JP2797263B2 - Magnetic recording media - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a magnetic recording medium that has high density recording, high output, and sufficiently reduced noise required for a small and high-quality video tape or the like. In particular, a magnetic recording medium of a type having a magnetic layer such as a magnetic tape and a back layer, a support, a magnetic coating film, and a total thickness of the back coat layer of 14 μm or less being wound on a tape. The purpose is to ensure reliability so that noise does not increase even after long-term storage.

[Prior art]

Generally, magnetic recording media are coated on a support such as a polyester film with a magnetic paint made by kneading and dispersing magnetic powder, binder, inorganic powder, lubricant, etc. in an organic solvent, and forming a dried magnetic coating film. It is made up of In particular, in the case of video tapes and 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 in order to improve running properties. In particular, it has been proposed to include an inorganic powder having a Mohs hardness of 5 or more in both the magnetic layer and the back coat layer in order to prevent abrasion from occurring when the magnetic layer contacts a magnetic head or a guide roller. Japanese Patent Application Laid-Open Nos. 62-112 and 62-3 describe the method of including an inorganic powder having a Mohs hardness of 5 or more in the back coat layer.
No. 8525, No. 62-38526, No. 62-38527, No. 62-38528
No. 47-18572 for the magnetic layer.
Nos. 48-15003, 49-39402, 52-28642,
Nos. 52-49961 and 55-15771. By the way, in order to achieve high-density recording, it is necessary to shorten the recording wavelength, and therefore, fine magnetic powder shorter than the recording wavelength is used. In addition, various proposals have been made to increase the magnetic characteristics in order to obtain a high output. For example, as represented by JP-A-61-37761, a ferromagnetic material having a high saturation magnetization in order to improve the magnetic characteristics has been proposed. High output is achieved by using metal powder. Furthermore, in order to reduce noise, surface finishing treatment of the magnetic layer by a super calendering treatment to smooth the magnetic layer surface and reduce noise is disclosed in JP-B-52-17404, 6
It has already been proposed in, for example, No. 0-12688.

[Problems to be solved by the invention]

However, a magnetic recording medium obtained through a surface finishing treatment using a ferromagnetic metal powder as a magnetic recording element is:
Initially, the noise is low, but there is a problem that the noise rises after storage. This problem is remarkable when the particle diameter of the magnetic powder is small and when a metal magnetic powder softer than the iron oxide-based magnetic powder is used, and noise for a signal having a frequency corresponding to a recording wavelength of about 0.8 μ or less is reduced. It was found to rise after storage. This problem occurred almost evenly for various prototypes, but only the magnetic recording medium manufactured without the backcoat layer maintained the originally reduced noise after storage. . Therefore, we considered this problem to be caused by the backcoat layer and conducted various studies.When the magnetic tape was wound, the unevenness of the surface of the backcoat layer was transferred to the surface of the magnetic layer. Was thought to be the cause. The increase in noise was found to be particularly remarkable in a magnetic recording medium of a type in which a tape having a total thickness of the support, the magnetic coating film, and the back coat layer of 14 μm or less was wound. This is compared to a thick one with a total thickness of about 20 μm,
One of the causes is considered to be that the number of windings per unit winding diameter is large, and particularly, the tightness of winding at a position close to 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, and the particle size of the inorganic powder having a Mohs hardness of 5 or more in the back coat layer does not exceed 0.2 μm and is larger than this. It is described that the use of a soft water inorganic powder in combination prevents the tape guide and the like from being damaged. Japanese Patent Application Laid-Open No. 63-255584 also recognizes the phenomenon that the unevenness of the surface of the back coat layer moves to the magnetic layer and lowers the S / N and C / N of the magnetic recording medium. Has been pointed out. However, the technology disclosed in these documents relates to the use of 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, which is a VHS type magnetic material. In the VHS method, the shortest recording wavelength of the white level is only 1.3 μm, and the center recording wavelength is 0.8 μm or less using a ferromagnetic metal powder as a magnetic recording element. It did not give any suggestions for noise reduction.

