JP2006155832A - Magnetic recording medium and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium and its manufacturing method adapted to high recording density, wherein a magnetic liquid produced by uniformly dispersing magnetic powder in a binder can be prepared even if the magnetic powder is atomized to a size of tens of nm. <P>SOLUTION: In the magnetic recording medium having at least one magnetic layer formed by dispersing ferromagnetic powder in a binder on a supporting body, the binder contains hyper branch polyester and the ferromagnetic powder is hexagonal ferrite powder. In the manufacturing method of the magnetic recording medium comprising a dispersed matter preparing step for preparing a uniform dispersed matter by mechanically mixing the ferromagnetic powder with a dispersant, a magnetic liquid preparing step for preparing the magnetic liquid by adding the binder to the obtained dispersed matter and uniformly dispersing the binder and an applying step for applying the obtained magnetic liquid on the supporting body, the dispersant is the hyper branch polyester and the ferromagnetic powder is the hexagonal ferrite powder. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a magnetic recording medium for high-density recording in which a magnetic layer contains a ferromagnetic fine powder and a method for manufacturing the same.

In general, in order to meet the demand for high-density recording on magnetic recording media for computers and the like, further improvement in electromagnetic conversion characteristics is required, and it is important to make ferromagnetic powder finer and to make the surface of the media super smooth. It has become.
In the finer magnetic material, recent magnetic materials use a ferromagnetic metal powder of 0.1 μm or less and a ferromagnetic hexagonal ferrite fine powder of a plate diameter of 40 nm or less. However, as the magnetic material becomes finer, it has become very difficult to uniformly disperse the magnetic material.
There has been proposed a method for manufacturing a magnetic recording medium (see Patent Document 1) in which a ferromagnetic powder, a binder, and a dispersant are mechanically mixed and then kneaded and dispersed. Also proposed is a magnetic recording medium (see Patent Documents 2 and 3) using a magnetic paint obtained by adding a dispersing agent to a magnetic kneaded material kneaded with a dispersing agent and further dispersing the dispersion. . However, these magnetic recording media have insufficient dispersibility.

JP 2001-076340 A JP 2002-025034 A JP 2002-329308 A

  The problem to be solved by the present invention is that a magnetic liquid uniformly dispersed in a binder can be prepared even when the magnetic powder size is reduced to several tens of nanometers, and a magnetic recording medium corresponding to high density and its manufacture Is to provide a method.

The above-mentioned problems to be solved by the present invention have been solved by the magnetic recording medium (1) and the method (2) for producing a magnetic recording medium.
(1) A magnetic recording medium having at least one magnetic layer in which a ferromagnetic powder is dispersed in a binder on a support, the binder containing hyperbranched polyester, and the ferromagnetic powder comprising a hexagonal system A magnetic recording medium characterized by being a ferrite powder;
(2) Dispersion preparation step in which a ferromagnetic powder is mechanically mixed with a dispersant to prepare a uniform dispersion, and a magnetic liquid is prepared by adding a binder to the obtained dispersion and uniformly dispersing it. And a coating method for coating the obtained magnetic liquid on a support, wherein the dispersant is hyperbranched polyester, and the ferromagnetic powder is a hexagonal ferrite powder. A method of manufacturing a magnetic recording medium.

According to the method for producing a magnetic recording medium of the present invention, the magnetic powder and the dispersant are dispersed in advance (hereinafter also referred to as “pre-dispersion step”), and the surface of the magnetic material is uniformly coated with the dispersant. It was found that by dissociating the agglomeration, the binder resin added in the post-process can be easily adsorbed on the surface of the magnetic material, and an extremely high dispersibility can be realized.
Furthermore, it was found that extremely high dispersibility and smoothness can be realized by using hyperbranched polyester as a dispersant used in pre-dispersion.

  Hyperbranched polyester is a polyester having a spherical structure with a high degree of branching, and since it has a small molecular size in solution and has OH groups at a number of molecular ends, it is easily adsorbed on the surface of fine magnetic particles. For this reason, by using hyperbranched polyester, a magnetic paint having excellent dispersibility and a low paint viscosity can be obtained, and a magnetic recording medium having an extremely smooth surface can be obtained.

The magnetic recording medium of the present invention is a magnetic recording medium having at least one magnetic layer in which a ferromagnetic powder is dispersed in a binder on a support, and includes a hyperbranched polyester as a binder, It is a hexagonal ferrite powder.
The magnetic recording medium of the present invention may have at least one nonmagnetic layer in which nonmagnetic powder is dispersed in a binder between the support and the magnetic layer.

(Hyperbranch polyester)
The magnetic recording medium of the present invention contains hyperbranched polyester as a binder.
In the method for producing a magnetic recording medium of the present invention, hyperbranched polyester is used as a dispersant.

  Hyperbranched polyester is a multi-branched polyester having a dendritic structure. Therefore, in the present invention, “hyperbranched polyester” is also referred to as “multi-branched polyester”. For example, it is described in documents such as “Science and function of dendrimers” (2000.7.20, published by ICP, Inc., p86). It is synthesized by self-condensation of a compound having three or more two kinds of substituents in one molecule, and grows while repeating branching during polymerization. In the case of hyperbranched polyester, the substituent is a combination of an OH group and a COOH group, but the OH group may be acetylated or trimethylsilylated. As the COOH group, an acid chloride or trimethylsilyl group can be used.

Specific examples of the monomer compound include aromatic compounds such as 3,5-dihydroxybenzoic acid, 5-hydroxyisophthalic acid, derivatives thereof, and substituent-modified products such as ethylene oxide and propylene oxide added to hydroxy groups. There are those with extended chain length, those with hydroxyl group acetylated or trimethylsilylated, and those with carboxyl group converted to acid chloride.
Among the aliphatic ones are dimethylolpropionic acid, dimethylolbutanoic acid, and their derivatives. Derivatives include those obtained by adding ε-caprolactone to extend the chain length.
Examples of monomer compounds are shown below.

The hyperbranched polyester can be obtained by polymerizing one kind of monomer, but a plurality of monomers may be combined, and it is also possible to polymerize using a small amount of polyfunctional alcohol as a core compound. It is preferable to use a trifunctional or tetrafunctional alcohol in combination.
For example, glycerin, trimethylolpropane, pentaerythritol, dipentaerythritol, and these ethylene oxide adducts and propylene oxide adducts are used as the core compound. The degree of branching, molecular weight, etc. can be controlled by the nuclear compound.
Examples of the nuclear compound are shown below.

