JP2006155693A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium which has superior electromagnetic conversion characteristics and running stability. <P>SOLUTION: The magnetic recording medium is provided with a magnetic layer on one of the surfaces of a supporting body and a back coat layer on the other surface. The magnetic layer includes magnetic particles in which diameters of the particles are 5 to 50nm and made of single crystals of rare-earth-transition metal-metalloid. The number of protrusions, which have a height from the surface of the back coat layer is ≥50nm, is ≤30 to 200(protrusions/(265x350μm<SP>2</SP>)). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

  Reducing the particle size of the magnetic particles is necessary for increasing the magnetic recording density. For example, in a magnetic recording medium widely used as a video tape, a computer tape, a disk, etc., when the weight of the ferromagnetic material is the same, the noise decreases as the particle size is reduced.

  If the particle size of the magnetic particle is reduced, it becomes superparamagnetic due to thermal fluctuation and cannot be used for a magnetic recording medium. Therefore, research is being conducted on materials having high magnetocrystalline anisotropy such as FePt. However, these magnetic materials contain Pt as a raw material and are expensive.

  Examples of magnetic materials having crystal magnetic anisotropy comparable to FePt magnetic particles include SmCo, NdFeB, and SmFeN. In general, heat treatment at 500 ° C. or higher is required to obtain these. When heat treatment is performed at such a high temperature, it is fused and becomes polycrystalline or coarse particles, so that it is not suitable for use in a magnetic recording medium.

  Magnetic particles made of NdFeB are manufactured by preparing a quenched ribbon containing NdFeB crystals of 5 to 50 nm by a molten metal quenching method and mechanically crushing the quenched ribbon. Such magnetic particles are used in bonded magnets and the like. The size of the mechanically pulverized magnetic particles was as large as a micron order and was not suitable for use in a magnetic recording medium. In addition, these are the ones in which the Nd—Fe phase is segregated at the grain boundaries of NdFeB crystals of 5 to 50 nm, and have the disadvantage of being easily corroded.

  Moreover, after synthesizing in the liquid phase, a method of reducing by heating in a hydrogen stream is also disclosed (for example, see Patent Documents 1 to 3). In these methods, it is difficult to produce particles having a uniform composition, and since nano-sized particles are synthesized and then heated, the particles are likely to be fused.

  On the other hand, in recent VTRs and computer drives, a so-called high transfer rate has been developed to increase the relative speed of the magnetic recording medium with respect to the magnetic head, together with an increase in capacity. In order to increase the capacity, it is necessary to improve the recording density, and a magnetic recording medium having excellent electromagnetic conversion characteristics is required. In addition, to increase the capacity and transfer rate, the tape feed speed is increased, and the drive that uses a rotating cylinder supports the increased number of rotations and the number of heads. Need to proceed.

  In order to improve the recording density, fine particles, high coercive force type ferromagnetic metal fine powders and hexagonal ferrite fine powders have been used. In a digital magnetic recording medium of a type in which a signal is recorded / reproduced by sliding a magnetic head on the medium surface, the surface property of the medium magnetic layer is extremely important. In general, when the surface property is improved, electromagnetic conversion characteristics are improved and the number of dropouts is reduced, but the contact area between the head and the medium is increased, which affects running durability. If the surface of the magnetic layer is provided with protrusions made of an abrasive or the like, the running durability will be good, but conversely the spacing loss will increase and the electromagnetic conversion characteristics will deteriorate. Thus, it can be said that it is difficult to achieve both high electromagnetic conversion characteristics and low dropout, and high running performance and high durability.

In order to solve such problems, it has been proposed to provide a depression on the surface of the magnetic layer (see, for example, Patent Documents 4 and 5). However, such a depression causes a recording defect, and causes a dropout increase in a high-density digital magnetic recording medium having a small reproduction bit area.
JP 2001-181754 A JP 2002-50509 A JP 2002-121027 A Japanese Patent Laid-Open No. 3-116413 JP-A-6-180835

  In view of the above, an object of the present invention is to solve the above conventional problems. That is, an object of the present invention is to provide a magnetic recording medium excellent in electromagnetic conversion characteristics and running stability.

The above problems can be solved by the present invention described below. That is, the present invention provides a magnetic recording medium having a magnetic layer on one side of a support and a backcoat layer on the other side, wherein the magnetic layer has a particle diameter of 5 to 50 nm and a rare earth element. The number of protrusions having a height of 50 nm or more from the surface of the back coat layer is 30 to 200 (pieces / (265 × 350 μm ² ). The magnetic recording medium is characterized by the following.

  In the magnetic recording medium of the present invention, carbon black is contained in the backcoat layer, and the carbon black includes fine carbon black having an average particle size of 10 to 20 nm and coarse particle carbon having an average particle size of 60 to 300 nm. A combination with black is preferred.

  According to the present invention, a magnetic recording medium excellent in electromagnetic conversion characteristics and running stability can be provided.

  The magnetic recording medium of the present invention has a magnetic layer on one side of the support and a backcoat layer on the other side. The magnetic layer contains magnetic particles made of a rare earth-transition metal-metalloid single crystal and a binder having a particle diameter of 5 to 50 nm. Hereinafter, first, the magnetic particles will be described.

[Magnetic particles]
As described above, the magnetic particles contained in the magnetic layer of the magnetic recording medium of the present invention have a particle diameter of 5 to 50 nm and are composed of a rare earth-transition metal-semimetal single crystal.

  As the rare earth, Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu, or the like is used. Among these, Y, Ce, Pr, Nd, Gd, Tb, Dy, and Ho exhibiting uniaxial magnetic anisotropy are preferably used, and Pr, Nd, Tb, and Dy having high crystal magnetic anisotropy are particularly preferably used. Furthermore, among these, Nd, which has the most abundant resources, can be preferably used industrially. It is also preferable to replace part of Nd with Dy or Tb for the purpose of improving the anisotropic magnetic field.

  As the transition metal, Fe, Ni, and Co are preferably used for forming a ferromagnetic material. When used alone, Fe having the largest magnetocrystalline anisotropy and saturation magnetization can be preferably used. When Fe is used alone, the Curie point is as low as about 310 ° C., which may deteriorate the thermal stability. Therefore, it is preferable to partially replace Fe with Co and Ni.

  Examples of the semimetal include boron, carbon, phosphorus, silicon, and aluminum. Among these, boron and aluminum are preferably used, and boron is most preferable.

  The rare earth in the magnetic particles is preferably 10 to 15 at%, the transition metal is preferably 70 to 85 at%, and the semimetal is preferably 5 to 15 at%.

When a plurality of metals are used as the transition metal, for example, when Fe, Co, or Ni is used, when expressed as Fe _(100-xy) Co _x Ni _y , the coercive force is 2000 Oe to 6000 Oe, which is suitable for a magnetic recording medium. The composition is such that x = 0 to 45 at%, y = 25 to 30 at%, or x = 45 to 50 at%, y = 0 to 25 at%.

  From the viewpoint of reducing the corrosiveness, x = 0 to 45 at%, y = 25 to 30 at%, or x = 45 to 50 at% and y = 10 to 25 at% are preferably used. From the viewpoint that the Curie point is 500 ° C. or higher and the temperature characteristics are excellent, regions of x = 20 to 45 at%, y = 25 to 30 at%, or x = 45 to 50 at% and y = 0 to 25 at% are preferably used. Therefore, x = 20 to 45 at%, y = 25 to 30 at%, x = 45 to 50 at%, and y = 10 to 25 at% are preferably used from the viewpoint of coercive force, corrosiveness, and temperature characteristics. More preferably, x = 30 to 45 at% and y = 28 to 30 at%.

  The magnetic particle is a single crystal composed of a rare earth-transition metal-semimetal without a transition metal phase such as a transition metal or a rare earth-transition metal substantially present at the crystal grain boundary. The single crystal makes it possible to make the magnetic properties in the particles uniform. Single crystals can be confirmed by X-ray diffraction.

  Since the SNR (signal to noise ratio) is improved by reducing the particle size of the magnetic material, the smaller the particle size, the better. However, if it is too small, it will be affected by thermal fluctuations and will not be a ferromagnetic material, so a certain size or more is required. Therefore, in the present invention, it is essential that the diameter of the rare earth-transition metal-metalloid single crystal particles is 5 to 50 nm, preferably 7 to 20 nm, and more preferably 7 to 15 nm. Moreover, by including such magnetic particles in the magnetic layer, a magnetic recording medium suitable for high-density digital recording can be obtained.

