JP2011216149A - Binder for magnetic recording medium, composition for magnetic recording medium, and magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a binder for a magnetic recording medium, with which a magnetic recording medium for high-density recording with excellent electromagnetic conversion characteristics can be produced, and which includes a vinyl polymer.SOLUTION: The binder for the magnetic recording medium is a vinyl copolymer including a structural unit represented by general formula [A]. In general formula [A], Rrepresents a hydrogen atom, a halogen atom or a methyl group, Lrepresents a single bond or a divalent linking group, Yrepresents a hydrogen atom, a 1-20C hydrocarbon group or an aromatic group, Zrepresents a divalent group having an ester bond, a thioester bond, or an amido bond, and n1 is an integer of 2 to 500.

Description

The present invention relates to a binder for a magnetic recording medium and a composition for a magnetic recording medium, and more specifically, a binder for a magnetic recording medium suitable for production of a magnetic recording medium capable of exhibiting excellent electromagnetic conversion characteristics, and a magnetic recording medium. The present invention relates to a recording medium composition.
Furthermore, the present invention relates to a magnetic recording medium containing the binder.

  In the coating type magnetic recording medium, the binder plays an important role in electromagnetic conversion characteristics.

  As binders for magnetic recording media, various resins such as vinyl chloride resin, polyurethane resin, polyester resin, and acrylic resin are used. Among them, vinyl polymers such as vinyl chloride resins and acrylic resins are widely used from the viewpoints of the high degree of freedom of the unit and the ease of synthesis reaction (see 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. Furthermore, as a means for increasing the smoothness of the magnetic layer surface, it is also effective to increase the dispersibility of the nonmagnetic powder contained in the nonmagnetic layer located below the magnetic layer. Therefore, it has been widely practiced to introduce an adsorption functional group (polar group) such as SO ₃ Na group into the binder for magnetic recording media. However, if the amount of the polar group introduced into the binder is increased in order to improve the dispersibility of the fine particle powder, the dispersibility may be lowered due to the association of polar groups. Therefore, it has become difficult to ensure sufficient dispersibility by simply introducing a polar group. That is, it is difficult to cope with higher density recording only by introducing polar groups into conventional vinyl polymers such as the polymers described in Patent Documents 1 to 3 described above.

  Accordingly, an object of the present invention is to provide a binder for a magnetic recording medium comprising a vinyl polymer, which can produce a magnetic recording medium for high density recording having excellent electromagnetic conversion characteristics.

As a result of intensive studies to achieve the above object, the present inventors have found that a vinyl polymer having a structural unit represented by the following general formula [A] can highly disperse ultrafine powder. Newly found. This is presumably because the repeating structure of “Z ^a ” included in the general formula [A], that is, the graft chain structure contributes to dispersion. To explain this point in more detail, the inventors of the present application speculate that the suppression of the aggregation of the particles contributes to dispersion by effectively spreading the graft chain structure contained in the general formula [A] in the dispersion medium. ing.
The present invention has been completed based on the above findings.

（一般式［Ａ］において、Ｒ^aは水素原子、ハロゲン原子またはメチル基を表し、Ｌ^aは単結合または２価の連結基を表し、Ｙ^aは水素原子、炭素数１以上２０以下の炭化水素基または芳香族基を表し、Ｚ^aはエステル結合、チオエステル結合またはアミド結合を有する２価の基を表し、ｎ１は２以上５００以下の整数を表す。）
＜２＞一般式［Ａ］で表される構造単位は、一般式［Ａ１］で表される構造単位である＜１＞に記載の磁気記録媒体用結合剤。

（一般式［Ａ１］において、Ｒ^aは一般式［Ａ］と同義であり、Ｌ^a1は主鎖の炭素原子と−Ｃ（Ｏ）−基を介して結合する２価の連結基を表し、Ｚ^a1は−Ｚ^bＣ（Ｏ）−Ｘ^a1−、−Ｃ（Ｏ）−Ｚ^bＸ^a1−、−Ｘ^a1Ｃ（Ｏ）−、またはこれらの２種以上の組み合わせを表し、Ｘ^a1は酸素原子、イオウ原子または−Ｎ（Ｒ⁰）−を表し、Ｚ^bは炭素数２以上１０以下の炭化水素基または芳香族基を表し、Ｒ⁰は水素原子または炭素数１以上８以下のアルキル基を表し、Ｙ^a1は水素原子、炭素数２以上１０以下の炭化水素基または芳香族基を表し、ｎ２は２以上５００以下の整数を表す。）
＜３＞一般式［Ａ１］で表される構造単位が一般式［Ａ２］で表される構造単位である＜２＞に記載の磁気記録媒体用結合剤。

（一般式［Ａ２］において、Ｒ^aは一般式［Ａ］と同義であり、Ｙ^a1は一般式［Ａ１］と同義であり、Ｘ^a2は一般式［Ａ１］中のＸ^a1と同義であり、Ｚ^a2は、

またはこれらの２種以上の組み合わせを表し、上記において、ｍは２以上１０以下の整数を表し、Ｘ^a3は酸素原子、イオウ原子または−ＮＨ−を表し、Ａｒはアリーレン基を表す。Ｌ^a2は単結合または２価の連結基を表し、ｎ３は２以上５００以下の整数を表す。）
＜４＞一般式［Ｂ］で表される構造単位を更に含むビニル系共重合体である＜１＞〜＜３＞のいずれかに記載の磁気記録媒体用結合剤。

［一般式［Ｂ］中、Ｒ³は水素原子、ハロゲン原子またはメチル基を表し、Ｌ³は単結合または２価の連結基を表す。］
＜５＞前記ビニル系共重合体は、スルホン酸（塩）基、カルボン酸（塩）基、およびリン酸（塩）基からなる群から選択される少なくとも一種の極性基を含む＜１＞〜＜４＞のいずれかに記載の磁気記録媒体用結合剤。
＜６＞＜１＞〜＜５＞のいずれかに記載の結合剤を含むことを特徴とする磁気記録媒体用組成物。
＜７＞ポリイソシアネートを更に含む＜６＞に記載の磁気記録媒体用組成物。
＜８＞非磁性支持体上に強磁性粉末および結合剤を含む磁性層を有する磁気記録媒体であって、
前記結合剤は、＜１＞〜＜５＞のいずれかに記載の結合剤を構成成分として含むことを特徴とする磁気記録媒体。
＜９＞非磁性支持体上に非磁性粉末および結合剤を含む非磁性層と強磁性粉末および結合剤を含む磁性層とをこの順に有する磁気記録媒体であって、前記磁性層に含まれる結合剤および／または前記非磁性層に含まれる結合剤は、＜１＞〜＜５＞のいずれかに記載の結合剤を構成成分として含むことを特徴とする磁気記録媒体。
＜１０＞前記結合剤は、＜１＞〜＜５＞のいずれか１項に記載の結合剤とポリイソシアネートとの反応生成物を含む＜８＞または＜９＞に記載の磁気記録媒体。
＜１１＞前記強磁性粉末は、平均板径が１０ｎｍ以上５０ｎｍ以下の六方晶フェライト粉末である＜８＞〜＜１０＞のいずれかに記載の磁気記録媒体。
＜１２＞前記強磁性粉末は、平均長軸長が２０ｎｍ以上５０ｎｍ以下の強磁性金属粉末である＜８＞〜＜１０＞のいずれかに記載の磁気記録媒体。 That is, the above object was achieved by the following means.
<1> A binder for a magnetic recording medium, which is a vinyl copolymer containing a structural unit represented by the general formula [A].

(In the general formula [A], R ^a represents a hydrogen atom, a halogen atom or a methyl group, La represents ^a single bond or a divalent linking group, Y ^a represents ^a hydrogen atom, a carbon atom having 1 to 20 carbon atoms. It represents a hydrogen group or an aromatic group, Z ^a represents a divalent group having an ester bond, a thioester bond, or an amide bond, n1 represents an integer of 2 or more and 500 or less.)
<2> The binder for magnetic recording media according to <1>, wherein the structural unit represented by the general formula [A] is a structural unit represented by the general formula [A1].

(In the general formula [A1], R ^a has the same meaning as in the general formula [A], L ^a1 represents a divalent linking group bonded to the carbon atom of the main chain via a —C (O) — group, Z ^a1 represents —Z ^b C (O) —X ^a1 —, —C (O) —Z ^b X ^a1 —, —X ^a1 C (O) —, or a combination of two or more thereof, and X ^a1 represents Represents an oxygen atom, a sulfur atom or —N (R ⁰ ) —, Z ^b represents a hydrocarbon group or an aromatic group having 2 to 10 carbon atoms, and R ⁰ represents a hydrogen atom or an alkyl having 1 to 8 carbon atoms. Y ^a1 represents a hydrogen atom, a hydrocarbon group having 2 to 10 carbon atoms or an aromatic group, and n2 represents an integer of 2 to 500.)
<3> The binder for magnetic recording media according to <2>, wherein the structural unit represented by the general formula [A1] is a structural unit represented by the general formula [A2].

(In the general formula [A2], R ^a have the same meanings as defined in formula [A], Y ^a1 have the same meanings as defined in formula [A1], X ^a2 have the same meanings as defined in formula [A1] X ^a1 in , Z ^a2 is

Or, it represents a combination of two or more of these. In the above, m represents an integer of 2 or more and 10 or less, X ^a3 represents an oxygen atom, a sulfur atom or —NH—, and Ar represents an arylene group. L ^a2 represents a single bond or a divalent linking group, and n3 represents an integer of 2 or more and 500 or less. )
<4> The binder for magnetic recording media according to any one of <1> to <3>, which is a vinyl copolymer further comprising a structural unit represented by the general formula [B].

