JPH05298666A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To provide a magnetic recording medium having satisfactory dispersibility of magnetic powder in the magnetic layer and excellent in durability as well as in electromagnetic transducing characteristics. CONSTITUTION:When a magnetic layer contg. magnetic powder and a binder is formed on a nonmagnetic substrate to obtain a magnetic recording medium, an alkyl (meth)acrylate polymer having a carboxyl group at one terminal and a block copolymer contg. polar groups in the molecule are used as the binder. The block copolymer consists of polyurethane and a polymer of a vinyl monomer based on alkyl (meth)acrylate, styrene or a deriv. thereof.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium such as a magnetic tape and a magnetic disk, and more particularly to the above medium excellent in electromagnetic conversion characteristics and durability.

[0002]

2. Description of the Related Art Generally, a magnetic recording medium is coated with a magnetic coating material comprising a magnetic powder, a binder, an organic solvent and other necessary components on a non-magnetic support such as a polyester film and dried to obtain a desired thickness. Is formed by forming a magnetic layer of.

In this type of magnetic recording medium, chroma S
In order to obtain excellent electromagnetic conversion characteristics such as / N ratio and C / N ratio, it is necessary that the magnetic powder be uniformly dispersed in the binder. Conventionally, in order to improve the dispersibility of magnetic powder, for example, the following methods (a) to (c) have been tried.

(A) As a binder, a resin component such as a vinyl chloride resin or a polyurethane resin having a hydrophilic polar group such as a carboxyl group or a sulfonic acid group in the molecule is used (Japanese Patent Laid-Open No. 55-38693). , JP-A-57-924
No. 22, JP-A-59-30235, etc.) b) Surface treatment of magnetic powder with fatty acid and various surfactants (JP-A-58-102504) C) Silane coupling agent for magnetic powder Surface treatment (Japanese Patent Application Laid-Open No. 55-125539).

In recent years, due to high performance of video tapes and audio tapes, high recording density and high reproducing output in short wavelength recording are required. In order to deal with this trend, magnetic powder is becoming finer and finer, but due to this fineness and the large magnetic moment of the powder itself, magnetic particles tend to agglomerate with each other, making it more likely than ever before to be contained in the binder. It is becoming difficult to disperse evenly.

[0006]

As described above, there are many known methods for improving the dispersibility of the magnetic powder, as described in the above (a)-(c). It was not possible to adequately meet the demand for high reliability in making the magnetic powder into fine particles, and therefore it could not be said that the electromagnetic conversion characteristics could still be satisfied.

In view of the above-mentioned conventional circumstances, the present invention provides a magnetic recording medium such as a magnetic tape and a magnetic disk which has a good dispersibility of the magnetic powder in the magnetic layer and is excellent in electromagnetic conversion characteristics and is also excellent in durability. Is intended to provide.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to achieve the above-mentioned object, and as a result, as a binder for magnetic powder, two kinds of specific resins each containing a polar group in the molecule were used. It was found that when the components are used in combination, the magnetic powder has improved dispersibility, electromagnetic conversion characteristics are improved, and a magnetic recording medium satisfying the durability can be obtained. Ivy.

That is, according to the present invention, in a magnetic recording medium comprising a magnetic layer containing a magnetic powder and a binder on a non-magnetic support, the binder has (A) a carboxyl group at one end (meta). ) Acrylic acid alkyl ester-based polymer (hereinafter referred to as an acrylic resin containing a carboxyl group at one end), and B) a polymer component of vinyl-based monomer containing (meth) acrylic acid alkyl ester, styrene or a derivative thereof as a main component. And a polyurethane component, and a block copolymer containing a polar group in the molecule (hereinafter referred to as a polar group-containing vinyl-urethane block copolymer). Is.

[0010]

In the present invention, the acrylic resin containing a carboxyl group at one end of the component A has a carboxyl group at one end of the molecule preferentially adsorbed on the surface of the particles of the magnetic powder, and there is no increase in viscosity after dispersion. Therefore, it exhibits good dispersibility in the magnetic powder. The polar group-containing vinyl-urethane block copolymer as the component B has a better affinity with the component A than the conventional polar group-containing binder resin, so that the mechanical strength of the magnetic layer is impaired. Disperses the magnetic powder well. Therefore, by using these A and B components together, a magnetic recording medium having excellent electromagnetic conversion characteristics and durability can be obtained.

