JPH11224417A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress noise to a low level and to obtain high electromagnetic conversion characteristics by forming an upper magnetic layer of specified thickness, forming a lower nonmagnetic layer comprising a nonmagnetic powder having a specified specific surface area and a carbon black, specifying the DBP oil absorption of the carbon black, and specifying the surface roughness. SOLUTION: The upper magnetic layer 3 is formed in 0.05 to 0.5 μm thickness. The lower nonmagnetic layer contains a nonmagnetic powder having 30 to 80 m<2> /g specific surface area and a carbon black having 100 to 400 m<2> /g specific surface area and 20 to 200 ml/100 g DBP oil absorption. The surface roughness of the medium is controlled to 3.5 to 3.9 nm. The upper magnetic layer 3 is formed by applying a magnetic coating material comprising a metal magnetic powder, a binder and an org. solvent. The metal magnetic material consists of metals or alloys such as Fe, Co and Ni, and is controlled to have 0.05 to 0.2 μm major axial length and 100 to 250 ÅX-ray particle size. The lower nonmagnetic layer 2 is formed by applying a nonmagnetic coating material comprising a nonmagnetlc powder, a binder and an org. solvent, and the nonmagnetic powder contains a carbon black.

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 multi-layer coated magnetic recording medium having an extremely thin magnetic layer of 0.05 to 0.5 .mu.m.

[0002]

2. Description of the Related Art As a magnetic recording medium, a magnetic layer formed by dispersing a ferromagnetic powder, a binder, and various additives together with an organic solvent onto a non-magnetic support is dried to form a magnetic layer. A so-called coating type magnetic recording medium is known.

[0003] The coating type magnetic recording medium is widely used as a magnetic tape for audio or video, a data cartridge for backup, a floppy disk and the like. Such coating-type magnetic recording media are required to have higher recording density in order to cope with recent shortening of recording wavelength and digitization of recording method, and have more excellent electromagnetic conversion characteristics. Is required.

[0004] As a technique for improving the electromagnetic conversion characteristics of a magnetic recording medium, there is a method of first reducing the thickness of a magnetic layer. By reducing the thickness of the magnetic layer, self-demagnetization loss during recording is reduced,
Electromagnetic conversion characteristics are improved.

However, when a thin magnetic layer of, for example, 0.05 to 0.5 μm is provided directly on a nonmagnetic support,
The surface shape of the non-magnetic support is likely to appear on the surface of the magnetic layer, making it difficult to smooth the surface of the magnetic layer. For this reason, in the case where the magnetic layer is thinned, a multi-layer coating type configuration in which a non-magnetic coating layer is interposed between the non-magnetic support and the magnetic layer is often adopted. With this non-magnetic layer interposed,
The thickness is increased between the surface of the nonmagnetic support and the surface of the magnetic layer, and the surface shape of the nonmagnetic support is less likely to appear on the surface of the magnetic layer. Therefore, a thin magnetic layer is formed with a smooth surface shape.

As a method of forming such two coating layers on a non-magnetic support, a die head having two slits through which a non-magnetic paint and a magnetic paint are extruded, respectively, is used. A simultaneous multilayer coating method (wet-on-wet coating method) in which a nonmagnetic paint and a magnetic paint are simultaneously applied has been proposed.
In this simultaneous multilayer coating method, a coating film having a uniform thickness with few coating defects and coating streaks can be formed. Therefore, a medium with excellent electromagnetic conversion characteristics and low noise can be obtained. Further, the formed lower layer and upper layer have high adhesion, and excellent durability can be obtained.

The electromagnetic conversion characteristics of a magnetic recording medium can be improved by reducing the thickness of the magnetic layer as described above. However, the reduction of spacing loss by smoothing the magnetic layer surface and the improvement of metal magnetic powder also improve the electromagnetic conversion characteristics. It is effective as a method to perform.

In particular, in high-density recording, since the recording wavelength used is short, it is easily affected by the roughness of the surface of the magnetic layer, and control of this surface roughness is particularly important.

