JP2657639B2 - Magnetic recording media - Google Patents

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,
Physical properties, especially dispersibility and storage stability under high temperature and high humidity environment,
The present invention relates to a magnetic recording medium having good running properties and electromagnetic conversion characteristics.

[0002]

2. Description of the Related Art In recent years, conversion from a conventional oxide-based magnetic powder to a metal magnetic powder has been promoted as a magnetic powder in order to increase the recording density.

[0003] Further, for the purpose of improving the magnetic characteristics of a magnetic recording medium, fine particles of magnetic powder have been developed. In particular, in the case of an audio medium, it is indispensable to reduce the size of metal magnetic powder to reduce noise in order to improve the S / N ratio. The use of alkaline metal magnetic powder is also effective in terms of electromagnetic conversion characteristics.

[0004] However, the fine metal magnetic powder has poor dispersibility when used as a magnetic paint, and therefore it is difficult to increase the content of the magnetic powder in the magnetic layer. In addition, since the reactivity of the magnetic powder itself becomes higher than that of the conventional magnetic powder, problems such as a change with time under high temperature and high humidity and problems such as reliability of physical properties arise.

By the way, the present inventors have previously described vinyl chloride-hydroxyl group-containing monomer-polar group-containing monomer or vinyl chloride-vinyl acetate-hydroxyl group-containing monomer-polar group-containing monomer. It has been proposed to use as a binder a copolymer which does not contain vinyl alcohol (no saponification step) as a component (filed on November 27, 1985).

This copolymer is produced without undergoing a saponification step and contains a polar group and an epoxy group, so that it is excellent in dispersibility of magnetic powder, especially in storage and running properties under high temperature and high humidity. And extremely good characteristics in terms of electromagnetic conversion characteristics and the like.

The inventors of the present invention have studied magnetic paints and magnetic layers in the case where the thermosetting resin having a polar group proposed by the present inventors is used as a binder. 60 m ² / g, pH 8-10
In the case where the above-mentioned fine metal magnetic powder was used, it was confirmed that the dispersibility was particularly improved, and the physical property reliability and the electromagnetic conversion characteristics were improved.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to use a vinyl chloride copolymer which does not include a saponification step in the production process as a binder, and use a metal magnetic powder whose specific surface area and pH are adjusted as a magnetic powder. An object of the present invention is to provide a magnetic recording medium which is excellent in dispersibility and thermal stability, particularly excellent in storage stability under high temperature and high humidity, durable running property and excellent electromagnetic conversion characteristics.

[0009]

This and other objects are achieved by the present invention which is defined below as (1) to (7). (1) In a magnetic recording medium in which a magnetic paint containing a binder and a magnetic powder is applied on a substrate, the binder comprises vinyl chloride, a hydroxyl group-containing monomer and a polar group-containing monomer as constituent units. Containing or vinyl chloride, vinyl acetate,
A copolymer containing a hydroxyl group-containing monomer and a polar group-containing monomer as constituent units, wherein the copolymer has a vinyl chloride unit content of 60 to 95% by weight, The hydroxyl group is not directly bonded to the vinyl group, and the polar group-containing monomer is one of a carboxylic acid group, a phosphoric acid group, a sulfuric acid group, a sulfonic acid group or a salt thereof, an amino group and an ammonium group. Having the above polar group,
Further, the magnetic powder has a BET value of 30 to 60 m ² / g and a pH of 8 to
10. A magnetic recording medium, which is a metal magnetic powder of 10. (2) The magnetic recording medium according to (1), wherein the copolymer contains 0.01 to 15% by weight of a polar group. (3) The magnetic recording medium according to (1) or (2), wherein the vinyl acetate unit content of the copolymer is 20% by weight or less. (4) The magnetic recording medium according to any one of the above (1) to (3), wherein the copolymer contains 0.01 to 5% by weight of a hydroxyl group. (5) The number average molecular weight of the copolymer is 10,000 to 3000
0. The magnetic recording medium according to any one of (1) to (4) above, wherein (6) The magnetic recording medium according to any one of (1) to (5) above, wherein the binder contains 70% by weight or less of a thermoplastic elastomer and / or resin. (7) The magnetic recording medium according to any one of (1) to (6), wherein the copolymer is crosslinked by a crosslinking agent.