The hyperbranched polyester used in the present invention is preferably a hyperbranched polyester obtained from an aliphatic monomer compound, and more preferably a hyperbranched polyester synthesized by condensation of AB ₂ type molecules. Here, A and B represent a functional group, which represents a hydroxyl group or a derivative group thereof, or a carboxyl group or a derivative group thereof. As the AB ₂ type molecule, it is particularly preferable that A is a carboxyl group or a derivative group thereof, and B is a hydroxyl group or a derivative group thereof. Other multi-branched polyester obtained by self-condensation of the AB ₂ type molecule, relative to 1 mole of the AB ₂ type molecule, an alcohol or a derivative thereof of trifunctional or tetrafunctional as nuclei compounds 0.01 to 0. A hyperbranched polyester obtained by cocondensation with 1 mol (preferably 0.02 to 0.05 mol) is also preferably used. With respect to 1 mol of dimethylolpropionic acid, dimethylolbutanoic acid, or a derivative thereof obtained by self-condensation, or 1 mol of these dimethylolcarboxylic acids, pentaerythritol, trimethylolpropane or a derivative thereof is reduced to 0. Multi-branched polyesters obtained by co-condensing 02 to 0.05 mol can be suitably used in the present invention.
The preferred molecular weight of the hyperbranched polyester used in the present invention is preferably a number average molecular weight of 500 to 20,000, more preferably 800 to 10,000.
The synthesis method is not particularly limited, and various methods described in the above-mentioned literatures and the like are used. Heat-melt polycondensation or polycondensation using a condensing agent or the like in a solution is used.

  The degree of branching of the hyperbranched polyester is preferably 0.3 to 0.9, and more preferably 0.4 to 0.8. Here, the definition of the degree of branching corresponds to the ratio of the total number of terminal units and branching units with respect to the total number of units according to Frechet's formula (see “Science and function of dendrimers” on pages 80 to 81).

The terminal group is preferably an OH group. Some COOH groups may remain.
The OH value is preferably 0.1 meq / g to 50 meq / g, more preferably 1 to 15 meq / g.

  The hyperbranched polyester is preferably added in an amount of 3 to 15 parts by weight, more preferably 5 to 10 parts by weight, based on 100 parts by weight of the ferromagnetic powder.

(Binder)
The magnetic recording medium of the present invention may use a known binder in addition to the hyperbranched polyester. Moreover, a well-known binder can be used also as a binder of a nonmagnetic layer.
The binder described in JP-A-10-222838 can be used for both the magnetic layer and the nonmagnetic layer.
Vinyl chloride binders include acrylics such as alkyl acrylates and alkyl methacrylates, methacrylic monomers, allyl ethers such as allyl alkyl ethers, fatty acid vinyl esters such as vinyl acetate and vinyl propionate, vinyls such as styrene, ethylene and butadiene. Monomers, further functional groups such as hydroxyl groups and epoxy groups, and monomers having polar groups described later may be copolymerized.

As the polyurethane binder, polyester urethane, polyether urethane, polycarbonate urethane, polyether ester urethane, acrylic polyurethane, and the like can be used.
The glass transition temperature (Tg) of the polyurethane is preferably −50 to + 200 ° C., more preferably +30 to + 150 ° C. When Tg is in the above range, the durability is good. Moreover, since the calendar moldability is good, the smoothness and the electromagnetic conversion characteristics are excellent.
Although it does not restrict | limit in particular, Since a high affinity with the compound used for this invention is high, a polyurethane-type binder is preferable.

The binder contains 1 × 10 ⁻⁵ eq / polar group as —COOM, —SO ₃ M, —SO ₄ M, —PO (OM) ₂ , —OPO (OM) ₂ , amino group, quaternary ammonium base and the like. It is preferable that g-2 × 10 ⁻⁴ eq / g is introduced. When the polar group amount is in the above range, the dispersibility is good.
In addition, an OH group is preferably introduced as a curing functional group with the isocyanate curing agent.
In addition, those having an epoxy group introduced are also preferred. The compound used in the present invention is preferred because of its affinity with the OH group or the possibility of binding.

  It is preferable that a polyisocyanate compound and an epoxy compound are included as the curing agent. A polyisocyanate compound is particularly preferable. Known polyisocyanate compounds can be used.

  The amount of the binder in the magnetic layer is preferably 5 to 25 parts by weight with respect to 100 parts by weight of the ferromagnetic fine powder including the curing agent. The amount of the binder in the nonmagnetic layer is preferably 10 to 30 parts by weight with respect to 100 parts by weight of the nonmagnetic powder.

(Magnetic material)
The magnetic recording medium of the present invention uses hexagonal ferrite powder as the ferromagnetic powder.
Examples of hexagonal ferrite include barium ferrite, strontium ferrite, lead ferrite, calcium ferrite substitution, and Co substitution. Specific examples include magnetoplumbite type barium ferrite and strontium ferrite, magnetoplumbite type ferrite whose particle surface is coated with spinel, and magnetoplumbite type barium ferrite and strontium ferrite partially containing spinel phase. Other than the predetermined atoms, Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg , Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge, and Nb may be included. In general, a material to which an element such as Co—Ti, Co—Ti—Zr, Co—Ti—Zn, Ni—Ti—Zn, Nb—Zn—Co, Sb—Zn—Co, or Nb—Zn is added is used. Can do. Some raw materials and manufacturing methods contain specific impurities.

The particle size is preferably 10 to 50 nm as a hexagonal plate diameter, more preferably 20 to 40 nm.
When reproducing with a magnetoresistive head, it is necessary to reduce the noise, and the plate diameter is preferably 40 nm or less. When the plate diameter is in the above range, thermal fluctuation is unlikely to occur, the magnetization is stable, and noise is reduced, which is suitable for high-density magnetic recording.
The plate ratio (plate diameter / plate thickness) is preferably 1-15. Preferably it is 2-7. When the plate ratio is in the above range, the orientation is sufficient, and noise due to stacking between particles hardly occurs. The specific surface area according to the BET method in this particle size range is 10 to ²⁰⁰ m ² / g. The specific surface area is generally signified as an arithmetic calculation value from the particle plate diameter and plate thickness. The crystallite size is 5 to 45 nm, preferably 10 to 35 nm. The distribution of the particle plate diameter and plate thickness is generally preferably as narrow as possible. Although numerical conversion is difficult, it can be compared by randomly measuring 500 particles from a particle TEM photograph.

  In many cases, the distribution is not a normal distribution, but when calculated and expressed as a standard deviation with respect to the average size, σ / average size = 0.1 to 2.0. In order to sharpen the particle size distribution, the particle generation reaction system is made as uniform as possible, and the generated particles are subjected to a distribution improvement process. For example, a method of selectively dissolving ultrafine particles in an acid solution is also known.