[In General Formula [B], R ³ represents a hydrogen atom, a halogen atom or a methyl group, and L ³ represents a single bond or a divalent linking group. ]
<5> The vinyl copolymer includes at least one polar group selected from the group consisting of a sulfonic acid (salt) group, a carboxylic acid (salt) group, and a phosphoric acid (salt) group. The binder for magnetic recording media according to any one of <4>.
<6> A magnetic recording medium composition comprising the binder according to any one of <1> to <5>.
<7> The composition for magnetic recording media according to <6>, further comprising a polyisocyanate.
<8> A magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a nonmagnetic support,
The binder includes the binder according to any one of <1> to <5> as a constituent component.
<9> A magnetic recording medium having a nonmagnetic layer containing a nonmagnetic powder and a binder and a magnetic layer containing a ferromagnetic powder and a binder in this order on a nonmagnetic support, the coupling contained in the magnetic layer The magnetic recording medium, wherein the binder and / or the binder contained in the nonmagnetic layer contains the binder according to any one of <1> to <5> as a constituent component.
<10> The magnetic recording medium according to <8> or <9>, wherein the binder includes a reaction product of the binder according to any one of <1> to <5> and a polyisocyanate.
<11> The magnetic recording medium according to any one of <8> to <10>, wherein the ferromagnetic powder is a hexagonal ferrite powder having an average plate diameter of 10 nm to 50 nm.
<12> The magnetic recording medium according to any one of <8> to <10>, wherein the ferromagnetic powder is a ferromagnetic metal powder having an average major axis length of 20 nm to 50 nm.

  ADVANTAGE OF THE INVENTION According to this invention, the magnetic recording medium which exhibits the outstanding electromagnetic conversion characteristic in a high-density recording area | region can be provided.

[Binder for magnetic recording media]
The binder for magnetic recording media of the present invention (hereinafter also referred to as “the binder of the present invention”) is a vinyl copolymer containing a structural unit represented by the general formula [A]. According to the binder of the present invention which is a vinyl copolymer containing the above structural unit, as described above, the fine particle powder can be highly dispersed to improve the surface smoothness of the magnetic layer.
Hereinafter, the binder of the present invention will be described in more detail. Hereinafter, the structural unit represented by the general formula [A] is also referred to as a structural unit [A]. The same applies to structural units represented by other general formulas. Moreover, the structural unit [A] contained in the binder of the present invention may be the same type or two or more different types. The same applies to other structural units. The repeating structure in the general formula may be the same structure or a combination of different structures.

Structural unit [A]
As described above, the binder of the present invention has a graft chain structure in the structural unit [A]. In relation to this, as for the graft copolymer applied to the magnetic recording medium, JP-A-7-9696 discloses a graft copolymer having a perfluoroalkyl group. However, the graft copolymer described in the publication is used as a lubricant and is not required for a copolymer used as a binder.
Details of the general formula [A] will be described below.

In the general formula [A], R ^a represents a hydrogen atom, a halogen atom or a methyl group, and preferably represents a hydrogen atom or a methyl group. As a halogen atom, a chlorine atom, a bromine atom, an iodine atom, etc. can be raised.

La represents ^a single bond or a divalent linking group, preferably a linking group containing any of an oxygen atom, a nitrogen atom or a sulfur atom, and is bonded to a carbon atom of the main chain via a -C (O)-group. A divalent linking group is more preferred.

Y ^a represents a hydrogen atom, a hydrocarbon group having 1 to 20 carbon atoms, or an aromatic group. In the present invention, both the hydrocarbon group and the aromatic group may have a substituent or may be unsubstituted. The hydrocarbon group includes a substituted or unsubstituted linear, branched, or cyclic saturated or unsaturated hydrocarbon group, and the aromatic group is an aryl group (aromatic hydrocarbon group). , Substituted aryl groups, aromatic heterocyclic groups and substituted aromatic heterocyclic groups.
In the present invention, unless otherwise specified, the described group may have a substituent or may be unsubstituted. When a group has 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), and a halogen atom (for example, fluorine Atom, chlorine atom, bromine atom), cyano group, amino group, nitro group, acyl group, carboxyl group and the like. In addition, the “carbon number” of the group having a substituent means the carbon number of a portion not including 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.

Hydrocarbon group represented by Y ^a can contribute to the improvement dispersibility by carbon atoms is 1 or more, it is possible to ensure the solubility by at 20 or less carbon atoms. The number of carbon atoms of the hydrocarbon group is preferably 4 or more and 20 or less from the viewpoint of dispersibility and solubility. Hydrocarbon group represented by Y ^a is more preferably an alkyl group having 8 to 18 carbon atoms, more preferably an alkyl group having 8 to 12 carbon atoms.

  As the aromatic ring contained in the aromatic group, a benzene ring, a furan ring, a thiophene ring, a pyrrole ring, an oxazole ring, a thiazole ring, an imidazole ring, a triazole ring, a pyridine ring, a pyrimidine ring and a pyrazine ring are preferable, and a benzene ring Is particularly preferred. From the viewpoint of improving dispersibility, the aromatic group is preferably an aryl group or a substituted aryl group.

In the general formula [A], Z ^a represents a divalent group having an ester bond, a thioester bond or an amide bond. The binding is intended to be included at least one in Z ^a, the binding of the same or different may be included two or more. The ester bond, thioester bond, and amide bond all have a graft structure (C = O) in the bond, and the binder of the present invention exhibits the effect of improving dispersibility by having a plurality of the graft structures. can do. Repeated several n1 of the Z ^a is 2 or more and 500 or less integer. When it is 2 or more, it contributes to dispersion, and when it is 500 or less, solubility can be ensured. From the viewpoint of achieving both dispersibility and solubility, n1 is preferably an integer of 10 or more and 100 or less.

  As a preferable embodiment of the structural unit represented by the general formula [A], a structural unit represented by the following general formula [A1] can be exemplified.

In the general formula [A1], R ^a has the same meaning as the general formula [A], and details thereof are as described above.

L ^a1 represents a divalent linking group bonded to a carbon atom of the main chain via a —C (O) — group, and a divalent linking group bonded via a —C (O) O— group is preferable.

Z ^{a1 is, -Z b C (O) -X} a1 -, - C (O) -Z b X a1 -, - X a1 C (O) -, or represents a combination of two or more of these, X ^a1 Represents an oxygen atom, a sulfur atom or —N (R ⁰ ) —, Z ^b represents a hydrocarbon group or aromatic group having 2 to 10 carbon atoms, and preferably a hydrocarbon group having 4 to 8 carbon atoms. The hydrocarbon group is preferably a linear alkylene group, and the aromatic group is preferably an arylene group. The arylene group is preferably an arylene group having 6 to 24 carbon atoms, more preferably 6 to 12 carbon atoms, such as phenylene, biphenylene and naphthylene. R ⁰ represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms. The alkyl group may be linear or branched and may have a substituent or may be unsubstituted.

Y ^a1 represents a hydrogen atom, a hydrocarbon group having 2 to 10 carbon atoms or an aromatic group, preferably a hydrogen atom or a hydrocarbon group having 2 to 10 carbon atoms.

  n2 represents an integer of 2 or more and 500 or less. The preferred range is the same as n1 above.

  As a preferable embodiment of the structural unit represented by the general formula [A1], a structural unit represented by the following general formula [A2] can be exemplified.

In the general formula [A2], R ^a have the same meanings as defined in formula [A], Y ^a1 have the same meanings as defined in formula [A1], X ^a2 have the same meanings as defined in formula [A1] X ^a1 in, Details of both are as described above.

またはこれらの２種以上の組み合わせを表す。上記において、ｍは２以上１０以下の整数を表し、Ｘ^a3は酸素原子、イオウ原子または−ＮＨ−を表し、Ａｒはアリーレン基を表す。アリーレン基としては、好ましくは炭素原子数６〜２４、より好ましくは６〜１２のもの、例えば、フェニレン、ビフェニレン、ナフチレンを挙げることができる。 Z ^a2 is

Or the combination of these 2 or more types is represented. In the above, m represents an integer of 2 to 10, X ^a3 represents an oxygen atom, a sulfur atom or —NH—, and Ar represents an arylene group. The arylene group is preferably an arylene group having 6 to 24 carbon atoms, more preferably 6 to 12 carbon atoms, such as phenylene, biphenylene and naphthylene.

L ^a2 represents a single bond or a divalent linking group, and a linking group having a urethane bond is preferable from the viewpoint of improving dispersibility. n3 represents an integer of 2 or more and 500 or less. The preferred range is the same as n1 above.

  The structural unit [A] described above can be derived from a vinyl monomer (graft chain monomer) represented by the following general formula [A ′], and the structural unit [A1] is represented by the following general formula [A1 ′]. The structural unit [A2] can be derived from a vinyl monomer represented by the following general formula [A3 ′].

