JP2006216149A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium which can exert superior electromagnetic conversion characteristics and durability in high-density magnetic recording/reproducing system which uses an MR head as a reproducing head. <P>SOLUTION: A magnetic recording medium is used to record and reproduce a magnetic signal using a magneto-resistive head. The magnetic recording medium has a magnetic layer including ferromagnetic powder, abrasive and binder on a non-magnetism support object. The Knoop hardness of the abrasive is in the extent of 2,500-7,000 kgf/mm<SP>2</SP>(24.5-68.6 GPa), and the mean particle diameter of the abrasive is ≤±20% of thickness of the magnetic layer. Indentation hardness of the magnetic layer is >50 kg/mm<SP>2</SP>(0.49 GPa) and ≤100 kg/mm<SP>2</SP>(0.98 GPa). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a magnetic recording medium suitable for high-density recording, particularly high-density digital recording.

  Magnetic recording media are used in a wide range of applications such as information recording in business and consumer fields, and data storage backup. In recent years, the amount of data handled has increased rapidly due to the spread of broadband networks and digital broadcasting. For this reason, there is a strong demand for a large capacity storage device for storing data, and high-density recording is becoming more and more essential.

  In recent years, a reproducing head that uses MR (magnetoresistance) as an operating principle has been proposed as a reproducing head suitable for high density, and has started to be used in a hard disk or the like. For example, Patent Documents 1 and 2 disclose a magnetic recording medium for use in a high-density recording / reproducing system using an MR head as a reproducing head. MR heads can produce a reproduction output several times that of an induction type magnetic head that has been used in the past, and do not use induction coils, so that equipment noise such as impedance noise is greatly reduced. By reducing the noise, a large SN ratio can be obtained, and the high density recording characteristics can be dramatically improved.

The MR head is an essential head for high-density recording. However, since a hard material such as Altic is used for the head substrate, only the MR element, shield layer, and insulating layer between the head substrates are selectively worn. Step wear occurs, and the output tends to decrease in the high-density recording area. Also, since the MR element is a thin film, if the entire head is worn much, the MR element itself is worn away. On the other hand, if the amount of abrasive in the magnetic layer is extremely reduced or an excessively soft abrasive is used to prevent head wear, head contamination will not be removed, leading to a decrease in electromagnetic conversion characteristics and an increase in error rate. . For this reason, there has been a demand for a magnetic recording medium that can remove head contamination without causing step wear on a part of the head and overall head wear.
JP 2003-22515 A JP 2003-272124 A

  An object of the present invention is to provide a magnetic recording medium that can exhibit excellent electromagnetic conversion characteristics and durability in a high-density magnetic recording / reproducing system using an MR head as a reproducing head.

Means for achieving the object of the present invention is as follows.
[1] A magnetic recording medium used for recording a magnetic signal and reproducing the signal using a magnetoresistive head (hereinafter referred to as “MR head”),
The magnetic recording medium has a magnetic layer containing a ferromagnetic powder, an abrasive and a binder on a nonmagnetic support.
The Knoop hardness of the abrasive is in the range of 2500 to 7000 kgf / mm ² (24.5 to 68.6 GPa),
The average particle diameter of the abrasive is within ± 20% of the thickness of the magnetic layer,
The magnetic recording medium, wherein the indentation hardness of the magnetic layer is greater than 50 kg / mm ² (0.49 GPa) and 100 kg / mm ² (0.98 GPa) or less.
[2] The magnetic recording medium according to [1], wherein an undercoat layer containing a radiation curable resin and carbon black is provided between the nonmagnetic support and the magnetic layer.
[3] The magnetic recording medium according to [1] or [2], wherein a distance between shields of the MR head is in a range of 100 to 250 nm.
[4] The magnetic recording medium according to any one of [1] to [3], wherein the ferromagnetic powder is a hexagonal ferrite powder.
[5] The magnetic recording medium according to any one of [1] to [4], wherein the thickness of the magnetic layer is in a range of 0.03 to 0.15 μm.

  According to the present invention, it is possible to provide a magnetic recording medium having both electromagnetic conversion characteristics and durability in a high recording density region.

Hereinafter, the present invention will be described in more detail.

The magnetic recording medium of the present invention is a magnetic recording medium used for recording a magnetic signal and reproducing the signal using a magnetoresistive head (MR head). The magnetic recording medium of the present invention has a magnetic layer containing a ferromagnetic powder, an abrasive and a binder on a nonmagnetic support, and the Knoop hardness of the abrasive is 2500 to 7000 kgf / mm ² (24.5 to 68.6 GPa), the average particle diameter of the abrasive is within ± 20% of the thickness of the magnetic layer, and the indentation hardness of the magnetic layer is 50 kg / mm ² (0.49 GPa) It is large and is 100 kg / mm ² (0.98 GPa) or less.

In general, an MR head includes a substrate made of a nonmagnetic material, an insulating layer made of an insulating material, a lower shield layer made of a magnetic material, a lower gap layer made of a nonmagnetic material, and a magnetoresistive resistor. An effect element, an upper gap layer made of a nonmagnetic material, an upper shield layer made of a magnetic material, and a protective layer made of an insulating material are sequentially laminated. Here, a portion sandwiched between the lower shield layer and the upper shield layer is a magnetic gap for reading. As the material of the substrate, alumina titanium carbide (AlTiC: Al ₂ O ₃ .TiC) which is a nonmagnetic material is often used. The insulating layer is made of an insulating material such as alumina (Al ₂ O ₃ ) or silica (SiO ₂ ). Altic constituting the substrate is a harder material than alumina or the like constituting the insulating layer. For this reason, when the MR head and the magnetic layer surface come into contact with each other during reproduction, step wear occurs in which only the MR element, shield layer, and insulating layer between the head substrates are selectively worn, resulting in a decrease in output in the high-density recording area. Cheap. Further, since the MR element is a thin film, if the entire head is worn much, the MR element itself is worn away. On the other hand, if the head is not polished at all, head dirt is not removed, which causes a decrease in electromagnetic conversion characteristics and an increase in error rate.
In contrast, in the present invention, polishing having a Knoop hardness in the range of 2500 to 7000 kgf / mm ² (24.5 to 68.6 GPa) and an average particle diameter within ± 20% of the thickness of the magnetic layer. The agent is used for a magnetic layer having an indentation hardness of greater than 50 kg / mm ² (0.49 GPa) and less than or equal to 100 kg / mm ² (0.98 GPa). Thus, by adding an abrasive that is harder than the insulating layer portion between the head substrates and having a hardness close to the hardness of the head substrate to the magnetic layer, and maintaining an appropriate cushioning property between the magnetic layer and the support, the head Thus, head contamination can be removed without causing step wear and overall wear, and both excellent electromagnetic conversion characteristics and durability can be achieved in a high recording density region.

In the magnetic recording medium of the present invention, Knoop hardness of the abrasive contained in the magnetic layer is 2500~7000kgf / mm ² (24.5~68.6GPa), preferably 2500~6500kgf / mm ² (24.5 -63.7 GPa), more preferably 2500-4500 kgf / mm ² (24.5-44.1 GPa), particularly preferably 3000-3500 kgf / mm ² (29.4-34.3 GPa). If the Knoop hardness of the abrasive contained in the magnetic layer is less than 2500 kgf / mm ² (24.5 GPa), it becomes difficult to remove the head dirt, and the Knoop hardness during the polishing exceeds 7000 kgf / mm ² (68.6 GPa). As a result, the total wear amount of the head is remarkably increased and the durability is deteriorated. Examples of the abrasive having Knoop hardness within the above range include diamond, silicon carbide, boron carbide (B ₄ C), α-alumina, SiO ₂ , and TiC.

  In the present invention, an abrasive having an average particle size within ± 20% of the thickness of the magnetic layer is used for the magnetic layer. In the present invention, as described later, by using an abrasive having an average particle diameter within ± 20% of the thickness of the magnetic layer while maintaining an appropriate cushioning property between the support and the magnetic layer. When the magnetic layer comes into contact with the head, the abrasive slightly protruding from the surface of the magnetic layer can be appropriately dented to the magnetic layer side. This makes it possible to remove head dirt without causing step wear or overall wear of the head. If the average particle size of the abrasive used in the magnetic layer exceeds the magnetic layer thickness + 20%, the abrasive may protrude excessively on the surface of the magnetic layer, which may cause head wear. If the thickness of the magnetic layer is less than -20%, it is difficult to remove the head dirt because the protrusion of the abrasive is small. The average particle diameter of the abrasive is preferably −10 to 10% of the thickness of the magnetic layer, and more preferably −5 to 5%. Moreover, it is preferable that the average particle diameter of an abrasive | polishing agent is 30-150 nm, It is more preferable that it is 40-100 nm, It is especially preferable that it is 50-80 nm.

  Knoop hardness can be calculated as (indentation surface area) / (test load) by pressing a quadrangular pyramid indenter. Further, the average particle size of the abrasive can be obtained as an average value obtained by a particle size distribution measuring device by laser scattering.

  The addition amount of the abrasive to the magnetic layer can be 1 to 10 parts by mass, preferably 1 to 5 parts by mass, and preferably 1 to 3 parts by mass with respect to 100 parts by mass of the ferromagnetic powder. Is more preferable.

