JPH1139641A - Magnetic recording medium and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium with further improved magnetic characteristics and traveling durability, especially electromagnetic conversion characteristics, and to provide its producing method. SOLUTION: This magnetic recording medium is produced by forming a magnetic layer containing a ferromagnetic powder and a binder on a nonmagnetic supporting body. The ferromagnetic powder is a hexagonal ferrite powder having <0.3 μm average particle size and having the average particle size twice as large as the thickness. The powder is uniformly dispersed in the magnetic layer. The medium is produced by kneading the hexagonal ferrite powder above described, vinyl chloride copolymers of 100 to 220 average polymn. degree having sulfonate groups or ammonium salt groups, hydroxyl groups and epoxy groups with a solvent to obtain a primary dispersion, adding a nonmagnetic hard inorg. powder, a binder and a solvent thereto to obtain a secondary dispersion, and then applying the obtd. magnetic coating material on a nonmagnetic supporting body.

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 a method for manufacturing the same, and more particularly, to a magnetic recording medium.
The present invention relates to a magnetic recording medium with further improved characteristics such as magnetic characteristics, running durability, and particularly electromagnetic conversion characteristics, and a method for manufacturing the same.

[0002]

2. Description of the Related Art In recent years, magnetic recording media have been used for audio equipment, video equipment, computers, and the like.
Its demand is growing significantly. This magnetic recording medium generally has a structure in which a magnetic layer composed of a ferromagnetic powder and a binder is provided on a non-magnetic support such as a polyester film, and the magnetic layer is usually formed of a binder. Is formed by dispersing a ferromagnetic powder together with a medium in a solvent containing, and applying or transferring the obtained magnetic paint to a non-magnetic support.

Conventionally, as binders, for example, polyester resins, cellulose resins, polyurethane resins, vinyl chloride resins, vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinyl acetate-vinyl alcohol copolymers, chloride resins Organic materials such as vinyl-vinylidene chloride copolymer, acrylonitrile-butadiene copolymer, phenolic resin, epoxy resin, polyamide resin, polyvinyl butyral, nitrocellulose, cellulose acetate butyrate, acrylic resin, and electron beam curable resin High molecular compounds are used.

[0004] These binders have advantages and disadvantages, and since it is difficult to obtain a magnetic layer having desirable properties when used alone, two or more binders are usually used in combination. For example, a relatively hard resin such as a vinyl chloride resin, polyvinyl butyral, and nitrocellulose, and a polyester resin, a polyurethane resin, and an acrylonitrile-butadiene copolymer are often used in combination with a soft resin. In many cases, a polyisocyanate compound is used as a curable component for the purpose of improving the durability of the magnetic layer.

In recent years, the recording density of magnetic recording media has been improved,
With the demand for improvement in the S / N ratio, ferromagnetic powders have become finer, have higher magnetic force, and have higher coercive force. Crystal oriented hexagonal ferrite powder such as barium ferrite (Ba)
Fe ₁₂ O ₁₉ ) powder attracts attention as both a perpendicular recording method and an in-plane recording method as a high-density recording method, and has been studied as a ferromagnetic powder of a coating type magnetic recording medium. In particular, with the improvement in recording density, powders having a small average particle size and a large ratio between the particle size and the thickness have been demanded and developed. However, when the above-mentioned barium ferrite is made into fine particles, dispersibility in a binder is deteriorated.

Conventionally, as a method for improving the dispersibility of ferromagnetic powder, an average degree of polymerization of 300 or more (GP
The weight average molecular weight in terms of styrene by C measurement is 22000.
The above method of using a high molecular weight binder or a method of using a dispersant such as a low molecular weight surfactant has been adopted. In the former case, however, the dispersibility is reduced when the ferromagnetic powder becomes finer. However, in the latter method, when a large amount of the dispersant is used, undesirable situations such as durability and head contamination of the magnetic recording medium are caused. Dispersibility cannot be obtained. Therefore, the binder itself is required to have a high level of dispersing performance from the viewpoint of improving the reliability of the magnetic recording medium.