The coercive force Hc measured with a magnetic material can be made up to about 39.8 to 398 kA / m (500 to 5,000 Oe). A higher Hc is advantageous for high-density recording, but is limited by the capacity of the recording head. Usually, it is about 63.7 to 318 kA / m (800 to 4,000 Oe), but preferably 119 kA / m (1,500 Oe) or more and 279 kA / m (3,500 Oe) or less. When the saturation magnetization of the head exceeds 1.4 Tesler, it is preferable to set it to 159 kA / m (2,000 Oe) or more. Hc can be controlled by the particle size (plate diameter / plate thickness), the type and amount of the contained element, the substitution site of the element, the particle generation reaction conditions, and the like. The saturation magnetization σs is 40 to 80 A · m ² / kg (emu / g). σs is preferably higher, but tends to be smaller as the particles become finer. In order to improve σs, it is well known to combine spinel ferrite with magnetoplumbite ferrite and to select the type and amount of elements contained. It is also possible to use W-type hexagonal ferrite.

When the magnetic material is dispersed, the surface of the magnetic material particles is also treated with a material suitable for the dispersion medium and the polymer. As the surface treatment material, an inorganic compound or an organic compound is used. Typical examples of the main compound include oxides or hydroxides such as Si, Al, and P, various silane coupling agents, and various titanium coupling agents. The amount is preferably 0.1 to 10% with respect to the magnetic substance. The pH of the magnetic material is also important for dispersion. Usually, about 4 to 12 has optimum values depending on the dispersion medium and polymer, but about 6 to 10 is selected from the chemical stability and storage stability of the medium. Water contained in the magnetic material also affects the dispersion. Although there is an optimum value depending on the dispersion medium and the polymer, 0.01 to 2.0% is usually selected.
As a manufacturing method of hexagonal ferrite,
(1) Barium oxide, iron oxide, a metal oxide that replaces iron, and boron oxide as a glass-forming substance are mixed so as to have a desired ferrite composition, melted, rapidly cooled to an amorphous body, and then recrystallized. Glass crystallization method to obtain barium ferrite crystal powder by washing and grinding after heat treatment,
(2) Hydrothermal reaction method in which barium ferrite composition metal salt solution is neutralized with alkali, by-products are removed, liquid phase heating is performed at 100 ° C or higher, washing, drying, and grinding to obtain barium ferrite crystal powder ,
(3) There is a coprecipitation method or the like in which a barium ferrite composition metal salt solution is neutralized with an alkali, removed by-products, dried, treated at 1,100 ° C. or less, and pulverized to obtain a barium ferrite crystal powder. However, the manufacturing method of the hexagonal ferrite used in the present invention is not limited.

(Non-magnetic powder)
The magnetic recording medium of the present invention may have a nonmagnetic layer comprising a binder and a nonmagnetic powder between the nonmagnetic support and the magnetic layer.
The nonmagnetic powder that can be used in the nonmagnetic layer may be an inorganic substance or an organic substance. Carbon black or the like can also be used. Examples of the inorganic substance include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, and the like. Specifically, titanium oxide such as titanium dioxide, cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO ₂ , SiO ₂ , Cr ₂ O ₃ , α-alumina, β-alumina having an α conversion of 90 to 100%, γ-alumina, α-iron oxide, goethite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide, copper oxide, MgCO ₃ , CaCO ₃ , BaCO ₃ , SrCO ₃ , BaSO ₄ , silicon carbide Titanium carbide or the like is used alone or in combination of two or more. Preferable are α-iron oxide and titanium oxide.

The nonmagnetic powder may be acicular, spherical, polyhedral, or plate-shaped. The crystallite size of the nonmagnetic powder is preferably 0.004 to 1 μm, and more preferably 0.04 to 0.1 μm. Within this range, it is preferable because good dispersibility can be obtained and a smooth surface can be obtained.
The average particle size of these nonmagnetic powders is preferably 0.005 to 2 μm, more preferably 0.01 to 0.2 μm. If necessary, nonmagnetic powders having different average particle diameters can be combined, or even a single nonmagnetic powder can have the same effect by widening the particle size distribution. Within this range, it is preferable because good dispersibility can be obtained and a smooth surface can be obtained.

The S _BET of the nonmagnetic powder is preferably 1 to 100 m ² / g, more preferably 5 to 70 m ² / g, and still more preferably 10 to 65 m ² / g. A specific surface area within the above range is preferable because it has a suitable surface roughness and can be dispersed in a desired amount of binder.
The DBP oil absorption is preferably 5 to 100 ml / 100 g, more preferably 10 to 80 ml / 100 g, and still more preferably 20 to 60 ml / 100 g.
The specific gravity is preferably 1 to 12, more preferably 3 to 6.
The tap density is 0.05 to 2 g / ml, preferably 0.2 to 1.5 g / ml. When the tap density is in the range of 0.05 to 2 g / ml, it is preferable because the number of scattered particles is small and the operation is easy, and it is difficult to adhere to the apparatus.

The pH of the nonmagnetic powder is preferably 2 to 11, but the pH is particularly preferably 6 to 9. When the pH is less than 2, the friction coefficient at high temperature and high humidity tends to increase. On the other hand, if the pH is higher than 11, the liberated amount of fatty acid tends to decrease and the friction coefficient tends to increase.
The water content of the nonmagnetic powder is preferably 0.1 to 5% by weight, more preferably 0.2 to 3% by weight, and still more preferably 0.3 to 1.5% by weight. A water content in the range of 0.1 to 5% by weight is preferable because the dispersion is good and the viscosity of the paint after dispersion is stable.
The ignition loss is preferably 20% by weight or less, and the ignition loss is preferably small.

When the nonmagnetic powder is an inorganic powder, the Mohs hardness is preferably 4 or more and 10 or less. If the Mohs hardness is less than 4, the durability tends to be unable to be secured.
The stearic acid adsorption amount of the nonmagnetic powder is preferably 1 to 20 μmol / m ² , more preferably ² to 15 μmol / m ² .
The heat of wetting of the nonmagnetic powder into water at 25 ° C. is preferably in the range of ^{20 to} 60 μJ / cm ² (200 to 600 erg / cm ² ). Moreover, the solvent which exists in the range of this heat of wetting can be used. The amount of water molecules on the surface at 100 to 400 ° C. is suitably 1 to 10 / 100Å. The pH of the isoelectric point in water is preferably between 3 and 9.

The surface of these nonmagnetic powders is preferably surface-treated with Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ and ZnO. Particularly preferred for dispersibility are Al ₂ O ₃ , SiO ₂ , TiO ₂ and ZrO ₂ , but more preferred are Al ₂ O ₃ , SiO ₂ and ZrO ₂ . These may be used in combination or may be used alone. Further, a surface-treated layer co-precipitated according to the purpose may be used, or a method of first treating with alumina and then treating the surface layer with silica, or vice versa. The surface treatment layer may be a porous layer depending on the purpose, but it is generally preferable that the surface treatment layer is homogeneous and dense.