  It is preferable that an oxide film is formed on the outer periphery of the single crystal particle which is a magnetic particle. Corrosion resistance can be improved by forming the oxide film. In order to exhibit a sufficient corrosion resistance effect, the thickness of the oxide film is preferably 1 to 7 nm, and more preferably 1 to 5 nm. If the oxide film is less than 1 nm, the effect of improving the corrosion resistance may not be sufficiently exerted, and if it exceeds 7 nm, the substantial filling degree of the magnetic material is lowered and the SNR (signal-to-noise ratio) is lowered. Sometimes.

  The magnetic particles are preferably produced through a step of producing a quenched ribbon made of rare earth-transition metal-metalloid particles.

  As a method of obtaining a rare earth-transition metal-boron ferromagnetic material, there is a method in which a raw metal is melted in a high-frequency melting furnace and then cast. However, in this method, many transition metals are contained as primary crystals, and a solution treatment immediately below the melting point is required to remove the excess transition metals. When this solution treatment is performed, the particle size increases. Therefore, with this method, it is difficult to obtain 5 to 50 nm particles necessary as the magnetic particles of the present invention.

  Therefore, in the present invention, it is preferable to produce magnetic particles through a step of producing a quenched thin body made of a rare earth-transition metal-metalloid. In the said process, it is preferable to employ | adopt the quenching method (molten alloy quenching method) which pours a molten metal on a rotating roll and produces a quenching thin body. In this rapid cooling method, not only the primary crystal Fe is generated, but also nanocrystals containing a single crystal state composed of rare earth-transition metal-metalloid having a particle size of 5 to 50 nm are obtained in the quenched ribbon. Also, in the method of depositing nanocrystals including a single crystal state by a heat treatment at 400 to 1000 ° C. after preparing a quenched thin body of an amorphous alloy by a rapid cooling method in which molten metal is poured onto a rotating roll, particles of 5 to 50 nm Magnetic particles composed of a rare earth-transition metal-metalloid of a size can be obtained.

When the molten metal quenching method is used, it is preferably performed in an inert gas atmosphere. This is to prevent oxidation. Specifically, He, Ar, H _{2 and the} like can be preferably used.

  In the molten metal quenching method, the cooling rate is determined by the rotational speed of the roll and the thickness of the quenching ribbon. It is preferable to use a roll rotation speed of 10 to 25 m / s when forming rare earth-transition metal-metalloid nanocrystals in the quenched ribbon immediately after quenching. Moreover, when obtaining an amorphous alloy once by rapid cooling, it is preferable to set it as 25-50 m / s.

  The thickness of the quenched ribbon is preferably 10 to 100 μm. The amount of molten metal poured is preferably controlled by an orifice or the like so that the thickness can be controlled.

After preparing the quenched thin body, the rare earth-transition metal phase can be dissolved and removed by immersing the quenched ribbon in an aqueous NaCl solution or an aqueous Na ₂ SO ₄ solution. It is possible to take out magnetic particles made of a single crystal. The water used for preparing the aqueous solution is preferably deoxygenated distilled water. Thereby, unexpected oxidation can be prevented. Deoxygenation can also be produced by a method of bubbling an inert gas such as Ar or N ₂ or a method of freezing and dissolving distilled water.

The concentration of such NaCl or Na ₂ SO ₄ is preferably set to 0.01~1kmol / m ^3, and more preferably to 0.05~0.5kmol / m ^3.

(Formation of oxide film (passive film))
As described above, when an oxide film (passive film) is formed on the surface of the magnetic particles, the magnetic particles can be heated in air to form an oxide film that is a passive film on the surface. it can. If the heating temperature is too low, the treatment time becomes long and industrially unsuitable, and if it is too high, it oxidizes to the inside and impairs magnetism, so it is preferably 40 ° C. to 80 ° C., more preferably 50 ° C to 70 ° C. The treatment time is preferably 5 hours to 10 hours.

[Magnetic recording medium]
As described above, the magnetic recording medium of the present invention has a magnetic layer on one side and a backcoat layer on the other side.

  As a specific example of the layer structure, a structure in which a nonmagnetic layer and a magnetic layer are sequentially formed on one surface side of a support, and a backcoat layer and an optional undercoat layer are formed on the other surface side. Is mentioned. In addition, it is essential that the magnetic layer contains the above-described magnetic particles and a binder, and various additives (such as a polar solvent and a nonpolar solvent) can be contained. Hereinafter, each layer including the support, the magnetic layer, and the back coat layer will be described.

<Support>
The support used in the present invention is not particularly limited, but is preferably substantially nonmagnetic and flexible.

  Examples of the flexible support include polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, aromatic polyamide, aliphatic polyamide, polyimide, polyamideimide, polysulfone, and polybenzoxapool. A known film can be used. It is preferable to use a high strength support such as polyethylene naphthalate or polyamide.

  Further, as required, in order to change the surface roughness of the surface on which the magnetic layer of the support is formed and the surface on which the backcoat layer is formed, as described in JP-A-3-224127. A laminated type support 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. Moreover, it is also possible to apply aluminum or a glass substrate as a support for the magnetic recording medium of the present invention.

  The center surface average surface roughness (Ra) measured with TOPO-3D manufactured by WYKO as the support is usually preferably 8.0 nm or less, more preferably 4.0 nm or less, and 2.0 nm or less. It is further preferable that 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 can be freely controlled by the size and amount of filler added to the support as required. Examples of these fillers include oxides such as Ca, Si, Ti, inorganic fine particles of carbonate, and organic fine powders such as acrylic. The maximum height Rmax of the protrusions present on the support is preferably 1 μm or less, and the ten-point average roughness Rz is preferably 0.5 μm or less. Further, the center plane peak height Rp is preferably 0.5 μm or less, the center plane valley depth Rv is preferably 0.5 μm or less, and the center plane area ratio Sr is 10% to 90%. 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, each having a size of 0.01 μm to 1 μm from 0 per 0.1 mm ² It can be controlled in the range of 2000.

The F-5 value (stress at 5% elongation) of the support is preferably 5 to 50 kg / mm ² (49 to 490 MPa). Furthermore, the heat shrinkage rate of the support at 100 ° C. for 30 minutes is preferably 3% or less, more preferably 1.5% or less. The thermal shrinkage at 80 ° C. for 30 minutes is preferably 1% or less, more preferably 0.5% or less.

The breaking strength is preferably 5 to 100 kg / mm ² (49 to 980 MPa), and the elastic modulus is preferably 100 to 2000 kg / mm ² (0.98 to 19.6 GPa).

The temperature expansion coefficient is preferably 10 ⁻⁴ to 10 ⁻⁸ / ° C., and more preferably 10 ⁻⁵ to 10 ⁻⁶ / ° C. The humidity expansion coefficient is preferably 10 ⁻⁴ / RH% or less, and 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.

  The thickness of the support is preferably 2 to 100 μm, and more preferably 2 to 80 μm. When the magnetic recording medium of the present invention is a computer tape, the thickness of the support is preferably 3.0 to 6.5 μm, more preferably 3.0 to 6.0 μm, and 4.0 to 5 More preferably, it is 5 μm.

<Magnetic layer>
The magnetic layer contains the above-described magnetic particles, a binder, and various additives (such as a polar solvent and a nonpolar solvent) as necessary.

  In the magnetic recording medium of the present invention, the magnetic layer described above may be provided only on one side of the support or on both sides. In addition, a nonmagnetic layer may be provided between the support and the magnetic layer from the viewpoint of supplying a lubricant and covering the protrusions of the support.

(Binder)
As the binder contained in the magnetic layer and the nonmagnetic layer, conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof are used. 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 degree of polymerization of about 50 to 1,000. .

  Examples 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 acetate. There are polymers or copolymers containing polyurethane, vinyl ether, etc. as structural units, 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, polyesters. Examples thereof include a mixture of resin and isocyanate prepolymer, a mixture of polyester polyol and polyisocyanate, a mixture of polyurethane and 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, but are preferably 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. A combination of at least one selected from the above and a polyurethane resin, or a combination of these with a polyisocyanate.

  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.

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, — It is preferable to use one in which at least one polar group selected from CN and the like is introduced by copolymerization or addition reaction. The amount of such a polar group is 10 ⁻¹ to 10 ⁻⁸ mol / g, preferably 10 ⁻² to 10 ⁻⁶ mol / g.

