JP2000231713A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a medium having excellent electromagnetic conversion characteristics and excellent travelling durability without producing a mixture region on an interface between a magnetic layer as an upper magnetic layer and a lower base layer and to accurately record and reproduce. SOLUTION: This medium consists of a nonmagnetic supporting body 1, a lower base layer 2 formed by applying a coating material of the base layer consisting of at least soft magnetic powder, nonmagnetic powder and a binder on the nonmagnetic supporting body 1, and an upper magnetic layer 3 formed by applying the coating material of the upper magnetic layer consisting of at least ferromagnetic powder and a binder on the lower base layer 2. In the coating material of the lower base layer, the ratio of the soft magnetic powder to the nonmagnetic powder ranges from 5:5 to 9:1. The ratio (r1/r2) of the major axial length r1 to the minor axial length r2 of the nonmagnetic powder in the coating material for the lower base layer ranges between >=2.5 and <=20.

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 lower underlayer containing a soft magnetic powder and an upper magnetic layer capable of magnetic recording.

[0002]

2. Description of the Related Art Magnetic recording media are widely used as recording tapes, video tapes, computer tapes, disks, and the like. In this magnetic recording medium, the recording density is increasing year by year, and analog and digital recording methods are being studied. In response to this demand for high-density recording, a magnetic recording medium formed by forming a thin metal magnetic thin film has been proposed as a magnetic recording medium. However, the magnetic recording medium having the metal magnetic thin film is not sufficient in practical reliability such as productivity and corrosiveness.

[0003] As a magnetic recording medium, a so-called coating type in which a magnetic layer formed by applying a magnetic coating material obtained by dispersing a magnetic powder in a binder onto a non-magnetic support and drying the coating is applied. is there. This coating type magnetic recording medium is
It is superior to a magnetic recording medium having the above-mentioned metal magnetic thin film in terms of practical reliability such as productivity and corrosiveness. However, the coating type magnetic recording medium is inferior in electromagnetic conversion characteristics because the filling degree of the magnetic material in the magnetic layer is lower than that of the metal magnetic thin film.

In order to improve the electromagnetic conversion characteristics of the coating type magnetic recording medium, improvement of the magnetic characteristics of the magnetic material to be used and smoothing of the surface have been studied and proposed. However, the conventional magnetic recording medium cannot be said to have sufficiently achieved high-density recording.

On the other hand, in a magnetic recording medium employing a digital recording system, it is known to reduce the thickness of a magnetic layer in order to improve performance. However, when a thin magnetic layer is formed by applying a magnetic paint on a non-magnetic support, coating defects such as pinholes and streaks are likely to occur due to the thinning of the magnetic layer, and a sufficient yield cannot be obtained. , There are production problems. Further, in this case, even if the formed magnetic layer is subjected to the calendering treatment, the molding effect by the calendering treatment is small, so that the surface properties are poor and the electromagnetic conversion characteristics are not good.

Conventionally, in order to solve such a problem, in a coating type magnetic recording medium, a lower non-magnetic layer having a predetermined thickness and a thin upper magnetic layer having a thickness of about 2 μm or less are simultaneously formed by multi-layer coating. After that, it is conceivable to perform a calendar process. In such a magnetic recording medium having a lower non-magnetic layer and an upper magnetic layer, as described above, it is possible to solve the deterioration of the electromagnetic conversion characteristics due to coating defects and poor surface properties of the upper magnetic layer.

On the other hand, in a magnetic recording medium having such a multi-layer coating, magnetic recording is performed by the upper magnetic layer containing a ferromagnetic material as disclosed in, for example, Japanese Patent Application Laid-Open No. 7-29153. It has been proposed to use a soft magnetic material in a lower non-magnetic layer to be used as a recording layer.
Thus, in the magnetic recording medium, the lower non-magnetic layer containing a soft magnetic material (hereinafter, referred to as a lower underlayer) acts to draw a magnetic field from a magnetic head. Therefore, the magnetic field applied from the magnetic head is applied to the upper magnetic layer in a narrow range. Therefore, in this magnetic recording medium, high-density recording can be performed on the upper magnetic layer.

[0008]

In a magnetic recording medium having a lower underlayer and an upper magnetic layer as described above, an orientation process for controlling the orientation of the upper magnetic layer is performed after the multilayer coating. Will be This orientation treatment forcibly controls the magnetization direction of the upper magnetic layer by making the magnetic recording medium exist in a predetermined magnetic field. By performing such an orientation treatment, the soft magnetic material of the lower underlayer and the ferromagnetic material of the upper magnetic layer rotate, and the magnetization direction of the upper magnetic layer is aligned in a predetermined direction.

However, in this magnetic recording medium, when the orientation treatment is performed, the soft magnetic material of the lower underlayer and the ferromagnetic material of the upper magnetic layer rotate in a magnetic field, so that the upper magnetic layer and the lower magnetic layer are rotated. A mixed region is easily formed at the interface with the formation. As a result, the above-described magnetic recording medium has a problem that sufficient surface properties cannot be obtained, and a problem that desired orientation cannot be obtained because sufficient orientation is not performed.

Accordingly, the present invention provides a magnetic recording medium having excellent electromagnetic conversion characteristics and excellent running durability while hardly causing a mixed region at the boundary between the upper magnetic layer and the lower underlayer. The purpose is to:

[0011]

A magnetic recording medium according to the present invention, which has achieved the above-mentioned object, comprises a non-magnetic support, a soft magnetic powder, a non-magnetic powder, and a binder on the non-magnetic support. A lower underlayer formed by applying a paint for a lower underlayer serving as a main component, and an upper magnetic layer formed by applying a paint for an upper magnetic layer containing a ferromagnetic powder and a binder as a main component on the lower sublayer. Wherein the ratio of the soft magnetic powder to the nonmagnetic powder is 5: 5 to 9: 1 in the lower underlayer coating, and the major axis length (r1) and the minor axis length (r2) of the nonmagnetic powder are included. ) And (r1
/ R2) is not less than 2.5 and not more than 20.

In the magnetic recording medium according to the present invention having the above-described structure, the mixing ratio of the soft magnetic powder and the nonmagnetic powder contained in the lower underlayer and the axial length ratio (r1
/ R2) in a predetermined range, the non-magnetic powder is aligned on the surface of the lower underlayer, so that the lower underlayer becomes a strong coating film even in an undried state. In this magnetic recording medium, the lower underlayer and the upper magnetic layer can be formed without mixing the boundaries between the lower underlayer and the upper magnetic layer.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the magnetic recording medium according to the present invention will be described in detail with reference to the drawings.