In the magnetic recording medium of the present invention, the indentation hardness of the magnetic layer is greater than 50 kg / mm ² (0.49 GPa) and 100 kg / mm ² (0.98 GPa) or less. When the indentation hardness of the magnetic layer is 50 kg / mm ² (0.49 GPa) or less, since the magnetic layer is excessively flexible, when the surface of the magnetic layer comes into contact with the head, the abrasive is recessed and buried in the magnetic layer. It becomes difficult to remove the head dirt, and when it exceeds 100 kg / mm ² (0.98 GPa), the cushioning property between the support and the magnetic layer is insufficient and head wear is caused. The indentation hardness is preferably 60~80kg / mm ² (0.59~0.78GPa), more preferably 60~70kg / mm ² (0.59~0.69GPa). The indentation hardness of the magnetic layer in the present invention is a value measured with a load of 6 mgf by a microindentation tester ENT-1100a manufactured by Elionix.

  In the present invention, by providing an undercoat layer containing a radiation curable compound and carbon black between the nonmagnetic support and the magnetic layer, the medium is imparted with an appropriate cushioning property, and the magnetic layer indentation hardness within the above range is provided. Can be obtained. In the present invention, by increasing the thickness of the undercoat layer, increasing the amount of carbon black added to the undercoat layer, and using soft carbon black for the undercoat layer, the magnetic layer indentation hardness is controlled to decrease. can do. The amount of carbon black in the undercoat layer is preferably 2 to 50 parts by mass, and more preferably 5 to 30 parts by mass with respect to 100 parts by mass of the radiation curable resin. The average particle size of carbon black used for the undercoat layer is preferably 10 to 30 nm. Specifically, carbon black that can be used for a magnetic layer and a nonmagnetic layer described later can be used. The thickness of the undercoat layer is preferably 0.1 to 0.7 μm, and more preferably 0.2 to 0.6 μm.

  The viscosity of the radiation curable resin used for the undercoat layer is preferably 40000 mPa · sec or less, more preferably 10,000 mPa · sec or less, more preferably 1000 to 6000 mPas, from the viewpoint of imparting appropriate cushioning properties to the medium. -More preferably in the range of sec. Here, the viscosity of the radiation curable resin refers to the viscosity measured at 25 ° C. of the resin component (without solvent) before radiation curing. The mass average molecular weight of the radiation curable resin is preferably 200 to 1000, and more preferably 200 to 500.

  Examples of radiation curable resins include acrylic acid esters, acrylamides, methacrylic acid esters, methacrylic acid amides, allyl compounds, vinyl ethers, and vinyl esters. Of these, acrylic acid esters and methacrylic acid esters are preferable, and acrylic acid esters having two or more radiation-curable functional groups are particularly preferable. Examples of the radiation curable functional group include an acryloyl group and a methacryloyl group. Among them, the radiation curable functional group is preferably an acryloyl group.

  The radiation curable resin preferably has an alicyclic ring structure. The alicyclic ring structure has a skeleton such as a cyclo skeleton, a bicyclo skeleton, a tricyclo skeleton, a spiro skeleton, or a dispiro skeleton. Among them, the alicyclic ring structure is preferably a structure composed of a plurality of rings sharing atoms, for example, a structure having a skeleton such as a bicyclo skeleton, a tricyclo skeleton, a spiro skeleton, or a dispiro skeleton. Examples of these skeletons include those that become residues such as polyols and polyamines for forming radiation curable resins such as esters and amides. The radiation curable resin can be obtained by bonding a radiation curable functional group to the residue.

  Since the radiation curable resin having an alicyclic ring structure has a higher glass transition temperature than that of an aliphatic resin, it is possible to reduce adhesion failure in a step after application of the undercoat layer. In addition, by having an alicyclic skeleton such as a cyclohexane ring, bicyclo, tricyclo, or spiro, shrinkage of the coating film due to curing can be reduced, and adhesion to the nonmagnetic support can also be improved.

Specific examples of the radiation curable resin include the following. Cyclopropane diacrylate, cyclopentane diacrylate, cyclohexane diacrylate, cyclobutane diacrylate, dimethylol cyclopropane diacrylate, dimethylol cyclopentane diacrylate, dimethylol cyclohexane diacrylate, dimethylol cyclobutane diacrylate, cyclopropane dimethacrylate, cyclo Pentanedimethacrylate, cyclohexanedimethacrylate, cyclobutanedimethacrylate, dimethylolcyclopropanedimethacrylate, dimethylolcyclopentanedimethacrylate, dimethylolcyclohexanedimethacrylate, dimethylolcyclobutanedimethacrylate, bicyclobutanediacrylate, bicyclooctanediacrylate, bicyclono Nanjiak Rate, bicycloundecane diacrylate, dimethylol bicyclobutane diacrylate, dimethylol bicyclooctane diacrylate, dimethylol bicyclononane diacrylate, dimethylol bicycloundecane diacrylate, bicyclobutane dimethacrylate, bicyclooctane dimethacrylate, bicyclononane dimethacrylate, Bicycloundecane dimethacrylate, dimethylol bicyclobutane dimethacrylate, dimethylol bicyclooctane dimethacrylate, dimethylol bicyclononane dimethacrylate, dimethylol bicycloundecane dimethacrylate, tricycloheptane diacrylate, tricyclodecane diacrylate, tricyclododecane diacrylate , Tricycloundecane Acryl , Tricyclotetradecane diacrylate, tricyclodecane tridecane diacrylate, dimethylol tricycloheptane diacrylate, dimethylol tricyclodecane diacrylate, dimethylol tricyclododecane diacrylate, dimethylol tricycloundecane diacrylate, dimethylol tricyclo Tetradecane diacrylate, dimethylol tricyclodecane tridecane diacrylate, tricycloheptane didimethacrylate, tricyclodecane dimethacrylate, tricyclododecane dimethacrylate, tricycloundecane dimethacrylate, tricyclotetradecane dimethacrylate, tricyclodecane tridecane dimethacrylate, Dimethylol tricycloheptane dimethacrylate, dimethylol trisi Chlodecane dimethacrylate, dimethylol tricyclododecane dimethacrylate, dimethylol tricycloundecane dimethacrylate, dimethylol tricyclotetradecane dimethacrylate, dimethylol tricyclodecane tridecane dimethacrylate, spirooctane diacrylate, spiroheptane diacrylate, spirode Candiacrylate, cyclopentane spirocyclobutane diacrylate, cyclohexane spirocyclopentane diacrylate, spirobicyclohexane diacrylate, dispiroheptadecane diacrylate, dimethylol spirooctane diacrylate, dimethylol spiroheptane diacrylate, dimethylol spirodecane diacrylate , Dimethylolcyclopentane spirocyclobutanedia Relate, dimethylolcyclohexane spirocyclopentane diacrylate, dimethylol spirobicyclohexane diacrylate, dimethylol dispiroheptadecane diacrylate, spirooctane dimethacrylate, spiroheptane dimethacrylate, spirodecane dimethacrylate, cyclopentane spirocyclobutane dimethacrylate, Cyclohexane spirocyclopentane dimethacrylate, spirobicyclohexane dimethacrylate, dispiroheptadecane dimethacrylate, dimethylol spirooctane dimethacrylate, dimethylol spiroheptane dimethacrylate, dimethylol spirodecane dimethacrylate, dimethylolcyclopentane spirocyclobutane dimethacrylate, Dimethylolcyclohexanes B cyclopentane dimethacrylate, dimethylol spirobicyclohexane cyclohexane dimethacrylate, dimethylol spiro hepta-decane dimethacrylate. Among them, preferred are dimethylol tricyclodecane diacrylate, dimethylol bicyclooctane diacrylate, and dimethylol spirooctane diacrylate. Particularly preferred is dimethylol tricyclodecane diacrylate. Specific examples of commercially available compounds include KAYARAD R-684 manufactured by Nippon Kayaku, light acrylate DCP-A manufactured by Kyoeisha Chemical, LUMICURE DCA-200 manufactured by Dainippon Ink and the like. is there.
Furthermore, in the present invention, for example, a radiation curable resin described in JP-A No. 2002-117520 can be used.

  The undercoat layer coating solution can be formed by dissolving a radiation curable resin and carbon black in a suitable solvent. As the solvent, it is preferable to use methyl ethyl ketone (MEK), methanol, ethanol, toluene or the like. The amount of the solvent used can be 2 to 50 on a mass basis, with 1 being a radiation curable resin.

  The undercoat layer coating solution can be cured by irradiating radiation on the nonmagnetic support after coating and drying. It is preferable that the glass transition temperature Tg after hardening is 80-150 degreeC, More preferably, it is 100-130 degreeC. If Tg is 80 ° C. or higher, adhesion failure does not occur in the coating step, and if Tg is 150 ° C. or lower, a high-strength coating film can be obtained.

