JP2004281025A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high density magnetic recording medium which is suitable to use with MR heads, and has excellent electromagnetic conversion capability, run durability, still characteristics and unbalanced head abrasion resistance. <P>SOLUTION: The magnetic recording medium has a magnetic layer made by dispersing ferromagnetic powder and polishing materials in a binder on a base, and the magnetic layer is 30-100nm thick. Further, the value of A is 1.25-3.80 in the expression A=D/T which shows the relation of the average size (D) of the polishing material and the thickness (T) of the magnetic layer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium excellent in electromagnetic conversion characteristics, running durability, and still durability.

  The coating type magnetic recording medium has a magnetic layer in which components such as a ferromagnetic powder and an inorganic additive are dispersed in a binder on a nonmagnetic support. One such magnetic recording medium is a magnetic recording medium in which a magnetic layer is provided on a flexible nonmagnetic support.

  The magnetic recording medium is required to have excellent wear resistance and durability because it comes into contact with the magnetic head and the hard side member during recording and reproduction. In recent years, the density of these magnetic recording media has been increased, and a magnetic recording medium having good durability even on a smooth surface has been demanded.

In general, one way to improve durability is to add an abrasive such as α-Al ₂ O ₃ or Cr ₂ O _{3 in} the magnetic layer to improve the internal strength of the coating and improve wear resistance. Has been tried.

  In recent years, further increase in density of magnetic recording media has been demanded, and the adoption of magnetoresistive (MR) heads has been advanced. MR heads have high reproduction sensitivity, and the amount of magnetization required for the magnetic layer is reduced as compared with the prior art, and an extremely thin magnetic layer is required.

  With the thinning of the magnetic layer, it has become difficult to ensure the uniformity of the magnetic layer thickness. Further, due to the thinning of the magnetic layer, the magnetic layer thickness is thinner than the grain size of the abrasive in a currently used abrasive. In this case, since the balance with the abrasive particle size is not appropriate, an appropriate polishing ability cannot be obtained and durability deteriorates, and electromagnetic conversion characteristics deteriorate.

  When using an abrasive that is significantly larger than the magnetic layer thickness due to thinning of the magnetic layer, the abrasive that is exposed larger than the coating promotes head wear, or the coating is scraped off due to the falling off of the abrasive. There was a problem.

  On the other hand, if the abrasive is smaller than necessary, the wear resistance of the coating film is significantly reduced.

Conventionally, attempts have been made to improve the uniformity of the magnetic layer thickness by a method in which the magnetic layer is applied after the nonmagnetic layer is applied and dried. For example, Patent Document 1 is cited.
On the other hand, the relationship between the magnetic layer thickness and the abrasive particle size has been studied. In Patent Document 2 and Patent Document 3, the magnetic layer thickness and the abrasive particle size are defined by numerical values. However, the relationship of the abrasive particle size to the magnetic layer thickness was not clear.

  Therefore, in Patent Document 4, Patent Document 5, and Patent Document 6, the relationship between the magnetic layer thickness and the abrasive particle size is clarified so that the abrasive particle size range relative to the magnetic layer thickness can be specified. However, these are so-called wet-on-wet coating methods in which the magnetic layer is applied while the nonmagnetic layer is in a wet state, and the design values take into consideration that the abrasive is embedded in the nonmagnetic layer. Yes. For this reason, when a thin magnetic layer is applied after drying the nonmagnetic layer, there is a problem such as excessive wear. Although Patent Document 7 shows the relationship between the magnetic layer thickness and the abrasive particle size in the wet-on-dry coating method, the magnetic layer thickness is at least 0.2 μm, and the conventional magnetic induction head is used. Designed for this. Therefore, the MR head medium having a smooth surface and a thin magnetic layer for higher density recording ensures electromagnetic conversion characteristics and satisfies running durability, still durability, and uneven wear resistance of the head. I couldn't.

Japanese Patent Laid-Open No. 11-203676 JP-A-5-282658 JP-A-8-263826 Japanese Patent Laid-Open No. 7-272256 JP 7-334835 A Japanese Patent Laid-Open No. 10-247316 JP 2001-101647 A

  It is an object of the present invention to provide a high recording density magnetic recording medium suitable for an MR head medium that satisfies excellent electromagnetic conversion characteristics and excellent running durability, still durability, and uneven wear resistance of the head.

The present invention solves the above problems by the following magnetic recording medium.
(1) In a magnetic recording medium provided with a magnetic layer in which a ferromagnetic powder and an abrasive are dispersed in a binder on a support, the magnetic layer has a thickness of 30 to 100 nm, and an average powder of the abrasive A magnetic recording medium, wherein a value A of a relational expression A = D / T between a body size (D) and a magnetic layer thickness (T) is 1.25 to 3.80.
(2) The magnetic recording medium as described in (1) above, wherein a substantially non-magnetic lower layer is provided between the support and the magnetic layer.
(3) The magnetic recording medium as described in (1) or (2) above, wherein the abrasive has a Mohs hardness of 5 or more and a relative polishing rate of 200 or more.
(4) The magnetic recording medium as described in any one of (1) to (3) above, wherein the average powder size of the ferromagnetic powder contained in the magnetic layer is 20 to 60 nm.
(5) The magnetic recording medium according to any one of (1) to (4), wherein the magnetic layer contains 2 to 15 parts by mass of an abrasive with respect to 100 parts by mass of the ferromagnetic powder.
(6) The magnetic recording medium according to any one of (1) to (5) above, wherein the magnetic recording medium is a magnetic tape having a back layer on the opposite surface of the support on which the magnetic layer is provided.
(7) Any of (1) to (6) above, wherein a magnetic layer is formed on the lower layer after applying and drying a substantially nonmagnetic lower layer containing a binder on the support. A magnetic recording medium according to claim 1.

  The present invention can improve electromagnetic conversion characteristics, running durability, and still durability, and can effectively reduce the amount of uneven head wear, and can achieve a high recording density suitable for an MR head medium. A recording medium can be provided.

  In the present invention, the thickness (T) of the magnetic layer is set to 30 to 100 nm, and the relationship between the magnetic layer thickness and the particle size of the abrasive is regulated, so that uneven wear of the head is suppressed, and excellent electromagnetic conversion characteristics in high-density recording. And excellent running durability and still durability.

The thickness (T) of the magnetic layer is a dry thickness and is controlled to 30 to 100 nm, preferably 40 to 90 nm. If it is thinner than 30 nm, there is a problem in quality assurance and productivity, and if it is larger than 100 nm, the electromagnetic conversion characteristics are not improved. When the magnetic layer is composed of two or more layers, T means the total thickness.
The relationship A = D / T between the average powder size (D) of the abrasive contained in the magnetic layer and the thickness (T) of the magnetic layer is A = 1.25 to 3.80, preferably A = 1.6. Adjusted to ~ 3.5.
From the above relational expression, D is in the range of 37.5 to 380 nm, preferably 50 to 250 μm, and more preferably 50 to 150 μm.
The abrasive is not particularly limited, but alumina is preferable.