The magnetic recording medium shown in the present embodiment has the structure shown in FIG.
As shown in FIG. 1, at least a non-magnetic support 1 and a lower underlayer 2 formed on one main surface of the non-magnetic support 1
And an upper magnetic layer 3 formed on the lower underlayer 2. This magnetic recording medium has a back coat layer (not shown) formed on the other main surface of the non-magnetic support 1.
Or a top coat layer (not shown) formed on the upper magnetic layer 3.

In this magnetic recording medium, the lower underlayer 2
Is a layer formed by applying a coating for a lower underlayer comprising at least a soft magnetic powder, a nonmagnetic powder, and a binder on one main surface of the nonmagnetic support 1. In the lower underlayer 2, the ratio between the soft magnetic powder and the nonmagnetic powder is 5: 5 to 9:
1, and more preferably in the range of 6: 4 to 8: 2. That is, when preparing the coating material for the lower underlayer, the soft magnetic powder and the nonmagnetic powder are mixed at a ratio of 5: 5 to 9: 1, more preferably at a ratio of 6: 4 to 8: 2. I do.

Further, in the nonmagnetic powder contained in the lower underlayer 2, the ratio (r) of the major axis length (r1) to the minor axis length (r2) is obtained.
1 / r2) is not less than 2.5 and not more than 20. Where r
1 is the longest axis length of the non-magnetic powder, and r2 is the shortest axis length of the non-magnetic powder. At this time, the shape of the nonmagnetic powder may be any shape such as a needle shape or a plate shape, and is measured by an electron microscope or the like. Here, the axis of the non-magnetic powder does not mean a symmetric axis in a strict sense. When the nonmagnetic powder is acicular, r1 usually indicates a portion called a major axis length, and r2 indicates a minor axis length or a thickness. At this time, r1 is preferably 3 μm or less, more preferably 1.5 μm or less. When the nonmagnetic powder is in the form of a scale or a plate, r1 usually indicates a portion called a plate diameter, and r2 indicates a plate thickness. At this time, r1 is preferably 0.01 to 3 μm, more preferably 0.05 to 1.5 μm.

For the soft magnetic powder contained in the lower underlayer 2, the needle ratio of the soft magnetic powder is defined in the same manner as r1 / r2 of the above-mentioned nonmagnetic powder, and is distinguished from that of the nonmagnetic powder. R1 and R2 to make
It is preferable that it is above. Also, R1 is 3 μm.
m, more preferably 1.5 μm or less.

The soft magnetic powder includes, but is not particularly limited to, powders of high magnetic susceptibility and low coercive force, such as metals, metal oxides, alloys, and amorphous alloys, which are used in so-called weak electric devices such as magnetic heads and electronic circuits. No. Specifically, as the soft magnetic powder, for example, soft magnetic materials such as iron-silicon alloy, iron-aluminum alloy, iron-nickel alloy, iron-cobalt alloy, iron-cobalt-nickel alloy, nickel-cobalt alloy, Sendust, manganese-
Examples include zinc-based ferrite, nickel-zinc-based ferrite, magnesium-zinc-based ferrite, and magnesium-manganese-based ferrite.

The soft magnetic powder has a coercive force (hereinafter, referred to as a coercive force).
Abbreviated as Hc. ) Is preferably 200 (Oe) or less.

The non-magnetic powder used for the lower underlayer 2 may be a non-magnetic metal (Cu, Cr, Ag, A
l, Ti, W, etc.) or oxides are preferred.
Al ₂ O ₃ (α, γ), Cr ₂ O ₃ , α ferrite, goethite, SiO ₂ (including glass), ZrO ₂ , CeO ₂ ,
TiO ₂ (rutile, anatase) and the like.

Further, as the binder used for the lower underlayer 2, a conventionally known thermoplastic resin, thermosetting resin, reactive resin, or a mixture thereof is used. These can be used in combination with the epoxy group-containing resin. As a thermoplastic resin, the glass transition temperature is -100 to 15
0 ° C., number average molecular weight of 1,000 to 200,000, preferably 10,000 to 100,000, and degree of polymerization of about 50 to 10
It is about 00. Such examples include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid esters, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic acid esters,
Styrene, butadiene, ethylene, vinyl butyral,
There are polymers or copolymers containing vinyl acetal, vinyl ether, and the like as constituent units, polyurethane resins, and various rubber resins. In addition, as a thermosetting resin or a reactive resin, a phenol resin, an epoxy resin, a polyurethane curing resin, a urea resin, a melamine resin, an alkyd resin,
Acrylic-based reaction resins, formaldehyde resins, silicone resins, epoxy-polyamide resins, mixtures of polyester resins and isocyanate prepolymers, mixtures of polyester polyols and polyisocyanates, mixtures of polyurethanes and polyisocyanates, and the like. Further, as the binder, a known electron beam-curable resin can be used.

The above resins can be used alone or in combination, but preferred are at least one selected from vinyl chloride resin, vinyl chloride vinyl acetate resin, vinyl chloride vinyl acetate vinyl alcohol resin, and vinyl chloride vinyl acetate maleic anhydride copolymer. A combination of one type with a polyurethane resin or a combination of these with a polyisocyanate can be given. Known polyurethane resin structures such as polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, and polycaprolactone polyurethane can be used. For all of the binders shown here, C is required to obtain better dispersibility and durability.
_{OOM, SO 3 M, OSO 3} M, P = O (OM) 2, O-
P = O (OM) ₂ , where M is a hydrogen atom or an alkali metal base, OH, NR ₂ , N ⁺ R ₃ , (R is a hydrocarbon group), SH, CN, etc. It is preferable to use one obtained by introducing the above polar group by a copolymerization reaction or an addition reaction. The amount of such a polar group is 1
0 ^-1 to 10 ^-8 mol / g, preferably 10 ^-2 to 10
^-6 mol / g. As the vinyl chloride-based copolymer, preferably, an epoxy group-containing vinyl chloride-based copolymer is mentioned, and a vinyl chloride repeating unit, a repeating unit having an epoxy group, and a repeating unit having a polar group as described above, if desired. And a unit containing a unit. In combination with the repeating unit having an epoxy group, an epoxy group-containing vinyl chloride copolymer preferably contains a repeating unit having an -SO ₃ Na. The content of the repeating unit having a polar group in the copolymer is usually in the range of 0.01 to 5.0 mol% (preferably, 0.5 to 3.0 mol%). Specifically, the content of the repeating unit having an epoxy group in the copolymer is 1.0 to 30 mol% (preferably 1 to 20 mol%).
Mol%). The vinyl chloride polymer is usually used in an amount of 0.0 mol per mol of the vinyl chloride repeating unit.
Those containing a repeating unit having 1 to 0.5 mol (preferably 0.01 to 0.3 mol) of an epoxy group are preferred. When a resin containing an epoxy group is used, the molecular weight of the resin is preferably 30,000 or more from the viewpoint of the dispersibility of the soft magnetic powder and the nonmagnetic powder described above. The epoxy group is contained at 1 × 10 ^{−5 to} 20 × 10 ⁻⁴ eq / g, preferably at 4 × 10 ^{−5 to} 16 × 10 ⁻⁴ eq / g. Known techniques can be applied as a method for introducing an epoxy group.
For example, there is a method of copolymerizing a vinyl monomer having a glycidyl group with another monomer.

When a polyurethane resin is used as the binder, the polyurethane resin has a glass transition temperature of-
50-100 ° C, elongation at break 100-2000%, stress at break 0.05-10 kg / cm ² , yield point 0.05
It is preferably from 10 to 10 kg / cm ² .

Further, as the vinyl chloride copolymer,
It may contain other monomers. Examples of other monomers include vinyl ethers (eg, methyl vinyl ether, isobutyl vinyl ether, lauryl vinyl ether), α-monoolefins (eg, ethylene, propylene), acrylates (eg, methyl (meth) acrylate, hydroxy (Meth) acrylate containing a functional group such as ethyl (meth) acrylate),
Examples include unsaturated nitriles (eg, (meth) acrylonitrile), aromatic vinyls (eg, styrene, α-methylstyrene), and vinyl esters (eg, vinyl acetate, vinyl propionate, etc.).

Specific examples of these binders include VAGH, VYHH, VMCH, manufactured by Union Carbide.
VAGF, VAGD, VROH, VYES, VYNC,
VMCC, XYHL, XYSG, PKHH, PKHJ,
PKHC, PKFE, manufactured by Nissin Chemical Industries: MPR-T
A, MPR-TA5, MPR-TAL, MPR-TS
N, MPR-TMF, MPR-TS, MPR-TM, manufactured by Denki Kagaku: 1000 W, DX80, DX81, DX8
2, DX83, manufactured by Zeon Corporation: MR110, MR10
0, 400X110A, manufactured by Nippon Polyurethane Co., Ltd .: Nipporan N2301, N2302, N2304, manufactured by Dainippon Ink Co., Ltd .: Pandex T-5105, T-R3080,
T-5201, Burnock D-400, D-210-8
0, Chris Bon 6109, 7209, manufactured by Toyobo: Byron UR8200, UR8300, RV530, RV2
80, manufactured by Dainichi Seika Co., Ltd .: Daifelamin 4020, 502
0, 5100, 5300, 9020, 9022, 702
0, manufactured by Mitsubishi Kasei: MX5004, manufactured by Sanyo Kasei: Samprene SP-150, manufactured by Asahi Kasei: Saran F310, F
210 and the like.

The above-mentioned binder is preferably used in a range of 5 to 50% by weight, more preferably 10 to 35% by weight, based on the nonmagnetic powder. When a vinyl chloride resin is used as the binder, the binder is used in an amount of 3 to 30% by weight based on the non-magnetic powder. Preferably, it is used in the range of 30 to 30% by weight. At this time, the polyisocyanate is preferably used in combination with a binder in the range of 0 to 20% by weight based on the nonmagnetic powder.

In particular, when a resin containing an epoxy group having a molecular weight of 30,000 or more is used as the binder, the binder is used in an amount of 3 to 30% by weight based on the non-magnetic powder, and the epoxy group is used. Resins other than the resin contained can be used in the range of 3 to 30% by weight based on the non-magnetic powder.
0% by weight and 0-20% by weight of polyisocyanate
Can be used in the range. At this time, the epoxy group is 4 × 10 ^{−5 to} 16 × 1 with respect to the total weight of the binder (including the curing agent).
It is preferably contained in the range of 0 ^-4 eq / g.

The polyisocyanate used together with the binder described above includes tolylene diisocyanate, 4
Isocyanates such as -4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, and triphenylmethane triisocyanate; A product with a polyalcohol, a polyisocyanate formed by condensation of isocyanates, and the like can be used. Commercially available trade names of these isocyanates include those manufactured by Nippon Polyurethane Co .: Coronate L, Coronate HL, Coronate 2030, Coronate 203
1, Millionate MR, Millionate MTL, manufactured by Takeda Pharmaceutical Co., Ltd .: Takenate D-102, Takenate D-110
N, Takenate D-200, Takenate D-202, manufactured by Sumitomo Bayer: Desmodur L, Desmodur I
L, death module N, death module HL, etc.
These can be used alone or in combination of two or more utilizing the difference in curing reactivity.

The lower underlayer 2 may optionally contain an optional additive such as an antistatic agent such as carbon black.
Colorants, lubricants, dispersants and the like can be used. As carbon black, furnace for rubber, thermal for rubber, black for color, acetylene black, and the like can be used.
Carbon black may be used after being surface-treated with a dispersant or the like or grafted with a resin, or may be one obtained by graphitizing a part of the surface. Before adding the carbon black to the magnetic paint, the carbon black may be dispersed in a binder in advance. These carbon blacks can be used alone or in combination. Further, an abrasive may be added to the lower underlayer 2 in order to improve durability. As the abrasive, it is preferable to mix powders having a Mohs hardness of 5 or more so that the mixing ratio of the soft magnetic powder and / or the nonmagnetic powder is powder / abrasive = 95/5 to 60/40. . When the amount of the abrasive is less than this range, sufficient durability cannot be obtained. On the other hand, when the amount of the abrasive is too large, the effect that the nonmagnetic powder orients the ferromagnetic powder contained in the upper magnetic layer 3 is obtained. Is impaired. By setting the abrasive within the above range, the mechanical strength of the lower underlayer 2 and hence the magnetic recording medium is enhanced, so-called powder drop is prevented, BER (block error rate), dropout And durability can be improved.