[Means to solve the problem]

Therefore, the present inventors first, the ease of damage of the magnetic layer and the back layer, not only the hardness of the powder contained in the coating film, depends on the particle size and if the particle size is smaller than a certain value. In view of the fact that the reinforcing effect of the coating film is reduced, in view of this fact, when the magnetic recording tape is wound, the inorganic powder having a Mohs hardness of 5 or more contained in the back coat layer may damage the surface of the magnetic layer. It has been found that the inorganic powder of such a back coat layer must be smaller than the major axis diameter of the metal magnetic powder in order to avoid such a problem. Second, it is necessary to include carbon black in the back coat layer in order to improve running properties. This in itself has already been achieved with carbon black of fine particles of 10 to 30 mμ and carbon black of coarse particles of 200 to 500 mμ.
A non-magnetic powder of 0.2 μm or less is included to suppress an increase in dropout and improve running durability.
It has already been described in JP-A-62-8328. However, this document does not mention the relationship between the particle diameter of the primary particles or the diameter of the aggregates of the carbon black and the major axis diameter of the ferromagnetic metal powder in the magnetic coating film. No study has been made on the relationship between a signal having a frequency corresponding to a recording wavelength of 0.8 μm or less and noise with respect to a signal when a ferromagnetic metal powder having a small particle diameter is used. Thus, in the present invention, as a result of examining the relationship between the diameter of the primary particles or the diameter of the aggregates of the primary particles of the carbon black of the back coat layer and the major axis diameter of the ferromagnetic metal powder in the magnetic coating film, the primary particles of the carbon black were determined. It has been clarified that when the particle size of the particles or the diameter of the aggregates is larger than that of the magnetic powder, the requirements for high output and low noise can be sufficiently satisfied. This fact becomes particularly remarkable in a magnetic recording medium having a magnetic layer having a thickness of about 3.0 μm or less corresponding to a demand for a high recording density, particularly, a thinner magnetic coating film. Therefore, in the present invention, a magnetic layer and a back coat layer are provided on both surfaces of the non-magnetic support, respectively, to exhibit high output, and in high-density recording with a central recording wavelength of 0.8 μm or less, even for long-term storage. Regardless, in order to provide a magnetic recording medium capable of suppressing the rise of noise as much as possible, it is preferable that the magnetic coating film has a squareness ratio of 0.85 or more under the applied magnetic field 10KOe measurement condition, The coercivity of the layer is 1500
Oe or more is preferable, the surface smoothness of the magnetic layer is preferably regulated to 0.004 μm or less in center line average roughness, and the center smoothness of the back coat layer is 0.01 μm or less in center line average roughness. And it is preferable to regulate the diameter of the metal magnetic powder to be smaller than the major axis diameter. In general, carbon black and the like are soft, and what was included in the back coat would damage the surface of the magnetic layer due to winding of the tape. In order to achieve high-density recording of about the following level, the major axis diameter of the magnetic powder is reduced to 0.2 μm or less, and the magnetic layer mainly containing the magnetic powder of the metal does not have a sufficient effect of reinforcing the coating film. In addition, strong tightening force can be applied under the harsh conditions of multiple turns of thin tape required by high-density recording, so even carbon black with a small particle size may damage the surface of the magnetic layer I found the fact that there was. In this case, when 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, those having a small particle diameter enter the gaps, so that the back coat layer It is preferable because it does not protrude to the surface. Further, it is preferable that the particle diameter of the inorganic powder having a Mohs hardness of 5 or more contained in the back coat film is smaller than the major axis diameter of the metal magnetic powder. When the back coat layer is wound inside around the hub, it is preferable to house it in a cassette body containing an ABS resin and a pigment having a Mohs' hardness of 2 to 4. Examples of the material of the nonmagnetic support used in the present invention include polyethylene terephthalate, polyesters such as polyethylene-2,6-naphthalate, polyethylene, polyolefins such as polypropylene, cellulose triacetate, and cellulose derivatives such as cellulose diacetate; Examples include vinyl resins such as polyvinyl chloride and polyvinylidene chloride, polycarbonates, polyimides, and polyamides.Especially, polyesters such as biaxially oriented polyethylene terephthalate and polyethylene-2,6-naphthalate have elasticity in the longitudinal direction. Rate is 700 kg / mm ²
