JP2006107623A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium having excellent dispersibility of ferromagnetic powder and non-magnetic powder, excellent electromagnetic conversion characteristics and durability. <P>SOLUTION: In the magnetic recording medium wherein at least one magnetic layer formed by dispersing ferromagnetic powder in a binder is provided on a substrate, the binder formed by curing a hyper branch polyester in which a radiation curing functional group is introduced by irradiation with a radiation is contained in the magnetic layer. A non-magnetic layer formed by dispersing non-magnetic powder in the binder may be provided between the substrate and the magnetic layer. <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 ferromagnetic fine powder.

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. In order to highly disperse the non-magnetic powder of the fine particles used in the non-magnetic layer and the fine particle magnetic material when the non-magnetic lower layer is provided on the surface of the support and the magnetic layer is provided as the upper layer, A magnetic recording medium containing a polyvalent carboxylic acid having a molecular weight of 300 or less or an anhydride thereof has been proposed (see Patent Document 1), but the scratch resistance of the magnetic layer was insufficient. Also, a magnetic recording medium containing an organic phosphorus compound in the magnetic layer has been proposed (see Patent Document 2), but the curability was insufficient. Furthermore, although a magnetic recording medium using a binder obtained by electron beam irradiation of a resin composition containing a compound containing an acryloyl group or methacryloyl group and a thermoplastic resin has been proposed (see Patent Document 3), dispersibility and durability are also proposed. Was insufficient.

Japanese Patent Laid-Open No. 9-212847 JP 2001-338414 A Japanese Patent Publication No. 4-058650

  The problem to be solved by the present invention is to provide a magnetic recording medium having excellent dispersibility, electromagnetic conversion characteristics and durability of ferromagnetic powder and non-magnetic powder.

The above-mentioned problems to be solved by the present invention have been solved by the following (1) and (2).
(1) A magnetic recording medium provided with at least one magnetic layer in which a ferromagnetic powder is dispersed in a binder on a support, wherein the hyperbranched polyester having a radiation-curable functional group introduced therein is irradiated with radiation. A magnetic recording medium comprising a binder cured by irradiation;
(2) A magnetic recording medium in which at least one magnetic layer in which ferromagnetic powder is dispersed in a binder is provided on a support via a nonmagnetic layer in which nonmagnetic powder is dispersed in a binder. And / or a magnetic recording medium comprising a binder obtained by curing a hyperbranched polyester having a radiation curable functional group introduced therein by irradiation with radiation.

  According to the present invention, a magnetic recording medium having improved magnetic layer smoothness and improved electromagnetic conversion characteristics could be obtained. In addition, a magnetic recording medium having both high smoothness and durability and improved durability at the tape edge portion could be obtained.

The magnetic recording medium of the present invention is characterized in that it contains a binder obtained by curing a hyperbranched polyester having a radiation curable functional group introduced therein by irradiation.
The magnetic recording medium of the present invention achieves both extremely high electromagnetic conversion characteristics and storability as compared with the prior art.
The hyperbranched polyester introduced with radiation curable functional groups used in the present invention is adsorbed on the surface of extremely fine magnetic materials and non-magnetic inorganic powders, exhibits extremely high dispersibility, and keeps the coating viscosity low. Therefore, a very smooth coating film can be obtained. For this reason, in addition to being applied to the magnetic layer, in the case of a magnetic recording medium in which a magnetic layer is provided on a support via a nonmagnetic lower layer, even if applied to the nonmagnetic lower layer, extremely smooth and advanced electromagnetic conversion characteristics Can be achieved.

  The hyperbranched polyester introduced with radiation-curable functional groups used in the present invention is a polyester having a highly branched dendritic structure, and unlike a linear polyester, the molecular size in the solution is small and OH groups are present at many molecular ends. Have. Adsorbs by interaction with magnetic material or non-magnetic powder surface and exhibits dispersibility. The hyperbranched polyester introduced with the radiation curable functional group used in the present invention has a radiation curable functional group introduced therein. Therefore, a crosslinking reaction is caused by irradiation with ultraviolet rays or electron beams, and a coating film having an extremely high crosslinking density can be formed, and a magnetic recording medium having excellent durability can be obtained.