  Examples of the radiation used in the present invention include electron beams and ultraviolet rays. When ultraviolet rays are used, it is necessary to add a photopolymerization initiator to the undercoat layer coating solution. In the case of electron beam curing, a polymerization initiator is unnecessary, and the penetration depth is also deep. Therefore, it is preferable to use an electron beam as radiation. As the electron beam accelerator, a scanning method, a double scanning method, or a curtain beam method can be adopted. A preferred one is a curtain beam system which is relatively inexpensive and can provide a large output. As the electron beam characteristics, the acceleration voltage is usually 30 to 1000 kV, preferably 50 to 300 kV, and the absorbed dose is usually 0.5 to 10 Mrad, preferably 2 to 5 Mrad. If the acceleration voltage is 30 kV or more, a sufficient amount of energy transmission can be obtained, and if it is 1000 kV or less, the efficiency of energy used for polymerization is high and economical. The atmosphere in which the electron beam is irradiated preferably has an oxygen concentration of 200 ppm or less by nitrogen purge. When the oxygen concentration is high, cross-linking and curing reactions near the surface are inhibited.

  A mercury lamp can be used as the ultraviolet light source. As the mercury lamp, for example, a lamp of 20 to 240 W / cm can be used, and it can be 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 can be used as a photopolymerization initiator used for ultraviolet curing. The details of the photopolymerization initiator are described in, for example, “New Polymer Experiments Vol. 2 Chapter 6 Light / Radiation Polymerization” (published by Kyoritsu Shuppan 1995, edited by Polymer Society). 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 photopolymerization initiator is usually 0.5 to 20 parts by mass, preferably 2 to 15 parts by mass, and more preferably 3 to 10 parts by mass with respect to 100 parts by mass of the radiation curable resin. 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.

  In the present invention, various methods can be used as a method for adjusting the indentation hardness of the magnetic layer. For example, as a binder that varies the three-component ratio (vinyl chloride-urethane-curing agent) of the binder resin in the magnetic layer, and varies the P / B ratio (ratio of inorganic powder such as magnetic substance and binder resin) Methods such as increasing the dispersibility of the ferromagnetic powder using a resin into which a polar functional group has been introduced, and increasing the elastic modulus and glass transition point (Tg) of the binder resin can be used. It is also possible to plasticize the binder by increasing the lubricant, improve the calendar moldability, and control the minute indentation. Further, the microindentation hardness can be adjusted by changing the kind and / or amount of the kneading solvent when preparing the magnetic layer coating solution to change the degree of kneading. Furthermore, by adjusting the calendar conditions (temperature, pressure, calender roll hardness, etc.), introducing a metal calender roll, etc., the calendering is performed relatively strongly, thereby adjusting the micro-indentation hardness of the magnetic layer. be able to.

[Magnetic layer]
In the present invention, examples of the ferromagnetic powder used in the magnetic layer include hexagonal ferrite powder and ferromagnetic metal powder, and it is preferable to use hexagonal ferrite powder. The hexagonal ferrite powder has a hexagonal magnetoplumbite structure, has a very large uniaxial crystal magnetic anisotropy, and has a very high coercive force (Hc). For this reason, the use of hexagonal ferrite powder makes it possible to produce a magnetic recording medium that is excellent in chemical stability, corrosion resistance, and friction resistance and has a high density.

  Examples of the hexagonal ferrite used in the present invention 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 a spinel phase. In addition to 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, elements added with Co-Zn, Co-Ti, Co-Ti-Zr, Co-Ti-Zn, Ni-Ti-Zn, Nb-Zn-Co, Sb-Zn-Co, Nb-Zn, etc. Can be used. Some raw materials and manufacturing methods contain specific impurities.

The particle size is preferably 10 to 100 nm as a hexagonal plate diameter, more preferably 10 to 60 nm, and particularly preferably 10 to 50 nm. In particular, when reproducing with an MR head in order to increase the track density, it is necessary to reduce the noise, so that the plate diameter is preferably 40 nm or less. If it is smaller than 10 nm, stable magnetization cannot be expected due to thermal fluctuation. If it exceeds 100 nm, the noise is high and none of them is suitable for high-density magnetic recording. The plate ratio (plate diameter / plate thickness) is preferably 1-15. More preferably, it is 1-7. When the plate ratio is small, the filling property in the magnetic layer is preferably high, but it is difficult to obtain sufficient orientation. When it is larger than 15, noise increases due to stacking between particles. The specific surface area according to the BET method in this particle size range is 10 to 100 m ² / g. The specific surface area is generally signified as an arithmetic calculation value from the particle plate diameter and plate thickness. 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.

In general, hexagonal ferrite powder having a coercive force Hc of about 500 to 5000 oersted (40 to 398 kA / m) can be produced. A higher Hc is advantageous for high-density recording, but is limited by the capacity of the recording head. The Hc of the hexagonal ferrite used in the present invention is preferably about 2000 to 4000 Oe (160 to 320 kA / m), more preferably 2200 to 3500 Oe (176 to 280 kA / m). When the saturation magnetization of the head exceeds 1.4 Tesla, it is preferably 2200 Oe (176 kA / m) 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. 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 hexagonal ferrite is dispersed, the surface of the hexagonal ferrite powder is treated with a material suitable for the dispersion medium and the polymer. As the surface treatment agent, an inorganic compound or an organic compound can be 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 can be 0.1 to 10% by mass relative to the hexagonal ferrite powder. The pH of the hexagonal ferrite powder is also important for dispersion. Usually, about 4 to 12 has optimum values depending on the dispersion medium and polymer, but about 6 to 11 is selected from the chemical stability and storage stability of the medium. Moisture contained in the hexagonal ferrite powder also affects the dispersion. Although there is an optimum value depending on the dispersion medium and polymer, 0.01 to 2.0% by mass is usually selected. The hexagonal ferrite can be manufactured by (1) mixing a metal oxide replacing barium oxide, iron oxide, iron and boron oxide as a glass forming substance so as to have a desired ferrite composition, and then melting and quenching. Glass crystallization method to obtain barium ferrite crystal powder by making it amorphous and then reheating treatment, (2) neutralizing barium ferrite composition metal salt solution with alkali, Hydrothermal reaction method to obtain barium ferrite crystal powder by liquid phase heating at 100 ° C or higher after removal, washing, drying and grinding, (3) neutralizing barium ferrite composition metal salt solution with alkali, by-product There is a coprecipitation method in which barium ferrite crystal powder is obtained by drying, treating at 1100 ° C. or less, and pulverizing, but the production method is not limited.

  In the present invention, the ferromagnetic metal powder used in the magnetic layer is not particularly limited, but it is preferable to use a ferromagnetic metal powder containing α-Fe as a main component. These ferromagnetic metal powders include Al, Si, S, Sc, Ca, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, other than predetermined atoms. Atoms such as Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, and B may be included. In particular, at least one of Al, Si, Ca, Y, Ba, La, Nd, Co, Ni, and B is preferably included in addition to α-Fe, and further includes at least one of Co, Y, and Al. preferable. The Co content is preferably 0 atom% or more and 40 atom% or less, more preferably 15 atom% or more and 35 atom% or less, more preferably 20 atom% or more and 35 atom% or less with respect to Fe. The Y content is preferably from 1.5 atomic percent to 12 atomic percent, more preferably from 3 atomic percent to 10 atomic percent, and particularly preferably from 4 atomic percent to 9 atomic percent. Al is preferably from 1.5 atomic percent to 12 atomic percent, more preferably from 3 atomic percent to 10 atomic percent, and even more preferably from 4 atomic percent to 9 atomic percent.

  These ferromagnetic metal powders may be treated in advance with a dispersant, lubricant, surfactant, antistatic agent, etc. described later before dispersion. Specifically, Japanese Patent Publication No. 44-14090, Japanese Patent Publication No. 45-18372, Japanese Patent Publication No. 47-22062, Japanese Patent Publication No. 47-22513, Japanese Patent Publication No. 46-28466, Japanese Patent Publication No. 46-38755. No. 47, No. 47-4286, No. 47-12422, No. 47-17284, No. 47-18509, No. 47-18573, No. 39-10307 No. 46-39639, U.S. Pat. Nos. 3,026,215, 3,031,341, 3,100,194, 3,342,005 and 3,389,014.

  The ferromagnetic metal powder may contain a small amount of hydroxide or oxide. As the ferromagnetic metal powder, those obtained by a known production method can be used, and the following methods can be mentioned. A method of reducing a complex organic acid salt (mainly oxalate) with a reducing gas such as hydrogen, a method of reducing iron oxide with a reducing gas such as hydrogen to obtain Fe or Fe-Co particles, a metal carbonyl compound A method of thermal decomposition, a method of reducing by adding a reducing agent such as sodium borohydride, hypophosphite, or hydrazine to an aqueous solution of a ferromagnetic metal, and evaporating the metal in a low-pressure inert gas to form a fine powder. How to get. The ferromagnetic metal powder obtained in this manner is a known gradual oxidation treatment, that is, a method of drying after immersing in an organic solvent, an oxygen-containing gas is sent after immersing in an organic solvent, and an oxide film is formed on the surface. Thereafter, either a drying method or a method of adjusting the partial pressures of oxygen gas and inert gas without using an organic solvent to form an oxide film on the surface can be applied.

