JPH0773452A - Magnetic recording medium - Google Patents

Abstract

(57) [Summary] [Structure] A first magnetic layer containing cobalt-containing iron oxide powder is formed on a substrate, and a coercive force of 130 is formed on the first magnetic layer.
Cobalt-coated iron oxide powder having a coercive force of 0 (Oe) or more and larger than that of the cobalt-containing iron oxide of the first magnetic layer and an average major axis length of 0.3 μm or less, and a ferromagnetic metal powder mainly containing iron A magnetic recording medium on which a second magnetic layer containing is formed. [Effect] Electromagnetic conversion characteristics such as output and noise, corrosion resistance,
Both physical properties such as runnability are improved in a well-balanced manner.

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 having a two-layer magnetic layer structure, and more particularly to a magnetic recording medium excellent in electromagnetic conversion characteristics and durability.

[0002]

2. Description of the Related Art A coating type magnetic recording medium is obtained by coating a magnetic coating material obtained by dispersing magnetic powder in a binder and an organic solvent on a substrate such as polyester and drying it. There is an increasing demand for higher density recording such as high quality video tapes and large capacity floppy disks.

Ferromagnetic metal powders composed mainly of iron are most often used as magnetic powders for achieving this purpose. Ferromagnetic metal powder composed mainly of iron has high saturation magnetization and coercive force, and has excellent output characteristics. However, since it is chemically unstable and easily rusts (is easily oxidized), the saturation magnetization is lowered by oxidation, and the noise characteristic is not always sufficient, and generally the noise level is higher than that of the oxide magnetic powder. Further, the ferromagnetic metal powder mainly composed of iron has a high cost and has some drawbacks in practical use.

A perpendicular magnetic recording method has been proposed as a method for realizing high-density recording, and hexagonal plate-shaped barium ferrite is used as a magnetic powder particularly suitable for this method. However, since barium ferrite has a low saturation magnetization, when used as a magnetic tape or a magnetic disk, the residual magnetic flux density becomes low and the output in the low range becomes insufficient. Further, when barium ferrite is used, there is a problem that the surface roughness of the formed magnetic layer becomes large.

In order to solve such a problem, it has been attempted to improve the output characteristics by making the single-layer magnetic layer, which has been the mainstream in the past, a double-layer structure. For example, in JP-A-62-78718, an oxide having a coercive force equal to or higher than the coercive force of the metal magnetic powder in the lower layer is used by using a metal magnetic powder (iron, cobalt, nickel, etc. or an alloy thereof) in the lower layer. A magnetic recording medium has been disclosed in which magnetic powder (γ-Fe ₂ O ₃ , Fe ₃ O _4, etc.) is used as an upper layer and the magnetic layer has a two-layer structure. However, the magnetic recording medium described in this publication has an insufficient output in a high frequency region, and its S / N is low, so that the noise characteristic is also insufficient.

In addition to this, as a magnetic recording medium having a magnetic layer having a two-layer structure, Japanese Examined Patent Publication No. 63-56608, Japanese Examined Patent Publication No. 3-6575, Japanese Examined Patent Publication No. 3-53689, and Japanese Examined Publication No. 3-
No. 53690 and the like, but these are intended to improve either the magnetic properties or the corrosion resistance, and it has not been achieved to improve both of them.

[0007]

As described above, in a magnetic recording medium intended for high density recording, it is still insufficient to improve both magnetic characteristics and physical characteristics such as running property in a well-balanced manner. However, the problem to be solved by the present invention is to further improve high frequency characteristics and S / N in a magnetic recording medium that enables high density recording, to improve running performance by lowering the friction coefficient, and It is to eliminate the head clogging caused by the above, and to reduce the decrease of the saturation magnetic flux density during storage more than the ferromagnetic metal powder mainly composed of iron.

[0008]

As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention have determined that the magnetic layer has a two-layer structure of upper and lower layers, a specific magnetic powder is used for the lower magnetic layer, and the coercive force is 1
Magnetic recording excellent in various characteristics by using cobalt-adhered iron oxide powder having an average major axis length of 0.3 μm or less and ferromagnetic metal powder mainly composed of iron in the upper magnetic layer of 300 (Oe) or more. The inventors have found that a medium can be obtained and completed the present invention.

