JP4094651B2 - Magnetic recording / reproducing system - Google Patents

Description

  The present invention relates to a coating type high recording density magnetic recording medium, and more particularly, to a magnetic recording medium for high density recording in which a magnetic layer contains a hexagonal ferrite ferromagnetic powder.

  In the field of magnetic disks, 2 MB MF-2HD floppy disks using Co-modified iron oxide have come to be standard installed in personal computers. However, today, when the data capacity to be handled is rapidly increasing, the capacity is not sufficient, and it has been desired to increase the capacity of the floppy disk.

  On the other hand, in the field of magnetic tape, in recent years, with the spread of office computers such as minicomputers, personal computers, and workstations, research on magnetic tapes (so-called backup tapes) for recording computer data as external storage media has been conducted. It is actively done. In the practical use of magnetic tapes for such applications, especially in connection with the downsizing of computers and the increase in information processing capability, there is a strong demand for improvement in recording capacity in order to achieve large recording capacity and miniaturization. It was.

  Furthermore, due to the expansion of the usage environment of magnetic tapes, magnetic tapes can be used under a wide range of environmental conditions (especially temperature and humidity conditions that fluctuate rapidly), reliability for data storage, and repeated use at high speed. In addition, the reliability of performance such as stable recording and reading of data in a large number of times of driving is required more than ever.

  Conventionally, magnetic tapes used in digital signal recording systems are determined for each system, and so-called DLT type, 3480, 3490, 3590, QIC, D8 type, or DDS type magnetic tapes are known. In any system, the magnetic tape used is a relatively thick single-layered ferromagnetic powder having a thickness of about 2.0 to 3.0 μm, a binder and an abrasive on one side of a nonmagnetic support. A magnetic layer containing an agent was provided. On the other side, a back coat layer is provided in order to prevent winding disturbance and to maintain good running durability. However, in general, a magnetic layer having a relatively thick single layer structure as described above has a problem of thickness loss in which output is reduced.

In order to improve the reduction in reproduction output due to the loss of thickness of the magnetic layer, a technique for thinning the magnetic layer is known. For example, a nonmagnetic support containing an inorganic powder and a lower nonmagnetic layer dispersed in a binder and a ferromagnetic metal powder dispersed in the binder while the nonmagnetic layer is in a wet state are 1.0 μm. A tape-like magnetic recording medium provided with an upper magnetic layer having the following thickness is disclosed (see Patent Document 1 below). Also disclosed is a disk-shaped magnetic recording medium having a magnetic layer having a coercive force (Hc) of 111 kA / m (1400 Oe) or more and a magnetic layer thickness of 0.5 μm or less and a nonmagnetic layer containing conductive particles. (See Patent Document 2 below).
JP-A-5-182178 (Claim 1, paragraphs [0009] to [0010] on page 3) JP-A-5-109061 (Claim 1, paragraphs [0009] to [0010] on page 3)

However, with the recent rapid increase in capacity and density of disk-shaped and tape-shaped magnetic recording media, it has become difficult to obtain satisfactory electromagnetic characteristics even with the above-described technology. In particular, when recording / reproduction is performed at a recording density of Gbit / in ² class or higher by increasing the linear recording density and the track density, it is difficult to obtain sufficient electromagnetic conversion characteristics with the conventional magnetic recording medium.
The present invention has been made in view of the above problems, and an object thereof is to provide a magnetic recording medium exhibiting excellent electromagnetic conversion characteristics in high-density recording.

The present inventors diligently studied to obtain a magnetic recording medium having excellent electromagnetic conversion characteristics even in high-density recording of a narrow track and Gbit / in ² class or higher. As a result, in magnetic recording media using hexagonal ferrite ferromagnetic powder as the ferromagnetic powder, the average plate diameter of the hexagonal ferrite ferromagnetic powder is adjusted by taking the recording track width and magnetic layer thickness into consideration. The inventors have found that a magnetic recording medium having excellent electromagnetic conversion characteristics can be obtained even when the density is increased, and have completed the present invention.

That is, the object of the present invention is achieved by the following magnetic recording medium.
(1) A magnetic recording medium used for magnetically recording a signal with a track width of 2.0 μm or less and reproducing the magnetically recorded signal,
The magnetic recording medium has a magnetic layer containing a hexagonal ferrite ferromagnetic powder and a binder on a nonmagnetic support, and the magnetic layer has a thickness of 0.2 μm or less, and the hexagonal ferrite ferromagnetic A magnetic recording medium characterized in that the average plate diameter of the powder is 1/30 or less of the track width of the magnetic recording and 1/2 or less of the thickness of the magnetic layer.
(2) A magnetic recording medium used for magnetically recording a signal with a track width of 2.0 μm or less and reproducing the magnetically recorded signal, the magnetic recording medium being non-magnetic on a nonmagnetic support. A nonmagnetic layer containing magnetic powder and a binder, and a magnetic layer containing a hexagonal ferrite ferromagnetic powder and a binder in this order, wherein the magnetic layer has a thickness of 0.2 μm or less, and the hexagonal ferrite strong A magnetic recording medium characterized in that the average plate diameter of the magnetic powder is 1/30 or less of the magnetically recorded track width and 1/2 or less of the thickness of the magnetic layer.
(3) The magnetic recording medium according to (1) or (2), wherein the bit length for magnetically recording a signal is 0.04 to 0.2 μm. (4) The magnetic layer has a coercive force of 143 to 398 kA / m (1800). The magnetic recording medium according to any one of (1) to (3), which is ˜5000 Oe).
(5) The magnetic recording medium according to any one of (1) to (4), wherein the squareness ratio (SQ) of the magnetic layer is 0.6 or more.
(6) The magnetic recording medium according to any one of (1) to (5) for use in a magnetic recording / reproducing system for reproducing a signal magnetically recorded on the magnetic layer by a magnetoresistive magnetic head (MR head).

  As described above, in the magnetic recording medium of the present invention, the average plate diameter of the hexagonal ferrite ferromagnetic powder used in the magnetic layer is 1/30 or less of the recording track width and 1/2 or less of the thickness of the magnetic layer. And Thus, the present invention can provide a magnetic recording medium that exhibits excellent electromagnetic conversion characteristics in high-density recording.

Embodiments of the magnetic recording medium of the present invention will be described in detail below.
[Recording track width]
The magnetic recording medium of the present invention is used for magnetically recording a signal with a track width of 2.0 μm or less and reproducing the magnetically recorded signal.
In order to achieve magnetic recording at a density higher than Gbit / in ² class, it is essential to improve the track density and the linear recording density. For this purpose, it is desirable to make the recording track width as small as possible. . However, the conventional magnetic recording medium has a problem that sufficient electromagnetic conversion characteristics (S / N ratio) cannot be obtained if the recording track width is reduced. The present invention is a magnetic recording medium capable of obtaining excellent electromagnetic conversion characteristics even when a signal magnetically recorded with a narrow track width of 2.0 μm or less is reproduced.