  Specific examples (trade names) of these binders 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; 1000W, DX80, DX81, DX82 manufactured by Denki Kagaku; , 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-5201 Barnock D-400, D-210-80, Crisbon 6109, 7209; Byron UR8200, UR8300, UR-8700, manufactured by Toyobo; RV530, RV280; Daiferamin 4020, 5020, 5100, 5300, 9020, 9022, manufactured by Dainichi Seika 7020; MX5004 manufactured by Mitsubishi Chemical; Samprene SP-150 manufactured by Sanyo Chemical; Saran F310 and F210 manufactured by Asahi Kasei;

  The binder used for the nonmagnetic layer and the magnetic layer is 5 to 50 based on the total of the nonmagnetic powder in the nonmagnetic layer and the nonmagnetic particles and the ferromagnetic metal powder (magnetic particles) in the magnetic layer. It is used in the range of mass%, preferably in the range of 10-30 mass%.

  When using a vinyl chloride resin, it is preferable to use 5-30% by weight, when using a polyurethane resin, 2-20% by weight, and polyisocyanate in a range of 2-20% by weight. When head corrosion occurs due to a small amount of dechlorination, it is possible to use only polyurethane or only polyurethane and polyisocyanate.

When polyurethane is used, the glass transition temperature is −50 to 150 ° C., preferably 0 to 100 ° C., the breaking elongation is 100 to 2000%, the breaking stress is 0.05 to 10 kg / mm ² , and the yield point is 0.05 to 10 kg / mm ² is preferable. In the magnetic recording medium of the present invention, the magnetic layer may be multilayered as long as the requirements of the present invention are satisfied, and the nonmagnetic layer may also be multilayered. Furthermore, an arbitrary layer having various functions can be provided as necessary. Therefore, the amount of binder, the amount of vinyl chloride resin, polyurethane resin, polyisocyanate or other resin in the binder, the molecular weight of each resin, the amount of polar groups, or the physical properties of the resin described above, etc. Of course, it is possible to change it in each layer such as a lower layer and a magnetic layer as needed, but rather it should be optimized in each layer, and a known technique relating to a multilayer magnetic layer can be applied.

  For example, when the amount of the binder is changed in each layer, in order to improve the head touch with respect to the head, for example, the amount of the binder in the nonmagnetic layer is increased to have flexibility.

  Polyisocyanates that can be 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, triphenyl. Isocyanates such as methane triisocyanate, products of these isocyanates and polyalcohols, polyisocyanates formed by condensation of isocyanates, and the like can be used.

  Commercially available product names of these isocyanates include Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR, Millionate MTL manufactured by Nippon Polyurethane; Takenate D-102, Takenate D-110N manufactured by Takeda Takenate D-200, Takenate D-202; Sumitomo Bayer's Death Module L, Death Module IL, Death Module N, Death Module HL, etc. Further combinations can be used for each layer.

  When a nonmagnetic layer is formed on a support, the magnetic layer (also referred to as an upper layer or an upper magnetic layer) is formed by applying the nonmagnetic layer, and then the nonmagnetic layer is in a wet state (W / W). It may be provided, or may be provided after the nonmagnetic layer is dried (W / D). From the viewpoint of production yield, simultaneous or sequential wet application is preferable, but in the case of a disk, application after drying can be used sufficiently. The nonmagnetic layer will be described later.

  Surface treatment such as calendering process because both layers can be formed at the same time in the multi-layer structure (structure of non-magnetic layer and magnetic layer), or the non-magnetic layer and the magnetic layer can be formed simultaneously by wet coating (W / W). The process can be used effectively, and even the ultra-thin layer can improve the surface roughness of the upper magnetic layer. Hereinafter, a method for forming the magnetic layer and the nonmagnetic layer will be described in detail.

  First, when preparing a coating liquid for a magnetic layer (magnetic liquid for a magnetic layer) or a coating liquid for a nonmagnetic layer (nonmagnetic liquid for a nonmagnetic layer), an open kneader, A kneading process using a continuous kneader, a pressure kneader, an extruder, or the like may be performed. Further, in order to disperse magnetic particles and nonmagnetic powder, dispersion media such as glass beads, zirconia beads, titania beads, and steel beads may be used. The magnetic liquid for the magnetic layer contains the magnetic particles of the present invention, the binder described above, and various additives as required. The content of the magnetic particles in the magnetic liquid for the magnetic layer is preferably 3 to 50% by mass.

  And a magnetic layer can be formed by apply | coating the magnetic liquid for magnetic layers on a support body by a well-known method. Here, when a magnetic recording medium having a multilayer structure in which a nonmagnetic layer and a magnetic layer are formed, the following method is preferably used.

  The first method is to apply a non-magnetic liquid for the non-magnetic layer first with a commonly used gravure coating, roll coating, blade coating, extrusion coating device, etc. A magnetic layer is formed by applying a magnetic liquid for a magnetic layer by a support pressure type extrusion coating apparatus disclosed in JP-A-46186, JP-A-60-238179, and JP-A-2-265672. It is a method.

  The second method is a single coating with two coating liquid passage slits as disclosed in JP-A-63-88080, JP-A-2-17971, and JP-A-2-265672. In this method, the nonmagnetic liquid for the nonmagnetic layer and the magnetic liquid for the magnetic layer are applied almost simultaneously by the head.

  The third method is a method in which a nonmagnetic liquid for a nonmagnetic layer and a magnetic liquid for a magnetic layer are applied almost simultaneously by an extrusion coating apparatus with a backup roll disclosed in JP-A-174965.

  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. Further, the viscosity of the coating solution for the magnetic layer and the nonmagnetic layer preferably satisfies the numerical range disclosed in Japanese Patent Laid-Open No. 3-8471. In order to realize a multilayer structure, a nonmagnetic layer may be applied and dried, and then a sequential multilayer coating may be performed in which a magnetic layer is provided thereon. 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 particular, in the case of a disk, a sufficiently isotropic orientation may be obtained even without non-orientation without using an orienting device. However, it is known that cobalt magnets are alternately arranged obliquely and an alternating magnetic field is applied by a solenoid. It is preferable to use a random orientation apparatus. In the case of ferromagnetic metal powder, the isotropic orientation is preferably the vertical orientation particularly when high density recording is performed. Further, circumferential orientation may be performed using spin coating.

  In the case of a magnetic tape, it is 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 preferably 20 m / min 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.

After the coating and drying, if necessary, the magnetic recording medium may be calendered. The calendering roll is treated with a heat-resistant plastic roll or metal roll such as epoxy, polyimide, polyamide, polyimide amide or the like. Particularly, when the 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 (196 kN / m) or more, more preferably 300 kg / cm (294 kN / m) or more.

  The thickness of the magnetic layer is preferably 0.005 μm to 0.20 μm, and more preferably 0.05 to 0.15 μm. By setting the thickness to 0.005 μm to 0.20 μm, it is possible to prevent a decrease in reproduction output, overwrite characteristics, and deterioration in resolution.

(Carbon black, abrasive)
The magnetic layer preferably contains carbon black. As the carbon black, furnace for rubber, thermal for rubber, black for color, acetylene black and the like can be used.

The SBET of carbon black is preferably 5 to 500 m ² / g, and the DBP oil absorption is preferably 10 to 400 ml / 100 g.

  The average particle size is preferably 5 to 300 nm, more preferably 10 to 250 nm, and still 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 / ml.

  Specific examples of the carbon black include BLACKPEARLS 2000, 1300, 1000, 900, 905, 800, 700, VULCAN XC-72 manufactured by Cabot Corporation; # 80, # 60, # 55, # 50 manufactured by Asahi Carbon Co., Ltd. # 35; # 2400B, # 2300, # 900, # 1000, # 30, # 40, # 10B manufactured by Mitsubishi Chemical Industries, Ltd. CONDUCTEX SC, Raven150, 50, 40, 15, Raven-MT-P manufactured by Columbian Carbon Ketjen Black EC manufactured by Japan EC Co .;

  Carbon black may be used after being surface-treated with a dispersant or the like and grafted with a resin. Moreover, you may use what made a part of the surface graphitized. Further, carbon black may be dispersed in advance with a binder before it is added to the magnetic paint.

  These carbon blacks can be used alone or in combination. When carbon black is used, it is preferably used in the range of 0.1 to 30% of the total mass with respect to the magnetic substance (magnetic particles). 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 are different in type, amount, and combination in the upper magnetic layer and the lower nonmagnetic layer, and the particle size, oil absorption, conductivity, pH, etc. Of course, it is possible to selectively use them according to the purpose based on the characteristics, but rather it should be optimized in each layer. For the carbon black that can be used in the magnetic layer, for example, “Carbon Black Handbook” (edited by Carbon Black Association) can be referred to.

  The magnetic layer preferably contains an abrasive. As an abrasive, α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, silicon carbide titanium carbide, titanium oxide having an α conversion rate of 90% or more, Known materials having a Mohs hardness of 6 or more, such as silicon dioxide and boron nitride, are 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. These abrasives may contain compounds or elements other than the main component, but the effect is not affected if the main component is 90% by mass or more.