  Specific examples of the nonmagnetic powder used in the nonmagnetic layer of the present invention include Showa Denko Co., Ltd. Nanotite, Sumitomo Chemical Co., Ltd. HIT-100, ZA-G1, Toda Kogyo Co., Ltd. DPN -250, DPN-250BX, DPN-245, DPN-270BX, DPB-550BX, DPN-550RX Titanium oxide made by Ishihara Sangyo TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D , SN-100, MJ-7, α-iron oxide E270, E271, E300, STT-4D, STT-30D, STT-30, STT-30, STT-65C, manufactured by Titanium Industry, MT-100S, MT-100T, MT- 150W, MT-500B, MT-600B, MT-100F, MT-500HD. FINEX-25, BF-1, BF-10, BF-20, ST-M, Dowa Mining DEFIC-Y, DEFIC-R, Nippon Aerosil AS2BM, TiO2P25, Ube Industries 100A, 500A, Titanium Industry Y-LOP manufactured and what baked it are mentioned. Particularly preferred nonmagnetic powders are titanium dioxide and α-iron oxide.

By mixing carbon black with nonmagnetic powder in the nonmagnetic layer, the surface electrical resistance (Rs) can be lowered, the light transmittance can be reduced, and a desired micro Vickers hardness can be obtained.
The micro-Vickers hardness of the nonmagnetic layer is usually, 25 to 60 kg / mm ^2, preferably in order to adjust the head contact, it is 30 to 50 kg / mm ^2. The micro Vickers hardness can be measured using a thin film hardness meter (HMA-400 manufactured by NEC) using a diamond triangular pyramid needle having a ridge angle of 80 degrees and a tip radius of 0.1 μm at the tip of the indenter.
It is standardized that the light transmittance is generally 3% or less for absorption of infrared rays having a wavelength of about 900 nm, for example, 0.8% or less for a VHS magnetic tape. For this purpose, rubber furnace, rubber thermal, color black, acetylene black and the like can be used.

The specific surface area of carbon black employed in the nonmagnetic layer of the present invention is preferably ^{100~500m 2 / g, 150~400m 2 /} g is more preferable. The DBP oil absorption of carbon black is preferably 20 to 400 ml / 100 g, more preferably 30 to 200 ml / 100 g. The particle size of carbon black is preferably 5 to 80 nm, more preferably 10 to 50 nm, and still more preferably 10 to 40 nm. Carbon black preferably has a pH of 2 to 10, a water content of 0.1 to 10%, and a tap density of 0.1 to 1 g / ml.
Specific examples of carbon black used in the present invention include Cabot Corporation, BLACKPEARLS 2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72, Mitsubishi Chemical Industries, Ltd., # 3050B, 3150B, 3250B. # 3750B, # 3950B, # 950, # 650B, # 970B, # 850B, MA-600, manufactured by Columbia Carbon Co., CONDUCTEX SC, RAVEN 8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255, 1250, manufactured by Akzo, Ketjen Black EC and the like.

  Carbon black may be surface-treated with a dispersant or the like, or may be used after being grafted with a resin, or may be obtained by graphitizing a part of the surface. Moreover, before adding carbon black to a coating material, you may disperse | distribute with a binder beforehand. These carbon blacks can be used in a range not exceeding 50% by weight and not exceeding 40% of the total mass of the nonmagnetic layer with respect to the inorganic powder. These carbon blacks can be used alone or in combination. The carbon black that can be used in the nonmagnetic layer of the present invention can be referred to, for example, “Carbon Black Handbook” edited by Carbon Black Association.

  In addition, an organic powder can be added to the nonmagnetic layer depending on the purpose. Examples include acrylic styrene resin powder, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment, but polyolefin resin powder, polyester resin powder, polyamide resin powder, polyimide resin powder, and polyfluorinated ethylene resin are also included. Can be used.

(Other additives)
In the magnetic recording medium of the present invention, the magnetic layer or the nonmagnetic layer may contain an additive for imparting a dispersing effect, a lubricating effect, an antistatic effect, a plasticizing effect, and the like. In addition to the hyperbranched polyester, a conventionally known dispersant may be used in combination as the dispersant.
The following examples can be given as these additives.
Molybdenum disulfide, tungsten disulfide, graphite, boron nitride, fluorinated graphite, silicone oil, silicone with polar group, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing ester, polyolefin, polyglycol, polyphenyl ether , Phenylphosphonic acid, benzylphosphonic acid group, phenethylphosphonic acid, α-methylbenzylphosphonic acid, 1-methyl-1-phenethylphosphonic acid, diphenylmethylphosphonic acid, biphenylphosphonic acid, benzylphenylphosphonic acid, α-cumylphosphonic acid, toluyl Phosphonic acid, xylylphosphonic acid, ethylphenylphosphonic acid, cumenylphosphonic acid, propylphenylphosphonic acid, butylphenylphosphonic acid, heptylphenylphosphonic acid, octylph Aromatic ring-containing organic phosphonic acids such as enylphosphonic acid and nonylphenylphosphonic acid and alkali metal salts thereof, octylphosphonic acid, 2-ethylhexylphosphonic acid, isooctylphosphonic acid, (iso) nonylphosphonic acid, (iso) decylphosphonic acid Alkylphosphonic acids such as (iso) undecylphosphonic acid, (iso) dodecylphosphonic acid, (iso) hexadecylphosphonic acid, (iso) octadecylphosphonic acid, (iso) eicosylphosphonic acid, and alkali metal salts thereof.

  Phenyl phosphate, benzyl phosphate, phenethyl phosphate, α-methylbenzyl phosphate, 1-methyl-1-phenethyl phosphate, diphenylmethyl phosphate, biphenyl phosphate, benzyl phenyl phosphate, α-cumyl phosphate, toluyl phosphate, xylyl phosphate, ethyl phenyl phosphate, Aromatic phosphates such as cumenyl phosphate, propylphenyl phosphate, butylphenyl phosphate, heptylphenyl phosphate, octylphenyl phosphate, nonylphenyl phosphate, and alkali metal salts thereof, octyl phosphate, 2-ethylhexyl phosphate, isooctyl phosphate, (iso) nonyl phosphate Alkyl phosphates such as phosphoric acid (iso) decyl, phosphoric acid (iso) undecyl, phosphoric acid (iso) dodecyl, phosphoric acid (iso) hexadecyl, phosphoric acid (iso) octadecyl, phosphoric acid (iso) eicosyl, and alkali metal salts thereof.