［一般式［Ａ’］中のＲ^a、Ｌ^a、Ｚ^a、ｎ１、Ｙ^aはそれぞれ一般式［Ａ］と同義である。］

[R ^a in the general formula ^{[A '], L a,} Z a, n1, Y a is the same meaning as in the general formula [A]. ]

［一般式［Ａ１’］中のＲ^a、Ｌ^a1、Ｚ^a1、ｎ２、Ｙ^a1はそれぞれ一般式［Ａ１］と同義である。］

[In the general formula [A1 ′], R ^a , L ^a1 , Z ^a1 , n2 and Y ^a1 have the same ^{meanings as those} in the general formula [A1]. ]

［一般式［Ａ２’］中のＲ^a、Ｘ^a2、Ｌ^a2、Ｚ^a2、ｎ３、Ｙ^a1はそれぞれ一般式［Ａ２］と同義である。］

[In the general formula [A2 ′], R ^a , X ^a2 , L ^a2 , Z ^a2 , n3 and Y ^a1 have the same meanings as those in the general formula [A2]. ]

  Specific examples of the graft chain monomer represented by the general formula [A ′] (further, the general formula [A1 ′] and the general formula [A2 ′]) are shown below. However, the present invention is not limited to the following specific examples.

Each monomer for obtaining the binder of the present invention containing the above graft chain monomer can be synthesized by a known method, and is also available as a commercial product. For example, the vinyl monomer (graft chain monomer) represented by the general formula [A ′] for introducing the structural unit [A] is described in, for example, “Macromonomer Chemistry and Industry” (IPC Publishing Department), page 57. According to a known method such as a method, it can be produced simply and with good yield.

Structural unit [B]
The binder of the present invention can be composed of only the above structural unit [A], but can also contain other structural units as necessary in order to obtain more desirable physical properties as a binder for magnetic recording media. Examples of such a coexisting structural unit include a structural unit represented by the following general formula [B]. By allowing the structural unit to coexist, it is possible to further improve dispersibility and further improve running durability by improving coating film strength. This is considered to be mainly due to the fact that the hydroxyl group contained in the general formula [B] contributes to the dispersion, and the coating strength contributes to the improvement by crosslinking with the polyisocyanate. For the same reason, those containing the structural unit [A] having a hydroxyl group in the side chain are also effective in improving running durability by improving the coating film strength.
Hereinafter, the structural unit [B] that can coexist will be described in more detail.

In the general formula [B], R ³ represents a hydrogen atom, a halogen atom or a methyl group. The details thereof are as set forth for R ^a in the previous general formula [A].

In the general formula [B], L ³ 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 ³ , those bonded to the main chain carbon atom via a —C (O) — group are preferred, and —C (O) in the general formula [B1] described below. A divalent linking group represented by X ³ R ³² — is more preferred.

  From the viewpoint of improving dispersibility, the structural unit [B] is preferably a structural unit represented by the following general formula [B1] (structural unit [B1]).

In the general formula [B1], R ³¹ represents a hydrogen atom or a methyl group, and preferably represents a methyl group.

X ³ represents a divalent linking group represented by —O—, —S—, or —N (R ³³ ) —, and is preferably —O—.

R ³³ represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms which may have a substituent. The carbon number of the alkyl group represented by R ³³ is preferably 1 or more and 4 or less. When the carbon number of the alkyl group represented by R ³³ is in the above range, dispersibility can be further improved while maintaining solubility.

R ³² 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 ³² is within the above range, the dispersibility can be further improved while maintaining the 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 ³² is preferably 1 to 3 inclusive.

  The structural unit [B] described above can be derived from a vinyl monomer represented by the following general formula [B ′], and the structural unit [B1] is derived from an acrylic monomer represented by the following general formula [B1 ′]. can do.

［一般式［Ｂ’］中のＲ³、Ｌ³はそれぞれ一般式［Ｂ］と同義である。］

[R ³ and L ³ in General Formula [B ′] have the same meanings as those in General Formula [B]. ]

［一般式［Ｂ１’］中のＲ³¹、Ｒ³²、Ｘ³はそれぞれ一般式［Ｂ１］と同義である。］

[R ³¹ , R ³² and X ³ in the general formula [B1 ′] have the same meanings as those in the general formula [B1]. ]

Specific examples of the monomer represented by the general formula [B ′] or the general formula [B1 ′] 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.

  The binder of the present invention can form a crosslinked structure with a crosslinking agent such as polyisocyanate used as a curing agent in a magnetic recording medium by the hydroxyl group contained in the structural unit [B]. Thereby, a magnetic recording medium having high coating strength can be obtained. Details of the curing agent such as polyisocyanate used in combination will be described later.

From the viewpoint of realizing excellent dispersibility, the vinyl-based copolymer that is the binder of the present invention is based on the structural unit [A] (preferably the structural unit [A1] based on the total polymerization units constituting the copolymer. More preferably, the unit structure [A2]) is contained in an amount of 0.1 mol% to 75 mol%, more preferably 1 mol% to 50 mol%, and more preferably 1 mol% to 30 mol%. More preferably.
Further, from the viewpoint of achieving both excellent dispersibility and high coating film strength, the structural unit [B] is preferably contained in an amount of 0.1 mol% or more and 50 mol% or less, based on the total polymerization units, and preferably 1 mol% or more. It is more preferably 30 mol% or less, and even more preferably 1 mol% or more and 10 mol% or less. Therefore, the blending ratio of each monomer during the polymerization reaction is preferably set so as to obtain a copolymer having the above preferred composition.

The binder of the present invention preferably has a functional group (polar group) that is adsorbed on the surface of the magnetic powder or nonmagnetic powder in the coating type magnetic recording medium. Thereby, the dispersibility of the powder in a coating material can be improved. Examples of the polar group preferably introduced into the binder of the present invention include a sulfonic acid (salt) group, a carboxylic acid (salt) group, and a phosphoric acid (salt) group. In the present invention, the sulfonic acid (salt) group includes sulfonic acid groups (—SO ₃ H), SO ₃ Na groups, SO ₃ K groups, SO ₃ Li and other sulfonate groups, and salts thereof. . The same applies to carboxylic acid (salt) groups and phosphoric acid (salt) groups.

  The polymerization reaction is carried out using the monomer having the polar group, or after the polymerization reaction, the obtained copolymer is subjected to an addition reaction with the monomer having the polar group, whereby the binder of the present invention has the above polarity. Groups can be introduced.

  As the monomer for introducing the sulfonic acid (salt) group, the following compounds can be used.

［上記において、Ｍは水素原子、アルカリ金属原子またはアンモニウム塩を表し、Ｒはアルキル基を表す。］

[In the above, M represents a hydrogen atom, an alkali metal atom or an ammonium salt, and R represents an alkyl group. ]

  Monomers for introducing carboxylic acid (salt) groups include unsaturated carboxylic acids such as (meth) acrylic acid, maleic acid and itaconic acid, unsaturated carboxylic acid anhydrides such as maleic anhydride and itaconic anhydride, and these And the like.

  Examples of monomers having a phosphoric acid (salt) group include mono [(meth) acryloyloxyethyl] acid phosphates and mono [(meth) acryloyloxypropyl] acid phosphates. , Mono [(meth) acryloyloxybutyl] acid phosphate, mono [(meth) acryloyloxyethoxynoethyl] acid phosphate, mono [(meth) acryloyloxypolyoxyethylene glycol] acid phosphate, (meth) acryloyloxyethyl methyl acid Phosphate, (meth) acryloyloxyethyl butyl acid phosphate, (meth) acryloyloxypropyl ethyl acid phosphate, (meth) acryloyloxyethyl methyl acid phosphate Eto, (meth) acryloyloxy polyoxyethylene glycol butyl acid phosphate, and vinyl acid phosphate and the like alkali metal salts thereof are. The (meth) acrylic acid includes methacrylic acid and acrylic acid, and the (meth) acryloyl includes methacryloyl and acryloyl. The following (meth) acrylates include methacrylate and acrylate.

  In order to further improve the dispersibility, the amount of the polar group introduced into the binder of the present invention is preferably 10 to 1000 μeq / g. For this purpose, the monomer having the polar group is preferably used in an amount of 0.1 to 10 mol%, preferably 0.5 to 6 mol%, based on the total amount of raw material monomers excluding the monomer. .

Examples of the structural unit that can coexist in order to further improve the dispersibility include a structural unit represented by the following general formula [1] and a structural unit represented by the following general formula [2]. Mainly, the alicyclic cyclic group contained in the general formula [1] and the long chain or polycyclic hydrocarbon group contained in the general formula [2] are considered to contribute to further improvement in dispersibility. .
Hereinafter, general formula [1] and general formula [2] will be described.

Structural unit [1]

In the general formula [1], R ¹ 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 ¹ preferably represents a hydrogen atom or a methyl group, more preferably a methyl group.

In general formula [1], L ¹ represents a single bond or a divalent linking group. The divalent linking group represented by L ¹ preferably contains a hetero atom, and preferably binds to the alicyclic cyclic group represented by Y by the hetero atom. Examples of the hetero atom include an oxygen atom, a nitrogen atom, and a sulfur atom. L ¹ 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) X ^{1 in} the general formula [4] described later. A divalent linking group represented by-is more preferred.

  In general formula [1], Y represents an alicyclic cyclic group. 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 monocyclic alicyclic group represented by Y is preferably a 5- to 6-membered group, and specific examples include a cyclohexyl group and a cyclopentyl group, and a polycyclic alicyclic ring. The formula 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, Y is preferably an alicyclic fused ring group. As the alicyclic condensed ring, an alicyclic condensed ring represented by Y ¹ in the general formula [4] described later is preferable.

  From the viewpoint of improving dispersibility, the structural unit [1] is preferably a structural unit (structural unit [4]) represented by the following general formula [4].

In the general formula [4], R ¹¹ represents a hydrogen atom or a methyl group, and preferably represents a methyl group.