On the other hand, in order to increase the durability and reliability of a magnetic recording medium, it is necessary to incorporate a curable compound such as a polyisocyanate compound into a magnetic paint and cross-link the magnetic layer. In the field. In this case, a binder containing a hydroxyl group having appropriate reactivity with a crosslinking agent such as a polyisocyanate compound is used. Such binders include, for example, SO ₃ M, SO ₄ M, PO ₄
Vinyl chloride resins containing hydrophilic groups such as salt-type strong acid groups containing sulfur or phosphorus such as M ₂ and PO ₃ M ₂ (where M represents an alkali metal or ammonium group) are widely used. The binder is excellent in the dispersion performance and dispersion stability of the ferromagnetic powder, and has high strength and the like based on the strength of the intermolecular force.

In the conventional magnetic paint production process, a primary dispersion process of dispersing ferromagnetic powder in a binder together with a medium by a shearing force between a stirring blade and a gap between a blade and a container; A secondary dispersion step of moving the medium to further disperse the ferromagnetic powder by the impact force between the media. These dispersion processes become more important as dispersion becomes more difficult. However, even if the above-mentioned good binder is selected and the kneading step is performed in the conventional manner, the finer the particle size of the ferromagnetic powder for high-density recording, the more difficult the dispersion becomes. This means that the conventional average degree of polymerization is 300
Due to the large molecular size of the above polymer binder, the number of binders adsorbed per ferromagnetic powder is reduced, and the wettability of the binder to the ferromagnetic fine particles is deteriorated. This is because high shearing force during kneading based on an increase in the solid content causes a large load on the machine.

[0009]

Accordingly, an object of the present invention is to provide a coating type magnetic recording medium in which finely divided ferromagnetic fine powder, particularly hexagonal ferrite powder, is well dispersed in a binder. Developed a magnetic coating material, whereby a hexagonal ferrite powder was uniformly dispersed in the magnetic layer, and a magnetic recording medium and a method of manufacturing the magnetic recording medium in which the magnetic characteristics, running durability, and especially the characteristics such as electromagnetic conversion characteristics were further improved. To provide.

[0010]

Means for Solving the Problems As a result of intensive studies, the present inventors have solved the above problems. That is, the present invention provides a magnetic recording medium comprising a nonmagnetic support provided with a magnetic layer containing a ferromagnetic powder and a binder, wherein the ferromagnetic powder has an average particle size of 0.3 μm or less, and Is a hexagonal ferrite powder having a thickness of twice or more, and the hexagonal ferrite powder is evenly dispersed in the magnetic layer. is there. The present invention also provides an average particle size of 0.3
μm or less, and a hexagonal ferrite powder having an average particle diameter of at least twice the thickness, and a vinyl chloride having a sulfonate group or an ammonium group, a hydroxyl group and an epoxy group, and having an average degree of polymerization of 100 to 220. And a primary copolymer kneaded with a solvent and primary-dispersed. Subsequently, an optional non-magnetic hard inorganic powder, a binder and a solvent are added to the primary dispersion and secondary-dispersed to obtain a magnetic coating material. Is applied to a non-magnetic support, and a method for manufacturing a magnetic recording medium is provided.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The ferromagnetic powder used in the present invention is a hexagonal ferrite powder having an average particle diameter of 0.3 μm or less and having an average particle diameter of twice or more the thickness.
In the present invention, since the average particle diameter is very small and the ratio of the average particle diameter to the thickness is very large, good magnetic properties, running durability, and especially electromagnetic conversion properties can be obtained. Ferrite having conventionally known various metal ions can be appropriately selected and used. The “average particle size” in the present invention means the average of the longest axis (longest size) of the plane of the crystal.

[0012] The magnetic recording medium of the present invention is characterized in that fine hexagonal ferrite powder having a specific size or less is uniformly dispersed in a magnetic layer. This can be achieved by applying a specific dispersion method using an agent. The dispersion method includes a primary dispersion and a secondary dispersion, and the primary dispersion is to knead the hexagonal ferrite powder with a specific binder and a solvent, and the secondary dispersion is obtained. It is to add and knead any non-magnetic hard inorganic powder, binder and solvent to the primary dispersion.

