same operation as in Example 1 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer L.

[Example 4]
The dispersed particle size was measured by the same operation as in Example 1 except that 0.10 parts by mass of trans-cinnamic acid was changed to 0.11 parts by mass of benzoic acid, and it was 38 nm.

[Example 5]
The dispersed particle size was measured by the same operation as in Example 4 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer B, and it was 38 nm.

[Example 6]
It was 39 nm when the dispersed particle diameter was measured by the same operation as in Example 4 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer C.

[Comparative Example 5]
The dispersed particle size was measured by the same operation as in Example 4 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer D, and it was 56 nm.

[Comparative Example 6]
It was 44 nm when the dispersed particle diameter was measured by the same operation as Example 4 except having changed the acrylic polymer A 1 mass part into the acrylic polymer E 1 mass part.

[Comparative Example 7]
The dispersed particle diameter was measured by the same operation as in Example 4 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer F, and it was 56 nm.

[Comparative Example 8]
It was 52 nm when the dispersed particle diameter was measured by the same operation as in Example 4 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer L.

[Example 7]
The dispersed particle diameter was measured by the same operation as in Example 1 except that 0.10 parts by mass of trans-cinnamic acid was changed to 0.21 parts by mass of oleic acid.

[Example 8]
The dispersed particle size was measured by the same operation as in Example 7 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer B, and it was 40 nm.

[Example 9]
It was 40 nm when the dispersed particle diameter was measured by the same operation as in Example 7 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer C.

[Comparative Example 9]
It was 68 nm when the dispersed particle diameter was measured by the same operation as Example 7 except having changed 1 mass part of acrylic polymer A into 1 mass part of acrylic polymer D.

[Comparative Example 10]
It was 49 nm when the dispersed particle diameter was measured by the same operation as in Example 7 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer E.

[Comparative Example 11]
It was 59 nm when the dispersed particle diameter was measured by the same operation as in Example 7 except that 1 part by mass of the acrylic polymer A was changed to 1 part by mass of the acrylic polymer F.

[Comparative Example 12]
It was 73 nm when the dispersed-particle diameter was measured by the same operation as Example 7 except having changed 1 mass part of acrylic polymer A into 1 mass part of acrylic polymer L.

  The results of the above examples and comparative examples are summarized in Table 1 below.

  As shown in Table 1 above, an example in which an anti-salt acrylic polymer containing a methyl group-containing 6-membered aromatic amine and a sulfonic acid group derived from an acrylic monomer in the form of a salt is used in combination with a carboxylic acid compound Then, the dispersed particle diameter was 40 nm or less, and it was possible to disperse the hexagonal ferrite powder in a highly dispersed state as compared with the comparative example. Thus, by forming a magnetic layer using a magnetic coating material in which fine magnetic particles are highly dispersed, a magnetic recording medium having excellent surface smoothness and suitable for high-density recording can be obtained.

  The magnetic recording medium of the present invention is suitable as a magnetic recording medium for high density recording.

Claims (17)

A magnetic recording medium having a magnetic layer comprising a ferromagnetic powder and a binder on a nonmagnetic support,
The binder is represented by the general formula (I):
[In the general formula (I), R ¹ represents a hydrogen atom or a methyl group, X ¹ represents an oxygen atom, a sulfur atom or a —N (R ¹¹ ) — group, R ¹¹ represents a hydrogen atom or a substituent, L ¹ represents an alkylene group which may have a substituent, and Y ¹ represents a salt formed from a sulfonic acid group and a methyl group-containing 6-membered aromatic amine. ]
A polymer having a structural unit represented by:
The magnetic recording medium, wherein the magnetic layer further contains a carboxylic acid compound. The magnetic recording medium according to claim 1, wherein the methyl group-containing 6-membered aromatic amine includes a pyridine ring as an aromatic ring. The magnetic recording medium according to claim 1, wherein the methyl group-containing 6-membered aromatic amine is selected from the group consisting of α-methylpyridine, β-methylpyridine, and γ-methylpyridine. The magnetic recording medium according to claim 1, wherein the carboxylic acid compound is selected from the group consisting of a fatty acid and an aromatic carboxylic acid. The magnetic recording medium according to claim 1, wherein the carboxylic acid compound is selected from the group consisting of cinnamic acid, benzoic acid, and oleic acid. The magnetic recording medium according to claim ¹ , wherein the polymer contains 10 to 1000 μeq / g of a salt represented by Y ¹ . The polymer has the general formula (II):
[In General Formula (II), R ² represents a hydrogen atom or a methyl group, and L ² represents a single bond or a divalent linking group. ]
The magnetic recording medium according to claim 1, further comprising a structural unit represented by: The polymer has the general formula (III):
[In general formula (III), R ³ represents a hydrogen atom or a methyl group, L ³ represents a single bond or a divalent linking group, and Z ³ represents a substituent having a cyclic structure. ]
The magnetic recording medium according to claim 1, further comprising a structural unit represented by: Formula (I):
[In the general formula (I), R ¹ represents a hydrogen atom or a methyl group, X ¹ represents an oxygen atom, a sulfur atom or a —N (R ¹¹ ) — group, R ¹¹ represents a hydrogen atom or a substituent, L ¹ represents an alkylene group which may have a substituent, and Y ¹ represents a salt formed from a sulfonic acid group and a methyl group-containing 6-membered aromatic amine. ]
A polymer having a structural unit represented by:
Carboxylic acid compounds,
A binder composition for magnetic recording media, comprising: The binder composition for a magnetic recording medium according to claim 9, wherein the methyl group-containing aromatic amine contains a pyridine ring as an aromatic ring. The binder composition for magnetic recording media according to claim 9 or 10, wherein the methyl group-containing 6-membered aromatic amine is selected from the group consisting of α-methylpyridine, β-methylpyridine, and γ-methylpyridine. . The binder composition for a magnetic recording medium according to any one of claims 9 to 11, wherein the carboxylic acid compound includes a benzene ring or a naphthalene ring selected from the group consisting of a fatty acid and an aromatic carboxylic acid. The binder composition for a magnetic recording medium according to any one of claims 9 to 12, wherein the carboxylic acid compound is selected from the group consisting of cinnamic acid, benzoic acid, and oleic acid. The binder composition for magnetic recording media according to any one of claims 9 to 13, further comprising a ketone solvent. The binder composition for magnetic recording media according to any one of claims 9 to 14, wherein the polymer contains 10 to 1000 µeq / g of a salt represented by Y ¹ . The polymer has the general formula (II):
[In General Formula (II), R ² represents a hydrogen atom or a methyl group, and L ² represents a single bond or a divalent linking group. ]
The binder composition for magnetic recording media according to claim 9, further comprising a structural unit represented by: The polymer has the general formula (III):
[In general formula (III), R ³ represents a hydrogen atom or a methyl group, L ³ represents a single bond or a divalent linking group, and Z ³ represents a substituent having a cyclic structure. ]
The binder composition for magnetic recording media according to claim 9, further comprising a structural unit represented by: