JP2001338417A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium having high output in short- wavelength recoding with <=0.8 μm median recording wavelength necessary for a miniature videotape of high image quality, etc., and free of the rise of noise due to storage. SOLUTION: The magnetic recording medium is a tape-shaped one used for high density recoding with <=0.8 μm median recording wavelength and obtained by disposing a magnetic coating film containing a magnetic metal powder and having a surface smoothness represented by <=0.004 ×m center line average roughness on one face of a base and a back coat layer containing carbon black as primary particles or aggregates and having a surface smoothness represented by <=0.01 μm center line average roughness on the other face.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-density recording, high-output, and sufficiently reduced noise magnetic recording medium required for a small and high-quality video tape. In particular, a tape having a magnetic coating such as a magnetic tape and a back coat layer, and having a total thickness of 14 μm or less of a support, a magnetic coating, and a back coat layer is wound. An object of the present invention is to ensure the reliability of a magnetic recording medium of the type so that noise does not increase even after long-term storage.

[0002]

2. Description of the Related Art In general, a magnetic recording medium is obtained by applying a magnetic paint obtained by kneading and dispersing a magnetic powder, a binder, an inorganic powder, a lubricant and the like in an organic solvent on a support such as a polyester film, and drying. It is formed by forming a magnetic coating film. In particular, in a video tape or the like, a back coat layer of a coating film containing a binder, an inorganic powder, a lubricant, and the like is formed on the back surface of the support to improve running properties.

[0003] In order to prevent abrasion when coming into contact with a magnetic head or a guide roller, an inorganic powder having a Mohs hardness of 5 or more is contained in both the magnetic coating film and the back coat layer. Has been proposed. The back coat layer is described in JP-A-62-112, JP-A-62-38525, JP-A-62-38526, JP-A-62-38527, and JP-A-62-38527.
Nos. 38,528, and JP-A-47-18572, JP-A-48-15003, and JP-A-48-15003.
Nos. 9-39402, 52-28642 and 52-4
Nos. 9961 and 55-15771.

In order to achieve high-density recording, it is necessary to shorten the recording wavelength. For this reason, fine magnetic powder shorter than the recording wavelength is used. Various proposals have been made to increase the magnetic characteristics in order to obtain a high output. For example, as typified by Japanese Patent Publication No. 61-37761, a high saturation magnetization is required to improve the magnetic characteristics. High output is achieved by using ferromagnetic metal powder. In order to reduce noise, it is known to reduce the noise by smoothing the surface of the magnetic coating by performing a surface finishing treatment on the magnetic coating by super calendaring to reduce the noise. It has been proposed in each publication such as No. 12688.

[0005]

However, a magnetic recording medium obtained by using a ferromagnetic metal powder as a magnetic recording element and undergoing a surface finishing treatment has low noise at first, but increases in noise after storage. There was a problem that would. This problem is remarkable when the particle size of the magnetic powder is small and when a metal magnetic powder softer than the iron oxide-based magnetic powder is used, and the noise for a signal having a frequency corresponding to a recording wavelength of about 0.8 μm or less is reduced. Became apparent after storage.

Although such a problem has occurred almost uniformly for various prototypes, only the magnetic recording medium manufactured without the back coat layer maintains the initially reduced noise even after storage. It was done. Therefore, it was thought that such a problem was caused by the back coat layer, and after various investigations, when the magnetic tape was wound, the unevenness of the back coat layer surface was transferred to the surface of the magnetic coating film. And
It was considered that the unevenness formed on the surface of the magnetic coating film caused the noise.

[0007] Such an increase in noise is particularly caused when the total thickness of the support, the magnetic coating film, and the backcoat layer is 14%.
It was found to be remarkable in a magnetic recording medium of a type in which a tape having a diameter of less than μm was wound. This is a total thickness of 20μm
It is considered that one of the causes is that the number of windings per unit winding diameter is larger than that of a thicker one, and the winding tightness particularly near the innermost side of the hub is remarkable.