In the present invention, the recording track width is 2.0 μm or less as described above, but preferably 0.5 to 1.5 μm. A recording track width of 0.5 μm or more is preferable because sufficient tracking can be performed even with the current technology.
In this specification, “magnetically recorded track width” is synonymous with “recorded track width”, and means a width when a signal is magnetically recorded on one track using a recording head. The track width is generally narrower than this, usually 50 to 95% of the track width. The track pitch is wider than this, and is usually 105 to 140% of the recording track width.

  The linear recording density during magnetic recording of the present invention is not particularly limited. The recording bit length is preferably 40 to 200 nm / bit, and more preferably 60 to 120 nm / bit. If the recording bit length is 40 nm / bit or more, it is difficult to be affected by space loss, and the relative size of the hexagonal ferrite ferromagnetic powder relative to the bit length is small, so that the generation of transition noise is reduced. It is preferable because a good SN ratio can be obtained. On the other hand, the recording bit length is preferably 200 nm / bit or less in order to obtain a desired recording density.

[Hexagonal ferrite ferromagnetic powder]
In the magnetic layer of the present invention, hexagonal ferrite ferromagnetic powder is used as the ferromagnetic powder. Examples of hexagonal ferrite ferromagnetic powders include barium ferrite, strontium ferrite, lead ferrite, calcium ferrite substitutes, and Co substitutes.

  Specifically, magnate plumbite type barium ferrite and strontium ferrite, magnate plumbite type ferrite whose particle surface is coated with spinel, and magnate plumbite 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, It may contain atoms such as Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge, and Nb. 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 production methods contain peculiar impurities, but these can also be used in the present invention.

In the present invention, the average plate diameter of the hexagonal ferrite ferromagnetic powder is 1/30 of the recording track width and 1/2 or less of the magnetic layer thickness.
When the recording track width is 2.0 μm or less, the non-uniformity of magnetization at both ends of the reproduction track due to the plate diameter of the hexagonal ferrite magnetic material cannot be relatively ignored. For example, assuming that a hexagonal ferrite ferromagnetic powder with a plate diameter A is arranged in a magnetic layer with a magnetic layer thickness A, the number of magnetic powders in the thickness direction of the magnetic layer is 1 Therefore, it can be easily understood that it is difficult to perform uniform film formation and uniform magnetic recording in the thickness direction of the magnetic layer.
On the other hand, when performing high-density recording, it is necessary to make the magnetic layer thin to reduce adjacent waveform interference, but if the magnetic layer is made thin, the SN ratio is drastically lowered.

  As a result of intensive studies on the above problems, the present inventors have found that the plate diameter of the hexagonal ferrite and the thickness of the magnetic layer are greatly related. That is, the present inventors have increased the number of plate-shaped hexagonal ferrite ferromagnetic powders arranged in the longitudinal direction in the magnetic recording medium for in-plane recording, so that the hexagonal ferrite can be arranged in the thickness direction in a thin magnetic layer. The number of ferromagnetic powders is limited. As a result, it was considered that the uniformity of the magnetic layer in the thickness direction of the magnetic layer was impaired and noise increased.

  Based on the above considerations, the inventors of the present invention have made further studies on the relationship between the average plate diameter of the hexagonal ferrite ferromagnetic powder used and the magnetic layer thickness. By adjusting the diameter to 1/30 or less of the recording track width and to 1/2 or less of the thickness of the magnetic layer, the width of the magnetic material relative to the magnetic layer can be made sufficiently small, and the magnetization at both ends of the track is not uniform. It has been found that noise can be reduced.

  The average plate diameter of the hexagonal ferrite ferromagnetic powder used in the present invention is not particularly limited as long as it is within the above range, but is preferably 10 to 50 nm, and more preferably 15 to 30 nm. The average plate diameter is preferably 1/40 or less of the recording track width, more preferably 1/50 or less, and 1/3 or less, more preferably 1/4 of the thickness of the magnetic layer. Further, the lower limit value of the average plate diameter is 1/150 of the recording track width and 1/15 of the thickness of the magnetic layer. The average plate thickness is preferably 5 to 15 nm, and more preferably 7 to 12 nm. An average plate diameter of 10 nm and an average plate thickness of 5 nm or more are preferable because magnetic anisotropy is maintained and good coercive force (Hc) and thermal stability can be obtained.

  The plate ratio (plate diameter / plate thickness) of the hexagonal ferrite ferromagnetic powder is preferably 2 to 5, and more preferably 2.5 to 4. When the plate ratio is small, the filling property in the magnetic layer becomes high, which is preferable, but sufficient orientation cannot be obtained. On the other hand, if it is too large, noise increases due to stacking between particles. From such a viewpoint, the plate ratio (plate diameter / plate thickness) is preferably in the range of 2-5.

The specific surface area of the hexagonal ferrite ferromagnetic powder according to the BET method is 20 to ²⁰⁰ m ² / g, which is almost the same as the 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. These can be compared by randomly measuring 500 particles from a particle TEM photograph. In many cases, the distribution is not a normal distribution, but when calculated and expressed as a standard deviation with respect to the average size, σ / average size = 0.1 to 2.0. In order to sharpen the particle size distribution, the particle generation reaction system is made as uniform as possible, and the generated particles are subjected to a distribution improvement process. For example, a method of selectively dissolving ultrafine particles in an acid solution is also known.

  The coercive force (Hc) of the hexagonal ferrite ferromagnetic powder is preferably 143 to 398 kA / m (1800 to 5000 Oe), more preferably 167 to 279 kA / m (2100 to 3500 Oe). When the coercive force (Hc) is 143 kA / m or more, it is difficult to receive recording demagnetization and there is no decrease in output. Moreover, if it is 398 kA / m or less, recording by a head is possible and output can be maintained. The coercive force (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) of the hexagonal ferrite ferromagnetic powder is 40 to 80 A · m ² / kg (40 to 80 emu / g). Higher saturation magnetization (σs) is preferable, but tends to be smaller as the particles become finer. In order to improve the saturation magnetization (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 dispersing the ferromagnetic powder, the particle surface of the magnetic powder is also treated with a material suitable for the dispersion medium and 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% by mass with respect to the mass of the magnetic powder. The pH of the magnetic 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. Water contained in the magnetic material also affects the dispersion. Although there is an optimum value depending on the dispersion medium and the polymer, 0.01 to 2.0% is usually selected.