Above, the elastic modulus in the width direction is 400 kg / mm ² or more, and the surface roughness is 0.0
It is desirable that the thickness be 1 μm or less. Then, the magnetic powder and the inorganic powder having a Mohs hardness of 5 or more are both contained in a binder resin dissolved in an organic solvent on the support, and a magnetic coating material in which the magnetic powder and the inorganic powder are dispersed is applied and dried while performing a magnetic field alignment treatment. Form a magnetic coating. As the magnetic powder used in the magnetic coating film, _Fe ,
C _o, metals such as N _i, F _e, form a C _o, coating N _i, Z alloy combination and _n and F _e on the surface of these metals and alloys, A _l, oxides such as S _i Ferromagnetic fine powder and the like. The major axis diameter of the ferromagnetic metal powder may be 0.8 μm or less in relation to the central recording wavelength, but 0.2 μm
It is desirable that: Among them, mainly iron and manganese in 0.1% by weight of iron
It is desirable to use a ferromagnetic metal powder containing the above. In particular, a magnetic metal powder containing manganese in an amount of 0.1% by weight or more based on iron and an alkaline earth metal such as calcium and magnesium in an amount of 1 to 50 times the amount of manganese is relatively hard and has magnetic properties and electromagnetic conversion properties. It is sufficiently hard and has the effect of preventing the magnetic layer, whose major axis diameter is less than 0.2 μm, which is not expected to have much reinforcing effect from being damaged by inorganic powder having a Mohs hardness of 5 or more on the surface of the back coat layer. It is preferable because it is high. In particular, when manganese is present as an oxide or an acid hydroxide on the surface of the ferromagnetic metal powder, a uniform and dense coating is formed. Therefore, it is preferable that manganese is present on the surface as manganese oxide, manganese hydroxide, or the like. Alternatively, if 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 a metal element of nickel or cobalt, the temperature is 60 ° C. In a 90% humidity environment
30% reduction in saturation magnetization (σs) even when left for days
The following is preferable. In addition, 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 be 3.2 or more in order to maintain the erasing characteristics. Mohs hardness of 5 contained in magnetic layer or back coat layer
Examples of the above inorganic powder include metal oxides, metal carbides, metal nitrides, etc., among which α-Fe2O3, Al2O3
, Cr2O3, SiO2, TiO2, ZrO2, SiC, TiC
, HBN, Si3N4, etc. (for example, indicating a Mohs' hardness of 6) are listed as more preferred. Since these inorganic powders are easily available in various particle sizes, they can be appropriately selected in accordance with the above findings of the present invention. In the present invention, any of black, furnace black, acetylene black and thermal black can be used as the carbon black used in the back coat layer, but acetylene black is particularly preferred. Graphitized carbon black whose surface is covered with a graphite layer as disclosed in JP-A-61-22424 can also be used. Specific examples of commercially available carbon black products include those manufactured by Cabot Corporation in the United States, Black Pearl 700 having a particle size of 18 μm, Mogal L having a particle size of 20 mμ, and ELFTEXpel having a particle size of 27 mμ.
Vulcan XC with lets-115, same legal 3001 and particle size 30mμ
72, Stirling NS with a particle size of 75mμ, Stirling R,
13mμ particle size as manufactured by Columbian Carbon
Raben 8000, particle size 20mμ raben 5250, particle size 30mμ
Raben 890, raben 450 with a particle size of 62mμ, raben 410 with a particle size of 70mμ, raben MT-P with a particle size of 280mμ (0.28μ)
Ravensebacarb MT with beads, particle size 300mμ (0.30μ)
-CI, HS manufactured by Asahi Carbon Co., Ltd.