That is, according to the present invention, a base material, a first magnetic layer formed on the base material and comprising a cobalt-containing iron oxide powder and a binder, a coercive force of 1300 (Oe) or more, and the cobalt-containing material. Cobalt-coated iron oxide powder having a coercive force equal to or greater than that of iron oxide and an average major axis length of 0.3 μm or less,
Consists of a ferromagnetic metal powder mainly composed of iron and a binder,
A magnetic recording medium having a second magnetic layer formed on the first magnetic layer (hereinafter abbreviated as magnetic recording medium (I)) is provided.

The present invention further comprises a base material, a first magnetic layer formed on the base material, the first magnetic layer comprising a ferromagnetic metal powder mainly containing iron and a binder, and the first magnetic layer formed on the first magnetic layer. Coercive force is 1
Magnetic recording medium having cobalt-deposited iron oxide powder having an average major axis length of not more than 300 (Oe) and not more than 0.3 μm, ferromagnetic metal powder containing iron as a main component, and a second magnetic layer containing a binder. (Hereinafter, abbreviated as magnetic recording medium (II)).

Further, the present invention comprises a base material, at least two kinds of magnetic powders selected from a ferromagnetic metal powder mainly composed of iron, a cobalt-containing iron oxide powder and a barium ferrite powder, and a binder. A first magnetic layer formed on the above, and a ferromagnetic metal powder mainly composed of iron and having a coercive force of 1300 (Oe) or more and a coercive force greater than or equal to the coercive force of the ferromagnetic metal powder mainly composed of iron. And the coercive force is 1300
(Oe) or more and a coercive force not less than the coercive force of the cobalt-containing iron oxide and having an average major axis length of 0.3 μm or less, and a cobalt-adhered iron oxide powder, and a binder, The present invention provides a magnetic recording medium having a second magnetic layer formed on one magnetic layer (hereinafter abbreviated as magnetic recording medium (III)).

The magnetic recording media (I) to (II of the present invention will be described below.
I) will be explained.

<Magnetic Recording Medium (I)> The magnetic recording medium (I) of the present invention has upper and lower two magnetic layers, and the lower magnetic layer is a cobalt-containing iron oxide powder (hereinafter abbreviated as Co—Fe). Do)
And an upper magnetic layer having a coercive force of 1300 (Oe) or more and an average major axis length of 0.3 μm or less, and a cobalt-coated iron oxide powder, and a ferromagnetic metal powder mainly composed of iron (hereinafter, metal powder). Abbreviated).

First, the Co--Fe used in the first magnetic layer of the magnetic recording medium (I) will be described. In the present invention,
Cobalt-containing iron oxides include Co surface-forming cobalt-deposited iron oxides, as well as Co-doped iron oxides containing Co inside,
It contains an adsorption type cobalt-containing iron oxide, cobalt ferrite, or the like, and any of these can be used for the first magnetic layer. Especially in the first magnetic layer,
Co-deposited Fe ₃ O ₄ , Co-deposited Fe ₂ O ₃ , Co-deposited FeO _x (1.34
≦ x <1.50), or coated with cobalt ferrite
Fe ₃ O ₄ , Fe ₂ O ₃ and FeO _x are preferably used.

Further, the cobalt-containing iron oxide used in the first magnetic layer of the magnetic recording medium of the present invention is produced by a conventional method of depositing cobalt with the acicular iron oxide as a nucleus. The magnetic characteristics can be adjusted to predetermined values by controlling the amount of cobalt added and the amount of Fe ²⁺ in the manufacturing conditions. The amount of cobalt added is usually 1 to 20% by weight (based on the raw iron oxide),
It is preferably 2 to 15% by weight, and the amount of Fe ²⁺ is usually 0.1.
It is 5 to 30% by weight (ratio in the total cobalt-containing iron oxide), preferably 1 to 25% by weight.

The coercive force of Co--Fe used for the first magnetic layer is not particularly limited and may be appropriately determined according to the desired performance of the magnetic recording medium, but is generally about 700 to 1800 (Oe). is there.

The particle size of Co--Fe is not particularly limited, but the average major axis length is generally 0.1 to 1.0 .mu.m, and the average minor axis length is generally 0.01 to 0.1 .mu.m. The shape of the particles is not limited, but needle-like particles are preferable.

