JP2007091517A - Hexagonal magnetoplumbite type ferrite, its manufacturing method and magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide hexagonal magnetoplumbite type ferrite having small average plate size, an adequate coefficient of variation of the plate size, an adequate ratio of the plate size to the plate thickness and adequate coercive force, to provide a method for manufacturing the hexagonal magnetoplumbite type ferrite and to provide a magnetic recording medium in which the hexagonal magnetoplumbite type ferrite is used in a magnetic layer and which shows high short-wavelength output and low medium noise when reproduced by using an MR head. <P>SOLUTION: The hexagonal magnetoplumbite type ferrite, which has 10-30 nm average plate size, 5-25% coefficient of variation of the plate size, 1.5-4.5 ratio of the plate size to the plate thickness and 125-400 kA/m coercive force, is obtained by forming a coprecipitated matter of a desired hexagonal magnetoplumbite type ferrite composition by a reverse micelle method, coating the surface of the coprecipitated matter with an alkaline-earth metal compound, heating the coated coprecipitated matter in the air at 550-650°C, subsequently heating the heated coprecipitated matter at 650-900°C to obtain ferrite and cleaning the obtained ferrite. The magnetic recording medium is obtained by incorporating the hexagonal magnetoplumbite type ferrite in the magnetic layer as magnetic powder. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a hexagonal magnetoplumbite type ferrite used for high-density magnetic recording media and a method for producing the same, and more particularly to a hexagonal magnetoplumbite type ferrite having a fine and excellent particle size distribution and a method for producing the same. The present invention also relates to a magnetic recording medium such as a magnetic tape, which is suitable for a system using the MR head having the hexagonal magnetoplumbite type ferrite in a magnetic layer and utilizing the magnetoresistive effect for reproduction.

Conventionally, as magnetic recording media such as video tapes, computer tapes, and flexible disks, ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO ₂ , ferromagnetic alloy metal powder, hexagonal ferrite, etc. are dispersed in a binder. A magnetic layer coated on a support is widely used. Among these, hexagonal ferrite is known to be excellent in high-density recording characteristics (for example, JP-A-60-157719, JP-A-62-109226, JP-A-3-280215). . In JP-A-5-12650, the thickness of the magnetic layer using the ferrite is set to 0.1 to 0.6 μm, and a nonmagnetic layer thicker than the magnetic layer is provided between the magnetic layer and the support. It is said to improve short wavelength output, erase characteristics, and durability. Japanese Patent Application Laid-Open No. 5-225547 discloses a magnetic recording medium in which a nonmagnetic layer is provided on a support and a magnetic layer containing a magnetic powder of 0.1 μm or less is provided thereon, which is excellent in high frequency characteristics. In addition, a magnetic recording medium having good signal overwriting characteristics and good durability is provided.

  Recently, high-sensitivity reproducing heads (MR heads) using the magnetoresistive effect have been used in data recording systems for computers, and system noise is dominated by noise originating from magnetic recording media. Has been. Japanese Laid-Open Patent Publication No. 7-182646 proposes reproducing a medium using Ba ferrite with an MR head. In order to reduce the medium noise, the magnetic powder is being miniaturized. However, it is estimated that the stability of the magnetization transition region becomes a problem due to the influence of thermal fluctuation accompanying the miniaturization of the magnetic powder. The stability of magnetization is evaluated by KuV / kT (Ku is a magnetic anisotropy constant, V is a particle volume, k is a Boltzmann constant, and T is an absolute temperature). Regarding the particle volume and thermal fluctuation of metal tape, there is a report by Toshiyuki Suzuki et al. (Science Technical Report MR97-55 P33-40 1997.11.21). In addition, there are reports from Toshiyuki Suzuki et al. Regarding Ba ferrite media and thermal fluctuations (Science Technical Report MR2000-45 2000.11.14, Journal of Japan Society of Applied Magnetics 25 1399-1404 (2001), 26 258-262 ( 2002)).

  Since the hexagonal ferrite has a saturation magnetization of about 1/3 to 1/2 that of the ferromagnetic metal powder, it is difficult to increase Ku, and the influence of thermal fluctuation is increased. In particular, when fine particles are included, the influence of thermal fluctuation increases, and the uniformity of the particle size distribution is required as the average particle size decreases. Further, it is said that a magnetic recording medium using hexagonal ferrite has a large interaction between particles and affects the noise level of the medium. When the interaction between particles is large, it is said that the stability of magnetization is excellent. However, if the particles are reversed in magnetization for some reason, the surrounding magnetic substance may also be dragged and reversed in magnetization. For this reason, there is a problem that it is difficult to ensure a sufficient C / N when reproducing a high-density recording medium prepared using hexagonal ferrite powder having a fine powder size with an MR head.

  There are glass crystallization method, coprecipitation method, hydrothermal reaction method, etc. as a method of producing hexagonal ferrite magnetic powder, but any method reduces the noise of magnetic recording media and refines the magnetic powder used. At the same time, it is continuously directed to reduce the particle size distribution. Even in hexagonal ferrite, attempts have been made to separate nucleation and growth reaction in order to reduce the particle size distribution (Japanese Patent Laid-Open Nos. 59-151339 and 10-92618). As for the crystallization conditions in the vitrification crystallization method, the amorphous flake size, thickness and heating temperature are prescribed in Japanese Patent Laid-Open Nos. 58-42203 and 60-262403, and the characteristics are uniform. It has been proposed to obtain hexagonal ferrite. However, as the target particle size decreases, it becomes difficult to separate nucleation and growth when the entire amorphous flakes are heated uniformly. Under the conventional conditions, the fine particles and particle size distribution that can achieve the desired high density recording are obtained. Good hexagonal ferrite cannot be produced. In Patent Document 1, as a production method improved from the coprecipitation method, a metal salt compound is dispersed in an organic solvent that does not mix with water in the presence of a surfactant to form a W / O emulsion and mixed with an aqueous alkali solution. The coprecipitate obtained by firing was fired to obtain Ba ferrite having an average diameter of about 0.1 μm. However, Ba ferrite described in Patent Document 1 has a large particle size and cannot be used for a high recording density magnetic recording medium.

Japanese Patent Laid-Open No. 3-204909

  The present invention has been made in view of the above-mentioned problems of the prior art, and has a small average plate diameter, an appropriate plate diameter variation coefficient, a plate ratio and a coercive force, and a hexagonal magnetoplumbite type ferrite and a method for producing the same. It is an object of the present invention to provide a magnetic recording medium in which the hexagonal magnetoplumbite type ferrite is used in a magnetic layer and has a short wavelength output and low medium noise when reproduced using an MR head.

   As a result of diligent studies to produce the magnetoplumbite type ferrite, the present inventor formed a fine coprecipitate having a desired magnetoplumbite type ferrite composition by the reverse micelle method, and this surface was formed with an alkaline earth metal compound. It was found that magnetoplumbite type ferrite having a particle size corresponding to the reverse micelle diameter and uniform particle size can be obtained by heat treatment after coating with.

That is, the present invention is as follows.
1) The following steps (1) to (3) have an average plate diameter of 10 to 30 nm, a plate diameter variation coefficient of 5 to 25%, a plate ratio of 1.5 to 4.5, and a coercive force. Of hexagonal magnetoplumbite-type ferrite having an A of 125 to 400 kA / m.
(1) A step of forming a coprecipitate having a desired hexagonal magnetoplumbite type ferrite composition by the reverse micelle method, and covering the surface with an alkaline earth metal compound; (2) obtained in the step (1) Drying the coating, followed by heating in air at 550-650 ° C. followed by heating at 650-900 ° C. to ferritize; and (3) washing the product obtained in step (2) above, The process of recovering hexagonal magnetoplumbite type ferrite.
2) Having the following steps (1 ′), (2) and (3), the average plate diameter is 10 to 30 nm, the plate diameter variation coefficient is 5 to 25%, and the plate ratio is 1.5 to 4. 5. A method for producing a hexagonal magnetoplumbite type ferrite having a coercive force of 125 to 400 kA / m.
(1 ') Starting from chlorides with the desired hexagonal magnetoplumbite type ferrite composition, co-precipitates are formed by reverse micelle method using alkali metal hydroxide, and coexisting alkali metal chlorides are removed. (2) The coating obtained in the step (1) is dried and then heated in air at 550 to 650 ° C., followed by 650 to (3) a step of washing the product obtained in the step (2) and recovering hexagonal magnetoplumbite type ferrite;
3) The average plate diameter produced by the production method described in 1) or 2) above is 10 to 30 nm, the variation coefficient of the plate diameter is 5 to 25%, the plate ratio is 1.5 to 4.5, and Hexagonal magnetoplumbite type ferrite having a coercive force of 125 to 400 kA / m.
4) A magnetic recording medium having a magnetic layer containing a magnetic powder and a binder on a nonmagnetic support, wherein the magnetic powder contains the hexagonal magnetoplumbite-type ferrite described in 3) above. Magnetic recording medium.

