JPH0991675A - Magnetic recording media - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the dispersibility of magnetic powder by using polyurethane resin having nitro groups in each molecule as a resin binder forming a magnetic layer. SOLUTION: This magnetic recording medium has a substrate and a magnetic layer consisting essentially of magnetic powder and a resin binder formed on the substrate and polyurethane resin having nitro groups in each molecule is used as the resin binder. The polyurethane resin has been blocked with an oxime blocking agent and has nitro groups by 10-1,000 equiv. per 10<6> g polymer. The number of nitro groups per one molecule represented by number average mol.wt. (expressed in terms of plystyrene) in measurement by GPC is <=5 and the number average mol.wt. is <=40,000. The magnetic powder in the magnetic layer has improved dispersibility and this magnetic recording medium has increased output and surface smoothness.

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, and more particularly to a magnetic recording medium having an improved magnetic layer dispersibility.

[0002]

2. Description of the Related Art The magnetic layer of a magnetic recording medium is usually formed mainly of a binder resin and magnetic powder. Then, in recent years, various proposals have been made to improve the binder resin to improve the dispersibility of the magnetic powder to enhance the output and surface smoothness of the magnetic recording medium. For example, Japanese Examined Patent Publication No. 58-41565 has a metal sulfonate group of 10 to 1000 per polymer.
It is described that the squareness ratio of a magnetic layer in a magnetic recording medium can be improved by using a polyurethane resin having an equivalent weight / 10 ⁶ g as a binder resin. However, with the recent increase in recording density of magnetic recording media, there is an increasing demand for the provision of magnetic recording media in which the dispersibility of magnetic powder is further improved.

Therefore, an object of the present invention is to provide a magnetic recording medium in which the dispersibility of magnetic powder is further improved.

DISCLOSURE OF THE INVENTION As a result of intensive studies on the binder resin forming the magnetic layer, the present inventors have found that a polyurethane resin having a specific structure can achieve the above object.

The present invention has been made on the basis of the above findings, and a magnetic recording medium having a support and a magnetic layer mainly composed of a magnetic powder and a binder resin provided on the support, The above object is achieved by providing a magnetic recording medium characterized by using a polyurethane resin having a nitro group in the molecule as the binder resin.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The magnetic recording medium of the present invention will be described below in detail. The magnetic recording medium of the present invention is used in the form of, for example, a magnetic disk or a magnetic drum, and is particularly preferably used in the form of a magnetic tape such as an 8 mm video tape or a DAT tape.

The magnetic recording medium of the present invention is characterized in that a polyurethane resin having a nitro group in the molecule is used as the binder resin in the magnetic layer. The polyurethane resin can be used in the invention without particular limitation as long as it has a nitro group in the molecule.

The above polyurethane resin preferably has 10 to 1000 equivalents / 10 ⁶ g of nitro groups per polymer. Nitro groups per polymer 10 equivalent / 10
If the amount is less than ⁶ g, the dispersibility of the magnetic powder may decrease, and a high squareness ratio may not be obtained, and the surface smoothness may decrease. Further, when the nitro group per polymer exceeds 1000 equivalents / 10 ⁶ g, the viscosity of the magnetic paint increases and the fluidity decreases. As a result, the coatability may be lowered, and the surface property and the squareness ratio may be lowered, so that it is preferably within the above range. The nitro group per polymer is more preferably 10 to 300 equivalent / 10 ⁶ g.

The above polyurethane resin has a polystyrene-reduced number average molecular weight of 1 in GPC measurement.
It is also preferable that the number of nitro groups contained per molecule is 5 or less. If the number of nitro groups contained exceeds 5, the viscosity of the magnetic coating material may increase and the fluidity may decrease, and the greater the molecular weight of the polyurethane resin, the greater the effect. The number of nitro groups contained is more preferably 0.3 to 5, and even more preferably 0.5 to 3.

The polyurethane resin has a number average molecular weight of 40 in terms of polystyrene in GPC measurement.
It is also preferably 000 or less. When the number average molecular weight of the polyurethane resin exceeds 40,000, the dispersibility of the coating composition is reduced, the surface smoothness of the coating film is reduced, the output of the magnetic recording medium may be reduced, and the durability is also reduced. There is. The number average molecular weight is 1500 to 30,000.
Is more preferable, and it is further preferable that it is 3000-28,000.