In the present invention, the specific binder used in the primary dispersion includes a sulfonate group or an ammonium group, a hydroxyl group and an epoxy group and has an average degree of polymerization of 10%.
It is a vinyl chloride copolymer in the range of 0 to 220. If the average degree of polymerization is less than 100, a sufficient polymer cannot be obtained even if a curing agent is mixed therein, which impairs durability such as level reduction.
Conversely, if it exceeds 220, the wettability of the hexagonal ferrite powder is poor, resulting in poor dispersibility. In the present invention, the average degree of polymerization refers to a solution of a resin dissolved in JIS K672.
The value refers to a value obtained by measuring the specific viscosity of a solution according to 1 and converting it to a JIS specific viscosity using nitrobenzene.

The vinyl chloride copolymer used in the present invention has a sulfonic acid group structure as a polar group in an amount of 0.3 to 3.0% by weight of SO 4 or an ammonium group as an N 2.
(Nitrogen) as 0.1 to 0.5% by weight,
0.8% by weight, 1.0 to 10.0% by weight of epoxy group
It is desirable to contain. When the content of the sulfonate group is less than 0.3% by weight or the content of the ammonium group is less than 0.1% by weight, the dispersibility of the ferromagnetic powder is poor, and the content of the sulfonate group is 3.
0% by weight or ammonium base
If the amount is more than 10% by weight, the solubility of the vinyl chloride resin in the solvent becomes poor and the practicality is lacking. On the other hand, if the amount of the hydroxyl group is less than 0.1% by weight, the crosslinking property is poor, and if it exceeds 0.8% by weight, the viscosity of the magnetic paint is too high, the application becomes poor and the practicality is lacking. If the amount of the epoxy group is less than 1.0% by weight, dehydrochlorination tends to occur from vinyl chloride, which may cause a problem of rust on the magnetic coating film. If the amount exceeds 10.0% by weight, the viscosity of the magnetic coating material is too high. Coating is poor and lacks practicality.

The vinyl chloride copolymer used in the present invention may be a vinyl chloride monomer, a copolymer monomer containing a sulfonic acid or an ammonium group, an epoxy group-containing monomer, a hydroxyl group-containing monomer and, if necessary, other copolymers. It can be obtained by copolymerizing the united monomer by a conventional method. The solvent used in the present invention is not particularly limited, and any solvent used in the production of conventional magnetic recording media can be used. For example, tetrahydrofuran (THF), methyl ethyl ketone (MEK), toluene, cyclohexanone and the like can be mentioned.

In the primary dispersion, the above-mentioned hexagonal ferrite powder, a vinyl chloride copolymer and a solvent are kneaded. This kneading is desirably performed such that the weight ratio of the hexagonal ferrite powder to the vinyl chloride copolymer (hexagonal ferrite powder / vinyl chloride copolymer) is in the range of 6 to 15. . This kneading has one feature in that non-magnetic hard inorganic powder used in the secondary dispersion described below is not mixed. Furthermore, the solid content in the primary dispersion step is 75 to 90 according to the calculation method of the following equation.
It is good to adjust to weight%. By adopting one or more of such conditions, more favorable dispersion of hexagonal ferrite powder can be obtained.

[0017]

## EQU2 ## solid content (% by weight) = ((total weight of primary dispersion-weight of solvent) / total weight of primary dispersion) .times.100

As the kneading machine used in the primary dispersion step, any conventionally known kneading machine such as a continuous twin-screw kneader, a continuous twin-screw kneader capable of multi-stage dilution, a co-kneader, a pressure kneader and the like is limited. Not be used. When the kneaded solid content in the primary dispersion step is 75% by weight or more,
The shear force received from the kneader increases, and the dispersing effect becomes particularly high. If the kneading solids content exceeds 90% by weight, the shearing force is too high and the kneading machine may be damaged. When the weight ratio between the hexagonal ferrite powder and the vinyl chloride-based copolymer is 6 or more, the shearing force received from the kneader increases, and the dispersing effect becomes particularly high. If the weight ratio exceeds 15, the shearing force is too high and the kneader may be damaged. Further, by not mixing the non-magnetic hard inorganic powder in the primary dispersion step, the shear force received from the kneader increases, and the dispersion effect is particularly enhanced.

In the secondary dispersion step, an arbitrary non-magnetic hard inorganic powder, a binder and a solvent are added to the obtained primary dispersion and kneaded. The conditions for the secondary dispersion can be the same as those in the conventional process, and are not particularly limited.