The hexagonal ferrite ferromagnetic powder may be subjected to surface treatment with Al, Si, P, or an oxide thereof as required. The amount is 0.1 to 10% by mass with respect to the mass of the ferromagnetic powder, and surface treatment is preferable because adsorption of a lubricant such as a fatty acid becomes 100 mg / m ² or less. The hexagonal ferrite ferromagnetic powder may contain soluble inorganic ions such as Na, Ca, Fe, Ni, and Sr. Although it is preferable that these are essentially not present, if they are 200 ppm or less, they do not particularly affect the characteristics.

  The hexagonal ferrite ferromagnetic powder is manufactured by mixing (1) a metal oxide that replaces barium oxide, iron oxide, and iron with boron oxide as a glass forming substance so as to have a desired ferrite composition, and then melting. Glass crystallization method to obtain barium ferrite crystal powder by washing and crushing after quenching to amorphous, then reheating treatment, (2) neutralizing barium ferrite composition metal salt solution with alkali After removing the by-products, liquid phase heating at 100 ° C. or higher, followed by washing, drying, and pulverization to obtain a barium ferrite crystal powder, (3) a barium ferrite composition metal salt solution with an alkali Neutralization and removal of by-products, followed by drying, treatment at 1100 ° C. or lower, and pulverization to obtain a barium ferrite crystal powder are available, but the present invention does not select a production method.

[Non-magnetic powder]
The magnetic recording medium of the present invention can have a nonmagnetic layer between the nonmagnetic support and the magnetic layer. As the nonmagnetic layer, a layer mainly composed of a nonmagnetic inorganic powder and a binder is preferable. The nonmagnetic inorganic powder used in the nonmagnetic layer can be selected from inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. Inorganic compounds include, for example, α-alumina, β-alumina, γ-alumina, θ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, hematite, goethite, corundum, 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. can be used alone or in combination. . In particular, titanium dioxide, zinc oxide, iron oxide, and barium sulfate are preferably used from the viewpoint of having a small particle size distribution and a large number of function imparting means, and more preferably titanium dioxide and α-iron oxide.

The particle size of the non-magnetic powder is preferably 0.005 to 2 μm, but if necessary, non-magnetic powders having different particle sizes can be combined, and even a single non-magnetic powder has a particle size. The distribution can be widened to have the same effect. The particle size of the nonmagnetic powder is particularly preferably 0.01 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 of the nonmagnetic powder is 0.05 to 2 g / ml, preferably 0.2 to 1.5 g / ml. The water content of the nonmagnetic powder is 0.1 to 5% by weight, preferably 0.2 to 3% by weight, and more preferably 0.3 to 1.5% by weight. The pH of the non-magnetic powder is 2 to 11, and the pH is particularly preferably between 5.5 and 10. The specific surface area of the non-magnetic powder is 1 to 100 m ² / g, preferably 5 to 80 m ² / g, more preferably 10 to 70 m ² / g. The crystallite size of the nonmagnetic powder is preferably 0.004 to 1 μm, and more preferably 0.04 to 0.1 μm. The oil absorption using DBP (dibutyl phthalate) is 5 to 100 ml / 100 g, preferably 10 to 80 ml / 100 g, and more preferably 20 to 60 ml / 100 g. The specific gravity is 1 to 12, preferably 3 to 6.

The shape of the nonmagnetic powder may be any of acicular, spherical, polyhedral, and plate shapes. The Mohs hardness is preferably 4 to 10. The non-magnetic powder has an SA (stearic acid) adsorption amount of 1 to 20 μmol / m ² , preferably ² to 15 μmol / m ² , and more preferably 3 to 8 μmol / m ² . The pH is preferably between 3-6. 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 , ZnO, Y ₂ O ₃ . 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 non-magnetic powder used in the non-magnetic layer of the present invention include Showa Denko: Nanotite, Sumitomo Chemical: HIT-100, ZA-G1, Toda Kogyo: α-hematite DPN-250, DPN. -250BX, DPN-245, DPN-270BX, DPN-500BX, DBN-SA1, DBN-SA3, manufactured by Ishihara Sangyo: Titanium oxide TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO -55D, SN-100, α-hematite E270, E271, E300, E303, manufactured by Titanium Industry: Titanium oxide STT-4D, STT-30D, STT-30, STT-65C, α-hematite α-40, manufactured by Teika: MT- 100S, MT-100T, MT-150W, MT-500B, MT-600B, MT-100F MT-500HD, Sakai Chemicals: FINEX-25, BF-1, BF-10, BF-20, ST-M, Dowa Mining: DEFIC-Y, DEFIC-R, Nippon Aerosil: AS2BM, TiO2P25, Ube Industries Manufactured: 100A, 500A, and those fired. Particularly preferred nonmagnetic powders are titanium dioxide and α-iron oxide.

<Binder>
In the magnetic layer, nonmagnetic layer, and back coat layer of the present invention, conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof can be used as binders.

  As the thermoplastic resin, those having 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 are used. it can.

  Examples of such thermoplastic resins include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl butyral. , Polymers or copolymers containing vinyl acetal, vinyl ether and the like as structural units, polyurethane resins, and various rubber resins.

  The thermosetting resin or reactive resin preferably has a mass average molecular weight of 200,000 or less in the state of a coating solution. Of these resins, those that do not soften or melt until the resin is thermally decomposed are preferable. Specifically, for example, phenol resin, epoxy resin, polyurethane curable resin, urea resin, melamine resin, alkyd resin, acrylic reaction resin, formaldehyde resin, silicone resin, epoxy-polyamide resin, mixture of polyester resin and isocyanate prepolymer , A mixture of polyester polyol and polyisocyanate, a mixture of polyurethane and polyisocyanate, and the like.

The aforementioned binding agents, in order to obtain a more excellent dispersibility and durability, if _{necessary, -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 ¹ R ² , N ⁺ R ¹ R ² R ³ (R ^{1 to} R ³ are hydrocarbon groups), epoxy group It is preferable to use one in which at least one polar group selected from, -SH, -CN and the like is introduced by copolymerization or addition reaction. In the case of using such a polar group, the amount of the polar group is 10 ⁻¹ to 10 ⁻⁸ mol / g, preferably 10 ⁻² to 10 ⁻⁶ mol / g.

  Details of the above resin are described in detail in “Plastic Handbook” issued by Asakura Shoten. Further, when an electron beam curable resin is used for each layer, not only the coating film strength is improved and the durability is improved, but also the surface is smoothed and the electromagnetic conversion characteristics can be further improved. These examples and their production methods are described in detail in JP-A No. 62-256219.

  Specific examples of commercially available product names of the binders include, for example, Union Carbide Corporation: VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ. , PKHC, PKFE, manufactured by Nissin Chemical Industry: MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, MPR-TAO, manufactured by Denki Kagaku: 1000W , DX80, DX81, DX82, DX83, 100FD, manufactured by Nippon Zeon Co., Ltd .: MR-104, MR-105, MR110, MR100, MR555, 400X-110A, manufactured by Nippon Polyurethanes Co., Ltd .: Nippon Poly N2301, N2302, N2304, Dainippon Ink Made by: Pandex T-5105, T R3080, T-5201, Barnock D-400, D-210-80, Crisbon 6109, 7209, manufactured by Toyobo Co., Ltd .: Byron UR8200, UR8300, UR-8700, RV530, RV280, manufactured by Dainichi Seika Co., Ltd .: Daiferamin 4020, 5020, 5100, 5300, 9020, 9022, 7020, manufactured by Mitsubishi Kasei Co., Ltd .: MX5004, Sanprene SP-150 manufactured by Sanyo Chemical Co., Ltd., Saran F310, F210 manufactured by Asahi Kasei Co., Ltd., and the like.

  The above resins can be used alone or in combination, but preferred are vinyl chloride resin, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-maleic anhydride copolymer. The combination of at least 1 sort (s) chosen from a polymer and a polyurethane resin, or what combined these with polyisocyanate is mentioned.

  As the structure of the polyurethane resin, known structures such as polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, and polycaprolactone polyurethane can be used.

  As a commercial name of the polyurethane resin which can be used by this invention, Toyobo Co., Ltd. UR8200, UR8300, UR8700 etc. are mentioned, for example.

  Examples of the polyisocyanate usable in the present invention include tolylene diisocyanate, 4-4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, triol. Isocyanates such as phenylmethane triisocyanate, products of these isocyanates and polyalcohols, and polyisocyanates produced by condensation of isocyanates can be used.

  Commercially available product names of the isocyanates include, for example, Nippon Polyurethane: Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR Millionate MTL, Takeda Pharmaceutical: Takenate D-102, Takenate D- 110N, Takenate D-200, Takenate D-202, manufactured by Sumitomo Bayer Co., Ltd .: Death module L, Death module IL, Death module N death module HL, etc., which are used alone or by utilizing the difference in curing reactivity Each layer can be used in combination of two or more.

The content of the binder used in the nonmagnetic layer, magnetic layer or backcoat layer of the present invention is in the range of 5 to 50% by mass with respect to the mass of the nonmagnetic powder or hexagonal ferrite ferromagnetic powder. It is preferable to be in the range of ˜30% by mass. When using a vinyl chloride resin, the content of the binder is 5 to 30% by mass. Moreover, when using a polyurethane resin, content of a polyurethane resin shall be 2-20 mass%, and content of polyisocyanate shall be 2-20 mass%. The vinyl chloride resin, polyurethane resin and polyisocyanate are preferably used in combination from the viewpoints of compatibility and cross-linking. However, for example, when head corrosion occurs due to a small amount of dechlorination, only polyurethane or only polyurethane and isocyanate can be used. When a polyurethane resin is used in the present invention, the glass transition temperature is −50 to 150 ° C., preferably 0 ° C. to 100 ° C., the breaking elongation is 100 to 2000%, and the breaking stress is 0.49 to 98 MPa (0.05 to 10 kg / kg). mm ² and the yield point are preferably 0.49 to 98 MPa (0.05 to 10 kg / mm ² ).

  Binder amount in the magnetic layer, nonmagnetic layer and back coat layer of the present invention, vinyl chloride resin, polyurethane resin, polyisocyanate or other resin in the binder, molecular weight of each resin, polar group amount Of course, it is possible to appropriately change the physical properties of the resin described above between the magnetic layer, the non-magnetic layer, and the backcoat layer as necessary. Rather, it should be optimized for each layer. It is possible to apply publicly known techniques relating to the above. 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.

[Additive]
Additives can be added to the magnetic layer, nonmagnetic layer, and backcoat layer of the present invention as needed. As such additives, carbon black, abrasives, lubricants, dispersion aids, antifungal agents, antistatic agents, antioxidants, solvents and the like can be added.

<Carbon black>
Carbon black can be added to the magnetic layer, nonmagnetic layer, and backcoat layer of the present invention for the purpose of preventing charging of the magnetic layer and nonmagnetic layer, reducing the friction coefficient, imparting light shielding properties, and improving the film strength. . As carbon black, for example, furnace for rubber, thermal for rubber, black for color, acetylene black and the like can be used. Specific surface area is 5 to 500 m ² / g, DBP oil absorption is 10 to 400 ml / 100 g, particle size is 5 to 300 nm (5 to 300 mμ), pH is 2 to 10, moisture content is 0.1 to 10%, tap density Is preferably 0.1 to 1 g / ml.

  Examples of commercially available carbon blacks that can be used in the present invention include, for example, Cabot Corporation: BLACKPEARLS 2000, 1300, 1000, 900, 905, 800, 700, VULCAN XC-72, Asahi Carbon Corporation: # 80, # 60, # 55, # 50, # 35, manufactured by Mitsubishi Chemical Industries: # 2400B, # 2300, # 900, # 1000, # 30, # 40, # 10B, manufactured by Colombian Carbon: CONDUCTEX SC, RAVEN 150, 50, 40, 15, RAVEN-MT-P, manufactured by Japan EC Co., Ltd .: Ketjen Black EC and the like. Carbon black may be surface-treated with a dispersant, grafted with a resin, or a part of the surface may be graphitized. Moreover, before adding carbon black to a magnetic layer solution coating material, you may disperse | distribute with a binder previously. 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 mass based on the mass of the hexagonal ferrite ferromagnetic powder. As described above, carbon black has functions such as antistatic, reduction of friction coefficient, provision of light shielding properties, and improvement of film strength, and these vary depending on the type of carbon black used. Therefore, these carbon blacks used in the present invention change the combination of type and mass in the magnetic layer and nonmagnetic layer, and have the above-mentioned characteristics such as particle size, oil absorption, conductivity, and PH. Of course, it is possible to use them according to the purpose, but rather they should be optimized in each layer. For the carbon black that can be used in the present invention, for example, “Carbon Black Handbook (Edited by Carbon Black Association)” can be referred to.

<Abrasive>
In the present invention, an abrasive can be added for the purpose of improving the durability of the magnetic layer. As an abrasive, for example, α-alumina, β-alumina, α carbide of 90% or more, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, silicon carbide titanium carbide, oxidation Known materials having a Mohs hardness of 6 or more, such as titanium, silicon dioxide and boron nitride, can be used alone or in combination. Further, a composite of these abrasives (a product obtained by surface-treating an abrasive with another abrasive) may be used.