  Conventional radiation curable compounds such as polyvalent acrylates are known to be those obtained by converting acryloyl to low molecular weight polyhydric alcohols or compounds with acryloyl groups added to linear polymers. There is a drawback that the curing shrinkage becomes large and brittle when trying to raise. On the other hand, when the crosslink density is lowered, it is difficult to increase the elastic modulus, and it is difficult to achieve both the problem of curing shrinkage and the coating strength such as the elastic modulus. The dendritic polymer introduced with the radiation-curable functional group used in the present invention has a high branch density, but if the radical-polymerizable functional group has a relatively small amount around the molecule, it can form a dense crosslink and suppress cure shrinkage. The magnetic layer and non-magnetic layer filled with magnetic material at high density can suppress brittleness and achieve high elastic modulus and coating strength. Durability at the center of the magnetic layer surface and cracks at the tape edge of the magnetic layer It was found that the destruction can be suppressed at the same time.

(1) Hyperbranched polyester into which radiation-curable functional groups are introduced Hyperbranched polyester is a multibranched 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.

式中、ｎは整数を表し、好ましくは１〜５である。２つのｎは同一分子内で同じでも異なっていてもよい。

In the formula, n represents an integer, preferably 1-5. Two n's may be the same or different in the same molecule.

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 co-condensation with 10 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 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.

  As a method for introducing a radiation curable functional group into the hyperbranched polyester, for example, a modification reaction can be performed using a compound having both a functional group that reacts with the OH group at the end of the hyperbranched polyester and a radiation curable functional group. .

The radiation curable functional group is preferably a radical polymerizable functional group having an ethylenically unsaturated double bond such as acryloyl group or methacryloyl group. In addition, a cyclic ether functional group that undergoes ring-opening polymerization with an acid generating catalyst such as an epoxy group or an oxetane group may be used.
Examples of the functional group that reacts with the OH group of the hyperbranched polyester include carboxylic acid, carboxylic acid anhydride, carboxylic acid chloride, carboxylic acid alkyl ester, and isocyanate group.

Specific compounds include acrylic acid, acrylic acid chloride, acrylic acid anhydride, methyl acrylate, methacrylic acid, methacrylic acid chloride, methacrylic acid anhydride, methyl methacrylate, acryloyloxyethyl isocyanate, methacryloyloxyethyl isocyanate. Acryloyloxypropyl isocyanate, methacryloyloxypropyl isocyanate, and the like.
In addition, 1 mol / 1 mol addition reaction product of acrylate or methacrylate having an OH group such as hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate and a diisocyanate compound may be used. Examples of the diisocyanate compound include hexamethylene diisocyanate, diphenylmethane diisocyanate, toluene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, p-phenylene diisocyanate, and hydrogenated hexamethylene diisocyanate.

  The amount of these radiation-curable functional groups introduced into the hyperbranched polyester is preferably 2 to 40, more preferably 5 to 20, per molecule. If it is in the above range, crosslinking and curing are sufficient, and durability is improved. In addition, since the magnetic layer and the nonmagnetic layer do not become brittle, cracks are unlikely to occur at the tape edge portion, and it is difficult for powder to fall off, dropout, and signal errors.

  The preferred molecular weight of the hyperbranched polyester introduced with the radiation-curable functional group used in the present invention is preferably 500 to 20,000, more preferably 800 to 10,000 in terms of number average molecular weight.

  The degree of branching of the hyperbranched polyester into which the radiation curable functional group has been introduced 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.