  According to the present invention, there is provided a hexagonal magnetoplumbite type ferrite having a small average plate diameter, an appropriate plate diameter variation coefficient, a plate ratio and a coercive force, and a method for producing the same, and the hexagonal magnetoplumbite type ferrite. Can be provided in the magnetic layer, and a magnetic recording medium having a high short wavelength output and a low medium noise when reproduced using an MR head can be provided.

Hereinafter, embodiments of the present invention will be described in detail.
Step (1) in the production method of the present invention is a step in which a coprecipitate having a desired hexagonal magnetoplumbite type ferrite composition is formed by a reverse micelle method, and this surface is coated with an alkaline earth metal compound.

In order to form the coprecipitate in the present invention, the following method may be mentioned.
(Preparation of reverse micelle solution (I))
First, a reverse micelle solution (I) is prepared by mixing a water-insoluble organic solvent containing a surfactant and an aqueous neutralizing agent and / or oxidizing agent solution.
An oil-soluble surfactant is used as the surfactant. Specifically, sulfonate type (for example, aerosol OT (manufactured by Wako Pure Chemical Industries)), quaternary ammonium salt type (for example, cetyltrimethylammonium bromide), ether type (for example, pentaethylene glycol dodecyl ether), sorbitan mono Nonionics having an HLB value of 4 to 6 such as oleate can be mentioned. The amount of the surfactant in the water-insoluble organic solvent is preferably 20 to 200 g / liter.

  Preferred examples of the water-insoluble organic solvent that dissolves the surfactant include alkanes, ethers, and alcohols. The alkane is preferably an alkane having 7 to 12 carbon atoms. Specifically, heptane, octane, isooctane, nonane, decane, undecane, dodecane and the like are preferable. As the ether, diethyl ether, dipropyl ether, dibutyl ether and the like are preferable. As the alcohol, ethoxyethanol, ethoxypropanol and the like are preferable.

As the neutralizing agent in the aqueous neutralizing agent solution, alkali metal hydroxide, alkaline earth hydroxide, ammonia water (NH ₄ OH), urea, an amine compound is used alone, or two or more kinds are used in combination. It is preferable. Of these, alkali metal hydroxides, NH ₄ OH, urea, and amine compounds are preferable. The amount of the neutralizing agent in the aqueous solution is preferably 1 to 5 times the amount of neutralization of the metal salt. As the oxidizing agent aqueous solution, it is preferable to use nitrate, perchlorate, and hydrogen peroxide water alone or in combination of two or more. The amount of the oxidizing agent in the aqueous solution is preferably 1.1 to 10 mol per 1 mol of the metal salt.

  Here, it is preferable that the mass ratio (water / surfactant) of water and the surfactant in the reverse micelle solution (I) is 20 or less. If the mass ratio exceeds 20, precipitation may easily occur and particles may be uneven. The mass ratio is preferably 15 or less, and more preferably 0.5 to 10.

(Preparation of reverse micelle solution (II))
A reverse micelle solution (II) is prepared by mixing a water-insoluble organic solvent containing a surfactant and a metal salt aqueous solution having a desired hexagonal magnetoplumbite type ferrite composition. The conditions (substance used, concentration, etc.) of the surfactant and the water-insoluble organic solvent are the same as in the case of the reverse micelle solution (I).
In addition, the same kind or different kind of reverse micelle solution (I) can be used. The mass ratio of water and surfactant in the reverse micelle solution (II) is the same as that of the reverse micelle solution (I), and may be the same as or different from the mass ratio of the reverse micelle solution (I). It may be.

The metal salt contained in the metal salt aqueous solution is preferably selected as appropriate so that the hexagonal magnetoplumbite type ferrite to be produced can be formed. It is preferable to add various known substitution elements in order to obtain desired magnetic properties.
The concentration in the metal salt aqueous solution (as the metal salt concentration) is preferably 0.1 to 1000 μmol / ml, and more preferably 1 to 200 μmol / ml.

(Mixing of reverse micelle solutions (I) and (II))
The prepared reverse micelle solutions (I) and (II) are mixed. Although it does not specifically limit as a mixing method, In consideration of the uniformity of reaction, reverse micelle solution (II) is added and mixed, stirring reverse micelle solution (I).
After mixing, the neutralization reaction and / or the oxidation reaction is allowed to proceed, and the temperature at that time is preferably in the range of 10 to 40 ° C.
When the reaction temperature is less than 10 ° C., the progress of the reaction is slow, and there is a problem that the reaction becomes non-uniform. When the temperature exceeds 40 ° C., aggregation or precipitation is likely to occur, and the system may become unstable. The preferred reaction temperature is 15 to 35 ° C, more preferably 20 to 30 ° C.
Here, the “constant temperature” means that when the set temperature is T (° C.), the T is in the range of T ± 3 ° C. Even in such a case, the upper limit and the lower limit of the T are in the above temperature range (10 to 40 ° C.).

  In the case of the neutralization reaction, the addition time needs to be appropriately set depending on the amount of the reverse micelle solution and the like, but is preferably 1 to 30 minutes, and more preferably 3 to 20 minutes. In the case of an oxidation reaction, the addition time is preferably 1 to 30 minutes, more preferably 3 to 20 minutes.

  Since the neutralization or oxidation reaction greatly affects the monodispersity of the particle size distribution, it is preferable to carry out the reaction while stirring as fast as possible. A preferred stirring device is a stirring device having a high shearing force. Specifically, the stirring blade basically has a turbine-type or paddle-type structure, and a sharp blade is provided at the end of the blade or at a position in contact with the blade. It is an attached structure and is a stirring device that rotates a blade by a motor. Specifically, devices such as a dissolver (manufactured by Special Machine Industries), an omni mixer (manufactured by Yamato Kagaku), a homogenizer (manufactured by SMT), and a jet flow mixing device are useful. By using these apparatuses, monodisperse particles can be synthesized as a stable dispersion.

  At least one dispersant having 1 to 3 amino groups or carboxy groups is added to at least one of the reverse micelle solutions (I) and (II) in an amount of 0.001 to 10 per mole of particles to be prepared. Mole may be added.

  By adding such a dispersing agent, it is possible to obtain particles that are more monodispersed and have no aggregation. If the addition amount is less than 0.001 mol, the monodispersity of the particles may not be improved, and if it exceeds 10 mol, aggregation may occur.

As the dispersant, an organic compound having a group that adsorbs to the particle surface is preferable. Specifically, it has 1 to 3 amino groups, carboxy groups, sulfonic acid groups or sulfinic acid groups, and these can be used alone or in combination.
As structural formulas, R—NH ₂ , NH ₂ —R—NH ₂ , NH ₂ —R (NH ₂ ) —NH ₂ , R—COOH, COOH—R—COOH, COOH—R (COOH) —COOH, R —SO ₃ H, SO ₃ H—R—SO ₃ H, SO ₃ H—R (SO ₃ H) —SO ₃ H, R—SO ₂ H, SO ₂ H—R—SO ₂ H, SO ₂ H— R (SO ₂ H) —SO ₂ H, wherein R is a linear, branched or cyclic saturated or unsaturated hydrocarbon.
It is also preferable to age after completion of the reaction, and the temperature of the solution after the reaction is raised to the age of aging. The aging temperature is preferably a constant temperature of 30 to 90 ° C., and the temperature is higher than the temperature of the reaction. The aging time is preferably 5 to 300 minutes.

(Removal of impurities)
The reaction product is washed with a mixed solution of water and a primary alcohol to remove impurities. The primary alcohol used for washing is not particularly limited, but methanol, ethanol and the like are preferable. The volume mixing ratio (water / primary alcohol) is preferably in the range of 10/1 to 2/1, and more preferably in the range of 5/1 to 3/1. When the ratio of water is high, the surfactant may be difficult to remove, and conversely, when the ratio of primary alcohol is high, aggregation may occur. After washing, the solvent is replaced to make an aqueous system.