As a method of smoothing the surface of the magnetic layer, the dispersibility of the powder component in the magnetic layer is improved, or when the calendering is performed as a surface smoothing treatment, the treatment temperature is increased, or a steel calender is used. Generally, a technique such as using only a roll is adopted. In addition, attempts have been made to use a binder having a low glass transition point for the purpose of improving the moldability during calendering. However, in this method, when the glass transition point of the binder is lowered, the running durability of the magnetic layer is impaired, so that there is an adverse effect on practical characteristics. For this reason, at present, there is a tendency to give priority to running durability and use a binder having a high glass transition point.

In order to improve the characteristics in the short wavelength region, it is effective to improve the ferromagnetic powder.

[0011] Improvements in ferromagnetic powder include (1) use of ferromagnetic alloy powder, (2) miniaturization of ferromagnetic powder, (3) increase of coercive force of ferromagnetic powder and uniform distribution of coercive force. Is mentioned.

As for (1) and (2), as a result of aggressive improvement of magnetic materials, ferromagnetic powders having a saturation magnetization of more than 140 Am ² / kg and a major axis length of 0.1 μm or less are now available. Have been developed.

Regarding the improvement in coercive force of (3), a ferromagnetic powder having a coercive force exceeding 160 kA / m has also been found, and the particle size distribution is reflected in the coercive force distribution of the ferromagnetic powder. However, the coercive force distribution is extremely uniform due to the uniformity of the particle size distribution.

[0014]

However, the improvement of these magnetic powders has been made on the assumption that the magnetic layer has a relatively large thickness, and is applied to a magnetic layer having a thickness of 0.5 μm or less as it is. However, the output characteristics cannot be sufficiently improved. Therefore, in order to obtain a synergistic effect by reducing the thickness of the magnetic layer and improving the magnetic powder, it is necessary to further study the magnetic powder.

Therefore, the present invention has been proposed in view of such a conventional situation, and has a magnetic layer having a thickness of 0.05.
It is an object of the present invention to provide a magnetic recording medium in which noise is kept low even when the thickness is set to 0.5 μm and high electromagnetic conversion characteristics are obtained.

[0016]

The inventors of the present invention have made intensive studies to achieve the above-mentioned object, and as a result, have a thickness of 0.0
By defining the specific surface area and the like of each of the nonmagnetic powder and carbon black contained in the lower nonmagnetic layer for the magnetic layer of 5 to 0.5 μm, the output characteristics of the magnetic recording medium in the short wavelength region are improved. That led to the finding.

The magnetic recording medium of the present invention has been completed on the basis of such findings, and is provided on a non-magnetic support.
After forming a lower non-magnetic layer by applying a non-magnetic paint comprising a non-magnetic powder, a binder and an organic solvent, while the lower non-magnetic layer is in an undried state, the metallic magnetic powder, the binder and the organic In a magnetic recording medium in which an upper magnetic layer is formed by applying a magnetic paint comprising a solvent, the upper magnetic layer has an average thickness of 0.05 μm to 0.5 μm, and the lower nonmagnetic layer comprises a nonmagnetic powder. And carbon black, wherein the specific surface area of the nonmagnetic powder is 30 to 80 m ² / g
And the specific surface area of the carbon black is 100 to 4
00m is a ² / g, DBP absorption of the carbon black is 20 to 200/100 g, in which the surface roughness is characterized in that it is a 3.5～3.9Nm.

In the magnetic recording medium according to the present invention configured as described above, the specific surface area of the nonmagnetic powder contained in the lower nonmagnetic layer, the specific surface area of the carbon black contained in the lower nonmagnetic layer, and the DBP oil absorption are determined. By defining and defining the surface roughness, the average thickness is 0.05 μm to 0.5 μm.
An upper magnetic layer as thin as μm has excellent output characteristics. In a magnetic recording medium, when the thickness of the upper magnetic layer is set to be as thin as 0.05 to 0.5 μm, the self-demagnetization loss in the upper magnetic layer is reduced, and the reproduction output in a short wavelength region is improved. In addition, when the thickness of the upper magnetic layer is reduced in this manner, unerased portions are less likely to occur during overwriting, and good overwriting characteristics can be obtained.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the magnetic recording medium according to the present invention will be described below.