X ¹ represents a divalent linking group represented by —O—, —S—, or —N (R ¹² ) —, and is preferably —O—.

R ¹² represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms which may have a substituent. The carbon number of the alkyl group represented by R ¹² is preferably 1 or more and 4 or less. When the carbon number of the alkyl group represented by R ¹² is in the above range, dispersibility can be further improved while maintaining solubility.

Y ¹ represents an alicyclic fused ring group. The alicyclic fused ring group represented by Y ¹ 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 [1] described above can be derived from a vinyl monomer represented by the following general formula [1 ′], and the structural unit [4] is derived from an acrylic monomer represented by the following general formula [4 ′]. can do.

[一般式[１’]中のＲ¹、Ｌ¹、Ｙはそれぞれ一般式[１]と同義である。]

[R ¹ , L ¹ and Y in the general formula [1 ′] have the same meanings as in the general formula [1]. ]

[一般式[４’]中のＲ¹¹、Ｘ¹、Ｙ¹はそれぞれ一般式[４]と同義である。]

[R ¹¹ , X ¹ and Y ¹ in the general formula [4 ′] have the same meanings as those in the general formula [4]. ]

  Specific examples of the monomer represented by the general formula [1 ′] or the general formula [4 ′] are shown below. However, the present invention is not limited to the following specific examples.

Structural unit [2]

In the general formula [2], R ² represents a hydrogen atom, a halogen atom or a methyl group. The details are as described for R ¹ in the general formula [1].

In the general formula [2], 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. L ² 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) X ^{2 in} the general formula [5] described later. A divalent linking group represented by-is more preferred.

  In the general formula [2], Z 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 Z is more preferably an alkyl group having 12 to 30 carbon atoms, and further preferably an alkyl group having 12 to 18 carbon atoms.

  From the viewpoint of improving dispersibility, the structural unit [2] is preferably a structural unit (structural unit [5]) represented by the following general formula [5].

In the general formula [5], R ²¹ 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 ² represents a divalent linking group represented by — (O) m ¹ —, — (S) m ² — or — {N (R ²² )} 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 ²² 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 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.
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 [2] described above can be derived from a vinyl monomer represented by the following general formula [2 ′], and the structural unit [5] is derived from an acrylic monomer represented by the following general formula [5 ′]. can do.

[一般式[２’]中のＲ²、Ｌ²、Ｚはそれぞれ一般式[２]と同義である。]

[R ² , L ² and Z in the general formula [2 ′] have the same meanings as the general formula [2]. ]

[一般式[５’]中のＲ²¹、Ｘ²、ｎはそれぞれ一般式[５]と同義である。]

[R ²¹ , X ² and n in the general formula [5 ′] are as defined in the general formula [5]. ]

  Specific examples of the monomer represented by the general formula [2 ′] or the general formula [5 ′] 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 strength, the vinyl-based copolymer that is the binder of the present invention has a structural unit [1] (preferably based on the total polymerized units constituting the copolymer). Preferably contains 5 mol% or more and 75 mol% or less, more preferably 15 mol% or more and 60 mol% or less, and still more preferably 30 mol% or more and 50 mol% or less.
Similarly, from the viewpoint of achieving both excellent dispersibility and high coating strength, the structural unit [2] (preferably the structural unit [5]) is contained in an amount of 5 mol% or more and 75 mol% or less based on the total polymerization units. Preferably, it is contained in an amount of 5 mol% or more and 50 mol% or less, more preferably 10 mol% or more and 30 mol% or less, and more preferably 5 mol% or more of structural unit [3] (preferably structural unit [6]). The content is preferably 80 mol% or less, more preferably 15 mol% or more and 70 mol% or less, and still more preferably 30 mol% or more and 60 mol% or less. Therefore, the blending ratio of each monomer during the polymerization reaction is preferably set so as to obtain a copolymer having the above preferred composition.

Copolymerizable monomer As a monomer copolymerizable with the vinyl monomer represented by the general formula [A ′] and further the vinyl monomer represented by the general formula [B] ′ in order to obtain the binder of the present invention. , Ethylenically unsaturated carboxylic acid ester monomers, aromatic vinyl monomers, ethylenically unsaturated nitrile monomers, ethylenically unsaturated acid monomers, alkyl vinyl ether monomers, vinyl ester monomers, ethylenically unsaturated polyvalent carboxylic acids An acid anhydride etc. are mentioned.

  Examples of the ethylenically unsaturated carboxylic acid ester monomer include alkyl (meth) acrylate monomers and alkoxyalkyl (meth) acrylate monomers.

  Alkyl (meth) acrylate monomers include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate, pentyl (meth) acrylate, hexyl (meth) Examples include acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate, and cyclohexyl (meth) acrylate. Among these, methyl (meth) acrylate is preferable.

  The alkyl group of the alkyl (meth) acrylate monomer may be an alkyl group having a substituent, and is preferably an aralkyl group. Specific examples of such compounds include benzyl (meth) acrylate, phenoxyethyl (meth) acrylate, phenoxypolyethylene glycol (meth) acrylate, and nonylphenol ethylene oxide adduct (meth) acrylate. Among these, benzyl (meth) acrylate and phenoxyethyl (meth) acrylate are preferable.

  Examples of the alkoxyalkyl (meth) acrylate monomer include methoxyethyl (meth) acrylate and butoxyethyl (meth) acrylate. Examples of the ethylenically unsaturated carboxylic acid ester monomer include glycidyl (meth) acrylate.

  Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, vinyltoluene, monochlorostyrene, p-methylstyrene, and hydroxymethylstyrene.

  Examples of the ethylenically unsaturated nitrile monomer include acrylonitrile, methacrylonitrile, 2-ethylpropenenitrile, 2-propylpropenenitrile, 2-chloropropenenitrile, 2-butenenitrile and the like.

  Examples of the alkyl vinyl ether monomers include allyl glycidyl ether, methyl vinyl ether, ethyl vinyl ether, isobutyl vinyl ether, n-butyl vinyl ether, 2-ethylhexyl vinyl ether, n-octyl vinyl ether, lauryl vinyl ether, cetyl vinyl ether, stearyl vinyl ether and the like.

  Examples of vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, isopropenyl acetate, vinyl valelate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl versatate, and vinyl pivalate. Can be mentioned.

  Examples of the ethylenically unsaturated polyvalent carboxylic acid anhydride include maleic anhydride and itaconic anhydride.

The said copolymerizable monomer may be used individually by 1 type, and can also use 2 or more types together.
Among these, an ethylenically unsaturated carboxylic acid monomer is preferable, and an alkyl (meth) acrylate monomer is more preferable.

  In order to polymerize the polymerization reaction system optionally containing the above copolymerizable compounds in order to obtain the binder of the present invention, it is preferable to use a known polymerization method such as solution polymerization.

  As a solvent used for solution polymerization, a water-miscible polar solvent is preferably used from the viewpoint of reactivity. In the present invention, the “water-miscible polar solvent” refers to a solvent that dissolves 5% by mass or more of water at 20 ° C. Specific examples of such a solvent include dimethylformamide (DMF), dimethylacetamide (DMAC), and N-methylpyrrolidone (NMP). 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, etc., but generally in an autoclave, the temperature is about 50 to 80 ° C., and the gauge pressure is about 4.0 to 1.0 MPa. The time is preferably about 5 to 30 hours. Polymerization is preferably performed in an atmosphere of a gas inert to the reaction from the viewpoint of ease of reaction control. Examples of such a gas include nitrogen and argon, and preferably nitrogen is used from the viewpoint of cost. In the polymerization, other components generally added to the polymerization reaction may be added to the polymerization reaction system in addition to the components described above.

  The weight-average molecular weight of the vinyl copolymer that is the binder of the present invention is preferably 1,000 or more from the viewpoint of obtaining a high-strength coating film, and a paint at a predetermined concentration in order to maintain good workability From the viewpoint of adjusting the viscosity to an appropriate range, it is preferably 1,000,000 or less. From the above viewpoint, the weight average molecular weight of the vinyl copolymer that is the binder of the present invention is more preferably 10,000 to 500,000. The average molecular weight in the present invention refers to a value determined in terms of standard polystyrene. The molecular weight of the binder of the present invention can be controlled by the raw material composition, reaction conditions, and the like.

  Specific examples of the binder of the present invention are shown below. However, the present invention is not limited to the following specific examples. Below, Mw represents a mass average molecular weight, and the numerical value attached | subjected to the right side of each structural unit represents the molar ratio of each structural unit with respect to all the polymer units in a copolymer.

[Composition for magnetic recording medium]
The composition for a magnetic recording medium of the present invention contains the binder of the present invention, preferably a composition for forming a coating type magnetic recording medium, specifically, a powder-containing layer of a coating type magnetic recording medium. For example, a binder resin composition used for preparing a coating solution for forming a magnetic layer containing a ferromagnetic powder and a nonmagnetic layer containing a nonmagnetic powder. According to the binder of the present invention, a powder-containing layer having high surface smoothness can be formed by highly dispersing various powders. Therefore, according to the composition for a magnetic recording medium of the present invention containing the binder of the present invention, a magnetic recording medium capable of exhibiting excellent electromagnetic conversion characteristics by having high surface smoothness can be formed.