The prior application of this application, Japanese Patent Application Laid-Open No.
No. 53368 discloses a copolymer which partially overlaps with the present invention, but does not disclose any metal magnetic powder in the present invention.

[0011]

[Specific Configuration] Hereinafter, a specific configuration of the present invention will be described in detail. The present invention comprises vinyl chloride and a hydroxyl group-containing monomer as a binder, or vinyl chloride, vinyl acetate and a hydroxyl group-containing monomer, and a polar group-containing monomer copolymerizable therewith. A copolymer which does not contain vinyl alcohol as a component (not subjected to a saponification step) is used.

The polar group-containing monomer used in the present invention is a polar group-containing vinyl monomer.

The polar group includes a carboxylic acid group, a phosphoric acid group, a sulfuric acid group and a sulfonic acid group or salts thereof, an amino group and an ammonium group.

In this case, a carboxylic acid group COOH, a phosphoric acid group PO ₄ H ₂ , a sulfate group SO ₄ H, a sulfonic acid group SO ₃ H
Alkali metals such as K and Na, and Ca and Mg
And the like, as well as alkaline earth metal salts and the like, and ammonium salts and the like.

The phosphate group may be a monophosphate group having an alkyl group, an aryl group and the like.

The amino group may be an unsubstituted one or a monosubstituted amino group substituted with an alkyl group, an aryl group or the like.

The ammonium group may be an unsubstituted one or a mono-, di-, or tri-substituted amino group substituted with an alkyl group, an aryl group or the like.

Further, an acid anion such as halogen may be bonded to the ammonium group.

The polar group-containing monomer used in the present invention has at least one of the above polar groups. And the polar group is
It may be directly bonded to a vinyl group, or may be bonded to a substituent bonded to a vinyl group.

The substituents bonded to the vinyl group to which the polar group is bonded include ROCO-, R-, RNHCO- and RSi-
-, RNCOOOCO-, etc. are preferred.

In this case, R is an alkyl group, particularly an alkyl group having 1 to 10 carbon atoms, such as an aryl group (phenyl group, naphthyl group, etc.). These may be further substituted with a substituent or the like.

Usually, only one of these polar groups is directly or indirectly bonded to the vinyl group, but two or more polar groups may be bonded.

Further, an alkyl group, particularly one having 1 to 10 carbon atoms, an aryl group or the like, preferably a lower alkyl group may be bonded to the vinyl group.

The following are specific examples of these polar group-containing monomers.

(1) Carboxylic acid group-containing monomer 1) acrylic acid 2) methacrylic acid 3) crotonic acid 4) isocrotonic acid 5) vinyl acetic acid 6) angelic acid 7) tiglic acid 8) allyl acetic acid 9) β, β -Dimethylacrylic acid 10) pyroterepic acid 11) undecylenic acid 12) maleic acid 13) itaconic acid.

(2) Phosphoric acid group-containing monomer 1) CH ₂ CHCHCOO (R) n-OPO (OH) ₂ where R: CH ₂ , C (CH ₃ ) _{2 and} other substituted or unsubstituted methylene n: about 1 to 10 2) CH ₂ = CHCOO [C (CH ₃ ) ₂ ] n (CH ₂ ) m-OPO (OH) ₂ where m: about 1 to 10 n: about 1 to 10 and so on.

(3) Sulfate group-containing monomer 1) The following compound

[0028]

Embedded image

Here, A: H or a cation such as Na, K or the like.

(4) Sulfonic acid group-containing monomer 1) Vinyl sulfonic acid 2) Vinyl sulfonic acid salt 3) Styrene sulfonic acid 4) Styrene sulfonic acid salt 5) CH ₂ = CH (R) nSO ₃ A where R : CH ₂ , C (CH ₃ ) ₂ etc. substituted or unsubstituted methylene n: about 1 to 10 A: H or cation such as Na, K 6) CH ₂ = CH [C (CH ₃ ) ₂ ] n ( CH ₂ ) mSO ₃ A where m: about 1 to 10 n: about 1 to 10 A: H or cation such as Na, K 7) The following compounds

[0031]

Embedded image

Here, A: H or a cation such as Na, K or the like.