A phenomenon similar to this problem has already been pointed out in Japanese Patent Application Laid-Open No. 60-32122, in which the inorganic powder having a Mohs hardness of 5 or more in the back coat layer has a particle size of 0.1. It is described that the use of a soft inorganic powder in an amount not exceeding 2 μm or more in combination with the soft inorganic powder prevents scratches such as a tape guide. JP-A-63-
Japanese Patent Application Laid-Open No. 255584/1999 also recognizes a phenomenon that the unevenness of the surface of the back coat layer moves to the magnetic coating film and lowers the S / N and C / N of the magnetic recording medium. It is pointed out that this is relevant.

However, the technology disclosed in these documents relates to a device using a so-called Co-containing iron oxide magnetic powder as the magnetic powder, and the total thickness of the support, the magnetic coating film and the back coat layer is about 20 μm. The above describes a VHS type magnetic recording medium.
In the HS system, the shortest recording wavelength of the white level is only 1.3 μm. That is, these documents use a ferromagnetic metal powder as a magnetic recording element and have a center recording wavelength of 0.
No suggestion was made about reduction of noise in a magnetic recording medium of 8 μm or less.

[0010]

Means for Solving the Problems Accordingly, the present inventors have:
As a result of intensive studies, first of all, the susceptibility of the magnetic coating film and the back coat layer to damage depends not only on the hardness of the powder contained in the coating film but also on the particle size of the powder. We found that a small size reduced the reinforcing effect of the coating film. In view of this fact, when a magnetic recording tape was wound,
In order to prevent the inorganic powder having a Mohs hardness of 5 or more contained in the coating layer from damaging the surface of the magnetic coating film, the inorganic powder of such a back coating layer should be coated with the long axis of the metal magnetic powder. I found that it had to be smaller than the diameter.

Second, it is necessary to include carbon black in the back coat layer in order to improve running properties. This itself is achieved by including carbon black of fine particles of 10 to 30 μm, carbon black of coarse particles of 200 to 500 μm, and non-magnetic powder of 0.2 μm or less, thereby suppressing an increase in dropout and improving running durability. It has already been described in Japanese Patent Application Laid-Open No. 62-8328 that it is improved. However, this document does not mention the relationship between the particle size of the primary particles and the size of the aggregates of the carbon black and the major axis diameter of the ferromagnetic metal powder in the magnetic coating film. When the ferromagnetic metal powder having a small diameter is used.
No study has been made on the relationship between a signal having a frequency corresponding to a recording wavelength of 8 μm or less and noise with respect to a signal having a recording wavelength.

The present inventors have studied the relationship between the particle diameter of primary particles and agglomerates of carbon black in the back coat layer and the major axis diameter of the ferromagnetic metal powder in the magnetic coating film. It has been found that when the particle size of the primary particles and the size of the aggregates of the carbon black are larger than that of the magnetic powder, the requirements for high output and low noise can be sufficiently satisfied. This remarkably appears in a magnetic recording medium having a magnetic coating having a thickness of about 3.0 μm or less corresponding to a demand for a high recording density, particularly, a thinner magnetic coating.

Further, the present inventors have provided a magnetic coating film and a back coat layer on both surfaces of the support, respectively, to provide a high output, and at the time of high-density recording with a central recording wavelength of 0.8 μm or less. In order to obtain a magnetic recording medium that can suppress the rise of noise as much as possible despite the storage of the period, it is necessary to give the magnetic coating a squareness ratio of 0.85 or more under the measurement condition of the applied magnetic field of 10 KOe. It is preferable that the coercive force of the magnetic coating be 1,500 Oe or more, that the surface smoothness of the magnetic coating be regulated to a center line average roughness of 0.004 μm or less. The surface smoothness of the layer is also 0.01 μm or less in center line average roughness,
It has also been found that it is preferable to regulate the diameter of the metal magnetic powder to be smaller than the major axis diameter.

In addition, carbon black and the like are generally soft, and it has been unexpectedly expected that what is contained in the back coat layer will damage the surface of the magnetic coating film by winding the tape. That is. However, in order to achieve high-density recording with a center recording wavelength of about 0.8 μm or less, the major axis diameter of the magnetic powder is reduced to 0.2 μm or less, and a magnetic coating film mainly containing metal magnetic powder is used. It can be said that carbon black is not enough because the effect of reinforcing the coating film is not sufficient and a strong tightening force can be applied under the harsh conditions of multiple turns of thin tape required by high density recording. It has been found that small particles may damage the surface of the magnetic coating film.