In the present invention, a particularly preferred polyurethane resin has 10 to 1000 equivalents / 10 ⁶ g of nitro groups per polymer, and the number of nitro groups contained per molecule is 5 expressed by polystyrene-reduced number average molecular weight in GPC measurement. And the number average molecular weight is 4000 or less
It is a polyurethane resin of 0 or less.

The above polyurethane resin having a nitro group in the molecule is generally obtained by a urethanization reaction of a polyhydroxy compound and a polyisocyanate. Also,
An amine compound such as ethylenediamine or diisopropanolamine may be partially used with the polyhydroxy compound as the main component. The polyhydroxy compound and polyisocyanate will be described in detail. As the polyhydroxy compound, a polyester polyol, a polyether polyol, an acrylic polyol, a castor oil derivative, a tall oil derivative, glycol, or an OH group-containing compound thereof can be used. The hydroxyl value of the polyhydroxy compound is preferably 40 to 1900 from the viewpoint of exhibiting abrasion resistance, which is a characteristic of polyurethane resins.

On the other hand, as the above polyisocyanate,
2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, p-phenylene diisocyanate,
Diphenylmethane diisocyanate, m-phenylene diisocyanate, hexamethylene diisocyanate, tetramethylene diisocyanate, 3,3'-dimethoxy-4,4'-biphenylene diisocyanate, 2,4-
Naphthalene diisocyanate, 3,3'-dimethyl-
4,4'-biphenylene diisocyanate, 4,4'-
Diphenylene diisocyanate, 4,4'-diisocyanate-diphenyl ether, 1,5-naphthalene diisocyanate, p-xylylene diisocyanate, m-
Preferable examples include xylylene diisocyanate, 1,3-diisocyanate methylcyclohexane, 1,4-diisocyanate methylcyclohexane, 4,4′-diisocyanate dicyclohexane, 4,4′-diisocyanate dicyclohexylmethane, and isophorone diisocyanate.

Introduction of a nitro group into the polyurethane resin can be achieved by, for example, using a polyhydroxy compound having a nitro group in a part or all of the polyhydroxy compound and carrying out a urethanization reaction with the polyisocyanate. However, the method is not limited to this.

The polyhydroxy compound having a nitro group has an OH group at both ends of the molecule and has a OH group in the molecule (side chain).
And a compound having a nitro group. Specifically, a nitro group-containing polyester polyol obtained by an ester exchange reaction using a dicarboxylic acid diester having a nitro group in the molecule and a polyol can be preferably used.

The nitro group-containing polyester polyol will be described in detail. The nitro group-containing polyester polyol preferably comprises a carboxylic acid component having no nitro group, a glycol component and a dicarboxylic acid component having a nitro group.

The carboxylic acid component and the glycol component will be described in detail. As the carboxylic acid component,
Terephthalic acid, isophthalic acid, orthophthalic acid, 1,5
An aromatic dicarboxylic acid such as naphthalene dicarboxylic acid;
Aliphatic dicarboxylic acids such as succinic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, citraconic acid, 3,3 ′ thiodipropionic acid, itaconic anhydride, itaconic acid; 5-sodium sulfoisophthalic acid, 5
-Potassium sulfoisophthalic acid, 2-sodium sulfoterephthalic acid, 2-potassium sulfoterephthalic acid, 3,
9-bis (2-carboxyethyl) -2,4,8,10
Examples thereof include carboxylic acid compounds such as tetroxyspiro [5,5] undecane, and esterified products of the carboxylic acid compounds and lower alcohols such as methanol and ethanol. These can be used alone or as a mixture.

On the other hand, examples of the glycol component include ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol and diethylene glycol. , Dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, 1,4-cyclohexanedimethanol, ethylene oxide adduct of bisphenol A and propylene oxide adduct, hydrogenated bisphenol A, ethylene oxide addition of bisphenol A Compounds and propylene oxide adducts, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, etc., and they can be used alone or as a mixture when used.

Examples of the dicarboxylic acid component having a nitro group include, but are not limited to, 5-nitrodimethylisophthalic acid. Further, the copolymerization amount of the dicarboxylic acid component having the nitro group in the nitro group-containing polyester polyol is determined so that the content of the nitro group in the polyurethane resin becomes the predetermined amount. However, as a general range, the copolymerization amount is preferably 1 to 50 mol%, and more preferably 5 to 30 mol% with respect to the total carboxylic acid component.

The nitro group-containing polyester polyol may be used alone or in combination of two or more.