  As described above, a magnetic recording medium having high coating strength can be obtained by using the binder of the present invention and a polyisocyanate in combination. Therefore, the composition for magnetic recording media of the present invention preferably contains polyisocyanate. Examples of polyisocyanates include tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, triphenylmethane triisocyanate and the like. Moreover, the product of these isocyanates and a polyalcohol, and polyisocyanate more than bifunctional, such as the polyisocyanate produced | generated by condensation of isocyanate, can be used. Commercially available product names of these isocyanates include Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR, Millionate MTL, Takeda Pharmaceutical Taketake D-102, Takenate D-110N, There are Takenate D-200, Takenate D-202, Sumitomo Bayer's Death Module L, Death Module IL, Death Module N, Death Module HL, etc., either alone or by utilizing the difference in curing reactivity. Each layer can be used in the above combination.

  From the viewpoint of improving the coating film strength, it is preferable to use a triisocyanate or higher polyisocyanate as the polyisocyanate. 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 may be, for example, 0 to 80 parts by mass with respect to 100 parts by mass of the binder of the present invention, and is preferably 50 to 80 parts by mass from the viewpoint of improving the coating film strength.

The composition for magnetic recording media of the present invention can contain various additives that are usually used in coating solutions for forming magnetic recording media, together with the binder and polyisocyanate of the present invention. Moreover, the composition for magnetic recording media of the present invention contains magnetic powder, non-magnetic powder or the like, or can be used by mixing with these powders.
Details of each component described above are as described later for the magnetic recording medium of the present invention.

[Magnetic recording medium]
The magnetic recording medium of the present invention includes the following embodiments 1 and 2.
Aspect 1: A magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a non-magnetic support, wherein the binder contains the binder of the present invention as a constituent component.
Aspect 2: A magnetic recording medium having a nonmagnetic layer containing a nonmagnetic powder and a binder and a magnetic layer containing a ferromagnetic powder and a binder in this order on a nonmagnetic support,
A magnetic recording medium in which the binder contained in the magnetic layer and / or the binder contained in the nonmagnetic layer contains the binder of the present invention as a constituent component.

  The magnetic recording medium of the present invention contains the binder of the present invention as a constituent component of the binder in the magnetic layer and / or the nonmagnetic layer. The phrase “containing the binder of the present invention as a constituent component of the binder” means that the binder of the present invention itself or a reaction product of the binder of the present invention and other binder components is contained in the binder. That means. As described above, the reaction product preferably includes a reaction product of the binder (vinyl copolymer) of the present invention and a polyisocyanate. By including such a reaction product, the coating strength can be further increased.

  The magnetic recording medium of the present invention comprises a known thermoplastic resin, thermosetting resin, reactive resin and a mixture thereof together with the binder of the present invention as a constituent component of the binder of the magnetic layer and / or the nonmagnetic layer. Although it can be included, it is possible to achieve high running durability that can withstand repeated running without using a resin component other than the binder (vinyl copolymer) of the present invention such as polyurethane resin.

  Examples of the resin component other than the binder of the present invention include a thermoplastic resin having a glass transition temperature of −100 to 150 ° C. and a number average molecular weight of 1,000 to 200,000, preferably 10,000 to 100,000. 000 can be mentioned. Specific examples include copolymers containing polyurethane resin, vinyl butyral, vinyl acetal, vinyl ether and the like as structural units, and various rubber resins. Thermosetting resins or reactive resins include phenol resins, phenoxy resins, epoxy resins, urea resins, melamine resins, alkyd resins, formaldehyde resins, silicone resins, epoxy-polyamide resins, polyester resins and isocyanate prepolymers. Etc. In the magnetic recording medium of the present invention, in the layer not containing the binder of the present invention, the above-mentioned known resins can be arbitrarily combined with polyisocyanate and used as a binder. The layer containing the binder of the present invention together with another resin and a polyisocyanate can contain a reaction product obtained by crosslinking the binder of the present invention and another resin with a polyisocyanate as a constituent component of the binder.

  When using the binder of this invention 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 binders of this invention, 10-100 It is more preferable to use a mass part. However, as described above, according to the binder of the present invention, a magnetic recording medium having a high coating strength can be obtained even when other resin components are not included.

  The binder of the present invention is preferably used in a range of 5 to 50 parts by mass, more preferably in a range of 7 to 45 parts by mass with respect to 100 parts by mass of powders such as ferromagnetic powder and nonmagnetic powder. preferable. The dispersibility of the powder can be enhanced by using the binder of the present invention in an amount within the above range with respect to various powders. The good dispersion state of the powder in the magnetic recording medium can be confirmed by increasing the surface smoothness of the magnetic recording medium. Moreover, it can be confirmed that the dispersion state of the ferromagnetic powder or the nonmagnetic powder is good by the phenomenon that the glossiness of the surface of the magnetic layer or the nonmagnetic layer is increased. Furthermore, electromagnetic conversion characteristics can be remarkably improved by using the binder of the present invention within a range of 10 to 40 parts by mass with respect to 100 parts by mass of the powder. If the content of the binder of the present invention is 5 parts by mass or more per 100 parts by mass of the powder, it is preferable that the ferromagnetic powder or the non-magnetic powder is not bonded and no powder falls off. As the amount of the binder in the magnetic layer increases, the filling degree of the ferromagnetic powder decreases and the electromagnetic conversion characteristics also deteriorate. However, in the magnetic layer, the content of the binder of the present invention with respect to 100 parts by mass of the ferromagnetic powder is 50. If the amount is less than or equal to part by mass, the decrease in the filling degree of the ferromagnetic powder in the magnetic layer is preferably small.

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

(Magnetic layer)
In the magnetic recording medium of the present invention, examples of the ferromagnetic powder contained in the magnetic layer include hexagonal ferrite powder. As for the size of the hexagonal ferrite powder, the average plate diameter is preferably 50 nm or less, particularly when reproducing with a magnetoresistive head in order to increase the track density. Therefore, stable magnetization cannot be expected. If it is larger than 200 nm, the noise is high and none is suitable for high-density magnetic recording. Therefore, the average plate diameter of the hexagonal ferrite powder is preferably 10 nm or more and 200 nm or less, and more preferably 10 nm or more and 50 nm or less. According to the binder of the present invention, finely divided hexagonal ferrite powder having an average plate diameter in the above range can be highly dispersed. The plate ratio (plate diameter / plate thickness) of the hexagonal ferrite powder is preferably 1-15. Preferably it is 1-7. When the plate ratio is small, the filling property in the magnetic layer is preferably high, but it is difficult to obtain sufficient orientation. When it is larger than 15, noise increases due to stacking between particles. The specific surface area according to the BET method in this particle size range is 10 to ²⁰⁰ m ² / g. The specific surface area roughly agrees with the arithmetic calculation value from the particle plate diameter and plate thickness. The distribution of the particle plate diameter and plate thickness is generally preferably as narrow as possible. Although it is difficult to digitize, it can be compared by randomly measuring 500 particles from a particle TEM photograph. In many cases, the distribution is not a normal distribution, but when calculated and expressed as a standard deviation with respect to the average size, for example, σ / average size = 0.1 to 2.0. In order to sharpen the particle size distribution, the particle generation reaction system is made as uniform as possible, and the generated particles are subjected to a distribution improvement process. For example, a method of selectively dissolving ultrafine particles in an acid solution is also known.

In general, hexagonal ferrite powder having a coercive force Hc of about 500 to 5000 oersted (≈40 to 398 kA / m) can be produced. A higher Hc is advantageous for high-density recording, but is limited by the capacity of the recording head. The Hc of the hexagonal ferrite used in the present invention is preferably about 2000 to 4000 Oe (≈160 to 320 kA / m), and more preferably 2200 to 3500 Oe (≈176 to 280 kA / m). When the saturation magnetization of the head exceeds 1.4 Tesla, it is preferable to be 2200 Oe (≈176 kA / m) or more. Hc can be controlled by the particle size (plate diameter / plate thickness), the type and amount of contained elements, the substitution site of the elements, the particle generation reaction conditions, and the like. The saturation magnetization σs is preferably 40 to 80 A · m ² / kg. σs is preferably higher, but tends to be smaller as the particles become finer. In order to improve σs, it is well known to combine spinel ferrite with magnetoplumbite ferrite and to select the type and amount of elements contained. It is also possible to use W-type hexagonal ferrite.

  JP, 2004-295926, A paragraphs [0042] and [0043] can also be referred to for the details of the hexagonal ferrite powder explained above.

  Examples of the ferromagnetic powder contained in the magnetic layer of the present invention include a ferromagnetic metal powder. With respect to the size of the ferromagnetic metal powder, the average major axis length is preferably 10 nm or more and 100 nm or less, and more preferably 20 nm or more and 50 nm or less, from the viewpoints of magnetization stability and noise reduction. According to the binder of the present invention, fine particulate ferromagnetic metal powder having an average major axis length in the above range can be highly dispersed.

The specific surface area (S _BET ) of the ferromagnetic metal powder by the BET method is preferably 30 m ² / g or more and less than 80 m ² / g, preferably 40 m ² / g or more, from the viewpoint of achieving both good surface properties and low noise. More preferably, it is 70 m ² / g.

The coercive force Hc of the ferromagnetic metal powder is preferably 1,500 to 7,000 Oe (≈119 to 557 kA / m), more preferably 2,000 to 6,000 Oe (≈159 to 478 kA / m), σs is preferably 80 to 170 emu / g (≈80 to 170 A · m ² / kg), more preferably 90 to 160 emu / g (≈90 to 160 A · m ² / kg).