The specific surface area by BET method of the ferromagnetic metal powder employed in the magnetic layer is preferably 45~80m ² / g, more preferably 50 to 70 m ² / g. If it is 45 m ² / g or more, low noise is obtained, and if it is 80 m ² / g or less, good surface properties can be obtained. The crystallite size of the ferromagnetic metal powder is preferably 80 to 180 mm, more preferably 100 to 180 mm, and still more preferably 110 to 175 mm. The major axis length of the ferromagnetic metal powder is preferably 0.01 μm or more and 0.15 μm or less, more preferably 0.03 μm or more and 0.15 μm or less, and further preferably 0.03 μm or more and 0.12 μm or less. . The acicular ratio of the ferromagnetic metal powder is preferably 3 or more and 15 or less, and more preferably 5 or more and 12 or less. Σs of the ferromagnetic metal powder is preferably from 100~180A · m ² / kg, more preferably 110~170A · m ² / kg, and more preferably from 125~160A · m ² / kg. The coercive force of the ferromagnetic metal powder is preferably 2000 to 3500 Oe (160 to 280 kA / m), more preferably 2200 to 3000 Oe (176 to 240 kA / m).

The moisture content of the ferromagnetic metal powder is preferably 0.01 to 2%. It is preferable to optimize the moisture content of the ferromagnetic metal powder depending on the type of the binder. The pH of the ferromagnetic metal powder is preferably optimized depending on the combination with the binder used. The range can be 4-12, preferably 6-10. The ferromagnetic metal powder may be surface-treated with Al, Si, P, or an oxide thereof as required. The amount thereof can be 0.1 to 10% with respect to the ferromagnetic metal powder, and the surface treatment is preferable because the adsorption amount of a lubricant such as a fatty acid is 100 mg / m ² or less. The ferromagnetic metal powder may contain inorganic ions such as soluble Na, Ca, Fe, Ni, and Sr. Although it is preferable that these are essentially not present, if they are 200 ppm or less, they hardly affect the characteristics. The ferromagnetic metal powder used in the present invention preferably has fewer vacancies, and its value is 20% by volume or less, more preferably 5% by volume or less. As for the shape, any of needle shape, rice grain shape, or spindle shape may be used as long as the above-mentioned characteristics regarding the particle size are satisfied. The SFD of the ferromagnetic metal powder itself is preferably small and is preferably 0.8 or less. It is preferable to reduce the Hc distribution of the ferromagnetic metal powder. When the SFD is 0.8 or less, the electromagnetic conversion characteristics are good, the output is high, the magnetization reversal is sharp, and the peak shift is small, which is suitable for high density digital magnetic recording. In order to reduce the distribution of Hc, there are methods such as improving the particle size distribution of goethite and preventing sintering in the ferromagnetic metal powder.

[Nonmagnetic layer]
The magnetic recording medium of the present invention can have a nonmagnetic layer containing a nonmagnetic powder and a binder between the nonmagnetic support and the magnetic layer. In the case of having a nonmagnetic layer, the undercoat layer described above can be provided between the support and the nonmagnetic layer, or can be provided between the nonmagnetic layer and the magnetic layer.

Details of the nonmagnetic layer will be described below.
The nonmagnetic layer is not particularly limited as long as it is substantially nonmagnetic, and may contain magnetic powder as long as it is substantially nonmagnetic. “Substantially non-magnetic” means that the non-magnetic layer is allowed to have magnetism within a range that does not substantially reduce the electromagnetic conversion characteristics of the magnetic layer. For example, the residual magnetic flux density is 0.01T. It shows below or that a coercive force is 7.96 kA / m (100 Oe or less), Preferably it has no residual magnetic flux density and a coercive force.

  The nonmagnetic powder used for the nonmagnetic layer can be selected from inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. Examples of inorganic compounds include α-alumina, β-alumina, γ-alumina, θ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, hematite, goethite, corundum, and nitridation with an α conversion rate of 90% or more. Silicon, titanium carbide, titanium oxide, silicon dioxide, tin oxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide, etc. are used alone or in combination. be able to. Particularly preferred are titanium dioxide, zinc oxide, iron oxide and barium sulfate because of their small particle size distribution and many means for imparting functions, and more preferred are titanium dioxide and α-iron oxide. The particle size of these non-magnetic powders is preferably 0.005 to 2 μm, but if necessary, non-magnetic powders having different particle sizes may be combined, or a single non-magnetic powder may have the same particle size distribution. It can also have an effect. The particle size of the nonmagnetic powder is particularly preferably 0.01 μm to 0.2 μm. In particular, when the nonmagnetic powder is a granular metal oxide, the average particle diameter is preferably 0.08 μm or less, and when the nonmagnetic powder is an acicular metal oxide, the major axis length is preferably 0.3 μm or less. Preferably, it is 0.2 μm or less. The tap density is preferably 0.05 to 2 g / ml, more preferably 0.2 to 1.5 g / ml. The water content of the nonmagnetic powder is preferably 0.1 to 5% by mass, more preferably 0.2 to 3% by mass, and still more preferably 0.3 to 1.5% by mass. The pH of the non-magnetic powder can be 2 to 11, and is particularly preferably between 5.5 and 10.

Preferably the specific surface area of the nonmagnetic powder is 1 to 100 m ² / g, more preferably 5~80m ² / g, more preferably from 10 to 70 m ² / g. The crystallite size of the nonmagnetic powder is preferably 0.004 μm to 1 μm, and more preferably 0.04 μm to 0.1 μm. The oil absorption amount using DBP (dibutyl phthalate) 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 shape may be any of a needle shape, a spherical shape, a polyhedron shape, and a plate shape. The Mohs hardness is preferably 4 or more and 10 or less. The SA (stearic acid) adsorption amount of the nonmagnetic powder is preferably 1 to ²⁰ μmol / m ² , more preferably ^{2 to} 15 μmol / m ² , and further preferably 3 to 8 μmol / m ² . The pH is preferably between 3-6. It is preferable that Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , ZnO, and Y ₂ O ₃ exist on the surface of these nonmagnetic powders by surface treatment. Particularly preferred for dispersibility are Al ₂ O ₃ , SiO ₂ , TiO ₂ and ZrO ₂ , and 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 making alumina exist, or vice versa can 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 non-magnetic powders include Showa Denko Nano Tight, Sumitomo Chemical HIT-100, ZA-G1, Toda Kogyo α Hematite DPN-250, DPN-250BX, DPN-245, DPN-270BX, DPN. -500BX, DBN-SA1, DBN-SA3, Ishihara Sangyo Titanium oxide TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100, α-hematite E270, E271, E300, E303, Titanium Industry Titanium Oxide STT-4D, STT-30D, STT-30, STT-65C, α Hematite α-40, Teika MT-100S, MT-100T, MT-150W, MT-500B, MT -600B, MT-100F, MT-500HD, Sakai Chemical FINEX- 5, BF-1, BF- 10, BF-20, ST-M, Dowa Mining Ltd. DEFIC-Y, DEFIC-R, manufactured by Japan Aerosil AS2BM, TiO ₂ P25, Ube Industries 100A, 500A, and the thing which baked it are mentioned. Particularly preferred nonmagnetic powders are titanium dioxide and α-iron oxide.

  By mixing carbon black with the nonmagnetic layer, the surface electrical resistance Rs, which is a known effect, can be reduced, the light transmittance can be reduced, and a desired micro Vickers hardness can be obtained. In addition, it is possible to bring about the effect of storing the lubricant by including carbon black in the nonmagnetic layer. 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 used for the nonmagnetic layer should be optimized for the following characteristics depending on the desired effect, and the effect may be obtained by using it in combination.

The specific surface area of carbon black used for the nonmagnetic layer is preferably 100 to 500 m ² / g, more preferably 150 to 400 m ² / g, and the DBP oil absorption is preferably 20 to 400 ml / 100 g. More preferably, it is 30-400 ml / 100g. The particle size of carbon black is, for example, 5 to 80 nm, preferably 10 to 50 nm, and 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's 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, MA-230, # 4000, # 4010, Columbian Carbon Corporation CONDUCTEX SC, RAVEN 8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255, 1250, Ketjen Black EC manufactured by Akzo, 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 mass relative to the inorganic powder and not exceeding 40% of the total mass of the nonmagnetic layer. These carbon blacks can be used alone or in combination. 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).

  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. As the production method, those described in JP-A Nos. 62-18564 and 60-255827 can be used.

  The binder resin, lubricant, dispersant, additive, solvent, dispersion method, etc. used for the nonmagnetic layer can be those of the magnetic layer described below. In particular, with respect to the binder resin amount, type, additive, and dispersant addition amount and type, known techniques relating to the magnetic layer can be applied.

[Binder]
In the present invention, examples of the binder used for the magnetic layer and the nonmagnetic layer include conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof. The thermoplastic resin has a glass transition temperature of −100 to 150 ° C., a number average molecular weight of 1,000 to 200,000, preferably 10,000 to 100,000, and a polymerization degree of about 50 to 1000. Can be used.

  Examples of such include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal, There are polymers or copolymers containing vinyl ether as a structural unit, polyurethane resins, and various rubber resins. Thermosetting resins or reactive resins include phenolic resins, epoxy resins, polyurethane curable resins, urea resins, melamine resins, alkyd resins, acrylic reactive resins, formaldehyde resins, silicone resins, epoxy-polyamide resins, polyester resins. And a mixture of an isocyanate prepolymer, a mixture of a polyester polyol and a polyisocyanate, a mixture of a polyurethane and a polyisocyanate, and the like. These resins are described in detail in “Plastic Handbook” published by Asakura Shoten. It is also possible to use a known electron beam curable resin for each layer. These examples and their production methods are described in detail in JP-A No. 62-256219. The above resins can be used alone or in combination. Preferred examples include a combination of at least one selected from vinyl chloride resin, vinyl chloride vinyl acetate copolymer, vinyl chloride vinyl acetate vinyl alcohol copolymer, vinyl chloride vinyl acetate vinyl maleic anhydride copolymer and polyurethane resin, or these And a combination of polyisocyanates.