Alkyl sulfonic acid esters and alkali metal salts thereof, fluorine-containing alkyl sulfates and alkali metal salts thereof, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, butyl stearate, oleic acid, linoleic acid, linolenic acid, elaidin Monobasic fatty acids which may contain or branch an unsaturated bond having 10 to 24 carbon atoms such as acid, erucic acid and acid, and metal salts thereof, or butyl stearate, octyl stearate, amyl stearate, Contains 10 to 24 carbon unsaturated bonds such as isooctyl stearate, octyl myristate, butyl laurate, butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan distearate, anhydrosorbitan tristearate But It is branched even if it contains monobasic fatty acids that may be branched and 1 to 6-valent alcohols that may be branched or contain unsaturated bonds of 2 to 22 carbon atoms, or those that contain unsaturated bonds of 12 to 22 carbon atoms. Mono-fatty acid ester, di-fatty acid ester or polyfatty acid ester, fatty acid amide having 2 to 22 carbon atoms, fat having 8 to 22 carbon atoms, which may be any one of alkoxy alcohols and monoalkyl ethers of alkylene oxide polymers Group amines can be used. In addition to the above hydrocarbon groups, alkyl groups, aryl groups, and aralkyl groups substituted with nitro groups and groups other than hydrocarbon groups such as halogen-containing hydrocarbons such as F, Cl, Br, CF ₃ , CCl ₃ , and CBr ₃ It may be one with

  In addition, nonionic surfactants such as alkylene oxide, glycerin, glycidol, and alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocycles, phosphonium or sulfoniums, etc. Amphoteric interfaces such as cationic surfactants, anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, and sulfate ester groups, amino acids, aminosulfonic acids, sulfuric or phosphate esters of aminoalcohols, and alkylbetaines An activator or the like can also be used. These surfactants are described in detail in “Surfactant Handbook” (published by Sangyo Tosho Co., Ltd.). These lubricants, antistatic agents, and the like are not necessarily pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposed products, and oxides in addition to the main components. These impurities are preferably 30% by weight or less, more preferably 10% by weight or less.

  Specific examples of these include: Nippon Oil & Fats Corporation: NAA-102, castor oil hardened fatty acid, NAA-42, cationic SA, Naimine L-201, Nonion E-208, Anon BF, Anon LG, Takemoto Yushi Co., Ltd .: FAL- 205, FAL-123, manufactured by Shin Nippon Chemical Co., Ltd .: NJ Lube OL, manufactured by Shin-Etsu Chemical Co., Ltd .: TA-3, manufactured by Lion Armor Co., Ltd .: Armido P, manufactured by Lion Co., Ltd., Duomin TDO, manufactured by Nisshin Oil Industries, Inc .: BA-41G, Sanyo Kasei Co., Ltd .: Profan 2012E, New Pole PE61, Ionette MS-400, and the like.

  These dispersants, lubricants, and surfactants used in the present invention can be used properly depending on the type and amount of the nonmagnetic layer and the magnetic layer. For example, of course, it is not limited to the examples shown here, but the dispersant has the property of adsorbing or binding with polar groups, and in the magnetic layer, the surface of the ferromagnetic fine powder is mainly nonmagnetic. In the layer, it is presumed that the organic phosphorus compound adsorbed or bonded mainly to the surface of the nonmagnetic powder with the polar group and once adsorbed is difficult to desorb from the surface of the metal or metal compound. Therefore, since the surface of the ferromagnetic fine powder or the nonmagnetic powder of the present invention is coated with an alkyl group, an aromatic group or the like, the affinity of the ferromagnetic fine powder or the nonmagnetic powder for the binder resin component. In addition, the dispersion stability of the ferromagnetic fine powder or non-magnetic powder is improved. In addition, since it exists in a free state as a lubricant, the non-magnetic layer and the magnetic layer use fatty acids with different melting points to control the bleeding to the surface, and use esters with different boiling points and polarities to bleed to the surface. It is conceivable that the stability of coating is improved by controlling the amount of the surfactant, and the lubricating effect is improved by increasing the amount of lubricant added in the nonmagnetic layer.

  All or some of the additives used in the present invention may be added in any step during the production of the coating solution for the magnetic layer or nonmagnetic layer. For example, when mixed with the ferromagnetic fine powder before the kneading step, when added during the kneading step with the ferromagnetic fine powder, the binder and the solvent, when added during the dispersing step, when added after dispersing, added immediately before coating There are cases.

  Known organic solvents can be used for the magnetic layer and the nonmagnetic layer of the present invention. The organic solvent is an arbitrary ratio of ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone and tetrahydrofuran, alcohols such as methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol and methylcyclohexanol. , Esters such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycol acetate, glycol ethers such as glycol dimethyl ether, glycol monoethyl ether, dioxane, benzene, toluene, xylene, cresol, chlorobenzene, etc. Aromatic hydrocarbons, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin Chlorinated hydrocarbons such as dichlorobenzene, N, N- dimethylformamide, may be used hexane.

  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 organic solvent used in the present invention is preferably the same in the magnetic layer and the nonmagnetic layer. The amount added may be changed. Use non-magnetic layers with high surface tension (cyclohexanone, dioxane, etc.) to improve coating stability. Specifically, the arithmetic average value of the magnetic layer solvent composition should not be lower than the arithmetic average value of the non-magnetic layer solvent composition. Is essential. In order to improve dispersibility, it is preferable that the polarity is somewhat strong, and it is preferable that 50% or more of a solvent having a dielectric constant of 15 or more is included in the solvent composition. Moreover, it is preferable that a solubility parameter is 8-11.

(Support)
In the magnetic recording medium of the present invention, a nonmagnetic layer or a magnetic layer is formed by applying a coating solution prepared from the above materials onto a support.
As the support that can be used in the present invention, known ones such as biaxially stretched polyethylene naphthalate, polyethylene terephthalate, polyamide, polyimide, polyamideimide, aromatic polyamide, and polybenzoxidazole can be used. Polyethylene naphthalate and aromatic polyamide are preferred. These supports may be subjected in advance to corona discharge, plasma treatment, easy adhesion treatment, heat treatment and the like.

In addition, the support that can be used in the present invention is a surface having excellent smoothness with a centerline average surface roughness of 0.1 to 20 nm, preferably 1 to 10 nm in a cutoff value of 0.25 mm. Is preferred. These non-magnetic supports preferably have not only a small center line average surface roughness but also no coarse protrusions of 1 μm or more.
The arithmetic average roughness of the obtained support is preferably 0.1 μm or less in terms of (Ra) [JIS B0660-1998, ISO 4287-1997] because noise of the obtained magnetic recording medium is reduced. .
The preferred thickness of the nonmagnetic support in the magnetic recording medium of the present invention is 3 to 80 μm.

(Back coat layer)
A back coat layer (backing layer) may be provided on the surface of the nonmagnetic support used in the present invention on which the magnetic paint is not applied. The back coat layer is provided by applying a back coat layer forming paint in which particulate components such as abrasives and antistatic agents and a binder are dispersed in an organic solvent on the surface of the nonmagnetic support on which the magnetic paint is not applied. Layer. Various inorganic pigments and carbon black can be used as the particulate component, and as the binder, resins such as nitrocellulose, phenoxy resin, and polyurethane can be used alone or as a mixture thereof. An adhesive layer may be provided on the application surface of the magnetic coating material and the backcoat layer forming coating material of the nonmagnetic support of the present invention.