The pH of the ferromagnetic metal powder is preferably optimized depending on the combination with the binder used. The range is preferably 4-12, more preferably 7-10. The ferromagnetic powder may be surface-treated with Al, Si, P, or an oxide thereof as required. The amount is preferably 0.1 to 10% by mass with respect to the ferromagnetic powder. When the surface treatment is applied, the adsorption of a lubricant such as a fatty acid is preferably 100 mg / m ² or less. The ferromagnetic metal powder may contain soluble inorganic ions such as Na, Ca, Fe, Ni, and Sr. However, if it is 200 ppm or less, the property is not particularly affected. Further, the ferromagnetic metal powder used in the present invention preferably has fewer vacancies, and the value is preferably 20% by volume or less, more preferably 5% by volume or less. In addition, the shape may be any of acicular, granular, rice granular, or plate-like shape as long as the above-described characteristics regarding the particle size are satisfied, but it is particularly preferable to use acicular ferromagnetic powder. In the case of acicular ferromagnetic metal powder, the acicular ratio is preferably 4 or more and 12 or less, more preferably 5 or more and 12 or less.

  JP, 2004-295926, A paragraphs [0033]-[0035] can be referred to for details of other ferromagnetic metal powders.

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 columnar (however, the height is the bottom) 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. (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 an 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).

(Nonmagnetic layer)
The magnetic recording medium according to aspect 2 has a nonmagnetic layer containing a nonmagnetic powder and a binder between the nonmagnetic support and the magnetic layer, and the binder of the magnetic layer and / or the nonmagnetic layer is the present one. Inventive binders are included as constituents. The binder of the present invention can contribute to the improvement of the coating strength and the dispersibility of the nonmagnetic powder in the nonmagnetic layer as well as the magnetic layer.

The nonmagnetic powder used for the nonmagnetic layer 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, metal sulfides, and the like. 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 can be used alone or in combination of two or more. Preferred nonmagnetic powders are α-iron oxide and titanium oxide.

The nonmagnetic powder may be acicular, spherical, polyhedral, or plate-shaped. The average particle size of these nonmagnetic powders is preferably 0.005 to 2 μm, and more preferably 0.01 to 0.2 μm. 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 crystallite size of the nonmagnetic powder is preferably 0.004 μm to 1 μm, and more preferably 0.04 μm to 0.1 μm. Preferably the specific surface area of the nonmagnetic powder is 1 to 100 m ² / g, more preferably 5 to 70 m ² / g, more preferably from 10~65m ² / g. According to the binder of the present invention, it is possible to highly disperse the fine non-magnetic powder having the above size.

  The oil absorption using non-magnetic powder dibutyl phthalate (DBP) is preferably 5 to 100 ml / 100 g, more preferably 10 to 80 ml / 100 g, and still more preferably 20 to 60 ml / 100 g. The specific gravity is preferably 1 to 12, more preferably 3 to 6. The tap density is preferably 0.05 to 2 g / ml, more preferably 0.2 to 1.5 g / ml. If it is less than 0.05 g / ml, there are many particles that scatter, and the operation tends to be difficult. When it is larger than 2 g / ml, it tends to be fixed to the apparatus and the operation tends to be difficult. The nonmagnetic powder preferably has a pH of 2 to 11, more preferably 6 to 9. If the pH is within the above range, an increase in the friction coefficient can be suppressed.

It is preferable that Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , and ZnO exist on the surface of these nonmagnetic powders by surface treatment. Particularly preferred for dispersibility are Al ₂ O ₃ , SiO ₂ , TiO ₂ and ZrO ₂ , and more preferred are Al ₂ O ₃ , SiO ₂ and ZrO ₂ . These may be used in combination or may be used alone. Further, a surface-treated layer co-precipitated according to the purpose may be used, or a method of treating the surface layer with silica after first making alumina exist, or vice versa can be employed. The surface treatment layer may be a porous layer depending on the purpose, but it is generally preferable that the surface treatment layer is homogeneous and dense.

  For details of other nonmagnetic powders, reference can be made to paragraphs [0047] to [0048] and [0050] of JP-A No. 2004-295926.

(Additive)
Additives can be added to the magnetic layer and the nonmagnetic layer as necessary. As the additive, a compound for imparting a dispersing effect, a lubricating effect, an antistatic effect, a plasticizing effect, and the like can be used. JP, 2004-295926, A paragraphs [0055]-[0060] can be referred to for details of a compound which can be used as an additive.

The magnetic recording medium of the present invention can contain carbon black in the magnetic layer and / or the nonmagnetic layer. Examples of usable carbon black include furnace for rubber, thermal for rubber, black for color, acetylene black and the like. The specific surface area is preferably ⁵ to 500 m ² / g, the DBP oil absorption is 10 to 400 ml / 100 g, and the average particle size is 5 to 300 nm, preferably 10 to 250 nm, more preferably 20 to 200 nm. The pH 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 / cc. When carbon black is used, it is preferably used in an amount of 0.1 to 30% by mass based on the ferromagnetic powder or nonmagnetic powder. Regarding the carbon black that can be used in the present invention, for example, “Carbon Black Handbook” (edited by the Carbon Black Association) can be referred to.

(Organic solvent)
Organic solvents include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone and tetrahydrofuran, alcohols such as methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol and methylcyclohexanol, acetic acid Esters such as methyl, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate and glycol acetate, glycol ethers such as glycol dimethyl ether, glycol monoethyl ether and dioxane, aromatics such as benzene, toluene, xylene, cresol and chlorobenzene Hydrocarbons, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin, dic Chlorinated hydrocarbons such as Rubenzen, N, N- dimethylformamide, a known organic solvent such as hexane may be used in any ratio.

  All or part of the additives used in the present invention may be added in any step during the production of the coating solution 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.

(Non-magnetic support)
As the nonmagnetic support, known films such as biaxially stretched polyethylene naphthalate, polyethylene terephthalate, polyamide, polyimide, polyamideimide, aromatic polyamide, and polybenzoxidazole can be used. Polyethylene naphthalate and aromatic polyamide are preferred. These nonmagnetic supports may be subjected in advance to corona discharge, plasma treatment, easy adhesion treatment, heat treatment and the like.

  The nonmagnetic support that can be used in the present invention has an excellent smoothness with a centerline average surface roughness of 0.1 to 20 nm, preferably 1 to 10 nm, at a cutoff value of 0.25 mm. It is preferable. These non-magnetic supports preferably have not only a small center line average surface roughness but also no coarse protrusions of 1 μm or more. The arithmetic average roughness of the obtained support is preferably 0.1 μm or less as Ra defined in JIS B0660-1998 and ISO 4287-1997 for noise reduction.

(Layer structure)
In the magnetic recording medium of the present invention, the thickness of the nonmagnetic support is, for example, 2 to 100 μm, preferably 3 to 80 μm. An undercoat layer for improving adhesion or smoothing for improving smoothness may be provided between the nonmagnetic support and the nonmagnetic layer or magnetic layer. The thickness of the undercoat layer and the smoothing layer is, for example, 0.01 to 0.5 μm, preferably 0.02 to 0.5 μm. The magnetic recording medium of the present invention may be a disk-shaped medium in which a nonmagnetic layer and a magnetic layer are provided on both sides of a support, or a tape-shaped medium or a disk-shaped medium provided only on one side. In this case, a backcoat layer may be provided on the side opposite to the nonmagnetic layer and the magnetic layer in order to obtain effects such as antistatic and curl correction. This thickness is, for example, 0.1 to 4 μm, preferably 0.3 to 2.0 μm. As these undercoat layer, smoothing layer and backcoat layer, known ones can be used, and details can be referred to paragraphs [0064] to [0066] of JP-A No. 2004-295926. The magnetic recording medium of the present invention can also contain the binder of the present invention in these layers.

  The thickness of the nonmagnetic layer is usually 0.2 to 5.0 μm, preferably 0.3 to 3.0 μm, more preferably 0.4 to 2.0 μm.

  The thickness of the magnetic layer is preferably 0.01 to 0.10 μm or less, more preferably 0.02 μm to 0.08 μm, still more preferably 0.03 μm to 0.08 μm, and saturation of the magnetic head used. It is preferable to optimize by the amount of magnetization, head gap length, and recording signal band. 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. In general, the thinner the magnetic layer, the lower the durability of the coating film, making it difficult to maintain good running durability, but increasing the coating strength of the magnetic layer by including the binder of the present invention in the magnetic layer. Therefore, excellent running durability can be realized in a magnetic recording medium having a thin magnetic layer having a thickness in the above range.

(Manufacture of coating solution)
The steps for producing a coating solution for forming each layer such as a magnetic layer coating solution, and further a non-magnetic layer coating solution and a backcoat layer coating solution are usually at least a kneading step, a dispersion step, and before and after these steps as necessary. The mixing step. 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 process. Absent. In addition, individual raw materials may be added in two or more steps. In order to achieve the object of the present invention, a conventional known manufacturing technique can be used as a partial process. Further, glass beads can be used to disperse each layer forming coating solution, and zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media, are preferably used. 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] in JP-A No. 2004-295926 can also be referred to.

  The medium after the coating step can be subjected to various post-treatments such as 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. 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. In the examples, “parts” indicates parts by mass unless otherwise specified. It should be noted that the components, ratios, operations, order, and the like shown here can be changed without departing from the spirit of the present invention, and should not be limited to the following examples. Moreover, the mass mean molecular weight as described below is a value determined in terms of standard polystyrene using a DMF solvent containing 0.3% by mass of lithium bromide.

