JP2007109370A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium superior in flatness and electromagnetic transducing properties and having the long-term storage property and superior dimension stability. <P>SOLUTION: The magnetic recording medium includes, on a nonmagnetic support, a radiation curing layer in which a radiation curing compound containing layer is cured by irradiation with radiation, and a magnetic layer in which ferromagnetic powder is dispersed in a binder in this order. The radiation curing compound includes hyperbranch polyester in which a radiation curing functional group is introduced. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a magnetic recording medium such as a magnetic tape or a magnetic disk.

  Ferromagnetic powders such as γ-iron oxide, Co-containing iron oxide, chromium oxide, and ferromagnetic metal powder are used as tape-like magnetic recording media for audio, video, and computers, and disk-like magnetic recording media such as flexible disks. A magnetic recording medium in which a magnetic layer dispersed in a binder is provided on a support is used. In general, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or the like is used as the support used in the magnetic recording medium. Since these supports are stretched and highly crystallized, they have high mechanical strength and excellent solvent resistance.

  The magnetic layer obtained by applying a coating solution in which ferromagnetic powder is dispersed in a binder to a support has a high degree of filling with high ferromagnetic powder, small elongation at break, and is brittle. And may be peeled off from the support. Therefore, an undercoat layer is provided on the support and the magnetic layer is strongly bonded onto the support.

On the other hand, a magnetic recording medium in which a compound having a functional group curable by radiation such as an electron beam, that is, a radiation curable compound is coated on a support and a magnetic layer is provided on the radiation curable layer cured by radiation irradiation. It has been reported (see Patent Documents 1 to 4).
However, these radiation-cured layers have insufficient durability because sufficient crosslinking of the coating film cannot be obtained. In particular, a magnetic recording medium using a fine particle magnetic material has a problem that sufficient durability cannot be obtained.

Japanese Patent Publication No. 5-57647 JP-A-60-133529 JP 60-133530 A Japanese Patent Application Laid-Open No. 60-133531

  The problem to be solved by the present invention is to provide a magnetic recording medium excellent in smoothness and electromagnetic conversion characteristics. Furthermore, it is to provide a magnetic recording medium having durability and excellent dimensional stability.

In order to achieve the above object, the present invention has the following configuration. That is, the present invention is as follows.
A magnetic recording medium having, in this order, a radiation-cured layer obtained by curing a radiation-curable compound-containing layer by radiation irradiation on a non-magnetic support, and a magnetic layer in which a ferromagnetic powder is dispersed in a binder. The magnetic recording medium, wherein the radiation curable compound comprises a hyperbranched polyester having a radiation curable functional group introduced therein.

  Further, it is preferable to provide a nonmagnetic layer in which a nonmagnetic powder is dispersed in a binder between the radiation-cured layer and the magnetic layer.

  According to the present invention, a magnetic recording medium having high magnetic layer smoothness and excellent electromagnetic conversion characteristics can be obtained. In addition, a magnetic recording medium having high durability between the support and the magnetic layer and having excellent durability can be obtained. Furthermore, a magnetic recording medium having excellent dimensional stability and durability can be obtained.

Hereinafter, the present invention will be described in more detail.
The magnetic recording medium of the present invention comprises a radiation-cured layer obtained by curing a radiation-curable compound-containing layer by radiation irradiation on a nonmagnetic support, and a magnetic layer in which a ferromagnetic powder is dispersed in a binder. The magnetic recording medium has an order, wherein the radiation curable compound includes a hyperbranched polyester having a radiation curable functional group introduced therein.
In the magnetic recording medium of the present invention, a nonmagnetic layer in which a nonmagnetic powder is dispersed in a binder is preferably provided between the radiation-cured layer and the magnetic layer.

I. Radiation-cured layer Hyperbranched polyester Hyperbranched polyester is a multibranched polyester with a dendritic structure. Therefore, in the present invention, “hyperbranched polyester” is also referred to as “multi-branched polyester”. Hyperbranched polyesters are described in documents such as “Science and function of dendrimers” (2000.7.20, issued by ICP Corporation, p86). Hyperbranched polyester is synthesized by, for example, self-condensation of a compound having a total of 3 or more of 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 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 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 molecular weight of the hyperbranched polyester used in the present invention is preferably 1,000 to 50,000, more preferably 1,500 to 30,000 in terms of weight average molecular weight.

  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 the above “Science and function of dendrimers”, 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 100 meq / g, more preferably 1 to 50 meq / g.

  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 curing functional group is preferably a radical polymerizable functional group having an ethylenically unsaturated double bond such as an acryloyl group or a 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 can also 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-curing functional groups introduced into the hyperbranched polyester is preferably 2 to 500, more preferably 5 to 100, per molecule. When the amount is in the above range, crosslinking and curing are sufficient, and this is preferable because it does not cause an adhesive failure in the edge portion. Moreover, since a favorable elastic modulus is obtained, it is preferable. Furthermore, since the radiation-cured layer does not become brittle, the adhesion with the support is good, and failure due to the magnetic layer falling off during repeated running is preferable.

  In the present invention, the thickness of the radiation-cured layer is preferably 0.1 to 1.0 μm. It is preferable for the thickness of the radiation-cured layer to be in the above range because sufficient smoothness can be obtained and adhesion to the support is good.

  The glass transition temperature (Tg) after curing of the radiation-cured layer is preferably 80 ° C to 150 ° C. More preferably, it is 100 degreeC-130 degreeC. It is preferable for the glass transition temperature of the radiation-cured layer to be in the above-mentioned range since no adhesion failure occurs in the coating process and the strength of the coating film is good.

The elastic modulus after curing of the radiation-cured layer is preferably 1.5 GPa to 4 GPa.
It is preferable for the elastic modulus to be within the above-mentioned range since the coating film does not cause adhesion failure and the strength of the coating film is good.

  The average roughness (Ra) of the radiation-cured layer is preferably 1 to 3 nm at a cutoff value of 0.25 nm. Within the above range, it is preferable that the application process does not cause a failure to stick to the pass roll and sufficient smoothness of the magnetic layer is obtained.

The radiation-cured layer preferably does not contain a binder, and it is preferable to substantially cure only the radiation-curable compound. However, it does not exclude that the radiation-cured layer contains additives such as a polymerization initiator and a pigment.
In the present invention, a known radiation curable compound can be used in combination with the radiation curable layer in addition to the hyperbranched polyester having a radiation curable functional group introduced therein.
Examples of radiation curable compounds to be used in combination include those known in “Applied Technology of Low Energy Electron Beam Irradiation” (issued by CMC) and “UV / EB Curing Technology” (issued by General Technology Center Co., Ltd.). Examples thereof include radiation curable compounds such as (meth) acrylate compounds.
The radiation curable compound used in combination is preferably one having two or more acryloyl groups.
Preferred for use in combination are 5-ethyl-2- (2-hydroxy-1,1′-dimethylethyl) -5- (hydroxymethyl) -1,3-dioxane diacrylate, tetrahydrofuran dimethanol diacrylate, 3 , 9-bis (1,1-dimethyl-2-hydroxyethyl) -2,4,8,10-tetraoxaspiro (5,5) undecanediacrylate or the like, or ethylene oxide of trimethylolpropane 4 or more functional groups such as modified triacrylate, trimethylolpropane propylene oxide modified triacrylate, dipentaerythritol tetraacrylate, dipentaerythritol pentaacrylate, dipentaerythritol hexaacrylate, ditrimethylolpropane tetraacrylate, etc. What has an acryloyl group can be illustrated.