As the polyurethane resin, known ones such as polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, polycaprolactone polyurethane can be used. For all binding agents indicated herein, if necessary in order to obtain more excellent dispersibility and _{durability, -COOM, -SO 3 M, -OSO} 3 M, -P = O (OM) 2, - O—P═O (OM) ₂ (wherein M is a hydrogen atom or an alkali metal base), OH, NR ₂ , N ⁺ R ₃ (R is a hydrocarbon group), epoxy group, SH, CN, etc. It is preferable to use one in which at least one selected polar group is introduced by copolymerization or addition reaction. The amount of such a polar group can be 10 ⁻¹ to 10 ⁻⁸ mol / g, preferably 10 ⁻² to 10 ⁻⁶ mol / g.

  Specific examples of these binders used in the present invention include VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, PKFE manufactured by Union Carbide. MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, MPR-TAO, manufactured by Nissin Chemical Industry Co., Ltd. 1000 W, DX80, DX81, DX82 DX83, 100FD, Nippon Zeon MR-104, MR-105, MR110, MR100, MR555, 400X-110A, Nippon Polyurethane Nipporan N2301, N2302, N2304, Dainippon Ink, Pandex T-5105, T -R3080, T-5 01, Vernock D-400, D-210-80, Crisbon 6109, 7209, Byron UR8200, UR8300, UR-8700, RV530, RV280, Toyobo Co., Ltd. Daiferamin 4020, 5020, 5100, 5300, 9020 9022, 7020, MX5004 manufactured by Mitsubishi Kasei Co., Ltd., Sanprene SP-150 manufactured by Sanyo Kasei Co., Ltd., Saran F310, F210 manufactured by Asahi Kasei Co., Ltd., and the like.

The binder used for the nonmagnetic layer and the magnetic layer can be used in the range of 5 to 50% by mass, preferably in the range of 10 to 30% by mass with respect to the nonmagnetic powder or the ferromagnetic powder. When using a vinyl chloride resin, it is preferable to use 5-30% by mass, when using a polyurethane resin, 2-20% by mass, and polyisocyanate in a range of 2-20% by mass. In the case where head corrosion occurs due to dechlorination, it is possible to use only polyurethane or only polyurethane and isocyanate. In the present invention, when polyurethane is used, the glass transition temperature is −50 to 150 ° C., preferably 0 ° C. to 100 ° C., more preferably 30 ° C. to 90 ° C., the breaking elongation is 100 to 2000%, and the breaking stress is 0.05. It is preferable to use a material having a yield point of 10 to 10 kg / mm ² (0.49 to 98 MPa) and a yield point of 0.05 to 10 kg / mm ² (0.49 to 98 MPa).

  The magnetic recording medium of the present invention can be composed of two or more layers. Therefore, the amount of the binder, the amount of vinyl chloride resin, polyurethane resin, polyisocyanate or other resin in the binder, the molecular weight of each resin forming the magnetic layer, the polar group amount, or the resin described above Of course, it is possible to change the physical characteristics of each layer as required, and rather, it should be optimized in each layer, and a known technique relating to a multilayer magnetic layer can be applied. For example, when changing the amount of binder in each layer, it is effective to increase the amount of binder in the magnetic layer in order to reduce scratches on the surface of the magnetic layer. The amount of binder in the magnetic layer can be increased to provide flexibility.

  Polyisocyanates used in the present invention include tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, triphenylmethane triisocyanate. And the like, the products of these isocyanates and polyalcohols, and the polyisocyanates produced by condensation of isocyanates. Commercially available product names of these isocyanates include Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR, Millionate MTL, Takeda Pharmaceutical Taketake D-102, Takenate D-110N, Takenate D-200, Takenate D-202, Sumitomo Bayer's Death Module L, Death Module IL, Death Module N, Death Module HL, etc. Each layer can be used in the above combination.

[Carbon black]
In the present invention, examples of the carbon black used in the magnetic layer include furnace for rubber, thermal for rubber, black for color, acetylene black and the like. The specific surface area is preferably ⁵ to 500 m ² / g, the DBP oil absorption is 10 to 400 ml / 100 g, and the average particle size is 5 to 300 nm, preferably 10 to 250 nm, more preferably 20 to 200 nm. The pH is preferably 2 to 10, the water content is preferably 0.1 to 10%, and the tap density is preferably 0.1 to 1 g / cc. As specific examples of carbon black used in the present invention, BLACKPEARLS 2000, 1300, 1000, 900, 905, 800, 700, VULCAN XC-72 manufactured by Cabot Corporation, # 80, # 60, # 55 manufactured by Asahi Carbon Co., Ltd. # 50, # 35, Mitsubishi Chemical Industries, Ltd. # 2400B, # 2300, # 900, # 1000 # 30, # 40, # 10B, Columbian Carbon Corporation CONDUCTEX SC, RAVEN 150, 50, 40, 15, RAVEN- Examples include MT-P, Ketjen Black EC manufactured by Japan EC. 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. Carbon black may be dispersed with a binder in advance before being added to the magnetic layer coating solution. 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% with respect to the ferromagnetic powder. Carbon black functions to prevent the magnetic layer from being charged, reduce the coefficient of friction, 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 of the upper magnetic layer and the lower non-magnetic layer, and have various characteristics such as particle size, oil absorption, conductivity, pH, etc. Of course, it is possible to use them according to the purpose, but rather they should be optimized in each layer. In the present invention, for the carbon black that can be used in the magnetic layer, for example, “Carbon Black Handbook” (edited by the Carbon Black Association) can be referred to.

  In the present invention, an abrasive can be added to the nonmagnetic layer, whereby the surface shape can be controlled and the protruding state of the abrasive can be controlled. A well-known abrasive | polishing agent can be used for a nonmagnetic layer, The abrasive | polishing agent which can be used for the magnetic layer described previously can also be used.

[Additive]
In the present invention, additives having a lubricating effect, an antistatic effect, a dispersing effect, a plasticizing effect, and the like can be used for the magnetic layer and the nonmagnetic layer. Specifically, molybdenum disulfide, tungsten disulfide graphite, boron nitride, graphite fluoride, silicone oil, silicone with polar group, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing ester, polyolefin, polyglycol Alkyl phosphate ester and alkali metal salt thereof, alkyl sulfate ester and alkali metal salt thereof, polyphenyl ether, phenylphosphonic acid, α-naphthyl phosphoric acid, phenyl phosphoric acid, diphenyl phosphoric acid, p-ethylbenzenephosphonic acid, phenylphosphinic acid, aminoquinones, Various silane coupling agents, titanium coupling agents, fluorine-containing alkyl sulfates and alkali metal salts thereof, monobasic fatty acids having 10 to 24 carbon atoms (including unsaturated bonds, And a metal salt thereof (Li, Na, K, Cu, etc.) or a monovalent, divalent, trivalent, tetravalent, pentavalent, hexavalent alcohol having 12 to 22 carbon atoms, ( It may contain an unsaturated bond or may be branched), an alkoxy alcohol having 12 to 22 carbon atoms, or a monobasic fatty acid having 10 to 24 carbon atoms (including an unsaturated bond or branched) Any one of monovalent, divalent, trivalent, tetravalent, pentavalent and hexavalent alcohols having 2 to 12 carbon atoms (which may contain unsaturated bonds or may be branched). Mono-fatty acid ester or di-fatty acid ester or tri-fatty acid ester consisting of, fatty acid ester of monoalkyl ether of alkylene oxide polymer, fatty acid amide having 8 to 22 carbon atoms, aliphatic amine having 8 to 22 carbon atoms, etc. can be used. The

  Specific examples of these fatty acids include capric acid, caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, isostearic acid, and the like. . For esters, butyl stearate, octyl stearate, amyl stearate, isooctyl stearate, butyl myristate, octyl myristate, butoxyethyl stearate, butoxydiethyl stearate, 2-ethylhexyl stearate, 2-octyldodecyl palmitate 2-hexyldecyl palmitate, isohexadecyl stearate, oleyl oleate, dodecyl stearate, tridecyl stearate, oleyl erucate, neopentyl glycol didecanoate, ethylene glycol dioleyl, oleyl alcohol, stearyl for alcohols Alcohol, lauryl alcohol, etc. are mentioned. 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. Cationic surfactants, anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, phosphoric acid, sulfate ester group, phosphate ester group, amino acids, aminosulfonic acids, sulfuric acid or phosphate esters of amino alcohol, Amphoteric surfactants such as alkylbedine type 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 100% 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 mass or less, and more preferably 10% by mass or less.