Further, in the present invention, it is also preferable that the polyurethane resin is a polyurethane resin blocked with an oxime-based blocking agent. Use of such a polyurethane resin is preferable because the dispersibility of the magnetic powder and the durability of the magnetic recording medium are further improved.
The oxime-based compound blocks an isocyanate group in the polyurethane resin, and examples of the oxime-based compound include formaldoxime, acetoaldoxime, acetoxime, methylethylketoxime, methylisopropylketoxime, and methylbutylketo. Examples include oxime, diethyl ketoxime, diacetyl monooxime, benzophenone oxime and cyclohexanone oxime. Of these, preferred oxime-based compounds include methylethylketoxime and cyclohexanone oxime. These oxime compounds can be used alone or in combination of two or more. The isocyanate group blocked by the oxime compound is unblocked and reacts under predetermined reaction conditions.

Next, the synthesis of the polyurethane resin will be described. The polyurethane resin can be obtained by a known method using the polyhydroxy compound and the polyisocyanate as main raw materials. For example, the polyhydroxy compound and the polyisocyanate in a solvent or without solvent in the presence or absence of a catalyst (for example, dibutyltin dilaurate) at 60 to 80 ° C.
It is obtained by reacting for 0 to 20 hours. At this time, the blending ratio of the polyhydroxy compound and the polyisocyanate is set to the NCO group of the polyisocyanate /
The OH group of the polyhydroxy compound is preferably 0.5 to 2 from the viewpoint of controlling the molecular weight.

When the polyurethane resin is a polyurethane resin blocked with an oxime blocking agent, the polyhydroxy compound and the polyisocyanate are reacted as described above, and then the oxime blocking agent is added.・ React. Such a reaction may be carried out in an organic solvent having no active hydrogen, or may be carried out without using a solvent. Preferably, the above oxime blocking agent is gradually added to the reaction system. The reaction temperature is generally about 50-100 ° C. The blocking of the isocyanate group by the oxime-based blocking agent can be measured by a method such as disappearance of the peak of the IR spectrum of the isocyanate group.

The number average molecular weight (Mn) in terms of polystyrene in GPC measurement of the polyurethane resin thus obtained is preferably 40,000 or less, more preferably 1,500 to 30,000.
More preferably, it is 3000-28000. The molecular weight distribution is preferably narrow, and specifically, the weight average molecular weight (Mw) / Mn <4 is preferable. When Mw / Mn is 4 or more, the dispersibility of the magnetic powder and the smoothness of the magnetic layer may be deteriorated, or the durability of the magnetic recording medium may be deteriorated. In particular, when Mw / Mn is 4 or more and Mn is more than 20000, adverse effects are likely to occur, so Mw / Mn <4 is preferable.

In the magnetic recording medium of the present invention, the above-mentioned polyurethane resin is used as the binder resin for forming the magnetic layer. The polyurethane resin is used in various magnetic layers usually used in magnetic recording media. It can also be used in combination with a binder resin. A preferable example of such a binder resin is a vinyl chloride copolymer. Such vinyl chloride copolymer is a copolymer obtained from vinyl chloride and other copolymerization components,
Those having a functional group such as a sulfone group or an epoxy group can be preferably used. Particularly preferable examples of the vinyl chloride copolymer include MR series manufactured by Nippon Zeon Co., Ltd. and the like.

The magnetic layer of the magnetic recording medium of the present invention is mainly composed of the above polyurethane resin (and preferably the above vinyl chloride copolymer) and magnetic powder. When the magnetic powder is described in detail, examples of the magnetic powder that can be used in the present invention include ferromagnetic iron oxide, ferromagnetic chromium dioxide, and ferromagnetic metal powder.

As the above-mentioned ferromagnetic iron oxide, FeOx
It is possible to use one in which a metal such as Cr, Mn, Co, or Ni is added to (1.33 ≦ x <1.5). As the ferromagnetic chromium dioxide, CrO ₂ or CrO ₂ containing metals such as Na, K, Fe and Mn, oxides of these metals or non-metal elements such as P can be used. The ferromagnetic metal powder has a metal content of 70% by weight or more and 80% by weight or more of at least one ferromagnetic metal (for example, Fe, Co, N).
The ferromagnetic metal powder of i) can be used. Specific examples of the ferromagnetic metal powder include Fe-Co and Fe-
Ni, Fe-Al, Fe-Ni-Al, Co-Ni, F
e-Co-Ni, Fe-Ni-Al-Zn, Fe-Al
—Si and the like.