  The hyperbranched polyester into which the radiation curable functional group is introduced when another radiation curable compound is used in combination is preferably 30 wt% or more, more preferably 50 wt% or more of the entire radiation curable compound.

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 1,000 kV, preferably 50 to 300 kV, and the absorbed dose is 0.5 to 20 Mrad, preferably 2 to 10 Mrad. It is preferable for the acceleration voltage to be within the above range since a sufficient amount of energy transmission and good energy efficiency can be obtained.
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. It is preferable for the oxygen concentration to be in the above-mentioned range since crosslinking and curing reactions in the vicinity of the surface are not inhibited.

As the ultraviolet light source, a mercury lamp is preferable. The mercury lamp is preferably 20 to 240 W / cm and 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.
An ultraviolet light source using a light emitting diode having ultraviolet light emission energy can also be used. For example, an LED light source having an emission peak wavelength of 365 nm can be used.

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, hydroxydimethylphenyl ketone, 1-hydroxycyclohexyl phenyl ketone, 2,2-diethoxyacetophenone, and the like. . 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.
For radiation curing equipment and conditions, see “UV / EB Curing Technology” (published by General Technology Center Co., Ltd.) and “Applied Technology for Low Energy Electron Beam Irradiation” (2000, issued by CMC Corporation). The known ones described can be used.

  In the present invention, the hyperbranched polyester introduced with a radiation-curing functional group has a low viscosity when applied to a support, and can be easily leveled during coating and drying to obtain a smooth surface. In addition, since the radiation-cured layer has a high crosslinking density and high elastic modulus, a magnetic recording medium in which thermal deformation and creep hardly occur and dimensional stability is high can be obtained. Furthermore, since the hyperbranched polyester introduced with the radiation curing functional group used in the present invention already has a crosslinked structure, the curing shrinkage rate upon radiation curing is low. For this reason, since there is little deformation such as cupping and the adhesiveness with the support is high, a magnetic recording medium excellent in storage stability, durability and dimensional stability can be obtained.

The magnetic recording medium of the invention has an extremely smooth magnetic layer surface as compared with the conventional one, and achieves high electromagnetic conversion characteristics and dimensional stability.
By having a radiation curable layer using a radiation curable compound containing hyperbranched polyester having a radiation curable functional group introduced on the surface of the support, an extremely smooth coating film can be formed, and excellent electromagnetic conversion characteristics can be obtained. It is considered that the hyperbranched polyester having a highly branched structure has a lower viscosity than the linear polymer, so that the protrusions of the support can be buried and leveled.

Immediately after applying this compound, a smooth coating film can be instantly cured by radiation irradiation. Further, a magnetic layer having a very excellent smoothness on the surface of the coating film can be obtained by coating the magnetic liquid directly or directly on the layer in which the nonmagnetic powder is dispersed.
Although it is conceivable to use a smooth support with very few protrusions, an extremely smooth support has a high coefficient of friction, and particularly in the case of a thin support having a thickness of 10 μm or less in the manufacturing process of the support or the magnetic tape coating process. Although there is a problem that the production yield is remarkably reduced, such as wrinkles occurring or meandering in the conveyance roll in the conveyance or winding process, this problem can be essentially avoided in the method of the present application.

Hyperbranched polyester has a highly branched structure, and is characterized by extremely high density, high elastic modulus, resistance to thermal deformation, and resistance to creep when stressed for a long time.
In order to improve the elastic modulus and creep properties with ordinary radiation curable compounds, and to increase the crosslink density, a structure with a high concentration of cured functional groups will result in extremely high cure shrinkage, tape capping phenomenon and support Decrease in adhesion to the body and deterioration of brittleness of the cured film become remarkable. In the present invention, the hyperbranched polyester already has a branched structure before curing, and a high crosslink density is formed only by curing the surrounding functional group by radiation, so that these problems can be cleared. it is conceivable that.
Further, according to the present invention, it is possible to improve the peeling and dropping of the magnetic layer at the tape edge portion in the step of slitting the magnetic recording tape. As a result, the small pieces of the magnetic layer that have dropped off are less likely to adversely affect the recording / reproducing characteristics. For example, a computer tape is effective in reducing an error rate and a video tape is effective in reducing a dropout failure.

II. Magnetic layer <ferromagnetic powder>
As the ferromagnetic powder contained in the magnetic layer in the present invention, both acicular and flat ferromagnetic powders can be used. Ferromagnetic metal powder is preferably used as the acicular ferromagnetic powder, and ferromagnetic hexagonal ferrite powder is preferably used as the flat ferromagnetic metal powder.

(Ferromagnetic metal powder)
The ferromagnetic metal powder used in the magnetic recording medium of the present invention is preferably acicular and cobalt-containing ferromagnetic iron oxide or ferromagnetic alloy powder. Preferably S _BET (BET specific surface area) is 40 to 80 m ² / g, more preferably 50 to 70 m ² / g. The crystallite size is preferably 9 to 25 nm, more preferably 10 to 22 nm, and particularly preferably 11 to 20 nm. The major axis length is preferably 20 to 70 nm, more preferably 30 to 50 nm.

  Examples of the ferromagnetic metal powder include yttrium-containing Fe, Fe—Co, Fe—Ni, and Co—Ni—Fe. The yttrium content in the ferromagnetic metal powder is a ratio of yttrium atoms to iron atoms, Y / Fe is preferably 0.5 to 20 atomic%, more preferably 5 to 10 atomic%. Within the above range, the ferromagnetic metal powder is increased in σs, and good magnetic characteristics and electromagnetic conversion characteristics can be obtained. Further, since the iron content is appropriate, it is preferable because good magnetic characteristics and electromagnetic conversion characteristics can be obtained.

  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.

An example of a method for producing a ferromagnetic metal powder introduced with cobalt or yttrium that can be used in the present invention will be described.
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 the present invention, neodymium, samarium, praseodymium, lanthanum and the like can be introduced into the ferromagnetic metal powder in addition to yttrium. These can be introduced using chlorides such as yttrium chloride, neodymium chloride, samarium chloride, praseodymium chloride, lanthanum chloride, nitrates such as neodymium nitrate and gadolinium nitrate, and these are used in combination of two or more. Also good.

(Ferromagnetic hexagonal ferrite powder)
In the present invention, it is preferable to use a ferromagnetic hexagonal ferrite powder as the tabular ferromagnetic powder.
Examples of ferromagnetic hexagonal ferrite powders include barium ferrite, strontium ferrite, lead ferrite, calcium ferrite substitutes, and Co substitutes. 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, Nb, and Zr 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 plate diameter of the tabular ferromagnetic powder is preferably 10 to 50 nm. The particle size is preferably 10 to 50 nm as a hexagonal plate diameter.
When reproducing with a magnetoresistive head, it is necessary to reduce the noise, and the plate diameter is preferably 10 to 40 nm. Within the above range, stable magnetization can be expected without being affected by thermal fluctuations, and noise is low, which is preferable for high-density magnetic recording.
The plate ratio (plate diameter / plate thickness) is preferably 1-15. More preferably, it is 2-7. It is preferable that the plate ratio is in the above range because the filling property in the magnetic layer is high and sufficient orientation is obtained. Further, it is preferable because noise due to stacking between particles is reduced.
S _BET (specific surface area by BET method) in this particle size range is 10 to ²⁰⁰ m ² / g. The specific surface area roughly agrees with the arithmetic calculation value from the particle plate diameter and plate thickness. The crystallite size is preferably 50 to 450 mm, more preferably 100 to 350 mm.