The first magnetic layer of the magnetic recording medium (I) is Co
It is formed by applying a magnetic coating material containing -Fe and a binder as a main component on a substrate, and the thickness of the first magnetic layer is 1 to 5 µm, preferably 2 to 4 µm.

Next, the second magnetic layer of the magnetic recording medium (I) will be described. In the second magnetic layer of the magnetic recording medium (I) of the present invention, the coercive force is 1300 (Oe) or more, and the coercive force is larger than that of Co—Fe used for the first magnetic layer, and the average major axis length is Two kinds of magnetic powders of cobalt-coated iron oxide powder of 0.3 μm or less and metal powder are used.

The second magnetic layer of the magnetic recording medium (I) of the present invention has a coercive force of 1300 (Oe) or more, preferably 130.
Cobalt-coated iron oxide of 0-2000 (Oe) is used. As the cobalt-coated iron oxide powder of the second magnetic layer, one having a coercive force larger than that of Co—Fe used for the first magnetic layer is used. When the coercive force of the cobalt-adhered iron oxide used for the second magnetic layer is less than 1300 (Oe), high output cannot be achieved. Specifically, as cobalt-deposited iron oxide, Co-deposited Fe ₃ O ₄ , Co-deposited Fe ₂ O ₃ , Co-deposited Fe
O _x (1.34 ≦ x <1.50) is used, especially Co deposition
FeO _x is preferred. Cobalt-doped iron oxide is not preferable because it has poor storage stability. The shape of the cobalt-coated iron oxide used in the second magnetic layer is not limited, but needle-shaped ones are preferably used. The average major axis length of the cobalt-coated iron oxide is 0.3 μm or less, and the axial ratio is 3
It is preferably about 20.

The cobalt-adhered iron oxide used in the second magnetic layer of the magnetic recording medium of the present invention is also produced by the ordinary cobalt-adhesion method using acicular iron oxide as the nucleus, as described above.
The magnetic characteristics can be adjusted to predetermined values by controlling the amount of cobalt added and the amount of Fe ²⁺ in the manufacturing conditions. The amount of cobalt added is usually 1 to 20% by weight (based on iron oxide as a raw material), preferably 2 to 15% by weight, and the amount of Fe ²⁺ is usually 0.5 to 30% by weight (cobalt deposition oxidation). The proportion of iron as a whole), preferably 1 to 25% by weight.

The metal powder used in the second magnetic layer of the magnetic recording medium of the present invention may be a conventionally known one mainly composed of iron, such as iron powder or an alloy powder of iron and cobalt, nickel or the like, In addition to these, aluminum, chromium, manganese, silicon, zinc, rare earth metal elements, lanthanoids,
Examples thereof include metal powders containing a transition metal element such as an actinide. Although the remanent magnetization and coercive force of the metal powder are not limited, for example, the coercive force is generally about 800 to 1900 (Oe). The particle size of the metal powder is also the same and is not particularly limited, but the average major axis length is generally 0.1 to 1.0 μm, and the average minor axis length is generally 0.01 to 0.1 μm. Also, the shape of the particles is not limited.

The weight ratio of the cobalt-coated iron oxide powder used for the second magnetic layer to the metal powder is [cobalt-coated iron oxide powder]: [metal powder] = 10: 90 to 80:20. preferable.

The second magnetic layer is formed by coating the above-described magnetic coating mainly composed of cobalt-coated iron oxide powder, metal powder and binder on the first magnetic layer. The thickness of the magnetic layer is preferably 0.1 to 3 μm.

In the present invention, the method of forming (coating) the magnetic layers may be either a method of simultaneously forming the first magnetic layer and the second magnetic layer or a method of sequentially forming one layer at a time, and one layer at a time. In this case, calendar processing may be performed for each layer.

The magnetic coating material for forming the magnetic layer of the magnetic recording medium of the present invention uses the above-mentioned magnetic powder for the first and second magnetic layers, and other components such as binder and organic solvent are common. Can be used.