-500, # 60H with a particle size of 35mμ, Seam 5H with a particle size of 20mμ for Tokai Carbon Co., Ltd., Ketjen Black E with a particle size of 30mμ for Akzo, the Netherlands
C, Mitsubishi Kasei's # 4040 with a particle size of 20mμ,
# 4330BS with 23mμ particle size, # 4350BS with 45mμ particle size, 80m particle size
μ # 4010 or the like can be preferably used. As described above, since carbon blacks having various particle diameters can be easily obtained, according to the above findings of the present invention,
Depending on the long axis diameter of the metal magnetic powder, it can be selected as appropriate in design, but when the particle diameter is relatively small, several primary particles are utilized by utilizing the structure forming ability of the carbon black itself. It is desirable to form aggregates in which In this case, the aggregated aggregate functions as if it were a single carbon black particle. These inorganic powder and carbon black may be mixed into the coating material of the magnetic layer and the back coat layer so as to satisfy the above-mentioned relationship of the particle size. Examples of the binder resin used in the magnetic coating film and the back coat coating film include vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-acrylate copolymer, and vinyl chloride. -Vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, vinyl chloride resin such as vinyl chloride-vinyl acetate-maleic acid copolymer, thermoplastic polyurethane resin, thermosetting polyurethane resin, polyester resin, phenoxy resin, polyvinyl Butyral resins, cellulose derivatives, epoxy resins or mixtures thereof are mentioned. In addition, by introducing hydrophilic polar groups such as carboxylic acid, sulfonic acid, sulfonic acid salt, phosphoric acid, phosphate, amine and ammonium salt into these resins, the dispersibility of powder particles as coating film constituent materials is improved. Alternatively, an acrylic double bond may be introduced and cured by irradiation with an electron beam. Solvents used for preparing paints for forming these coating films include alcohols such as ethanol, propanol and butanol, esters such as methyl acetate, ethyl acetate and butyl acetate, ketones such as methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone. , Tetrahydrofuran, ethers such as dioxane, benzene, toluene, aromatic hydrocarbons such as xylene, heptane, hexane, aliphatic hydrocarbons such as cyclohexane,
Examples thereof include chlorinated hydrocarbons such as methylene chloride, ethylene chloride, and chloroform, and a mixed solvent of cyclohexanone-toluene is preferable. In addition, lubricants such as saturated and unsaturated higher fatty acids, higher fatty acid amides, fatty acid esters, higher alcohols, silicone oils, mineral oils, edible oils, and fluorine oils can be used as additives in the coating film. . Such a composition is achieved by dispersing a predetermined amount in a ball mill or a sand mill to form a magnetic paint and a back coat paint, and applying the magnetic paint and the back coat paint on the non-magnetic support. One of the most important things to keep in mind when dispersing paint is
The purpose is to apply extra force to the particles of the inorganic powder or carbon black so as not to change the particle size of the carbon black compounded as designed. In application, it is preferable to apply a magnetic layer 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. The order of the steps is described in detail in JP-B-58-23647. The magnetic layer and the back coat layer may be applied alone or as a plurality of layers, and may be applied simultaneously. The coated magnetic recording medium is cut into a predetermined tape width by a slit device, usually wound around individual bub with the back coat layer inside, and can be housed in the tape cassette body. The total thickness of the magnetic coating, support, and backcoat layer is 14μ
When it is less than m, 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. In addition, the elastic modulus in the longitudinal direction is 1000 kg / mm ²
It is desirable that this is the case. Magnetic coating is also 6m in diameter
When a steel ball of m is pressed with a load of 5 grams and run for 23 meters, the volume reduction due to wear of the steel ball
It is preferable to regulate the size to 20 × 10 ⁻⁵ mm ³ or less. 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 requires proper support selection and mechanical control by magnetic field orientation of magnetic particles. Can be achieved. Further, setting the elastic modulus in the longitudinal direction to 1000 kg / mm ² or more can be achieved by appropriately selecting the curing conditions of the binder resin of each coating film. In order to regulate the volume reduction due to the wear of the magnetic coating film to 20 × 10 ⁻⁵ mm ³ or less, an appropriate amount of inorganic powder having a Mohs hardness of 5 or more should be included, and the conditions for the surface smoothing treatment should be selected. This can be achieved by selecting drying conditions and the like. Since a strong radial tightening force is applied to the hub serving as the core, it is desirable to control the sink of the tape winding peripheral surface to 5 μm or less in order to prevent curling of the tape. Since the back coat side of such a magnetic recording tape is in contact with the tape guide member of the tape cassette body, it is preferable to use an ABS resin for the tape cassette body or at least the tape guide portion in order to reduce noise. It is also desirable to include a pigment having a Mohs' hardness of 2 to 4 such as calcium carbonate and barium sulfate to prevent damage to the back coat. The cassette body further includes carbon black and polyoxygen to reduce electrostatic noise. It is preferable to include an antistatic agent such as a quaternary ammonium salt such as ethylene alkylamine and a molten resin fluidizing agent such as ethylene bis stearamide. To manufacture such a cassette body, these compositions are usually melted and injected into a mold by injection molding. As described above, in short-wavelength recording with a center recording wavelength of about 0.8 μm or less, which is required for small and high-quality video tapes, the phenomenon that noise is increased by storage at high output is suppressed as much as possible. Magnetic recording medium can be obtained. In particular, in the present invention, the major axis diameter is 0.