FIG. 1 shows a magnetic recording medium to which the present invention is applied.
As shown in (1), after a lower non-magnetic layer 2 is formed by applying a non-magnetic paint composed of a non-magnetic powder, a binder and an organic solvent on a non-magnetic support 1, the lower non-magnetic layer 2 is uncoated. This is a multilayer coating type magnetic recording medium in which the upper magnetic layer 3 is formed by applying a magnetic paint comprising a metal magnetic powder, a binder and an organic solvent while it is in a dry state.

According to the present invention, in such a multilayer coating type magnetic recording medium, the average thickness of the upper magnetic layer is set to 0.05 to 0.05.
0.5 μm, the major axis length of the metal magnetic powder is set to 0.05 to 0.2 μm, and the X-ray particle size is set to 100 to 250 μm.
Regulate to Angstrom.

The thickness of the upper magnetic layer is 0.05 to 0.5 μm
When the thickness is set to be thin, the self-demagnetization loss in the upper magnetic layer is reduced, and the reproduction output in the short wavelength region is improved. Further, when the thickness of the upper magnetic layer is reduced in this manner, unerased portions are less likely to occur during overwriting, and good overwriting characteristics can be obtained.

Further, the long axis length of metal magnetic powder and X
When the particle diameter is regulated within the above range, the reproduction output is increased and the noise is reduced, so that excellent output characteristics are obtained.

In the present invention, the thickness of the upper magnetic layer and the dimensions of the metallic magnetic powder are regulated as described above. The material of the upper magnetic layer and other conditions are those which are usually used in this type of magnetic recording medium. Can be used.

First, as the metal magnetic powder, Fe, C
o, metals such as Ni, Fe-Co, Fe-Ni, Fe-A
1, Fe-Ni-Al, Fe-Al-P, Fe-Ni-
Si-Al, Fe-Ni-Si-Al-Mn, Fe-M
n-Zn, Fe-Ni-Zn, Co-Ni, Co-P,
Fe-Co-Ni, Fe-Co-Ni-Cr, Fe-C
o-Ni-P, Fe-Co-B, Fe-Co-Cr-
An alloy of B, Mn-Bi, Mn-Al, Fe-Co-V, or the like is used.

In order to prevent sintering at the time of reduction or maintain the shape of the metal magnetic powder, Al, Si, C
a, Mg, P, B, Zr, Y and rare earth elements may be contained in the surface layer portion. These elements may be used alone or in combination of two or more. .

Next, as the binder, a known thermoplastic resin conventionally used as a binder for a magnetic recording medium,
It is desirable to use a thermosetting resin, a reactive resin, or the like, which can impart running durability, flexibility, and toughness to the magnetic recording medium and has good adhesion to the nonmagnetic support. The weight average molecular weight is preferably 15,000 to 200,000.

Examples of the thermoplastic resin include vinyl chloride, vinyl acetate, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylate-acrylonitrile copolymer, Acrylate-vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylate-acrylonitrile copolymer,
Acrylate-vinylidene chloride copolymer, methacrylate-vinylidene chloride copolymer, methacrylate-vinyl chloride copolymer, methacrylate-ethylene copolymer, polyvinyl fluoride, vinylidene chloride-acrylonitrile copolymer , Acrylonitrile-
Butadiene copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose), styrene butadiene copolymer, polyurethane resin, polyester resin, amino resin, And synthetic rubber.

The thermosetting resin or the reactive resin includes a phenol resin, an epoxy resin, a polyurethane curable resin, a urea resin, a melamine resin, an alkyd resin,
Silicone resins, polyamine resins, urea-formaldehyde resins and the like can also be used.