[Reference Example 1]
Synthesis of graft chain monomer (Exemplary Compound A-3)

  To a reactor equipped with a stirrer and a reflux condenser, 1250 g of ε-caprolactam and 71.3 g of 2-ethylhexanol were added and heated to 90 ° C. for 8 hours under a nitrogen atmosphere to complete the reaction. Further, 86.3 g of 2-isocyanatoethyl acrylate and 0.35 g of dibutylhydroxytoluene were added and heated at 80 ° C. for 2 hours to obtain Exemplified Compound A-3 as a pale yellow solid.

  Graft chain monomers used in the following synthesis examples were also synthesized in the same manner as in Reference Example 1 by changing the type and amount ratio (mol%) of the monomers used.

[Synthesis Example 1]
Synthesis of vinyl copolymer having structural unit [A] (Exemplary Compound P-6)
In a reactor equipped with a stirrer and a reflux condenser, 17.4 g of Exemplified Compound A-2, 17.4 g of norbornyl methacrylate, 8.8 g of stearyl methacrylate, 3.4 g of 2-hydroxyethyl methacrylate, 0.23 g of methacrylic acid, Dimethyl 2,2′-azobisisobutyrate (0.60 g) and 2-butanone (55.1 g) were added, and the mixture was heated to 78 ° C. for 9 hours under a nitrogen atmosphere to complete the reaction. The mass average molecular weight was 1.0 × 10 ⁶ . As a result of GPC analysis after the reaction, no peak of residual monomer and residual oligomer was observed, and it was confirmed that a copolymer having each structural unit introduced at a charging ratio was obtained.

[Synthesis Examples 2 to 10]
Synthesis of acrylic copolymer having structural unit [A] Acrylic copolymers P-1 and P- were prepared in the same manner as in Synthesis Example 1 except that the type and amount ratio (mol%) of the monomers used were changed. 4, P-9, P-15, P-18, P-20, P-23, P-26, P-29 were obtained. The weight average molecular weight of each copolymer was measured and confirmed to be the value shown above. Further, as a result of GPC analysis after the reaction, no peaks of residual monomer and residual oligomer were observed, so that it was confirmed that a copolymer having each structural unit introduced at a charging ratio was obtained.

[Example 1-1]
(1) Preparation of magnetic layer coating solution Ferromagnetic metal powder: 100 parts Composition Fe / Co = 100/25
Hc 2450 Oe (≒ 195kA / m)
Specific surface area by BET method 65m ² / g
Surface treatment agent Al ₂ O ₃ , SiO ₂ , Y ₂ O ₃
Particle size (average major axis length) 45nm
Needle ratio 5
σs 110 emu / g (≈110 A · m ² / kg)
Phenylphosphonic acid: 3 parts Acrylic copolymer P-1: 15 parts Methyl ethyl ketone: 200 parts Cyclohexanone: 100 parts α-Al ₂ O ₃ Mohs hardness 9 (average particle size 0.1 μm): 15 parts Carbon black (average particle size) Diameter 0.08 μm): 0.5 part

The above components were kneaded with an open kneader and then dispersed using a sand mill. The following components were added to the obtained dispersion and stirred, followed by sonication and filtration using a filter having an average pore size of 1 μm to prepare a magnetic layer coating solution.
Butyl stearate: 1.5 parts Stearic acid: 0.5 parts Methyl ethyl ketone: 75 parts Cyclohexanone: 25 parts Toluene: 3 parts Polyisocyanate compound (Coronate 3041 manufactured by Nippon Polyurethane Industry Co., Ltd.): 5 parts

(2) Preparation of coating solution for nonmagnetic layer Nonmagnetic powder (αFe ₂ O ₃ hematite): 80 parts
Average long axis length 0.15μm
Specific surface area by BET method 52m ² / g
pH 6
Tap density 0.8
DBP oil absorption 27-38 g / 100 g,
Surface treatment agent Al ₂ O ₃ , SiO ₂
Carbon black: 20 parts
Average primary particle size 0.020μm
DBP oil absorption 80ml / 100g
pH 8.0
Specific surface area by BET method: 250 m ² / g
Volatile content: 1.5%
Acrylic copolymer P-1: 19 parts Methyl ethyl ketone: 200 parts Cyclohexanone: 100 parts

The above components were kneaded with an open kneader and then dispersed using a sand mill. The following components were added to the obtained dispersion and stirred, followed by filtration using a filter having an average pore size of 1 μm to prepare a coating solution for the lower coating layer (nonmagnetic layer).
Butyl stearate: 1.5 parts Stearic acid: 1 part Methyl ethyl ketone: 50 parts Cyclohexanone: 50 parts Toluene: 3 parts Polyisocyanate compound (Coronate 3041 manufactured by Nippon Polyurethane Industry Co., Ltd.): 5 parts

(3) Preparation of Backcoat Layer Coating Solution Carbon black (average particle size 40 nm): 85 parts Carbon black (average particle size 100 nm): 3 parts Nitrocellulose: 28 parts Polyurethane resin: 58 parts Copper phthalocyanine dispersant: 2. 5 parts Nippon Run 2301 (manufactured by Nippon Polyurethane Industry Co., Ltd.): 0.5 part Methyl isobutyl ketone: 0.3 part Methyl ethyl ketone: 860 parts Toluene: 240 parts

The above components were pre-kneaded with a roll mill and then dispersed with a sand mill, and 4 parts of a polyester resin (Byron 500 manufactured by Toyobo Co., Ltd.), 14 parts of a polyisocyanate compound (Nihon Polyurethane Industry Co., Ltd. Coronate 3041), α-Al ₂ O ₃ 5 parts) (manufactured by Sumitomo Chemical Co., Ltd.) was added and stirred and filtered to prepare a backcoat layer coating solution.

(4) Preparation of magnetic tape The non-magnetic layer coating solution is dried so that the thickness after drying becomes 1.0 μm, and immediately after that, the thickness of the magnetic layer becomes 0.1 μm thereon. Both layers are coated simultaneously on a polyethylene naphthalate resin support having a thickness of 5 μm and a magnetic layer coating surface having a center line surface roughness of 0.001 μm and having a base surface made hydrophilic by pre-corona treatment. Was still wet and oriented with a cobalt magnet having a magnetic force of 0.5T (5000G) and a solenoid having a magnetic force of 0.4T (4000G) and dried. Then, after applying a corona treatment in advance to the surface opposite to the base surface, the backcoat layer coating solution is applied so that the thickness after drying is 0.5 μm, and then composed of a metal roll. Processing was performed at a temperature of 100 ° C. at a speed of 80 m / min with a seven-stage calendar, and a magnetic tape was produced by slitting to a width of ½ mm.

(Examples 1-2 to 1-10)
Magnetic tapes of Examples 1-2 to 1-9 in the same manner as in Example 1-1 except that the types of acrylic copolymers used in the magnetic layer and the nonmagnetic layer are changed to those shown in Table 1. Was made.

(Comparative Example 1-1)
Comparative Example 1 was carried out in the same manner as in Example 1-1, except that the type of acrylic copolymer used in the magnetic layer and nonmagnetic layer was changed to that described in Example 1 of JP-A-8-67855. -1 magnetic tape was produced.

(Comparative Example 1-2)
Comparative Example 1 was carried out in the same manner as in Example 1-1 except that the type of acrylic copolymer used in the magnetic layer and nonmagnetic layer was changed to that described in Example 1 of JP-A-2004-295926. -2 magnetic tape was produced.

(Comparative Example 1-3)
Comparative Example 1 was carried out in the same manner as in Example 1-1, except that the type of acrylic copolymer used in the magnetic layer and nonmagnetic layer was changed to that described in Example 1 of JP-A-6-111277. 3 magnetic tape was produced.

(Comparative Example 1-4)
Comparative Example 1 was carried out in the same manner as in Example 1-1, except that the type of acrylic copolymer used in the magnetic layer and the nonmagnetic layer was changed to that described in Reference Example 1 of JP-B-7-9696. -4 magnetic tape was produced.

(Comparative Example 1-5)
Comparative Example 1 was carried out in the same manner as in Example 1-1, except that the type of acrylic copolymer used in the magnetic layer and nonmagnetic layer was changed to that described in Reference Example 7 of JP-B-7-9696. -4 magnetic tape was produced.

(Example 2-1)
(1) Preparation of ferromagnetic hexagonal ferrite magnetic layer coating solution Ferromagnetic tabular hexagonal ferrite powder: 100 parts Composition excluding oxygen (molar ratio): Ba / Fe / Co / Zn = 1/9 / 0.2 / 1
Hc: 160 kA / m (2000 Oe)
Average plate diameter: 20nm
Average plate ratio: 2.7
BET specific surface area: 60 m ² / g
σs: 46 A · m ² / kg (46 emu / g)
Acrylic copolymer P-1: 12 parts α-Al ₂ O ₃ (particle size 0.1 μm): 8 parts Carbon black (average particle size: 20 nm): 0.5 parts Cyclohexanone: 110 parts

The above components were kneaded with an open kneader and then dispersed using a sand mill. The following components were added to the obtained dispersion and stirred, followed by sonication and filtration using a filter having an average pore size of 1 μm to prepare a magnetic layer coating solution.
Butyl stearate: 2 parts Stearic acid: 0.5 parts Methyl ethyl ketone: 75 parts Cyclohexanone: 25 parts Toluene: 3 parts Polyisocyanate compound (Coronate 3041 manufactured by Nippon Polyurethane Industry Co., Ltd.): 5 parts

  The non-magnetic layer coating solution prepared in the same manner as in Example 1-1 was further dried so that the thickness after drying was 1.0 μm, and immediately after that, the magnetic layer coating solution was coated thereon with the thickness of the magnetic layer. On a polyethylene naphthalate resin support having a thickness of 5 μm and a magnetic layer coated surface having a center line surface roughness of 0.001 μm and having a base surface made hydrophilic by pre-corona treatment The two layers were coated simultaneously, and were oriented by a cobalt magnet having a magnetic force of 0.5T (5000G) and a solenoid having a magnetic force of 0.4T (4000G) while both layers were still wet. Then, the thickness after drying the coating liquid for the backcoat layer prepared in the same manner as in Example 1-1 on the surface opposite to the base surface, which has been subjected to corona treatment in advance, becomes 0.5 μm. Then, a magnetic tape of Example 2-1 was produced by performing a treatment with a seven-stage calendar composed of metal rolls at a temperature of 100 ° C. at a speed of 80 m / min and slitting to a width of ½ mm.