In this case, if carbon black having a diameter larger than the major axis diameter of the metal magnetic powder is contained as primary particles or aggregates in the back coat layer, the particles having a small particle diameter have a smaller gap. It is preferable that the inorganic powder having a Mohs hardness of 5 or more contained in the back coat layer is smaller than the major axis diameter of the metal magnetic powder. Preferably smaller,
Further, when the magnetic tape thus configured is wound around a hub of a polyoxymethylene resin or the like with the back coat layer inside, the ABS and the ABS have a Mohs hardness of 2 to 4.
It has been found that it is preferable to house the ink in a cassette body containing the above pigment.

The present invention has been made with further studies based on the above findings. That is, the present invention
A magnetic coating containing metal magnetic powder on one surface of the support and having a surface smoothness of 0.004 μm or less in center line average roughness,
A center recording wavelength of 0.1 mm, characterized in that a back coat layer containing carbon black of primary particles or aggregates and having a center line average roughness of 0.01 μm or less is provided on the other surface. An object of the present invention is to provide a tape-shaped magnetic recording medium (claim 1) used for high-density recording of 8 μm or less.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the support is made of a non-magnetic material such as polyethylene terephthalate.
Polyesters such as polyethylene-2,6-naphthalate; polyolefins such as polyethylene and polypropylene; cellulose derivatives such as cellulose triacetate and cellulose diacetate; polyvinyl chloride; polyvinylidene chloride; Vinyl resin, polycarbonate
Bones, polyimides, polyamides, and the like.
Among them, polyesters such as biaxially oriented polyethylene terephthalate and polyethylene-2,6-naphthalate and having an elastic modulus in the longitudinal direction of 700 kg.
/ Mm ² or more, an elastic modulus in the width direction of 400 kg / mm ² or more, and a surface roughness of 0.01 μm or less are particularly desirable.

On such a support, a magnetic resin is contained in a binder resin dissolved in an organic solvent, and a magnetic coating material in which the magnetic powder is well dispersed is applied and dried while performing a magnetic field orientation treatment. Form a magnetic coating.

Examples of the metal magnetic powder used for the magnetic coating film include metals such as Fe, Co, and Ni, Fe, Co, Ni, and the like.
Alloys obtained by combining Zn and the like, and ferromagnetic fine powders in which an oxide film such as Fe, Al, and Si is formed on the surface of these metals and alloys are exemplified. The major axis diameter of these metal magnetic powders is preferably 0.8 μm or less in relation to the central recording wavelength, and is preferably 0.2 μm or less from the viewpoint of resolution.

Among these, it is desirable to use a metal magnetic powder mainly composed of iron and containing manganese in an amount of 0.1% by weight or more with respect to iron. In particular, manganese is 0.1% by weight of iron
The metal magnetic powder containing 1 to 50 times the amount of alkaline earth metals such as calcium and magnesium with respect to manganese with respect to manganese is relatively hard, has excellent magnetic properties and electromagnetic conversion properties, and is sufficiently hard. It is preferable since the magnetic coating film having a reinforcing effect of less than 0.2 μm can be prevented from being damaged by an inorganic powder having a Moth hardness of 5 or more on the surface of the back coat layer.

In particular, when manganese is present as an oxide or an acid hydroxide on the surface of the metal magnetic powder, a uniform and dense coating is formed. Therefore, manganese should be present on the surface as manganese oxide, manganese hydroxide, or the like. preferable. When the composition is adjusted so that the weight ratio of nickel is 2% by weight or more and the weight ratio of cobalt to nickel is 110% by weight or more in an alloy powder containing nickel and cobalt metal elements, It is preferable because the reduction rate of the saturation magnetization (σs) is 30% or less even when the device is left in an environment of 90% humidity for 7 days. Further, it is desirable that the value of the quotient obtained by dividing the half width of the anisotropic magnetic field distribution by the coercive force is 3.2 or more in order to maintain the erasing characteristics.

In the present invention, examples of the inorganic powder having a Mohs hardness of 5 or more that can be used as required include metal oxides, metal carbides, and metal nitrides. Among them, α-Fe ₂ O ₃ , Al ₂ O ₃ , Cr ₂ O
₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SiC
, TiC, hBN, SiN ₄ (for example, indicates a Mohs hardness of 6) and the like can be preferably used. As these inorganic powders, those having various particle diameters can be easily obtained, and can be appropriately selected in accordance with the above findings of the present invention.