As the binder used in the present invention, urethane resin, particularly polyurethane containing polar groups such as sulfonic acid group, sulfonic acid metal base, sulfobetaine group, carbobetaine group, amino group, hydroxyl group, epoxy group and the like. Resins, vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinylidene chloride copolymers, vinyl chloride-acrylonitrile copolymers, and other vinyl chloride-based copolymers, particularly sulfonic acid groups, sulfonic acid metal bases, amino Chloride copolymers containing polar groups such as groups, butadiene-acrylonitrile copolymers, polyamide resins, polyvinyl butyral, cellulose derivatives (cellulose acetate butyrate, cellulose propionate, nitrocellulose, etc.), styrene-butadiene copolymer Coalesced, polyester resin, various synthetic rubber, Nol resin, epoxy resin, urea resin, melamine resin, phenoxy resin, silicone resin, acrylic reaction resin, mixture of high molecular weight polyester resin and isocyanate prepolymer, mixture of polyester polyol and polyisocyanate, urea formaldehyde resin, low molecular weight glycol / Examples include high molecular weight diol / isocyanate mixtures, and mixtures thereof, and similar materials can be used for the first magnetic layer and the second magnetic layer. Usually, the binder is 3.0-
About 10.0% by weight is blended.

As the organic solvent, cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate, benzene, toluene, xylene, dimethyl sulfoxide, tetrahydrofuran, dioxane and the like are suitable solvents for dissolving the binder resin to be used. There is no particular limitation and they may be used alone or in combination of two or more. Usually, the organic solvent is 20 to 80% by weight in the magnetic paint.
The degree is mixed.

In the magnetic coating composition, various commonly used additives such as dispersants, abrasives and lubricants may be appropriately added and used. As the dispersant, lecithin,
Nonionic surfactants, anionic surfactants, cationic surfactants and the like can be used. As an abrasive, α-
Fine powders having an average particle size of 0.05 to 1 μm, such as alumina, fused alumina, chromium oxide (Cr ₂ O ₃ ), iron oxide, silicon carbide, corundum, and diamond can be used, and usually 100 parts by weight of the binder as described above. 0.5 to 100 parts by weight is added. Examples of lubricants include silicone oils such as various polysiloxanes, inorganic powders such as graphite and molybdenum disulfide, fine plastic powders such as polyethylene and polytetrafluoroethylene, higher fatty acids, higher alcohols, higher fatty acid esters, and fluorocarbons. Is 0.1 to 100 parts by weight of the above-mentioned binder.
It is added in a proportion of 50 parts by weight.

The substrate used in the magnetic recording medium of the present invention includes synthetic resins (for example, polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyamides, polyolefins, cellulose derivatives), non-magnetic metals, glass, ceramics and papers. Etc., and the form thereof is used in films, tapes, sheets, cards, disks and the like.

<Magnetic Recording Medium (II)> The magnetic recording medium (II) of the present invention has upper and lower two magnetic layers, the lower first magnetic layer contains metal powder, and the upper second magnetic layer. The layer contains a cobalt-deposited iron oxide powder having a coercive force of 1300 (Oe) or more and an average major axis length of 0.3 μm or less, and a ferromagnetic metal powder mainly containing iron.

The metal powder used in the first magnetic layer in the magnetic recording medium (II) may be the conventionally known one as described in the magnetic recording medium (I) above. The thickness and the like are similar to those of the magnetic recording medium (I).

In the magnetic recording medium (II), the cobalt-coated iron oxide powder and the metal powder used in the second magnetic layer are the same as those described in the magnetic recording medium (I). The thickness and the like are similar to those of the magnetic recording medium (I).

In the magnetic recording medium (II), the coercive force of the metal powder used in the first magnetic layer and the metal powder used in the second magnetic layer is larger in the metal powder used in the second magnetic layer. desirable.

Also in the magnetic recording medium (II), the weight ratio of the cobalt-coated iron oxide powder used in the second magnetic layer to the metal powder is [cobalt-coated iron oxide powder]: [metal powder] =
The range of 10:90 to 80:20 is preferable.

<Magnetic Recording Medium (III)> The magnetic recording medium (III) of the present invention has upper and lower magnetic layers, and the first lower magnetic layer is made of metal powder, Co--Fe and barium ferrite powder. Mainly iron, containing at least two kinds of magnetic powders selected, the second magnetic layer of the upper layer is a cobalt-coated iron oxide powder having a coercive force of 1300 (Oe) or more and an average major axis length of 0.3 μm or less. Containing a ferromagnetic metal powder.