(Undercoat layer)
In the magnetic recording medium of the present invention, an undercoat layer may be provided. By providing the undercoat layer, the adhesive force between the support and the magnetic layer or the nonmagnetic layer can be improved. As the undercoat layer, a polyester resin soluble in a solvent is used. The undercoat layer has a thickness of 0.5 μm or less.

(Smoothing layer)
The magnetic recording medium of the present invention may be provided with a smoothing layer. The smoothing layer is a layer for embedding protrusions on the surface of the nonmagnetic support. In the case of a magnetic recording medium in which a magnetic layer is provided on the nonmagnetic support, the nonmagnetic support is provided between the nonmagnetic support and the magnetic layer. In the case of a magnetic recording medium in which a nonmagnetic layer and a magnetic layer are provided in this order on a support, it is provided between the nonmagnetic support and the nonmagnetic layer.
The smoothing layer can be formed by curing a radiation curable compound by radiation irradiation. The radiation curable compound refers to a compound having a property of starting to polymerize or crosslink when irradiated with radiation such as ultraviolet rays or an electron beam, to be polymerized and cured.

(Production method)
The method for producing a magnetic recording medium of the present invention comprises a dispersion preparation step (hereinafter also referred to as “pre-dispersion”) in which a ferromagnetic powder is mechanically mixed with a dispersant to prepare a uniform dispersion, and the resulting dispersion is obtained. A magnetic liquid preparation step in which a binder is added to the product and uniformly dispersed to prepare a magnetic liquid, and a coating step in which the obtained magnetic liquid is applied onto a support. Each process may be divided into two or more stages.

  In the dispersion preparation step, the ferromagnetic powder is mechanically mixed with the hyperbranched polyester as a dispersant to prepare a uniform dispersion. In order to prepare the dispersion, known ones such as a media type disperser and an ultrasonic disperser can be used. As the media type disperser, either a batch type or a continuous type can be used, and it is preferable to use micro beads as the dispersion medium. The dispersion medium used in the disperser preferably has a high specific gravity, and glass beads, zircon beads, zirconia beads, titania beads, and steel beads are suitable. The combination of factors such as the type of dispersion medium, particle size and filling rate, and peripheral speed of the disk is optimized. The particle size of the dispersion medium is typically 0.1 to 3 mm. As the media type disperser, a known one such as a sand mill can be used.

After the dispersion preparation step, a kneading step of kneading all or part of the magnetic layer binder and a dispersion obtained by mixing the ferromagnetic powder and the dispersant may be taken, but in the production method of the present invention, It is preferable not to have a kneading step.
For kneading, it is preferable to use a conventionally known apparatus having a strong kneading force, such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. When these kneaders are used, it is preferable to knead all or part of the ferromagnetic fine powder and the binder (however, 30% or more of the total binder is preferable). The kneading ratio of the ferromagnetic fine powder and the binder is preferably 10 to 500 parts by weight of the binder with respect to 100 parts by weight of the ferromagnetic fine powder. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274.

Subsequently, a binder is added to the obtained dispersion, and a magnetic liquid preparation step is performed in which a magnetic liquid is prepared by uniform dispersion. In the magnetic liquid preparation process, similarly to the dispersion preparation process, conventionally known media-type dispersers, ultrasonic dispersers, etc. are used, and the mixture of the ferromagnetic fine powder and the dispersant obtained by the dispersion preparation process. A binder and a coating solvent are added to the mixture, and the fine ferromagnetic powder is completely dispersed in the binder solution using a sand mill or the like.
Further, glass beads can be used to disperse the coating solution for the magnetic layer or the nonmagnetic layer solution. Such glass beads are preferably zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media. The particle diameter and filling rate of these dispersion media are optimized. As the disperser, a known one such as a sand mill can be used.

  All raw materials such as ferromagnetic fine powder (ferromagnetic hexagonal ferrite powder), non-magnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant, solvent used in the present invention are from the beginning of any process. Alternatively, it may be added in the middle. In addition, individual raw materials may be added in two or more steps.

In the present invention, as the coating step for coating the magnetic coating material on the nonmagnetic support, for example, a magnetic coating solution is applied to the surface of the nonmagnetic support under running so as to have a predetermined film thickness. Here, a plurality of magnetic layer coating solutions may be applied sequentially or simultaneously, or a nonmagnetic layer coating solution and a magnetic layer coating solution may be applied sequentially or simultaneously. As the coating machine for applying the above magnetic coating solution or lower layer coating solution, air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure coat, kiss coat Cast coat, spray coat, spin coat, etc. can be used.
For example, “Latest coating technology” (May 31, 1983) issued by the General Technology Center Co., Ltd. can be referred to. When applied to the magnetic recording medium of the present invention, the following can be proposed as an example of a coating apparatus and method.

(1) A lower layer is first applied by a coating apparatus such as a gravure, a roll, a blade, or an extrusion that is generally applied in the application of a magnetic layer coating solution. The upper layer is applied by a support pressure type extrusion coating apparatus as disclosed in Japanese Patent No. 46186, Japanese Patent Laid-Open No. 60-238179, Japanese Patent Laid-Open No. 2-265672, and the like.
(2) Upper and lower layers by one coating head having two coating liquid passage slits as disclosed in JP-A-63-88080, JP-A-2-17971, and JP-A-2-265672. Are applied almost simultaneously.
(3) The upper and lower layers are applied almost simultaneously by an extrusion coating apparatus with a backup roll as disclosed in JP-A-2-174965.

  The thickness of the magnetic layer of the magnetic recording medium of the present invention is optimized depending on the saturation magnetization amount of the head used, the head gap length, and the band of the recording signal, but is generally 0.01 to 0.10 μm, preferably Is 0.02-0.08 μm, more preferably 0.03-0.08 μm. The magnetic layer may be separated into two or more layers having different magnetic characteristics, and a configuration related to a known multilayer magnetic layer can be applied.

In the present invention, when the nonmagnetic layer is provided, the thickness is preferably 0.2 to 3.0 μm, more preferably 0.3 to 2.5 μm, and more preferably 0.4 to 2.0 μm. Further preferred. The non-magnetic layer of the magnetic recording medium of the present invention exhibits its effect if it is substantially non-magnetic. For example, even if it contains a small amount of magnetic material as an impurity or intentionally, This shows the effect of the invention and can be regarded as substantially the same configuration as the magnetic recording medium of the invention. Note that “substantially the same” means that the residual magnetic flux density of the nonmagnetic layer is 10 mT (100 G) or less or the coercive force is 7.96 kA / m (100 Oe) or less. It means not having.
The polyurethane resin (A) can be used as all or part of the binder of the nonmagnetic layer. It is preferable to use as all of the binder of the nonmagnetic layer.

  In the present invention, in order to stably coat the magnetic layer, it is desirable to interpose a lower layer containing an inorganic powder on the support, and to coat the magnetic layer thereon by wet-on-wet.