When the hyperbranched polyester having the radiation curable functional group introduced therein is added to the magnetic layer, it is preferably added in an amount of 0.1 to 15 parts by weight, more preferably 1 to 10 parts by weight with respect to 100 parts by weight of the magnetic material. .
When the hyperbranched polyester introduced with the radiation curable functional group is added to the nonmagnetic layer, it is preferably added in an amount of 0.1 to 15 parts by weight, more preferably 1 to 10 parts by weight based on 100 parts by weight of the nonmagnetic powder. Parts by weight.

(2) Radiation-curing compounds that can be used in combination The well-known (meta) described in “Applied technology of low-energy electron beam irradiation (issued by CMC)” “UV / EB curing technology (issued by General Technology Center Co., Ltd.)” An acrylate compound may be used in combination.

(3) Irradiation In the magnetic recording medium of the present invention, the binder is cured by irradiation.
As the radiation used in the present invention, an electron beam or ultraviolet rays can be used. When ultraviolet rays are used, it is necessary to add a photopolymerization initiator to the above compound. In the case of electron beam curing, a polymerization initiator is unnecessary, and the penetration depth is also deep, which is preferable.

  As the electron beam accelerator, a scanning system, a double scanning system, or a curtain beam system can be adopted, but a curtain beam system that can obtain a large output at a relatively low cost is preferable. As the electron beam characteristics, the acceleration voltage is 30 to 1000 kV, preferably 50 to 300 kV, and the absorbed dose is 0.5 to 20 Mrad, preferably 2 to 10 Mrad. When the applied voltage is in the above range, the amount of transmitted energy is moderate and efficient, which is economical.

It is preferable that the atmosphere in which the electron beam is irradiated has an oxygen concentration of 200 ppm or less by nitrogen purge. When the oxygen concentration is within the above range, crosslinking and curing reaction in the vicinity of the surface is sufficiently performed.
A mercury lamp is used as the ultraviolet light source. The mercury lamp is a 20 to 240 W / cm lamp and is used at a speed of 0.3 m / min to 20 m / min. In general, the distance between the substrate and the mercury lamp is preferably 1 to 30 cm.

  A radical photopolymerization initiator is used as a photopolymerization initiator used for ultraviolet curing. For details, for example, those described in “New Polymer Experimental Science Vol. 2 Chapter 6 Light / Radiation Polymerization” (published by Kyoritsu Shuppan 1995, edited by Polymer Society) can be used. Specific examples include acetophenone, benzophenone, anthraquinone, benzoin ethyl ether, benzyl methyl ketal, benzyl ethyl ketal, benzoin isobutyl ketone, hydroxydimethyl phenyl ketone, 1-hydroxycyclohexyl phenyl ketone, 2-2 diethoxyacetophenone, and so on. The mixing ratio of the aromatic ketone is preferably 0.5 to 20 parts by weight, more preferably 2 to 15 parts by weight, and further preferably 3 to 10 parts by weight with respect to 100 parts by weight of the radiation curable compound.

  Radiation curing equipment and conditions are described in “UV / EB Curing Technology” (issued by General Technology Center Co., Ltd.) and “Applied Technology for Low Energy Electron Beam Irradiation” (2000, issued by CMC Corporation). The well-known thing can be used.

  The radiation irradiation is preferably performed after the nonmagnetic layer and the magnetic layer are applied, dried and calendered. The magnetic layer before irradiation is soft and easily smoothed by calendaring. Winding up after calendar treatment and storing for a long time in a state where radiation has not been irradiated may cause unevenness of the back surface to be transferred and the surface to become rough. Therefore, it is preferable to cure the magnetic layer by irradiating radiation as soon as possible after the calendar treatment. It is more preferable to perform the calendar process and the radiation irradiation process consistently.

(4) Binder Next, the binder that can be used in combination with the magnetic layer and the nonmagnetic layer will be described.
For both the magnetic layer and the nonmagnetic layer, the binder described in JP-A-10-222838 can be used in addition to the hyperbranched polyester into which the radiation-curable functional group is introduced.
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, polyester urethane, polyether urethane, polycarbonate urethane, polyether ester urethane, acrylic polyurethane, or 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 not particularly limited, a polyurethane-based binder is preferable because of its high affinity with the hyperbranched polyester having a radiation-curable functional group introduced therein, which is used in the present invention.