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, it is preferable that 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 the ferromagnetic hexagonal ferrite powder 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 to 279 kA / m (1,500 to 3,500 Oe). 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 preferably 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 ferromagnetic hexagonal ferrite powder is dispersed, the surface of the ferromagnetic hexagonal ferrite powder is 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 0.1 to 10% with respect to the ferromagnetic hexagonal ferrite powder. The pH of the ferromagnetic hexagonal ferrite powder is also important for dispersion. Usually, the optimum value is about 4 to 12 depending on the dispersion medium and polymer, but about 6 to 10 is preferable from the viewpoint of chemical stability and storage stability of the medium. Moisture contained in the ferromagnetic hexagonal ferrite powder also affects the dispersion. There are optimum values depending on the dispersion medium and the polymer, but 0.01 to 2.0% is usually preferable.

As a manufacturing method of the ferromagnetic hexagonal ferrite powder,
(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) A hydrothermal reaction method in which a barium ferrite composition metal salt solution is neutralized with an alkali, and by-products are removed, followed by liquid phase heating at 100 ° C. or higher, followed by washing, drying and pulverization to obtain a barium ferrite crystal powder.
(3) There is a coprecipitation method 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, this invention does not choose a manufacturing method.

<Binder>
Binders used in the magnetic layer include polyurethane resins, polyester resins, polyamide resins, vinyl chloride resins, acrylic resins copolymerized with styrene, acrylonitrile, methyl methacrylate, cellulose resins such as nitrocellulose, epoxy resins, A single resin or a mixture of a plurality of resins can be used from polyvinyl alkyl resins such as phenoxy resin, polyvinyl acetal, and polyvinyl butyral. Among these, polyurethane resins, acrylic resins, cellulose resins, and vinyl chloride resins are preferable.

The binder preferably has a functional group (polar group) that is adsorbed on the surface of the powder in order to improve the dispersibility of the ferromagnetic powder and the nonmagnetic powder described later. Preferred -SO ₃ M is as a functional _{group, -SO 4 M, -PO (OM} ) 2, -OPO (OM) 2, -COOM,> NSO 3 M,> NRSO 3 M, -NR 1 R 2, -N ⁺ R ¹ R ² R ³ X- ^{and the} like. Here, M is hydrogen or an alkali metal such as Na or K, R is an alkylene group, R ¹ , R ² or R ³ is an alkyl group or a hydroxyalkyl group or hydrogen, and X is a halogen such as Cl or Br. The amount of the functional group in the binder is preferably 10 to 200 μeq / g, more preferably 30 to 120 μeq / g. Within this range, it is preferable because good dispersibility can be obtained.

In addition to the adsorptive functional group, the binder is preferably provided with a functional group having an active hydrogen such as an —OH group in order to react with an isocyanate curing agent to form a crosslinked structure and improve the coating film strength. A preferred amount is 0.1 to 2 meq / g.
The molecular weight of the binder is preferably 10,000 to 200,000, more preferably 20,000 to 100,000 in terms of weight average molecular weight. Within this range, the coating film strength is sufficient, the durability is good, and the dispersibility is improved, which is preferable.

  Polyurethane resins, which are preferred binders, are described in detail in, for example, “Polyurethane Resin Handbook” (edited by Keiji Iwata, 1986, Nikkan Kogyo Shimbun). Usually, long-chain diols and short-chain diols (called chain extenders) And an addition polymerization of a diisocyanate compound. Polyester diol, polyether diol, polyether ester diol, polycarbonate diol, polyolefin diol having a molecular weight of 500 to 5,000 is used as the long chain diol. Depending on the type of this long-chain polyol, it is called polyester urethane, polyether urethane, polyether ester urethane, polycarbonate urethane or the like.

The polyester diol can be obtained by polycondensation of an aliphatic dibasic acid such as adipic acid, sebacic acid or azelaic acid, or an aromatic dibasic acid such as isophthalic acid, orthophthalic acid, terephthalic acid or naphthalenedicarboxylic acid with glycol. Examples of the glycol component include ethylene glycol, 1,2-propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6- There are hexanediol, 2,2-dimethyl-1,3-propanediol, 1,8-octanediol, 1,9-nonanediol, cyclohexanediol, cyclohexanedimethanol, hydrogenated bisphenol A and the like. As the polyester diol, polycaprolactone diol and polyvalerolactone diol obtained by ring-opening polymerization of lactones such as ε-caprolactone and γ-valerolactone can also be used.
The polyester diol is preferably one having a branched side chain or one obtained from an aromatic or alicyclic raw material from the viewpoint of hydrolysis resistance.

Polyether diols include polyethylene glycol, polypropylene glycol, polytetramethylene glycol, aromatic glycols such as bisphenol A, bisphenol S, bisphenol P, and hydrogenated bisphenol A, and alicyclic diols and alkylene oxides such as ethylene oxide and propylene oxide. There are addition-polymerized products.
These long-chain diols can be used in combination of a plurality of types.

  The short chain diol can be selected from the same group of compounds as exemplified in the glycol component of the polyester diol. In addition, when a small amount of trifunctional or higher polyhydric alcohols such as trimethylolethane, trimethylolpropane, pentaerythritol, etc. are used in combination, a branched polyurethane resin can be obtained, and the solution viscosity can be reduced or the OH group at the end of the polyurethane can be increased. Curability with an isocyanate curing agent can be increased.

As the diisocyanate compound, MDI (diphenylmethane diisocyanate), 2,4-TDI (tolylene diisocyanate), 2,6-TDI, 1,5-NDI (naphthalene diisocyanate), TODI (tolidine diisocyanate), p-phenylene diisocyanate, XDI ( Aromatic diisocyanates such as xylylene diisocyanate), transcyclohexane-1,4-diisocyanate, HDI (hexamethylene diisocyanate), IPDI (isophorone diisocyanate), H ₆ XDI (hydrogenated xylylene diisocyanate), H ₁₂ MDI (hydrogenated diphenylmethane) Aliphatic such as diisocyanate), alicyclic diisocyanate and the like are used.

A preferred composition of the long chain diol / short chain diol / diisocyanate in the polyurethane resin is (80 to 15% by weight) / (5 to 40% by weight) / (15 to 50% by weight).
The urethane group concentration of the polyurethane resin is preferably 1 to 5 meq / g, more preferably 1.5 to 4.5. It is preferable for the urethane group concentration to be in the above range since the mechanical strength is excellent, the solution viscosity is good, and high dispersibility is obtained.
The glass transition temperature of the polyurethane resin is preferably 0 to 200 ° C, more preferably 40 to 160 ° C. When the glass transition temperature is within the above range, it is preferable because durability is excellent, calendar moldability is excellent, and good electromagnetic characteristics can be obtained.