The particle size of these abrasives is preferably 0.01 to 2 μm, more preferably 0.05 to 1.0 μm, and still more preferably 0.05 to 0.5 μm. In particular, in order to enhance electromagnetic conversion characteristics, it is preferable that the particle size distribution is narrow. Further, in order to improve the durability, it is possible to combine abrasives having different particle sizes as necessary, or to obtain the same effect by widening the particle size distribution even with a single abrasive. The tap density of the abrasive is preferably 0.3-2 g / ml, the water content is preferably 0.1-5%, the pH is preferably 2-11, and the SBET is 1-30 m. ² / g is preferred. 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.

  Specifically, AKP-12, AKP-15, AKP-20, AKP-30, AKP-50, HIT20, HIT-30, HIT-55, HIT60A, HIT70, HIT80, HIT100 manufactured by Sumitomo Chemical Co., Ltd .; Reynolds ERC-DBM, HP-DBM, HPS-DBM; WA10000 manufactured by Fujimi Abrasives Co., Ltd .; UB20 manufactured by Uemura Kogyo Co., Ltd .; G-5 manufactured by Nippon Chemical Industry Co., Ltd .; Chromex U2, Chromex U1; ; Beta random ultra fine manufactured by Ibiden; B-3 manufactured by Showa Mining Co .; These abrasives can be added to the nonmagnetic layer as required. 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.

(Additive)
The magnetic layer and the nonmagnetic layer described later preferably contain various additives such as a polar solvent and a nonpolar solvent.

  When a magnetic layer is formed using magnetic particles and a binder such as urethane resin is used, it cannot be dissolved unless a polar solvent is used. Therefore, it is conceivable to dissolve the binder using the polar solvent. However, when the polar solvent is added, the magnetic particles are aggregated, and it is necessary to mechanically redisperse. Since the magnetic particle has a very small particle size, sufficient dispersibility cannot be obtained by mechanical redispersion.

  On the other hand, when a binder (urethane resin) is mixed in both solutions after mixing the polar solvent and the nonpolar solvent, the binder does not dissolve in the solution containing the nonpolar solvent, but contains the polar solvent. In the solution, it can be confirmed that the binder is dissolved.

  Therefore, it is also possible to prepare a coating liquid (magnetic liquid for magnetic layer) by adding a nonpolar solvent to a liquid containing magnetic particles and then adding a polar solvent and a binder. By containing a non-polar solvent, it is considered that the effect of relaxing the stability of the particles due to the valence electricity of the polar solvent is generated, and aggregation of the magnetic particles is prevented.

  Since the nonpolar solvent is contained in order to prevent aggregation of the magnetic particles, it is preferable to add it before the polar solvent, but in a state mixed with the polar solvent or in a state mixed with the polar solvent and the binder. It may be added.

  As the nonpolar solvent, it is preferable to use at least one kind of aromatic hydrocarbons such as toluene, benzene and xylene, octane, decane, hexane, nonane and the like. The nonpolar solvent is preferably contained in an amount of 1 to 95% by mass, more preferably 5 to 85% by mass in the state before the coating liquid is formed.

  As polar solvents, ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; alcohols such as methanol, ethanol, and propanol; esters such as methyl acetate, butyl acetate, and isobutyl acetate; glycols such as glycol dimethyl ether and glycol monoethyl ether Ether type; it is preferable to use at least one kind of cyclohexanone. The polar solvent is preferably contained in an amount of 20 to 95% by mass, more preferably 30 to 85% by mass, in a state prior to the coating liquid.

  From the viewpoint of achieving both the solubility of the binder and the prevention of solidification of the magnetic particles, the mass ratio of the polar solvent to the nonpolar solvent (polar solvent / nonpolar solvent) is 1/20 to 20/1. Preferably, it is 1/9 to 9/1.

  As other additives, those having at least one effect such as a lubrication effect, an antistatic effect, a dispersion effect, and a plastic effect are appropriately used.

  For example, molybdenum disulfide, tungsten disulfide graphite, boron nitride, fluorinated graphite, silicone oil, silicone with polar groups, 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 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 or branched) And a metal salt thereof (Li, Na, K, Cu, etc.) or a monovalent, divalent, trivalent, tetravalent, pentavalent, hexavalent alcohol (unsaturated bond) having 12 to 22 carbon atoms. Or may be branched), an alkoxy alcohol having 12 to 22 carbon atoms, a monobasic fatty acid having 10 to 24 carbon atoms (which may contain an unsaturated bond or may be branched) and carbon Mono-fatty acid ester comprising any one of monovalent, divalent, trivalent, tetravalent, pentavalent and hexavalent alcohols (which may contain an unsaturated bond or may be branched). Alternatively, difatty acid esters or trifatty acid esters, fatty acid esters of monoalkyl ethers of alkylene oxide polymers, fatty acid amides having 8 to 22 carbon atoms, aliphatic amines having 8 to 22 carbon atoms, and the like can be used.

  More specifically, examples of 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. Esters include butyl stearate, octyl stearate, amyl stearate, isooctyl stearate, butyl myristate, octyl myristate, butoxyethyl stearate, butoxydiethyl stearate, 2-ethylhexyl stearate, 2-octyldodecyl palmitate 2-hexyl decyl 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, grindole, alkylphenol ethylene oxide adducts, etc .; cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocycles, phosphonium or sulfoniums, Cationic surfactants such as: 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 aminoalcohols Amphoteric surfactants such as alkylbedine type, etc. 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% or less.

  These lubricants and surfactants have different physical actions, and the type, amount, and combination ratio of lubricants or surfactants that produce a synergistic effect should be optimally determined according to the purpose. Is. For this purpose, (1) control the bleeding to the surface using fatty acids having different melting points in the nonmagnetic layer and the magnetic layer, and (2) bleeding to the surface using esters having different boiling points, melting points and polarities. Controlling, (3) Improving the coating stability by adjusting the amount of surfactant, (4) Increasing the lubricating effect by increasing the amount of lubricant added in the intermediate layer, of course here It is not limited to the example shown in. Generally, the total amount of the lubricant is preferably 0.1 to 50% by mass, more preferably 2 to 25% by mass with respect to the magnetic substance (magnetic particles) or nonmagnetic powder.

  Further, all or a part of the additives used in the present invention may be added in any step of magnetic coating production and non-magnetic coating production. For example, when mixing with a magnetic body before the kneading step, when adding at a kneading step with a magnetic body, a binder and a solvent, when adding at a dispersing step, when adding after dispersing, when adding just before coating, etc. is there.

  In addition, depending on the purpose, after the magnetic layer is formed by coating, the purpose may be achieved by coating a part or all of the additive simultaneously or sequentially. Further, depending on the purpose, a lubricant can be applied to the surface of the magnetic layer after the calendar treatment or after the slit is finished.

<Nonmagnetic layer>
Next, detailed contents regarding the nonmagnetic layer will be described. The configuration of the nonmagnetic layer should not be limited as long as it is substantially nonmagnetic, but it is usually composed of at least a resin, and preferably a powder, for example, an inorganic powder or an organic powder is dispersed in the resin. Can be mentioned. The inorganic powder is preferably a non-magnetic powder, but a magnetic powder can also be used as long as the non-magnetic layer is substantially non-magnetic.

  The particle size (particle size) of these nonmagnetic powders is preferably in the range of 0.005 to 2 μm. However, if necessary, nonmagnetic powders having different particle sizes can be combined, or even a single nonmagnetic powder can have a wide particle size distribution. Thus, the same effect can be achieved. The particle size of the particularly preferred nonmagnetic powder is in the range of 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. In the case of an acicular metal oxide, the major axis length is preferably 0.3 μm or less, and more preferably 0.2 μm or less. The tap density is preferably 0.05 to 2 g / ml, and 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 is preferably 2 to 11, and particularly preferably 5.5 to 10.

The non-magnetic powder SBET (specific surface area) is preferably from 1 to 100 m ² / g, more preferably 5~80m ² / g, more preferably from 10 to 70 m ² / g. The crystallite size (crystallite diameter) 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 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 of the nonmagnetic powder is preferably 1 to 12, and 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. Preferably SA (stearic acid) adsorption amount of nonmagnetic powders is 1 to 20 [mu] mol / m ^2, more preferably from 2 to 15 [mu] mol / m ^2, further preferably 3 to 8 [mu] mol / m ^2. The pH is preferably between 3-6.

  The nonmagnetic powder can be selected from inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides.