As an abrasive having a Mohs hardness of 5 or more, α-Al ₂ O ₃ , β-Al ₂ O ₃ , Cr ₂ O ₃ ,
α-Fe ₂ O ₃ , ZrO ₂ , TiO ₂ , TiC, SiO ₂ ,
SiC, CeO ₂ , corundum, artificial diamond, silicon nitride, silicon carbide titanium carbide, boron nitride and the like. The particle size of the abrasive is preferably equal to or less than the coating thickness, and is about 0.1 to 5 μm, more preferably 0.1 to 2 μm. The shape of the particles may be either granular or acicular. Further, a composite of these abrasives (abrasive whose surface is treated with another abrasive) may be used. These abrasives may contain compounds or elements other than the main component, but the main component is 9
If it is 0% or more, the effect is not changed. The particle size of these abrasives is preferably from 0.01 to 2 μm. However, if necessary, abrasives having different particle sizes can be combined, or even a single abrasive can have the same effect by widening the particle size distribution. . The shape of the abrasive may be any of a needle shape, a spherical shape, and a dice shape, but a shape having a part of a corner is preferable because of high abrasiveness. Specific examples of the abrasive include AKP-20, AKP-30, and AKP-5 manufactured by Sumitomo Chemical Co., Ltd.
0, HIT-50, manufactured by Nippon Chemical Industry: G5, G7, S
-1, manufactured by Toda Kogyo KK: 100ED, 140ED and the like. These abrasives may be previously dispersed in a binder and then added to the coating for the lower underlayer. In the lower underlayer 2, it is preferable to use these abrasives in an amount of 20% by weight or less based on the nonmagnetic powder.

Further, the following are listed as additives having a lubricating effect, an antistatic effect, a dispersing effect, a plasticizing effect and the like. Solid lubricants such as molybdenum disulfide, tungsten disulfide, graphite, boron nitride, graphite fluoride, carbon black, etc., silicone oil, polar group-containing silicone, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing Esters, polyolefins, polyglycols, alkyl phosphates and alkali metal salts thereof, alkyl sulfates and alkali metal salts thereof, polyphenyl ethers, fluorine-containing alkyl sulfates and alkali metal salts thereof,
A monobasic fatty acid having 10 to 24 carbon atoms (which may contain an unsaturated bond or may be branched) and metal salts thereof (such as Li, Na, K, and Cu) or 12 to 24 carbon atoms; 22 mono-, di-, tri-, tetra-, penta-, hexa-hydric alcohols (which may contain unsaturated bonds or may be branched), alkoxy alcohols having 12 to 22 carbon atoms, 10 carbon atoms ~ 24 monobasic fatty acids (which may contain unsaturated bonds or may be branched) and have 2 carbon atoms
Mono-fatty acid ester consisting of any one of monohydric, dihydric, trihydric, tetrahydric, pentahydric, and hexahydric alcohols (which may contain an unsaturated bond or may be branched) or Difatty acid ester or trifatty acid ester, fatty acid ester of monoalkyl ether of alkylene oxide polymer, fatty acid amide having 8 to 22 carbon atoms, 8 to 22 carbon atoms
And 22 organic lubricants such as aliphatic amines.

Specific examples thereof include lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid,
Butyl stearate, oleic acid, linoleic acid, linolenic acid, elaidic acid, octyl stearate, amyl stearate, isooctyl stearate, octyl myristate, butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan distearate , Anhydrosorbitan tristearate,
Oleyl alcohol, lauryl alcohol and the like can be mentioned. Also, nonionic surfactants such as alkylene oxides, glycerin, glycidol, alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocycles, phosphoniums or sulfoniums, Such as cationic surfactants, carboxylic acids, sulfonic acids,
Also used are anionic surfactants containing acidic groups such as phosphoric acid, sulfate ester groups, phosphate ester groups, etc., amino acids, aminosulfonic acids, sulfuric acid or phosphate esters of amino alcohols, and amphoteric surfactants such as alkylbedine-type surfactants. it can.

These lubricants, antistatic agents and the like are not necessarily 100% pure, and may contain impurities such as isomers, unreacted materials, by-products, decomposition products and oxides in addition to the main components. Absent. These impurity components are preferably 30% or less, more preferably 10% or less.

In preparing the coating for the lower underlayer, an organic solvent which is usually used can be used. Examples of the organic solvent include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, and tetrahydrofuran, methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, methyl cyclohexanol, and the like in an arbitrary ratio. Alcohols, methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, esters such as glycol acetate, glycol dimethyl ether, glycol monoethyl ether, glycol ethers such as dioxane, benzene, toluene, xylene, cresol, chlor Aromatic hydrocarbons such as benzene, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chloride Emissions, chlorinated hydrocarbons such as dichlorobenzene, N, N- dimethylformamide,
Hexane or the like can be used.

On the other hand, the upper magnetic layer 3 is formed by applying a paint for an upper magnetic layer comprising at least a ferromagnetic powder and a binder onto the lower underlayer 2. The upper magnetic layer 3 preferably has a thickness of 2 μm or less.

As the ferromagnetic powder constituting the upper magnetic layer 3, any material may be used as long as it is generally used for a magnetic layer of a magnetic recording medium.
The size is not limited. Specifically, the acicular ferromagnetic powder includes γ-Fe ₂ O ₃ , Fe ₃ O ₄ , Co-γ-Fe ₂ O ₃ ,
Alloy ferromagnetic powders such as CrO ₂ , Fe—Ni alloy, and Fe—Ni—Co alloy can be exemplified. Examples of plate-like ferromagnetic powders include hexagonal ferrites such as barium ferrite and strontium ferrite and Co alloy powders. And the like. Further, as a ferromagnetic powder, γ-
FeOx (x = 1.33 to 1.5), Co-modified γ-Fe
Known ferromagnetic powders such as Ox (x = 1.33 to 1.5), ferromagnetic alloy fine powder containing Fe or Ni or Co as a main component (75% or more) can be used. These ferromagnetic powders include Al, Si, S, Sc, Ti,
V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn,
Sb, Te, Ba, Ta, W, Re, Au, Hg, P
b, Bi, La, Ce, Pr, Nd, P, Co, Mn,
It may contain atoms such as Zn, Ni, Sr, and B. Further, the ferromagnetic alloy fine powder may contain a small amount of hydroxide or oxide. Furthermore, it is particularly preferable to use an alloy mainly composed of Fe or hexagonal ferrite as the ferromagnetic powder.

Here, the axial ratio r1 / 1 /
When the axial ratio of the ferromagnetic powder is expressed as φ1 / φ2 following r2, φ1 / φ2 is preferably 2.5 or more,
In particular, in the case of a plate-like ferromagnetic powder, φ1 / φ2 is preferably 2.5 or more. Further, when the ferromagnetic powder is acicular, φ1 is preferably 0.3 μm or less,
More preferably, it is 0.25 μm or less. When the ferromagnetic powder is plate-shaped, φ1 is preferably 0.01 to 0.3 μm, and more preferably 0.05 to 0.2 μm. The ferromagnetic powder preferably has a small number of vacancies, and its value is preferably 20% by volume or less, more preferably 5% by volume.
It is as follows.