  As a method of introducing the above-mentioned adsorptive functional group (polar group) into the polyurethane resin, a method using a functional group as a part of a monomer of a long-chain diol, a method using a part of a short-chain diol, or after polymerizing a polyurethane, There is a method of introducing a polar group by molecular reaction.

As the vinyl chloride resin, those obtained by copolymerizing various monomers with a vinyl chloride monomer are used.
Copolymer monomers include fatty acid vinyl esters such as vinyl acetate and vinyl propionate, and acrylates such as methyl (meth) acrylate, ethyl (meth) acrylate, isopropyl (meth) acrylate, butyl (meth) acrylate, and benzyl (meth) acrylate. , Methacrylates, allyl methyl ether, allyl ethyl ether, allyl propyl ether, allyl butyl ether and other alkyl allyl ethers, other styrene, α-methyl styrene, vinylidene chloride, acrylonitrile, ethylene, butadiene, acrylamide, and co-functional groups. As polymerization monomers, vinyl alcohol, 2-hydroxyethyl (meth) acrylate, polyethylene glycol (meth) acrylate, 2-hydroxypropyl (meth) acrylate Relate, 3-hydroxypropyl (meth) acrylate, polypropylene glycol (meth) acrylate, 2-hydroxyethyl allyl ether, 2-hydroxypropyl allyl ether, 3-hydroxypropyl allyl ether, p-vinylphenol, maleic acid, maleic anhydride Acrylic acid, methacrylic acid, glycidyl (meth) acrylate, allyl glycidyl ether, phosphoethyl (meth) acrylate, sulfoethyl (meth) acrylate, p-styrene sulfonic acid, and Na salts and K salts thereof are used.
The composition of the vinyl chloride monomer in the vinyl chloride resin is preferably 60 to 95% by weight. Within the above range, good mechanical strength can be obtained, solvent solubility is good, and good dispersibility can be obtained for a suitable solution viscosity.

The preferred amount of the functional group for enhancing the curability with the adsorption functional group (polar group) and the polyisocyanate curing agent is as described above. These functional groups may be introduced by copolymerizing the above functional group-containing monomers, or by copolymerizing a vinyl chloride resin and then introducing a functional group by a polymer reaction.
The polymerization degree is preferably 200 to 600, more preferably 240 to 450. Within this range, it is preferable because good mechanical strength can be obtained and good dispersibility can be obtained for a suitable solution viscosity.

A curing agent can be used to crosslink and cure the binder used in the present invention to increase the mechanical strength and heat resistance of the coating film. A preferred curing agent is a polyisocyanate compound. The polyisocyanate compound is preferably a triisocyanate or higher polyisocyanate.
Specifically, a compound obtained by adding 3 moles of TDI (tolylene diisocyanate) to trimethylolpropane (TMP), a compound obtained by adding 3 moles of HDI (hexamethylene diisocyanate) to TMP, and 3 moles of IPDI (isophorone diisocyanate) to TMP Adduct type polyisocyanate compounds such as a compound obtained by adding 3 moles of XDI (xylylene diisocyanate) to TMP. TDI condensed isocyanurate type trimer, TDI condensed isocyanurate pentamer, TDI condensed isocyanurate heptamer, and mixtures thereof. HDI isocyanurate type condensate, IPDI isocyanurate type condensate. There is also Crude MDI.
Among these, a compound obtained by adding 3 moles of TDI to TMP, an isocyanurate type trimer of TDI, and the like are preferable.

In addition to the isocyanate curing agent, a radiation curable curing agent such as an electron beam or an ultraviolet ray may be used. In this case, a curing agent having two or more, preferably three or more acryloyl groups or methacryloyl groups in the molecule as a radiation curing functional group can be used. Examples include TMP (trimethylolpropane) triacrylate, pentaerythritol tetraacrylate, and urethane acrylate oligomer. In this case, it is preferable to introduce a (meth) acryloyl group into the binder in addition to the curing agent. In the case of ultraviolet curing, a photosensitizer is also used in combination.
It is preferable to add 0 to 80 parts by weight of the curing agent with respect to 100 parts by weight of the binder. Within the above range, dispersibility is good, which is preferable.

  In the case of a magnetic layer, the amount of binder added is preferably 5 to 30 parts by weight, more preferably 10 to 20 parts by weight with respect to 100 parts by weight of the ferromagnetic powder.

Additives can be added to the magnetic layer in the present invention as necessary. Examples of the additive include an abrasive, a lubricant, a dispersant / dispersion aid, an antifungal agent, an antistatic agent, an antioxidant, a solvent, and carbon black.
Examples of these additives include molybdenum disulfide, tungsten disulfide, graphite, boron nitride, graphite fluoride, silicone oil, silicone having a 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, benzylphenylphosphon Acid, α-cumylphosphonic acid, toluylphosphonic acid, xylylphosphonic acid, ethylphenylphosphonic acid, cumenylphosphonic acid, propylphenylphosphonic acid, butylphenylphosphonic acid, heptyl Aromatic ring-containing organic phosphonic acids such as phenylphosphonic acid, octylphenylphosphonic acid, nonylphenylphosphonic acid and alkali metal salts thereof, octylphosphonic acid, 2-ethylhexylphosphonic acid, isooctylphosphonic acid, isononylphosphonic acid, isodecyl Alkylphosphonic acids such as phosphonic acid, isoundecylphosphonic acid, isododecylphosphonic acid, isohexadecylphosphonic acid, isooctadecylphosphonic acid, isoeicosylphosphonic acid and alkali metal salts thereof, phenyl phosphate, benzyl phosphate, phosphorus Phenethyl phosphate, α-methylbenzyl phosphate, 1-methyl-1-phenethyl phosphate, diphenylmethyl phosphate, biphenyl phosphate, benzylphenyl phosphate, α-cumyl phosphate, toluyl phosphate, xylyl phosphate, phosphate Ethylphenyl, 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, isononyl phosphate, isodecyl phosphate, isoundecyl phosphate, isododecyl phosphate, isohexadecyl phosphate, isooctadecyl phosphate, isoeicosyl phosphate, and alkali metal salts thereof, alkylsulfonic 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, oleic acid, linoleic acid, linolenic acid, elaidic acid, erucic acid, etc. Monobasic fatty acids which may contain or be branched by an unsaturated bond having a prime number of 10 to 24 and metal salts thereof, or butyl stearate, octyl stearate, amyl stearate, isooctyl stearate, isohexadecyl stearate, Even if it contains an unsaturated bond having 10 to 24 carbon atoms such as octyl myristate, butyl laurate, butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan distearate, anhydrosorbitan tristearate, etc. A monobasic fatty acid that may be included and a 1 to 6-valent alcohol that may be branched or include an unsaturated bond having 2 to 22 carbon atoms, or may be branched or include an unsaturated bond that has 12 to 22 carbon atoms. Monoalkyl ether of alkoxy alcohol or alkylene oxide polymer Mono-fatty acid esters consisting of any one of, di-fatty acid ester or polyhydric fatty acid esters, fatty acid amides having 2 to 22 carbon atoms, and aliphatic amines having 8 to 22 carbon atoms 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 You may have something.