Here, the average powder size is obtained from the following powder sizes.
In the present specification, the sizes of various powders such as the abrasive, the ferromagnetic powder, and the carbon black (hereinafter referred to as “powder size”) are obtained from a high-resolution transmission electron micrograph and an image analyzer. It is done. The powder size in the high-resolution transmission electron micrograph (magnification 20000 times) can be traced with an image analyzer to determine the size of the powder. That is, the powder size is as follows: 1) When the powder shape is needle-like, spindle-like, columnar (however, the height is larger than the maximum major axis of the bottom surface), the length of the long axis constituting the powder, That is, it is represented by the long axis length. 2) When the shape of the powder is plate-like or columnar (however, the thickness or height is smaller than the maximum major axis of the plate surface or bottom surface) like hexagonal ferrite magnetic powder, It is expressed by the maximum major axis of the plate surface or the bottom surface, that is, the plate diameter. 3) The shape of the powder is spherical, polyhedral, unspecified, etc., and the major axis constituting the powder cannot be specified from the shape. Is represented by the equivalent circle diameter. The equivalent circle diameter is a value obtained by a circle projection method.

Further, the average powder size of the powder is an arithmetic average of the above powder sizes, and is obtained by performing the measurement as described above on 100 primary particles. Primary particles refer to an independent powder without aggregation.
In the case where the shape of the powder is specific, for example, in the case of definition 1) of the above powder size, the average powder size is referred to as the average major axis length, and the arithmetic of the value of (major axis length / minor axis length) is performed. The average is called the average needle ratio. The short axis length is the maximum axis that is perpendicular to the long axis. In the case of definition 2), the average powder size is called the average plate diameter, and the arithmetic average of (plate diameter / plate thickness) is called the average plate ratio. Here, the plate thickness is a thickness or a height. In the case of definition 3), the average powder size is called the average particle size.
In powder size measurement, a standard deviation / average value expressed as a percentage is defined as a coefficient of variation. The coefficient of variation of the abrasive particle diameter is preferably 0 to 50%, more preferably 0 to 30%.

In addition, it is preferable that the average powder size D of an abrasive | polishing agent is represented by the average particle diameter in description of the said average powder size.
T is calculated | required by the following.
The longitudinal direction of the magnetic recording medium is cut into a thickness of about 0.1 μm with a diamond cutter, observed with a transmission electron microscope at a magnification of 30,000, and photographed. The photo print size is A4 size. Then, pay attention to the shape difference between the magnetic layer and the ferromagnetic powder of the non-magnetic layer and the non-magnetic powder, and visually determine the interface, and the surface of the magnetic layer is similarly blacked. Then, the image analysis apparatus (Carl Zeiss) The distance between the edged lines was measured with KS400. A sample photograph is taken over a range of 21 cm, and 500 measurement points are taken for measurement. The simple addition average of the measured values at that time is divided by the magnification to obtain the thickness of the magnetic layer.

The abrasive preferably has a Mohs hardness of 5 or more, more preferably 8 or more.
The relative polishing rate of the abrasive is preferably 200 or more, more preferably 200 to 600, and particularly preferably 200 to 350.
In the present invention, the relative polishing rate means that the Mn-Zn ferrite single crystal (111) surface is polished on a buff while quantitatively supplying an abrasive slurry, and the amount of polishing (length) per unit time is the standard sample. This is a value obtained by comparison with the polishing amount. The composition of the abrasive slurry is 2 parts by weight of the sample, 98 parts by weight of water, the standard sample is AKP20 manufactured by Sumitomo Chemical Co., Ltd., 2 parts by weight, 98 parts by weight of water, and the polishing rate of the standard sample is 100. Represent.
The abrasive is preferably contained in the magnetic layer in an amount of 2 to 15 parts by mass, more preferably 3 to 7.5 parts by mass with respect to 100 parts by mass of the ferromagnetic powder. If it is less than 2 parts by mass, running durability and still durability are difficult to improve, and if it exceeds 15 parts by mass, head wear increases and electromagnetic conversion characteristics tend to deteriorate.

The magnetic recording medium of the present invention preferably has a magnetic layer on one side of the support and a back layer on the opposite side. The magnetic recording medium of the present invention includes those having layers other than the magnetic layer and the back layer. For example, it may have a soft magnetic layer containing soft magnetic powder, a second magnetic layer, a cushion layer, an overcoat layer, an adhesive layer, and a protective layer. These layers can be provided at appropriate positions so that their functions can be effectively exhibited.
In the present invention, a configuration in which a magnetic layer is provided on a substantially non-magnetic lower layer (hereinafter also simply referred to as a lower layer or a non-magnetic layer) is preferable. The magnetic layer in this case is also referred to as an upper layer or an upper magnetic layer.
The upper layer can be formed by wet-on-wet (W / W) provided while the lower layer is wet after applying the lower layer simultaneously or sequentially, or wet-on-dry (W / D) provided after the lower layer is dried. .
In the present invention, it is preferable to provide the lower layer and the upper layer by wet-on-dry in order to suppress the interface fluctuation.
As the coating means, gravure coating, reverse roll coating, die nozzle coating or the like can be used.
The thickness of the layer can be 0.5 to 3 μm for the nonmagnetic layer. The nonmagnetic layer is preferably thicker than the magnetic layer.

The ferromagnetic powder used in the magnetic layer of the magnetic recording medium of the present invention is not particularly limited, but ferromagnetic metal powder or hexagonal ferrite powder is preferable.
The average powder size of the ferromagnetic powder is preferably 20 to 60 nm. When the ferromagnetic powder used in the present invention is acicular or the like, the average powder size is represented by an average major axis length, preferably 30 to 45 nm, and the average acicular ratio is 3 to 7. In the case of a plate shape, the average powder size is represented by an average plate diameter, preferably 25 to 35 nm, and the average plate ratio is preferably 2 to 5.