All of the above binders include -SO ₃ M and -OSO ₃ in order to improve the dispersibility of the metal magnetic powder.
M, -COOM, P = O ( OM) 2 ( where, M in the formula represents an alkali metal such as a hydrogen atom or a lithium, potassium, sodium) _{or, -NR 1 R 2, -NR 1} R 2 R 3 + X
^- side chain amine having terminal ^{_{groups,> NR 1 R 2 + X}} - main-chain amine represented by (wherein, wherein R _1, R _2, R ₃ represents a hydrogen atom or a hydrocarbon group, X ^- is fluorine, chlorine,
Represents a halogen element ion such as bromine or iodine or an inorganic ion or an organic ion), and further represents -OH, -SH, -C
A polar functional group such as N or an epoxy group may be introduced. The amount of these polar functional groups introduced into the binder is 10 ⁻⁸ to
It is preferably 10 ^-1 mol / g, and 10 ^-6 to 10 ^-2.
More preferably, it is mol / g. Note that these binders may be used alone or in combination of two or more.

The amount of the binder used in the upper magnetic layer 3 is preferably 1 to 100 parts by weight, more preferably 10 to 50 parts by weight, based on 100 parts by weight of the metal magnetic powder. If the binder is used in a larger amount than this range, the output of the magnetic layer is reduced because the amount of the metal magnetic powder contained in the magnetic layer is relatively small. Further, when the magnetic layer is repeatedly slid on the drive device, the magnetic layer is liable to cause plastic flow, and the running durability is deteriorated. On the other hand, when the binder is used in an amount smaller than the above range, the dispersibility of the metal magnetic powder decreases, and the mechanical strength of the coating film decreases.

As a solvent used for coating, acetone,
Methyl ethyl ketone, methyl isobutyl ketone, ketone solvents such as cyclohexanone, alcohol solvents such as methanol, ethanol, propanol, methyl acetate,
Ethyl acetate, butyl acetate, propyl acetate, ethyl lactate,
Ester solvents such as ethylene glycol acetate, diethylene glycol dimethyl ether, 2-ethoxyethanol, ether solvents such as tetrahydrofuran and dioxane, benzene, toluene, aromatic hydrocarbon solvents such as xylene, methylene chloride, ethylene chloride, carbon tetrachloride, Halogenated hydrocarbon solvents such as chloroform and chlorobenzene are used.

The magnetic paint is a metal magnetic powder as described above,
It is prepared by kneading, mixing and dispersing a binder and an organic solvent. As the kneading and dispersing device, a roll mill,
A ball mill, a sand mill, an agitator, a kneader, an extruder, a homogenizer, an ultrasonic disperser and the like are used.

On the other hand, the following materials are used as the material of the lower nonmagnetic layer 2.

First, as the nonmagnetic powder, α-Fe ₂ O ₃
Such as non-magnetic iron oxide, goethite, rutile type titanium oxide,
Anatase type titanium oxide, tin oxide, tungsten oxide,
Silicon oxide, zinc oxide, chromium oxide, cerium oxide, titanium carbide, BN, α-alumina, β-alumina, γ
-Alumina, calcium sulfate, barium sulfate, molybdenum disulfide, magnesium carbonate, calcium carbonate, barium carbonate, strontium carbonate, barium titanate and the like. These non-magnetic powders may be doped with an appropriate amount of impurities depending on the purpose. For the purpose of improving dispersibility, imparting conductivity, and improving color tone, Al, S
The surface treatment may be performed with a compound containing i, Ti, Sn, Sb, Zr, or the like. The specific surface area of these nonmagnetic powders is preferably 30 to 80 m ² / g,
More preferably, it is 40 to 70 m ² / g. These non-magnetic powders may be used alone or in combination of two or more.

The non-magnetic powder may contain carbon black such as furnace furnace for rubber, pyrolytic carbon, black for color, and acetylene black as required. These carbon blacks have a specific surface area of 100
400 m ² / g, DBP oil absorption 20-200 ml /
Preferably, it is 100 g.