  These lubricants and surfactants used in the present invention have different physical actions, and the type, amount, and combination ratio of lubricants that produce a synergistic effect are optimally determined according to the purpose. It should be done. Adjusting the amount of surfactant to control bleeding on the surface using fatty acids with different melting points in the nonmagnetic layer and magnetic layer, and controlling bleeding on the surface using esters with different boiling points, melting points and polarities. In this case, it is conceivable to improve the stability of coating and increase the lubricating effect by increasing the additive amount of the lubricant in the intermediate layer. Of course, it is not limited to the examples shown here. Generally, the total amount of the lubricant can be in the range of 0.1 to 50% by mass, preferably 2 to 25% by mass, with respect to the ferromagnetic powder or nonmagnetic powder.

  Further, all or a part of the additives used in the present invention may be added in any step of producing the magnetic layer coating solution and the nonmagnetic layer coating solution, for example, mixed with the ferromagnetic powder before the kneading step. In the case of adding in a kneading step with a ferromagnetic powder, a binder and a solvent, in a case of adding in a dispersing step, in a case of adding after dispersing, or in a case of adding just before coating. Moreover, after applying a magnetic layer according to the purpose, the object may be achieved by applying a part or all of the additive by simultaneous or sequential application. Depending on the purpose, a lubricant can be applied to the surface of the magnetic layer after calendering or after completion of the slit. In the present invention, a known organic solvent can be used, and for example, a solvent described in JP-A-6-68453 can be used.

[Layer structure]
In the magnetic recording medium of the present invention, the thickness of the nonmagnetic support is, for example, 2 to 100 μm, preferably 2 to 80 μm. In the case of computer tape, the non-magnetic support has a thickness in the range of 3.0 to 6.5 μm (preferably 3.0 to 6.0 μm, more preferably 4.0 to 5.5 μm). Can be used.

  The magnetic recording medium of the present invention may be a disk-shaped medium in which a nonmagnetic layer and a magnetic layer are provided on both sides of a support, or a tape-shaped medium or a disk-shaped medium provided only on one side. In this case, a back coat layer may be provided on the side opposite to the non-magnetic layer and the magnetic layer in order to obtain effects such as antistatic and curl correction. This thickness is, for example, 0.1 to 4 μm, preferably 0.3 to 2.0 μm. Known undercoat layers and backcoat layers can be used.

  In the magnetic recording medium of the present invention, the thickness of the magnetic layer is optimized by the saturation magnetization amount of the head used, the head gap length, and the band of the recording signal, preferably 0.03 to 0.15 μm, more preferably 0.03 to 0.07 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.

  The thickness of the nonmagnetic layer is usually 0.2 to 5.0 μm, preferably 0.3 to 3.0 μm, and more preferably 1.0 to 2.5 μm. The non-magnetic layer exhibits its effect if it is substantially non-magnetic. For example, even if a small amount of magnetic material is included as an impurity or intentionally, the effect of the present invention is exhibited. Needless to say, the configuration can be considered substantially the same as the invention.

[Back coat layer]
In general, a magnetic tape for recording computer data is strongly required to have repeated running characteristics as compared with a video tape and an audio tape. In order to maintain such high running durability, the backcoat layer preferably contains carbon black and inorganic powder.

  It is preferable to use a combination of two types of carbon black having different average particle sizes. In this case, it is preferable to use a combination of fine particle carbon black having an average particle size of 10 to 20 nm and coarse particle carbon black having an average particle size of 50 to 300 nm. In general, by adding fine carbon black as described above, the surface electrical resistance of the backcoat layer can be set low, and the light transmittance can also be set low. Some magnetic recording devices utilize the light transmittance of the tape and are used for the operation signal. In such a case, the addition of particulate carbon black is particularly effective. In addition, particulate carbon black is generally excellent in retention of liquid lubricants, and contributes to reduction of the friction coefficient when used in combination with lubricants. On the other hand, coarse particulate carbon black having an average particle diameter of 50 to 300 nm has a function as a solid lubricant, and also forms microprotrusions on the surface of the back layer, reducing the contact area, and reducing the friction coefficient. Contributes to the reduction of However, when coarse particle carbon black is used alone, in a harsh running system, the tape is liable to fall off from the backcoat layer, which may increase the error ratio. In the present invention, in consideration of the above points, it is preferable to select carbon black to be used for the backcoat layer.

  Specific products of the particulate carbon black include the following. The average particle size is shown in parentheses. RAVEN2000B (18nm), RAVEN1500B (17nm) (above, Columbia Carbon Co., Ltd.), BP800 (17nm) (Cabot Corp.), PRINTEX90 (14nm), PRINTEX95 (15nm), PRINTEX85 (16nm), PRINTEX75 (17nm) (above, Degussa), # 3950 (16 nm) (Mitsubishi Kasei Kogyo Co., Ltd.).

  Moreover, as an example of a concrete product of coarse particle carbon black, thermal black (270 nm) (made by Khan Kalb), RAVEN MTP (275 nm) (made by Columbia Carbon Co.) can be mentioned.

  In the case where two types of back coat layers having different average particle diameters are used, the content ratio (mass ratio) of fine particle carbon black of 10 to 20 nm and coarse particle carbon black of 50 to 300 nm is the former: the latter = It is preferably in the range of 98: 2 to 75:25, and more preferably in the range of 95: 5 to 85:15.

  The content of carbon black in the backcoat layer (the total amount when two types are used) is usually in the range of 30 to 80 parts by mass with respect to 100 parts by mass of the binder, It is the range of 45-65 mass parts.

  It is preferable to use two types of inorganic powders having different hardnesses. Specifically, it is preferable to use a soft inorganic powder having a Mohs hardness of 3 to 4.5 and a hard inorganic powder having a Mohs hardness of 5 to 9. By adding a soft inorganic powder having a Mohs hardness of 3 to 4.5, the friction coefficient can be stabilized by repeated running. Moreover, when the hardness is within this range, the sliding guide pole is not scraped. Moreover, it is preferable that the average particle diameter of this inorganic powder exists in the range of 30-50 nm.

  Examples of the soft inorganic powder having a Mohs hardness of 3 to 4.5 include calcium sulfate, calcium carbonate, calcium silicate, barium sulfate, magnesium carbonate, zinc carbonate, and zinc oxide. These can be used alone or in combination of two or more.

  The content of the soft inorganic powder in the backcoat layer is preferably in the range of 10 to 140 parts by mass, more preferably 35 to 100 parts by mass with respect to 100 parts by mass of carbon black.

  By adding a hard inorganic powder having a Mohs hardness of 5 to 9, the strength of the backcoat layer is enhanced and the running durability is improved. When these inorganic powders are used together with carbon black or the soft inorganic powder, there is little deterioration even with repeated sliding, and a strong backcoat layer is obtained. In addition, the addition of the inorganic powder provides an appropriate polishing force and reduces the adhesion of shavings to the tape guide pole and the like. In particular, when used in combination with soft inorganic powder, the sliding characteristics with respect to the guide pole having a rough surface are improved, and the friction coefficient of the backcoat layer can be stabilized. The average particle size of the hard inorganic powder is preferably in the range of 80 to 250 nm (more preferably 100 to 210 nm).

Examples of the hard inorganic powder having a Mohs hardness of 5 to 9 include α-iron oxide, α-alumina, and chromium oxide (Cr ₂ O ₃ ). These powders may be used alone or in combination. Of these, α-iron oxide or α-alumina is preferred. Content of hard inorganic powder is 3-30 mass parts normally with respect to 100 mass parts of carbon black, Preferably, it is 3-20 mass parts.

  When the soft inorganic powder and the hard inorganic powder are used in combination in the backcoat layer, the difference in hardness between the soft inorganic powder and the hard inorganic powder is 2 or more (more preferably 2.5 or more, particularly preferably 3 or more). It is preferable to select and use a soft inorganic powder and a hard inorganic powder. The back coat layer preferably contains the two types of inorganic powders having different specific Mohs hardnesses having specific average particle sizes and the two types of carbon blacks having different average particle sizes.

  The back coat layer can contain a lubricant. The lubricant can be appropriately selected from the lubricants listed above as the lubricant that can be used for the nonmagnetic layer or the magnetic layer. In the backcoat layer, the lubricant is usually added in the range of 1 to 5 parts by mass with respect to 100 parts by mass of the binder.

[Non-magnetic support]
In the present invention, as the non-magnetic support, polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, aromatic polyamide, polybenzoxazole and the like are known. Can be used. It is preferable to use a support having a glass transition temperature of 100 ° C. or higher, and it is particularly preferable to use a high-strength support such as polyethylene naphthalate or aramid. Moreover, in order to change the surface roughness of a magnetic surface and a base surface as needed, the laminated type support body as shown in Unexamined-Japanese-Patent No. 3-224127 can also be used. These supports may be subjected in advance to corona discharge treatment, plasma treatment, easy adhesion treatment, heat treatment, dust removal treatment, and the like.

As the non-magnetic support, the center surface average surface roughness Ra measured with a light interference type surface roughness meter HD-2000 manufactured by WYKO is 8.0 nm or less, preferably 4.0 nm or less, more preferably 2.0 nm or less. Are preferably used. These supports preferably have not only a small center plane average surface roughness but also no coarse protrusions of 0.5 μm or more. The surface roughness shape is freely controlled by the size and amount of filler added to the support as required. Examples of these fillers include organic fine powders such as acrylic, in addition to oxides and carbonates such as Ca, Si, and Ti. The maximum height Rmax of the support is 1 μm or less, the ten-point average roughness Rz is 0.5 μm or less, the center surface peak height is Rp of 0.5 μm or less, the center face valley depth Rv is 0.5 μm or less, and the center surface area The rate Sr is preferably 10% to 90%, and the average wavelength λa is preferably 5 μm to 300 μm. In order to obtain the desired electromagnetic conversion characteristics and durability, the surface protrusion distribution of these supports can be arbitrarily controlled by a filler, and each of those having a size of 0.01 μm to 1 μm can be reduced from 0 per 0.1 mm ^2. It can be controlled in the range of 2000.