The ferromagnetic metal powder usually has an S _BET (BET specific surface area) of 40 to 80 m ² / g, preferably 50 to 70 m ² / g. The crystallite size is usually 10 to 25 nm, preferably 11 to 22 nm. The pH of the ferromagnetic metal powder is preferably 7 or more. Examples of the ferromagnetic metal powder include a simple substance or an alloy such as Fe, Ni, Fe—Co, Fe—Ni, Co—Ni, and Co—Ni—Fe, and within a range of 20% by mass or less of the metal component, aluminum, Silicon, sulfur, scandium, titanium, vanadium, chromium, manganese, copper, zinc, yttrium, molybdenum, rhodium, palladium, gold, tin, antimony, boron, barium, tantalum, tungsten, rhenium, silver, lead, phosphorus, lanthanum, Cerium, praseodymium, neodymium, tellurium, bismuth, and the like can be included. Further, the ferromagnetic metal powder may contain a small amount of water, hydroxide or oxide. Methods for producing these ferromagnetic metal powders are already known, and the ferromagnetic metal powders used in the present invention can also be produced according to known methods. There are no particular restrictions on the shape of the ferromagnetic metal powder, but needle-shaped, granular, dice-shaped, rice-grained and plate-shaped ones are usually used. It is particularly preferable to use acicular ferromagnetic powder.
The coercive force Hc of the ferromagnetic metal powder is preferably 144 to 300 kA / m, and more preferably 160 to 224 kA / m. Further, the saturation magnetization is preferably 85~150A · m ² / kg, more preferably 100~130A · m ² / kg.

  Examples of the hexagonal ferrite powder include barium ferrite, strontium ferrite, lead ferrite, calcium ferrite, and various substitutes thereof such as a Co substitute. Specific examples include magnetoplumbite-type barium ferrite and strontium ferrite, magnetoplumbite-type ferrite whose particle surface is coated with spinel, and composite magnetoplumbite-type barium ferrite and strontium ferrite partially containing a spinel phase. In addition to predetermined atoms, Al, Si, S, Nb, Sn, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, W, Re, Au, Bi, It may contain atoms such as La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, B, Ge, and Nb. Generally, Co—Zn, Co—Ti, Co—Ti—Zr, Co—Ti—Zn, Ni—Ti—Zn, Nb—Zn—Co, Sn—Zn—Co, Sn—Co—Ti, Nb—Zn, etc. The thing which added these elements can be used. Some raw materials and manufacturing methods contain specific impurities. The hexagonal ferrite powder has a hexagonal plate shape.

In particular, when reproducing with a magnetoresistive head (MR head) in order to increase the track density, it is necessary to reduce noise. If the average plate diameter is too small, stable magnetization cannot be expected due to thermal fluctuation. Also, if the average plate diameter is too large, the noise is high and none is suitable for high density magnetic recording. When the average plate ratio is small, the filling property in the magnetic layer is preferably increased, but sufficient orientation cannot be obtained. If the average plate ratio is too large, noise increases due to stacking between powders. The specific surface area according to the BET method is usually 30 to ²⁰⁰ m ² / g, and preferably 50 to 100 m ² / g. The specific surface area is generally referred to as an arithmetic calculation value from the powder plate diameter and plate thickness. The narrower the distribution of plate diameter and plate thickness, the better. In many cases, the distribution is not a normal distribution, but when calculated and expressed as a standard deviation with respect to the powder size, σ / (average plate diameter or average plate thickness) = 0.1 to 0.5. In order to sharpen the powder size distribution, the powder production reaction system is made as uniform as possible, and the produced powder is subjected to a distribution improving process. For example, a method of selectively dissolving ultrafine powder in an acid solution is also known. In the vitrification crystal method, a more uniform powder is obtained by performing heat treatment a plurality of times to separate nucleation and growth. The coercive force Hc measured with the magnetic powder can be made up to about 40 to 400 kA / m, but preferably 144 to 300 kA / m. Higher Hc is more advantageous for high density recording, but is limited by the capacity of the recording head. Hc can be controlled by powder size (plate diameter / plate thickness), type and amount of contained elements, element substitution site, powder generation reaction conditions, and the like.

The saturation magnetization σs of the hexagonal ferrite powder is preferably 30 to 70 A · m ² / kg. σs tends to decrease as the powder becomes finer. Production methods include a method of reducing the crystallization temperature or heat treatment temperature time, a method of increasing the amount of the compound to be added, and a method of increasing the surface treatment amount.
It is also possible to use W-type hexagonal ferrite. When the magnetic material is dispersed, the surface of the magnetic material powder is treated with a material suitable for the dispersion medium and polymer. As the surface treating agent, an inorganic compound or an organic compound is used. Typical examples of the main compound include oxides or hydroxides such as Si, Al, and P, various silane coupling agents, and various titanium coupling agents. The amount is 0.1 to 10% by mass relative to the magnetic substance. The pH of the magnetic material is also important for dispersion. Usually, about 4 to 12 has optimum values depending on the dispersion medium and polymer, but about 6 to 11 is selected from the chemical stability and storage stability of the medium. Water contained in the magnetic material also affects the dispersion. Although there is an optimum value depending on the dispersion medium and polymer, 0.1 to 2.0% by mass is usually selected. The hexagonal ferrite production method is as follows: (1) Barium carbonate, iron oxide, metal oxide that replaces iron, and boron oxide as a glass-forming substance are mixed so as to have a desired ferrite composition, and then melted and quenched. And then re-heat treatment, washing and pulverization to obtain barium ferrite crystal powder, (2) neutralizing barium ferrite composition metal salt solution with alkali, by-product Hydrothermal reaction method to obtain barium ferrite crystal powder after liquid phase heating at 100 ° C. or higher after removal of water, (3) neutralizing barium ferrite composition metal salt solution with alkali, by-product There is a coprecipitation method in which the product is removed, dried, treated at 1100 ° C. or lower, and pulverized to obtain a barium ferrite crystal powder, but the present invention does not select a production method.

Examples of the abrasive that can be used in the magnetic layer of the present invention include α-alumina, β-alumina, γ-alumina, α-alumina of 90% or more, fine particle diamond, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, γ -Known materials such as iron oxide, goethite, corundum, silicon nitride, silicon carbide, titanium carbide, titanium oxide, zinc oxide, tin oxide, zirconium oxide, silicon dioxide, boron nitride, etc., mainly known materials having a Mohs hardness of 5 or more are used alone or in combination. Used in.
Moreover, you may use the composite_body | complex (what surface-treated the abrasive | polishing agent with the other abrasive | polishing agent) of these abrasive | polishing agents. 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 powder size of these abrasives preferably has a narrow particle size distribution, particularly in order to improve electromagnetic conversion characteristics. Further, in order to improve the durability, it is possible to combine abrasives having different powder sizes as necessary, or to obtain the same effect by widening the particle size distribution even with a single abrasive. The tap density is preferably 0.3 to 1.5 g / cc, the water content is 0.1 to 5% by mass, the pH is 2 to 11, and the specific surface area is preferably 1 to 40 m ² / g. The shape of the abrasive used in the present invention may be any of a needle shape, a spherical shape, and a dice shape, but those having a corner in a part of the shape are preferable because of high polishing properties.