The specific surface area of carbon black and other non-magnetic powder is regulated for the following reasons.

That is, in the powder component, the larger the specific surface area, the finer the particle.

Therefore, when the specific surface area of the carbon black and the other non-magnetic powder is smaller than the above range, since the powder has a large particle size, the powder shape easily appears on the surface of the lower non-magnetic layer 2, Reflecting this, the surface smoothness of the upper magnetic layer 3 is impaired. In addition, since non-magnetic powder does not have magnetic cohesive force, dispersibility is hardly reduced due to miniaturization as compared with magnetic powder, but when the specific surface area is larger than the above range, dispersion is also difficult. . Thereby, the surface smoothness of the lower non-magnetic layer 2 and, consequently, the upper magnetic layer 3 is impaired.

By setting the specific surface area of the powder in the above range, the surface of the lower non-magnetic layer is smoothed, whereby the upper magnetic layer is also formed with a smooth surface, so that the modulation noise characteristics are improved and the spacing loss is improved. Will be suppressed.

As the binder and the organic solvent for the lower non-magnetic layer 2, any of those exemplified for the upper magnetic layer 3 can be used. The amount of the binder used in the lower non-magnetic layer 2 is
The amount is preferably 1 to 100 parts by weight, more preferably 10 to 50 parts by weight, based on 100 parts by weight of the nonmagnetic powder. If the binder is used in a larger amount than this range,
When the medium is repeatedly slid on the drive device, the non-magnetic layer easily causes plastic flow, and the running durability is deteriorated. On the other hand, when the binder is used in an amount smaller than the above range, the dispersibility of the non-magnetic powder decreases, and the mechanical strength of the coating film decreases.

The non-magnetic paint is a non-magnetic powder as described above,
It is prepared by kneading, mixing and dispersing a binder and an organic solvent. As the kneading and dispersing device, any of those exemplified for the upper magnetic layer 3 can be used.

The upper layer and the lower layer are formed of the above-mentioned materials, but may contain additives such as abrasive particles, a lubricant, a surfactant and the like in addition to this material.

Abrasive particles are added to the upper layer, and aluminum oxide (α, β, γ), chromium oxide, silicon carbide, diamond, garnet, emery, boron nitride, titanium carbide, titanium carbide, titanium oxide (rutile, anatase) ) Etc. are used. The abrasive particles have a Mohs hardness of 4 or more, preferably 5 or more, a specific gravity of 2 to 6, preferably 3 to 5, and an average particle size of 0.5 μm or less, preferably 0.3 μm or less. Is good. The amount of the abrasive particles added is 20 parts by weight based on 100 parts by weight of the metal magnetic powder.
It is appropriate that the content is not more than 10 parts by weight, preferably not more than 10 parts by weight.

Examples of the lubricant include solid lubricants such as graphite, molybdenum disulfide and tungsten disulfide, silicone oil, fatty acids having 10 to 22 carbon atoms, and 10 to 22 carbon atoms.
Fatty acid esters, terpene-based compounds, and oligomers thereof, which are synthesized from the above-mentioned fatty acids and alcohols having 2 to 26 carbon atoms. These lubricants may be added only to the upper layer, or may be added to both the upper layer and the lower layer.

As the surfactant, any of nonionic, anionic, cationic and amphoteric surfactants can be used. These surfactants may be added only to the upper layer or to both the upper layer and the lower layer. In this case, the type of the surfactant may be the same or different between the upper layer and the lower layer.

Further, a polyisocyanate for crosslinking and curing the binder may be used in combination to increase the strength of the coating film. As this polyisocyanate, toluene diisocyanate and its adduct, alkylene diisocyanate, and their adducts can be used. The addition amount of these polyisocyanates is
5 to 80 parts by weight, preferably 10 to 5 parts by weight per 0 parts by weight
0 parts by weight is suitable. This polyisocyanate is
It may be used for both upper and lower layers, or may be used for only the upper layer. When used in both upper and lower layers, the amount may be equal in each layer or may be changed at an arbitrary ratio.