For example, as a dispersing machine, any of a vertical sand mill, a horizontal sand mill, a spike mill, a ball mill, a tower mill, a pal mill, a DCP and the like can be used. As the dispersion medium, any of steel balls and ceramic beads can be used. As the nonmagnetic hard inorganic powder, a known abrasive such as alumina or chromium oxide, carbon black, or the like can be used. Other optional binders include, for example, polyester resin, cellulose resin, polyurethane resin, vinyl chloride resin, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride- Organic polymers such as vinylidene chloride copolymer, acrylonitrile-butadiene copolymer, phenolic resin, epoxy resin, polyamide resin, polyvinyl butyral, nitrocellulose, cellulose acetate butyrate, acrylic resin, and electron beam curable resin And the like. Examples of the solvent include the above-described solvents and the like, and are not particularly limited.

If the vinyl chloride copolymer used as a binder in the primary dispersion step has a low softening point, it may be softened by friction due to running or frictional heat due to contact with the head depending on the application. In some cases, blocking may occur while the tape is wound, or other resins may be added as long as the dispersibility of the ferromagnetic powder is not impaired.
A compound that crosslinks with the vinyl chloride copolymer may be mixed. The mixing amount is preferably 0.05 to 0.5 times the weight of the vinyl chloride copolymer, but this amount is not limited at all. Examples of the crosslinking compound include, for example, epoxy resin, isocyanate resin, melamine resin,
In addition to polyol resins, compounds such as triazine thiol and dibutyl tin dilaurate can also be used.
Examples of other resins include a polyurethane resin, a polyester resin, a cellulosic resin, an acrylic resin, and an acetal resin.

The magnetic recording medium of the present invention may contain a plasticizer such as dibutyl phthalate and triphenyl phosphate, dioctyl sulfo sodium succinate, t-butyl phenol polyethylene ether, sodium ethyl naphthalene sulfonate, and dilauryl succinate, if necessary. Hydrocarbons such as carboxylate, metal stearate, and stearic acid esters; silicon-based lubricants such as silicone oil; fluorine-based lubricants such as perfluoropolyether and perfluorocarboxylic acid; and carbon black. Can be added.

The material of the non-magnetic support used in the present invention is not particularly limited. For example, polyesters such as polyethylene terephthalate, polyolefins such as polyethylene and polypropylene, cellulose triacetate, cellulose diacetate, cellulose acetate butyrate Such as cellulose derivatives, polyvinyl chloride,
Vinyl resins such as polyvinylidene chloride, plastics such as polyethylene naphthalate, polyaramid, polycarbonate, polyimide, polyamide, polyamideimide, paper, metals such as aluminum and steel, light metals such as aluminum alloys and titanium alloys, ceramics, and single crystal silicon And the like. The form of these non-magnetic supports may be any of films, tapes, sheets, disks, cards, drums and the like. When a rigid substrate such as an Al alloy plate or a glass plate is used for the non-magnetic support, an oxide film such as alumite treatment or a Ni-PP film is formed on the substrate surface and the surface is formed. You may make it hard.

The magnetic recording medium of the present invention may be provided with a top coat layer or a back coat made of a rust inhibitor or the like, if necessary.

[0025]

[Function] For the purpose of high-density recording of a magnetic recording medium using hexagonal ferrite powder, especially when a hexagonal ferrite powder having a small average particle size and a large ratio of particle size to thickness is adopted, it is combined with this. The wettability with the agent deteriorates, and the dispersibility deteriorates. In the case of a conventional hexagonal ferrite powder having an average particle diameter of more than 0.3 μm, the dispersibility is good without using a special binder, but the average particle diameter is 0.3 μm as in the present invention. When the amount is below the above, no matter how the kneading is performed, the cohesive force of the powder is strong and the powder is not uniformly dispersed. In the present invention, a good dispersion of hexagonal ferrite powder having an average particle diameter of 0.3 μm or less can be achieved by using a specific binder and applying a specific dispersion method. High-density recording on a magnetic recording medium has become possible while maintaining the electromagnetic conversion characteristics.

[0026]

EXAMPLES The present invention will be further described below with reference to examples, but the present invention is not limited to these examples.