The acrylic resin containing a carboxyl group at one end of the component A is, for example, a monomer containing a (meth) acrylic acid alkyl ester as a main component, a chain transfer agent containing a carboxyl group in the molecule, or It can be obtained by radical polymerization in the presence of a carboxyl group-containing polymerization initiator.

Examples of the (meth) acrylic acid alkyl ester include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, hexyl methacrylate, lauryl methacrylate, stearyl methacrylate, and alkyl groups similar to these. Examples thereof include alkyl acrylates. If necessary, a monomer having no carboxyl group in the molecule such as styrene or a derivative thereof may be used in combination as a monomer copolymerizable with these (meth) acrylic acid alkyl esters.

Examples of the chain transfer agent containing a carboxyl group in the molecule include mercaptoacetic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, o-
Examples include mercaptobenzoic acid and thiomalic acid.
Further, examples of the polymerization initiator having a carboxyl group in the molecule include 4.4'-azobis (4-cyanopentanoic acid) and diglutaric acid peroxide.

When a polymerization initiator containing a carboxyl group in the molecule is not used, radical polymerization causes
Needless to say, for example, a usual polymerization initiator such as an azo compound such as α · α′-azobisisobutyronitrile and a peroxide such as benzoyl peroxide can be used.

The acrylic resin having a carboxyl group at one end as the component A obtained by such a method has a number average molecular weight of 1,000 to 20,000, particularly 2,000 to 10,000.
It is preferably 00. If it is less than 1,000, the properties of the acrylic resin will not be exhibited, and if it exceeds 20,000, the degree of freedom of the acrylic resin will be reduced, and the dispersibility of the magnetic powder will be reduced when it is made into a magnetic paint. Neither is preferable.

The polar group-containing vinyl-urethane block copolymer as the component B is, for example, I) a method of polymerizing a vinyl-type monomer using a polyurethane type polymer initiator, I
I) It can be obtained by a method of reacting a vinyl polymer having hydroxyl groups at both ends with an organic diisocyanate to form a urethane.

In the above method I, a) azodiol is added to
b) an organic diisocyanate and, if necessary, c) a chain extender and d) a polyol are reacted to obtain a polyurethane type polymer initiator, and then the polymer initiator is used to e) By polymerizing a vinyl-based monomer, and in carrying out these reactions and polymerizations, each raw material component is added so that a polar group is contained in either the polyurethane component or the polymer component of the vinyl-based monomer. By selective use, a polar group-containing vinyl-urethane block copolymer can be obtained.

Here, as the azodiol as the component a,
For example, 2.2'-azobis (2-cyanopropano-
2,2'-azobis (2-cyanopentanol)
Examples of the organic diisocyanate of component b include tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, and hexamethylene diisocyanate.

Examples of the chain extender of the component c include ethylene glycol, 1.3-propanediol, 1.4
Examples thereof include butanediol, neopentyl glycol, and other diols, ethylenediamine, tolylenediamine, and other diamines, and water. Also, 2,2-dimethylolpropionic acid, bis (4-hydroxybutyl)-
5-Sulfoisophthalic acid sodium salt, β-glycerophosphoric acid sodium salt, N-methyldiethanolamine, 2.3
A polar group can be contained in the polyurethane component by using a material having a polar group such as potassium dihydroxy-1-propanesulfonate.

Examples of the d-component polyol include, for example,
Polycarbonate dioles such as poly 1,6-hexanecarbonate diol, polybutadiene diole, polyolefin diolefins such as hydrogenated butadiene diol, 1,6-hexamethylene lactone dione -Le, poly ε
-Polycaprolactone diols such as caprolactone diol, polyethylene glycerol, polyethylene glycol such as polypropylene glycol, polyester butyl such as polybutylene adipate and polybutylene phthalate And the like.

Examples of the vinyl monomer of the component e include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, hexyl methacrylate, lauryl methacrylate, stearyl methacrylate, and alkyl groups similar to these. A (meth) acrylic acid alkyl ester such as an acrylic acid alkyl ester and styrene or a derivative thereof such as α-methylstyrene are used as main components.