The upper magnetic coating and the lower non-magnetic coating prepared from the above-mentioned materials are coated on a non-magnetic support 1 and dried to form an upper magnetic layer 3 and a lower non-magnetic layer 2.

Examples of the non-magnetic support 1 include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate; polyolefins such as polypropylene; celluloses such as cellulose triacetate and cellulose diacetate; vinyl resins; A support made of a polymer material typified by polycarbonates, or a metal, glass, ceramics or the like is used.

The prepared two kinds of paints were applied to a non-magnetic support 1
For application on the upper layer, a so-called wet-on-wet coating method (wet multilayer coating method) is used, in which an upper layer coating is overlaid on a lower layer coating in a wet state.

FIG. 2 shows an example of a coating apparatus for applying a paint by the wet-on-wet coating method.

This coating apparatus has two slits (a slit 11 for lower layer paint,
Die head 18 having slit 12) for upper paint
(4 lip type die head). That is, in this die head, the two slit portions 11, 1
A lower paint reservoir 13 and an upper paint reservoir 14 to which a lower paint and an upper paint are supplied, respectively, are formed on the back side of 2.
The lower layer paint and the upper layer paint supplied to the paint pools 13 and 14 are extruded through the slits 11 and 12 to the tip of the die head. On the other hand, the support 15 to which the paint is applied
Travels in the direction A in the figure from the slit 11 for the lower layer paint to the slit 12 for the upper layer paint along the tip surface of the die head.

The non-magnetic support 15 running as described above
First, when passing through the slit portion 11 for the lower layer paint, the lower layer paint extruded from the slit portion 11 is applied to the surface to form the lower layer coating film 16. Then, when passing through the slit portion 12 for the upper layer paint, the upper layer paint extruded from the slit portion 12 is applied on the lower layer coating film 16 in a wet state, and two layers of the coating films 16 and 17 are formed. . Then, the two-layered coating film in a wet state is dried and subjected to a surface smoothing treatment such as a calendering treatment as necessary, whereby a multilayer coating type magnetic recording medium is produced.

As the die head, besides the four-lip method, there are also a three-lip method and a two-lip method.

Since the lower layer and the upper layer formed by the wet-on-wet coating method are formed by applying the upper layer paint on the wet lower layer coating film, the surface of the lower layer, that is, The boundary between the lower layer and the upper layer is formed smoothly. Therefore, the surface properties of the upper layer are also very good, dropout is suppressed, high output,
It is suitable for high density recording where low noise is strictly required. Further, since the adhesion between the lower layer and the upper layer is high, film peeling hardly occurs and excellent durability can be obtained.

It is to be noted that a clear boundary substantially exists between the lower layer and the upper layer formed by the wet-on-wet coating method, and a boundary where the components of both layers are mixed with a certain thickness. There may be regions. In the present invention, when such a boundary region exists, a layer below the boundary region is defined as a lower layer and an upper layer is defined as an upper layer except for the boundary region.

The basic structure of the magnetic recording medium of the present invention has been described above. This magnetic recording medium is characterized by having an additional structure usually used in a multilayer coating type magnetic recording medium. May be improved.

For example, a back coat layer 4 may be provided on the surface opposite to the side on which the upper and lower layers of the non-magnetic support 1 are formed for the purpose of improving running properties, preventing charge, preventing transfer, and the like. Further, an undercoat layer may be provided between the lower layer 2 and the nonmagnetic support 1 for the purpose of enhancing the adhesiveness between the lower layer 2 and the support 1.

[0059]

EXAMPLES Hereinafter, specific examples and comparative examples of the present invention will be described, but it goes without saying that the present invention is not limited to these examples.

Example 1 First, an upper layer paint was prepared based on the following composition.