  In the case of a magnetic tape, the magnetic layer coating liquid coating layer is obtained by subjecting the ferromagnetic fine powder contained in the magnetic layer coating liquid coating layer to a magnetic field orientation treatment in the longitudinal direction using a cobalt magnet or a solenoid. In the case of a disk, a sufficiently isotropic orientation may be obtained even without non-orientation without using an orientation device, but known random methods such as alternately arranging cobalt magnets obliquely and applying an alternating magnetic field with a solenoid. It is preferable to use an alignment device. In the case of a ferromagnetic metal powder, the isotropic orientation is generally preferably in-plane two-dimensional random, but can also be three-dimensional random with a vertical component. In the case of the ferromagnetic hexagonal ferrite, in general, it tends to be in-plane and vertical three-dimensional random, but in-plane two-dimensional random is also possible. Further, isotropic magnetic characteristics can be imparted in the circumferential direction by using a well-known method such as a counter-polarized magnet and making it vertically oriented. In particular, when performing high density recording, vertical alignment is preferable. Further, circumferential orientation may be performed using spin coating.

  It is preferable that the drying position of the coating film can be controlled by controlling the temperature, air volume, and coating speed of the drying air, the coating speed is preferably 20 to 1,000 m / min, and the temperature of the drying air is preferably 60 ° C. or higher. Also, moderate pre-drying can be performed before entering the magnet zone.

After being dried, the coating layer is subjected to a surface smoothing treatment. For the surface smoothing process, for example, a super calendar roll or the like is used. By performing the surface smoothing treatment, voids generated by removing the solvent during drying disappear and the filling rate of the ferromagnetic fine powder in the magnetic layer is improved, so that a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. Can do.
As the calendering roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, polyamideimide or the like is used. Moreover, it can also process with a metal roll.
The magnetic recording medium of the present invention has a surface having extremely excellent smoothness with a center line average roughness of 0.1 to 4 nm, preferably 1 to 3 nm, at a cutoff value of 0.25 mm. preferable. As the method, for example, as described above, a magnetic layer formed by selecting a specific ferromagnetic fine powder and a binder is subjected to the calendering process.

  As the calendering conditions, the temperature of the calender roll is in the range of 60 to 100 ° C, preferably 70 to 100 ° C, more preferably 80 to 100 ° C. The pressure of the calendar roll is 100 to 500 kg / cm, preferably 200 to 450 kg / cm, more preferably 300 to 400 kg / cm. The obtained magnetic recording medium can be cut into a desired size using a cutting machine or the like.

EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to an Example.
In addition, the display of “parts” in the examples indicates “parts by weight”.

(Synthesis example of hyperbranched polyester)
In a reaction vessel equipped with a thermometer, a stirrer, and a partial reflux condenser, 0.5 mol (136 g) of pentaerythritol (molecular weight 136), 2 mol (296 g) of dimethylolbutanoic acid (molecular weight 148) and p-toluenesulfonic acid 0 .5 mmol (87 mg) was charged, and the temperature was raised to 130 ° C., and the temperature was raised while reducing the pressure to 140 ° C. over 1 hour. Thereafter, 12 mol (1776 g) of dimethylolbutanoic acid was added to the reaction mixture and reacted at 140 ° C. under reduced pressure for 5 hours while stirring.
The obtained hyperbranched polyester had an OH value of 4.1 meq / g, a GPC molecular weight (in terms of polystyrene) of 3,900 in number average, and a weight average molecular weight of 12,000.

[Preparation of magnetic liquid for magnetic layer]
The following components were dispersed by a sand mill using Zr beads having a particle diameter of 0.5 mm (pre-dispersion).
Barium ferrite powder (plate diameter: 25 nm, σs: 50 emu / g) 100 parts by weight Hyperbranched polyester (cyclohexanone 10% solution) 60 parts by weight With respect to the obtained dispersion,
Sulfonic acid group-containing polyurethane resin (cyclohexanone 30% solution) 50 parts by weight Methyl ethyl ketone 60 parts by weight Cyclohexanone 60 parts by weight Diamond powder MD100 (Tomei Dia Co., Ltd.) 20 parts by weight Carbon black # 50 (Asahi Carbon) 1 part by weight The Zr beads having a particle diameter of 0.5 mm were added and dispersed by a sand mill.
to this,
Polyisocyanate (Coronate 3041 manufactured by Nippon Polyurethane Industry Co., Ltd.) 3 parts by weight Methyl ethyl ketone 100 parts by weight Cyclohexanone 100 parts by weight Stearic acid 1 part by weight Butyl stearate 1 part by weight After stirring for 30 minutes, an average pore size of 1 μm A magnetic paint was prepared by filtration using a filter having

(Preparation of nonmagnetic liquid for nonmagnetic layer)
The nonmagnetic liquid for the nonmagnetic layer was prepared as follows.
Acicular iron-iron oxide (major axis length 150 nm) 80 parts by weight Carbon black (average particle diameter 20 nm) 20 parts by weight was ground in an open kneader for 10 minutes,
Polyvinyl chloride copolymer MR110 (manufactured by Nippon Zeon Co., Ltd.) 60 parts by weight of cyclohexanone 30 parts polyurethane resin MEK (methyl ethyl ketone) / cyclohexanone = 1/1 30% solution 60 parts by weight Addition of 20 parts by weight of cyclohexanone 60 parts Knead for a minute, then
200 parts by weight of cyclohexanone was added and dispersed with a sand mill using Zr beads having a particle diameter of 1 mm. to this,
Polyisocyanate (Coronate 3041 manufactured by Nippon Polyurethane Industry Co., Ltd.) 15 parts by weight Stearic acid 1 part by weight Butyl stearate 1 part by weight Cyclohexanone 50 parts by weight, and after stirring for 30 minutes, a filter having an average pore size of 1 μm And filtered to prepare a non-magnetic paint.

(Preparation of back coat layer paint)
The coating material for the back coat layer was prepared as follows.
First,
Nonmagnetic inorganic powder: α-iron oxide (average major axis length: 0.15 μm) 80 parts by weight Carbon black (average particle diameter: 20 nm) 20 parts by weight Phenylphosphonic acid 3 parts by weight Carbon black (average particle diameter: 100 nm) 3 Grind parts by weight with an open kneader for 10 minutes,
Vinyl chloride copolymer MR110 (manufactured by Nippon Zeon Co., Ltd.) 13 parts by weight Sulfonic acid group-containing polyurethane resin 6 parts by weight Cyclohexanone 20 parts by weight and kneaded for 60 minutes, then cyclohexanone 140 parts by weight Methyl ethyl ketone 170 parts by weight Dispersion was performed with a sand mill using Zr beads having a particle diameter of 1 mm. To this was added 15 parts by weight of polyisocyanate (Coronate 3041 manufactured by Nippon Polyurethane Industry Co., Ltd.), 100 parts by weight of cyclohexanone, and the mixture was further stirred and mixed for 30 minutes, followed by filtration using a filter having an average pore size of 1 μm for back coat layer use. A paint was prepared.