Examples of monomers copolymerizable with these main components include acrylic acid, methacrylic acid, dimethylaminoethyl methacrylate, 2-acrylamido-2-methyl-1-propanesulfonic acid, 2- Polymers of vinyl monomers by using polar group-containing monomers such as methacryloxyethyl acid, 3-sulfopropyl acrylate, 3-sulfopropyl acrylate / potassium salt, and phosphate. A polar group can be contained in the component. Furthermore, by copolymerizing a monomer containing a reactive functional group such as glycidyl methacrylate, and then reacting these reactive functional groups with another compound, a vinyl-based monomer polymer component is obtained. A new polar group may be introduced.

A monomer containing a hydroxyl group in the molecule, such as 2-hydroxyethyl methacrylate, is used as one type of the vinyl monomer of the component e, and is added to the polymer component of the vinyl monomer. By including a hydroxyl group,
It is desirable because the reactivity of the magnetic coating material with the curing agent component is improved and the mechanical strength of the magnetic layer is increased. In this case, the hydroxyl group content is usually 0.
05 mmol / g or more, preferably 0.2 mmol / g
It should be above.

In the method II, the vinyl monomer is subjected to group transfer polymerization by using ketene silyl acetal as a polymerization initiator to obtain a vinyl polymer having hydroxyl groups at both terminals, and then an organic compound is added to the vinyl polymer. By reacting a diisocyanate with a chain extender or polyol as necessary to form a urethane, and when polymerizing or urethane-forming these, either the polymer component of the vinyl monomer or the polyurethane component is polarized. A polar group-containing vinyl-urethane block copolymer can be obtained by selectively using each raw material component so that the group is contained.

The raw material components used in this method, which are vinyl monomers, organic diisocyanates, chain extenders and polyols, are the same as those exemplified as components b to e in the above method I. be able to.

The polar group-containing vinyl-urethane block copolymer of the component B obtained by such a method has a number average molecular weight of usually 5,000 to 500,000, particularly 8,000 to 300,000. Is most preferable, and the most preferable range is 10,000 to 50,000. 5,000
If less than 500, the mechanical properties of the magnetic layer deteriorate, and
If it exceeds 000, the viscosity when dissolved in an organic solvent or the like is high, and it becomes difficult to form a magnetic coating, so that both are not preferable.

The polar group-containing vinyl of the component B is
In the urethane block copolymer, the proportion of the polymer component of the vinyl monomer contained in the urethane block copolymer is in the total amount with the polyurethane component, that is, 5 to 90% by weight, especially 10 to 70% by weight of the whole block copolymer. It is desirable to be within the range. If it is less than 5% by weight, the properties as a vinyl polymer will not be exhibited, and if it exceeds 90% by weight, the resin becomes brittle and the mechanical strength of the magnetic layer is lowered, so that both are not preferable.

Further, the polar group-containing vinyl-urethane block copolymer of the component B has a polar group introduced into either or both of the vinyl monomer polymer component and the polyurethane component. as the polar group, -SO ₃ M group, -OSO ₃ M group, -COOM group, -
A PO (OM) ₂ group or a tertiary amino group (M is hydrogen, an alkali metal or a hydrocarbon group) can be mentioned.

The amount of these polar groups introduced is usually 0.0001 to 0.5 mmol / mol based on the block copolymer.
It is preferably in the range of g, particularly 0.001 to 0.1 mmol / g. When it is less than 0.0001 mmol / g, the dispersibility of the magnetic powder cannot be sufficiently improved, and when it exceeds 0.5 mmol / g, the viscosity when dissolved in an organic solvent due to aggregation of polar groups. Is high, and it becomes difficult to form a magnetic paint, which is not preferable.

In the present invention, the ratio of the component A and the component B used is such that the ratio of the acrylic resin containing a carboxyl group at one terminal of the component A in the total amount of both is usually 5 to 5.
It is preferably in the range of 50% by weight, particularly 10 to 30% by weight. If it is less than 5% by weight, the dispersibility of the magnetic powder cannot be sufficiently improved, and if it exceeds 50% by weight, the durability of the magnetic layer is deteriorated.

The total amount of the above-mentioned components A and B to be used is usually 10 to 35% by weight, preferably 15 to 30% by weight, based on the magnetic powder. With such an amount used, the dispersibility of the magnetic powder in the magnetic layer can be sufficiently improved.