The coating can be made by a conventional method using metal magnetic powder,
After mixing a binder, an additive, and a solvent, the mixture was kneaded with an extruder, and further dispersed for 4 hours by a sand mill.

The metal magnetic powder has a major axis length of 0.11.
μm, X-ray particle size is 153 Å, coercive force is 1
72.8 kA / m, saturation magnetization is 131.2 Am ² / k
g.

<Upper layer coating composition> Metal magnetic powder 100 parts by weight Vinyl chloride-based copolymer 14 parts by weight (trade name: MR-110 manufactured by Zeon Corporation) Polyester polyurethane resin 3 parts by weight (weight average molecular weight 35,000 manufactured by Toyobo Co., Ltd.) α -Al ₂ O ₃ 5 parts by weight Stearic acid 1 part by weight Heptyl stearate 1 part by weight Methyl ethyl ketone 150 parts by weight Cyclohexanone 150 parts by weight Next, a lower layer coating material was prepared based on the following composition.

According to a conventional method, a non-magnetic powder, a binder, an additive, a solvent, and a surfactant were mixed, kneaded with an extruder, and dispersed in a sand mill for 4 hours.

<Lower layer coating composition> 100 parts by weight of α-Fe ₂ O ₃ (specific surface area: 53 m ² / g, major axis length: 0.15 μm) 16 parts by weight of vinyl chloride copolymer (weight average molecular weight: 10,000, Functional group-SO ₃ K: 7 × 10 ⁻⁵ mol / g) Stearic acid 1 part by weight Heptyl stearate 1 part by weight Methyl ethyl ketone 105 parts by weight Cyclohexanone 105 parts by weight Polyisocyanate is added to the prepared upper layer paint and lower layer paint, respectively. After adding 3 parts by weight, these paints
Using a lip-type die coater, simultaneous multi-layer coating was performed on a 7 μm-thick polyethylene terephthalate film. Then, in an undried state, the upper coating film was subjected to an orientation treatment using a solenoid coil, and then dried, calendered, and cured to form an upper magnetic layer and a lower nonmagnetic layer. The coating thickness of each layer is 0.2 μm for the upper layer and 2.0 μm for the lower layer after calendering.

On the other hand, a back coating was prepared based on the following composition.

<Back coating composition> Carbon black (Asahi # 50) 100 parts by weight Polyester polyurethane 100 parts by weight (trade name: Nipporan N-2304) Methyl ethyl ketone 500 parts by weight Toluene 500 parts by weight This back coating is applied to the upper layer of a non-magnetic support. A back coat layer was formed by applying the composition on the side opposite to the side on which the lower layer was formed.

Then, the raw tape on which the upper layer, the lower layer and the back coat layer were formed was cut into a tape by slitting it to a width of 8 mm.

Examples 2 to 6 In the same manner as in Example 1 except that the metal magnetic powder having the major axis length, X-ray particle size, coercive force and saturation magnetization shown in Table 1 was used. A magnetic tape was produced.

Comparative Examples 1 to 6 Metal magnetic powders having a major axis length, an X-ray particle size, a coercive force, and a saturation magnetization value shown in Table 1, ie, having an X-ray particle size outside a predetermined range. A magnetic tape was produced in the same manner as in Example 1 except that the magnetic tape was used.

Comparative Examples 7 to 18 The thickness of the upper layer was changed as shown in Table 1, and the major axis length, X-ray particle size, coercive force and saturation magnetization were as shown in Table 1, ie, the upper layer A magnetic tape was produced in the same manner as in Example 1 except that the thickness of the magnetic tape was out of the predetermined range.

Comparative Examples 19 and 20 Metallic magnetic powders having a major axis length, an X-ray particle size, a coercive force, and a saturation magnetization value shown in Table 1, ie, having a major axis length outside a predetermined range, were used. A magnetic tape was produced in the same manner as in Example 1 except that the magnetic tape was used.

With respect to the magnetic tape produced as described above, the surface roughness Ra, the reproduced output at 10 MHz,
The / N ratio was measured.