The magnetic powder may contain, if necessary,
A rare earth element or a transition metal element can be contained. Further, as the magnetic powder, fine tabular barium ferrite and some of its Fe atoms are Ti, Co, Z.
Magnetic powders substituted with atoms such as n and V can also be used. The magnetic powder may be surface-treated in order to improve its dispersibility and the like.

The magnetic powder has a specific surface area (B
It is also preferable that the (measured by ET) is 40 m ² / g or more. It is preferable to use the magnetic powder having such a specific surface area value because the dispersion effect of the magnetic powder is further exhibited. Further, magnetic powder having a specific surface area of 40 m ² / g or less can be dispersed by the conventional formulation. The specific surface area value is 40 to 60 m
More preferably, it is ² / g.

In the magnetic layer of the magnetic recording medium of the present invention, in addition to the above polyurethane resin (and preferably the above vinyl chloride copolymer) and the above magnetic powder, various additives used in ordinary magnetic recording media are added. It is also possible to mix agents and the like. Examples of such additives include dispersants, abrasives,
Antistatic agent (carbon black, etc.), rust preventive agent, antifungal agent,
Examples of the additives include lubricants (fatty acids, fatty acid esters, etc.) and curing agents (isocyanate compounds, silane coupling agents, aluminum chelate compounds, amine compounds, etc.) used in ordinary magnetic recording media. Specifically, it is described in JP-A-57-162128, page 2, upper left column, line 6 to page 2, upper right column, line 10 and page 3, upper left column, line 6 to third page upper right column, line 18 and the like. Various additives may be mentioned.

The thickness of the magnetic layer containing the components as described above is not particularly limited, but the thickness as a general range is
The thickness is preferably 0.05 to 3 μm, more preferably 0.05 to 2.5 μm.

Next, the support on which the magnetic layer is formed will be described. As the support, a known magnetic or non-magnetic support can be used without particular limitation. Specifically, polyethylene terephthalate, polyethylene naphthalate, polyphenylene sulfide, polyaramid, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, aramid, a film formed from a known resin such as aromatic polyamide, glass, aluminum thin film, etc. Can be used. In the above support, before providing a magnetic layer, corona discharge treatment in advance,
Plasma treatment, easy adhesion treatment, heat treatment, dust removal treatment and the like can also be performed. The preferred thickness of the support is 1 to 3.
It is 00 μm.

As described above, the magnetic recording medium of the present invention has a support and a magnetic layer provided on the support. In addition to this, other layers such as the above support and the above support are also provided. It has a non-magnetic layer provided between the magnetic layer and another magnetic layer (such as a soft magnetic layer) and a back coat layer provided on the back side of the support (the side opposite to the side on which the magnetic layer is provided). May be. In the present invention, the polyurethane resin may be used in these layers. In particular,
A magnetic recording medium having a combination of (magnetic layer / magnetic layer), a magnetic recording medium having a combination of (magnetic layer / non-magnetic layer), and a magnetic recording medium having a back coat layer are coated with the above polyurethane resin. The polyurethane resin can be used or used for those layers in a magnetic recording medium having any combination thereof.

Next, a preferable method for manufacturing the magnetic recording medium of the present invention will be described. First, a magnetic paint is applied onto the support. The coating amount is such that the dry thickness of the magnetic layer becomes a predetermined thickness. When the non-magnetic layer is provided in addition to the magnetic layer, the dry thicknesses of the non-magnetic coating material and the magnetic coating material on the support become the predetermined thickness. As described above, simultaneous multi-layer coating is performed by the wet-on-wet method to form a coating film for the non-magnetic layer and the magnetic layer. Next, the magnetic powder in the coating film is subjected to a magnetic field orientation treatment, then dried and wound. After that, calendering is performed if necessary, and then a back coat layer is further formed if necessary. Then, if necessary, for example, when a magnetic tape is to be obtained, it is subjected to an aging treatment at 50 to 70 ° C. for 12 to 72 hours and then slit to a desired width.

To prepare the above magnetic paint, for example, the polyurethane resin, abrasive, carbon such as carbon black and the above magnetic powder are put into a Nauter mixer or the like together with a solvent and premixed to obtain a mixture. The obtained mixture is kneaded by a continuous pressure kneader or the like, then, the above solvent is added to dilute, and after dispersion treatment using a sand mill or the like, additives such as a lubricant are mixed and filtered, and the remaining A method of adding a solvent and mixing is used. The mixing ratio of the polyurethane resin in the magnetic paint is preferably 3 to 20 parts by weight with respect to 100 parts by weight of the magnetic powder. When the vinyl chloride copolymer is used in combination, the blending ratio of the vinyl chloride copolymer is preferably 3 to 15 parts by weight based on 100 parts by weight of the magnetic powder.