Example 1 As a ferromagnetic powder, a barium ferrite powder having an average particle diameter of 0.2 μm and a particle diameter twice or more the thickness was used. As a binder used at the time of kneading, a vinyl chloride copolymer shown in A of Table 1 was used. First, 100 parts by weight of ferromagnetic powder, a vinyl chloride-based copolymer, and a solvent ((methyl ethyl ketone: cyclohexanone: toluene = 1: 1: 1)) were extruded (Kurimoto Iron Works, Ltd.) so as to satisfy the condition of a in Table 2. Then, a polyester polyurethane having a molecular weight of 30,000 containing a sulfonate group was added as a binder in the secondary dispersion step so that the total binder was 20 parts by weight, and alumina (slurry) was used as an abrasive. 3 parts by weight), diluted with the above-mentioned solvent to a solid content of 35%, and dispersed using a sand mill for 5 hours. 4 parts by weight, 100 parts by weight of ferromagnetic powder, 1 part by weight of myristic acid as a lubricant, stearyl By adding an acid butyl 1 part by weight 1
The mixture was stirred for 0 minutes to obtain a magnetic paint. The magnetic paint thus obtained was applied on a polyethylene terephthalate support having a thickness of 10 μm so that the thickness after drying became 2 μm, and after orientation in a magnetic field, dried and wound up. This was subjected to a calendering treatment and a curing treatment, and a back coat was applied so that the thickness after drying became 0.5 μm, and then cut into 8 mm width to prepare a sample tape.

(Examples 2 to 3 and Comparative Examples 1 and 2) The binders used in the primary dispersion step are shown in Table 1 from A to Table B (Example 2), Table 1 C (Example 3), Sample tapes were prepared under the same conditions as in Example 1 except that D in Table 1 (Comparative Example 1) and E in Table 1 (Comparative Example 2) were used. That is, Examples 2 to 3 and Comparative Examples 1 and 2 are intended to evaluate the characteristics of a magnetic recording medium depending on the change in the average degree of polymerization of a vinyl chloride-based strong polymer as a binder.

(Comparative Examples 3 to 5) The binders used in the primary dispersion step were changed from A in Table 1 to F in Table 1 (Comparative Example 3) and G in Table 1.
(Comparative Example 4) A sample tape was prepared under the same conditions as in Example 1 except that H was changed to H (Comparative Example 5) in Table 1.
That is, Comparative Examples 3 to 5 are intended to evaluate the dependence of the functional group of the binder.

(Examples 4 to 6) The binder used in the secondary dispersion step was A in Table 1 (Example 4), and a vinyl chloride copolymer containing a sulfonate group and having a polymerization degree of 300 (Example 5) ) And a polybutyral containing a sulfonate group and having a polymerization degree of 300 (Example 6), and a sample tape was prepared under the same conditions as in Example 1 except for the above. That is, Examples 4 to 6 are intended to show the grounds that the binder used in the secondary dispersion step is not particularly limited in the present invention.

(Examples 7 to 10 and Comparative Examples 6 to 11)
The kneading conditions were as follows: b in Table 2 (Example 7), c in Table 2 (Example 8), d in Table 2 (Example 9), and e in Table 2 (Example 1).
0), f in Table 2 (Comparative Example 6), g in Table 2 (Comparative Example 7),
Sample tapes were prepared under the same conditions as in Example 1 except that h in Table 2 (Comparative Example 8), i in Table 2 (Comparative Example 9), and k in Table 2 (Comparative Example 11) were used. In these examples and comparative examples, the solid content in the primary dispersion step and the ratio between the hexagonal ferrite powder and the binder are examined. In Comparative Examples 10 and 11, kneading conditions were changed to j and k in Table 2, respectively, and non-magnetic hard inorganic powder was added in the primary dispersion step, and this was not added in the secondary dispersion step. Is a sample tape prepared in the same manner as in Example 1. Comparative Examples 10 and 11 are intended to evaluate the presence or absence of non-magnetic hard inorganic powder in the primary dispersion step.

(Examples 11 to 13 and Comparative Examples 12 to 1)
3) The binders used in the primary dispersion step are shown in Table 1 from A to Table 1.
I (Example 11), J in Table 1 (Example 12), K in Table 1 (Example 13), L in Table 1 (Comparative Example 12), M in Table 1
(Comparative Example 13) A sample tape was prepared under the same conditions as in Example 1 except for the above. That is, Examples 11 to
13 and Comparative Examples 12 to 13 are intended to evaluate the characteristics of the magnetic recording medium by changing the average degree of polymerization by changing the polar group of the vinyl chloride copolymer as the binder from a sulfonic acid group to an ammonium group. is there.