The surface roughness Ra is JIS B 06
The center line average roughness Ra defined by 01 is measured using a non-contact type surface roughness meter based on an optical interference method.

The reproduction output and the C / N ratio were measured using a fixed head type electromagnetic conversion characteristic measuring device. The measuring machine has a rotating drum and a head in contact with the rotating drum, and a magnetic tape is wound around the drum.

The reproduction output was measured by recording a 10-MHz rectangular wave signal on a magnetic tape at an optimum recording current on this measuring instrument, and then detecting the 10-MHz output level with a spectrum analyzer. Also, the C / N ratio is determined by the output level at 10 MHz and the
It was determined as a ratio to the output level (noise) at 9 MHz separated by 9 Hz.

Table 1 shows the measurement results together with the thickness of the upper layer and the characteristics of the metal magnetic powder.

[0078]

[Table 1]

As can be seen from Table 1, the thickness of the upper layer, the major axis length of the metallic magnetic powder, and the X-ray particle size are within the predetermined ranges (the thickness of the upper layer: 0.05 to 0.5 μm, the major axis length: 0.05-0.
2 μm, X-ray particle size: 100 to 250 angstroms)
The magnetic tape of Examples 1 to 6 was 10 MHz.
, The reproduction output and the C / N ratio are high values.

On the other hand, the magnetic tapes of Comparative Examples 1 to 3 in which the X-ray particle size of the metal magnetic powder was larger than the predetermined range, and the comparative examples in which the X-ray particle size of the metal magnetic powder was smaller than the predetermined range. 4-Magnetic tapes of Comparative Example 6 and Comparative Examples 7 to 12 in which the thickness of the upper layer is thicker than a predetermined range, and Comparative Examples 13 to 13 in which the thickness of the upper layer is thinner than a predetermined range.
The magnetic tape of Comparative Example 19, in which the major axis length of the metal magnetic powder was longer than the predetermined range, and the magnetic tape of Comparative Example 20, in which the major axis length of the metal magnetic powder was shorter than the predetermined range, were 10 MHZ. , The reproduction output and C / N are low values.

From this, it is understood that regulating the thickness of the upper layer, the major axis length of the metallic magnetic powder, and the X-ray particle size is effective in improving the output characteristics in the short wavelength region of the magnetic tape. Was.

[0082]

As is clear from the above description, the magnetic recording medium of the present invention is a multi-layer coating type magnetic recording medium in which the average thickness of the upper magnetic layer is defined, Since the specific surface area of the non-magnetic powder, the specific surface area of the carbon black, and the DBP oil absorption amount are specified, and the surface roughness is also specified, a high reproduction output can be obtained in a short wavelength region and the noise is suppressed low. And good output characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a main part showing a configuration example of a magnetic recording medium to which the present invention is applied.

FIG. 2 is a schematic view showing an application device for applying a non-magnetic paint and a magnetic paint in a multilayer manner.

[Explanation of symbols]

1 Non-magnetic support, 2 Lower non-magnetic layer, 3 Upper magnetic layer, 4 Back coat layer

Claims (1)

[Claims] 1. A non-magnetic paint comprising a non-magnetic powder, a binder and an organic solvent is applied on a non-magnetic support to form a lower non-magnetic layer, and the lower non-magnetic layer is dried in an undried state. Meanwhile, in a magnetic recording medium in which an upper magnetic layer is formed by applying a magnetic paint comprising a metal magnetic powder, a binder and an organic solvent, the average thickness of the upper magnetic layer is 0.05 μm to 0.5 μm.
Wherein the lower nonmagnetic layer has a nonmagnetic powder and carbon black, the specific surface area of the nonmagnetic powder is 30 to 80 m ² / g, and the specific surface area of the carbon black is 100 to 400.
a m ² / g, DBP oil absorption amount of the carbon black is 20 to 200/100 g, the magnetic recording medium, wherein the surface roughness of 3.5～3.9Nm.