(Coating with alkaline earth metal compound)
Next, an alkaline earth metal compound and an alkali are added, and the coprecipitate is covered with the alkaline earth metal compound. Alkaline aqueous solution used is an alkali metal hydroxide, NH ₄ OH, urea is preferred. As the alkaline earth metal compound, calcium chloride, calcium nitrate, barium chloride, strontium chloride and the like are preferable.
The addition amount of the alkaline earth metal compound is preferably 50 to 500% by mass, and more preferably 75 to 300% by mass with respect to the coprecipitate.
In this way, the alkaline earth metal compound is coated on the surface of the coprecipitate. The coating is preferably washed, filtered, separated, granulated, dried and the like.

  In addition, as a metal salt contained in the aqueous metal salt solution, a chloride having a desired hexagonal magnetoplumbite type ferrite composition is used as a starting material, and an alkali metal hydroxide is used by the reverse micelle method as described above. If this surface is coated with an alkaline earth metal compound without forming a coprecipitate and removing the coexisting alkali metal chloride, the produced alkali metal chloride is obtained in the following step (2). This is preferable because it is used as a flux when crystallization of magnetoplumbite type ferrite and the crystallization temperature can be lowered (step (1 ′) of the present invention).

In step (2) in the production method of the present invention, the coating obtained in the step (1) or (1 ′) is dried and then heated in air at 550 to 650 ° C. to convert the surface coating into an oxide. Subsequently, it is a step of heating and crystallizing at 650 to 900 ° C. to obtain a magnetoplumbite type ferrite.
If the crystallization temperature is less than 650 ° C., the crystallization time is long, and the particle size distribution deteriorates during this time. If the crystallization temperature exceeds 900 ° C., the crystal grows excessively and becomes coarse.
The oxidation temperature is preferably 570 to 630 ° C, and the crystallization temperature is preferably 660 to 880 ° C. The oxidation time is preferably 30 to 120 minutes, and the crystallization time is preferably 30 to 360 minutes. (2) After completion of the process, the product may be pulverized as necessary.

Step (3) in the production method of the present invention is a step of washing the product obtained in step (2) to recover hexagonal magnetoplumbite type ferrite.
At this time, it is preferable to use a weak acid having a pH of about 4 to promote dissolution of an alkaline earth metal oxide covering the surface. When recovering magnetoplumbite type ferrite, the organic substance having a pKa smaller than the fatty acid (pKa is ~ 4.2) added during the production of the magnetic recording medium is allowed to coexist and adsorbed on the recovered magnetoplumbite type ferrite surface. It is also preferable to prevent stacking in the meantime and improve dispersibility. Examples of the functional group having a pKa of 4 or less include PO (OH) ₂ , OPO (OH) ₂ , SO ₃ H, CONHOH, and SO ₂ NHOH. The amount of adsorption is preferably completely covered with the particle surface, but these effects were recognized even when a part of the surface was covered.

  A method in which the above process is partially simplified may be used. Specifically, (a) a reverse micelle solution (II ') is prepared by mixing a water-insoluble organic solvent containing a surfactant and a metal salt aqueous solution having a magnetoplumbite type ferrite composition. An oil-soluble surfactant is used as the surfactant. Specifically, sulfonate type (for example, aerosol OT (manufactured by Wako Pure Chemical Industries)), quaternary ammonium salt type (for example, cetyltrimethylammonium bromide), ether type (for example, pentaethylene glycol dodecyl ether), sorbitan mono Nonionics having an HLB value of 4 to 6 such as oleate are listed. The amount of the surfactant in the water-insoluble organic solvent is preferably 20 to 200 g / liter.

Preferable examples of the water-insoluble organic solvent for dissolving the surfactant include alkanes, ethers and alcohols. The alkane is preferably an alkane having 7 to 12 carbon atoms. Specifically, heptane, octane, isooctane, nonane, decane, undecane, dodecane and the like are preferable. As the ether, diethyl ether, dipropyl ether, dibutyl ether and the like are preferable. As the alcohol, ethoxyethanol, ethoxypropanol and the like are preferable.
As a step (b) following the step (a), an aqueous solution in which a neutralizing agent is dissolved is added to the reverse micelle solution (II ′) prepared while stirring, and the slurry is allowed to settle. Next, as a step (c), the supernatant is removed, and the coprecipitate is washed with a mixed solution of water and a primary alcohol to remove impurities. After washing, the solvent is replaced to obtain an aqueous system. Subsequently, as the step (d), an alkaline earth metal compound and an alkali are added to cover the coprecipitate. Alkaline aqueous solution used is an alkali metal hydroxide, NH ₄ OH, urea is preferred. Wash, filter, separate, granulate and dry. Thereafter, magnetoplumbite type ferrite is manufactured under the same process conditions as described above.

The hexagonal magnetoplumbite type ferrite obtained by the production method of the present invention has an average plate diameter of 10 to 30 nm, a plate diameter variation coefficient of 5 to 25%, a plate ratio of 1.5 to 4.5, and an anti-resistance. Magnetic force (Hc) is 125-400 kA / m.
When the average plate diameter is less than 10 nm, it is not preferable as a magnetic powder for high-density recording because desired magnetic properties cannot be obtained. When the average plate diameter exceeds 30 nm, the volume of the magnetic material is increased, noise is increased, and the variation coefficient is 25%. If it exceeds 1, the noise due to volume fluctuation increases, which is not preferable. If the plate ratio is out of the above range, it is not preferable because a desired residual magnetic flux density cannot be obtained. If the current exceeds 400 kA / m, the recording current of the head becomes insufficient, and recording cannot be performed or overwriting cannot be performed.
Preferably, the average plate diameter is 15 to 25 nm, the coefficient of variation is 5 to 20%, the plate ratio is 1.8 to 4.0, and the coercive force is 140 to 350 kA / m.
The plate ratio in the present invention is an arithmetic average of (plate diameter / plate thickness), and the plate diameter and plate thickness are obtained by randomly measuring 500 particles from a particle TEM photograph. The average plate diameter is the arithmetic average thereof, and the coefficient of variation is a percentage obtained by dividing the standard deviation by the average value.

   Next, the magnetic recording medium of the present invention will be described. The magnetic recording medium of the present invention is a magnetic recording medium having a magnetic layer containing a magnetic powder and a binder on a nonmagnetic support, and contains the magnetoplumbite type ferrite as the magnetic powder in the magnetic layer.

  In order to manufacture a magnetic recording medium, a normal coating type magnetic recording medium manufacturing method can be applied. As the binder for the magnetic layer in the magnetic recording medium of the present invention, conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof can be used. The thermoplastic resin has a glass transition temperature of −100 to 150 ° C., a number average molecular weight of 1,000 to 200,000, preferably 10,000 to 100,000, and a degree of polymerization of about 50 to 1,000.

  Such binder resins include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl butyral. , Vinyl acetal, vinyl ether, etc. as a constituent unit, polymers or copolymers, polyurethane resins, and various rubber resins.

  Thermosetting resins or reactive resins include phenolic resins, epoxy resins, polyurethane curable resins, urea resins, melamine resins, alkyd resins, acrylic reactive resins, formaldehyde resins, silicone resins, epoxy-polyamides. Resin, a mixture of polyester resin and isocyanate prepolymer, a mixture of polyester polyol and polyisocyanate, a mixture of polyurethane and polyisocyanate, and the like.

In order to obtain a better dispersion effect of the magnetic powder and durability of the magnetic layer in the binder resin, COOM, SO ₃ M, OSO ₃ M, P═O (OM) ₂ , O— P = O (OM) ₂ (wherein M is a hydrogen atom or an alkali metal base), OH, NR ₂ , N ⁺ R ₃ (R is a hydrocarbon group) an epoxy group, SH, CN, etc. One or more polar groups are preferably introduced by copolymerization or addition reaction. The amount of such a polar group is 10 ⁻¹ to 10 ⁻⁸ mol / g, preferably 10 ⁻² to 10 ⁻⁶ mol / g.

  The binder resin used in the magnetic recording medium of the present invention is used in the range of 5 to 50% by mass, preferably in the range of 10 to 30% by mass with respect to the magnetic powder. It is preferable to use a combination of 5 to 100% by mass when using a vinyl chloride resin, 2 to 50% by mass when using a polyurethane resin, and 2 to 100% by mass of polyisocyanate.

In the present invention, when a polyurethane resin is used, the glass transition temperature is −50 to 100 ° C., the breaking elongation is 100 to 2000%, the breaking stress is 0.05 to 10 kg / cm ² (4.9 to 980 kPa), and the yield point is 0.05-10 kg / cm < ² > (4.9-980 kPa) is preferable.