A configuration in which a metal magnetic powder having a particle diameter of 2 μm or less and an inorganic powder having a Mohs hardness of 5 or more are included, and carbon black of primary particles or agglomerates having a diameter larger than the major axis of the metal magnetic powder is included in the back coat layer; By doing so, it is possible to provide a magnetic recording medium excellent in the above characteristics. Hereinafter, details of the present invention will be described along with examples.

【Example】

Example 1 A. Preparation of magnetic layer Coercive force 1600 Oe, saturation magnetization 120 emu / g, major axis diameter 0.18μ
m, axis ratio 10, coated with alumina on the surface, mainly iron, 0.2 wt% manganese and 1.0 wt% based on iron
% Ferromagnetic metal iron powder containing 100% by weight of calcium, 10 parts by weight of a hydroxyl group-containing vinyl chloride resin having a polymerization degree of 340, 7 parts by weight of a thermoplastic polyurethane resin, and a particle size of 0.2 μm.
8 parts by weight of alumina, 2 parts by weight of myristic acid, 2 parts by weight of bengara (α-Fe2O3) having a particle size of 0.8 μm, and # 4010 manufactured by Mitsubishi Kasei Co., Ltd. having a particle size of 80 μm as carbon black.
A mixture of 1 part by weight and 2 parts by weight of a seam 5H manufactured by Tokai Carbon Co., Ltd. having a particle size of 20 mμ was mixed with 70 parts by weight of cyclohexanone and toluene, and kneaded and dispersed in a ball mill for 96 hours. Then, 5 parts by weight of a trifunctional polyisocyanate compound was further added and stirred to prepare a magnetic paint. This magnetic paint was applied on a support of a biaxially oriented polyethylene tetaphthalate film having a thickness of 10 μm, applied to a thickness of 2.5 μm, dried and calendered to prepare a magnetic recording medium. B. Preparation of Back Coat Layer On the other hand, 60 parts by weight of Tokai Carbon Co., Ltd.
7.5) parts by weight of Raven MT-P beads manufactured by Columbian Carbon Co., Ltd., 30 parts by weight of calcium carbonate having a particle size of 0.05 μm, 2.5 parts by weight of bengara having a particle size of 0.1 μm, and 45 parts by weight of a thermoplastic polyurethane resin And nitrocellulose 40 parts by weight, trifunctional isocyanate crosslinking agent 15 parts by weight, 33
The composition was mixed with 0 parts by weight of cyclohexanone and toluene, and this composition was kneaded and dispersed in a ball mill for 96 hours to prepare a back coating. This was applied on the back surface of the magnetic recording medium coated with the magnetic layer to a thickness of 1.0 μm and dried.