(Examples 2-2 to 2-10)
Magnetic tapes of Examples 2-2 to 2-9 in the same manner as in Example 2-1, except that the types of acrylic copolymers used in the magnetic layer and the nonmagnetic layer are changed to those shown in Table 2 Was made.

(Comparative Example 2-1)
Comparative Example 1 was carried out in the same manner as in Example 1-1, except that the type of acrylic copolymer used in the magnetic layer and nonmagnetic layer was changed to that described in Example 1 of JP-A-8-67855. -1 magnetic tape was produced.

(Comparative Example 2-2)
Comparative Example 2 in the same manner as in Example 2-1, except that the type of acrylic copolymer used in the magnetic layer and the nonmagnetic layer was changed to that described in Example 1 of JP-A-2004-295926. -2 magnetic tape was produced.

(Comparative Example 2-3)
Comparative Example 2 was carried out in the same manner as in Example 2-1, except that the type of acrylic copolymer used in the magnetic layer and nonmagnetic layer was changed to that described in Example 1 of JP-A-6-111277. 3 magnetic tape was produced.

(Comparative Example 2-4)
A comparison was made in the same manner as in Example 2-1, except that the type of acrylic copolymer used in the magnetic layer and the nonmagnetic layer was changed to that described in Reference Example 1 of JP-B-7-9696. The magnetic tape of Example 2-4 was produced.

(Comparative Example 2-5)
A comparison was made in the same manner as in Example 2-1, except that the type of acrylic copolymer used in the magnetic layer and the nonmagnetic layer was changed to that described in Reference Example 7 of JP-B-7-9696. The magnetic tape of Example 2-4 was produced.

[Measuring method]
<Average surface roughness of tape>
Using an atomic force microscope (AFM: NANOSCOPE III manufactured by DIGITAL INSTRUMENT), an area of 40 μm × 40 μm was measured on the magnetic layer surface in the contact mode, and the centerline average surface roughness (Ra) was measured.

<Electromagnetic conversion characteristics: SN ratio>
Using an LTO-Gen4 drive, a signal having a recording track of 11.5 μm, a reproduction track width of 5.3 μm, a linear recording density of 172 kfci and 86 kfci is recorded, a frequency analysis of the reproduction signal is performed by a spectrum analyzer, and a carrier signal at the time of recording a 172 kfci signal Of the output and the integrated noise of the entire spectrum band during 86 kfci signal recording was defined as the SN ratio. Fujifilm LTO-Gen4 tape was used as a reference tape. The S / N ratio of the reference tape was 0 dB, and the S / N ratio of each tape was obtained as a relative value. If the S / N ratio is 0 dB or more, it can be determined that the magnetic recording medium has excellent electromagnetic conversion characteristics as a high-density recording magnetic recording medium.

<Amount of dirt on the tape surface>
The tape is passed at an angle of 150 degrees so that the magnetic layer surface is brought into contact with the edge of a prismatic bar made of Al ₂ O ₃ / TiC and having a cross section of 7 mm × 7 mm. By sliding the path, the edge part of the prismatic bar was observed with a microscope, and the adhesion state of the dirt was evaluated. Evaluation was made into sensory evaluation and evaluated in 10 steps. 10 is less dirty and 1 is most dirty.
The stain evaluated by the above method is mainly caused by the abrasion of the magnetic layer surface, and the smaller the evaluation result value, the more the surface of the magnetic layer is scraped and the running durability is poor.

Evaluation results From the results shown in Tables 1 and 2, the following points can be confirmed.
(1) The magnetic tape of the example formed using the binder of the present invention had high surface smoothness. Thereby, it can be confirmed that the fine particle powder was highly dispersed by the binder of the present invention. As a result, the magnetic tape of the example showed excellent electromagnetic conversion characteristics.
(2) On the other hand, magnetic tapes of Comparative Examples 1-1 to 1-5 and Comparative Examples 2-1 to 2-5 using vinyl copolymers that have been conventionally used as binders for magnetic recording media Showed results inferior to the magnetic tapes of the examples in the evaluation items of the surface smoothness and electromagnetic conversion characteristics.
(3) Further, it was confirmed that the running durability was improved in a binder containing a hydroxyl group in the structural unit [A] and a magnetic tape containing the structural unit [A] together with the structural unit [A].
(4) A conventional vinyl polymer is difficult to form a high-strength coating film alone, and the coating film strength is secured by using a polyurethane resin in combination (for example, Patent Document 2, 3), the magnetic tape of the example was able to realize excellent running durability without using a polyurethane resin together with the vinyl copolymer (the binder of the present invention).
From the above results, it was shown that the binder of the present invention is a vinyl copolymer for producing a magnetic recording medium having both excellent electromagnetic conversion characteristics and running durability.

  Since the magnetic recording medium of the present invention can achieve both excellent electromagnetic conversion characteristics and running durability, it is suitable as a backup tape that requires high reliability.

Claims (12)

A binder for magnetic recording media, which is a vinyl copolymer containing a structural unit represented by the general formula [A].

(In the general formula [A], R ^a represents a hydrogen atom, a halogen atom or a methyl group, La represents ^a single bond or a divalent linking group, Y ^a represents ^a hydrogen atom, a carbon atom having 1 to 20 carbon atoms. It represents a hydrogen group or an aromatic group, Z ^a represents a divalent group having an ester bond, a thioester bond, or an amide bond, n1 represents an integer of 2 or more and 500 or less.) The binder for magnetic recording media according to claim 1, wherein the structural unit represented by the general formula [A] is a structural unit represented by the general formula [A1].

(In the general formula [A1], R ^a has the same meaning as in the general formula [A], L ^a1 represents a divalent linking group bonded to the carbon atom of the main chain via a —C (O) — group, Z ^a1 represents —Z ^b C (O) —X ^a1 —, —C (O) —Z ^b X ^a1 —, —X ^a1 C (O) —, or a combination of two or more thereof, and X ^a1 represents Represents an oxygen atom, a sulfur atom or —N (R ⁰ ) —, Z ^b represents a hydrocarbon group or an aromatic group having 2 to 10 carbon atoms, and R ⁰ represents a hydrogen atom or an alkyl having 1 to 8 carbon atoms. Y ^a1 represents a hydrogen atom, a hydrocarbon group having 2 to 10 carbon atoms or an aromatic group, and n2 represents an integer of 2 to 500.) The binder for magnetic recording media according to claim 2, wherein the structural unit represented by the general formula [A1] is a structural unit represented by the general formula [A2].

(In the general formula [A2], R ^a have the same meanings as defined in formula [A], Y ^a1 have the same meanings as defined in formula [A1], X ^a2 have the same meanings as defined in formula [A1] X ^a1 in , Z ^a2 is

Or, it represents a combination of two or more of these. In the above, m represents an integer of 2 or more and 10 or less, X ^a3 represents an oxygen atom, a sulfur atom or —NH—, and Ar represents an arylene group. L ^a2 represents a single bond or a divalent linking group, and n3 represents an integer of 2 or more and 500 or less. ) The binder for magnetic recording media according to any one of claims 1 to 3, which is a vinyl copolymer further comprising a structural unit represented by the general formula [B].

[In General Formula [B], R ³ represents a hydrogen atom, a halogen atom or a methyl group, and L ³ represents a single bond or a divalent linking group. ] The vinyl copolymer includes at least one polar group selected from the group consisting of a sulfonic acid (salt) group, a carboxylic acid (salt) group, and a phosphoric acid (salt) group. The binder for magnetic recording media according to claim 1. A composition for magnetic recording media, comprising the binder according to claim 1. The composition for magnetic recording media according to claim 6, further comprising a polyisocyanate. A magnetic recording medium having a magnetic layer comprising a ferromagnetic powder and a binder on a nonmagnetic support,
The magnetic recording medium, wherein the binder contains the binder according to any one of claims 1 to 5 as a constituent component. A magnetic recording medium having, in this order, a nonmagnetic layer containing nonmagnetic powder and a binder and a magnetic layer containing ferromagnetic powder and a binder on a nonmagnetic support, wherein the binder contained in the magnetic layer and / or Or the binder contained in the said nonmagnetic layer contains the binder of any one of Claims 1-5 as a structural component, The magnetic recording medium characterized by the above-mentioned. The magnetic recording medium according to claim 8 or 9, wherein the binder contains a reaction product of the binder according to any one of claims 1 to 5 and a polyisocyanate. The magnetic recording medium according to claim 8, wherein the ferromagnetic powder is a hexagonal ferrite powder having an average plate diameter of 10 nm to 50 nm. The magnetic recording medium according to claim 8, wherein the ferromagnetic powder is a ferromagnetic metal powder having an average major axis length of 20 nm to 50 nm.