  Examples of the polyisocyanate used in the present invention include tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1, 5- Isocyanates such as diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, triphenylmethane triisocyanate, and products of these isocyanates and polyalcohols In addition, polyisocyanate produced by condensation of isocyanates can be used. Commercially available product names of these isocyanates include: Nippon Polyurethane, Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR Millionate MTL, Takeda Pharmaceutical Co., Ltd. Takenet D-102, Takenet D-110N, Takenet D-200, Takenet D-202, manufactured by Sumika Bayer Co., Ltd., Death Module L, Death Module IL, Death Module N, There is a desmodulation HL, etc., which can be used alone or in combination of two or more utilizing the difference in curing reactivity.

  In the magnetic layer of the magnetic recording medium of the present invention, materials having various functions such as a lubricant, an abrasive, a dispersant, an antistatic agent, a dispersant, a plasticizer, an antifungal agent and the like are usually used. It is contained depending on the purpose.

  As the lubricant used in the magnetic layer of the present invention, dialkylpolysiloxane (alkyl is 1 to 5 carbon atoms), dialkoxypolysiloxane (alkoxy is 1 to 4 carbon atoms), monoalkyl monoalkoxypolysiloxane (alkyl) Are silicon oil such as phenyl polysiloxane and fluoroalkyl polysiloxane (alkyl is 1 to 5 carbon atoms); conductive fine powder such as graphite; Inorganic powder such as molybdenum sulfide and tungsten disulfide; Plastic fine powder such as polyethylene, polypropylene, polyethylene vinyl chloride copolymer and polytetrafluoroethylene; α-olefin polymer; Saturated fatty acid solid at normal temperature (10 to 22 carbon atoms) ); Unsaturated aliphatic hydrocarbon liquid at room temperature (n-olefin double) Compounds in which the bond is bonded to the terminal carbon, carbon number of about 20); fatty acid esters composed of monobasic fatty acids having 12 to 20 carbon atoms and monohydric alcohols having 3 to 12 carbon atoms, fluorocarbons, etc. are used it can.

  Among the above, saturated fatty acid and fatty acid ester are preferable, and it is more preferable to use both together. Examples of alcohols used as fatty acid ester raw materials include ethanol, butanol, phenol, benzyl alcohol, 2-methylbutyl alcohol, 2-hexyldecyl alcohol, propylene glycol monobutyl ether, ethylene glycol monobutyl ether, dipropylene glycol monobutyl ether, diethylene glycol monobutyl ether, Examples thereof include monohydric alcohols such as s-butyl alcohol, and polyhydric alcohols such as ethylene glycol, diethylene glycol, neopentyl glycol, glycerin, and sorbitan derivatives. Similarly, fatty acids include acetic acid, propionic acid, octanoic acid, 2-ethylhexanoic acid, lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, arachidic acid, oleic acid, linoleic acid, linolenic acid, elaidic acid, palmitoleic acid. Aliphatic carboxylic acids such as, or a mixture thereof.

  Specific examples of fatty acid esters include butyl stearate, s-butyl stearate, isopropyl stearate, butyl oleate, amyl stearate, 3-methylbutyl stearate, 2-ethylhexyl stearate, 2-hexyldecyl stearate, Butyl palmitate, 2-ethylhexyl myristate, a mixture of butyl stearate and butyl palmitate, butoxyethyl stearate, 2-butoxy-1-propyl stearate, dipropylene glycol monobutyl ether acylated with stearic acid, diethylene glycol Examples thereof include various ester compounds such as dipalmitate, hexamethylenediol acylated with myristic acid to obtain a diol, and glycerin oleate.

Furthermore, in order to reduce the hydrolysis of fatty acid esters that often occur when the magnetic recording medium is used under high humidity, the branched / straight chain, cis / trans and other isomeric structures and branch positions of the starting fatty acid and alcohol are selected. Things are done.
These lubricants are added in the range of 0.2 to 20 parts by mass with respect to 100 parts by mass of the binder.

  As the lubricant, the following compounds can also be used. That is, silicon oil, graphite, molybdenum disulfide, boron nitride, fluorinated graphite, fluorine alcohol, polyolefin, polyglycol, alkyl phosphate ester, tungsten disulfide, and the like.

As the abrasive used in the magnetic layer of the present invention, commonly used materials are α, γ alumina, fused alumina, corundum, artificial corundum, silicon carbide, chromium oxide (Cr ₂ O ₃ ), diamond, artificial diamond, pomegranate. Stone, αFe ₂ O ₃ or the like is used. These abrasives have a Mohs hardness of 6 or more. Specific examples include Sumitomo Chemical Co., Ltd., AKP-30, AKP-50, HIT-50, HIT60A, HIT60G, HIT70, HIT80, HIT82, HIT-100, Nippon Chemical Industry Co., Ltd., G5, G7, S-1 , Chrome oxide K, Uemura Kogyo UB40B, Fujimi Abrasive WA8000, WA10000, LANDS LS600F 0 / -1 / 4, Tomei Dia MD-200, MD-150, MD-100, MD- 70, IRM 0-1 / 4F, IRM 0-1 / 4FF, GE 0-1 / 10, 0-1 / 4, DuPont Mypolex 1 / 10QG, 1 / 8QG, Toda Kogyo TF100 , TF140, TF180, etc. An average particle diameter of 0.05 to 0.5 μm is effective, and preferably 0.05 to 0.3 μm.
In addition to using the abrasive alone, it is also suitable to use two or more kinds of abrasives together. In the case of fine-grain diamond, the amount added to the magnetic powder is 0.1% by using it together with other abrasives. Can be reduced to a degree. The total amount of these abrasives is added in the range of 1 to 20 parts by mass, preferably 1 to 15 parts by mass with respect to 100 parts by mass of the magnetic powder. When the amount is less than 1 part by mass, sufficient durability cannot be obtained. When the amount is more than 20 parts by mass, the surface property and the filling degree deteriorate. These abrasives may be added to the magnetic paint after being dispersed with a binder in advance.

The magnetic layer of the magnetic recording medium of the present invention may contain conductive particles as an antistatic agent in addition to the nonmagnetic powder. However, in order to maximize the saturation magnetic flux density of the uppermost layer, it is preferable to add it to the uppermost layer as much as possible and add it to a coating layer other than the uppermost layer. In particular, the addition of carbon black as an antistatic agent is preferable in terms of reducing the surface electrical resistance of the entire medium. The carbon black that can be used in the present invention may be rubber furnace, rubber thermal, color black, conductive carbon black, acetylene black, or the like. Specific surface area is 5 to 500 m ² / g, DBP oil absorption is 10 to 1500 ml / 100 g, particle size is 5 to 300 nm, PH is 2 to 10, moisture content is 0.1 to 10%, tap density is 0.1 to 1 g / cm ³ is preferable. Specific examples of carbon black used in the present invention include Cabot Corporation, BLACKPEARLS 2000, 1300, 1000, 900, 800, 700, VULCAN XC-72, Asahi Carbon Corporation, # 80, # 60. # 55, # 50, # 35, manufactured by Mitsubishi Chemical Corporation, # 3030B, # 3040B, # 3050B, # 3230B, # 3350B, # 9180B, # 2700, # 2650, # 2600B, # 2400B, # 2300, # 950B # 900, # 1000, # 95, # 30, # 40, # 10B, MA230, MA220, MA77, manufactured by Columbian Carbon, CONDUCTEX SC, RAVEN 150, 50, 40, 15, Ketjen Black International Ketjen Black EC, Ketjen Black EC DJ-500, ketjen black ECDJ-600, and the like. Carbon black may be surface-treated with a dispersant, carbon black may be oxidized, or may be grafted with a resin, or may be obtained by graphitizing a part of the surface. Further, carbon black may be dispersed in advance with a binder before being added to the magnetic paint. When carbon black is used for the magnetic layer, the amount based on the magnetic powder is preferably 0.1 to 30% by mass. Further, the nonmagnetic layer is preferably contained in an amount of 3 to 20% by mass based on the total nonmagnetic powder.

  In general, carbon black not only serves as an antistatic agent, but also functions to reduce the coefficient of friction, impart light shielding properties, and improve film strength. These differ depending on the carbon black used. Therefore, these carbon blacks used in the present invention are changed in type, amount and combination, and depending on the purpose based on the various characteristics shown above, such as particle size, oil absorption, conductivity, and pH. Of course, it is possible to use them properly. Carbon black that can be used can be referred to, for example, “Carbon Black Handbook” edited by Carbon Black Association.