In the present invention, as the carbon black used in the back coat layer, any of a channel black, a furnace black, an acetylene black and a thermal black can be used, but acetylene black is particularly preferable. Also, Japanese Patent Application Laid-Open No. Sho 61-22424
It is also possible to use a graphitized carbon black whose surface is wrapped with a graphite layer, as disclosed in Japanese Patent Application Laid-Open No. H10-209,086.

[0024] Commercially available carbon blacks include black particles having a particle size of 18 µm as manufactured by CABOTT, USA.
700, Mogal L having a particle size of 20 μm, ELFTEXpellets-115 having a particle size of 27 μm, and the same legal 30
01, Vulcan XC-72 with a particle size of 30 mμ, particle size of 75 m
μ Staring NS, Staring R, etc. made by Columbia Carbon Co., Ltd.
Ben 8000, Raben 5250 with a particle size of 20 μm, Raben 890 with a particle size of 30 μm, Raben 4 with a particle size of 62 μm
50, Raben 410 with a particle size of 70 mμ, particle size of 280 mμ
(0.28 μm) Raben MT-P bead, particle size 30
0mμ (0.30μm) Rabensevacarb MT-C
I and the like are preferably used.

Similarly, as a product of Asahi Carbon Co., Ltd., HS-500 having a particle size of 75 mμ and # 6 having a particle size of 35 mμ are used.
0H, manufactured by Tokai Carbon Co., Ltd., and having a particle size of 20 μm, 5H, manufactured by Akzo, the Netherlands, and having a particle size of 30 μm, Ketjen Black EC, manufactured by Mitsubishi Kasei Co., Ltd. # 4040, particle size 23mμ
# 4330BS, # 4350BS with a particle size of 45 μm, # 4010 with a particle size of 80 μm, and the like are preferably used.

As described above, carbon blacks having various particle sizes can be easily obtained. Therefore, according to the above findings of the present invention, the carbon black is appropriately selected in design in consideration of the long axis diameter of the metal magnetic powder. However, when the particle size is relatively small, it is desirable to form an aggregate of several primary particles by utilizing the ability of the carbon black itself to form a structure. In this case, the aggregates function as if they were one carbon black particle.

The carbon black and, if necessary, the inorganic powder may be mixed into the paint for forming the back coat layer so as to satisfy the above-mentioned particle size relationship. Further, if necessary, it can be mixed in a paint for forming a magnetic coating film.

The binder resin used for the magnetic coating film and the back coat layer includes vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-acrylate copolymer. , A vinyl chloride resin such as a vinyl chloride-vinylidene chloride copolymer, a vinyl chloride-acrylonitrile copolymer, a vinyl chloride-vinyl acetate-maleic acid copolymer, a thermoplastic polyurethane resin, a thermosetting polyurethane resin, a polyester resin, Examples include phenoxy resin, polyvinyl butyral resin, cellulose derivative, epoxy resin, and mixtures thereof.

Further, these resins may be provided with a hydroxyl group, a carboxylic acid, a sulfonic acid, a sulfonate, a phosphoric acid, a phosphate,
Amine, a hydrophilic polar group such as ammonium salt is introduced to improve the dispersibility of powder particles as a coating material,
An acrylic double bond may be introduced to cure by irradiation with an electron beam. In particular, as the binder resin of the magnetic coating film, a resin containing a hydrophilic polar group-containing vinyl chloride-based resin obtained by introducing the above-mentioned hydrophilic polar group into the vinyl chloride-based resin is preferably used.

Solvents used for preparing paints for forming these coating films and the like include ethanol and propanol.
Alcohols such as toluene and butanol, esters such as methyl acetate, ethyl acetate and butyl acetate, ketones such as methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, ethers such as tetrahydrofuran and dioxane, benzene and toluene , Aromatic hydrocarbons such as xylene, aliphatic hydrocarbons such as heptane, hexane and cyclohexane, and chlorinated hydrocarbons such as methylene chloride, ethylene chloride and chloroform. A mixed solvent of cyclohexanone-toluene is preferred.

In the above magnetic coating film, as other additives,
Lubricants such as saturated or unsaturated higher fatty acids, higher fatty acid amides, fatty acid esters, higher alcohols, silicone oils, mineral oils, edible oils, and fusso oils can be used.