  Also, 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 sulfonium cations 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 amino alcohols, and amphoteric surfactants such as alkylbetaine type Agents can also be used. These surfactants are described in detail in “Surfactant Handbook” (published by Sangyo Tosho Co., Ltd.).

The dispersant, lubricant, and the like are not necessarily pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposition 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 additives include, for example, NAF-102, castor oil hardened fatty acid, NAA-42, cation SA, Naimine L-201, Nonion E-208, Anon BF, Anon LG, Takemoto, manufactured by NOF Corporation. Oils and fats: FAL-205, FAL-123, Shin Nippon Chemical Co., Ltd .: NJELBU OL, Shin-Etsu Chemical: TA-3, Lion Armor: Armide P, Lion: Duomin TDO, Nisshin Oil : BA-41G, Sanyo Kasei Co., Ltd .: Profan 2012E, New Pole PE61, Ionette MS-400, and the like.

  In the present invention, known organic solvents can be used in the magnetic layer. Organic solvents can be used in any ratio, such as ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, alcohols such as methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, methylcyclohexanol, acetic acid Esters such as methyl, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate and glycol acetate, glycol ethers such as glycol dimethyl ether, glycol monoethyl ether and dioxane, and aromatic hydrocarbons such as benzene, toluene, xylene and cresol , Methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin, chlorobenzene, dichlorobenzene, etc. Fluorinated hydrocarbons, N, can be used N- dimethylformamide, hexane, tetrahydrofuran or the like.

  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.

  These dispersants, lubricants, and surfactants used in the magnetic layer in the present invention can be properly used in the magnetic layer and the nonmagnetic layer described later as needed. For example, of course, the dispersant is not limited to the example shown here, but the dispersant has a property of adsorbing or binding with a polar group, and in the magnetic layer, mainly on the surface of the ferromagnetic powder, as will be described later. In the nonmagnetic layer, it is presumed that the organophosphorus compound adsorbed or bonded to the surface of the nonmagnetic powder mainly by the polar group and once adsorbed is difficult to desorb from the surface of metal or metal compound. Accordingly, the surface of the ferromagnetic powder of the present invention or the nonmagnetic powder surface described later is in a state of being coated with an alkyl group, an aromatic group, etc., so the affinity of the ferromagnetic powder or nonmagnetic powder for the binder resin component In addition, the dispersion stability of the ferromagnetic 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 a part 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 mixing with a ferromagnetic powder before the kneading step, when adding at a kneading step with a ferromagnetic powder, a binder and a solvent, when adding at a dispersing step, when adding after dispersing, when adding just before coating, etc. There is.

In addition, carbon black can be added to the magnetic layer in the present invention as necessary.
As the type of carbon black, furnace for rubber, thermal for rubber, black for color, acetylene black and the like can be used. The carbon black of the magnetic layer should be optimized for the following characteristics depending on the desired effect, and may be more effective when used in combination.

The specific surface area of the carbon black is preferably 100 to 500 m ² / g, more preferably 150 to 400 m ² / g, preferably DBP oil absorption 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 BLACKPEARLS 2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72, manufactured by Cabot Corporation, # 3050B, # 3150B, # 3250B manufactured by Mitsubishi Kasei Kogyo Co., Ltd. , # 3750B, # 3950B, # 950, # 650B, # 970B, # 850B, MA-600, MA-230, # 4000, # 4010, CONDUCTEX SC, Raven 8800, 8000, 7000, 5750, 5250, manufactured by Columbia Carbon Co., Ltd. , 3500, 2100, 2000, 1800, 1500, 1255, 1250, Ketjen Black EC manufactured by Akzo Corporation, and the like.

  Carbon black may be surface-treated with a dispersant or the like, or may be 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. The carbon black that can be used in the present invention can be referred to, for example, “Carbon Black Handbook” (edited by Carbon Black Association).

  These carbon blacks can be used alone or in combination. When carbon black is used, it is preferably used in an amount of 0.1 to 30% by weight based on the weight of the ferromagnetic powder. Carbon black functions to prevent the magnetic layer from being charged, reduce the friction coefficient, impart light-shielding properties, and improve the film strength. These differ depending on the carbon black used. Therefore, these carbon blacks used in the present invention change the type, amount, and combination in the magnetic layer, and depending on the purpose based on the above-mentioned various characteristics such as particle size, oil absorption, conductivity, pH, etc. Of course, it is possible to use them properly, but they should be optimized in each layer.

III. Nonmagnetic Layer The magnetic recording medium of the present invention may have a nonmagnetic layer containing a binder and a nonmagnetic powder on a nonmagnetic support. The nonmagnetic powder that can be used in the nonmagnetic layer may be an inorganic substance or an organic substance. Moreover, you may mix carbon black with a nonmagnetic powder as needed with a nonmagnetic layer.

(Non-magnetic powder)
Next, detailed contents regarding the nonmagnetic layer will be described.
The magnetic recording medium of the present invention may have a nonmagnetic layer (lower layer) containing a binder and nonmagnetic powder on a nonmagnetic support provided with a radiation-cured layer.
For the nonmagnetic layer, magnetic powder may be used as long as the lower layer is substantially nonmagnetic, but it is preferable to use nonmagnetic powder.
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, and metal sulfides.

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 shape of the nonmagnetic powder may be any of acicular, spherical, polyhedral and plate shapes.
The crystallite size of the nonmagnetic powder is preferably 4 nm to 1 μm, and more preferably 40 to 100 nm. A crystallite size in the range of 4 nm to 1 μm is preferred because it does not become difficult to disperse and has a suitable surface roughness.
The average particle size of these non-magnetic powders is preferably 5 nm to 2 μm. However, if necessary, non-magnetic powders having different average particle sizes may be combined, or even a single non-magnetic powder may have a wide particle size distribution. It can also have an effect. The average particle size of the particularly preferred nonmagnetic powder is 10 to 200 nm. The range of 5 nm to 2 μm is preferable because the dispersion is good and the surface roughness is suitable.

The specific surface area of the non-magnetic 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 in the range of 1 to 100 m ² / g is preferred because it has a suitable surface roughness and can be dispersed with a desired amount of binder.
The oil absorption using dibutyl phthalate (DBP) 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 preferably 0.05 to 2 g / ml, more preferably 0.2 to 1.5 g / ml. When the tap density is in the range of 0.05 to 2 g / ml, there are few particles to be scattered, the operation is easy, and there is a tendency that 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 between 6 and 10. A pH in the range of 2 to 11 is preferable because the friction coefficient does not increase due to high temperature, high humidity, or liberation of fatty acids.
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 4-10. A Mohs hardness in the range of 4 to 10 is preferable because durability can be secured. The amount of stearic acid adsorbed by the nonmagnetic powder is 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 ). Further, it is preferable to use a solvent within the range of heat of wetting.
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 treating the surface layer with silica after first treating with alumina, or vice versa may be employed. 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 for the nonmagnetic layer in the present invention include, for example, 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 manufactured by Teica -600B, MT-100F, MT-500HD and the like. 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.