Cure at 20 ° C. for 20 hours. The total thickness of this magnetic recording medium was 13.5 μm. The tape was cut into a predetermined width to form a tape, and the tape was wound around a hub made of an injection molded product of a polyoxymethylene resin having a sink of 0.1 μm or less on the peripheral surface of the tape, with the back coat layer inside. C. Production of cassette body On the other hand, 100 parts by weight of ABS resin NA-1060 manufactured by Denki Kagaku Kogyo Co., 23 parts by weight of a colorant obtained by treating carbon black with ferrocyanine blue, and calcium carbonate (particle size: 0.5 μm) 3
5 parts by weight, 12 parts by weight of polyoxyethylene alkylamine and 3 parts by weight of ethylene bis stearamide were added, kneaded at 110 ° C. for 1 minute by a Henschel mixer, and extruded at 220 ° C. by a twin-screw extruder to be pelletized. . Further, together with 1500 parts by weight of the above-mentioned 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 magnetic recording medium was assembled by housing the hub-wound tape in the cassette body. Example 2 Instead of 7.5 parts by weight of Raven MT-P beads manufactured by Columbian Carbon Co., Ltd. having a particle size of 280 mμ (0.28 μm) as the carbon black for preparing the back coat layer in Example 1,
Ravensebacarb MT- with the same amount of particle size 300mμ (0.30μ)
A magnetic recording medium was manufactured in the same manner as in Example 1 except that the magnetic recording medium was changed to CI. Example 3 Instead of 7.5 parts by weight of Raven MT-P beads manufactured by Columbian Carbon Co., Ltd. having a particle size of 280 mμ (0.28 μm) as the carbon black for preparing the back coat layer in Example 1,
The same amount was changed to # 4010 manufactured by Mitsubishi Kasei Co., Ltd. having a particle size of 80 mμ.In addition, instead of kneading and dispersing the composition in a ball mill for 96 hours in preparing a back coating, except kneading and dispersing in a ball mill for 22 hours, A magnetic recording medium was manufactured in the same manner as in Example 1. When the magnetic paint was observed at the time of kneading and dispersion in a ball mill for 22 hours, the carbon black particles formed a structure with an average of 13 particles and the diameter of the aggregate was
210 mμ. Reference Example 1 Instead of 7.5 parts by weight of Raven MT-P beads manufactured by Columbian Carbon Co., Ltd. having a particle size of 280 mμ (0.28 μm) as the carbon black for preparing the back coat layer in Example 1,
A magnetic recording medium was manufactured in the same manner as in Example 1 except that the same amount was changed to # 4010 manufactured by Mitsubishi Kasei Co., Ltd. having a particle size of 80 μm. When the magnetic paint 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 diameter of the primary particles was 80 mμ. Example 4 The same amount of particle size as in Example 1 was used in place of 2.5 parts by weight of 0.1 μm particle size bengara used in the preparation step of the back coat layer in Example 1.
A magnetic recording medium was manufactured in the same manner as in Example 1 except that the red iron oxide of 0.8 μm was used. Comparative Example 1 The same amount of coercive force of 1600 Oe and saturation magnetization of 12
0 emu / g, a ferromagnetic metal powder of a major axis diameter 0.35μm axis ratio of 1 _0, instead of the red iron oxide 2.5 parts by weight of particle size 0.1μm used in the manufacturing stage of the backcoat layer of Example 1, the same amount of A magnetic recording medium was produced in the same manner as in Example 1 except that bengara having a particle size of 0.8 μm was used. Comparative Example 2 The same amount of particle size was used in place of 2.5 parts by weight of 0.1 μm particle size used in the step of forming the back coat layer in Example 1.
A 0.8 μm bengara was used, and instead of 7.5 parts by weight of Raven MT-P beads manufactured by Columbian Carbon Co., Ltd. having a particle size of 280 mμ (0.28 μm) as carbon black, the same amount of # 80 mμ manufactured by Mitsubishi Kasei Co., Ltd. was used. A magnetic recording medium was manufactured in the same manner as in Example 1 except that the magnetic recording medium was changed to 4010. When the elastic modulus of the magnetic recording medium of Example 1 was measured, the elastic modulus in the longitudinal direction was 1100 kg / mm ² and the elastic modulus in the width direction was 700 kg / mm ² . Further, when a steel ball having a diameter of 6 mm was pressed against the magnetic coating film with a load of 5 g and allowed to travel for 23 meters, the volume reduction due to the wear of the steel ball was 15%.