As the binder constituting the upper magnetic layer 3, the binder used in the lower underlayer 2 described above can be used. In the upper magnetic layer 3, the binder is
It is preferably used in the range of 5 to 50% by weight, more preferably 10 to 35% by weight, based on the ferromagnetic powder. When a vinyl chloride resin is used as a binder, it is preferably used in a range of 5 to 30% by weight, and when a polyurethane resin is used as a binder, it is used in a range of 2 to 20% by weight. Preferably, a polyisocyanate is used in combination in the range of 2 to 20% by weight based on the vinyl chloride resin or the polyurethane resin.

Further, in the upper magnetic layer 3, the content of the ferromagnetic powder is preferably 70% or more. If the content of the ferromagnetic powder is less than 70%, the degree of filling may be reduced and the electromagnetic conversion characteristics may be degraded. Here, the content of the ferromagnetic powder means
(Ferromagnetic powder) / (ferromagnetic powder + binder + additive, etc., contained in the upper magnetic layer 3).

Further, a dispersant, a lubricant, a surfactant, an antistatic agent and the like may be added to the upper magnetic layer 3 as in the lower underlayer 2. These lubricants, surfactants, and the like can be used in the lower underlayer 2 or the upper magnetic layer 3 in different types and amounts as needed. For example, the lower non-magnetic layer 2 and the upper magnetic layer 3 control bleeding to the surface using fatty acids having different melting points, control bleeding to the surface using esters having different boiling points and polarities, and the amount of surfactant. It is possible to improve the stability of the coating by adjusting the amount of the lubricant, and to improve the lubrication effect by increasing the amount of the lubricant added to the intermediate layer. Of course, the present invention is not limited to the examples shown here. In addition, all or a part of these additives may be added at any step of the lower underlayer paint, the upper magnetic layer paint production, for example, when mixed with the ferromagnetic powder before the kneading step, When adding in the kneading step with ferromagnetic powder, binder and solvent, when adding in the dispersing step,
There is a case where it is added after the dispersion or a case where it is added just before the application.

Similarly, for the upper magnetic layer 3, the abrasive used for the lower underlayer 2 as described above can be used. The amount of the abrasive used is preferably 20% by weight or less based on the ferromagnetic powder.

In particular, in the upper magnetic layer 3, the carbon black effectively acts to prevent charging of the magnetic recording medium, reduce the friction coefficient, impart light-shielding properties, improve film strength, and the like. And since these effects differ depending on the type of carbon black used, the type, amount, combination, etc. of the carbon black used are appropriately changed, and based on various properties such as particle size, oil absorption, conductivity, pH, etc. It is preferable to use them properly according to the purpose.

On the other hand, in this magnetic recording medium, a commonly used nonmagnetic support 1 can be used. As the nonmagnetic support 1, polyethylene terephthalate, polyethylene naphthalate, polyphenylene sulfide, polyaramid, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, aramid, aromatic polyamide, and the like can be used. Various treatments such as corona discharge treatment, plasma treatment, easy adhesion treatment, heat treatment, and dust removal treatment may be performed on the surface of the non-magnetic support 1 in advance.

The thickness of the nonmagnetic support 1 is 1 to 100
μm, more preferably 6 to 20 μm. In the nonmagnetic support 1, the center line average surface roughness is preferably 0.1 μm or less, more preferably 0.05 μm or less. Further, it is preferable that the nonmagnetic support 1 not only has a small center line average surface roughness but also has no coarse protrusions of 1 μm or more. The surface roughness of the non-magnetic support 1 can be freely controlled by the size and amount of the filler added to the non-magnetic support 1. These fillers include oxides and carbonates such as Ca, Si, and Ti,
An organic fine powder such as an acryl type can be exemplified.
Furthermore, F-5 in the tape running direction of the nonmagnetic support 1
The value is preferably ⁵ to 50 kg / mm ² , and the F-5 value of the nonmagnetic support 1 in the tape width direction is 3 to 30 kg / mm ^2.
mm ² is preferred. In general, the F-5 value in the tape running direction is higher than the F-5 value in the tape width direction, but this is not particularly necessary when it is necessary to increase the strength in the tape width direction. Furthermore, the non-magnetic support 1
100 ° C3 in the tape running direction and tape width direction
The heat shrinkage at 0 minutes is preferably 3% or less, more preferably 1.5% or less, and the heat shrinkage at 80 ° C. for 30 minutes is preferably 1% or less, more preferably 0.5% or less. is there. The breaking strength of the nonmagnetic support 1 is 5 to 100 in both directions.
kg / mm ² , and the elastic modulus is 100 to 100 kg / mm ^2.
It is preferably 2000 kg / mm ² .

By the way, when producing this magnetic recording medium, the paint for the lower underlayer and the paint for the upper magnetic layer described above are prepared, and these are applied by a wet-on-wet coating method or a wet-on-dry coating method. The lower base layer paint and the upper magnetic layer paint are applied. After that, an orientation process for magnetizing the upper magnetic layer 2 is performed, and thereafter, a calendar process, a cutting process, and the like are performed.

The coating for the lower underlayer and the coating for the upper magnetic layer are produced through at least a kneading step, a dispersing step, and a mixing step performed before and after these steps as necessary. Each of these steps may be divided into two or more steps. Raw materials such as a ferromagnetic powder or a soft magnetic powder and a non-magnetic powder, a binder, carbon black, an abrasive, an antistatic agent, a lubricant, and a solvent may be added at the beginning or during any step. Further, individual raw materials may be added in two or more steps in a divided manner.
For example, polyurethane, which is a binder, may be divided and supplied in a kneading step, a dispersing step, and a mixing step for adjusting viscosity after dispersion.

In producing a magnetic recording medium,
By using a wet-on-wet coating method, productivity can be further improved. At this time,
The thickness of the lower underlayer 2 is 0.5 μm or more, especially 0.5 to
3.5 μm is preferred. If the thickness of the lower base layer 2 is less than 0.5 μm, the productivity is reduced and the moldability by the calendering process is deteriorated, so that the electromagnetic conversion characteristics may be deteriorated. However, even if the lower underlayer is thinner than 0.5 μm, it can be practically used depending on the application.

Further, in the orientation treatment, it is preferable to use a solenoid of about 1000 G or more and a cobalt magnet of 2000 G or more in combination. In particular, in this alignment treatment, it is preferable to provide an appropriate drying step before the alignment so that the alignment after drying is the highest.