  Specifically, AKP-10, AKP-15, AKP-20, AKP-30, AKP-50, HIT-20, HIT-30, HIT-50, HIT-60A, HIT-50G, HIT- manufactured by Sumitomo Chemical Co., Ltd. 70, HIT-80, HIT-82, HIT-100, Reynolds ERC-DBM, HP-DBM, HPS-DBM, Fujimi Abrasives WA10000, Uemura Kogyo UB20, Nippon Kagaku Kogyo G- 5, Chromex U2, Chromex U1, TF100, TF140 manufactured by Toda Kogyo Co., Ltd., Beta Random Ultra Fine manufactured by Ibiden, B-3 manufactured by Showa Mining Co., Ltd., and the like. 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 powder size and amount of the abrasive added to the nonmagnetic layer should of course be set to optimum values.

As the binder used in the magnetic layer and the nonmagnetic layer of the magnetic recording medium of the present invention, 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 of such include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl butyral, vinyl 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, Examples thereof include a mixture of polyester 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, 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), OH, NR ₂ , N ⁺ R ₃ (R is a hydrocarbon group), epoxy group, SH, CN, etc. It is preferable to use one in which at least one polar group 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 of these binders used in the present invention include VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, PKFE, Union Carbide, Nissin Chemical Industry, MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, MPR-TAO, Electrochemical 1000W, DX80, DX81, DX82, DX83 , 100FD, Nippon Zeon MR-104, MR-105, MR110, MR100, MR555, 400X-110A, Nippon Polyurethane Nipporan N2301, N2302, N2304, Dainippon Ink Pandex T-5105, T-R3080, T- 5201, bar Knock D-400, D-210-80, Crisbon 6109, 7209, Toyobo's Byron UR8200, UR8300, UR-8700, RV530, RV280, manufactured by Daiichi Seika, Daiferamin 4020, 5020, 5100, 5300, 9020, 9022, 7020 , Mitsubishi Chemical, MX5004, Sanyo Kasei Samprene SP-150, Asahi Kasei Saran F310, F210, and the like.

  The binder used in the nonmagnetic layer and the magnetic layer of the present invention is usually used in the range of 5 to 50% by mass, preferably in the range of 10 to 30% by mass with respect to the nonmagnetic powder or the ferromagnetic powder. In the case of using a vinyl chloride resin, it is preferable to use a combination of 5 to 30% by mass, in the case of using a polyurethane resin, 2 to 20% by mass, and polyisocyanate in the range of 2 to 20% by mass. In the case where head corrosion occurs due to dechlorination, it is possible to use only polyurethane or only polyurethane and isocyanate. In the present invention, when polyurethane is used, the glass transition temperature is usually −50 to 150 ° C., preferably 0 to 100 ° C., the breaking elongation is 100 to 2000%, the breaking stress is 0.49 to 98 MPa, and the yield point is 0.2. 49-98 MPa is preferable.

  In the present invention, at least a binder, an abrasive, and a ferromagnetic powder are kneaded and dispersed together with a solvent such as methyl ethyl ketone, dioxane, cyclohexanone, and ethyl acetate, which are usually used in the preparation of magnetic paints. Use paint. The kneading and dispersing can be performed according to a usual method. In addition to the above components, the magnetic layer-forming coating material may contain commonly used additives or fillers such as antistatic agents such as carbon black, lubricants such as fatty acids, fatty acid esters and silicone oils, and dispersants. Good.

Next, the nonmagnetic layer will be described. The nonmagnetic layer of the present invention preferably contains at least a nonmagnetic powder and a binder.
The nonmagnetic powder used in the nonmagnetic layer of the present invention can be selected from inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, and nonmagnetic metals. . As an inorganic compound, for example, titanium oxide (TiO ₂ , TiO), α-alumina, β-alumina, γ-alumina, α-iron oxide, chromium oxide, zinc oxide, tin oxide, oxidized with an α conversion rate of 90-100% Tungsten, vanadium oxide, silicon carbide, cerium oxide, corundum, silicon nitride, titanium carbide, silicon dioxide, magnesium oxide, zirconium oxide, boron nitride, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide, goethite, hydroxide Aluminum or the like can be used alone or in combination. Particularly preferred are titanium dioxide, zinc oxide, iron oxide and barium sulfate, and more preferred are titanium dioxide described in JP-A-5-182177, and JP-A-6-60362 and JP-A-9-170003. Α-iron oxide described in the publication. Nonmagnetic metals include Cu, Ti, Zn, Al and the like. These nonmagnetic powders preferably have an average particle size of 0.005 to 2 μm. However, if necessary, nonmagnetic powders having different average particle sizes may be combined or a single nonmagnetic powder may have a wide particle size distribution. The same effect can be given. Particularly preferred is a nonmagnetic powder having an average particle size of 0.01 μm to 0.2 μm. The pH of the nonmagnetic powder is particularly preferably 6-9. The specific surface area of the non-magnetic powder is 1 to 100 m ² / g, preferably 5 to 50 m ² / g, more preferably 7 to 40 m ² / g. The crystallite size of the nonmagnetic powder is preferably 0.01 μm to 2 μm. The oil absorption using DBP is 5 to 100 ml / 100 g, preferably 10 to 80 ml / 100 g, and more preferably 20 to 60 ml / 100 g. The specific gravity is 1 to 12, preferably 3 to 6. The shape may be any of a needle shape, a spherical shape, a polyhedron shape, and a plate shape.

The surface of the nonmagnetic powder is preferably surface-treated so that at least a part thereof is coated with Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , and ZnO. Of these, particularly providing good dispersibility is _{Al 2 O 3, SiO 2,} TiO 2, ZrO 2, further preferred is _{Al 2 O 3, SiO 2,} ZrO 2. These may be used in combination or may be used alone. Further, a surface-treated layer co-precipitated depending on the purpose may be used, or a method of first treating the surface layer with alumina and then treating the surface layer with silica, or vice versa. May be. 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.

  By mixing carbon black with the nonmagnetic layer, the surface electrical resistance Rs can be lowered and the desired micro Vickers hardness can be obtained. The average particle diameter of carbon black is usually 5 nm to 80 nm, preferably 10 to 50 nm, and more preferably 10 to 40 nm. Specifically, the same carbon black that can be used for the above-described back layer can be used.

Examples of the support that can be used in the present invention include biaxially stretched polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polyamide, polyimide, polyamideimide, aromatic polyamide, and polybenzoxidazole. it can. These supports may have been previously subjected to corona discharge, plasma treatment, easy adhesion treatment, heat treatment and the like. Further, the support that can be used in the present invention has a centerline average surface roughness of 0.1 to 20 nm, preferably 1 to 10 nm, with a cut-off value of 0.25 mm, and an excellent smooth surface. It is preferable to have the property. These supports preferably have not only a small center line average surface roughness but also no coarse protrusions of 1 μm or more. In the case of a magnetic tape, the thickness of the support is 4 to 15 μm, preferably 4 to 9 μm. When it is thin, the unevenness of the back layer is easily captured by handling tension, and this can be effectively suppressed by using the above-mentioned polyurethane resin as the uppermost layer. When the thickness is 7 μm or less, it is preferable to use an aromatic polyamide such as PEN or aramid. Most preferred is aramid.
In the case of a flexible disk, the thickness of the support is usually 20 to 80, preferably 30 to 70 μm.