The first magnetic recording medium (III) of the present invention is described below.
The magnetic powder used in the magnetic layer will be described.

Metal Powder The metal powder used in the first magnetic layer of the magnetic recording medium (III) of the present invention is not particularly limited, and may be the magnetic recording medium (I) described above.
Conventionally known ones as described above can be used. However, it is necessary to use one having a coercive force smaller than that of the metal powder used for the second magnetic layer.

Co-Fe Used for the first magnetic layer of the magnetic recording medium (III) of the present invention.
Co-Fe is not particularly limited, and the conventionally known one as described in the magnetic recording medium (I) can be used. However,
It is necessary to use a material having a coercive force smaller than that of the cobalt-coated iron oxide used for the second magnetic layer.

Barium ferrite powder (hereinafter abbreviated as Ba) Used in the first magnetic layer of the magnetic recording medium (III) of the present invention.
A hexagonal plate-shaped particle is preferably used as Ba, and the particle diameter (diagonal length) is not particularly limited, but is 0.0
2 to 1.0 μm is general, and the thickness is 0.001
It is about 0.1 μm. The coercive force of Ba is not particularly limited and may be appropriately determined according to the desired performance of the magnetic recording medium, but is generally about 700 to 2000 (Oe).

The magnetic powder used in the first magnetic layer of the magnetic recording medium (III) may be selected in any combination,
It is possible to use all three types, and the weight ratio of the magnetic powders is arbitrary, but each magnetic powder used in the first magnetic layer is in the total amount of all magnetic powders used in the first magnetic layer. 1
It preferably accounts for 0% by weight or more.

When Co--Fe and metal powders are used among the above magnetic powders, the reduction of the saturation magnetic flux density during storage is reduced even when the metal powders are used alone. Further, since the surface properties of both the first and second magnetic layers are improved, one layer of metal powder or
S / N is improved as compared with the case where Ba alone is used. Further, the cost is reduced and the economical efficiency is improved. In this case, the weight ratio of Co-Fe and metal powder is Co-Fe: metal powder = 10: 90-9.
The range of 0:10 is preferable.

When Co--Fe and Ba are used, the surface property of the first magnetic layer is better than when Ba is used alone.
The surface property of the second magnetic layer is also improved, and the S / N is improved.
When Ba and Co-Fe are used, low-frequency output is improved by selecting Co-Fe having a larger saturation magnetization than Ba. In addition, since the decrease in the saturation magnetization amount is smaller than that when the metal powder is used alone, the low frequency output is stable for a long period of time. In this case, the weight ratio of Co-Fe and Ba is Co-Fe: Ba.
= 10: 90 to 90:10 is preferable.

If metal powder and Ba are used, Ba
Since the surface property of the first magnetic layer is better than that in the case of using only, the surface property of the second magnetic layer is also improved and the S / N is improved. When using metal powder and Ba, by selecting a metal powder having a larger saturation magnetization than Ba,
The low range output is improved. Further, the corrosion resistance is improved as compared with the case where the metal powder is used alone. In this case, the metal powder and Ba
The weight ratio of is preferably in the range of metal powder: Ba = 10: 90 to 90:10.

It is also possible to use all three kinds of Ba, Co--Fe and metal powder, and in this case, a magnetic recording medium which is more excellent in output in addition to the above effects can be obtained. When all three types of magnetic powders are used, it is preferable to use each magnetic powder in an amount of at least 10% by weight based on the total amount of the magnetic powders. When the amount of each magnetic powder is less than 10% by weight, for example, the output is reduced when the metal powder is less than 10% by weight, and S / when the content of Co-Fe or Ba is less than 10% by weight.
N gets worse.

For the first magnetic layer, at least two kinds of magnetic powders are selected from metal powders, Co--Fe and Ba, and the respective magnetic powders are preferably blended in the above weight ratio, and these are combined with a binder. It is formed by coating a base material with a magnetic paint containing as a main component. The weight ratio of the magnetic powder is not limited. The thickness of the first magnetic layer is 1 to 5 μm, preferably 2 to 4 μm.