The F-5 value of the support used in the present invention is preferably 5 to 50 kg / mm ² (49 to 490 MPa). The heat shrinkage rate of the support at 100 ° C. for 30 minutes is preferably 3% or less, more preferably 1.5% or less, and the heat shrinkage rate at 80 ° C. for 30 minutes is preferably 1% or less, more preferably 0. .5% or less. Breaking strength 5~100kg / mm ² (49~980MPa), it is preferred that each elastic modulus is 100~2000kg / mm ² (0.98~19.6GPa). The temperature expansion coefficient is preferably 10 ⁻⁴ to 10 ⁻⁸ / ° C., more preferably 10 ⁻⁵ to 10 ⁻⁶ / ° C. The humidity expansion coefficient is preferably 10 ⁻⁴ / RH% or less, more preferably 10 ⁻⁵ / RH% or less. These thermal characteristics, dimensional characteristics, and mechanical strength characteristics are preferably substantially equal with a difference within 10% in each in-plane direction of the support.

[Production method]
The process for producing the magnetic layer coating liquid and further the nonmagnetic layer coating liquid for the magnetic recording medium of 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. All raw materials such as ferromagnetic powder, non-magnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant, and solvent used in the present invention may be added at the beginning or during any step. In addition, individual raw materials may be added in two or more steps. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion. 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. When the kneader is used, the ferromagnetic powder or nonmagnetic powder and the binder are all or a part thereof (preferably 30% by mass or more of the total binder) and 100 parts of the ferromagnetic powder in the range of 15 to 500 parts. It can be kneaded. 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 magnetic layer coating solution and the nonmagnetic layer coating solution, and it is preferable to use 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.

When a magnetic recording medium having a multi-layer structure is applied in the present invention, a coating solution for forming a nonmagnetic layer is applied and dried, and then a coating solution for forming a magnetic layer is applied and dried (dry on wet). ) Is preferably used.
In addition, when using a method for applying and drying the magnetic layer forming coating solution on the wet surface of the nonmagnetic layer coating solution (wet on wet), the following method may be used. preferable. First, a nonmagnetic layer is first applied by a gravure coating, a roll coating, a blade coating, an extrusion coating apparatus or the like generally used in the coating of a magnetic paint, and the nonmagnetic layer is in a wet state. A method of applying a magnetic layer by a support pressure type extrusion coating apparatus disclosed in Japanese Patent No. 46186, Japanese Patent Laid-Open No. 60-238179, and Japanese Patent Laid-Open No. 2-265672; No. 2, JP-A-2-17971, JP-A-2-265672, and the like, wherein the upper and lower layers are applied almost simultaneously by one application head incorporating two coating liquid passage slits; Thirdly, the upper and lower layers are applied almost simultaneously by an extrusion coating apparatus with a backup roll disclosed in JP-A-2-174965. It is a method. In order to prevent the deterioration of the electromagnetic conversion characteristics of the magnetic recording medium due to the aggregation of the magnetic particles, the inside of the coating head is formed by a method as disclosed in Japanese Patent Application Laid-Open Nos. 62-95174 and 1-2236968. It is desirable to apply shear to the coating solution. Furthermore, it is preferable that the viscosity of the coating solution satisfies the numerical range disclosed in JP-A-3-8471.

  After the coating and drying, the magnetic recording medium is usually subjected to a calendar process. As the calendering roll, a heat-resistant plastic roll or metal roll such as epoxy, polyimide, polyamide, polyimide amide or the like can be used. In particular, in the case of a double-sided magnetic layer, it is preferable to treat with metal rolls. The treatment temperature is preferably 50 ° C. or higher, more preferably 100 ° C. or higher. The linear pressure is preferably 200 kg / cm (196 kN / m) or more, more preferably 300 kg / cm (294 kN / m) or more.

  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 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.

  In the case of a magnetic tape, it can be oriented in the longitudinal direction using a cobalt magnet or solenoid. 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 20 m / min to 1000 m / min, and the temperature of the drying air is 60 ° C. or higher. It is preferred that moderate pre-drying can be performed before entering the magnet zone.

  After the coating and drying, the magnetic recording medium is usually subjected to a calendar process. As the calendering roll, a heat-resistant plastic roll or metal roll such as epoxy, polyimide, polyamide, polyimide amide or the like can be used. In particular, in the case of a double-sided magnetic layer, it is preferable to treat with metal rolls. The treatment temperature is preferably 50 ° C. or higher, more preferably 100 ° C. or higher. The linear pressure is preferably 200 kg / cm (196 kN / m) or more, more preferably 300 kg / cm (294 kN / m) or more.

[Physical properties]
In the magnetic recording medium of the present invention, the saturation magnetic flux density of the magnetic layer is preferably 0.2 to 0.5 T when a ferromagnetic metal powder is used, and preferably 0.1 to 0.5 when a hexagonal ferrite powder is used. 0.3T. The coercive force Hc of the magnetic layer is preferably 159 kA / m (2000 Oe) or more, more preferably 159 kA / m (2000 Oe) to 398 kA / m (5000 Oe), and 159 to 239 kA / m (2000 to 3000 Oe). More preferably. The coercive force distribution is preferably narrow, and the SFD is preferably 0.6 or less. The squareness ratio is preferably 0.55 or more and 0.67 or less in the case of two-dimensional random, more preferably 0.58 or more and 0.64 or less, and in the case of three-dimensional random, 0.45 or more and 0.55 or less. Preferably, in the case of vertical alignment, it is preferably 0.6 or more in the vertical direction, more preferably 0.7 or more, and 0.7 or more when demagnetizing field correction is performed. Preferably, it is 0.8 or more. The orientation degree ratio is preferably 0.8 or more for both two-dimensional random and three-dimensional random. In the case of two-dimensional randomness, the squareness ratios Br, Hc in the vertical direction are preferably within 0.1 to 0.5 times the in-plane direction.

In the case of a magnetic tape, the squareness ratio is preferably 0.7 or more, more preferably 0.8 or more. The friction coefficient with respect to the head of the magnetic recording medium of the present invention is preferably 0.5 or less, more preferably 0.3 or less, in a temperature range of −10 ° C. to 40 ° C. and humidity of 0% to 95%. Preferably, the magnetic surface is 10 ⁴ to 10 ¹² ohm / sq, and the charging position is within −500V to + 500V. The elastic modulus at 0.5% elongation of the magnetic layer is preferably 100 to 2000 kg / mm ² (0.98 to 19.6 GPa) in each in-plane direction, and the breaking strength is preferably 10 to 70 kg / mm ² (98 to 686 MPa), the elastic modulus of the magnetic recording medium is preferably 100 to 1500 kg / mm ² (0.98 to 14.7 GPa) in each in-plane direction, the residual spread is preferably 0.5% or less, and any temperature of 100 ° C. or less The thermal contraction rate at 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 in dynamic viscoelasticity measurement measured at 110 Hz) is preferably 50 ° C. or higher and 120 ° C. or lower, and that of the nonmagnetic layer is 0 ° C. to 100 ° C. Is preferred. The loss elastic modulus is preferably in the range of 1 × 10 ⁹ to 8 × 10 ¹⁰ μN / cm ² , and the loss tangent is preferably 0.2 or less. If the loss tangent is too large, adhesion failure is likely to occur. It is preferable that these thermal characteristics and mechanical characteristics are substantially 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, 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, a larger void ratio is often preferable for running durability.

  The center surface average surface roughness Ra of the magnetic layer is preferably 4.0 nm or less, more preferably 3 nm or less, as measured by an optical interference surface roughness meter HD-2000 manufactured by WYKO in an area of about 250 μm × 250 μm. It is 8 nm or less, more preferably 3.5 nm or less. The maximum height Rmax of the magnetic layer is 0.5 μm or less, the ten-point average roughness Rz is 0.3 μm or less, the central surface peak height Rp is 0.3 μm or less, the central surface valley depth Rv is 0.3 μm or less, the central surface The area ratio Sr is preferably 20% or more and 80% or less, and the average wavelength λa is preferably 5 μm or more and 300 μm or less. The surface protrusions of the magnetic layer can be arbitrarily set in the range of 0.01 to 1 μm in the range of 0 to 2000, and it is preferable to optimize the electromagnetic conversion characteristics and the friction coefficient. 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 process, and the like. The curl is preferably within ± 3 mm.

  When the magnetic recording medium of the present invention has a nonmagnetic layer and a magnetic layer, 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 running durability, 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 used for reproduction using an MR head. The distance between the shields of the MR head used for reproduction is preferably 100 to 250 nm, and more preferably 100 to 200 nm. If the distance between the shields of the MR head used for reproduction is within the above range, it is difficult to be affected by gap loss at a high recording density (for example, a linear recording density of 300 kfci or more), and the influence of output reduction due to head contamination is reduced. Can do.