(5) Amino group-containing monomer 1) acrylamide 2) CH ₂ = CHCOO (CH ₂ ) ₂ -NH ₂ 3) dimethylaminoethyl methacrylate 4) CH ₂ = CHCONH (CH ₂ ) ₂ -SO ₃ A Where A: H or cation such as Na, K, etc.

(6) Ammonium group-containing monomer 1) CH ₂ CHCH—COO (CH ₂ ) ₃ N— (R ′) ₃ ⁺ X ^— where R ′: alkyl such as CH ₃ and C ₂ H ₅ X: anions such as Cl and I 2) diallyldimethyl-ammonium bromide and the like.

One or more such polar group-containing monomers are contained in the copolymer, and a total of 0.02 to
The content is about 40% by weight.

As a result, the polar group is contained in the copolymer in a total amount of 0.01 to 15% by weight, more preferably 0.01 to 15% by weight.
5% by weight is contained.

When the content of the polar group is less than 0.01% by weight, the dispersibility is deteriorated, and when it exceeds 15% by weight, disadvantages are caused in terms of solubility, durability and the like.

The copolymer containing such a polar group-containing monomer as a constituent unit contains vinyl chloride and a hydroxyl group-containing monomer as essential constituent units.

The vinyl chloride unit content is 60 to 95% by weight, more preferably 70 to 90% by weight.

If the vinyl chloride unit content exceeds 95% by weight, the solubility will deteriorate.

On the other hand, if it is less than 60% by weight, the dispersibility deteriorates.

The hydroxyl group of the hydroxyl group-containing monomer used is not directly bonded to a vinyl group but is bonded to a substituent bonded to the vinyl group.

More specifically, ROCO- and R- are preferred as the substituent to which the hydroxyl group is bonded and which is bonded to the vinyl group. In this case, R is an alkyl group, particularly an alkyl group having 2 to 10 carbon atoms, an aryl group (such as a phenyl group or a naphthyl group), and particularly preferably an alkyl group having 2 to 5 carbon atoms.

These may be further substituted by other substituents and the like.

The carbon atom in R to which the hydroxyl group is bonded is preferably a carbon atom at the terminal side, which is bonded to an ester group.

Usually, only one of these substituents is bonded to the vinyl group, but two or more may be bonded.

Further, an alkyl group, particularly one having 2 to 10 carbon atoms, an aryl group or the like, preferably a lower alkyl group may be bonded to the vinyl group.

Among these hydroxyl group-containing monomers, hydroxyalkyl (meth) acrylate is preferred.

The following are specific examples of these hydroxyl group-containing monomers.

1) 2-hydroxyethyl acrylate 2) 2-hydroxyethyl methacrylate 3) 2-hydroxypropyl acrylate 4) 2-hydroxypropyl methacrylate 5) 3-hydroxypropyl acrylate 6) 3-hydroxypropyl methacrylate 7) 2-hydroxy Butyl acrylate 8) 2-hydroxybutyl methacrylate 9) 3-hydroxybutyl acrylate 10) 3-hydroxybutyl methacrylate 11) 4-hydroxybutyl acrylate 12) 4-hydroxybutyl methacrylate 13) 5-hydroxypentyl acrylate 14) 5-hydroxypentyl Methacrylate 15) 6-hydroxyhexyl acrylate 16) 6-hydroxyhexyl methacrylate 17) neopentyl glycol acrylate 18) neopentyl glycol Methacrylate 19) allyl carbinol

The hydroxyl group-containing monomer is contained in the copolymer in an amount of 0.02 to 40% as a structural unit.

As a result, the hydroxyl groups are contained in the copolymer in an amount of 0.01 to 5% by weight.

When the hydroxyl group content is less than 0.01% by weight, the dispersibility deteriorates, and the degree of crosslinking by the crosslinking agent described below decreases. On the other hand, if it exceeds 5% by weight, disadvantages are caused in terms of solubility and durability.

Further, vinyl acetate may be contained as a structural unit in the copolymer.

The solubility tends to be improved by the addition of vinyl acetate, but the content of vinyl acetate units is 20% by weight.
If the content exceeds the above range, the dispersibility will be deteriorated, so that the content of the vinyl acetate unit is 20% by weight or less.

The number average molecular weight of such a copolymer is usually from 10,000 to 30,000.

If the number average molecular weight is less than 10,000, the physical properties deteriorate, and if it exceeds 30,000, the dispersibility deteriorates.