For the first magnetic layer, at least two kinds of magnetic powders are selected from metal powders, Co--Fe and Ba, and the respective magnetic powders are preferably blended in the above-mentioned weight ratio, and these are combined with a binder. It is formed by coating a base material with a magnetic paint containing as a main component. The weight ratio of the magnetic powder is not limited. The thickness of the first magnetic layer is 1 to 5 μm, preferably 2 to 4 μm.

In the magnetic recording medium (III), the cobalt-coated iron oxide powder and the metal powder used in the second magnetic layer are the same as those described in the magnetic recording medium (I). The thickness and the like are similar to those of the magnetic recording medium (I). Also in the magnetic recording medium (III), the weight ratio of the cobalt-coated iron oxide powder used in the second magnetic layer to the metal powder is [cobalt-coated iron oxide powder]: [metal powder] = 10: 90-
The range of 80:20 is preferable.

[0049]

According to the present invention, high density recording is possible,
A magnetic recording medium having a well-balanced improvement in electromagnetic conversion characteristics and physical characteristics can be obtained. In particular, the magnetic recording medium of the present invention is
S / N is improved, friction coefficient is low, running property is good, and head clogging is small. In addition, the magnetic recording medium of the present invention has a smaller decrease in the saturation magnetic flux density and a lower manufacturing cost than the magnetic recording medium using only metal powder.

[0050]

The present invention will be further described in the following examples, but the present invention is not limited to these examples.

Examples 1 to 5 and Comparative Examples 1 to 4 The following components were dispersed in a sand mill to prepare a magnetic coating material for the first magnetic layer or a magnetic coating material for the second magnetic layer. In addition,
The magnetic powders shown in Table 1 are used, and the first magnetic layer and the second magnetic layer are used.
The magnetic powder used for the magnetic layer is shown in Table 2. <Magnetic paint component> • Magnetic powder (A to D, see Table 1) 100 parts by weight • Vinyl chloride resin (containing -SO ₃ Na group) 10 · F parts by weight · Polyurethane resin (containing -SO ₃ Na group) 5 · F part by weight ・ Coronate L 3 parts by weight [hardening agent manufactured by Nippon Polyurethane Industry Co., Ltd., added before coating] ・ Butyl stearate 2 parts by weight ・ Stearic acid 2 parts by weight ・ Carbon black (average particle size 0.02 μm) 3 parts by weight Alumina (average particle size 0.3 μm) 7 parts by weight Methyl ethyl ketone 80 parts by weight Toluene 80 parts by weight Cyclohexanone 120 parts by weight

In the above formulation, "F" of vinyl chloride resin and polyurethane resin is a value calculated by the following formula. F = (A / 45 × v / 100) + (B / 55 × w / 10
0) A: Specific surface area of metal powder shown in Table 1 (m ² / g) B: Specific surface area of Co—Fe shown in Table 1 (m ² / g) v: Weight ratio of metal powder shown in Table 2 w: The weight ratio of Co-Fe shown in Table 2 is v + w = 100.

[0053]

[Table 1]

Note) Co-Fe: FeO _x (1.34 ≦ x <1.
5) ... FeO _x is used as a raw material, and predetermined properties are obtained by controlling the amount of cobalt and the amount of Fe ²⁺ . Metal powder: Metal powder containing 95% by weight of iron and 5% by weight of nickel Particle size: Mean major axis length.

[0055]

[Table 2]

The thickness after drying with a gravure roll is 2.5.
The magnetic coating material for the first magnetic layer was applied onto a polyethylene terephthalate (PET) film having a thickness of 7 μm so as to have a thickness of μm, magnetic field orientation (8000 G) was performed, and then the first coating was dried.
A magnetic layer was formed. Then, the magnetic coating material for the second magnetic layer is applied on the first magnetic layer so that the thickness after drying becomes 0.5 μm, the magnetic field is oriented (8000 G), and then dried to form the second magnetic layer. , After calendaring (80 ℃), 50
Aged at 24 ° C. for 24 hours.