  The magnetic recording medium of the present invention is preferably produced by coating and drying the magnetic layer on the nonmagnetic layer after coating and drying the nonmagnetic layer. In addition to the above-mentioned coating machines for coating each paint or the following back layer, air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, transfer roll coat, kiss coat Cast coat, spray coat, spin coat, etc. can be used. As for these, for example, “Latest Coating Technology” (May 31, 1983) issued by General Technology Center Co., Ltd. can be referred to.

The magnetic recording medium of the present invention is preferably provided with a back layer, and the magnetic recording medium provided with the back layer is suitable for a magnetic tape.
The back layer preferably uses inorganic oxide powder and carbon black. As the inorganic oxide powder, it is preferable to use any of titanium oxide, α-iron oxide, or a mixture thereof. As titanium oxide and α-iron oxide, those usually used can be used. Further, the shape of the particles is not particularly limited. When spherical titanium oxide or α-iron oxide is used, one having a particle size of 0.01 to 0.1 μm is preferable for securing the film strength of the back layer itself. When acicular titanium oxide or α-iron oxide is used, an acicular ratio of 2 to 20 is appropriate, and an acicular ratio of 3 to 10 is more preferable. Further, it is preferable that the major axis length is 0.05 to 0.3 μm and the minor axis length is 0.01 to 0.05 μm. At least a part of the surface of the inorganic oxide powder may be modified with another compound or coated with another compound. In particular, at least a part of the surface of the inorganic oxide powder is at least one compound selected from Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ and ZnO ₂ , especially Al ₂ Those coated with at least one compound selected from O ₃ , SiO _2, and ZrO ₂ are preferred from the viewpoint of excellent dispersibility in the binder. Such an inorganic oxide powder is prepared by synthesizing titanium oxide or α-iron oxide particles and then treating the surface so that another compound as described above is deposited or coated on the surface. - can be obtained by a method of coprecipitating at least one compound selected from iron oxide and _{Al 2 O 3, SiO 2,} TiO 2, ZrO 2, SnO 2, Sb 2 O 3 and ZnO _2.

  Moreover, such inorganic oxide powder can also be obtained from a commercial item. For example, DPN-245, DPN-250, DPN-250BX, DPN-270BX, DPN-550BX, DPN-550RX, TF-100, TF-120 manufactured by Toda Kogyo Co., Ltd., TTO- manufactured by Ishihara Sangyo Co., Ltd. 51A, TTO-51B, TTO-51C, TTO-53B, TTO-55A, TTO-55B, TTO-55C, TTO-55D, TTO-55N, TTO-55S, TTO-S-1, TTO-S-2, TTO-M-1, TTO-M-2, TTO-D-1, TTO-D-2, SN-100, E270, E271, STT-4D, STT-30D and STT-30 manufactured by Titanium Industry Co., Ltd. , STT-65C, MT-100F, MT-100S, MT-100T, MT-150W, MT-500B, MT-500HD manufactured by Teika Co., Ltd. MT-600B, manufactured by Nippon Aerosil Co., Ltd. of TiO2P25, mention may be made of a super Thailand Tania, such as manufactured by Showa Denko (Ltd.).

It is preferable to use carbon black for the back layer to prevent electrification. As the carbon black used for the back layer, those commonly used for magnetic recording media can be widely used. For example, furnace black for rubber, thermal black for rubber, carbon black for color, acetylene black, and the like can be used. In order to prevent the irregularities of the back layer from appearing on the magnetic layer, the particle size of the carbon black is preferably 0.3 μm or less. A particularly preferable particle diameter is 0.01 to 0.1 μm. Carbon black preferably has a pH of 2 to 10, a moisture content of 0.1 to 10%, and a tap density of 0.1 to 1 g / ml. The specific surface area is usually, 100~500m ² / g, preferably from 150~400m ² / g, DBP oil absorption 20 to 400 ml / 100 g, preferably 30 to 200 ml / 100 g. As specific examples, BLACK PEARL S2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72 manufactured by Cabot Corporation; # 3050B, # 3150B, # 3250B, # 3250B, #, manufactured by Mitsubishi Chemical Corporation 3750B, # 3950B, # 950, # 650B, # 970B, # 850B, MA-600; Columbian Carbon's CONDUCTEX SC, RAVEN8800, 8000, 7000, 5750, 5250, 3500, 2100, same 2000, 1800, 1500, 1255, 1250; Akzo Ketchen Black EC, Asahi Carbon Corporation # 55, # 50, # 30; Columbia Carbon Corporation Raven450, 430; Thermax MT Can.

  The mass ratio of the inorganic oxide powder and carbon black is preferably 60/40 to 90/10, more preferably 70/30 to 80/20. As described above, by containing the inorganic oxide powder in a larger amount than the carbon black, it is possible to form a back layer having good powder dispersibility and a smooth surface. Further, the height of the protrusion from the back layer surface is preferably 30 nm or less in order to reduce the back image, and the above-mentioned blending ratio, type, size, etc. are selected. Such protrusions are preferably formed mainly from carbon black. The back layer forming paint having such a composition has higher thixotropic properties than the conventional back layer forming paint mainly composed of carbon black. For this reason, it is possible to perform application of an extrusion method or a gravure method at a high concentration. By applying such a high-concentration paint, it is possible to form a back layer having a high mechanical strength and a high adhesive strength with the support despite its thin film thickness. In addition, if a coating material for forming a back layer with a high concentration is used, even if an undercoat layer mainly composed of a low molecular weight polyester is formed between the back layer and the support, the low molecular weight polyester leaches out to the back layer surface. Can be suppressed. For this reason, it is possible to effectively avoid adhesion and running failure due to leaching, which has been regarded as a problem when the undercoat layer is conventionally formed. Therefore, by forming the adhesive undercoat layer, it becomes easy to adjust the adhesive strength between the back layer and the support to a desired range. However, the undercoat layer is not necessarily formed. By using vinyl chloride resin or urethane resin as the binder for the back layer forming paint, adjusting the ratio, or selecting and using a solvent, the difference in interfacial energy from the support surface is reduced, and the adhesive strength Can also be raised. The adhesive strength between the support and the back layer in the present invention can be adjusted to a desired range by appropriately selecting or combining these means. If a coating for forming a back layer containing a larger amount of inorganic oxide powder than carbon black is used, the adsorbability of the inorganic oxide powder with respect to the binder is good. It can also be reduced. The usage-amount of a binder is chosen from the range of 10-30 mass parts by making the total mass of inorganic oxide powder and carbon black into 100 mass parts, More preferably, it is 20-28 mass parts. The back layer thus formed has high film strength and low surface electrical resistance. The magnetic recording medium of the present invention having a back layer having such an excellent function has better running durability and still durability and superior electromagnetic conversion characteristics as compared with conventional products.