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.

  The amount of the binder in the magnetic layer is preferably 0 to 25 parts by weight with respect to 100 parts by weight of the ferromagnetic fine powder. The amount of binder in the nonmagnetic layer is preferably 0 to 20 parts by weight with respect to 100 parts by weight of the nonmagnetic powder.

(5) Magnetic material In the magnetic recording medium of the present invention, the following acicular ferromagnetic material or flat magnetic material can be used as the magnetic material.
(Ferromagnetic iron oxide or ferromagnetic metal powder)
One shape of the ferromagnetic powder used in the magnetic recording medium of the present invention is needle-shaped, and examples thereof include cobalt-containing ferromagnetic iron oxide or ferromagnetic alloy powder.
The specific surface area by the BET method (hereinafter, “S _BET ” means the specific surface area by the BET method) is preferably 40 to 80 m ² / g, more preferably 50 to 70 m ² / g.
The major axis length is preferably 20 nm to 200 nm, more preferably 25 to 60 nm.
The long axis length is obtained by taking a transmission electron microscope photograph, directly reading the short axis length and the long axis length of the ferromagnetic metal powder from the photograph, and a transmission electron microscope using an image analysis apparatus Carl Zeiss IBASSI. It is required to use the method of photo tracing and reading.

The crystallite size is preferably 5 to 25 nm, more preferably 8 to 20 nm, and particularly preferably 10 to 15 nm.
As the crystallite size, an X-ray diffractometer (RINT2000 series manufactured by Rigaku Corporation) was used, and an average value obtained by the Scherrer method from the half-value width of the diffraction peak under the conditions of the radiation source CuKα1, the tube voltage 50 kV, and the tube current 300 mA was used. .

Examples of the ferromagnetic powder include Fe, Fe—Co, Fe—Ni, and Co—Ni—Fe containing yttrium. The yttrium content in the ferromagnetic powder is such that the ratio of yttrium atoms to iron atoms, Y / Fe is preferably 0.5 atom% to 20 atom%, more preferably 5 to 10 atom%. . When the yttrium content is in the above range, the ferromagnetic powder can be increased in σS. Moreover, since the iron content is appropriate, the magnetic properties are good, and the electromagnetic conversion properties are good.
Furthermore, within a range of 20 atomic% or less with respect to 100 atomic% of iron, aluminum, silicon, sulfur, scandium, titanium, vanadium, chromium, manganese, copper, zinc, molybdenum, rhodium, palladium, tin, antimony, boron, Barium, tantalum, tungsten, rhenium, gold, lead, phosphorus, lanthanum, cerium, praseodymium, neodymium, tellurium, bismuth, and the like can be included. Further, the ferromagnetic metal powder may contain a small amount of water, hydroxide or oxide.

  In addition, the shape may be any of acicular, granular, rice granular, or plate-like shapes as long as the above-described characteristics regarding the particle size are satisfied, but it is particularly preferable to use acicular ferromagnetic metal powder. In the case of acicular ferromagnetic metal powder, the acicular ratio is preferably 4-12, more preferably 5-12.

The coercive force (Hc) of the ferromagnetic metal powder is preferably 159 to 239 kA / m (2,000 to 3,000 Oe), more preferably 167 to 231 kA / m (2,100 to 2,900 Oe). The saturation magnetic flux density is preferably 100 to 300 mT (1,000 to 3,000 G), more preferably 160 to 280 mT (1,600 to 2,800 G). The saturation magnetization (σs) is preferably 100 to 170 A · m ² / kg (emu / g), more preferably 100 to 160 A · m ² / kg (emu / g).