An inductive head can be used to record a signal on the magnetic recording medium of the present invention. In order to use at a high recording density (for example, a linear recording density of 300 kfci or more), the gap length of the inductive head used for recording is preferably 0.3 μm or less. In order to record a signal on a magnetic recording medium having a high coercive force, it is preferable to use an inductive head having a high saturation magnetic flux density (Bs) of 1.6 T or more, for example.
The magnetic recording medium of the present invention can exhibit excellent electromagnetic characteristics and durability, for example, in a high recording density region having a linear recording density of 300 kfci or more.

  Specific examples and comparative examples of the present invention will be described below, but the present invention is not limited to these examples. In addition, unless otherwise indicated, the display of "part" in an Example shows "mass part".

[Example 1]
For each of the magnetic layer coating solution, the nonmagnetic layer coating solution, and the backcoat layer coating solution, each component described below was kneaded with an open kneader and then subjected to a dispersion treatment with a sand mill. The obtained dispersion was filtered using a filter having an average pore size of 1 μm to prepare a coating solution for forming each layer.

100 parts of magnetic layer coating liquid barium ferrite magnetic powder (Hc: 199 kA / m (2500 Oe), average plate diameter: 0.03 μm)
Sulfonic acid group-containing polyurethane resin 15 parts Carbon black (average particle diameter: 20 nm) 0.5 part Diamond powder 2 parts (Knoop hardness: 6500 kgf / mm ² (63.7 GPa), average particle diameter: 100 nm)
Cyclohexanone 150 parts Methyl ethyl ketone 150 parts Butyl stearate 0.5 parts Stearic acid 1 part

Nonmagnetic layer coating solution Nonmagnetic powder α iron oxide 100 parts Average primary particle size: 0.09 μm
Specific surface area by BET method: 50 m ² / g
pH: 7
DBP oil absorption: 27-38 g / 100 g,
Surface treatment agent: Al ₂ O ₃ (8% by mass)
Carbon black 20 parts Conductex SC-U (manufactured by Colombian Carbon)
Vinyl chloride copolymer 13 parts MR104 (manufactured by Nippon Zeon)
Polyurethane resin 5 parts UR8200 (manufactured by Toyobo)
Phenylphosphonic acid 3.5 parts Butyl stearate 1 part Stearic acid 2 parts Methyl ethyl ketone 205 parts Cyclohexanone 135 parts

Backcoat layer coating solution Nonmagnetic inorganic powder: α-iron oxide 80 parts Average major axis length: 0.15 μm, average needle ratio: 7, BET specific surface area: 52 m ² / g
Carbon black (average particle size: 20 nm) 20 parts Carbon black (average particle size: 100 nm) 3 parts Vinyl chloride copolymer 13 parts Sulfonic acid group-containing polyurethane resin 6 parts Phenylphosphonic acid 3 parts Cyclohexanone 140 parts Methyl ethyl ketone 170 parts Stearic acid 3 parts

The undercoat layer coating solution was prepared as follows.
Radiation curable resin Tripropylene glycol acrylate 100 parts MEK / toluene 400 parts Carbon black (average particle size 20 nm) 10 parts CONDUCTEX SC-U (manufactured by Colombian Carbon)

  An undercoat layer was applied onto a polyethylene phthalate base having a thickness of 5.8 μm using a coil bar so that the thickness after drying was 0.2 μm, and then dried, and an electron beam with an acceleration voltage of 150 kV was applied to the surface with an absorbed dose of 3 Mrad. Irradiated to be cured. A non-magnetic layer coating solution is applied and dried so that the thickness after drying is 1.5 μm, and then a magnetic layer coating solution is applied thereon so that the thickness of the magnetic layer is 0.1 μm. Then, while the magnetic layer was still wet, it was oriented and dried with a magnet having a magnetic force of 0.3T. Thereafter, a surface smoothening treatment was performed at a speed of 100 m / min, a linear pressure of 300 kg / cm (294 kN / m), and a temperature of 90 ° C. with a calendar composed of only metal rolls, and then cured. Then, after applying a 0.5μm thick backcoat layer, slit it to 1/2 inch width, send out the slit product, and attach it so that the nonwoven fabric and razor blade are pressed against the magnetic surface to the device with the winding device The surface of the magnetic layer was cleaned with a tape cleaning device to obtain a tape sample.

[Examples 2-4, 6-8, Comparative Examples 1-3]
Each tape sample was prepared in the same manner as in Example 1 except that the abrasive added to the magnetic layer coating solution was changed from diamond powder to SiC powder, Al ₂ O ₃ powder, and the average particle size was changed as shown in Table 1. Produced.

[Comparative Example 4]
A tape sample was produced in the same manner as in Example 1 except that a nonmagnetic layer was formed without providing an undercoat layer on a 6.0 μm thick polyethylene phthalate base.

[Example 5]
Implemented except that an undercoat layer coating solution in which carbon black was changed to ketjen black (average particle size 20 nm) was applied to a polyethylene phthalate base with a thickness of 5.5 μm so that the thickness after drying was 0.5 μm. A tape sample was prepared in the same manner as in Example 1.

[Comparative Example 5]
Sample 1 except that carbon black was changed to ketjen black (average particle size 20 nm) on a polyethylene phthalate base with a thickness of 5.2 μm, and 20 parts were added so that the thickness after drying was 0.8 μm. A tape sample was prepared in the same manner as described above.

Evaluation Method (1) Indentation Hardness The indentation hardness was measured with a load of 6 mgf using a microindentation tester ENT-1100a manufactured by Elionix.
(2) Step wear After using a 1/2 inch linear system (recording head: inductive head with saturation magnetic flux density (Bs) 1.6T, reproducing head: MR head) for 200 hours, between MR head substrate and substrate The difference in level from the MR element was measured (the dents are indicated by a minus sign). A step wear of less than −20 nm was judged as a practically acceptable level.
(3) Head wear (whole wear), head dirt Using the above system, the wear amount of the MR head after running for 1000 hours was measured, and the head dirt was observed with an optical microscope. A head wear amount of 250 nm or less was judged to be a practically acceptable level.
Regarding head contamination, the level up to Δ was judged to be a practically acceptable level.
○: No contamination occurred in the head sliding portion Δ: No contamination occurred in the vicinity of the head gap portion ×: Contamination occurred in the vicinity of the head gap portion (4) SNR measurement The SNR after running for 200 hours in the above system was measured. S is a signal having a recording density (300 kfci), and N is an integrated value up to a band twice the recording density. The SNR of the magnetic tape of Example 1 at 300 kfci is shown as a relative value with 0 dB.
The results are shown in Table 1.

Evaluation results A Knoop hardness is in the range of 2500 to 7000 kgf / mm ² , an abrasive having an average particle size within ± 20% of the thickness of the magnetic layer is used for the magnetic layer, and the indentation hardness of the magnetic layer is In Examples 1 to 8, which were larger than 50 kg / mm ² (0.49 GPa) and 100 kg / mm ² (0.98 GPa) or less, there was little step wear, head wear and head contamination, and the SNR was good.
On the other hand, in the magnetic tape of Comparative Example 1 in which the abrasive having a Knoop hardness of 1600 kgf / mm ² was used for the magnetic layer, step wear was noticeably generated and the SNR was lowered. In Comparative Example 2, since the abrasive particle size was excessively large with respect to the magnetic layer thickness, the wear amount of the entire head was large and the SNR was lowered. On the other hand, in Comparative Example 3 in which the abrasive particle diameter was excessively small with respect to the magnetic layer thickness, although the step wear and the head wear were small, the head was dirty and the running was impossible. The magnetic tape of Comparative Example 4 in which the magnetic layer indentation hardness exceeded 100 kg / mm ² (0.98 GPa) had a large amount of wear on the entire head. SNR decreased. The magnetic tape of Comparative Example 5 had a magnetic layer indentation hardness of 50 kg / mm ² (0.49 GPa), and although step wear and head wear were suppressed, head contamination occurred remarkably and was unable to run.

  The magnetic recording medium of the present invention is suitable for recording and reproduction in a high-density region.

Claims (5)

A magnetic recording medium used for recording a magnetic signal and reproducing the signal using a magnetoresistive head (hereinafter referred to as “MR head”),
The magnetic recording medium has a magnetic layer containing a ferromagnetic powder, an abrasive and a binder on a nonmagnetic support.
The Knoop hardness of the abrasive is in the range of 2500 to 7000 kgf / mm ² (24.5 to 68.6 GPa),
The average particle diameter of the abrasive is within ± 20% of the thickness of the magnetic layer,
The magnetic recording medium, wherein the indentation hardness of the magnetic layer is greater than 50 kg / mm ² (0.49 GPa) and 100 kg / mm ² (0.98 GPa) or less. The magnetic recording medium according to claim 1, wherein an undercoat layer containing a radiation curable resin and carbon black is provided between the nonmagnetic support and the magnetic layer. The magnetic recording medium according to claim 1, wherein a distance between shields of the MR head is in a range of 100 to 250 nm. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder is a hexagonal ferrite powder. The magnetic recording medium according to claim 1, wherein a thickness of the magnetic layer is in a range of 0.03 to 0.15 μm.