When the nonmagnetic layer is formed below the magnetic layer of the magnetic recording medium of the present invention, the nonmagnetic layer is a layer in which nonmagnetic powder is dispersed in a binder resin. Various nonmagnetic powders can be used for the nonmagnetic layer. For example, α-alumina, β-alumina, γ-alumina, boehmite, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, goethite, corundum, silicon nitride, titanium carbide having an α conversion rate of 90% or more, Titanium oxide, silicon dioxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate and the like are used alone or in combination. Α-iron oxide, goethite, titanium oxide, zinc oxide, and boehmite are preferable as those having a fine particle size. The particle size of these non-magnetic powders is preferably 0.01 to 0.6 μm. However, if necessary, non-magnetic powders with different particle sizes can be combined, or even a single non-magnetic powder can have the same effect by widening the particle size distribution. Can also be given. In order to increase the interaction with the binder resin to be used and improve the dispersibility, the nonmagnetic powder to be used may be surface-treated. The surface-treated product may be treated with an inorganic material such as silica, alumina, zirconia, or silica-alumina, or may be treated with a coupling agent. The tap density is preferably 0.3 to ² g / cm ³ , the water content is 0.1 to 5% by mass, the pH is 2 to 11, and the specific surface area is preferably 5 to 150 m ² / g. The nonmagnetic powder may be acicular, spherical, dice, or plate-shaped. Specific examples of the non-magnetic powder used in the lower layer of the magnetic recording medium obtained by the present invention include Showa Denko Nanotite, Sumitomo Chemical HIT-100, HIT-80, Toda Kogyo α-iron oxide DPN-250BX. , DPN-245, DPN-270BX, DPN-550RX, DBN-450BX, DBN-650RX, DAN-850RX, manufactured by Ishihara Sangyo TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100, Titanium Industry Titanium Oxide STT-4D, STT-30D, STT-30, STT-65C, Teica Titanium Oxide MT-100S, MT-100T, MT-150W, MT-500B, MT -600B, MT-100F, MT-500HD, Sakai Chemical FINEX-2 BF-1, BF-10, BF-20, ST-M, Dowa Mining Iron Oxide DEFIC-Y, DEFIC-R, Nippon Aerosil AS2BM, TiO2 P25, Ube Industries 100A, 500A, and fired Things.

  Forming two or more coating layers on a non-magnetic support is also effective for producing high recording density magnetic recording media, and the simultaneous coating method can produce ultra-thin magnetic layers. Efficiency is excellent. As a specific method of wet-on-wet method as the simultaneous application method,

(1) First, a lower layer is first applied by a gravure coating, roll coating, blade coating, and extrusion coating device generally used in magnetic paints, and while the layer is still wet, for example, Japanese Patent Publication No. 1-44686 A method of applying an upper layer with a non-magnetic support pressure type extrusion coating apparatus disclosed in Japanese Patent Laid-Open No. 60-238179 and Japanese Patent Laid-Open No. 2-265672,

(2) A coating head incorporating two coating liquid passage slits as disclosed in JP-A-63-88080, JP-A-2-17971 and JP-A-2-265672, A method of applying the coating solution and the upper layer coating solution almost simultaneously,

(3) A method in which an upper layer and a lower layer are applied almost simultaneously with an extrusion coating apparatus with a backup roll disclosed in JP-A-2-174965,
Etc.

  When applying the wet-on-wet method, the flow characteristics of the magnetic layer coating solution and non-magnetic layer coating solution should be as close as possible, but the thickness of the coated magnetic layer and non-magnetic layer interface will not be disturbed. A uniform magnetic layer with little thickness variation can be obtained. Since the flow characteristics of the coating solution strongly depend on the combination of the powder particles and the binder resin in the coating solution, it is necessary to pay particular attention to the selection of the nonmagnetic powder used for the nonmagnetic layer. Further, after forming the nonmagnetic layer, a magnetic layer may be formed on the nonmagnetic layer. Since this method can further reduce the disturbance of the interface, it is a preferable method when forming a thin magnetic layer.

  The nonmagnetic support of this magnetic recording medium is usually 3 to 100 μm, preferably 2 to 15 μm when used in a tape form, and preferably 20 to 80 μm when used as a flexible disk. The non-magnetic layer provided in is 0.5 to 3.0 μm, preferably 0.5 to 2.5 μm. The thickness of the magnetic layer is 30 to 200 nm, preferably 50 to 150 nm, and more preferably 50 to 100 nm. In addition, the formation of layers other than the magnetic layer and the nonmagnetic layer according to the purpose is permitted as long as the magnetic layer is the uppermost layer and the nonmagnetic layer is the lower layer. For example, an undercoat layer may be provided between the nonmagnetic support and the lower layer to improve adhesion. This thickness is 0.01 to 0.7 μm, preferably 0.05 to 0.5 μm. Further, a backcoat layer may be provided on the side opposite to the nonmagnetic support side magnetic layer side. This thickness is 0.1 to 1.0 μm, preferably 0.2 to 0.8 μm. As these intermediate layer and backcoat layer, known ones can be used. In the case of a disk-shaped magnetic recording medium, the above layer configuration can be provided on both sides or one side.

  There is no restriction | limiting in particular in the nonmagnetic support body used by this invention, The normally used thing can be used. Examples of materials forming the nonmagnetic support include polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamideimide, polyimide, polysulfone, polyethersulfone, and other synthetic resin films, and aluminum foil. And metal foil such as stainless steel foil.

In order to effectively achieve the object of the present invention, the surface roughness of the non-magnetic support is 0.03 μm or less, desirably 0.02 μm or less, in terms of center plane average surface roughness Ra (cutoff value 0.25 mm), Desirably, it is 0.01 micrometer or less. In addition, these nonmagnetic supports preferably have not only a small average surface roughness of the center plane but also no coarse protrusions of 1 μm or more. The roughness of the surface can be freely controlled according to the size and amount of filler added to the nonmagnetic support as required. Examples of these fillers include oxides and carbonates such as Ca, Al, Si, and Ti and fine organic resin powders such as acrylic. The F-5 value in the web running direction of the nonmagnetic support used in the present invention is preferably 5 to 50 kg / mm ² (49 to 490 MPa), and the F-5 value in the web width direction is preferably 3 to 30 kg / mm ^2. (29.4 to 294 MPa), and the F-5 value in the web running direction is generally higher than the F-5 value in the web width direction, but particularly when the strength in the width direction needs to be increased. Not so.

The heat shrinkage rate at 100 ° C. for 30 minutes in the web running direction and width direction of the support is preferably 3% or less, more desirably 1.5% or less, and the heat shrinkage rate at 80 ° C. for 30 minutes is preferably 1%. % Or less, more preferably 0.5% or less. The breaking strength is preferably 5 to 100 kg / mm ² (49 to 980 MPa) in both directions, and the elastic modulus is preferably 100 to 2000 kg / mm ² (0.98 to 19.6 GPa).

  The organic solvent used in the present invention may be any ratio of ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, tetrahydrofuran, methanol, ethanol, propanol, butanol, Alcohols such as isobutyl alcohol, isopropyl alcohol, methylcyclohexanol, esters such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycol acetate, glycol dimethyl ether, glycol mono Glycol ethers such as ethyl ether, dioxane, aromatic hydrocarbons such as benzene, toluene, xylene, cresol, chlorobenzene, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, Chi Ren chlorohydrin, dichlorobenzene, chlorinated hydrocarbons and the like, N, N- dimethylformamide, those hexane and the like can be used. These organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposition products, oxides, and moisture in addition to the main components. These impurities are preferably 30% or less, more preferably 10% or less. The type and amount of the organic solvent used in the present invention may be changed between the magnetic layer and the intermediate layer if necessary. The first layer uses a highly volatile solvent to improve surface properties. The first layer uses a high surface tension solvent (cyclohexanone, dioxane, etc.) to increase the coating stability. Examples of such a method include increasing the filling degree using a high solvent, but it is needless to say that the present invention is not limited to these examples.

  The magnetic recording medium of the present invention is kneaded and dispersed using an organic solvent together with the magnetic powder, binder resin, and other additives if necessary, and a magnetic coating is applied onto a nonmagnetic support, and if necessary. Obtained by orientation and drying.