The SFD (switching field distribution) of the magnetic material itself is preferably small and is preferably 0.8 or less. When the SFD is 0.8 or less, the electromagnetic conversion characteristics are good, the output is high, the magnetization reversal is sharp, the peak shift is small, and it is suitable for high-density digital magnetic recording. In order to reduce the Hc distribution, there are methods such as improving the particle size distribution of goethite in the ferromagnetic metal powder, using monodispersed α-Fe ₂ O ₃ , and preventing sintering between particles.

An example of the manufacturing method of the ferromagnetic powder which introduce | transduced cobalt and yttrium used for this invention is shown.
An example in which iron oxyhydroxide obtained by blowing an oxidizing gas into an aqueous suspension in which a ferrous salt and an alkali are mixed is used as a starting material can be given. As the type of this iron oxyhydroxide, α-FeOOH is preferable, and as its production method, ferrous salt is neutralized with an alkali hydroxide to form an aqueous suspension of Fe (OH) _2. There is a first production method in which an oxidizing gas is blown into a needle-like α-FeOOH. On the other hand, there is a second production method in which ferrous salt is neutralized with alkali carbonate to form an aqueous suspension of FeCO ₃ , and oxidizing gas is blown into this suspension to form spindle-shaped α-FeOOH. Such an iron oxyhydroxide is obtained by reacting a ferrous salt aqueous solution and an alkaline aqueous solution to obtain an aqueous solution containing ferrous hydroxide and oxidizing it by air oxidation or the like. Is preferred. In this case, Ni salt, alkaline earth element salt such as Ca salt, Ba salt, Sr salt, Cr salt, Zn salt, etc. may coexist in the ferrous salt aqueous solution, and such salt is appropriately selected. Thus, the particle shape (axial ratio) and the like can be prepared.

As the ferrous salt, ferrous chloride, ferrous sulfate and the like are preferable. As the alkali, sodium hydroxide, aqueous ammonia, ammonium carbonate, sodium carbonate and the like are preferable. Moreover, as a salt which can coexist, chlorides, such as nickel chloride, calcium chloride, barium chloride, strontium chloride, chromium chloride, zinc chloride, are preferable.
Next, when introducing cobalt into iron, before introducing yttrium, an aqueous solution of a cobalt compound such as cobalt sulfate or cobalt chloride is stirred and mixed into the iron oxyhydroxide slurry. After preparing a slurry of iron oxyhydroxide containing cobalt, an aqueous solution containing a compound of yttrium can be added to the slurry and mixed by stirring.

  In addition to yttrium, neodymium, samarium, praseodymium, lanthanum, and the like can be introduced into the ferromagnetic powder of the present invention. These can be introduced using chlorides such as yttrium chloride, neodymium chloride, samarium chloride, praseodymium chloride, lanthanum chloride, and nitrates such as neodymium nitrate and gadolinium nitrate. Also good.

(Ferromagnetic hexagonal ferrite powder)
Another shape of the ferromagnetic powder used in the magnetic recording medium of the present invention is a flat plate shape, and a hexagonal ferrite fine powder can be preferably exemplified.
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. A glass crystallization method in which barium ferrite crystal powder is obtained 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 production method of the hexagonal ferrite used in the present invention is not limited.

(6) Nonmagnetic 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 non-magnetic powder used in the non-magnetic layer of the present invention include Showa Denko Nanotite, Sumitomo Chemical HIT-100, ZA-G1, Toda Kogyo 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-65C, manufactured by Titanium Industry, MT-100S, MT-100T, MT-150W, MT-500B, MT-600B manufactured by Teika , 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.

(7) Other additives, etc. In the magnetic recording medium of the present invention, the magnetic layer or nonmagnetic layer may contain additives for imparting a dispersing effect, a lubricating effect, an antistatic effect, a plasticizing effect, and the like.
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 in the nonmagnetic layer and the magnetic layer as needed. 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.

(8) Support In the magnetic recording medium of the present invention, a coating solution prepared from the above materials is applied on a nonmagnetic support to form a nonmagnetic layer or a magnetic layer.
As the nonmagnetic 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 polybenzoxydazole can be used. Polyethylene naphthalate and aromatic polyamide are preferred. These nonmagnetic supports may be subjected in advance to corona discharge, plasma treatment, easy adhesion treatment, heat treatment and the like.