The above-mentioned abrasive may contain a compound or element other than the main component, but the effect is not changed if the main component is 90% or more. The particle size of these abrasives is preferably in the range of 0.01 to 2 μm. In particular, in order to improve electromagnetic conversion characteristics, it is preferable that the particle size distribution is narrow. In addition, to improve durability, abrasives with different particle sizes can be combined as needed, and even single abrasives can have the same effect by widening the particle size distribution. It is. It is preferable that the tap density is 0.3 to ² g / ml, the water content is 0.1 to 5%, the pH is 2 to 11, and the specific surface area is 1 to 30 m ² / g. The shape of the abrasive may be any of acicular, spherical and dice, but those having a corner in a part of the shape are preferable because of high polishing properties.

  Examples of commercially available product names of abrasives include, for example, Sumitomo Chemical Co., Ltd .: AKP-12, AKP-15, AKP-20, AKP-30, AKP-50, HIT20, HIT-30, HIT-55, HIT60, HIT70, HIT80, HIT100, manufactured by Reynolds: ERC-DBM, HP-DBM, HPS-DBM, Fujimi Abrasives: WA10000, Uemura Kogyo: UB20, Nippon Kagaku Kogyo: G-5, Kromex U2, Kromex U1, Toda Kogyo Co., Ltd .: TF100, TF140, Ibiden Co., Ltd .: Beta Random Ultra Fine, Showa Mining Co., Ltd .: B-3, Tomei Dia Co., Ltd .: MD150, Lands Co., Ltd .: LS-600F, etc. Is mentioned.

  The above abrasive may be added to the nonmagnetic layer as necessary. By adding to the nonmagnetic layer, the surface shape can be controlled, and the protruding state of the abrasive can be controlled. The particle size and amount of the abrasive added to these magnetic and nonmagnetic layers should of course be set to optimum values.

<Other additives>
As other additives that can be used in the magnetic layer and the nonmagnetic layer of the present invention, those having a lubricating effect, an antistatic effect, a dispersing effect, a plasticizing effect, and the like are used.
For example, molybdenum disulfide, tungsten disulfide graphite, boron nitride, fluorinated graphite, silicone oil, silicone with polar group, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing ester, polyolefin, polyglycol, alkyl phosphoric acid Esters and alkali metal salts thereof, alkyl sulfates and alkali metal salts thereof, polyphenyl ether, phenylphosphonic acid, α-naphthyl phosphoric acid, phenyl phosphoric acid, diphenyl phosphoric acid, p-ethylbenzenephosphonic acid, phenylphosphinic acid, aminoquinones, various silane cups Ring agent, titanium coupling agent, fluorine-containing alkyl sulfate ester and alkali metal salt thereof, monobasic fatty acid having 10 to 24 carbon atoms (which may contain an unsaturated bond or may be branched) And these metal salts (Li, Na, K, Cu, etc.), or monovalent, divalent, trivalent, tetravalent, pentavalent, hexavalent alcohols (including unsaturated bonds) having 12 to 22 carbon atoms And may be branched), alkoxy alcohols having 12 to 22 carbon atoms, monobasic fatty acids having 10 to 24 carbon atoms (which may contain unsaturated bonds or may be branched) and carbon number Mono-fatty acid ester comprising any one of 2 to 12 monovalent, divalent, trivalent, tetravalent, pentavalent and hexavalent alcohols (which may contain an unsaturated bond or may be branched). , Difatty acid ester or trifatty acid ester, 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, and the like.

  Specific examples of these additives include fatty acids such as capric acid, caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, and isostearic acid. Can be mentioned. For esters, butyl stearate, octyl stearate, amyl stearate, isooctyl stearate, butyl myristate, octyl myristate, butoxyethyl stearate, butoxydiethyl stearate, 2-ethylhexyl stearate, 2-octyldodecyl palmi Tate, 2-hexyldecyl palmitate, isohexadecyl stearate, oleyl oleate, dodecyl stearate, tridecyl stearate, oleyl erucate, neopentyl glycol didecanoate, ethylene glycol dioleyl and the like. Examples of alcohols include oleyl alcohol, stearyl alcohol, and lauryl alcohol.

  Surfactants include alkylene oxide, glycerin, glycidol, nonionic surfactants such as alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocycles, phosphonium or sulfonium. Cationic surfactants such as carboxylic acids, sulfonic acids, phosphoric acids, sulfate ester groups, anionic surfactants containing acidic groups such as phosphate ester groups, amino acids, aminosulfonic acids, amino alcohol sulfuric acid or phosphate esters Also, amphoteric surfactants such as alkylbedine type can be used. These surfactants are described in detail in “Surfactant Handbook” (published by Sangyo Tosho Co., Ltd.).

  The above-mentioned 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% or less, and more preferably 10% or less.

The lubricants and surfactants that can be used in the present invention have different physical actions, and the type, mass, and 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. Thus, it is conceivable to improve the stability of coating and to increase the lubrication effect by increasing the amount of lubricant added in the intermediate layer. Of course, it is not limited to the examples shown here.
Generally, the total amount of the lubricant is selected in the range of 0.1 to 50% by mass, preferably 2 to 25% by mass, based on the mass of the hexagonal ferrite ferromagnetic powder or nonmagnetic powder.

  In addition, all or a part of the additives that can be used in the present invention may be added in any step of manufacturing the magnetic layer coating material and the nonmagnetic layer coating material, for example, before the kneading step When mixed with a ferrite ferromagnetic powder, it may be added in a kneading step with the ferromagnetic powder, a binder and a solvent, added in a dispersing step, added after dispersion, or added just before coating. Further, after applying the magnetic layer according to the purpose, the purpose may be achieved by applying a part or all of the additive by simultaneous or sequential application. Further, depending on the purpose, a lubricant can be applied to the surface of the magnetic layer after calendering or after completion of the slit.

[Organic solvent]
Known organic solvents can be used in the present invention. Specifically, for example, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, tetrahydrofuran, etc., methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, methylcyclohexanol at an arbitrary ratio , Alcohols such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycol acetate, glycol ethers such as glycol dimethyl ether, glycol monoethyl ether, dioxane, benzene, toluene, xylene, cresol , Aromatic hydrocarbons such as chlorobenzene, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chloride Hydrin, chlorinated hydrocarbons such as dichlorobenzene, N, N- dimethylformamide, may be used hexane.

  These organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposition products, oxides, and moisture in addition to the main components. These impurities are preferably 30% or less, more preferably 10% or less. The amount of the organic solvent used in the present invention may be changed. 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.