  For the binder for the back layer of the present invention, conventionally known thermoplastic resins, thermosetting resins, reactive resins and the like can be used. Preferred binders are vinyl chloride resins, vinyl chloride-vinyl acetate resins, fibrous resins such as nitrocellulose, phenoxy resins, and polyurethane resins. Among them, the same vinyl chloride resin, vinyl chloride-vinyl acetate resin and polyurethane resin as those used for the magnetic layer are used because the hardness of the back layer becomes close to the hardness of the magnetic layer and the back shot can be reduced. More preferable.

Polyurethane resin, -SO ₃ M in the _{molecule, -OSO 3 M, -COOM, -PO} 3 MM ', - OPO 3 MM', - NRR ', - N + RR'R "COO - ( wherein, M And M ′ are each independently a hydrogen atom, an alkali metal, an alkaline earth metal, or ammonium, R and R are each independently an alkyl group having 1 to 12 carbon atoms, and R ″ is an N bond having 1 to 12 carbon atoms. It preferably contains at least one polar group selected from (which represents an alkylene group), particularly preferably —SO ₃ M or —OSO ₃ M. The amount of these polar groups is preferably 1 × 10 ⁻⁵ to 2 × 10 ⁻⁴ eq / g, particularly preferably 5 × 10 ⁻⁵ to 1 × 10 ⁻⁴ eq / g. When the amount is less than 1 × 10 ⁻⁵ eq / g, the powder tends to be insufficiently adsorbed, so that the dispersibility tends to decrease. When the amount exceeds 2 × 10 ⁻⁴ eq / g, the solubility in the solvent decreases. Therefore, dispersibility tends to decrease.

  The number average molecular weight (Mn) of the polyurethane resin is preferably 5,000 to 100,000, more preferably 10,000 to 50,000, and particularly preferably 20,000 to 40,000. If it is less than 5000, the strength and durability of the coating film are low. Moreover, when more than 100,000, the solubility to a solvent and a dispersibility are low. The cyclic structure of the polyurethane resin contributes to rigidity, and the ether group contributes to flexibility.

  In the back layer of the magnetic recording medium of the present invention, components other than inorganic oxide powder, carbon black, and binder can be appropriately contained. The back layer preferably contains a lubricant. Examples of the lubricant include fatty acids, fatty acid esters, and fatty acid amides. In particular, the inclusion of a fatty acid is essential for suppressing an increase in the coefficient of friction during repeated running while maintaining the strength. Further, by containing a fatty acid ester or an abrasive having a Mohs hardness of 8 or more, an increase in the coefficient of friction during repeated running can be suppressed and sliding durability can be improved. Furthermore, an increase in friction coefficient can be suppressed by adding an aromatic organic acid compound or a titanium coupling agent to improve dispersibility and increase strength. Moreover, an organic powder can be contained to suppress an increase in the coefficient of friction and to reduce the reflection. Examples of fatty acids that can be added include monobasic fatty acids having 8 to 24 carbon atoms. Among these, monobasic fatty acids having 8 to 18 carbon atoms are preferable. Specific examples thereof include lauric acid, caprylic acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, and elaidic acid. In addition, amides of these fatty acids are also used.

Examples of fatty acid esters include monobasic fatty acids having 10 to 24 carbon atoms (which may include unsaturated bonds or branched) and monovalent, divalent, trivalent, and tetravalent carbon atoms having 2 to 12 carbon atoms. And mono-fatty acid ester, di-fatty acid ester or tri-fatty acid ester formed of any one of monovalent, pentavalent, and hexahydric alcohols (which may include an unsaturated bond or may be branched). . Specific examples thereof include butyl stearate, octyl stearate, amyl stearate, isooctyl stearate, octyl myristate, butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan distearate, anhydrosorbitan tristearate. Rate.
The addition amount of the lubricant is preferably 0.1 to 5 parts by mass, more preferably 0.1 to 3 parts by mass, when the total amount of the inorganic oxide powder and carbon black is 100 parts by mass.

  Examples of the abrasive having a Mohs hardness of 8 or more include α-alumina, chromium oxide, artificial diamond, and carbon-modified boron nitride (CBN). Among them, it is preferable to use those having an average particle size of 0.2 μm or less and a particle size of the back layer or less. In the present invention, since the back layer can be made thin, sufficient sliding durability can be ensured only by adding a small amount of abrasive. As the aromatic organic acid compound, phenylphosphonic acid is preferable. The amount used is usually 0.03 to 10 parts by mass, preferably 0.5 to 5 parts by mass, when the total amount of the inorganic oxide powder and carbon black is 100 parts by mass.

  Examples of the organic powder include acrylic-styrene copolymer resin powder, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment.

  The glass transition temperature of the back layer is preferably 60 to 120 ° C., and the dry thickness is usually about 0.05 to 1.0 μm.

  The magnetic recording medium of the present invention can reduce the thickness of the tape to 4 to 9 μm because the back layer is difficult to be reflected on the magnetic layer even when it is wound and stored with high tension.

  The back layer can be prepared by applying a coating material for forming a back layer in which particulate components such as an abrasive and an antistatic agent and a binder are dispersed in an organic solvent to the surface opposite to the magnetic layer. As in the above preferred embodiment, if the amount of inorganic oxide powder used is larger than that of carbon black, sufficient dispersibility can be ensured, so that the back layer can be obtained without roll kneading, which has been conventionally required. Forming paints can be prepared. Moreover, if the carbon black content ratio is low, the amount of residual cyclohexanone after drying can be reduced even if cyclohexanone is used as a solvent. The applied magnetic layer is dried after subjecting the ferromagnetic powder contained in the magnetic layer to a magnetic field orientation treatment. The magnetic field orientation treatment can be appropriately performed by a method well known to those skilled in the art. The magnetic layer is subjected to a surface smoothing treatment using a super calender roll or the like after drying. By performing the surface smoothing treatment, voids generated by the removal of the solvent at the time of drying disappear and the filling rate of the ferromagnetic powder in the magnetic layer is improved. For this reason, a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. As the calendering roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, polyamideimide or the like is used. Moreover, it can also process with a metal roll.