Subsequently, the coating material for the back coat layer prepared by mixing the following components in a sand mill was applied to the surface of the film opposite to the surface provided with the magnetic layer so as to have a dry thickness of 0.5 μm, Then, it aged at 50 degreeC for 24 hours. <Backcoat layer coating composition> Carbon black (average particle size 0.02 μm) 32 parts by weight Carbon black (average particle size 0.06 μm) 8 parts by weight Polyurethane resin 20 parts by weight (Nippon Polyurethane Co., Ltd., Nipolan 2301) ・ Nitrocellulose 20 parts by weight (viscosity indication by Hercules Powder CO. Is 1/2 second) ・ Polyisocyanate 4 parts by weight [Coronate HX, a curing agent manufactured by Nippon Polyurethane Industry Co., Ltd.] ・ Stearic acid 1 Parts by weight: 320 parts by weight of solvent.

As described above, the film on which the magnetic layer and the back coat layer were formed was cut into a tape having a width of 8 mm, loaded into an 8 mm cassette case, and the recording time was 8/60 minutes.
mm video tape was produced. The obtained 8 mm video tape was evaluated for friction coefficient, reduction rate of saturated magnetic flux density (ΔBs), S / N, and head clogging by the following methods. The results are shown in Table 3.

(1) Coefficient of Friction A tape running tester manufactured by Kyowa Tech Co., Ltd. was used, and the friction body was measured using a guide pin made of stainless steel for VTR having an outer diameter of 5 mm. (2) Saturation magnetic flux density reduction rate (ΔBs) 8mm video cassette 14 at 60 ° C, 90% RH
The decrease rate of the saturation magnetic flux density after storage for a day [ΔBs, the decrease amount is shown as a ratio (%) to the initial value. ] Was measured with a vibrating magnetometer. (3) The S / N 8 mm video cassette was set in a commercially available 8 mm VTR device to which a noise meter was connected, and the luminance S / N was measured by recording and reproducing a 50% white signal and measuring with a noise meter. The chroma S / N (AM, PM) was also measured according to this. (4) Clogged Head After running the 8 mm video cassette for 120 minutes and repeating the running test for 30 times, the magnetic head and the microscope were observed with a microscope to determine the dirt attached to the head as follows. ◯: Almost no stain Δ: Slight stain x: Severe stain

[0060]

[Table 3]

Examples 6 to 9 and Comparative Examples 5 to 8 The following components were dispersed in a sand mill to prepare a magnetic coating material for the first magnetic layer or a magnetic coating material for the second magnetic layer. In addition,
The magnetic powder used in Examples 1 to 5 (see Table 1) was used, and the magnetic powders used in the first magnetic layer and the second magnetic layer are as shown in Table 4. <Magnetic paint components> Magnetic powder (to D, see Table 1) 100 parts by weight Vinyl chloride resin (-SO ₃ Na group-containing) 10-F parts by weight Polyurethane resin (-SO ₃ Na group-containing) 5 - F parts by weight (F has the same meaning as above.) ・ Coronate L 3 parts by weight [Curing agent from Nippon Polyurethane Industry Co., Ltd., added before coating] ・ Butyl stearate 2 parts by weight ・ Stearic acid 2 parts by weight Parts-Carbon black (average particle size 0.02 μm) 3 parts by weight-Alumina (average particle size 0.3 μm) 7 parts-methyl ethyl ketone 80 parts-toluene 80 parts-cyclohexanone 120 parts

[0062]

[Table 4]

Then, the first magnetic layer, the second magnetic layer and the back coat layer were formed in the same manner as in Examples 1 to 5 to prepare an 8 mm video tape. The same evaluation as described above was performed on the obtained 8 mm video tape. The results are shown in Table 5.

[0064]

[Table 5]

Examples 10 to 19 and Comparative Examples 9 to 15 The following components were dispersed in a sand mill to prepare a magnetic coating material for the first magnetic layer or a magnetic coating material for the second magnetic layer. In addition,
The magnetic powders shown in Table 6 are used, and the first magnetic layer and the second magnetic layer are used.
Table 7 shows the magnetic powder used in the magnetic layer. <Magnetic paint components> Magnetic powder (to h, see Table 6) 100 parts by weight Vinyl chloride resin (-SO ₃ Na group-containing) 10 - F 'parts by weight Polyurethane resin (-SO ₃ Na group-containing) 5 -F 'parts by weight-Coronate L 3 parts by weight [hardening agent manufactured by Nippon Polyurethane Industry Co., Ltd., added before coating] -butyl stearate 2 parts by weight-stearic acid 2 parts by weight-carbon black (average particle size of 0. 02 μm) 3 parts by weight Alumina (average particle size 0.3 μm) 7 parts by weight Methyl ethyl ketone 80 parts by weight Toluene 80 parts by weight Cyclohexanone 120 parts by weight In addition, in the above formulation, “F ′” of vinyl chloride resin and polyurethane resin is used. Is a value calculated by the following formula.