In order to synthesize such a copolymer, each monomer is usually copolymerized according to a conventional method.

Alternatively, an acid group or an ammonium group can be separately introduced according to a conventional method after the synthesis of the copolymer.

Such a copolymer is used as a main binder (binder) of a magnetic recording medium.

In the present invention, the binder contains a thermoplastic elastomer and / or resin in an amount of 70% by weight or less, particularly 20 to 70% by weight, based on the total resin content in the binder.

The following are suitable as the thermoplastic elastomer.

I) Polyurethane Elastomer As a thermoplastic elastomer, the balance of abrasion resistance of a magnetic coating film required for a magnetic recording medium, adhesion to a PET film, and wettability with a magnetic material particle is excellent. In this respect, urethane compounds are particularly suitable for the purpose of the present invention.

Examples of such urethane compounds include:
As the isocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 1,3-xylene diisocyanate, 1,4-xylene diisocyanate, 1,5-naphthalene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, 3, 3'-dimethyl-4,4'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 3,3'-dimethylbiphenylene diisocyanate, 4,4'-biphenylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane diisocyanate, Various polyisocyanates such as Desmodur L and Desmodur N, and network-like saturated polyester (ethylene glycol, die Glycol, glycerol, trimethylolpropane, 1,4-butanediol, 1,6
Polyhydric alcohols such as hexanediol, pentaerythritol, sorbitol, neopentyl glycol, 1,4-cyclohexanedimethanol, and phthalic acid;
Isophthalic acid, terephthalic acid, maleic acid, succinic acid,
Condensation polymerization with a saturated polybasic acid such as adipic acid or sebacic acid), network saturated polyether (polyethylene glycol, polypropylene glycol, polytetraethylene glycol), caprolactam, hydroxy-containing acrylate, hydroxy-containing methacrylic acid Polyurethane elastomers composed of condensation polymers of various polyesters such as esters are effective.

Ii) Acrylonitrile-butadiene copolymer elastomer Elastomer such as acrylonitrile-butadiene copolymer having a terminal hydroxyl group and commercially available as poly-BD liquid resin manufactured by Sinclair Petrochemical Co., Ltd. or Hiker 1432j manufactured by Zeon Corporation.

Iii) Polybutadiene Elastomer A prepolymer having a low molecular weight terminal hydroxyl group, such as poly-BD Liquid Resin R-15 manufactured by Sinclair Petrochemical Co., Ltd., is particularly suitable in terms of compatibility with thermoplastics and affinity with magnetic powder. is there.

The cyclized product of polybutadiene, CBR-M901 manufactured by Nippon Synthetic Rubber, also exhibits excellent performance in combination with thermoplastics.

Other suitable thermoplastic elastomers include chloride rubber, acrylic rubber, isoprene rubber and its cyclized product (CIR701 manufactured by Nippon Synthetic Rubber), epoxy-modified rubber, internally plasticized saturated linear polyester (Toyobo Byron #). 300).

The thermoplastic resin which can be used in place of or in addition to such an elastomer component is a linear saturated polyester resin, desmophen # 8 manufactured by Bayer AG.
Low-molecular-weight polyester resin, epoxy resin, phenoxy resin, chlorosulfonated polyethylene resin, vinyl isobutyl ether, polyamide resin, polyvinyl butyral resin, polyvinyl resin which is versatile as a two-liquid type with isocyanates such as 00, # 1100 and # 2200 A pyrrolidone resin or the like can be used.

The binder can be cross-linked by a compound cross-linking agent which can react particularly with the hydroxyl group of the copolymer.

Examples of such a cross-linking agent include a polyfunctional isocyanate compound having an isocyanate group or an isocyanate-containing compound as described above, and a butylated melamine resin.

In order to cure such a binder, the binder may be generally heated in a heating oven at 50 to 80 ° C. for 6 to 100 hours.

The magnetic powder used for the magnetic paint of the magnetic recording medium of the present invention has a specific surface area (BET value) of 30 to 60 m ² / g, pH 8
10 to 10 are magnetic metal powders.

The specific surface area of the magnetic powder may be measured by an automatic measuring apparatus for specific surface area by one-point adsorption method (Micromeritics 2200-01 manufactured by Shimadzu Corporation).