The nonmagnetic support that can be used in the present invention has a surface having excellent smoothness with a centerline average surface roughness of 0.1 to 20 nm, preferably 1 to 10 nm, at a cutoff value of 0.25 mm. Preferably there is. 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.

(9) Backcoat layer A backcoat 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.

(10) 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.

(11) 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 being polymerized or cross-linked when irradiated with radiation such as ultraviolet rays or an electron beam, and polymerized and cured.

(12) Manufacturing Method The process of manufacturing the magnetic layer coating liquid for the magnetic recording medium used in the present invention comprises at least a kneading process, a dispersing process, and a mixing process provided before and after these processes as necessary. Each process may be divided into two or more stages. What are all raw materials such as ferromagnetic fine powder (ferromagnetic hexagonal ferrite powder or ferromagnetic metal powder), non-magnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant, solvent used in the present invention? It may be added from the beginning or during the process. In addition, individual raw materials may be added in two or more steps.
In the method for producing a magnetic recording medium of the present invention, at the time of producing a magnetic coating material that is a coating solution for a magnetic layer, at least one magnetic material in which a ferromagnetic fine powder is dispersed in a binder solution containing a polyurethane resin (A). Prepare paint. In the preparation of the magnetic coating material, a kneading step is adopted in which the polyurethane resin (A) and the ferromagnetic fine powder are kneaded as all or part of the magnetic layer binder. 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.
Following the kneading step, a dispersing step is performed. A coating solvent is added to the mixture of the ferromagnetic fine powder and binder obtained by the kneading step, and the ferromagnetic fine 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.

In the present invention, as a method for applying the magnetic coating material on the nonmagnetic support, for example, a magnetic coating solution is applied on 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 a coating machine for applying the magnetic coating solution or the non-magnetic coating solution, an air doctor coat, a blade coat, a rod coat, an extrusion coat, an air knife coat, a squeeze coat, an impregnation coat, a reverse roll coat, a transfer roll coat, a 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 nonmagnetic layer containing an inorganic powder on the support, and 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 magnetic recording medium of the present invention is irradiated with radiation in order to cure the hyperbranched polyester having the radiation curable functional group introduced therein. Radiation irradiation is preferably performed after calendar treatment.
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 1)
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 (A) had an OH value of 4.1 meq / g, a GPC molecular weight (polystyrene equivalent) of 3900 in number average, and a weight average molecular weight of 12000.

(Synthesis Example 2)
In a reaction vessel equipped with a thermometer, a stirrer, and a partial reflux condenser, 0.5 mol (136 g) of trimethylolpropane (molecular weight 134), 1.5 mol (201 g) of dimethylolpropionic acid (molecular weight 134) and p-toluene 0.5 mmol (87 mg) of sulfonic acid was charged, the temperature was raised to 120 ° C., and the temperature was raised while reducing the pressure to 140 ° C. over 1 hour. Thereafter, 9 mol (1206 g) of dimethylolpropionic acid was added to the reaction mixture and reacted at 140 ° C. under reduced pressure for 5 hours while stirring.
The obtained hyperbranched polyester (B) had an OH value of 4.7 meq / g, a GPC molecular weight (in terms of polystyrene) of 2600 in number average, and a weight average molecular weight of 6900.

(Synthesis Example 3)
A reaction vessel equipped with a thermometer, a stirrer, and a partial reflux condenser was charged with 390 g of hyperbranched polyester (A) of Synthesis Example 1, 195 g of methyl ethyl ketone (MEK), and 195 g of toluene, and stirred at 60 ° C. under a nitrogen stream. The solution was heated to melt. Next, 46 mg of dibutyltin dilaurate was added as a catalyst and dissolved for another 15 minutes. Further, 233 g of a 30 wt% MEK / toluene (= 1/1) solution of acryloyloxyethyl isocyanate (molecular weight 141) was added and reacted by heating at 90 ° C. for 6 hours to obtain a hyperbranched polyester (C) introduced with an acryloyl group. . It was confirmed from the IR measurement of the obtained compound that the NCO group had disappeared.