[Non-magnetic support]
The nonmagnetic support used in the present invention is not particularly limited. For example, polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, polyamide, polyimide, polyamideimide, and polysulfone. Known films such as polyaramid, aromatic polyamide, and polybenzoxazole can be used. Among them, it is preferable to use a high-strength support such as polyethylene naphthalate or polyamide. Moreover, in order to change the surface roughness of a magnetic layer surface and a support body surface as needed, a laminated type support body as shown by Unexamined-Japanese-Patent No. 3-224127 can also be used.
These nonmagnetic supports may be subjected in advance to corona discharge treatment, plasma treatment, easy adhesion treatment, heat treatment, dust removal treatment and the like. It is also possible to apply an aluminum or glass substrate as the nonmagnetic support of the present invention.

In order to achieve the object of the present invention, the average surface roughness measured by the Mirau method of TOPO-3D manufactured by WYKO as a nonmagnetic support is SRa of 8.0 nm or less, preferably 4.0 nm or less, more preferably Must be 2.0 nm or less. These nonmagnetic 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 can be freely controlled by the size and content of the filler added to the nonmagnetic support as required. Examples of these fillers include organic fine powders such as acrylic, as well as oxides and carbonates such as Ca, Si, and Ti. The maximum height SRmax of the non-magnetic support is 1 μm or less, the ten-point average roughness SRz is 0.5 μm or less, the center plane peak height is SRp of 0.5 μm or less, the center plane valley depth SRv is 0.5 μm or less, the center The surface area ratio SSr is preferably 10 to 90%, and the average wavelength Sλa is preferably 5 to 300 μm. In order to obtain desired electromagnetic conversion characteristics and durability, the distribution of surface protrusions of these nonmagnetic supports can be arbitrarily controlled by a filler. For example, protrusions having a size of 0.01 to 1 μm are each 0 per 0.1 mm ^2. It can be controlled in the range of ~ 2000.

The F-5 value of the nonmagnetic support used in the present invention is preferably 0.49 to 4.9 MPa (5 to 50 kg / mm ² ), and the heat shrinkage rate of the nonmagnetic 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 0.5% or less, more preferably 0.1% or less. The nonmagnetic support preferably has a breaking strength of 49 to 980 MPa (5 to 100 kg / mm ² ) and an elastic modulus of 980 to 19600 MPa (100 to 2000 kg / mm ² ). The temperature expansion coefficient of the nonmagnetic support is 10 ⁻⁴ to 10 ⁻⁸ / ° C., preferably 10 ⁻⁵ to 10 ⁻⁶ / ° C. The humidity expansion coefficient is 10 ⁻⁴ / RH% or less, 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.

[Back coat layer]
In the case of a tape-like magnetic recording medium, a backcoat layer can be provided on the surface opposite to the surface on which the magnetic layer of the nonmagnetic support is provided in order to obtain effects such as antistatic and curl correction. The backcoat layer is useful for stable running of the tape-like magnetic recording medium. The backcoat layer usually has a thickness of about 0.1 to 1 μm and preferably has conductivity. The backcoat layer can contain carbon black and a binder. As the carbon black and the binder, those described above as the carbon black and the binder can be used.

  The back coat layer further contains a metal oxide having a Mohs hardness of 5 to 9 such as α-alumina and α-iron oxide and having an average particle size in the range of 100 to 210 μm. Thus, even when the tape guide of the recording / reproducing apparatus or the tape guide of the stored cassette and the backcoat layer are repeatedly slid, the fluctuation of the dynamic friction coefficient is small and the backcoat layer has excellent durability. Is preferable. The metal oxide having a Mohs hardness of 5 to 9 is used in the range of 3 to 20 parts by mass with respect to 100 parts by mass of carbon black.

  In the case where the light transmittance is detected by the photosensitive layer through the backcoat layer, the backcoat layer needs to be a transparent layer. Therefore, in this case, it is preferable to adjust the amount of carbon black added in accordance with the light transmittance. When detecting the reflectance, a reflective film such as a metal vapor deposition film can be provided between the nonmagnetic support and the photosensitive layer.

[Layer structure]
In the thickness structure of the magnetic recording medium of the present invention, the thickness of the nonmagnetic support is 2 to 100 μm, preferably 2 to 80 μm. In the case of computer magnetic recording 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. Things are used.

  The thickness of the magnetic layer of the present invention is 0.2 μm or less, preferably 0.03 to 0.15 μm. The thickness variation rate is preferably within ± 20%, and more preferably within ± 5%. 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.

When the magnetic layer is provided on the nonmagnetic support, the thickness of the nonmagnetic layer is 0.2 to 5.0 μm, preferably 0.3 to 3.0 μm, more preferably 1.0 to 2.5 μm. It is.
The nonmagnetic layer of the present invention exhibits its effect if it is substantially a nonmagnetic layer. For example, even if a small amount of magnetic material is intentionally included as an impurity, the effect of the present invention is exhibited. Needless to say, it can be regarded as substantially the same configuration as the present invention. Here, the substantially nonmagnetic layer means that the residual magnetic flux density of the nonmagnetic layer is 50 T · m (500 G) or less, or the coercive force (Hc) is 39.8 kA / m (500 Oe) or less, Preferably it means having no residual magnetic flux density and coercive force.

  An undercoat layer for improving adhesion may be provided between the nonmagnetic support and the nonmagnetic layer or magnetic layer. When providing an undercoat layer, the thickness of the undercoat layer is 0.01 to 0.5 μm, preferably 0.02 to 0.5 μm. Moreover, when providing a backcoat layer, the thickness is 0.1-4 micrometers, Preferably it is 0.3-2.0 micrometers.

  The magnetic recording medium of the present invention is a double-sided magnetic layer disk-like magnetic recording medium in which a non-magnetic layer and a magnetic layer are provided on both sides of a non-magnetic support. It may be a recording medium.

[Physical properties]
The saturation magnetic flux density of the magnetic layer in the magnetic recording medium of the present invention is 100 to 300 T · m (1000 to 3000 G). The coercive force (Hc) of the magnetic layer is 143 to 398 kA / m (1800 to 5000 Oe), preferably 167 to 279 kA / m (2100 to 3500 Oe). The distribution of coercive force (Hc) is preferably narrow, and SFD and SFDr are 0.6 or less, more preferably 0.2 or less.

  In a deep magnetic recording medium, the squareness ratio (SQ) is preferably 0.6 or more because of high output. There is no particular upper limit for SQ, but if it exceeds 0.90, noise increases due to stacking of hexagonal ferrite ferromagnetic powder, which is not preferable. In the disk-shaped magnetic recording medium, when random orientation is performed, SQ is preferably 0.45 to 0.65, and SQ is preferably isotropic in the disk. When the circumferential orientation is performed, SQ is preferably 0.6 or more in the circumferential direction as in the tape-like magnetic recording medium.