Such a composition is achieved by dispersing a predetermined amount in a ball mill or a sand mill to form a magnetic coating material and a coating material for a back coat layer, and applying the coating material on the support. One of the most important things to consider when dispersing paint is to apply extra force to the particles of inorganic powder and carbon black so as not to change the particle size of the carbon black formulated as designed. Be careful.

At the time of coating, it is preferable to apply a magnetic coating film first, orient the magnetic field before drying, perform a surface smoothing treatment on the coating film, wind the film once, and then apply a back coat layer on the back surface. desirable. The order of this process is described in
No. 23647. The magnetic coating film and the back coat layer may be applied alone or as a plurality of layers, or may be applied simultaneously.

The coated magnetic recording medium is cut into a predetermined tape width by a slit device, usually wound around an individual bubble with the backcoat layer inside, and housed in a tape cassette body. Is done. Since a strong radial tightening force is applied to the hub serving as the core, the sink of the tape winding peripheral surface is controlled within 5 μm to prevent the tape from being curled. It is desirable.

The total thickness of the support, the magnetic coating film and the back coat layer is preferably set to 14 μm or less. In this case, it is desirable that the elastic modulus in the longitudinal direction is larger than the elastic modulus in the width direction and does not exceed twice the elastic modulus in the width direction. Further, it is desirable that the elastic modulus in the longitudinal direction is 1,000 kg / mm ² or more. The magnetic coating also has a volume reduction of 20 × 10 ⁻⁵ mm ³ or less due to abrasion of the steel balls when the steel balls having a diameter of 6 mm are pressed with a load of 5 g and run over 23 meters. It is preferable to regulate such that

Making the modulus of elasticity in the longitudinal direction larger than the modulus of elasticity in the width direction and not exceeding twice the modulus of elasticity in the width direction depends on the selection of an appropriate support and the magnetic field orientation of magnetic particles. This can be achieved by mechanical control. Further, setting the elastic modulus in the longitudinal direction to 1,000 kg / mm ² or more can be achieved by appropriately selecting the curing conditions of the binder resin of each coating film. further,
In order to regulate the volume reduction due to the wear of the magnetic coating film to 20 × 10 ⁻⁵ mm ³ or less, it is necessary to include an appropriate amount of an inorganic powder having a Mohs hardness of 5 or more and to set conditions for the surface smoothing treatment. It can be achieved by selection, selection of drying conditions, and the like.

Such a magnetic recording tape is formed by a backing tape.
Since the tape layer contacts the tape guide member of the tape cassette main body, it is preferable to use ABS resin for the tape cassette main body and at least a portion of the tape guide in order to reduce noise. It is preferable to include a pigment having a moth hardness of 2 to 4 such as barium sulfate to prevent damage to the back coat.

The cassette body further contains an antistatic agent such as carbon black and a quaternary ammonium salt such as polyoxyethylene alkylamine, and a molten resin such as ethylene bisstearamide to further reduce electrostatic noise. It is preferable to include an agent. To manufacture such a cassette body, these compositions are usually melted and cast into a mold by injection molding.

As described above, according to the above configuration of the present invention, small and high quality video tapes are required.
In short-wavelength recording with a central recording wavelength of about 0.8 μm or less, it is possible to obtain a magnetic recording medium which has high output and suppresses as much as possible a phenomenon that noise increases upon storage.

[0040]

The present invention will be described more specifically with reference to several experiments including examples and comparative examples. Hereinafter, "parts" means parts by weight.

Experiment 1 (A) Formation of Magnetic Coating Film Alumina was coated on the surface with a coercive force of 1600 Oe, a saturation magnetization of 120 emu / g, a major axis diameter of 0.18 μm, and an axial ratio of 10.
0.2% by weight based on iron
100 parts of a ferromagnetic metal iron powder containing manganese and 1.0% by weight of calcium, 10 parts of a hydroxyl group-containing vinyl chloride resin having a degree of polymerization of 340, 7 parts of a thermoplastic polyurethane resin, and a particle size of 0.2 μm. 8 parts of alumina, 2 parts of myristic acid, and bengara (α-Fe ₂ O) having a particle size of 0.8 μm
₃ ) A mixture of 2 parts, 1 part of # 4010 manufactured by Mitsubishi Kasei Co., Ltd. having a particle size of 80 mμ, and 2 parts of Seam 5H manufactured by Tokai Carbon Co., Ltd. having a particle size of 20 mμ as a carbon black. A composition prepared by mixing 70 parts each of cyclohexanone and toluene was kneaded and dispersed in a ball mill for 96 hours, and then 5 parts of a trifunctional polyisocyanate compound was added. Prepared.