In the magnetic recording medium of the present invention, the above-mentioned components A and B are mixed with and dispersed in a magnetic powder and an organic solvent, if necessary, together with other binder components to prepare a magnetic coating material. It can be prepared by coating on a non-magnetic support such as polyester film by means such as spraying or roll coating and drying to form a magnetic layer having a thickness of usually 0.1 to 5 μm. ..

At this time, the magnetic layer may be provided on one side of the non-magnetic support, and the back coat layer may be provided on the opposite side.
A magnetic layer may be provided on both sides of the non-magnetic support. When a back coat layer is provided, the A component and B
And the component can be preferably used as the binder component of the back coat layer.

As the magnetic powder, for example, γ-Fe ₂
O ₃ powder, Fe ₃ O ₄ powder, γ-Fe ₂ O ₃ and Fe ₃ O
Iron oxide powder in an intermediate oxidation state with ₄ , Co-containing γ-Fe ₂
In addition to oxide-based magnetic powders such as O ₃ powder, Co-containing Fe ₃ O ₄ powder, CrO ₂ powder, and barium ferrite powder,
Various conventionally known magnetic powders such as nitride-based magnetic powders such as iron nitride and metallic magnetic powders such as Fe powder, Co powder, and Fe-Ni-Cr alloy powder can be widely used.

When these magnetic powders are acicular, the average particle diameter (average major axis diameter) is usually 1 μm or less, and the average axial ratio (average major axis diameter / average minor axis diameter) is usually 5 to 10. In the case of a plate, the average major axis diameter is usually preferably 1 μm or less.

As a binder, in addition to the components A and B,
When other binder component is used, for example, polyurethane resin, vinyl chloride-vinyl acetate copolymer, fibrin resin, polyvinyl butyral resin, polyester resin, polyacrylic resin, epoxy resin. Resin, pheno
Conventionally known various resins such as resin and the like can be used, and a curing agent component such as a low molecular weight isocyanate compound can also be used.

As the organic solvent, for example, methylisobutylketone, methylethylketone, cyclohexanone, ethyl acetate, tetrahydrofuran, dioxane, dimethylformamide, etc. which are generally used for magnetic recording media can be used alone or in admixture of two or more. To be done.

It should be noted that the magnetic paint may optionally contain various additives that are usually used, such as lubricants, abrasives, antistatic agents, and dispersants.

[0039]

As described above, in the present invention, as the binder of the magnetic powder, the acrylic resin containing the carboxyl group at one terminal of the component A and the vinyl-urethane block copolymer containing the polar group of the component B are added. By using the combination with the combination, it is possible to provide a magnetic recording medium in which the dispersibility of the magnetic powder is excellent, the electromagnetic conversion characteristics are excellent, and the durability is also satisfied.

[0040]

EXAMPLES Next, examples of the present invention will be described to explain more specifically. In the following, "parts" means "parts by weight".

Synthesis Example 1 Methyl methacrylate 50 parts α · α′-azobisisobutyronitrile 0.8 parts Mercaptopropionic acid 1.6 parts Tetrahydrofuran 60 parts The above composition was placed in a flask equipped with a condenser. ,
After sufficiently substituting with nitrogen, the mixture was reacted at 60 ° C. for 3 hours while stirring.

Next, the obtained reaction product was reprecipitated in petroleum ether and water for purification, dried under reduced pressure at 60 ° C. for 48 hours, and acrylic resin A containing a carboxyl group at one end.
Got 1. This resin A1 has a number average molecular weight of 6.0 × 10 ^{3 as} measured by vapor pressure permeation and a carboxyl group content of 0.1.
It was 7 mmol / g.

Synthesis Example 2 The same procedure as in Synthesis Example 1 was repeated except that 1.2 parts of thiomalic acid was used instead of 1.6 parts of mercaptopropionic acid.
An acrylic resin A2 containing a carboxyl group at one end was obtained.
This resin A2 has a number average molecular weight of 5.8 by vapor pressure permeation method.
The content was × 10 ³ , and the carboxyl group content was 0.32 mmol / g.

Synthesis Example 3 2.2'-Azobis (2-cyanopropanol) 5.8 parts Hexamethylene diisocyanate 50 parts Neopentyl glycol 31 parts Tin octylate 0.1 part Methyl ethyl ketone 80 parts Put the composition of in a flask equipped with a condenser,
After sufficiently substituting with nitrogen, the reaction was carried out at 20 ° C. for 80 hours while stirring.