  The magnetic recording medium of the present invention preferably has a surface with good smoothness. In order to improve the smoothness, for example, it is effective to subject the magnetic layer formed by selecting a specific binder as described above to the calendar treatment. In the calendering, the temperature of the calender roll is set to 60 to 100 ° C., preferably 70 to 100 ° C., particularly preferably 80 to 100 ° C., and the pressure is usually 100 to 500 kg / cm (98 to 490 kN / m), preferably 200 450 kg / cm (196 to 441 kN / m), particularly preferably 300 to 400 kg / cm (294 to 392 kN / m). The obtained magnetic recording medium can be cut into a desired size using a cutting machine or the like. The magnetic recording medium that has undergone the calendar process is generally heat-treated. Recently, it is important to reduce the thermal contraction rate for the linearity of the high-density magnetic recording medium (to ensure off-track margin). In particular, as the track becomes narrower, the shrinkage rate in the MD direction (longitudinal direction) under the use environment is required to be suppressed to 0.07% or less. As a means for reducing the heat shrinkage rate, there are a method in which heat treatment is performed in a web shape while handling at a low tension, and a method in which heat treatment is performed in a form in which tapes are laminated as in a bulk or cassette (thermo treatment). When the former is used, there is little risk of unevenness on the back surface, but the heat shrinkage rate cannot be greatly reduced. Although it varies somewhat depending on the annealing temperature, staying time, tape thickness, and handling tension, 0.1 to 0.12% is the limit in terms of thermal shrinkage after 70 ° C. and 48 hours. The latter thermo-treatment can greatly improve the heat shrinkage ratio, but the unevenness of the back surface is considerably reflected, so that the magnetic layer may become rough and cause a decrease in output and an increase in noise.

  By adopting the configuration of the magnetic recording medium of the present invention, it is possible to form a layer that is highly elastic and hardly remains plastically deformed. Therefore, particularly in a magnetic recording medium with a thermo process, a high-output, low-noise magnetic recording medium. Can be supplied. In the magnetic recording medium of the present invention, since the surface of the back layer can be smoothed, the friction coefficient of the back layer can be set to a moderate level. This increases the inter-layer friction coefficient between the back layer and the magnetic layer, so that the tape wound up on the roll, slit pancake, or built-in reel is good even when handled at a high speed during the production of the magnetic recording medium. . Similarly, the winding form of the tape on the reel after the video cassette recorder is fast-forwarded or rewinded at high speed is also good.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these Examples. In addition, unless otherwise indicated, the display of "part" in an Example shows a "mass part".
Examples 1-10, Comparative Examples 1-6
Magnetic layer coating 1 (ferromagnetic metal powder)
100 parts of ferromagnetic acicular metal powder (average major axis length: 60 nm, crystallite size: 120 mm, Hc: 2300 Oe (184 kA / m), S _BET : 70 m ² / g)
18 parts of polyurethane resin (UR8200: Toyobo, sulfonic acid group-containing polyurethane resin)
Abrasives Number of parts listed in Table 1 (α-Al ₂ O ₃ , D (average particle diameter): listed in Table 1, Mohs hardness: 9)
Carbon black (average particle size: 20 nm) 0.5 part cyclohexanone 110 parts methyl ethyl ketone 100 parts toluene 100 parts butyl stearate 2 parts stearic acid 1 part

Magnetic layer coating 2 (hexagonal ferrite powder)
Barium ferrite magnetic powder 100 parts (Hc: 2500 Oe (200 kA / m), average plate diameter: 30 nm)
18 parts of polyurethane resin (UR8200: Toyobo, sulfonic acid group-containing polyurethane resin)
Abrasives Number of parts listed in Table 1 (α-Al ₂ O ₃ , D (average particle diameter): listed in Table 1, Mohs hardness: 9)
Carbon black (average particle size: 20 nm) 0.5 part cyclohexanone 110 parts methyl ethyl ketone 100 parts toluene 100 parts butyl stearate 2 parts stearic acid 1 part

Nonmagnetic layer coating material Nonmagnetic inorganic powder: 85 parts of α-iron oxide (average major axis length: 0.15 μm, average needle ratio: 7, S _BET : 52 m ² / g)
Carbon black (average particle size: 20 nm) 15 parts Vinyl chloride copolymer 13 parts (MR110, manufactured by Zeon Corporation)
6 parts of polyurethane resin (UR8200: Toyobo, sulfonic acid group-containing polyurethane resin)
α-Al ₂ O ₃ (average particle size: 0.2 μm) 1 part cyclohexanone 140 parts methyl ethyl ketone 170 parts butyl stearate 2 parts stearic acid 1 part

Back layer coating material Nonmagnetic inorganic powder: 80 parts of α-iron oxide (average major axis length: 0.15 μm, average needle ratio: 7, S _BET : 52 m ² / g)
Carbon black (average particle diameter: 20 nm) 20 parts Carbon black (average particle diameter: 100 nm) 3 parts Vinyl chloride copolymer 13 parts (MR110 manufactured by Nippon Zeon Co., Ltd.)
6 parts of polyurethane resin (UR8200: Toyobo, sulfonic acid group-containing polyurethane resin)
α-Al ₂ O ₃ (average particle size: 0.2 μm) 3 parts cyclohexanone 140 parts methyl ethyl ketone 170 parts stearic acid 3 parts

In each of the magnetic layer coating materials, as shown in Table 1, the α-Al ₂ O ₃ abrasive was added by changing the average particle diameter and the addition amount. For each of the magnetic layer coating material, the nonmagnetic layer coating material, and the back layer coating material, each component was kneaded with an open kneader for 60 minutes and then dispersed with a sand mill for 240 minutes. To the obtained dispersion, trifunctional low molecular weight polyisocyanate compound was added 4 parts for the magnetic layer paint, 5 parts each for the non-magnetic layer paint and back layer paint, and 40 parts cyclohexanone for each. In addition, the mixture was filtered using a filter having an average pore size of 1 μm to prepare a coating solution for the magnetic layer, nonmagnetic layer, and back layer.
The obtained coating solution for the nonmagnetic layer is coated and dried so that the thickness of the nonmagnetic layer after drying is 1.5 μm, and the thickness of the magnetic layer is further adjusted to the thickness shown in Table 1. In addition, coating was performed on an aramid support having a thickness of 3.6 μm and a center surface average surface roughness of 2 nm, and oriented with a magnet having a magnetic force of 0.3 T while the magnetic layer was still wet. After drying, a surface smoothing treatment was performed at a speed of 100 m / min, a linear pressure of 300 kg / cm (294 kN / m), and a temperature of 90 ° C. with a calendar composed only of metal rolls. Then, after applying a 0.5μm thick back layer, slitting to 3.8mm width, feeding the slit product, tape cleaning attached so that the nonwoven fabric and razor blade pressed against the magnetic surface to the device with the winding device The surface of the magnetic layer was cleaned with an apparatus to obtain a tape sample.