This magnetic paint was applied to a support made of a biaxially oriented polyethylene terephthalate film having a thickness of 10 μm so as to have a thickness of 2.5 μm, dried, and calendered. A magnetic coating was formed.

(B) Formation of Back Coat Layer As a carbon black, 60 parts of a seaweed 5H manufactured by Tokai Carbon Co., Ltd. having a particle size of 20 mμ and a particle size of 280 mμ (0.
Raven MT manufactured by Columbian Carbon Co., Ltd.
7.5 parts of P bead, 30 parts of calcium carbonate having a particle size of 0.05 μm, 2.5 parts of bengara having a particle size of 0.1 μm, 45 parts of a thermoplastic polyurethane resin, and 40 parts of nitrocellulose And 15 parts of a trifunctional isocyanate crosslinking agent were mixed with 330 parts of cyclohexanone and toluene, and this composition was kneaded and dispersed in a ball mill for 96 hours to prepare a paint for a back coat layer. did.

This backcoat layer paint is applied to the back surface of the support on which the magnetic coating film has been formed so as to have a thickness of 1.0 μm and dried to form a backcoat layer. At 60 ° C. for 20 hours. The magnetic recording medium thus obtained had a total thickness of 13.5 μm. This is cut into a predetermined width to form a tape, and is made of an injection molded article of a polyoxymethylene resin in which the sink of the tape winding surface is processed to within 0.1 μm with the back coat layer inside. Wound around the hub.

(C) Preparation of cassette body ABS resin NA-1060 manufactured by Denki Kagaku Kogyo Co., Ltd.
Part, colorant obtained by treating carbon black with ferrocyanine blue, calcium carbonate (particle size: 0.5 μm)
35 parts, 12 parts of polyoxyethylene alkylamine and 3 parts of ethylene bis stearamide were added, kneaded at 110 ° C. for 1 minute by a Hensiel mixer, and extruded at 220 ° C. by a twin screw extruder to pelletize. Further, together with 1,500 parts of the ABS resin, the mixture was melted at 240 ° C. in a heating furnace, and the cassette body was injection molded at a mold temperature of 30 ° C. The above-described hub-wound tape was accommodated in the cassette body to assemble a magnetic recording medium.

Experiment 2 In the formation of the back coat layer in Experiment 1, 7.5 parts of a Raben MT-P bead manufactured by Columbian Carbon Co., Ltd. having a particle size of 280 mμ (0.28 μm) were used as the carbon black. A magnetic recording medium was produced in the same manner as in Experiment 1, except that the same amount of Labensevacarb MT-CI having a particle size of 300 μm (0.30 μm) was used instead.

Experiment 3 In the formation of the back coat layer in Experiment 1, 7.5 parts of Raben MT-P beads having a particle size of 280 mμ (0.28 μm) manufactured by Columbia Carbon Co., Ltd. were used as the carbon black. In place of the above, # 40 of Mitsubishi Kasei Co., Ltd.
In the same manner as in Experiment 1, except that the composition was kneaded and dispersed in a ball mill for 22 hours instead of using the composition for kneading and dispersing in a ball mill for 96 hours. Thus, a magnetic recording medium was manufactured. When the coating was observed at the time of kneading and dispersion in the ball mill for 22 hours, the carbon black particles formed a structure of 13 particles each on average, and the diameter of the aggregate was 210 mμ.

Experiment 4 In the formation of the back coat layer in Experiment 1, 7.5 parts of Raven MT-P beads having a particle size of 280 mμ (0.28 μm) manufactured by Columbia Carbon Co., Ltd. were used as carbon black. In place of the above, # 40 of Mitsubishi Kasei Co., Ltd.
A magnetic recording medium was manufactured in the same manner as in Experiment 1 except that Sample No. 10 was used. When the coating was observed at the time of kneading and dispersion in a ball mill for 96 hours, the carbon black particles hardly formed a structure, and the primary particles had a diameter of 80 mμ.