  The process for producing the magnetic coating material of the magnetic recording medium of the present invention comprises at least a kneading process, a dispersing process, and a mixing process provided as necessary before and after these processes. Each process may be divided into two or more stages. All raw materials such as magnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant and solvent used in the present invention may be added at the beginning or during any step. In addition, individual raw materials may be added in two or more steps. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion.

  Various kneaders are used for kneading and dispersing the magnetic paint. For example, two roll mill, three roll mill, ball mill, pebble mill, tron mill, sand grinder, Segvari, attritor, high speed impeller disperser, high speed stone mill, high speed impact mill, disper, kneader, high speed mixer, homogenizer, ultra A sonic disperser or the like can be used.

  In order to achieve the object of the present invention, the manufacturing technique known so far can be used as a part of the process, but the kneading process is strong such as continuous kneader or pressure kneader. The high Br of the magnetic recording medium of the present invention can be obtained only by using a material having a kneading force. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-64-79274. In the present invention, it is possible to produce more efficiently by using a simultaneous multilayer coating method as disclosed in JP-A-62-212933.

The residual solvent contained in the magnetic layer of the magnetic recording medium of the present invention is preferably 100 mg / m ² or less, more preferably 10 mg / m ² or less, and the residual solvent contained in the magnetic layer is contained in the nonmagnetic layer. Less than the solvent is preferred.

  The porosity of the magnetic layer is preferably 30% by volume or less, more preferably 10% by volume or less, for both the lower layer and the uppermost layer. The porosity of the nonmagnetic layer is preferably larger than the porosity of the magnetic layer, but may be small as long as the porosity of the nonmagnetic layer is 5% by volume or more.

  Although the magnetic recording medium of the present invention has a lower layer and an uppermost layer, it is easily estimated that these physical characteristics can be changed between the lower layer and the uppermost layer depending on the purpose. For example, the elastic modulus of the uppermost layer is increased to improve running durability, and the elastic modulus of the lower layer is made lower than that of the magnetic layer to improve the contact of the magnetic recording medium with the head.

  By such a method, the magnetic layer coated on the nonmagnetic support is subjected to a treatment for orienting the magnetic powder in the layer, if necessary, and then the formed magnetic layer is dried. Further, the magnetic recording medium of the present invention is produced by performing a surface smoothing process or cutting into a desired shape as necessary. The above composition for the uppermost layer and the composition for the lower layer are dispersed together with a solvent, and the obtained coating solution is coated on a nonmagnetic support and oriented and dried to obtain a magnetic recording medium.

The elastic modulus at 0.5% elongation of the magnetic layer is preferably 100 to 2000 kg / mm ² (0.98 to 19.6 GPa) in both the web application direction and the width direction, and the breaking strength is preferably 1 to 30 kg / cm ² ( 98 to 2940 KPa), the elastic modulus of the magnetic recording medium is preferably 100 to 1500 kg / mm ² (0.98 to 14.7 GPa) in both the web coating direction and the width direction, and the residual spread is preferably 0.5% or less, 100 ° C. The thermal shrinkage at any of the following temperatures is desirably 1% or less, more desirably 0.5% or less, and most desirably 0.1% or less.

  The magnetic recording medium of the present invention may be a tape for video applications, data recording applications, or a flexible disk or magnetic disk for data recording applications, but signal loss due to the occurrence of dropout is fatal. This is particularly effective for a digital recording medium. Furthermore, by making the lower layer a non-magnetic layer and the thickness of the uppermost layer to be 0.3 μm or less, it is possible to obtain a high-density and large-capacity magnetic recording medium having high electromagnetic conversion characteristics and excellent overwrite characteristics. it can.

  Hereinafter, the present invention will be described more specifically with reference to examples.

<Production example of magnetic powder>
Examples 1-11
An alkane solution in which 108 g of aerosol OT and 800 ml of decane were mixed was added to an aqueous solution in which 80 mmol of NaOH was dissolved in 160 ml of pure water to prepare a reverse micelle solution (I).

Pure BaCl ₂ · 2H ₂ O 2 mmol, FeCl ₃ · 6H ₂ O 22.7 mmol, CoCl ₂ · 6H ₂ O 0.56 mmol, ZnCl ₂ 0.50 mmol, Nb (NO ₃ ) ₃ 0.24 mmol An alkane solution obtained by mixing aerosol OT (amount shown in Table 1) and decane 500 ml was added to a metal salt aqueous solution dissolved in 140 ml of water and mixed to prepare a reverse micelle solution (II).

  While the reverse micelle solution (I) was stirred at a high speed with an omni mixer (manufactured by Yamato Kagaku) at 22 ° C., the reverse micelle solution (II) brought to 22 ° C. was added over 3 minutes. After stirring for 8 minutes with a magnetic stirrer, the mixture was heated to 50 ° C. and aged for 30 minutes. It was cooled to room temperature and taken out into the air after cooling. In order to destroy the reverse micelle, a mixed solution of 500 ml of water and 500 ml of methanol was added to separate into an aqueous phase and an oil phase. A state where coprecipitated particles were dispersed on the oil phase side was obtained. The oil phase side was washed twice with a mixed solution of 600 ml of water and 200 ml of methanol.

Thereafter, 2000 ml of methanol was added to cause co-precipitation particles to flocculate and settle. The supernatant was removed and 100 ml of heptane was added to redisperse.
Furthermore, sedimentation by adding 1000 ml of methanol and sedimentation dispersion of 100 ml of heptane were repeated twice, and methanol was added to precipitate the coprecipitated particles. The supernatant was removed, and then 1000 ml of water was added to disperse, settle and remove the supernatant twice.

  The composition was analyzed about the obtained coprecipitate. The composition was calculated by dissolving the coprecipitate-containing liquid obtained by the reaction with hydrochloric acid, and measuring it with ICP (inductively coupled high frequency plasma emission spectrometry). The magnetoplumbite type ferrite composition added within the experimental error was contained.

The obtained coprecipitate was re-dispersed in 1 L of pure water, 0.1 mol of CaCl ₂ · 6H ₂ O dissolved in pure water was added, and then neutralized with NH ₄ OH, Ba ferrite composition A coating of Ca (OH) ₂ was formed on the surface of the product particles. The precipitate was filtered and washed with water, and then dried at 120 ° C. The dried product was dry pulverized.
The dried product was heated in air at 600 ° C. to convert the surface coating into an oxide, and further heat-treated at the temperature shown in Table 1 to obtain Ba ferrite. The product was placed in a 5% aqueous acetic acid solution to remove the surface coating, and magnetoplumbite type ferrite was recovered. The obtained Ba ferrite was observed by TEM to determine the average plate diameter, coefficient of variation, and average plate ratio. Moreover, it deaerated at 250 degreeC in nitrogen, and measured the specific surface area by BET method. In addition, using a vibrating sample magnetometer, the magnetic characteristics were measured with an applied magnetic field of 796 kA / m (10 kOe). These measured values are shown in Table 1.

Example 12
An alkane solution obtained by mixing 108 g of aerosol OT and 800 ml of decane was added to an aqueous solution obtained by dissolving 80 mmol of NH ₄ OH in 160 ml of pure water to prepare a reverse micelle solution (I).

An alkane solution in which 54 g of aerosol OT and 500 ml of decane are mixed is added to a metal salt aqueous solution in which 2 mmol of BaCl ₂ · 2H ₂ O and 24.0 mmol of FeCl ₃ · 6H ₂ O are dissolved in 140 ml of pure water, and mixed to form a reverse micelle. Solution (II) was prepared.

  While the reverse micelle solution (I) was stirred at a high speed with an omni mixer (manufactured by Yamato Kagaku) at 22 ° C., the reverse micelle solution (II) brought to 22 ° C. was added over 3 minutes. After stirring for 8 minutes with a magnetic stirrer, the mixture was heated to 50 ° C. and aged for 30 minutes. It was cooled to room temperature and taken out into the air after cooling. In order to destroy the reverse micelle, a mixed solution of 500 ml of water and 500 ml of methanol was added to separate into an aqueous phase and an oil phase. A state where coprecipitated particles were dispersed on the oil phase side was obtained. The oil phase side was washed twice with a mixed solution of 600 ml of water and 200 ml of methanol.

Thereafter, 2000 ml of methanol was added to cause co-precipitation particles to flocculate and settle. The supernatant was removed and 100 ml of heptane was added to redisperse.
Furthermore, sedimentation by adding 1000 ml of methanol and sedimentation dispersion of 100 ml of heptane were repeated twice, and methanol was added to precipitate the coprecipitated particles. The supernatant was removed, and then 1000 ml of water was added to disperse, settle and remove the supernatant twice.