In particular, when the magnetic powder having a specific surface area of 40 m ² / g or more is used as the magnetic powder,
0.05 to 5 parts by weight of carbon black and 100 to 1 parts by weight of the magnetic powder, and the polyurethane resin 3 to 1
It is preferable to form the magnetic layer using a magnetic coating material containing 0 part by weight, from the viewpoint of dispersibility of the magnetic powder and output and durability of the magnetic recording medium.

Examples of the above-mentioned solvent for preparing the above-mentioned magnetic paint include a ketone solvent, an ester solvent, an ether solvent, an aromatic hydrocarbon solvent, and a chlorinated hydrocarbon solvent. Is mentioned. Specifically, JP-A-5
The solvents described in JP-A No. 7-162128, page 3, lower right column, line 17 to page 4, lower left column, line 10 can be used. The solvent is preferably blended in an amount of about 10 to 1200 parts by weight, particularly preferably 15 to 800 parts by weight, based on 100 parts by weight of the magnetic powder.

The simultaneous multilayer coating method for forming the magnetic layer is described in JP-A-5-73883, column 42, column 3.
1st line to column 43, 31st line. This coating method is a method of coating the magnetic coating material forming the magnetic layer before the nonmagnetic coating material forming the nonmagnetic layer is dried. According to such a coating method, the boundary surface between the non-magnetic layer and the magnetic layer is smoothed and the surface property of the magnetic layer is improved, so that there are few dropouts and high density recording is possible and the coating film ( A magnetic recording medium having excellent durability of the magnetic layer and the non-magnetic layer) can be obtained.

The magnetic field orientation treatment of the coating film after the application of the magnetic paint is performed before the magnetic paint is dried. For example, when the magnetic recording medium of the present invention is a magnetic tape, the magnetic paint is applied. Parallel to the plane, preferably about 5
00 Oe or more, more preferably about 1000 to 10,000
It can be carried out by a method of applying a magnetic field of Oe, a method of passing through a solenoid or the like of preferably 1000 to 10000 Oe before the magnetic paint is dried.

The drying treatment is carried out, for example, at 30 to 120 ° C.
It is possible to control the degree of drying of the coating film by controlling the temperature of the gas and the supply amount thereof.

The above calendering treatment can be carried out by a super calendering method in which a metal roll is passed through a cotton roll or a synthetic resin roll, or between a metal roll and the like. In addition, the conditions of the calendar treatment are 60 to 140 ° C. and 100 to 500 kg / c.
m.

In the production of the magnetic recording medium of the present invention, finishing steps such as polishing and cleaning of the magnetic layer surface may be carried out if necessary. Further, the application of the above-mentioned non-magnetic coating material can also be carried out by a known sequential multi-layer coating method.

[0042]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but it goes without saying that the present invention is not limited to such Examples.

[Production Example] Production of Polyester Polyol In a reaction vessel equipped with a thermometer, a stirrer and a partial reflux condenser, 438 parts of dimethyl terephthalate, 342 parts of ethylene glycol, 430 parts of neopentyl glycol, 0.5 part of zinc acetate, And 0.1 part of sodium acetate were added 14
The transesterification reaction was carried out at 0 to 220 ° C. for 3 hours. Then, add 330 parts of adipic acid to this reaction system and
After reacting at 20 ° C for 5 hours, the reaction system was allowed to react for 2 minutes over 2 minutes.
Reduce the pressure to 0 mmHg, and further 5 to 20 mmHg at 230 ℃ 1
Polycondensation reaction was carried out for an hour to obtain a polyester polyol (A). The obtained polyester polyol (A) had a hydroxyl value of 110. When the NMR analysis of this polyester polyol (A) was carried out, the composition was as follows.

Terephthalic acid; 50 mol%, adipic acid;
50 mol%, ethylene glycol; 54 mol%, neopentyl glycol; 46 mol%

Further, polyester polyols (B) to (D) shown in Table 1 were obtained by the same production method.