  As an inorganic compound, for example, α-alumina, β-alumina, γ-alumina, θ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, hematite, goethite, corundum Use silicon nitride, 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. alone or in combination Is done. 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 most preferred are titanium dioxide and alpha iron oxide.

  Specific examples (product names) of non-magnetic powders 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 TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100, α Hematite E270, E271, E300, E303; Titanium Industrial 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 F NEX-25, BF-1, BF-10, BF-20, ST-M; Dowa Mining Ltd. DEFIC-Y, DEFIC-R; manufactured by Japan Aerosil AS2BM, TiO2P25; manufactured by Ube Industries, Ltd. 100A, 500A; and the like. Particularly preferred nonmagnetic powders are titanium dioxide and α-iron oxide.

The surface of these nonmagnetic powders is subjected to a surface treatment so that at least one of Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , ZnO, and Y ₂ O ₃ is applied. Preferably it is present. In consideration of dispersibility, Al ₂ O ₃ , SiO ₂ , TiO ₂ and ZrO ₂ are preferable, but Al ₂ O ₃ , SiO ₂ and ZrO ₂ are more preferable. 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 in which alumina is first present and then the surface layer is treated with silica, or vice versa may be employed. Furthermore, 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.

  Carbon black can be mixed in the nonmagnetic layer to lower the surface electrical resistance Rs, light transmittance can be reduced, and 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 of the nonmagnetic layer should be optimized for the following characteristics depending on the desired effect, and the effect may be obtained more when used in combination.

Preferably SBET of the carbon black in the nonmagnetic layer is 100 to 500 m ² / g, more preferably 150 to 400 m ² / g. The DBP oil absorption is preferably 20 to 400 ml / 100 g, and more preferably 30 to 400 ml / 100 g. The particle size of carbon black is preferably 5 nm to 80 nm, more preferably 10 to 50 nm, and even more preferably 10 to 40 nm. The pH of the carbon black 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 / ml.

  As specific examples (product names) of carbon black used in the present invention, BLACKPEARLS 2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72 manufactured by Cabot Corporation; # 3050B, manufactured by Mitsubishi Kasei Kogyo Co., Ltd., # 3150B, # 3250B, # 3750B, # 3950B, # 950B, # 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;

  Carbon black may be used by surface treatment with a dispersant or the like, grafted with a resin, or a part of its surface graphitized. Furthermore, carbon black may be dispersed in advance with a binder before being added to the paint. These carbon blacks can be used within 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, methods described in JP-A Nos. 62-18564 and 60-255827 can be used.

  The thickness of the nonmagnetic layer is preferably 0.2 μm to 5.0 μm, more preferably 0.3 μm to 3.0 μm, and even more preferably 1.0 μm to 2.5 μm.

  The nonmagnetic layer exhibits its effect if it is substantially nonmagnetic. For example, the nonmagnetic layer may contain a small amount of a magnetic material (magnetic material) as an impurity or intentionally. “Substantially non-magnetic” means that the residual magnetic flux density is 0.01 T or less or the coercive force is 7.96 kA / m (100 Oe or less), and preferably has no residual magnetic flux density and coercive force. Show.

  The binder of the nonmagnetic layer (binder resin), lubricant, dispersant, additive, solvent, dispersion method, etc. can be applied to those of the magnetic layer. 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.

  An undercoat layer for improving adhesion may be provided between the support and the nonmagnetic layer or the magnetic layer. The thickness of the undercoat layer is preferably from 0.01 to 0.5 μm, more preferably from 0.02 to 0.5 μm.

<Back coat layer>
The back coat layer is formed on the surface of the support opposite to the surface on which the magnetic layer is formed. The thickness of the back coat layer is preferably 0.1 to 4 μm, and more preferably 0.3 to 2.0 μm.

In the back coat layer, protrusions having a height of 50 nm or more from the surface thereof are 30 to 200 (pieces / (265 × 350 μm ² )) or less. If it is less than 30 pieces / (265 × 350 μm ² ), the friction coefficient becomes high in a high-temperature and high-humidity environment, and / or stick slip is likely to occur due to repeated running durability in a drive. If it exceeds 200 pieces / (265 × 350 μm ² ), the protrusions of the backcoat layer are transferred to the surface of the magnetic layer during storage at high temperature and high humidity, resulting in dents, and the output decreases and DO (dropout) increases. Further, protrusions with a height of 50 nm or more are transferred to the surface of the magnetic layer and become dents, causing DO, and therefore the number of such protrusions must be defined. The number of the protrusions is preferably in the 35 to 150 (number ^{/ (265 × 350μm 2))} , and more preferably in the 40-120 (number ^{/ (265 × 350μm 2))} .

  In the present invention, “the number of protrusions having a height of 50 nm or more on the surface of the backcoat layer” refers to the number of protrusions having a height of 50 nm or more from the root mean square surface of the backcoat layer. Here, the “square mean surface” is a surface in which the sum of the squares of deviations of the back coat layer surface is equal in the vertical direction. The position of the root mean square surface can be obtained by a surface roughness measuring instrument such as an atomic force microscope or an optical surface roughness meter. The number of protrusions having a height of 50 nm or more from the root mean square surface of the back coat layer was measured within a range of 265 μm × 350 μm using a surface roughness meter (NewView 5000) manufactured by ZYGO, and squared by image analysis software. It can be obtained by integrating the number of regions having a height of 50 nm or more from the average surface.

  As a method for controlling the number of protrusions having a height of 50 nm or more on the back coat layer surface, a polyether polyester polyurethane having excellent dispersibility as a polyurethane can be introduced by introducing a polar group into the polyurethane contained as a binder in the back coat layer. The method of using can be mentioned. Furthermore, the particle diameter and addition amount of the carbon black used for the back coat layer, the dispersion condition of the back coat layer coating solution, the number of protrusions of the support, the calender temperature, and the calender pressure can be exemplified. In general, when the calender conditions are increased (calendar pressure, temperature, roll hardness is increased, speed is decreased), the protrusions on the surface of the backcoat layer can be reduced.

  In particular, in the present invention, in order to control the number of back coat layer protrusions, it is important to highly disperse the back coat layer coating solution. In the present invention, in the step of dispersing the backcoat layer coating solution, multi-stage dispersion is performed while sequentially reducing the concentration of the dispersion containing carbon black, and the dispersion medium has high specific gravity, high hardness, and a small diameter (for example, 0.5 mmφ). ) Dispersion media, high dispersion media filling, high peripheral speed of the disperser, and long dispersion time can be combined appropriately to disperse the coating solution for the backcoat layer to a very high degree. .

As the disperser, for example, a media disperser such as a ball mill, an attritor or a sand grinder can be used, and a vibration mill or an ultrasonic disperser can also be used. When using a media-type disperser, it is preferable to increase the filling rate of the dispersion medium, for example, 60 to 100%. As the dispersion medium, steel balls, glass beads, ceramic beads (zirconia or the like) can be used, and it is particularly preferable to use small diameter and high specific gravity ceramic beads such as zirconia beads. Furthermore, when it is desired to apply shear to the carbon black on the powder, a kneader can be used. For example, an open kneader, a pressure kneader, a helical rotor, a continuous kneader, a roll mill, a taper roll, an internal mixer, a Banbury mixer and the like can be used. The dispersed liquid is usually adjusted in viscosity so as to be easily applied and then supplied to the application section. When the liquid is sent, it is usually filtered in a circulation, single or multiple stages. The filter used at this time can be selected depending on the liquid quality, and either a depth type or a pleat type can be used. The filtration accuracy is preferably selected according to the accuracy according to the desired level.

  The protrusions existing on the surface of the backcoat layer are classified into protrusions caused by the support and protrusions related to the coating layer. Usually, fine protrusions are often formed on the support surface by containing fine inorganic or organic fillers. These minute protrusions are reflected on the surface of the backcoat layer to form minute surface protrusions, but the number of protrusions on the surface of the backcoat layer can be changed by changing the particle size of the filler contained in the support. Further, the influence of the protrusions on the support surface on the backcoat layer can be reduced by increasing the thickness of the backcoat layer.

  In addition, by polishing the surface of the backcoat layer, the number of protrusions having a height of 50 nm can be reduced. Examples of the method for surface treatment of the back coat layer include a method of polishing by pressing a polishing tape or a diamond wheel against the surface of the back coat layer. At that time, surface protrusions can be controlled by controlling the count and pressing pressure of the polishing tape and diamond wheel. In the case of a tape, it is possible to use a polishing method using a polishing tape (wrapping tape blade method), a sapphire blade method, a diamond wheel method, etc. disclosed in JP-A-63-259830. The surface projections on the backcoat layer can be controlled by selecting these and setting each processing condition. Even when there are many protrusions on the surface of the support or on the backcoat layer after coating, the protrusions on the surface of the backcoat layer can be reduced by applying this surface treatment.