The friction coefficient with respect to the head of the magnetic recording medium of the present invention is 0.5 or less, preferably 0.3 or less in the range of temperature -10 to 40 ° C. and humidity 0 to 95%, and the surface resistivity is preferably 10 ⁴ to 10. 10 ¹² Ω / sq and the charging position is preferably −500 to 500V. The elastic modulus at 0.5% elongation of the magnetic layer is preferably 980 to 19600 MPa (100 to 2000 kg / mm ² ) in each in-plane direction, and the breaking strength is preferably 98 to 686 MPa (10 to 70 kg / mm ² ). The elastic modulus of the magnetic recording medium is preferably 980 to 14700 MPa (100 to 1500 kg / mm ² ) in each in-plane direction, the residual elongation is preferably 0.5% or less, and the heat shrinkage rate at any temperature of 100 ° C. or less is preferred. Is 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 120 ° C, and that of the nonmagnetic layer is preferably 0 to 100 ° 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. If the loss tangent is too large, adhesion failure is likely 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, more preferably 20% by volume or less for both the nonmagnetic lower 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 surface roughness Ra of the magnetic layer measured by the TOPO-3D mirau method is 4.0 nm or less, preferably 3.0 nm or less, more preferably 2.0 nm or less. The maximum height SRmax 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, the center plane valley depth SRv is 0.3 μm or less, the center plane The 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 0.01 to 1 [mu] m 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.

[Production method]
The process for producing the coating material for the magnetic layer of the magnetic recording medium of the present invention comprises at least a kneading process, a dispersing process, and a mixing process provided as necessary before and after these processes. Each process may be divided into two or more stages. All raw materials such as magnetic materials, nonmagnetic powders, binders, carbon black, abrasives, antistatic agents, lubricants and solvents 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 material having a strong kneading force such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. When using a kneader, hexagonal ferrite ferromagnetic powder or non-magnetic powder and all or a part of the binder (however, 30% or more of the total binder is preferable) and 15 to 500 parts by weight with respect to 100 parts by mass of the ferromagnetic powder. The kneading process is performed in the range of parts by mass. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Glass beads can be used to disperse the magnetic layer coating material and the nonmagnetic layer coating material, but zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media, are preferred. The particle diameter and filling rate of these dispersion media are optimized. A well-known thing can be used for a disperser.

When applying a magnetic recording medium having a multi-layer structure in the present invention, the following method is preferably used.
(1) First, apply a nonmagnetic layer with a gravure coating, roll coating, blade coating, extrusion coating device, etc., which are generally used for coating a magnetic layer coating. A method of applying a magnetic layer coating with a support pressure type extrusion coating apparatus disclosed in JP-A-46186, JP-A-60-238179, or JP-A-2-265672.
(2) One coating head containing two coating liquid passage slits as disclosed in JP-A-63-88080, JP-A-2-17971, and JP-A-2-265672 is used to A method of applying the lower layer almost simultaneously.
(3) A method in which a magnetic layer and a nonmagnetic layer are applied almost simultaneously by an extrusion coating apparatus with a backup roll disclosed in JP-A-2-174965.

  In order to prevent a decrease in electromagnetic conversion characteristics of the magnetic recording medium due to the aggregation of the magnetic particles, a coating head is used by a method as disclosed in JP-A-62-95174 and JP-A-1-236968. It is desirable to apply shear to the internal coating solution. Furthermore, the viscosity of the coating solution needs to satisfy the numerical range disclosed in Japanese Patent Laid-Open No. 3-8471.

  In order to realize the configuration of the present invention, it is of course possible to use a sequential multilayer coating in which a magnetic layer is provided on the nonmagnetic layer coating material after it has been dried, and the effects of the present invention are not lost. Absent. However, in order to reduce coating defects and improve quality such as dropout, it is preferable to use the above-mentioned simultaneous multilayer coating.

  In the case of a disk-shaped magnetic recording medium, a sufficiently isotropic orientation may be obtained even without orientation without using an orientation device, but cobalt magnets are arranged alternately at an angle, and an alternating magnetic field is applied by a solenoid. It is preferable to use a known random orientation device. The isotropic orientation generally tends to be in-plane and vertical three-dimensional random in the case of hexagonal ferrite ferromagnetic powder, but it can also be in-plane two-dimensional random. Alternatively, circumferential orientation may be performed using spin coating.

  In the case of a tape-like magnetic recording medium, it is oriented in the longitudinal direction using a cobalt magnet or a 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 preferably 20 to 1000 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.

  The calendering roll is treated with a heat-resistant plastic roll or metal roll such as epoxy, polyimide, polyamide, polyimide amide, etc. In particular, when a double-sided magnetic layer is used, it is preferably treated 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 or more, more preferably 300 kg / cm or more.

  In addition, it is preferable to perform surface treatment with a polishing tape made of alumina, chromium oxide, diamond or the like for both the tape-like magnetic recording medium and the disk-like magnetic recording medium because protrusions and foreign matters can be removed.

The need for image recording has become stronger not only in the industrial world but also at home, and the magnetic recording medium of the present invention is not only data such as characters and numbers, but also functions as an image recording medium. It has the ability to fully meet the cost requirements.
The magnetic recording medium of the present invention can be suitably used in a magnetic recording / reproducing system using a magnetoresistive reproducing head (MR head). The type of MR head is not particularly limited, and a GMR head or a TMR head can also be used. The head used for recording is not particularly limited, but the saturation magnetization is preferably 1.2 T or more, more preferably 2.0 T or more.
The magnetic recording medium of the present invention is suitable for computer data recording.

  Hereinafter, specific examples of the present invention will be described, but the present invention should not be limited thereto. In addition, "part" in an Example shows a mass part unless otherwise indicated.

( Reference Example 1)
Preparation of coating material for magnetic layer and coating material for non-magnetic layer The barium ferrite magnetic powder of the coating material for magnetic layer was as shown in Table 1 below.