Next, the obtained reaction product, a polyurethane type polymer initiator, was added to ethyl methacrylate 74.1 parts 2-hydroxyethyl methacrylate 12 parts 3-sulfopropyl acrylate potassium salt 0. 0.8 part Methyl ethyl ketone 80 parts was added and the polymerization reaction was carried out at 80 ° C. for 20 hours.

The obtained polar group-containing vinyl-urethane block copolymer B1 had a number average molecular weight of 2.0 × 10 ^{4 in} terms of polystyrene by GPC method (gel permeation chromatography method) and hydroxyl group. Content of 0.5
The content of 2 mmol / g and potassium sulfonate base was 0.020 mmol / g. The weight ratio of vinyl monomer polymer component / polyurethane component is 50 /
It was 50.

Synthetic Example 4 2.2'-Azobis (2-cyanopropanol) 5.8 parts Hexamethylene diisocyanate 50 parts Dipropylene glycol 34.5 parts Dimethylol propionic acid 0.8 parts Tin octylate 0.1 part Methyl ethyl ketone 80 parts The above composition was placed in a flask equipped with a condenser,
After sufficiently substituting with nitrogen, the reaction was carried out at 20 ° C. for 80 hours while stirring.

The resulting reaction product, a polyurethane type polymer initiator, was then added with styrene 53.4 parts 2-hydroxyethyl methacrylate 7 parts 3-sulfopropyl acrylate sodium salt 0.35 parts. 80 parts of methyl ethyl ketone was added, and a polymerization reaction was carried out at 80 ° C. for 20 hours.

The polar group-containing vinyl-urethane block copolymer B2 thus obtained had a number average molecular weight of 2.5 × 10 ^{4 in} terms of polystyrene by GPC method, a hydroxyl group content of 0.34 mmol / g, and a sulfone. The content of sodium acid salt base was 0.010 mmol / g, and the content of carboxyl group was 0.04 mmol / g. Further, the weight ratio of polymer component / polyurethane component of vinyl monomer is 40.
It was / 60.

Example 1 α-Fe magnetic powder (coercive force 1,500 oersteds, saturation magnetization 120 emu / g, average major axis diameter 0.5 μm) 300 parts Acrylic resin A1 containing a carboxyl group at one end (Synthesis Example 1) 5 Parts polar group-containing vinyl-urethane block copolymer B1 (Synthesis example 3) 35 parts polyurethane (N-2301 manufactured by Nippon Polyurethane Industry Co., Ltd.) 20 parts carbon black 8 parts alumina powder 20 parts methyl ethyl ketone 400 parts toluene 400 parts

The above composition was mixed and dispersed in a high speed disperser for 2 hours, and then mixed and dispersed in a sand grinder mill for 7 hours, and then a trifunctional low molecular weight isocyanate compound (Cornerate L manufactured by Nippon Polyurethane Co., Ltd.) was used. ) Was added and further mixed and dispersed for 30 minutes with a sand grinder mill.

Thereafter, a predetermined amount of a mixed solvent of methyl ethyl ketone / toluene was added to adjust the solid content concentration to 30% by weight to prepare a magnetic paint. This magnetic paint was applied onto a polyester film having a thickness of 7.5 μm so that the thickness after drying was 3 μm, and dried to form a magnetic layer, which was calendered and cut into 8 mm width. Then, a magnetic tape was produced.

Example 2 Example 5 except that 5 parts of the acrylic resin A1 containing one terminal carboxyl group (Synthesis example 1) was replaced with 5 parts of the acrylic resin A2 containing one terminal carboxyl group (Synthesis example 2). 1
A magnetic tape was produced in the same manner as in.

Example 3 Polar group-containing vinyl-urethane block copolymer B1
(Synthesis Example 3) A magnetic tape was produced in the same manner as in Example 1 except that 35 parts of a vinyl-urethane block copolymer B2 (Synthesis Example 4) containing a polar group was used instead of 35 parts. did.

Comparative Example 1 The use of 5 parts of acrylic resin A1 (Synthesis Example 1) containing a carboxyl group at one end was omitted, and the amount of polyurethane (N-2301 manufactured by Nippon Polyurethane Industry Co., Ltd.) was changed to 25 parts. Example 1 except that 35 parts of a vinyl chloride-based resin (MR-110 manufactured by Nippon Zeon Co., Ltd.) was used in place of 35 parts of the vinyl-urethane block copolymer B1 (Synthesis Example 3) containing a polar group. A magnetic tape was produced in the same manner.