  The obtained nonmagnetic coating material was applied to one side of a 6.0 μm thick polyethylene naphthalate support so that the thickness after drying was 1.0 μm, dried, and then coated on the nonmagnetic layer. The magnetic paint was applied so that the thickness after drying was 0.1 μm, and magnetic field orientation was performed with a 6000 gauss Co magnet and a 4,000 gauss solenoid magnet with the magnetic paint undried. Furthermore, the coating material for the back coat layer was applied to the other surface of the support so that the thickness after drying was 0.5 μm.

  The coated material was calendered by a combination of metal roll-metal roll-metal roll-metal roll-metal roll-metal roll-metal roll at a speed of 80 m / min, a linear pressure of 300 kg / cm, and a temperature of 80 ° C. The heat treatment was performed at 48 ° C. for 48 hours, and slit into a 1/2 inch width. Thereafter, the surface of the magnetic layer was cleaned with a tape cleaning device attached to the apparatus having a slitting device and a winding device so that the nonwoven fabric and the razor blade were pressed against the magnetic surface, and a tape sample was obtained.

(Example 2)
The following components were dispersed by a sand mill using Zr beads having a particle diameter of 0.5 mm (pre-dispersion).
Barium ferrite powder (plate diameter: 25 nm, σs: 50 emu / g) 100 parts by weight Hyperbranched polyester (cyclohexanone 10% solution) 60 parts by weight With respect to the obtained dispersion,
50 parts by weight of a sulfonic acid group-containing polyurethane resin (cyclohexanone 30% solution) was added and kneaded for 60 minutes using an open kneader. afterwards,
Diamond powder MD100 (manufactured by Tomei Dia Co., Ltd.) 20 parts by weight Carbon black # 50 (manufactured by Asahi Carbon) 1 part by weight Methyl ethyl ketone 60 parts by weight Cyclohexanone 60 parts by weight Sand mill using Zr beads having a particle diameter of 0.5 mm Dispersed.
to this,
Polyisocyanate (Coronate 3041 manufactured by Nippon Polyurethane Industry Co., Ltd.) 3 parts by weight Methyl ethyl ketone 100 parts by weight Cyclohexanone 100 parts by weight Stearic acid 1 part by weight Butyl stearate 1 part by weight After stirring for 30 minutes, an average pore size of 1 μm A magnetic paint was prepared by filtration using a filter having Subsequent steps were performed in the same manner as in Example 1 to obtain a tape sample.

(Example 3)
The following components were kneaded for 60 minutes using an open kneader.
Barium ferrite powder (plate diameter 25 nm, σs 50 emu / g) 100 parts by weight Hyperbranched polyester (cyclohexanone 10% solution) 60 parts by weight Sulfonic acid group-containing polyurethane resin (cyclohexanone 30% solution) 50 parts by weight
Diamond powder MD100 (manufactured by Tomei Dia Co., Ltd.) 20 parts by weight Carbon black # 50 (manufactured by Asahi Carbon) 1 part by weight Methyl ethyl ketone 60 parts by weight Cyclohexanone 60 parts by weight Sand mill using Zr beads having a particle diameter of 0.5 mm Dispersed. to this,
Polyisocyanate (Coronate 3041 manufactured by Nippon Polyurethane Industry Co., Ltd.) 3 parts by weight Methyl ethyl ketone 100 parts by weight Cyclohexanone 100 parts by weight Stearic acid 1 part by weight Butyl stearate 1 part by weight After stirring for 30 minutes, an average pore size of 1 μm A magnetic paint was prepared by filtration using a filter having Subsequent steps were performed in the same manner as in Example 1 to obtain a tape sample.

(Comparative Example 1)
As a dispersant, a tape sample was obtained by the same production method as in Example 1 except that 5 parts by weight of phenylphosphonic acid was added instead of the hyperbranched polyester in Example 1.

(Comparative Example 2)
A tape sample was obtained by the same production method as in Example 1 except that 5 parts by weight of citric acid was added as a dispersant instead of the hyperbranched polyester in Example 1.

(Comparative Example 3)
A tape sample was obtained in the same manner as in Example 1 except that 100 parts by weight of acicular metal powder (major axis length 45 nm, σs: 110 emu / g) was added as magnetic powder instead of barium ferrite powder.

(Evaluation methods)
(1) Measurement of surface roughness of magnetic layer Using a surface roughness meter TOPO3D manufactured by WYKO, centerline average roughness Ra of an area of about 250 μm × 250 μm was measured.

(2) Electromagnetic conversion characteristic measurement A 1/2 inch tape was run at a speed of 3 m / sec with a linear tester and pressed against the head for recording and reproduction. Recording was performed using a MIG head (g) with a saturation magnetization of 1.4T = 0.2 μm and a track width of 14 μm, and the recording current was set to the optimum recording current for each tape. An anisotropic MR head having an element thickness of 25 nm and a shield interval of 0.2 μm (track width of 7 μm) was used as the reproducing head. S / N measurement with a recording wavelength of 0.3 μm was performed in the above evaluation system. Comparative Example 3 is 0 dB.

  Table 1 shows methods for preparing the magnetic powder, the dispersant, and the magnetic paint used in Examples 1 to 3 and Comparative Examples 1 to 3. Table 1 shows the evaluation results of the produced magnetic tape.

  By not performing the kneading step after the dispersion preparation step (pre-dispersion), it was found that the surface smoothness and the S / N ratio were improved compared to the case where the conventional kneading step was performed (Example 2). . This is because the use of hyperbranched polyester lowers the viscosity of the liquid, so in a kneading method such as an open kneader, sufficient shear force is not applied during kneading, and the magnetic powder reaggregates during kneading. It seems to be because.

  Furthermore, it was found that the surface properties and S / N ratio were inferior to those of the examples when the pre-dispersion treatment was not performed and a dispersant and a binder were added in the conventional kneading process (Example 3). This is presumably because the agglomerated state is difficult to disintegrate even when kneaded and dispersed because the dispersant and binder are added in a state where the magnetic powder is not sufficiently disintegrated.

Claims (2)

A magnetic recording medium having at least one magnetic layer in which a ferromagnetic powder is dispersed in a binder on a support,
The binder comprises hyperbranched polyester;
A magnetic recording medium, wherein the ferromagnetic powder is a hexagonal ferrite powder. A dispersion preparation step of mechanically mixing the ferromagnetic powder together with the dispersant to prepare a uniform dispersion;
A method for producing a magnetic recording medium comprising a magnetic liquid preparation step of preparing a magnetic liquid by adding a binder to the obtained dispersion and uniformly dispersing it, and a coating step of applying the obtained magnetic liquid on a support. And
The dispersant is a hyperbranched polyester;
A method for producing a magnetic recording medium, wherein the ferromagnetic powder is a hexagonal ferrite powder.