When the BET value exceeds 60 m ² / g, dispersibility
Electromagnetic conversion characteristics and transfer deteriorate, and when it is less than 30 m ² / g, the S / N ratio deteriorates.

Therefore, the BET value is 30 to 60 m ² / g.

The pH of the magnetic powder is determined according to JIS K5101.
It may be performed according to. That is, 5 g of the magnetic powder was added to distilled water 1
The mixture is placed in a beaker together with 00 ml, heated to boiling, then cooled to room temperature, the magnetic powder is settled, and the pH of the supernatant is measured with a pH meter (for example, HORIBAF-7).

When the pH is less than 8, for example, in the case of audio, the output in a low frequency range and the sensitivity are reduced, such as a decrease in sensitivity. If the pH exceeds 10, the durability tends to decrease.

The composition of the metal magnetic powder used was Fe,
A simple substance of Co and Ni and their alloys, or a metal obtained by adding Cr, Mn, Co, Ni, and Zn, Cu, Zr, Al, Ti, Bi, Ag, Pt, etc. to these simple substances and alloys Can be used. The effect of the present invention is not lost even if a small amount of a nonmetallic element such as B, C, Si, P, or N is added to these metals.

The pH of these metal magnetic powders is adjusted to 8 to
In order to adjust to 10, the preparation method and composition may be selected.

The metal magnetic powder may be subjected to an alkali treatment using an amine, an ammonia or the like.

The metal magnetic powder may be in the form of needles or particles, and may be selected according to the use as a magnetic recording medium.

The mixing ratio between the metal magnetic powder and the above-mentioned binder is about 10/1 to 1/10 by weight.

For the magnetic paint, it is effective to use various abrasives, lubricants, antistatic agents, dispersants, and the like according to the intended use.

The thickness of the recording layer containing the magnetic powder and the binder is usually about 1 to 7 μm.

As the substrate, any of various resins such as polyester film, polyamide, and polyimide, aluminum metal, and glass may be used according to the purpose.

Further, various known back coat layers may be provided on the side of the substrate opposite to the surface on which the recording layer is provided.

Further, recording layers may be provided on both sides of the substrate.

[0089]

The magnetic recording medium of the present invention comprises, as a binder, a polar monomer and a vinyl chloride-hydroxyl group-containing monomer or a vinyl chloride-vinyl acetate-hydroxyl group-containing monomer. A polymer which has not been subjected to a saponification step is used, and has a specific surface area (BET) of 30 as a magnetic powder.
~ 60m ² / g, to use metal magnetic powder of pH 8 ~ 10,
It has the following excellent effects.

(1) Since the binder contains a polar group, the dispersibility of the magnetic powder in the magnetic coating material is very good, and the magnetic coating material has very good stability over time.

(2) Since the magnetic powder having a specific surface area of 30 to 60 m ² / g is used, the affinity with the binder is very good.
The magnetic layer exhibits excellent running durability and also exhibits excellent electromagnetic conversion characteristics such as a high S / N ratio.

(3) Since magnetic powder having a pH of 8 to 10 is used, the electromagnetic conversion characteristics (f characteristics) are very good.

(4) By using a copolymer which has not been subjected to a saponification step as a binder, chemical change of the resin itself due to dehydrochlorination and dehydrochlorination can be prevented, and magnetic powder and other powders formed by HCl released from the resin can be prevented. Of the magnetic recording medium is prevented from chemically changing, and the magnetic layer exhibits excellent temporal stability.

[0094]

Next, specific examples of the present invention will be shown, and the present invention will be described in more detail.

Example 1 A metal magnetic powder having the specific surface area (BET value) and pH shown in Table 1 was prepared.

[0096]

[Table 1]

The specific surface area of the magnetic powder was measured by a specific surface area automatic measuring device (Micromeritics Model 2200-01 manufactured by Shimadzu Corporation) by a one-point adsorption method.

The pH of the magnetic powder is determined according to JIS K5101.
5 g of magnetic powder was put into a beaker together with 100 ml of distilled water according to the method described in the above, and the pH after boiling was measured with a pH meter (HORIB).
AF-7).

Further, the polymers shown in Table 2 were prepared.

[0100]

[Table 2]

Using the above-described metal magnetic powder and copolymer in the combinations shown in Table 3, magnetic coating materials were prepared as follows.