Further, in the calendering process, a conventionally known calendar roll is used. As the calendar roll, it is preferable to use a heat-resistant plastic roll such as epoxy, polyimide, polyamide, or polyimide amide. Further, the calendering process can be performed by a pair of metal rolls disposed opposite to each other. The calendering temperature is preferably 70 ° C. or higher, more preferably 80 ° C. or higher. The linear pressure of the calendering treatment is preferably 200 kg / cm, more preferably 300 k / cm.
g / cm or more.

In the magnetic recording medium manufactured as described above, the coefficient of friction with respect to SUS420J is 0.1.
It is preferably 5 or less, more preferably 0.3 or less. The surface resistivity of the magnetic recording medium is
It is preferably 10 ^-5 to 10 ^-12 ohm / sq,
The elastic modulus at 0.5% elongation of the upper magnetic layer 3 is preferably 100 to 2000 kg / mm ² in both the running direction and the width direction, and the breaking strength is preferably 1 to 30 kg / cm ² , The elastic modulus of the magnetic recording medium is preferably 100 to 1500 kg / mm ² in both the running direction and the longitudinal direction, and the residual elongation is preferably 0.5% or less. Further, the heat shrinkage at a temperature of 100 ° C. or lower is preferably 1% or lower, more preferably 0.5% or lower, and further preferably 0.1% or lower.

Also, in the manufactured magnetic recording medium,
The residual solvent contained in the upper magnetic layer 3 is 100 mg / m
Preferably ² or less, more preferably 10 mg / m ² or less. It is preferable that the residual solvent in the upper magnetic layer 3 is smaller than the residual solvent in the lower underlayer 2. The porosity of the upper magnetic layer 3 is preferably 30% by volume or less, more preferably 10% by volume or less.

Further, the magnetic properties of the magnetic recording medium, when measured at a magnetic field of 5000 Oe, are preferably such that the squareness ratio in the tape running direction is 0.70 or more, more preferably 0.80 or more. More preferably, it is 90 or more. The squareness ratio in two directions perpendicular to the tape running direction is preferably 80% or less of the squareness ratio in the running direction. Furthermore, the SFD of the upper magnetic layer 3 is preferably 0.6 or less.

In the above-described magnetic recording medium, an undercoat layer for improving adhesion may be formed between the nonmagnetic support 1 and the lower underlayer 2. The thickness of the undercoat layer is preferably 0.01 to 2 μm,
More preferably, it is 0.5 μm. When forming a back coat layer, the thickness of the back coat layer is 0.1 to 2
μm, more preferably 0.3 to 1.0 μm. Known undercoating layers and backcoat layers can be used.

In the magnetic recording medium configured as described above, the axial ratio (r1) of the nonmagnetic powder contained in the lower underlayer 2
/ R2) is not less than 2.5 and not more than 20 and the ratio of the soft magnetic powder to the nonmagnetic powder contained in the lower underlayer 2 is 5: 5 to 5: 5.
9: 1. And in the paint for the lower underlayer,
By containing a nonmagnetic powder having a prescribed axial ratio, the rheological properties and dispersibility are controlled. Therefore, when the coating for the lower underlayer is applied, the non-magnetic powder is oriented in the longitudinal direction of the non-magnetic support due to the flow orientation at the time of application.

Further, in the paint for the lower underlayer, the rheological properties such as the thixotropic properties are improved by containing the soft magnetic powder. And, in this lower layer underlayer coating, the ratio between the soft magnetic powder and the non-magnetic powder is specified, thereby ensuring the effect of improving the rheological properties and the effect of improving the dispersibility by the non-magnetic powder having the specified shape. The effect of improving the rheological properties by the soft magnetic powder can be ensured.

For this reason, when the coating material for the lower underlayer is applied on the non-magnetic support 1, the non-magnetic powder is present in a line and forms a strong coating film even in an undried state. Therefore, even if the ferromagnetic powder in the upper magnetic layer 3 is rotated by performing the above-described orientation treatment, it is possible to prevent the upper magnetic layer 3 and the lower underlayer 2 from being greatly mixed. That is, in this magnetic recording medium, the occurrence of disturbance at the interface between the lower underlayer 2 and the upper magnetic layer 3 can be reliably prevented, and the mixed region can be minimized. Here, the mixed region at the interface between the lower underlayer and the upper magnetic layer refers to a region where the upper magnetic layer paint component and the lower underlayer paint component are mixed.

As described above, in the above-described magnetic recording medium,
Since there is little disturbance at the interface between the lower underlayer 2 and the upper magnetic layer 3, the surface properties can be improved and the orientation of the upper magnetic layer 3 can be improved. Therefore, this magnetic recording medium can greatly improve the RF output, and can effectively reduce BER (block error rate), dropout, and the like.

In particular, even when the lower underlayer 2 and the upper magnetic layer 3 are formed by a wet-on-wet coating method, the mixed region can be made as small as possible. That is, in the wet-on-wet coating method, generally,
The interface between the lower underlayer 2 and the upper magnetic layer 3 is likely to be disturbed, but in this magnetic recording medium, the mixed area can be made as small as possible. Therefore, a magnetic recording medium with improved RF output characteristics, BER, dropout,
It can be manufactured with high productivity by employing an on-wet coating method.

Also, in this magnetic recording medium, by making the thickness of the upper magnetic layer 3 2 μm or less, it becomes suitable for recording at a short wavelength. In particular, in this magnetic recording medium,
Since the mixed area at the interface between the lower underlayer 2 and the upper magnetic layer 3 can be reduced, recording can be reliably performed even when the thickness of the upper magnetic layer 3 is 2 μm or less. In other words, in this magnetic recording medium, the upper magnetic layer 3 having a thickness of 2 μm or less can be formed without being damaged by the mixed region.

Further, in this magnetic recording medium, by using an epoxy group-containing resin having a molecular weight of 30,000 or more as a binder constituting the lower underlayer 2, the soft magnetic powder and the non-magnetic powder in the lower underlayer paint are used. Dispersibility can be improved.

[0061]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments to which the present invention is applied will be described below, but the present invention is not limited to the following embodiments.

Example 1 In Example 1, a lower underlayer having a soft magnetic powder was formed on a nonmagnetic support made of polyethylene terephthalate, and an upper magnetic layer having a ferromagnetic powder was formed on the lower underlayer. The provided magnetic recording medium (magnetic tape) was manufactured, and its characteristics were evaluated.

First, each component was weighed according to the following composition and kneaded using a kneader and a sand mill, respectively.
By dispersing, a paint for the upper magnetic layer and a paint for the lower underlayer were prepared.