Experiment 5 In the formation of the back coat layer in Experiment 1, the particle size was 0.1 μm.
The same amount of particle size 0.8 μm instead of 2.5 parts
A magnetic recording medium was produced in the same manner as in Experiment 1 except that the bengara was used.

Experiment 6 In the formation of the magnetic coating film of Experiment 1, the same amount of coercive force 1600 Oe and saturation magnetization 12
A ferromagnetic metal powder having 0 emu / g, a major axis diameter of 0.35 μm, and an axial ratio of 10 was used. In forming the back coat layer in Experiment 1, 2.5 parts of bengara having a particle size of 0.1 μm were used instead. , Except that the same amount of bengara with a particle size of 0.8 μm was used.
A magnetic recording medium was manufactured in the same manner as in Experiment 1.

Experiment 7 In the formation of the back coat layer in Experiment 1, the particle size was 0.1 μm.
The same amount of particle size 0.8 μm instead of 2.5 parts
The same amount of particle diameter of 80 mμ was used instead of 7.5 parts of Raven MMT-P beads manufactured by Columbian Carbon Co., Ltd. having a particle diameter of 280 mμ (0.28 μm). A magnetic recording medium was manufactured in the same manner as in Experiment 1 except that # 4010 manufactured by Mitsubishi Kasei Co., Ltd. was used.

Experiment 8 In Experiment 1, after forming the backcoat layer, the tape cut to a predetermined width was wound around a hub with the backcoat layer inside, and the tape was wound around the tape as a hub. Except for using an injection-molded product of a polyoxymethylene resin having a surface sink of 13 μm (the post-molding process was not performed and the sink was not controlled), the same as in Experiment 1 except that A magnetic recording medium was manufactured.

Next, the elastic modulus of the magnetic recording medium of Experiment 1 was measured. The result is that the elastic modulus in the longitudinal direction is 1,100 kg / mm ² and the elastic modulus in the width direction is 70 kg / mm ^2.
It was 0 kg / mm ² . In addition, a diameter of 6
When a steel ball of mm was pressed with a load of 5 grams and allowed to run over 23 meters, the volume reduction due to wear of the steel ball was 15 × 10 ⁻⁵ mm ³ .

For each of the magnetic recording media of Experiments 1 to 8, the surface smoothness of the magnetic coating film and the back coat layer was measured using a stylus type surface roughness tester. It was measured under the condition of .08 mm and expressed as the center line average roughness. In addition, the squareness ratio and the coercive force of the magnetic coating under an applied magnetic field of 10 KOe were measured. Further, the noise level at 6 MHz when a 7 MHz signal was input and reproduced was measured immediately after winding the hub and after storage for 60 days, and the change in the noise level before and after storage was measured in Experiment 1 after the hub winding. The level was set to 0 dB as a standard, and expressed as a noise ratio in dB. The results were as shown in Table 1 below.

[0055]

It should be noted that in the above experiment, the magnetic coating was
Although alumina and red iron oxide are included as inorganic powders having a hardness of 5 or more, even when an experiment similar to the above is performed without including these inorganic powders, the same result as above can be obtained. Wakata. As is clear from these results, the magnetic recording medium employing the configuration of the present invention was able to suppress an increase in noise even after storage for 60 days. On the other hand, in the magnetic recording medium not employing the configuration of the present invention, noise increased after storage.

[0057]

As described above, according to the present invention, by adopting the above-mentioned specific structure, the magnetic recording medium suitable for short-wavelength recording does not increase in noise even when stored for a long time. Can be provided.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Akira Kato 1-88 Ushitora, Ibaraki-shi, Osaka F-term within Hitachi Maxell, Ltd. 5D006 BA01 BA19 CC02 CC03 EA01 FA02 FA09

Claims (1)

[Claims] 1. A magnetic coating containing metal magnetic powder on one surface of a support and having a surface smoothness of 0.004 μm or less in center line average roughness, and a carbon black of primary particles or aggregates on the other surface. A tape coating used for high-density recording at a central recording wavelength of 0.8 μm or less, characterized in that a back coat layer having a surface smoothness of 0.01 μm or less in center line average roughness is provided. Magnetic recording medium.