<Coating composition for magnetic layer>
Barium ferrite magnetic powder 100 parts Polyurethane resin UR8200 (manufactured by Toyobo Co., Ltd.) 8 parts UR8300 (manufactured by Toyobo Co., Ltd.) 4 parts α-alumina HIT55 (manufactured by Sumitomo Chemical Co., Ltd.) 4 parts Diamond MD150 (manufactured by Tomei Diamond Co., Ltd.) 1 part Carbon black # 50 ( 1 part Phenylphosphonic acid 1 part Butyl stearate 10 parts Butoxyethyl stearate 5 parts Isohexadecyl stearate 3 parts Stearic acid 2 parts Methyl ethyl ketone 125 parts Cyclohexanone 125 parts

<Composition for nonmagnetic layer>
Nonmagnetic powder α-Fe ₂ O ₃ hematite 80 parts Long axis length: 0.08 μm,
Specific surface area by BET method: 60 m ² / g
pH: 9
Surface treatment agent Al ₂ O ₃ : 8% by mass
Carbon black Conductex SC-U (Columbian Carbon) 15 parts Polyurethane resin UR8200 (Toyobo) 12 parts UR8300 (Toyobo) 6 parts Phenylphosphonic acid 3 parts Butyl stearate 8 parts Butoxyethyl stearate 5 parts Isohexadecyl stearate 2 parts Stearic acid 3 parts Methyl ethyl ketone / cyclohexanone (8/2 mixed solvent) 250 parts

Production of Magnetic Recording Medium Each of the magnetic layer coating material and the nonmagnetic layer coating material was kneaded with a kneader and then dispersed using a sand mill. To the obtained dispersion, 10 parts of polyisocyanate is added to the non-magnetic layer coating material and 10 parts to the magnetic layer coating material, and 40 parts of cyclohexanone is added to each, and the mixture is filtered using a filter having an average pore size of 1 μm. Then, coating solutions for forming the nonmagnetic layer and for forming the magnetic layer were prepared, respectively.
The obtained coating for the nonmagnetic layer was applied on a polyethylene naphthalate support having a thickness of 62 μm and a center surface average surface roughness of 1.8 nm so that the thickness after drying was 1.5 μm. Dried. Immediately thereafter, a magnetic layer coating is applied on the nonmagnetic layer by a blade method so that the magnetic layer has a predetermined thickness, and the frequency is 50 Hz, the magnetic field strength is 25 T · m (250 gauss), and the frequencies are 50 Hz, 12 T · Random orientation treatment was performed by passing through two magnetic field strength AC magnetic field generators of m (120 gauss). After drying, it was treated with a seven-stage calendar at a temperature of 90 ° C. and a linear pressure of 300 kg / cm, punched to 3.7 inches, and further subjected to surface polishing treatment. Next, after punching into a disk shape, a thermo treatment at 70 ° C. was performed, and a burnishing process was performed with an abrasive tape to accelerate the curing process of the coating layer.

( Reference Examples 2-11, Examples 12-17)
A magnetic recording medium was produced in the same manner as in Reference Example 1 except that the thickness of the magnetic layer or the recording track width in Reference Example 1 was changed.

(Comparative Examples 1-14)
A magnetic recording medium was produced in the same manner as in Reference Example 1 except that the average plate diameter of the barium ferrite magnetic powder of Reference Example 1 and the thickness of the magnetic layer were changed.

<Evaluation of magnetic recording media>
The characteristics of the magnetic recording media obtained in the examples and comparative examples were measured by the following method.
(1) Magnetic properties (Hc, σs)
It measured by Hm796kA / m (10KOe) using the vibration sample type magnetometer (made by Toei Kogyo Co., Ltd.).
(2) 500 particles were randomly measured from the TEM photograph of barium ferrite plate diameter and plate thickness, and the average value was obtained.
(3) Thickness of magnetic layer A magnetic recording medium is cut out in a radial direction to a thickness of about 0.1 μm with a diamond cutter, and a magnification of 10,000 to 100,000, preferably 20,000 to The photo was taken at 50,000 times. Focusing on the difference in shape of the magnetic layer, the ferromagnetic powder in the nonmagnetic layer, and the nonmagnetic powder, the interface was visually determined.
(4) Each disk sample is set on a spin stand to which an SN ratio head is attached, and the rotational speed is adjusted so that the relative speed between the medium and the head is 4 m / s at the radial position to be measured. Next, a 20 MHz rectangular wave signal was recorded on the disk by an inductive head, reproduced by an AMR head, the signal output was measured, noise in the range of 0 to 40 MHz was integrated, and the ratio was defined as SN.
Table 2 shows the characteristics of the magnetic disk obtained in (1) to (4).

From Table 2, the average plate diameter (X) of the barium ferrite magnetic powder is 1/2 or less of the thickness (Y) of the magnetic layer, that is, the value of Y / X is 2 or more, and the average plate diameter (X) is When the recording track width (Z) is 1/30 or less, that is, when the value of Z / X is 30 or more, a good SN ratio of 20 dB or more was obtained ( Reference Examples 1 to 11 and Examples 12 to 17). .
On the other hand, when only the value of Y / X is smaller than 2 (Comparative Examples 2 and 6), when only the value of Z / X is smaller than 30 (Comparative Examples 1, 3, 4, 10-13), or Y When / X is smaller than 2 and the value of Z / X is smaller than 30 (Comparative Examples 5, 7 to 9 and 14), the SN ratio rapidly decreased and the SN ratio was less than 20 dB. (Comparative Examples 1-14).
From this, good electromagnetic characteristics can be obtained when the average plate diameter of barium ferrite is 1/30 or less of the recording track width and 1/2 or less of the thickness of the magnetic layer as in the present invention. I understand.

Claims (4)

A signal is magnetically recorded on a magnetic recording medium with a track width of 0.5 to 1.5 μm and a bit length of 0.04 to 0.1 μm, and the magnetically recorded signal is reproduced by a magnetoresistive magnetic head (MR head). A magnetic recording / reproducing system
The magnetic recording medium has a magnetic layer containing a hexagonal ferrite ferromagnetic powder having a plate ratio of 2 to 5 and a binder on a nonmagnetic support, and the thickness of the magnetic layer is 0.03 to 0. .2 μm, the average plate diameter of the hexagonal ferrite ferromagnetic powder is 1/150 or more and 1/30 or less of the magnetic recording track width, and is 1/15 or more and 1/2 or less of the thickness of the magnetic layer. A magnetic recording / reproducing system characterized by the above. A signal is magnetically recorded on a magnetic recording medium with a track width of 0.5 to 1.5 μm and a bit length of 0.04 to 0.1 μm, and the magnetically recorded signal is reproduced by a magnetoresistive magnetic head (MR head). A magnetic recording / reproducing system
The magnetic recording medium has a nonmagnetic layer containing a nonmagnetic powder and a binder on a nonmagnetic support, and a magnetic layer containing a hexagonal ferrite ferromagnetic powder having a plate ratio of 2 to 5 and a binder in this order. The magnetic layer has a thickness of 0.03 to 0.2 μm, and the average plate diameter of the hexagonal ferrite ferromagnetic powder is 1/150 or more and 1/30 or less of the magnetically recorded track width, And a magnetic recording / reproducing system having a thickness of 1/15 to 1/2 of the thickness of the magnetic layer. 3. The magnetic recording / reproducing system according to claim 1, wherein the magnetic layer has a coercive force of 143 to 398 kA / m (1800 to 5000 Oe). The magnetic recording / reproducing system according to claim 1, wherein a squareness ratio (SQ) of the magnetic layer is 0.6 or more.