Carbon black can be mixed in the nonmagnetic layer together with nonmagnetic powder to lower the surface electrical resistance, reduce the light transmittance, and obtain the desired μ Vickers hardness. The μ Vickers hardness of the nonmagnetic layer is preferably 25 to 60 kg / mm ² , more preferably 30 to 50 kg / mm ² in order to adjust the head contact, and a thin film hardness meter (NEC HMA-400) is used. By using a diamond triangular pyramid needle having a ridge angle of 80 degrees and a tip radius of 0.1 μm, the tip can be measured. 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 the carbon black used in the non-magnetic layer in the present invention is preferably 100 to 500 m ² / g, more preferably 150~400m ² / g, DBP oil absorption preferably 20 to 400 ml / 100 g, more preferably 30 ~ 200ml / 100g. 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 that can be used for the nonmagnetic layer in the present invention include BLACKPEARLS 2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72, manufactured by Mitsubishi Kasei Kogyo Co., Ltd. 3050B, # 3150B, # 3250B, # 3750B, # 3950B, # 950, # 650B, # 970B, # 850B, MA-600, Columbia Carbon Corporation CONDUCTEX SC, RAVEN8800, 8000, 7000, 5750, 5250, 3500, 2100 2000, 1800, 1500, 1255, 1250, Ketjen Black EC manufactured by Akzo Corporation, and the like.

  Carbon black may be surface-treated with a dispersant, or may be 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 are preferably used in a range not exceeding 50% by weight relative to the nonmagnetic powder and not exceeding 40% of the total weight of the nonmagnetic layer. 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.

  Further, an organic powder can be added to the nonmagnetic layer according to the purpose. Examples of such organic powder 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, and the like. Resin powder and polyfluorinated ethylene resin can also be used. As the production method, those described in JP-A Nos. 62-18564 and 60-255827 can be used.

IV. Nonmagnetic Supports Examples of the nonmagnetic support that can be used in the present invention include known ones such as biaxially stretched polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamideimide, and aromatic polyamide. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferable.
These supports may be subjected in advance to corona discharge, plasma treatment, easy adhesion treatment, heat treatment and the like. The surface roughness of the nonmagnetic support that can be used in the present invention is preferably a center average roughness Ra of 3 to 10 nm at a cutoff value of 0.25 mm.

V. Backcoat layer Generally, magnetic tape for computer data recording is strongly required to have repeated running characteristics as compared with video tape and audio tape. In order to maintain such high storage stability, a backcoat layer can be provided on the surface of the nonmagnetic support opposite to the surface on which the radiation-cured layer and the magnetic layer are provided. The coating material for the back coat layer disperses a particle component such as an abrasive and an antistatic agent and a binder in an organic solvent. Various inorganic pigments and carbon black can be used as the particle component. Moreover, as a binder, resin, such as a nitrocellulose, a phenoxy resin, a vinyl chloride resin, a polyurethane, can be used individually or in mixture, for example.

VI. Layer Configuration In the configuration of the magnetic recording medium used in the present invention, the thickness of the radiation-cured layer is preferably in the range of 0.1 to 1.0 μm, more preferably 0.3 to 0.7 μm, as described above. Moreover, the preferable thickness of a nonmagnetic support body is 3-80 micrometers. The thickness of the backcoat layer provided on the surface opposite to the surface on which the radiation-cured layer and the magnetic layer of the nonmagnetic support are provided is preferably 0.1 to 1.0 μm, more preferably 0. .2 to 0.8 μm.

  The thickness of the magnetic layer is optimized according to the saturation magnetization amount, head gap length, and recording signal band of the magnetic head to be used, but is preferably 0.01 to 0.12 μm, more preferably 0.00. It is 02-0.10 micrometer. The thickness variation rate of the magnetic layer is preferably within ± 50%, more preferably within ± 40%. There may be at least one magnetic layer, and 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, the thickness of the nonmagnetic layer is preferably 0.2 to 3.0 μm, more preferably 0.3 to 2.5 μm, and further preferably 0.4 to 2.0 μm. . 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 T · m (100 G) or less or the coercive force is 7.96 kA / m (100 Oe) or less. It means no coercive force.

VII. Manufacturing Method The step of manufacturing the magnetic layer coating liquid for the magnetic recording medium used in the present invention comprises at least a kneading step, a dispersing step, and a mixing step provided as necessary before and after these steps. Each process may be divided into two or more stages. All raw materials such as ferromagnetic hexagonal ferrite powder or ferromagnetic metal powder, non-magnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant, solvent, etc. used in the present invention are at the beginning or middle of any process. It may be added. In addition, individual raw materials may be added in two or more steps. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion. In order to achieve the object of the present invention, a conventional known manufacturing technique can be used as a partial process. In the kneading step, it is preferable to use a kneading force such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. In the case of using a kneader, all or a part of the ferromagnetic powder or nonmagnetic powder and the binder (provided that 30% or more of the total binder is preferred) is 15 to 100 parts by weight of the ferromagnetic powder or nonmagnetic powder. The kneading process is performed in the range of 500 parts by weight. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Further, glass beads can be used to disperse the coating liquid for magnetic layer and the coating liquid for nonmagnetic layer. 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. A well-known thing can be used for a disperser.

In the method for producing a magnetic recording medium of the present invention, for example, a magnetic layer coating solution is applied to the surface of a nonmagnetic support under running so as to have a predetermined film thickness. Here, a plurality of coating solutions for magnetic layers may be applied sequentially or simultaneously, and a lower nonmagnetic layer coating solution and an upper magnetic layer coating solution may be applied sequentially or simultaneously.
As the coating machine for applying the coating solution for the magnetic layer or the coating solution for the nonmagnetic layer, an 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 and the like can be used. As for these, for example, “Latest Coating Technology” (May 31, 1983) issued by General Technology Center Co., Ltd. can be referred to.

In the case of a magnetic tape, the magnetic layer coating solution is subjected to magnetic field orientation treatment in the longitudinal direction using a cobalt magnet or a solenoid on the ferromagnetic powder contained in the magnetic layer coating solution. 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 fine 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 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 different-pole counter magnet and making the vertical orientation. In particular, when performing high density recording, vertical alignment is preferable. Moreover, it is good also as circumferential orientation using a spin coat.
It is preferable to be able to control the drying position of the coating film 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. . Moreover, moderate preliminary drying can also be performed before entering a magnet zone.

After drying, it is preferable to subject the coating layer 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 the removal of the solvent during drying disappear and the filling rate of the ferromagnetic powder in the magnetic layer is improved, so that a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. Is preferable.
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 very excellent center surface average roughness of 0.1 to 4.0 nm, more preferably 0.5 to 3.0 nm, with a cut-off value of 0.25 mm. A surface having smoothness is preferred. As the method, for example, as described above, a magnetic layer formed by selecting a specific ferromagnetic powder and a binder is subjected to the calendar treatment. As the calendering conditions, the temperature of the calender roll is preferably in the range of 60 to 100 ° C., more preferably in the range of 70 to 100 ° C., particularly preferably in the range of 80 to 100 ° C., and the pressure is preferably 100 to 500 kg / kg. It is preferably carried out by operating under conditions in the range of cm, preferably in the range of 200 to 450 kg / cm, particularly preferably in the range of 300 to 400 kg / cm.