(Examples 14 to 17 and Comparative Examples 14 to 1)
7) The kneading conditions are shown in Table 2 as b (Example 14), Table 2 as c (Example 15), Table 2 as d (Example 16), Table 2 as e (Example 17), and Table 2 as f (Example 17). Comparative Example 14), g in Table 2 (Comparative Example 15), h in Table 2 (Comparative Example 16), i in Table 2 (Comparative Example 1)
7), and the polar group of the vinyl chloride copolymer as the binder is changed from a sulfonate group to an ammonium group,
Otherwise, a sample tape was prepared under the same conditions as in Example 1. In these Examples and Comparative Examples, the solid content and the ratio of the hexagonal ferrite powder to the binder in the primary dispersion step when the binder contains an ammonium base are studied.

(Comparative Examples 18 to 19) As a hexagonal ferrite powder, the average particle size was 0.4 μm, and the ratio of the particle size to the thickness was 2:
1.1 barium ferrite was used, and the binder used in the primary dispersion step was changed to A in Table 1 (Comparative Example 18) and B in Table 1 (Comparative Example 19), and otherwise the same as in Example 1. A sample tape was made under the conditions. This is to evaluate the case where the ferromagnetic powder has a conventional size and is relatively large.

[0035]

[Table 1]

[0036]

[Table 2]

In the evaluation of the sample tape obtained in the above example, the magnetostatic property, the electromagnetic conversion property, and the running durability were used.
Measurement of static magnetic properties is performed using a vibrating sample magnetometer (Toei Kogyo)
Was used. The electromagnetic conversion characteristics are EXABYTE data 8mm
The tape was run with a tape drive EXB-8505XL, and the signal output generated in the magnetic head was measured. The results are shown for an output component of 8.5 MHz. The running durability was measured by measuring the output drop after running the tape 1000 times with EXABYTE EXB-8505XL data 8 mm tape drive. The measurement results are shown in Tables 3 and 4.

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

As is apparent from the above results, according to the present invention, an ultrafine hexagonal ferrite powder having an average particle size of 0.3 μm or less and having an average particle size of not less than twice the thickness. Can be uniformly dispersed in the magnetic layer,
The porosity is reduced, and high orientation is possible. Therefore, according to the present invention, there is provided a magnetic recording medium further improved in characteristics such as magnetic characteristics, running durability, and especially electromagnetic conversion characteristics, and a method for manufacturing the same.

Claims (4)

[Claims] 1. A magnetic recording medium comprising a nonmagnetic support provided with a magnetic layer containing a ferromagnetic powder and a binder, wherein the ferromagnetic powder has an average particle size of 0.3 μm or less, and Is a hexagonal ferrite powder having a thickness of at least twice the thickness, and wherein the hexagonal ferrite powder is uniformly dispersed in the magnetic layer. 2. An average polymerization having a hexagonal ferrite powder having an average particle size of 0.3 μm or less and having an average particle size of at least twice the thickness, and having a sulfonate group or an ammonium group, a hydroxyl group and an epoxy group. A vinyl chloride-based copolymer having a degree in the range of 100 to 220 is kneaded with a solvent and primary-dispersed. Subsequently, an optional non-magnetic hard inorganic powder, a binder and a solvent are added to the primary dispersion. A method for producing a magnetic recording medium, comprising applying a magnetic dispersion obtained by secondary dispersion to a non-magnetic support. 3. The weight ratio of the hexagonal ferrite powder and the vinyl chloride copolymer (hexagonal ferrite powder / vinyl chloride copolymer) in the primary dispersion step is in the range of 6 to 15. 3. The method for manufacturing a magnetic recording medium according to claim 2, wherein: 4. The method for producing a magnetic recording medium according to claim 2, wherein the solid content in the primary dispersion step is adjusted to 75 to 90% by weight by a calculation method of the following equation. ## EQU1 ## Solid content (% by weight) = ((total weight of primary dispersion−weight of solvent) / total weight of primary dispersion) × 100