  The composition was analyzed about the obtained coprecipitate. The composition was calculated by dissolving the coprecipitate-containing liquid obtained by the reaction with hydrochloric acid, and measuring it with ICP (inductively coupled high frequency plasma emission spectrometry). The magnetoplumbite type ferrite composition added within the experimental error was contained.

The obtained coprecipitate was re-dispersed in 1 L of pure water, 0.1 mol of CaCl ₂ · 6H ₂ O dissolved in pure water was added, and then neutralized with NH ₄ OH, Ba ferrite composition A coating of Ca (OH) ₂ was formed on the surface of the product particles. The precipitate was filtered and washed with water, and then dried at 120 ° C. The dried product was dry pulverized.
The dried product was heated in air at 600 ° C. to convert the surface coating into an oxide, and then heat treated at 850 ° C. for 2 hours to obtain Ba ferrite. The product was placed in a 5% aqueous acetic acid solution to remove the surface coating, and magnetoplumbite type ferrite was recovered. The measured values of the obtained Ba ferrite are shown in Table 1.

Example 13
To an aqueous metal salt solution in which BaCl ₂ · 2H ₂ O 2 mmol, FeCl ₃ · 6H ₂ O 22.0 mmol and AlCl ₃ 2 mmol were dissolved in 140 ml of pure water, an alkane solution obtained by mixing 54 g of aerosol OT and 500 ml of decane was added, Synthesis was performed in the same manner as in Example 12 except that the reverse micelle solution (II) was prepared by mixing. The measured values of the obtained Ba ferrite are shown in Table 1.

Example 14
BaCl ₂ · 2H ₂ O 0.05 mol, ZnCl ₂ 0.025 mol, Nb (NO ₃ ) ₃ 0.0015 mol and FeCl ₃ · 6H ₂ O 0.56 mol were dissolved in 1 L of pure water to obtain a metal chloride. A solution was made. A metal chloride solution was added to 1.5 L of n-peptane in which 30 mL of sorbitan monooleate was previously dissolved and brought to 25 ° C., and dispersed strongly using a homogenizer to form a W / O emulsion. While stirring, an alkaline solution obtained by dissolving 100 g of NaOH and 10.6 g of Na ₂ CO _{3 in 2} L of pure water was brought to 25 ° C. and mixed with a W / O emulsion to obtain a coprecipitate slurry. After stirring for 15 minutes and holding at 45 ° C. for 30 minutes, an aqueous HCl solution was added to adjust the pH of the slurry to 7.8, and the slurry was allowed to settle and the supernatant was removed. A solution prepared by dissolving 3 mol of CaCl ₂ .6H ₂ O in 1 L of pure water was added to the slurry, neutralized with NH ₄ OH, and the surface of the coprecipitate was coated with Ca (OH) ₂ . The coprecipitate was filtered and washed with water, and then dried at 120 ° C. The dried product was dry pulverized. The dried product was heated in air at 625 ° C. for 30 minutes to convert the surface coating into an oxide, and further heat-treated at 800 ° C. for 2 hours to obtain Ba ferrite. The obtained Ba ferrite was observed by TEM to determine the average plate diameter, coefficient of variation, and plate ratio. Moreover, it deaerated at 250 degreeC in nitrogen, and measured the specific surface area by BET method. Further, using a vibrating sample magnetometer, magnetic characteristics (SQ is square ratio) were measured with an applied magnetic field of 796 kA / m (10 kOe). These measured values are shown in Table 1.

Comparative Example 1
BaCl ₂ · 2H ₂ O 0.05 mol, ZnCl ₂ 0.025 mol, Nb (NO ₃ ) ₃ 0.0055 mol and FeCl ₃ · 6H ₂ O 0.56 mol were dissolved in 1 L of pure water to obtain a metal chloride. A solution was made. A metal chloride solution was added to 1.5 L of n-peptane in which 30 mL of sorbitan monooleate was previously dissolved and brought to 25 ° C., and dispersed strongly using a homogenizer to form a W / O emulsion. While stirring, an alkaline solution obtained by dissolving 100 g of NaOH and 10.6 g of Na ₂ CO _{3 in 2} L of pure water was brought to 25 ° C. and mixed with a W / O emulsion to obtain a coprecipitate slurry. After stirring for 15 minutes and holding at 45 ° C. for 30 minutes, an aqueous HCl solution was added to adjust the pH of the slurry to 7.8, and the slurry was allowed to settle and the supernatant was removed. The coprecipitate was filtered and washed with water, and then dried at 120 ° C. The dried product was dry pulverized. The dried product was heated in air at 625 ° C. for 30 minutes to convert the surface coating into an oxide, and further heat-treated at 800 ° C. for 2 hours to obtain Ba ferrite. The obtained Ba ferrite was observed by TEM to determine the average plate diameter, coefficient of variation, and plate ratio. Moreover, it deaerated at 250 degreeC in nitrogen, and measured the specific surface area by BET method. In addition, using a vibrating sample magnetometer, the magnetic characteristics were measured with an applied magnetic field of 796 kA / m (10 kOe). These measured values are shown in Table 1.

<Preparation of paint>
In the examples, the display of “parts” indicates “parts by mass”.
Magnetic liquid formula 1
Ba ferrite (magnetic powder is shown in Table 2) 100 parts Binder resin Vinyl chloride copolymer 14 parts (-SO ₃ K group: 1 × 10 ⁻⁴ eq / g contained, degree of polymerization 300)
Polyester polyurethane resin 5 parts (Neopentyl glycol / Caprolactone polyol / MDI =
0.9 / 2.6 / 1, —SO ₃ Na group: 1 × 10 ⁻⁴ eq / g)
α-alumina (average particle size: 0.10 μm) 2 parts carbon black (average particle size: 30 nm) 4 parts butyl stearate 2 parts stearic acid 2 parts methyl ethyl ketone 125 parts cyclohexanone 125 parts

Magnetic liquid formula 2
Ba ferrite (magnetic powder is shown in Table 3) 100 parts Binder resin Vinyl chloride copolymer 15 parts (-SO ₃ K group: 1 × 10 ⁻⁴ eq / g contained, degree of polymerization 300)
Polyester polyurethane resin 5 parts (Neopentyl glycol / Caprolactone polyol / MDI =
0.9 / 2.6 / 1, —SO ₃ Na group: 1 × 10 ⁻⁴ eq / g)
α-alumina (average particle size: 0.10 μm) 2 parts carbon black (average particle size: 30 nm) 5 parts butyl stearate 10 parts butoxyethyl stearate 5 parts isohexadecyl stearate 2 parts stearic acid 3 parts methyl ethyl ketone 125 parts cyclohexanone 125 parts

Non-magnetic liquid formula 1
80 parts of acicular hematite (BET specific surface area: ^65m 2 / g, average major axis length: 0.10 .mu.m,
(Average needle ratio: 7, pH: 8.8, aluminum treatment layer: 1% by mass as Al ₂ O ₃ )
20 parts of carbon black (average particle size: 17 nm, DBP oil absorption: 80 ml / 100 g,
Surface area according to BET method: 240 m ^{2 /} g, pH 7.5)
Binder resin 13 parts of vinyl chloride copolymer (-SO ₃ K group: 1 × 10 ⁻⁴ eq / g contained, degree of polymerization 300)
Polyester polyurethane resin 5 parts (Neopentyl glycol / Caprolactone polyol / MDI =
0.9 / 2.6 / 1, —SO ₃ Na group: 1 × 10 ⁻⁴ eq / g)
Phenylphosphonic acid 3 parts Butyl stearate 3 parts Stearic acid 3 parts Methyl ethyl ketone and cyclohexanone 1: 1 mixed solvent 280 parts

Non-magnetic liquid formulation 2
80 parts of acicular hematite (specific surface area by BET method: 65 m ² / g, average major axis length: 0.10 μm,
(Average needle ratio: 7, pH: 8.8, aluminum treatment layer: 1% by mass as Al ₂ O ₃ )
20 parts of carbon black (average particle size: 17 nm, DBP oil absorption: 80 ml / 100 g,
Surface area according to BET method: 240 m ^{2 /} g, pH 7.5)
Binder resin Vinyl chloride copolymer 15 parts (-SO ₃ K group: 1 × 10 ⁻⁴ eq / g contained, degree of polymerization 300)
Polyester polyurethane resin 5 parts (Neopentyl glycol / Caprolactone polyol / MDI =
0.9 / 2.6 / 1, —SO ₃ Na group: 1 × 10 ⁻⁴ eq / g)
Phenylphosphonic acid 3 parts α-alumina (average particle diameter: 0.15 μm) 2 parts carbon black (average particle diameter: 30 nm) 5 parts butyl stearate 10 parts butoxyethyl stearate 5 parts isohexadecyl stearate 2 Part stearic acid 3 parts methyl ethyl ketone and cyclohexanone 1: 1 mixed solvent 280 parts

  For each of the magnetic liquid formulations 1 and 2 and the non-magnetic liquid formulations 1 and 2, after kneading the magnetic powder or pigment, vinyl chloride resin, phenylphosphonic acid and 50% by mass of each solvent with a kneader, The polyurethane resin and the remaining components were added and dispersed with a sand grinder. To the obtained dispersion, 15 parts of isocyanate is added to the non-magnetic liquid, 14 parts to the magnetic liquid, and 30 parts of cyclohexanone is added to each dispersion, followed by filtration using a filter having an average pore diameter of 1 μm. Coating solutions for forming and for forming the magnetic layer were prepared, respectively.