  As heat shrinkage reduction means, there are a method in which heat treatment is performed in a web shape while handling at a low tension, and a method in which heat treatment is performed in a form in which tapes are laminated such as a state of being incorporated in a bulk or a cassette (thermo treatment). The former is less affected by protrusions on the surface of the backcoat layer, but the heat shrinkage rate cannot be greatly reduced. On the other hand, the latter thermo-treatment can greatly improve the heat shrinkage rate, but if it is strongly influenced by the projections on the surface of the backcoat layer, the magnetic layer becomes rough, causing a decrease in output and an increase in noise. In particular, a high-output, low-noise magnetic recording medium can be supplied as a magnetic recording medium with a thermo process. The obtained magnetic recording medium can be cut into a desired size using a cutting machine, a punching machine, or the like.

VIII. Physical Properties The saturation magnetic flux density of the magnetic layer of the magnetic recording medium used in the present invention is preferably 100 to 300 T · m (1,000 to 3,000 G). The coercive force (Hc) of the magnetic layer is preferably 143.3 to 318.4 kA / m (1,800 to 4,000 Oe), more preferably 159.2 to 278.6 kA / m ( 2,000 to 3,500 Oe). The coercive force distribution is preferably narrow, and SFD and SFDr are preferably 0.6 or less, and more preferably 0.2 or less.

The friction coefficient with respect to the head of the magnetic recording medium used in the present invention is preferably 0.5 or less, more preferably 0.3 or less, in the temperature range of −10 to 40 ° C. and humidity of 0 to 95%. The charging position is preferably within −500 to + 500V. The elastic modulus at 0.5% elongation of the magnetic layer is preferably 0.98 to 19.6 GPa (100 to 2,000 kg / mm ² ) in each in-plane direction, and the breaking strength is preferably 98 to 686 MPa (10 ˜70 kg / mm ² ), the elastic modulus of the magnetic recording medium is preferably 0.98 to 14.7 GPa (100 to 1,500 kg / mm ² ) in each in-plane direction, and the residual spread is preferably 0.5%. Hereinafter, the thermal contraction rate at any temperature of 100 ° C. or less is preferably 1% or less, more preferably 0.5% or less, and most preferably 0.1% or less.

The glass transition temperature of the magnetic layer (maximum point of loss elastic modulus measured by dynamic viscoelasticity measured at 110 Hz) is preferably 50 to 180 ° C, and that of the nonmagnetic layer is preferably 0 to 180 ° C. The loss elastic modulus is preferably in the range of 1 × 10 ⁷ to 8 × 10 ⁸ Pa (1 × 10 ⁸ to 8 × 10 ⁹ dyne / cm ² ), and the loss tangent is preferably 0.2 or less. A loss tangent of 0.2 or less is preferred because adhesion failure is unlikely to occur. These thermal characteristics and mechanical characteristics are preferably almost equal within 10% in each in-plane direction of the medium.
The residual solvent contained in the magnetic layer is preferably 100 mg / m ² or less, more preferably 10 mg / m ² or less. The porosity of the coating layer is preferably 30% by volume or less, and more preferably 20% by volume or less for both the nonmagnetic layer and the magnetic layer. The porosity is preferably small in order to achieve high output, but it may be better to ensure a certain value depending on the purpose. For example, in the case of a disk medium in which repeated use is important, the storage stability is often preferable when the porosity is large.

The center surface surface roughness Ra of the magnetic layer measured by the TOPO-3D mirau method is preferably 4.0 nm or less, more preferably 3.0 nm or less, and still more preferably 2.0 nm or less. The maximum height SR _max of the magnetic layer is 0.5 μm or less, the ten-point average roughness SRz is 0.3 μm or less, the center plane peak height SRp is 0.3 μm or less, and the center plane valley depth SRv is 0.3 μm or less. The center surface area ratio SSr is preferably 20 to 80%, and the average wavelength Sλa is preferably 5 to 300 μm. The surface protrusions of the magnetic layer can be arbitrarily set within the range of 0 to 2,000 per 100 μm ² with the size of 0.01 to 1 μm, thereby optimizing electromagnetic conversion characteristics and friction coefficient. It is preferable to do. These can be easily controlled by controlling the surface properties by the filler of the support, the particle size and amount of the powder added to the magnetic layer, the roll surface shape of the calendering treatment, and the like. The curl is preferably within ± 3 mm.

Between the nonmagnetic layer and the magnetic layer in the magnetic recording medium of the present invention, it is easily estimated that these physical characteristics can be changed between the nonmagnetic layer and the magnetic layer according to the purpose. For example, the elastic modulus of the magnetic layer is increased to improve storage stability, and at the same time, the elastic modulus of the nonmagnetic layer is made lower than that of the magnetic layer to improve the contact of the magnetic recording medium with the head.
The magnetic recording medium of the present invention is not particularly limited with respect to a head for reproducing a signal magnetically recorded on the magnetic recording medium, but is preferably used for an MR head. When the MR head is used for reproducing the magnetic recording medium of the present invention, the MR head is not particularly limited, and for example, a GMR head or a TMR head can be used. The head used for magnetic recording is not particularly limited, but the saturation magnetization is preferably 1.0 T or more, and more preferably 1.5 T or more.

  Hereinafter, the present invention will be described more specifically with reference to examples. In the examples, “parts” means “parts by weight” unless otherwise specified.

(Synthesis Example 1)
0.5 mol (68 g) of pentaerythritol (molecular weight 136), 2 mol (296 g) of dimethylolbutanoic acid (molecular weight 148) and p-toluenesulfonic acid in a reaction vessel equipped with a thermometer, a stirrer and a partial reflux condenser 0.5 mmol (87 mg) was charged, 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 (1,776 g) of dimethylolbutanoic acid was added to the reaction mixture and reacted for 5 hours at 140 ° C. under reduced pressure while stirring.
The obtained hyperbranched polyester (A) had an OH value of 4.1 meq / g, a GPC molecular weight (polystyrene equivalent) of 3,900 in terms of number average, and a weight average molecular weight of 12,000.

(Synthesis Example 2)
In a reaction vessel equipped with a thermometer, a stirrer, and a partial reflux condenser, 0.5 mol (67 g) of trimethylolpropane (molecular weight 134), 1.5 mol (201 g) of dimethylolpropionic acid (molecular weight 134) and p- Toluenesulfonic acid 0.5 mmol (87 mg) 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 (1,206 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 2,600 in number average, and a weight average molecular weight of 6,900.

(Synthesis Example 3)
A reaction vessel equipped with a thermometer, a stirrer, and a partial reflux condenser was charged with 390 g of the hyperbranched polyester (A) of Synthesis Example 1, 195 g of MEK (methyl ethyl ketone), and 195 g of toluene, and stirred while stirring under a nitrogen stream. The temperature was raised to ° C and dissolved. Next, 46 mg of dibutyltin dilaurate was added as a catalyst, and further dissolved for 15 minutes. Further, 233 g of a 30 wt% solution of acryloyloxyethyl isocyanate (molecular weight 141) in MEK / toluene (= 1/1) was added and reacted by heating at 90 ° C. for 6 hours to introduce a hyperbranched polyester solution (solid content concentration). 45.5 wt%) (C) was obtained. 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, a stirrer, and a partial reflux condenser was charged with 260 g of the hyperbranched polyester (B) of Synthesis Example 2, 20 g of 4-dimethylaminopyridine, and 63 g of acrylic anhydride (molecular weight 126). Stir at 90 ° C. for 24 hours. After the reaction, the reaction mixture was cooled to room temperature, and the resulting solid was washed with chloroform and methylene dichloride three times and dried to obtain acryloyl group-introduced hyperbranched polyester (D).
This was dissolved in MEK / toluene = 1/1 to prepare a 45.5 wt% hyperbranched polyester solution (E).