Metallic magnetic powder 100 parts by weight Copolymer 10 parts by weight Polyurethane resin 10 parts by weight (Nipporan 2304 manufactured by Nippon Polyurethane) 1 part by weight Abrasive 1 part by weight 70 parts by weight methyl ethyl ketone 70 parts by weight methyl isobutyl ketone 70 parts by weight toluene 70 Parts by weight

This mixture was dispersed in a ball mill for about 20 hours.

Next, Coronate L (a polyisocyanate manufactured by Nippon Polyurethane Co., Ltd.) was added as a crosslinking agent, and 8 μm
Was applied to a polyester base. After the surface treatment, a thermosetting treatment was performed to obtain a magnetic tape having a coating thickness of 5 μm. The gloss of these coating compositions before surface processing was measured with a glow meter (Murakami formula).

Table 3 shows the results.

The following measurements were made on the above magnetic tape.

(1) Running Accident Rate (Thermal Stability) The tape was treated at 50 ° C. and 80% RH for 24 hours and 4 hours.
The running accident rate when running continuously for 8 hours was measured.

(2) Electromagnetic conversion characteristics The maximum output level (MOL333) (dB) at 333 Hz, the sensitivity (S333) (dB) and the sensitivity (S16k) (dB) at 333 Hz and 16 kHz were measured.

The measured values are the same as those of Sample No. 3 (metal magnetic powder category C, copolymer category I) relative to the measured values.

Table 3 shows the results.

However, the magnetic powder of the metal magnetic powder category H (comparative) had a high S / N ratio, though high in sensitivity, and was unsuitable for a magnetic tape.

[0112]

[Table 3]

The effects of the present invention are clear from the results shown in Table 3.

Continuation of the front page (56) References JP-A-59-5424 (JP, A) JP-A-59-221830 (JP, A) JP-A-53-28298 (JP, A) JP-A 60-235814 (JP) JP-A-58-177524 (JP, A) JP-A-60-238309 (JP, A) JP-A-60-238371 (JP, A) JP-A-60-238306 (JP, A) 57-154618 (JP, A) JP-B-53-30001 (JP, B1) Oil Chemistry, Vol. 33, No. 9 (1984) 62-68 “Development of Magnetic Materials and Technology for Highly Dispersing Magnetic Powder” (57-4-30), Sogo Gijutsu Center, p. 55 "Magnetic Recording Media Comprehensive Data Collection" (Showa 60-8-15), Comprehensive Electronic Research, p. 124 to 125

Claims (7)

(57) [Claims] 1. A magnetic recording medium having a magnetic coating material containing a binder and magnetic powder applied on a substrate, wherein the binder comprises vinyl chloride, a hydroxyl group-containing monomer and a polar group-containing monomer. A copolymer containing vinyl chloride, vinyl acetate, a hydroxyl group-containing monomer and a polar group-containing monomer as constituent units, or the vinyl chloride unit content of the copolymer is 60 to 95. % By weight, the hydroxyl group of the hydroxyl group-containing monomer is not directly bonded to a vinyl group, and the polar group-containing monomer is a carboxylic acid group, a phosphate group,
It has one or more polar groups among a sulfate group and a sulfonic acid group or a salt thereof, an amino group and an ammonium group, and the magnetic powder has a BET value of 30 to 60 m ² / g and a pH of 8 to
10. A magnetic recording medium, which is a metal magnetic powder of 10. 2. The magnetic recording medium according to claim 1, wherein the copolymer contains 0.01 to 15% by weight of a polar group. 3. The copolymer has a vinyl acetate unit content of 20.
3. The magnetic recording medium according to claim 1, wherein the content of the magnetic recording medium is not more than weight%. 4. The magnetic recording medium according to claim 1, wherein the copolymer contains 0.01 to 5% by weight of a hydroxyl group. 5. A copolymer having a number average molecular weight of 10,000 to 10,000.
5. The magnetic recording medium according to claim 1, wherein the number is 30,000. 6. The magnetic recording medium according to claim 1, wherein the binder contains 70% by weight or less of a thermoplastic elastomer and / or resin. 7. The magnetic recording medium according to claim 1, wherein the copolymer is cross-linked by a cross-linking agent.