(Synthesis Example 4)
A reaction vessel equipped with a thermometer, stirrer, and partial reflux condenser was charged with 260 g of hyperbranched polyester (B) of Synthesis Example 1, 20 g of 4-dimethylaminopyridine, and 63 g of acrylic anhydride (molecular weight 126) at 90 ° C. For 24 hours. After the reaction, the resulting solid was cooled to room temperature, washed with chloroform and methylene dichloride three times, and dried to obtain an acryloyl group-introduced hyperbranched polyester (D).

Example 1
[Preparation of magnetic liquid for magnetic layer]
Ferromagnetic alloy powder A (composition: Fe: 100 atomic%, Co: 20%, Al: 9%, Y: 6%, Hc: 175 kA / m (2,200 Oe), crystallite size: 11 nm, S _BET : 70 m ² / G, long axis length: 45 nm, σs: 111 A · m ² / kg (emu / g)) 100 parts were pulverized with an open kneader for 10 minutes,
20% by weight of a 30% solution of acryloyl group-introduced hyperbranched polyester (C) in 30% MEK / toluene (= 1/1) vinyl chloride copolymer MR110 (manufactured by Nippon Zeon Co., Ltd.) 20% by weight polyurethane resin UR8200 (manufactured by Toyobo Co., Ltd.) MEK / toluene = 1/1 30% solution 20 parts by weight is added and kneaded for 60 minutes, then α-alumina HIT55 (manufactured by Sumitomo Chemical Co., Ltd.) 10 parts by weight carbon black # 50 (manufactured by Asahi Carbon) 3 parts by weight of methyl ethyl ketone / toluene = 1/1 200 parts by weight were added and dispersed in a sand mill for 120 minutes. 1 part by weight stearic acid 1 part by weight myristic acid 3 parts by weight isohexadecyl stearate 3 parts by weight MEK 100 parts by weight were further stirred and mixed for 20 minutes, and then filtered using a filter having an average pore size of 1 μm to obtain a magnetic paint. Was prepared.

[Preparation of nonmagnetic liquid for nonmagnetic layer]
Opened acicular α-iron oxide (major axis length 100 nm, surface treatment layer; alumina, SBET 52 m ² / g, pH 9.4) 85 parts by weight and carbon black Ketjen black EC (manufactured by Japan EC) 15 parts by weight Mill with a kneader for 10 minutes, then
Polyethylene resin UR8200 (manufactured by Toyobo Co., Ltd.) MEK / toluene = 1/1 30% solution 60 parts by weight cyclohexanone 20% Then, 200 parts by weight of methyl ethyl ketone / cyclohexanone = 6/4 was added and dispersed in a sand mill for 120 minutes. Polyisocyanate (Coronate 3041 made by Nippon Polyurethane) MEK / toluene = 1/1 30% solution 15 parts by weight Stearic acid 1 part by weight Myristic acid 1 part by weight Isooctyl stearate 3 parts by weight MEK 50 parts by weight After stirring and mixing for a minute, the mixture was filtered using a filter having an average pore size of 1 μm to prepare a nonmagnetic paint.

Simultaneous application of a multilayer coating on the surface of a 6.0 μm thick polyethylene naphthalate support so that the resulting non-magnetic coating is 1.2 μm and immediately after that the dried magnetic coating is 0.1 μm thick. did. With the magnetic paint in an undried state, magnetic field orientation was performed with a 5000 gauss Co magnet and a 4000 gauss solenoid magnet, and the coated material was metal roll-metal roll-metal roll-metal roll-metal roll-metal roll-metal roll. Calendar processing by combination was performed at a speed of 100 m / min, a linear pressure of 300 kg / cm, and a temperature of 90 ° C.
Thereafter, an electron beam with an accelerating voltage of 150 kV was irradiated in an atmosphere having an oxygen concentration of 200 ppm or less so that the absorbed dose was 5 Mrad, further heat-treated at 50 ° C. for 7 days, and slit to a width of 3.8 mm to prepare a magnetic tape.