<Preparation of Tape: Examples 15 to 19 and Comparative Example 2>
The obtained coating liquid 1 for forming a lower nonmagnetic layer comprising the nonmagnetic liquid formulation 1 is applied onto a polyethylene terephthalate support having a thickness of 7 μm so that the thickness after drying is 1.5 μm. While the coating layer for the lower nonmagnetic layer is still in a wet state, the thickness of the magnetic layer after drying is about 0.080 μm by controlling the coating amount of the coating solution 1 for forming the magnetic layer composed of the magnetic fluid formulation 1. The wet simultaneous multi-layer coating was performed so that the two layers were still wet and passed through an aligning device for longitudinal alignment. At this time, the orientation magnet was passed through a rare earth magnet (surface magnetic flux 500 mT) and then through a solenoid magnet (magnetic flux density 500 mT), dried to such an extent that the orientation did not return in the solenoid, and further dried and wound up the magnetic layer. . Thereafter, a 7-stage calendar composed of metal rolls was used to perform a calendar process at a roll temperature of 90 ° C. to obtain a web-like magnetic recording medium, which was slit to 8 mm width to prepare an 8 mm video tape sample. Using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.), the magnetic properties of the sample were measured parallel to the orientation direction with an applied magnetic field of 796 kA / m. Furthermore, the surface roughness was measured. For the surface roughness, a 250 μm square sample area was measured using an optical interference three-dimensional roughness meter “TOPO-3D” manufactured by WYKO (Arizona, US). In calculating the measurement values, corrections such as tilt correction, spherical correction, and cylinder correction were performed in accordance with JIS-B601, and the center plane average roughness Ra was set as the surface roughness value.

<Production of Flexible Disk: Examples 20 to 22 and Comparative Example 3>
After applying the coating solution of the obtained nonmagnetic liquid formulation 2 comprising the nonmagnetic liquid formulation 2 on a polyethylene naphthalate support having a thickness of 35 μm to a thickness of 1.4 μm after drying, after drying, A magnetic layer with a thickness of about 0.1 μm after drying is formed by adjusting the coating amount of the magnetic layer using the coating liquid 2 for forming the magnetic layer comprising the magnetic liquid formulation 2, and the magnetic layer is still wet. The magnetic field strength was 24 kA / m at a frequency of 50 Hz, and then passed through two magnetic field strength AC magnetic field generators having a frequency of 50 kHz and 12 kA / m, and a random orientation process was performed. Thereby, an orientation degree ratio of 98% or more could be obtained.
Similarly, a coating solution for forming the lower nonmagnetic layer and the magnetic layer is applied to the other support surface, oriented, dried, and then dried at a temperature of 90 ° C. and a linear pressure of 300 kg / cm (294 kN / m) using a seven-stage calendar. Was processed. It was punched into 3.7 mm, thermotreated (70 ° C., 24 hours) to accelerate the curing treatment of the coating layer, burnished with an abrasive tape, and post-treated to remove surface protrusions. Using a vibrating sample magnetometer (manufactured by Toei Kogyo), the magnetic properties of the sample were measured in parallel with the longitudinal direction of the support with an applied magnetic field of 796 kA / m. Furthermore, the surface roughness was measured. For the surface roughness, a 250 μm square sample area was measured using an optical interference three-dimensional roughness meter “TOPO-3D” manufactured by WYKO (Arizona, US). In calculating the measurement values, corrections such as tilt correction, spherical correction, and cylinder correction were performed in accordance with JIS-B601, and the center plane average roughness Ra was set as the surface roughness value.
The results are shown in Tables 2 and 3. The unit of the error rate in Table 3 is (× 10 ⁻⁵ ).
The method of measuring the electromagnetic conversion characteristics of the tape system was as follows. 8mm deck for data recording, MIG head (head gap 0.2μm, track width 17μm, saturation magnetic flux density 1.5T, azimuth angle 20 °) and playback MR head (SAL bias, MR element is Fe-Ni, track width 6μm) And a gap length of 0.2 μm and an azimuth angle of 20 °). Using the MIG head, the relative speed of the tape and the head is 10.2 m / sec, the optimum recording current is determined from the input / output characteristics of 1/2 Tb (λ = 0.5 μm), and the signal is recorded with this current. Played with. C / N is from the peak of the reproduced carrier to the demagnetizing noise, and the resolution bandwidth of the spectrum analyzer is 100 kHz. The measurement results are shown with Comparative Example 2 set to 0 dB.
The flexible disk was evaluated under the following conditions.
The output was measured at a linear recording density of 144 kbpi and a track density of 200 tpi. Comparative example 3 was used for the output ref. The linear recording density is the number of bits of a signal recorded per inch in the recording direction. The track density is the number of tracks per inch. The surface recording density is obtained by multiplying the linear recording density and the track density. The error rate of the disk was measured by recording the signal of the above linear recording density on the disk using the (2, 7) RLL modulation method.

  As can be seen from the above examples, a coprecipitate having a desired hexagonal magnetoplumbite type ferrite composition is formed by the reverse micelle method, this surface is coated with an alkaline earth metal compound, and the coating is appropriately heat-treated. As a result, magnetoplumbite-type ferrite particles having a small particle diameter variation coefficient while maintaining a fine particle diameter were obtained. Moreover, the magnetic recording medium using these has a small SFD, and the output is large reflecting this.

Claims (4)

It has the following steps (1) to (3), the average plate diameter is 10 to 30 nm, the plate diameter variation coefficient is 5 to 25%, the plate ratio is 1.5 to 4.5, and the coercive force is 125. A method for producing a hexagonal magnetoplumbite type ferrite of ˜400 kA / m.
(1) A step of forming a coprecipitate having a desired hexagonal magnetoplumbite type ferrite composition by the reverse micelle method, and covering the surface with an alkaline earth metal compound; (2) obtained in the step (1) Drying the coating, followed by heating in air at 550-650 ° C. followed by heating at 650-900 ° C. to ferritize; and (3) washing the product obtained in step (2) above, The process of recovering hexagonal magnetoplumbite type ferrite. It has the following steps (1 ′), (2) and (3), the average plate diameter is 10 to 30 nm, the plate diameter variation coefficient is 5 to 25%, the plate ratio is 1.5 to 4.5, And the manufacturing method of the hexagonal magnetoplumbite type ferrite whose coercive force is 125-400 kA / m.
(1 ') Starting from chlorides with the desired hexagonal magnetoplumbite type ferrite composition, co-precipitates are formed by reverse micelle method using alkali metal hydroxide, and coexisting alkali metal chlorides are removed. (2) The coating obtained in the step (1) is dried and then heated in air at 550 to 650 ° C., followed by 650 to (3) a step of washing the product obtained in the step (2) and recovering hexagonal magnetoplumbite type ferrite;   An average plate diameter of 10 to 30 nm, a plate diameter variation coefficient of 5 to 25%, a plate ratio of 1.5 to 4.5, and a coercive force manufactured by the manufacturing method according to claim 1 or 2. Hexagonal magnetoplumbite type ferrite of 125-400 kA / m.   A magnetic recording medium having a magnetic layer containing a magnetic powder and a binder on a nonmagnetic support, wherein the magnetic powder contains the hexagonal magnetoplumbite type ferrite according to claim 3. Magnetic recording medium.