Comparative Example 2 Polar group-containing vinyl-urethane block copolymer B1
(Synthesis Example 3) 35 parts by weight of vinyl chloride resin (MR-110 manufactured by Nippon Zeon Co., Ltd.) was used instead of 35 parts by weight.
A magnetic tape was produced in the same manner as in Example 1.

Comparative Example 3 Except that 5 parts of the acrylic resin A1 containing one terminal carboxyl group (Synthesis Example 1) was omitted and the amount of polyurethane (N-2301 manufactured by Nippon Polyurethane Industry Co., Ltd.) was changed to 25 parts. In the same manner as in Example 1, a magnetic tape was manufactured.

For each of the magnetic tapes obtained in Examples 1 to 3 and Comparative Examples 1 to 3, the chroma S / N ratio, C /
The N ratio (electromagnetic conversion characteristics) and still durability were measured by the following methods. The results are shown in Table 1 below.

<Chroma S / N ratio> A 0.75 MHz chroma signal of 0.71 is applied to the magnetic tape by using an 8 mm type VTR.
4Vp-p, recorded on the brightness modulation signal, measure the noise part of the reproduced signal with a chroma noise measuring machine to obtain the chroma S / N ratio, and use the magnetic tape of Comparative Example 1 as a reference (0 dB ).

<C / N ratio> Using a VTR of 8 mm system, a chroma signal of 7 MHz was recorded on a magnetic tape, and noise generated in the range of 7 ± 1 MHz when this signal was reproduced was measured, The ratio of the reproduced signal to this noise was measured using an ED-V900 type output level measuring device manufactured by Sony Corporation, and shown as a relative value with the magnetic tape of Comparative Example 1 as a reference (0 dB).

<Still durability> Using an 8 mm type VTR, after recording a 7 MHz chroma signal on a magnetic tape at a constant level, the output at the time of reproducing at 0 ° C. is measured until the output drops by 3 dB. Was measured.

[0062]

[Table 1]

As is clear from Table 1 above, Example 1
The magnetic tapes Nos. 3 to 3 have higher chroma S / N ratios and C / N ratios than the magnetic tapes of Comparative Examples 1 to 3, and the acrylic resin containing a carboxyl group at one end of the A component and the B component. It can be seen that a magnetic recording medium having a good dispersibility of the magnetic powder and excellent electromagnetic conversion characteristics can be obtained by the combination with the polar group-containing vinyl-urethane block copolymer of.

Further, since the above-mentioned A component and B component are excellent in compatibility, the still durability of the magnetic tapes of Examples 1 to 3 is lowered as in the magnetic tape of Comparative Example 2. It is also understood that a magnetic recording medium excellent in durability can be obtained without doing so.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kenji Kono Inventor Kenji Kono 1-88 1-Tora Tora, Ibaraki City, Osaka Prefecture Hitachi Maxell Co., Ltd.

Claims (4)

[Claims] 1. A magnetic recording medium comprising a magnetic layer containing a magnetic powder and a binder on a non-magnetic support, wherein the binder comprises (A) (meth) acrylic acid containing a carboxyl group at one end. It is composed of an alkyl ester polymer and a polymer component of B) a vinyl monomer whose main component is (meth) acrylic acid alkyl ester, styrene or its derivative, and a polyurethane component, and contains a polar group in the molecule. A magnetic recording medium containing a block copolymer. 2. The magnetic recording medium according to claim 1, wherein the average particle diameter of the magnetic powder is 1 μm or less. 3. The number average molecular weight of the (meth) acrylic acid alkyl ester-based polymer of the component A is 1,000 to 20,
3. The magnetic recording medium according to claim 1, wherein the block copolymer of the component B has a number average molecular weight of 5,000 to 500,000. 4. The polar group contained in the block copolymer of the component B is a —SO ₃ M group, a —OSO ₃ M group or a —COOM.
Group, -PO (OM) ₂ group, or a tertiary amino group (M is hydrogen, an alkali metal, or a hydrocarbon group) at least one kind. Magnetic recording medium.