The obtained samples were evaluated by the following, and the results are shown in Table 1.
〔Measuring method〕
(1) Abrasive (particle size distribution): An appropriate amount of α-alumina was taken, and 500 particles randomly extracted from the electron micrograph were measured by the above method to determine the average particle size and coefficient of variation.
(2) Relative polishing rate: Mn-Zn ferrite single crystal (111) surface is polished on a buff while quantitatively supplying an alumina slurry, and the polishing amount (length) per unit time is compared with the polishing amount of a standard sample. did.
(3) Electromagnetic conversion characteristics The electromagnetic conversion characteristics were measured using a drum tester. A signal with a recording wavelength of 0.4 μm is written using a 1.5T MIG head, reproduced by an MR head, measured at a position ± 0.5 MHz away from a carrier output obtained by a spectrum analyzer as noise, and C / N The value was determined. The relative speed of the tape and the head is 5.1 m / s, the C / N of Example 10 is 0 dB, and 0 dB or more is considered good.
(4) Measurement of uneven wear amount of MR head After initialization with a dedicated cleaning tape using Sony DCR-IP7, the tape was run continuously for 200 hours. Thereafter, the head was removed, and the three-dimensional shape of the head surface was measured using an atomic force microscope (SPA500 manufactured by Seiko Instruments Inc.) to determine the vertical step between the vicinity of the MR element and the outer ceramic member. Here, the measurement was performed in a contact mode and a scanning speed of 1 Hz in a range of 20 μm × 20 μm. 45 nm or less is considered good.
(5) Durability Evaluation Method Running Durability: Using the above drum tester, a signal with a recording wavelength of 0.4 μm was written using a 1.5T MIG head in a room temperature environment, and then the reproduction output was continuously monitored. The time until the tape output reached 80% of the initial value was measured and used as an index of durability. 180 minutes or more is considered good.
Still characteristics: Reproduced in still mode using FUJIX8, 8 mm video cassette manufactured by Fuji Photo Film Co., Ltd. The measurement was performed in three environments of 23 ° C., 50% RH, 5 ° C., 20% RH, and 40 ° C., 80% RH.
When all three environments were able to reproduce well for 30 minutes or more, when at least one environment of three environments was successfully reproduced for 15 to 30 minutes, Δ when at least one environment of three environments was able to reproduce for less than 15 minutes The case was marked with x.

  From the above table, the examples of the present invention are excellent in electromagnetic conversion characteristics, still characteristics and running durability, and the uneven wear amount of the MR head is suppressed, but in the comparative example, the uneven wear amount of the MR head, the electromagnetic conversion. It turns out that at least any one of a characteristic and running durability is inferior.

Example 11
<Creation of paint>
Magnetic paint 100 parts of ferromagnetic metal fine powder (Co / Fe = 25 atomic%, Al / Fe = 5 atomic%, Y / Fe = 13 atomic%, Hc: 179 kA / m (2250 Oe), S _BET : 73 m ² / g , S: 125 A · m ² / kg, crystallite size: 120 mm, average major axis length: 45 nm, average needle ratio: 6, surface oxide film thickness: 25 mm)
Vinyl chloride polymer MR110 (manufactured by Nippon Zeon) 12 parts Polyurethane resin UR8200 (manufactured by Toyobo) 4 parts α-alumina (see Table 2) 5 parts Carbon black (average particle size: 40 nm) 5 parts Butyl stearate 5 parts Stearic acid 6 parts methyl ethyl ketone 180 parts cyclohexanone 180 parts

<Non-magnetic paint>
Nonmagnetic inorganic powder: 85 parts of α-iron oxide (average major axis length: 0.15 μm, average needle ratio: 7, S _BET : 52 m ² / g)
Carbon black (average particle size: 20 nm) 15 parts Vinyl chloride copolymer 13 parts (MR110, manufactured by Zeon Corporation)
6 parts of polyurethane resin (UR8200: Toyobo, sulfonic acid group-containing polyurethane resin)
α-Al ₂ O ₃ (average particle size: 0.2 μm) 1 part cyclohexanone 140 parts methyl ethyl ketone 170 parts butyl stearate 2 parts stearic acid 1 part

  About said magnetic coating material and nonmagnetic coating material, after kneading each component with a kneader, it was made to disperse | distribute for 4 hours using the sand mill. To the obtained dispersion, 2.5 parts of polyisocyanate is added to non-magnetic paint, 3 parts to magnetic paint, 40 parts of cyclohexanone is added to each, and the mixture is filtered using a filter having an average pore size of 1 μm. A non-magnetic paint and a magnetic paint were prepared, respectively. Using the obtained nonmagnetic paint and magnetic paint, on the aramid support having a nonmagnetic paint thickness of 4.4 μm and a center plane average surface roughness of 2 nm so that the thickness of the lower layer after drying is 1.5 μm. Cobalt magnet having a magnetic force of 600 mT while the two layers are still wet while applying the magnetic coating on the magnetic layer so that the thickness of the magnetic layer is 0.05 μm. And a solenoid having a magnetic force of 600 mT. After drying, a seven-stage calendar composed only of a metal roll at a temperature of 85 ° C. and a speed of 200 m / min. Then, a back layer having a thickness of 0.5 μm was applied. Slit to a width of 8mm, feed the slit product, attach the nonwoven fabric and razor blade to the magnetic surface against the magnetic surface, and clean the surface of the magnetic layer with a tape cleaning device. And tape samples were obtained.

Examples 12-14, Comparative Examples 7-10
A magnetic tape was prepared in the same manner as in Example 11 except that the thickness of the abrasive and the magnetic layer was changed as shown in Table 2.

  The performance of each of the prepared magnetic tapes was evaluated by the same measurement method as described above.

  In the examples of the present invention, each of the magnetic layer thickness D and A = D / T is set within a predetermined range, so that T and / or D is S / N, running durability, and still as compared with comparative examples that are out of the present invention. It can be seen that the characteristics are excellent. The embodiment is compatible with head wear.

Claims (7)

  In a magnetic recording medium provided with a magnetic layer in which a ferromagnetic powder and an abrasive are dispersed in a binder on a support, the magnetic layer has a thickness of 30 to 100 nm and an average powder size of the abrasive ( A magnetic recording medium characterized in that the value A of the relational expression A = D / T between D) and the thickness (T) of the magnetic layer is 1.25 to 3.80.   2. The magnetic recording medium according to claim 1, wherein a substantially non-magnetic lower layer is provided between the support and the magnetic layer.   The magnetic recording medium according to claim 1 or 2, wherein the abrasive has a Mohs hardness of 5 or more and a relative polishing rate of 200 or more.   The magnetic recording medium according to claim 1, wherein an average powder size of the ferromagnetic powder contained in the magnetic layer is 20 to 60 nm.   The magnetic recording medium according to claim 1, wherein the magnetic layer contains 2 to 15 parts by mass of an abrasive with respect to 100 parts by mass of the ferromagnetic powder.   6. The magnetic recording medium according to claim 1, wherein the magnetic recording medium is a magnetic tape having a back layer on the opposite surface of the support on which the magnetic layer is provided.   7. The magnetic recording according to claim 1, wherein a magnetic layer is formed on the lower layer after coating and drying a substantially nonmagnetic lower layer containing a binder on the support. Medium.