Example 1
<Preparation of radiation curable paint>
100 parts by weight of acryloyl-modified hyperbranched polyester solution (C) (solid concentration, 45.5 wt%)
MEK 100 parts by weight This was mixed and stirred for 20 minutes, and then filtered using a filter having an average pore size of 1 μm to prepare a radiation curable coating.

<Preparation of magnetic coating for upper 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 , Major axis length 45 nm, σs111A · m ² / kg (emu / g)) 100 parts were pulverized with an open kneader for 10 minutes,
Vinyl chloride copolymer MR110 30 parts by weight (manufactured by Nippon Zeon Co., Ltd., 30% cyclohexanone solution)
30 parts by weight of polyurethane resin UR8200 (manufactured by Toyobo Co., Ltd., MEK / toluene = 1/1 30% solution)
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 Co., Ltd.) 3 parts by weight Methyl ethyl ketone / toluene = 1/1 200 parts by weight Was added and dispersed in a sand mill for 120 minutes. Polyisocyanate (Coronate 3041) 15 parts by weight (manufactured by Nippon Polyurethane Industry Co., Ltd., MEK / toluene = 1/1 30% solution Stearic acid 1 part by weight Myristic acid 1 part by weight Isohexadecyl stearate 3 parts by weight MEK 100 Part by weight was added, and the mixture was further stirred and mixed for 20 minutes, followed by filtration using a filter having an average pore size of 1 μm to prepare a magnetic paint (magnetic layer coating solution).

<Preparation of nonmagnetic coating for lower layer>
85 parts by weight of acicular iron oxide (long axis length 100 nm, surface treatment layer; alumina, S _BET 52 m ² / g, pH 9.4)
15 parts by weight of carbon black ketjen black EC (manufactured by Nippon EC) was pulverized for 10 minutes with an open kneader, and then 30 parts by weight of vinyl chloride copolymer MR110 (manufactured by Nippon Zeon, 30% cyclohexanone solution)
30 parts by weight of polyurethane resin UR8200 (Toyobo, MEK / toluene = 1/1 30% solution)
20 parts by weight of cyclohexanone was added and kneaded for 60 minutes, and then 200 parts by weight of methyl ethyl ketone / cyclohexanone = 6/4 was added and dispersed in a sand mill for 120 minutes.
15 parts by weight of polyisocyanate coronate 3041 (manufactured by Nippon Polyurethane, MEK / toluene = 1/1 30% solution)
1 part by weight of stearic acid 1 part by weight of myristic acid 3 parts by weight of isooctyl stearate 3 parts by weight of MEK 50 parts by weight After stirring for 20 minutes, the mixture was filtered using a filter having an average pore size of 1 μm, and a non-magnetic paint (non-magnetic paint) Magnetic layer coating solution) was prepared.

  A radiation curable coating is applied to the surface of a polyethylene naphthalate (PEN) support having a thickness of 7 μm and a center average surface roughness Ra of 6.2 nm using a coil bar so that the thickness after drying is 0.5 μm and then dried. In an atmosphere with an oxygen concentration of 200 ppm or less, an electron beam with an acceleration voltage of 100 kV was irradiated and cured so that the absorbed dose was 1 Mrad. Immediately thereafter, a non-magnetic coating was applied on the radiation-cured layer, and a magnetic coating was further coated thereon using a reverse roll so that the thickness after drying was 1.5 μm and 0.1 μm, respectively. With magnetic coating in an undried state, magnetic field orientation is performed with a 5,000 gauss Co magnet and a 4,000 gauss solenoid magnet, and the solvent is dried to form a metal roll-metal roll-metal roll-metal roll-metal roll- The calender treatment by the combination of metal roll and metal roll was performed (speed 100 m / min, linear pressure 300 kg / cm, temperature 90 ° C.), further heat treated at 50 ° C. for 7 days, and slit to a width of 3.8 mm.

(Example 2)
A sample was prepared in the same manner as in Example 1 except that the hyperbranched polyester solution (C) was replaced with the hyperbranched polyester solution (E).

(Comparative Example 1)
In Example 1, a sample was prepared in the same manner except that no radiation curable coating was applied and no electron beam irradiation was performed.

(Comparative Example 2)
A sample was prepared in the same manner as in Example 1 except that 100 parts by weight of the hyperbranched polyester solution (C) was replaced with 45.5 parts by weight of trimethylolpropane triacrylate.

(Comparative Example 3)
A sample was prepared in the same manner as in Example 1 except that 100 parts by weight of the hyperbranched polyester solution (C) was replaced with 45.5 parts by weight of polyester acrylate (modified acryloyl of neopentylglycol adipate, molecular weight 1,000). .

〔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) 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.

(3) Adhesive force A double-sided adhesive tape was applied on the glass plate, a tape sample was applied thereon so that the magnetic layer surface was in contact with the adhesive tape, and the force when peeled off by the 180 ° peeling method was measured with a spring burr.
1 gf (gram weight) is about 9.8 mN.

(4) Elastic modulus Tensile elastic modulus was obtained from a tensile test at 25 ° C. of a sample obtained by applying a radiation-curing coating onto a PEN support, drying and electron beam curing, and a PEN support alone.

(5) Tape dimensional change When the tape was aged in 10 ° C and 10% RH atmosphere for 24 hours and then set in TMA (thermomechanical analyzer) and changed to 30 ° C and 80% RH atmosphere over 1 hour, The dimensional change was determined, and the rate of change relative to the initial length was determined.

(6) Durability of edge part The tape edge part after observing the tape edge after being run 1,000 times repeatedly with DDS3 drive of (2) in 40 ° C and 30% RH environment, and cracks are observed. △, a case where the magnetic layer has fallen off from the crack generation part, and a case where no crack or drop occurred.

In the magnetic recording medium of the present invention, the smoothness of the magnetic layer was improved and the electromagnetic conversion characteristics could be improved. Further, it was revealed that the magnetic recording medium of the present invention has high adhesion and improved durability of the tape edge portion.
Furthermore, the magnetic recording medium of the present invention can achieve both dimensional stability and durability.

Claims (2)

A magnetic recording medium having, in this order, a radiation-cured layer obtained by curing a radiation-curable compound-containing layer by radiation irradiation on a non-magnetic support, and a magnetic layer in which a ferromagnetic powder is dispersed in a binder. ,
The magnetic recording medium, wherein the radiation curable compound includes a hyperbranched polyester having a radiation curable functional group introduced therein.   The magnetic recording medium according to claim 1, further comprising a nonmagnetic layer in which a nonmagnetic powder is dispersed in a binder between the radiation-cured layer and the magnetic layer.