× 10 ⁻⁵ mm ³ . Further, for each of the magnetic recording media of Examples 1 to 4, Reference Example 1, and Comparative Examples 1 and 2, the surface smoothness of the magnetic coating film and the back coat layer was measured using a stylus type surface roughness meter. = 2μ
m and a cutoff of 0.08 mm, and expressed as a center line average roughness. The squareness ratio and the coercive force of the magnetic layer were measured under the conditions of an applied magnetic field of 10KOe. 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 immediately after winding the hub in Example 1. The noise level was set to 0 dB as a standard, and expressed as a noise ratio in dB. The results are shown in the table at the end. The magnetic recording media of Examples 1 to 3 have the same configuration as that of Example 1 except that the back coat layer contains carbon black having a diameter smaller than the long axis diameter (0.18 μm) of the metal magnetic powder. It can be seen that the rise in noise after storage for 60 days can be reduced by 0.8 dB as compared with the magnetic recording medium of No. 1. The magnetic recording medium of Example 4 has the same structure as that of Example 4 except that the back coat layer contains a carbon black having a diameter smaller than the major axis of the metal magnetic powder. Compared to the magnetic recording medium of Example 2, 60
It can be seen that the rise in noise after storage for a day can be reduced by as much as 1.0 dB. Comparative Example 1 in which a carbon black having a major axis diameter of the metal magnetic powder contained in the magnetic coating film as large as 0.35 μm and a diameter (0.28 μm) smaller than the major axis diameter was included in the back coat layer. The magnetic recording medium of
It can also be seen that the rise in noise after storage is significantly large.

【The invention's effect】

Thus, according to the present invention, the major axis diameter is 0.2 μm in the magnetic coating film.
m or less, and a back coat layer containing carbon black of primary particles or agglomerates having a diameter larger than the major axis of the metal magnetic powder in the back coat layer. It is clear that the magnetic recording medium obtained by this method is suitable for short-wavelength recording without increasing noise even after long-term storage.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-2328 (JP, A) JP-A-61-172215 (JP, A) JP-A-1-112520 (JP, A) JP-A-60-1985 36603 (JP, A) JP-A-59-117768 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G11B 5/706 G11B 5/704 G11B 23/04 G11B 5/714

Claims (4)

(57) [Claims] 1. A magnetic coating containing both a magnetic metal powder having a major axis diameter of 0.2 μm or less and an inorganic powder having a Mohs' hardness of 5 or more on one surface of a support, and a magnetic coating having a major axis diameter of not less than 5 on the other surface. A magnetic recording medium provided with a back coat layer containing carbon black of primary particles or aggregates having a large diameter. 2. A magnetic recording medium comprising a support having a magnetic coating film on one surface and a back coat layer on the other surface, which is used for recording information having a center recording wavelength of 0.8 μm or less. The film contains both a magnetic metal powder having a major axis diameter of 0.2 μm or less and an inorganic powder having a Mohs hardness of 5 or more, and the back coat layer has primary particles or aggregates having a diameter larger than the major axis diameter of the metallic magnetic powder. A magnetic recording medium comprising: a carbon black. 3. A magnetic coating containing both a magnetic metal powder having a major axis diameter of 0.2 μm or less and an inorganic powder having a Mohs hardness of 5 or more on one surface of the support, and a magnetic coating having a major axis diameter of not less than 5 on the other surface. A magnetic tape for recording information having a central recording wavelength of 0.8 μm or less, provided with a back coat layer containing carbon black of primary particles or aggregates having a large diameter, with the back coat layer inside the hub around the hub. Rolled up,
A magnetic recording medium housed in a cassette body containing an ABS resin and a pigment having a Mohs hardness of 2 to 4. 4. A magnetic recording medium comprising a support having a magnetic coating film on one surface and a back coat layer on the other surface, wherein the magnetic coating film is a metal magnetic powder having a major axis diameter of 0.2 μm or less; The back coat layer contains both the above inorganic powders and the carbon black of primary particles or aggregates having a diameter larger than the major axis diameter of the metal magnetic powder, and the total of the support, the magnetic coating film and the back coat layer Has a thickness of 14
A magnetic recording medium having a tape shape of μm or less.