<Coating Material for Upper Magnetic Layer> 100 parts by weight of ferromagnetic iron fine powder (Fe alloy powder) (Hc = 1600 Oe, σS = 135 emu / g, major axis length 0.18 μm, needle ratio = 9) Polymer 10 parts by weight (-SO ₃ Na, containing epoxy group) Polyurethane resin 5 parts by weight (-SO ₃ Na, molecular weight 45000) α-alumina (average particle size 0.2 μm) 5 parts by weight Cyclohexanone 150 parts by weight Methyl ethyl ketone 150 parts by weight 5 parts by weight of stearic acid 10 parts by weight of butyl stearate

<Coating for Lower Underlayer> Soft magnetic powder (Mn—Zn ferrite powder) 70 parts by weight (Hc = 150 Oe) Nonmagnetic powder (acicular Fe ₂ O ₃ ) 30 parts by weight (major axis length 0.5 μm) , Needle ratio 10) carbon black 5 parts by weight (average particle diameter 20 μm) vinyl chloride copolymer 8 parts by weight (-SO ₃ Na, containing epoxy group, molecular weight 45000) polyurethane resin 5 parts by weight (-SO ₃ Na containing, (Molecular weight 45,000) cyclohexane 100 parts by weight methyl ethyl ketone 100 parts by weight butyl stearate 1 part by weight

5 parts by weight of a polyisocyanate compound (trade name: Coronate L, manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to each of the paint for the upper magnetic layer and the paint for the lower underlayer thus prepared. Then, the paint for the upper magnetic layer and the paint for the lower underlayer were applied in a wet state using two doctors having different gaps (wet-on-wet coating method), and then subjected to an orientation treatment and dried. Thereafter, a calendar process was performed.

The thickness of the upper magnetic layer thus formed was 0.5 μm, and the thickness of the lower underlayer was 3.0 μm.
The raw material thus obtained was cut into 3.81 mm width to produce a digital data storage tape (DDS).

Example 2 A magnetic tape was produced in the same manner as in Example 1 except that the thickness of the upper magnetic layer was changed to 1.0 μm and the soft magnetic powder of the lower underlayer was changed to Mn-Zn ferrite powder of Hc = 200 Oe. did.

Example 3 A magnetic tape was produced in the same manner as in Example 1 except that the thickness of the upper magnetic layer was changed to 2.0 μm and the ferromagnetic powder used for the upper magnetic layer was changed to Fe alloy powder having a long axis of 0.25 μm. Was prepared.

Example 4 A magnetic tape was produced in the same manner as in Example 1, except that the nonmagnetic powder of the lower underlayer was changed to ZrO ₂ having a needle ratio of 3.

Example 5 A magnetic tape was manufactured in the same manner as in Example 1 except that the nonmagnetic powder of the lower underlayer was changed to Fe ₂ O ₃ having a needle ratio of 18.

Example 6 Except that the coating method was changed to wet-on-dry,
A magnetic tape was produced in the same manner as in Example 1.

Example 7 The magnetic powder of the lower underlayer was coated with Co having a coercive force Hc of 950 Oe.
A magnetic tape was produced in the same manner as in Example 1 except that the magnetic tape was changed to modified iron oxide.

Example 8 A magnetic tape was produced in the same manner as in Example 1 except that the ferromagnetic powder of the upper magnetic layer was changed to Ba ferrite having a plate diameter of 0.05 μm. In addition, this Ba ferrite has Hc =
1100 Oe, σS = 70 emu / g, and plate ratio = 5.

Example 9 A magnetic tape was produced in the same manner as in Example 1 except that the ratio of the soft magnetic powder to the nonmagnetic powder in the lower underlayer was changed to 5: 5.

Example 10 A magnetic tape was produced in the same manner as in Example 1 except that the ratio of the soft magnetic powder to the nonmagnetic powder in the lower underlayer was changed to 9: 1.

Example 11 A magnetic tape was produced in the same manner as in Example 1 except that the thickness of the upper magnetic layer was changed to 1.0 μm and the ferromagnetic powder used for the upper magnetic layer was changed to Fe alloy powder having a long axis of 0.34 μm. Was prepared.

Example 12 A magnetic tape was produced in the same manner as in Example 1 except that the thickness of the upper magnetic layer was changed to 2.2 μm.

Example 13 The soft magnetic powder of the lower underlayer was coated with M having a coercive force Hc = 250 Oe.
A magnetic tape was produced in the same manner as in Example 1 except that the powder was changed to n-Zn ferrite powder.

Comparative Example 1 A magnetic tape was produced in the same manner as in Example 1 except that the nonmagnetic powder of the lower underlayer was changed to granular alumina having a needle ratio of 2.1.

Comparative Example 2 A magnetic tape was produced in the same manner as in Example 1, except that the nonmagnetic powder of the lower underlayer was changed to Fe ₂ O ₃ having a needle ratio of 23.

Comparative Example 3 The ratio of the soft magnetic powder to the nonmagnetic powder in the lower underlayer was 95: 5.
A magnetic tape was produced in the same manner as in Example 1 except that the tape was changed to.

Comparative Example 4 A magnetic tape was produced in the same manner as in Example 1 except that the ratio of the soft magnetic powder to the nonmagnetic powder in the lower underlayer was changed to 4: 6.

Comparative Example 5 The ratio of the soft magnetic powder to the nonmagnetic powder in the lower underlayer was 0:10.
A magnetic tape was produced in the same manner as in Example 1 except that the tape was changed to.

<Characteristics Evaluation> The magnetic tapes of Examples 1 to 13 and Comparative Examples 1 to 5 manufactured as described above were subjected to the following characteristic evaluations.

Presence or absence of mixed region Particles (soft magnetic powder and the like) constituting the lower magnetic underlayer are measured near the interface with the lower magnetic underlayer in the upper magnetic layer, and the lower magnetic layer and the lower magnetic layer constituting the upper magnetic layer are measured. The existence ratio (%) of the particles constituting the underlayer was determined. When the presence ratio was 0.5% or less, it was determined that there was no mixed region, and when the ratio exceeded 0.5%, there was a mixed region. When measuring particles (soft magnetic powder and the like) constituting the lower underlayer, the following cases were classified.

I) When the particle shapes of the upper magnetic layer and the lower underlayer are different, for example, when the ferromagnetic powder of the upper magnetic layer is acicular and the nonmagnetic powder of the lower underlayer is granular or scaly, or When the ferromagnetic powder of the magnetic layer was plate-shaped and the non-magnetic powder of the lower underlayer was granular or acicular, the presence or absence of a mixed region was evaluated by checking the mixing of different shapes.