[0066]

[Equation 1]

[0067]

[Table 6]

Note) Co-Fe: FeO _x (1.34 ≦ x <1.
5) ... FeO _x is used as a raw material, and predetermined properties are obtained by controlling the amount of cobalt and the amount of Fe ²⁺ . Metal powder: Metal powder containing 95% by weight of iron and 5% by weight of nickel Particle size: Mean major axis length. Shape ratio: * (a to f) are needle-like ratios, ** (g, h) are plate-like ratios

[0069]

[Table 7]

Then, the first magnetic layer, the second magnetic layer and the back coat layer were formed in the same manner as in Examples 1 to 5 to prepare an 8 mm video tape. The same evaluation as described above was performed on the obtained 8 mm video tape. The results are shown in Table 8.

Table 8 also shows the magnetostatic characteristics (coercive force and residual magnetic flux density) of the first magnetic layer and the second magnetic layer, which are obtained by applying each magnetic coating alone onto a PET film. (These are the dry film thickness of 2.5 μm), and the values are measured for the magnetic layer calendered after drying.

[0072]

[Table 8]

Claims (6)

[Claims] 1. A base material, a first magnetic layer formed on the base material, which comprises a cobalt-containing iron oxide powder and a binder, and a coercive force of 1300 (Oe) or more and the cobalt-containing iron oxide. Has a coercive force equal to or greater than the coercive force, and the average major axis length is 0.3μ
A magnetic recording medium comprising a cobalt-coated iron oxide powder having a size of m or less, a ferromagnetic metal powder containing iron as a main component, and a binder, and a second magnetic layer formed on the first magnetic layer. 2. The weight ratio of the cobalt-coated iron oxide powder used in the second magnetic layer to the iron-based ferromagnetic metal powder is [cobalt-coated iron oxide powder]: [iron-based ferromagnetic metal]. Powder] = 10: 90 to 80:20 The magnetic recording medium according to claim 1. 3. A base material, a first magnetic layer formed on the base material, comprising a ferromagnetic metal powder mainly composed of iron and a binder, and a coercive force formed on the first magnetic layer. Is 1300 (O
e) A magnetic recording medium having a cobalt-deposited iron oxide powder having an average major axis length of 0.3 μm or less, a ferromagnetic metal powder containing iron as a main component, and a second magnetic layer containing a binder. 4. The weight ratio of the cobalt-coated iron oxide powder used for the second magnetic layer to the iron-based ferromagnetic metal powder is [cobalt-coated iron oxide powder]: [iron-based ferromagnetic metal]. Powder] = 10: 90 to 80:20 The magnetic recording medium according to claim 3. 5. A base material, a ferromagnetic metal powder containing iron as a main component, at least two kinds of magnetic powders selected from cobalt-containing iron oxide powder and barium ferrite powder, and a binder, and formed on the base material. The first magnetic layer, and a ferromagnetic metal powder mainly composed of iron and having a coercive force of 1300 (Oe) or more and a coercive force not less than the coercive force of the ferromagnetic metal powder mainly composed of iron. Magnetic force is 1300 (O
e) or more and a coercive force not less than the coercive force of the above-mentioned cobalt-containing iron oxide and having an average major axis length of 0.3 μm or less, and a cobalt-adhered iron oxide powder, and a binder.
A magnetic recording medium having a second magnetic layer formed on the magnetic layer. 6. The weight ratio of the cobalt-coated iron oxide powder used in the second magnetic layer to the iron-based ferromagnetic metal powder is [cobalt-coated iron oxide powder]: [iron-based ferromagnetic metal]. Powder] = 10: 90 to 80:20 The magnetic recording medium according to claim 5.