  As described above, various methods can be used to control the number of protrusions on the surface of the backcoat layer, and a magnetic recording medium having desired characteristics can be obtained by combining and optimizing these methods. Can do.

  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 the present invention, carbon black is contained in the back coat layer in order to maintain such high running durability. The back coat layer is preferably used in combination of two types having different average particle sizes (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 50 nm and coarse particle carbon black having an average particle size of 60 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.

  Specific products of the particulate carbon black include the following. In addition, the average particle size is shown in parentheses.

  BLACK PEARLS 800 (17 nm), BLACK PEARLS 1400 (13 nm), BLACK PEARLS 1300 (13 nm), BLACK PEARLS 1100 (14 nm), BLACK PEARLS 1000 (16 nm), BLACK PEARLS 900 (15 nm), BLACK PEARLS 8 PEARLS 4630 (19 nm), BLACK PEARLS 460 (28 nm), BLACK PEARLS 430 (28 nm), BLACK PEARLS 280 (45 nm), MONARCH 800 (17 nm), MONARCH 14000 (13 nm), MONARCH 1300 (13 nm), MONARCH 1 , MONARCH 000 (16 nm), MONARCH 900 (15 nm), MONARCH 880 (16 nm), MONARCH 630 (19 nm), MONARCH 430 (28 nm), MONARCH 280 (45 nm), REGAL 330 (25 nm), REGAL 250 (34 nm), REGAL 99 ( 38 nm), REGAL 400 (25 nm), REGAL 660 (24 nm) (above, manufactured by Cabot Corporation),

Raven 2000B (18 nm), Raven 1500 B (17 nm), Raven 7000 (11 nm), Raven 5750 (12 nm), Raven 5250 (16 nm), Raven 3500 (13 nm), Raven 2500 ULTRA (13 nm), Raven 2000 (18 nm), Raven 2000 (18 nm) 17 nm), Raven 1255 (21 nm), Raven 1250 (20 nm), Raven 1190 ULTRA (21 nm), Raven 1170 (21 nm), Raven 1100 ULTRA (32 nm), Raven 1080 ULTRA (28 nm), RavenU 30A, Raven10 1040 (28 nm), Raven 880 ULTRA (30 nm) Raven 860 (39nm), Raven 850 (34nm), Raven 820 (32nm), Raven 790 ULTRA (30nm), Raven 780 ULTRA (29nm), Raven 760 ULTRA (30nm) (manufactured by Columbian Carbon Co., Ltd.),

Asahi # 90 (19 nm), Asahi # 80 (22 nm), Asahi # 70 (28 nm), Asahi F-200 (35 nm), Asahi # 60 HN (40 nm), Asahi # 60 (45 nm), HS-500 (38 nm), Asahi # 51 (38 nm) (Asahi Carbon Co., Ltd.), # 2700 (13 nm), # 2650 (13 nm), # 2400 (14 nm), # 1000 (18 nm), # 950 (16 nm), # 850 (17 nm) , # 750 (22 nm), # 650 (22 nm), # 52 (27 nm), # 50 (28 nm), # 40 (24 nm), # 30 (30 nm), # 25 (47 nm), # 95 (40 nm), CF9 (40 nm) (Mitsubishi Chemical Co., Ltd.), PRINTEX90 (14 nm), PRINTEX95 (15 nm), PRINTEX85 (16 nm), PRINT X75 (17nm) (manufactured by Degussa AG), # (manufactured by Mitsubishi Chemical Industries (Co., Ltd.)) 3950 (16nm).

  Examples of specific products of coarse particulate carbon black include BLACK PEARLS 130 (75 nm), MONARCH 120 (75 nm), Regal 99 (100 nm) (above, manufactured by Cabot Corporation), Raven 450 (75 nm), Raven 420 (86 nm). ), Raven 410 (101 nm), Raven 22 (83 nm), Raven MTP (275 nm) (above Colombian Carbon), Asahi 50H (85 nm), Asahi # 51 (91 nm), Asahi # 50 (80 nm), Asahi # 35 (78 nm), Asahi # 15 (122 nm) (above, manufactured by Asahi Carbon Co., Ltd.), # 10 (75 nm), # 5 (76 nm), # 4010 (75 nm) (above, manufactured by Mitsubishi Chemical Corporation), thermal black ( 270 nm) (manufactured by Khan Calve) The

  When two types of backcoat layers having different average particle sizes are used, the content ratio (mass ratio) of fine particle carbon black having an average particle size of 10 to 50 nm and coarse particle carbon black having an average particle size of 60 to 300 nm. The former / the latter is preferably in the range of 100 / 0.5 to 100/100, and more preferably in the range of 100/1 to 100/50. 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 100 parts by mass with respect to 100 parts by mass of the binder, It is the range of 45-95 mass parts.

An inorganic powder can be used for the back coat layer. As the inorganic powder, it is preferable to use two types having different hardnesses in combination.
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. The average particle size of the inorganic powder is preferably in the range of 30 to 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. Among these, calcium carbonate is particularly preferable.

  The content of the soft inorganic powder in the back coat layer is preferably in the range of 0 to 140 parts by mass, more preferably 0 to 100 parts by mass with respect to 100 parts by mass of the 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 a soft inorganic powder (especially calcium carbonate), the sliding characteristics with respect to the guide pole having a rough surface can be improved, and the friction coefficient of the backcoat layer can be stabilized. The hard inorganic powder preferably has an average particle size 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 0-30 mass parts normally with respect to 100 mass parts of carbon black, Preferably, it is 0-20 mass parts.

  When the soft inorganic powder and the hard inorganic powder are used in combination in the back coat 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 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. In particular, in this combination, it is preferable that calcium carbonate is contained as the soft inorganic powder.

  The back coat layer can contain a lubricant. The lubricant can be appropriately selected from the lubricants mentioned as the lubricant that can be used for the nonmagnetic layer or the magnetic layer described later. 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. Note that the binder (binder resin), lubricant, dispersant, additive, solvent, dispersion method, and the like that can be applied to the backcoat layer can be those of the magnetic layer. In particular, the technology relating to the magnetic layer can be applied to the binder resin amount, type, additive, and dispersant addition amount and type.

<Protective film>
In addition, by forming a very thin protective film on the magnetic layer, the wear resistance is improved, and a lubricant is applied on the protective film to increase slipperiness, thereby providing sufficient reliability. It can be a magnetic recording medium.

  Examples of the material for the protective film include oxides such as silica, alumina, titania, zirconia, cobalt oxide and nickel oxide; nitrides such as titanium nitride, silicon nitride and boron nitride; carbides such as silicon carbide, chromium carbide and boron carbide; Examples of the carbon include carbon and carbon such as graphite and amorphous carbon, and particularly preferable is a hard amorphous carbon generally called diamond-like carbon.

  A carbon protective film made of carbon is suitable as a material for the protective film because it has a very thin film thickness and sufficient wear resistance, and hardly causes seizure on the sliding member.

  As a method for forming a carbon protective film, sputtering is generally used for hard disks, but many methods using plasma CVD with a higher film formation rate have been proposed for products that require continuous film formation such as video tape. ing. Therefore, it is preferable to apply these methods.

  In particular, it has been reported that the plasma injection CVD (PI-CVD) method has a very high film formation rate, and that the obtained carbon protective film is also hard and has a good quality protective film with few pinholes (for example, Japanese Patent Laid-Open No. Sho-sho). 61-130487, JP-A-63-279426, JP-A-3-113824, etc.).

The carbon protective film preferably has a Vickers hardness of 1000 kg / mm ² or more, and more preferably 2000 kg / mm ² or more. The crystal structure is preferably an amorphous structure and non-conductive. When a diamond-like carbon (diamond-like carbon) film is used as the carbon protective film, this structure can be confirmed by Raman light spectroscopic analysis. That is, when a diamond-like carbon film is measured, it can be confirmed by detecting a peak at 1520 to 1560 cm ⁻¹ . When the structure of the carbon film deviates from the diamond-like structure, the peak detected by Raman light spectroscopic analysis deviates from the above range, and the hardness as a protective film also decreases.

  As the carbon raw material for forming this carbon protective film, it is preferable to use carbon-containing compounds such as alkanes such as methane, ethane, propane, and butane; alkenes such as ethylene and propylene; and alkynes such as acetylene. Further, a carrier gas such as argon or an additive gas such as hydrogen or nitrogen for improving the film quality can be added as necessary.