[0046]

[Table 1]

Polyurethane synthesis In a reaction vessel equipped with a thermometer, a stirrer and a cooler, 2160 g of methyl ethyl ketone, 675 g of diphenylmethane diisocyanate, 49 of polyester polyol (A).
2 g, 60 g of polyester polyol (C) and 0.1 g of dibutyltin dilaurate were added, and the mixture was heated at 70 to 80 ° C. for 5 minutes.
A time reaction was performed. In addition, 1,4 butanediol 99
g and neopentyl glycol 114 g,
The reaction was carried out at 80 ° C. for 20 hours to obtain polyurethane resin (I). The number average molecular weight (Mn) of the polyurethane resin (I) obtained by GPC measurement was 24,000. Table 2 shows the nitro group content in this polyurethane resin (I).

Further, the polyurethane resins (II) to (V) shown in Table 2 were obtained by the same manufacturing method.

[0049]

[Table 2]

Example 1 Methyl ethyl ketone, toluene and cyclohexanone were added to the composition shown below.
A mixed solvent mixed in a ratio of 1: 1 was added, the solid content concentration was adjusted to 35%, the mixture was diluted and mixed, and then dispersed using a sand mill. Furthermore, polyisocyanate compound ["Coronate L" trade name, Nippon Polyurethane Co., Ltd.
4 parts) was added to obtain a magnetic paint. The obtained magnetic paint was coated with polyethylene terephthalate (PE
T) It was applied by gravure coating on the film so that the dry film thickness was 3 μm, and magnetic field orientation (4000 Oe) was performed.
After drying, the surface was smoothed to form a magnetic layer. Then, a back coat paint was applied and the obtained film was cut into a width of 3.8 mm to prepare a sample of a magnetic tape as a magnetic recording medium.

(Compound) Magnetic powder (ferromagnetic metal magnetic powder, BET value 45 m ² /
g); 100 parts by weight; carbon black (average primary particle size 50 nm); 0.1
Parts by weight Alumina (particle size 100 to 300 nm); 5 parts by weight Vinyl chloride copolymer (trade name MR110, manufactured by Zeon Corporation); 10 parts by weight Polyurethane resin (I) shown in [Table 2]; 10 parts by weight Parts-myristic acid; 1 part by weight-n-butyl stearate; 1 part by weight

Examples 2 to 4 and Comparative Example 1 Instead of the polyurethane resin (I) in Example 1, polyurethane resins (II) to (V) shown in Table 2 (respectively from Examples 2 to 2)
4 and corresponding to Comparative Example 1), respectively.
A magnetic tape sample was prepared in the same manner as in Example 1.

<Performance Evaluation> In order to evaluate the performance of the magnetic tapes obtained in Examples 1 to 4 and Comparative Example 1, the smoothness and squareness ratio (Sq) of the magnetic layers of these magnetic tapes were measured. Table 3 shows the results.

[0054]

[Table 3]

As is clear from the results shown in Table 3, the magnetic recording media of the present invention obtained in Examples 1 to 4 had a high Sq value of the magnetic layer and had good smoothness. On the other hand, in the magnetic recording medium (Comparative Example 1) produced by using the polyurethane resin having no nitro group, the high S
No q value was obtained, and the smoothness was poor.

[0056]

As described above in detail, according to the magnetic recording medium of the present invention, by using a polyurethane resin having a nitro group in the molecule as a binder resin for forming a magnetic layer, a magnetic powder of The dispersibility is improved, and the output and surface smoothness of the magnetic recording medium are improved.

Claims (4)

[Claims] 1. A magnetic recording medium comprising a support and a magnetic layer mainly composed of a magnetic powder and a binder resin provided on the support, wherein the binder resin comprises a nitro group in the molecule. A magnetic recording medium characterized by using the polyurethane resin having. 2. The polyurethane resin has 10 to 1000 equivalents / 10 ⁶ g of nitro groups per polymer, and GP
The number of nitro groups contained per molecule represented by polystyrene equivalent number average molecular weight in C measurement is 5 or less,
The magnetic recording medium according to claim 1, which has a number average molecular weight of 40,000 or less. 3. The polyurethane resin is a polyurethane resin blocked with an oxime-based blocking agent,
The magnetic recording medium according to claim 1. 4. The magnetic layer comprises 0.05-5 parts by weight of carbon black and 100 parts by weight of magnetic powder having a specific surface area of 40 m ² / g or more, and the polyurethane resin 3-1.
2. Formed from a magnetic paint containing 0 parts by weight.
4. The magnetic recording medium according to any one of claims 1 to 3.