(Example 2)
A sample was prepared in the same manner as in Example 1 except that the acryloyl group-introduced hyperbranched polyester (C) in Example 1 was replaced with a 30% MEK / toluene (1/1) solution of the acryloyl group-introduced hyperbranched polyester (D). .

(Example 3)
In Example 1, in the preparation of the nonmagnetic liquid for the nonmagnetic layer, a cyclohexanone 30% solution of vinyl chloride copolymer MR110 (manufactured by Nippon Zeon Co., Ltd.) 60 parts by weight polyurethane resin UR8200 (manufactured by Toyobo Co., Ltd.) MEK / Toluene = 1/1 30% solution MEK / toluene (= 1/1) 30% solution of acryloyl group-introduced hyperbranched polyester (C) instead of 60 parts by weight Vinyl chloride copolymer MR110 (manufactured by Nippon Zeon) ), 20 parts by weight of polyurethane resin UR8200 (manufactured by Toyobo Co., Ltd.) MEK / toluene = 1/1 30% solution 20 parts by weight was added, and polyisocyanate was not added. Samples were prepared in the same manner.

(Comparative Example 1)
A sample was prepared in the same manner as in Example 1 except that the acryloyl group-introduced hyperbranched polyester was not added and the electron beam irradiation was not performed.

(Comparative Example 2)
A sample was prepared in the same manner as in Example 3 except that the acryloyl group-introduced hyperbranched polyester (C) added to the magnetic liquid for the magnetic layer and the nonmagnetic liquid for the nonmagnetic layer in Example 3 was replaced with pentaerythritol tetraacrylate. Created.

〔Measuring method〕
(1) Magnetic layer surface roughness Ra
The center line average roughness Ra was measured by a light interference method using a digital optical profilometer (manufactured by WYKO) under the condition of a cutoff of 0.25 mm.
(2) Squareness ratio SQ
The magnetostatic characteristics (hysteresis curve) of the tape were measured using a vibrating sample magnetometer VSM, and the squareness ratio (residual magnetization Br / saturated magnetization Bm) was determined.
(3) Electromagnetic conversion characteristics A 4.7 MHz single frequency signal was recorded with an optimum recording current using a DDS3 drive, and the reproduction output was measured. The reproduction output of Comparative Example 1 is shown as a relative value with 0 dB.
(4) Durability The decrease in output after the tape was run 1000 times repeatedly for 1 minute with the DDS3 drive in (3) in an environment of 40 ° C. and 30% RH was shown relative to the initial output.
(5) Edge endurance (4) The tape edge portion after running was observed and cracks were observed Δ, the case where the magnetic layer was missing from the crack occurrence portion ×, cracks and dropout occurred The ones that did not exist were marked as ◯.

  Table 1 shows the types and evaluation results of the acryloyl group-introduced hyperbranched polyesters used in Examples 1 to 3 and Comparative Examples 1 and 2.

Claims (2)

A magnetic recording medium comprising at least one magnetic layer in which a ferromagnetic powder is dispersed in a binder on a support,
A magnetic recording medium characterized in that the magnetic layer contains a binder obtained by curing a hyperbranched polyester having a radiation curable functional group introduced therein by irradiation. A magnetic recording medium comprising at least one magnetic layer in which ferromagnetic powder is dispersed in a binder via a nonmagnetic layer in which nonmagnetic powder is dispersed in a binder on a support,
A magnetic recording medium comprising a binder obtained by curing a hyperbranched polyester having a radiation-curable functional group introduced into the nonmagnetic layer and / or magnetic layer by irradiation with radiation.