  When the carbon protective film is thick, the electromagnetic conversion characteristics are deteriorated and the adhesion to the magnetic layer is deteriorated. When the carbon protective film is thin, the wear resistance is insufficient. Accordingly, the film thickness is preferably 2.5 to 20 nm, and more preferably 5 to 10 nm.

  In order to improve the adhesion between the protective film and the magnetic layer serving as the substrate, it is preferable to etch the surface of the magnetic layer with an inert gas in advance or to modify the surface by exposure to a reactive gas plasma such as oxygen. .

  In order to improve running durability and corrosion resistance, it is preferable to apply a lubricant or a rust preventive agent on the magnetic layer or protective film. As the lubricant to be added, known hydrocarbon lubricants, fluorine lubricants, extreme pressure additives and the like can be used.

  Examples of hydrocarbon-based lubricants include carboxylic acids such as stearic acid and oleic acid; esters such as butyl stearate; sulfonic acids such as octadecyl sulfonic acid; phosphate esters such as monooctadecyl phosphate; stearyl alcohol and oleyl alcohol. Alcohols such as stearamide, amines such as stearylamine, and the like.

  Examples of the fluorine-based lubricant include a lubricant in which part or all of the alkyl group of the hydrocarbon-based lubricant is substituted with a fluoroalkyl group or a perfluoropolyether group.

Examples of perfluoropolyether groups include perfluoromethylene oxide polymer, perfluoroethylene oxide polymer, perfluoro-n-propylene oxide polymer (CF ₂ CF ₂ CF ₂ O) _n , perfluoroisopropylene oxide polymer (CF (CF ₃ ) CF ₂ O) _n or a copolymer thereof.

  In addition, a compound having a polar functional group such as a hydroxyl group, an ester group, or a carboxyl group at the terminal or in the molecule of the alkyl group of the hydrocarbon-based lubricant is preferable because of its high effect of reducing frictional force.

  Furthermore, this molecular weight is 500-5000, Preferably it is 1000-3000. By setting it as 500-5000, volatilization can be suppressed and the fall of lubricity can be suppressed. Further, it is possible to prevent the viscosity from becoming high, and it is possible to prevent the slider and the disk from being easily adsorbed to cause a stoppage of travel or a head crash.

  Specifically, this perfluoropolyether is commercially available under trade names such as FOMBLIN manufactured by Augmond and KRYTOX manufactured by DuPont.

  Examples of extreme pressure additives include phosphate esters such as trilauryl phosphate; phosphites such as trilauryl phosphite; thiophosphites and thiophosphates such as trilauryl trithiophosphite; dibenzyl disulfide; And sulfur-based extreme pressure agents such as

  The lubricants may be used alone or in combination. As a method for applying these lubricants on the magnetic layer or the protective film, the lubricant is dissolved in an organic solvent and applied by a wire bar method, a gravure method, a spin coating method, a dip coating method, or a vacuum deposition method. Can be attached.

  Examples of rust inhibitors include nitrogen-containing heterocycles such as benzotriazole, benzimidazole, purine, and pyrimidine, and derivatives in which an alkyl side chain is introduced into the mother nucleus; benzothiazole, 2-mercapton benzothiazole, tetrazaindene And nitrogen- and sulfur-containing heterocycles such as ring compounds and thiouracil compounds and derivatives thereof.

<Physical properties>
The magnetic recording medium of the present invention preferably has physical characteristics as described below.

  The saturation magnetic flux density of the magnetic layer of the magnetic recording medium according to the present invention is preferably 0.1 to 0.3T. The coercive force Hc of the magnetic layer is preferably 159 kA / m (2000 Oe) to 796 kA / m (10000 Oe), more preferably 159 to 239 kA / m (2000 to 3000 Oe). The coercive force distribution is preferably narrow, and the SFD is preferably 0.6 or less.

  In the case of a magnetic disk, the squareness ratio is 0.55 or more and 0.67 or less in the case of two-dimensional random, preferably 0.58 or more and 0.64 or less, and in the case of three-dimensional random, 0.45 or more, 0.55. In the case of vertical alignment, it is 0.6 or more, preferably 0.7 or more in the vertical direction, and 0.7 or more, preferably 0.8 or more when demagnetizing field correction is performed. The orientation 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 usually 0.55 or more, preferably 0.7 or more. 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 ° C to 40 ° C and humidity 0% to 95%. The surface specific resistance is preferably 10 ⁴ to 10 ¹² ohm / sq of the magnetic layer surface, and the charged potential is preferably 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 lower nonmagnetic layer is preferably 0 ° C. to 100 ° C.

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 average surface roughness Ra of the center plane of the magnetic layer is 4.0 nm or less, preferably 3.8 nm or less, more preferably 3.5 nm or less when measured in an area of 250 μm × 250 μm using TOPO-3D manufactured by WYKO. is there. 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% to 80%, and the average wavelength λa is preferably 5 μm to 300 μm. It is preferable to optimize the electromagnetic conversion characteristics and the friction coefficient by setting the surface protrusions of the magnetic layer as described above. 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 as described above, the roll surface shape of the calendar treatment, and the like. The curl is preferably within ± 3 mm.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto. In the following description, “parts” refers to parts by mass.

[Manufacture of magnetic particles]
(Process for producing quenched ribbon)
The following operations were performed in an Ar atmosphere. First, the raw material (Nd—Fe—B alloy powder) was melted in an arc furnace so as to have the composition shown in Table 1 below, and the molten alloy was cooled and placed in a quartz tube. The alloy was melted at high frequency, pressure was applied with Ar gas, and the molten metal was sprayed from a quartz tube through a quartz orifice onto a rotating copper roll. The rotational speed of the roll was 20 m / s.

  Thereby, a rapidly cooled ribbon (thickness: 10 μm) could be obtained. This was heated at 500 ° C. until the particle diameter was 25 nm. As a result, nanocrystals composed of rare earth-transition metal-metalloid single crystals shown in Table 1 below, and rare earth-transition metal phases were present at the grain boundaries.

(Removal of rare earth-transition metal-metalloid single crystals from quenched ribbon)
The rare earth-transition metal phase was dissolved and removed by immersing the quenched ribbon in a 0.1 kmol / m ³ aqueous NaCl solution prepared using distilled water deoxygenated by bubbling with N ₂ gas. Thereafter, NaCl was removed by washing with deoxygenated distilled water. The particle diameter of the single crystal particles after removal (magnetic particles before forming the oxide film) was 14 nm.

(Formation of oxide film)
By heating the single crystal particles in air at 60 ° C. for 4.8 hours, an oxide film as a passive film was formed on the surface to produce magnetic particles. When confirmed with a transmission electron microscope, the thickness of the oxide film was 5 nm.

  The composition and magnetic properties of the obtained magnetic particles are shown in Table 1 below. Evaluation of magnetic properties (coercivity and saturation magnetization measurement) was performed under the condition of applied magnetic field of 790 kA / m (10 kOe) using Toei Kogyo's highly sensitive magnetization vector measuring machine and the company's DATA processing device. It was.

[Example 1]
(Preparation of magnetic liquid for magnetic layer)
The following materials were mixed to prepare a magnetic liquid for the magnetic layer. The “magnetic material” below is the above magnetic particle.

"Magnetic substance": 100 parts "The following compound": 7.5 parts Copolymer of vinyl chloride / vinyl acetate / glycidyl methacrylate / 2-hydroxypropyl allyl ether = 86/5/5/4 sodium hydroxyethylsulfonate Compound added with salt (SO ₃ Na = 6 × 10 ⁻⁵ eq / g, epoxy = 10 ⁻³ eq / g, weight average molecular weight (Mw) = 30,000)
“Polyurethane resin”: 5 parts (polyester polyurethane having SO ₃ Na = 7 × 10 ⁻⁵ eq / g, containing terminal OH group, Mw = 40,000, Tg 90 ° C.)
“Cyclohexanone”: 60 parts “Alumina abrasive with particle size of 0.1 μm”: 1.5 parts “Carbon black (particle size 40 nm)”: 2 parts “Methyl ethyl ketone”: 51 parts “Toluene”: 1200 parts “Polyisocyanate ( Nippon Polyurethane Coronate 3041) ”: 5 parts (solid content)
“Butyl stearate”: 5 parts “Isohexadecyl stearate”: 5 parts “Stearic acid”: 1 part “Oleic acid”: 1 part

(Preparation of nonmagnetic liquid for nonmagnetic layer)
The following materials were mixed to prepare a nonmagnetic liquid for the nonmagnetic layer.