Specifically, a sample tape was prepared using each magnetic tape. The sample tape is manufactured by sandwiching a magnetic tape with an epoxy resin, cooling with liquid nitrogen, and cutting the tape with a microtome in the longitudinal direction and the width direction of the tape. Then, the cut surface of this sample tape was observed at a magnification of 50,000 times using a TEM (transmission electron microscope). Then, from the observation result of the TEM, the number of particles having different shapes was visually measured, and the existence ratio of the particles constituting the lower underlayer as described above was obtained.

Ii) When the particle shapes are the same but the average diameters of the major axis lengths are different. For example, the ferromagnetic powder of the upper magnetic layer and the nonmagnetic powder of the lower nonmagnetic layer are both acicular but have different average diameters of the major axis length. In the case, or when the ferromagnetic powder of the upper magnetic layer is plate-shaped, and the non-magnetic powder of the lower non-magnetic layer is scaly but the average diameter of the major axis is different, how many different average diameters are mixed. The presence or absence of the mixed area was evaluated depending on whether the area was mixed.

Specifically, as in the case of i), the TEM
The number of particles having different average diameters was visually measured based on the observation, and the abundance ratio of the particles constituting the lower underlayer as described above was obtained.

Iii) When both the particle shape and the average diameter are equal In this case, since it is difficult to visually measure the number of particles constituting the lower underlayer, the lower underlayer is formed using a so-called micro Auger electron spectroscopy. The existence ratio of the constituent particles was determined.

More specifically, by detecting a specific element contained in each layer near the interface between the upper magnetic layer and the lower nonmagnetic layer by micro Auger electron spectroscopy, the lower underlayer as described above is formed. The existence ratio of particles to be mixed was determined, and the presence or absence of a mixed region was evaluated.

RF output The RF output is a recording / reproducing device (DTC-1) manufactured by Sony Corporation.
000), a signal having a single reproduction frequency of 4.7 MHz was input, the reproduced signal was output to a spectrum analyzer, and the peak value of the signal was read. The spectrum analyzer is HP-358
5A (made by Hewlett-Packard Company), and the output of a commercially available magnetic tape (trade name: DG90M) is 0d
The relative output as B was measured.

A BER (block error rate) computer converted the random signal by 24-25. The scrambled interleaved NRZ-I was used as a test signal, an error was detected in data obtained by recording / reproducing this test signal, and the error ratio was defined as BER.

A 4.7 MHz single frequency signal is input to the dropout magnetic tape.
Threshold (DO) A dropout of 0.5 μSEC in length at a level of −10 dB was measured with a dropout counter.

Note that, in the above-described examples and comparative examples,
The dry thickness of the upper magnetic layer was measured from a cut surface observed with a TEM by preparing a sample in the same manner as in the method for determining the presence or absence of a mixed region.

Tables 1 and 2 show the compounding ratios and the results of the evaluation of the properties of Examples 1 to 13 and Comparative Examples 1 to 5.

[0098]

[Table 1]

[0099]

[Table 2]

As is clear from the results of Tables 1 and 2, in Examples 1 to 13, the ratio of the soft magnetic powder to the non-magnetic powder was 5: 5 to 9: 1, and Since the needle ratio (r1 / r2) of the magnetic powder was 2.5 or more and 20 or less, it was found that the RF output was excellent, the dropout was small, and the BER was low. On the other hand, in Comparative Examples 1 to 5, good results were not obtained in at least one of BER, dropout, and RF output. The target level of the BER is 10 ⁻⁴ or less, the number of dropouts is 100 or less, and the RF output is 3.0 dB or more.

Particularly, when Examples 1 and 2 are compared with Examples 7 and 13, various characteristics are obtained by specifying the coercive force of the soft magnetic powder contained in the lower underlayer to be 200 Oe or less. It turns out that it becomes more excellent. In addition, comparing Example 1 with Example 11, it can be seen that various characteristics are excellent by regulating the major axis length of the ferromagnetic powder contained in the upper magnetic layer to 0.3 μm or less. .

[0102]

As described above in detail, the magnetic recording medium according to the present invention regulates the ratio between the soft magnetic powder and the non-magnetic powder contained in the lower underlayer within a predetermined range,
By defining the value of (r1 / r2) of the nonmagnetic powder in a predetermined range, the mixed region at the interface between the lower underlayer and the upper magnetic layer can be reduced. Therefore, this magnetic recording medium has excellent electromagnetic conversion characteristics, excellent running durability, low dropout, and low block error rate.

[Brief description of the drawings]

FIG. 1 is a sectional view of a main part of a magnetic recording medium according to the present invention.

[Explanation of symbols]

 1 Non-magnetic support, 2 Underlayer, 3 Upper magnetic layer

Claims (8)

[Claims] A non-magnetic support; a lower base layer formed by applying a coating for a lower base layer mainly composed of a soft magnetic powder, a non-magnetic powder, and a binder on the non-magnetic support; An upper magnetic layer formed by applying a coating for an upper magnetic layer mainly composed of a ferromagnetic powder and a binder, on the lower underlayer, wherein the soft magnetic powder and the non-magnetic powder And the ratio (r1 / r2) of the major axis length (r1) to the minor axis length (r2) of the nonmagnetic powder is 2.5 or more and 20 or less. A magnetic recording medium characterized by the above-mentioned. 2. The magnetic recording medium according to claim 1, wherein the thickness of the upper magnetic layer is 2 μm or less. 3. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder of the upper magnetic layer has an average major axis length of 0.3 μm or less. 4. The ferromagnetic powder of the upper magnetic layer comprises Fe,
2. The magnetic recording medium according to claim 1, wherein the magnetic recording medium is a ferromagnetic alloy fine powder containing Ni or Co as a main component (75% or more). 5. The magnetic recording medium according to claim 1, wherein the soft magnetic powder in the coating material for the lower underlayer has a coercive force (Hc) of 200 (Oe) or less. 6. The magnetic recording medium according to claim 1, wherein the non-magnetic powder in the coating for the lower underlayer has a major axis length of 3 μm or less. 7. The non-magnetic particles of the lower underlayer are Fe ₂
O ₃ , ZrO ₂ , Al ₂ O ₃ , Cr ₂ O ₃ , SiO ₂ and Ti
_2. The magnetic recording medium according to claim 1, comprising at least one selected from O2. 8. The magnetic recording medium according to claim 1, wherein the lower underlayer and the upper magnetic layer are applied by a wet-on-wet coating method or a wet-on-dry coating method. .