“Titanium oxide”: 85 parts (average particle size 0.035 μm, crystalline rutile, TiO ₂ content 90% or more, surface treatment layer is alumina, SBET: 35 to 42 m ² / g, true specific gravity 4.1, pH 6. 5 to 8.0)
“The following compound”: Compound obtained by adding hydroxyethyl sulfonate sodium salt to a copolymer of 11 parts vinyl chloride / vinyl acetate / glycidyl methacrylate = 86/9/5 (SO ₃ Na = 6 × 10 ⁻⁵ eq / g Epoxy = 10 ⁻³ eq / g, Mw = 30,000)
"Sulfonic acid-containing polyurethane resin (UR8700 manufactured by Toyobo Co., Ltd.)": 10 parts (solid content)
“Cyclohexanone”: 4260 parts “Methyl ethyl ketone”: 56 parts “Butyl stearate”: 5 parts “Isohexadecyl stearate”: 5 parts “Polyisocyanate (Coronate 3041 from Nippon Polyurethane)”: 5 parts (solid content)
"Stearic acid": 1 part "Oleic acid": 1 part

(Preparation of coating solution for backcoat layer formation)
The materials used for preparing the coating solution for forming the backcoat layer are as follows.

“Fine particle carbon black powder (average particle size: 38 nm)”: 100 parts “Coarse particle carbon black powder (average particle size 80 nm)”: 4 parts “Polyurethane resin (neopentyl glycol / caprolactone polyol / MDI = 0.9) /2.6/1, -SO ₃ Na group 1 × 10 ^-4 eq / g) ": 90 parts" Polyinocyanate (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) ": 15 parts" Methyl ethyl ketone ": 1330 parts" Cyclohexanone " : 420 parts

  Fine carbon black powder, coarse carbon black powder and a prescribed amount of 50% polyurethane resin are kneaded together with 300 parts of methyl ethyl ketone and 200 parts of cyclohexanone, and then 6 hours using a sand mill (dispersion medium: zirconia φ0.5 μm). First dispersed. Subsequently, 130 parts of 50% polyurethane resin and methyl ethyl ketone in the prescribed amounts were further added, followed by secondary dispersion over 6 hours using a sand mill (dispersion medium: zirconia φ0.5 μm) to prepare a secondary dispersion.

  To the obtained secondary dispersion, 200 parts of methyl ethyl ketone was further added and then subjected to tertiary dispersion using a sand mill (dispersion medium: zirconia φ0.5 μm) over 3 hours to prepare a tertiary dispersion.

  After adding 15 parts of polyisocyanate, 700 parts of methyl ethyl ketone and 220 parts of cyclohexanone to the resulting tertiary dispersion, the resulting dispersion is subjected to quaternary dispersion over 1 hour using a sand mill (dispersion medium: zirconia φ0.5 μm). Prepared. The quaternary dispersion was filtered using a filter having an average pore diameter of 1 μm to prepare a backcoat layer coating solution.

The thickness of the magnetic layer forming surface is 6 μm, the center line surface roughness is 0.002 μm, the Young's modulus in the longitudinal direction is 6.86 GPa (700 kg / mm ² ), and the Young's modulus in the width direction is 4.9 GPa (500 kg / mm ^2). ) On the polyethylene naphthalate support of the lower layer coating layer (nonmagnetic liquid for nonmagnetic layer) after drying so that the thickness after drying becomes a predetermined thickness (1.5 μm). A magnetic layer magnetic liquid was simultaneously applied to the magnetic layer so that the thickness of the magnetic layer was 0.1 μm. While both layers are still wet, they are oriented by a cobalt magnet having a magnetic force of 3000 G and a solenoid having a magnetic force of 1500 G, dried, and then heated to 70 ° C. with a seven-stage calendar composed of a metal roll and an epoxy resin roll. At a speed of 200 m / min. The process was performed. Thereafter, a coating solution for forming a backcoat layer was applied to the surface on which the magnetic layer was not formed to form a backcoat layer having a thickness of 0.5 μm. Next, the roll is slit to a 1/2 inch width, and the tape after slitting is wound into a DLT type system compatible cartridge (Compact Tape IV) by 557 m to produce a DLT4000 computer data recording magnetic tape (magnetic recording medium) according to the present invention. did.

[Example 2]
A magnetic recording medium was produced in the same manner as in Example 1 except that the average particle size of the coarse-particle carbon black in the back coat layer was changed to 90 nm.

[Example 3]
A magnetic recording medium was produced in the same manner as in Example 2 except that the average particle size of the fine carbon black in the backcoat layer was changed to 25 nm.

[Example 4]
A magnetic recording medium was produced in the same manner as in Example 1 except that a smooth base was used and the number of protrusions having a height of 50 nm or more on the surface of the backcoat layer was as shown in Table 2 below.

[Example 5]
A magnetic recording medium was manufactured in the same manner as in Example 3 except that the calendar conditions were set at 70 ° C. to 40 ° C. and the number of protrusions having a height of 50 nm or more on the surface of the backcoat layer was as shown in Table 2 below.

[Comparative Example 1]
A magnetic recording medium was prepared in the same manner as in Example 2 except that the average particle size of the fine particle carbon black of the back coat layer was changed to 17 nm and the average particle size of the coarse particle carbon was changed to 350 nm.

[Comparative Example 2]
Magnetic field is the same as in Example 2 except that a base (support) in which only the roughness of the back surface is changed (the number of protrusions having a height of 50 nm or more is changed as shown in Table 2) without using a backcoat layer is used. A recording medium was produced.

[Comparative Example 3]
A magnetic recording medium was produced in the same manner as in Example 2 except that the magnetic material (magnetic particles) was changed to MP: 100 nm, axial ratio: 6, σs: 130 emu / g, and HC: 2300 Oe.

[Evaluation]
(Method for measuring thickness of magnetic layer)
About each magnetic recording medium of an Example and a comparative example, it cut out to the thickness of about 0.1 micrometer with the diamond cutter over the longitudinal direction, and it observed with the magnification of 30,000 times with the transmission electron microscope, and performed the photography. . The photo print size is A4 size. After that, focusing on the difference in the shape of the magnetic layer, the ferromagnetic powder of the nonmagnetic layer, and the nonmagnetic powder, the interface was visually judged and blackened, and the magnetic layer surface was similarly blackened. Thereafter, the interval between the lines bordered by an image processing apparatus IBAS2 manufactured by Zeiss was measured. The sample photograph was taken over a range of 21 cm and measured by taking measurement points. The simple average value of the measured values at that time was taken as the thickness of the magnetic layer.

(Particle size of ferromagnetic powder (magnetic particles) and non-magnetic powder)
Taking a transmission electron micrograph, and directly reading the short axis diameter and long axis diameter of the ferromagnetic powder from the photograph, and a method of tracing and reading the transmission micrograph with the image analysis apparatus IBASS1 manufactured by Carl Zeiss. The average particle size was determined by appropriately using in combination.

(SNR and noise)
Using an LTO drive, noise, SNRsk and error rate were measured according to ECMA standards. An MR element having a width of 3 μm was used for the reproducing head. The noise in Table 2 was evaluated for each magnetic recording medium of the example and other comparative examples, with Comparative Example 1 being 0 dB.

(Winding characteristics and running durability)
The tape was wound on an LTO1 cartridge for 600 m, the full length was recorded with the same drive (manufactured by IBM) at a conveyance speed of 5 m / sec, and the wound surface of the tape was observed to evaluate the winding characteristics. Specifically, the number of protruding points on the tape winding surface was counted, and the case where the number of protruding points was 3 or less was set as an allowable range (“good” in Table 2), and when exceeding the allowable range, it was determined as “bad”. The running durability was a 200-pass full length running test in 5 environments of 5 ° C. 80% RH, 5 ° C. 10% RH, RT, 40 ° C. 80% RH, 40 ° C. 10% RH. “○” indicates that the running durability is good and the error rate does not increase very much (within one digit) in all environments, and “×” indicates that the driving has stopped or the error rate has increased by two digits or more. "

  From Table 2 above, it was clear that all of the examples were superior in balance in terms of SNR and winding shape performance after running compared to the comparative example.

Claims (2)

A magnetic recording medium having a magnetic layer on one side of a support and a backcoat layer on the other side,
The magnetic layer contains magnetic particles composed of a rare earth-transition metal-metalloid single crystal having a particle diameter of 5 to 50 nm and a binder;
The magnetic recording medium, wherein the number of protrusions having a height of 50 nm or more from the surface of the back coat layer is 30 to 200 (pieces / (265 × 350 μm ² )) or less.   Carbon black is contained in the back coat layer, and the carbon black is composed of a combination of fine particle carbon black having an average particle size of 10 to 50 nm and coarse particle carbon black having an average particle size of 60 to 300 nm. The magnetic recording medium according to claim 1.