JP2005149621A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium wherein high recording density is realized and electromagnetic conversion characteristics and an error rate are improved by improving dispersibility of ferromagnetic metal powder and realizing satisfactory surface properties when especially the ferromagnetic metal powder is used as magnetic powder. <P>SOLUTION: The magnetic recording medium has a non-magnetic layer containing non-magnetic powder and a binder resin, and a magnetic layer containing ferromagnetic metal powder and a binder resin on one surface of a non-magnetic substrate. The ferromagnetic metal powder has ≤80 nm average major axis length, the surface of the magnetic layer has ≤3.0 nm three-dimensional center surface average roughness in 100 μm<SP>2</SP>region measured by an atomic force microscope and the surface of the magnetic layer has ≤15% occupation area of a projecting and recessed part having ≥±5.0 nm height from an average height plane in 100 μm<SP>2</SP>region measured by the atomic force microscope. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium in which electromagnetic conversion characteristics and error rate are improved by improving the surface properties of a magnetic layer.

  In recent years, magnetic recording media used in video, audio equipment, computers, etc. have become increasingly higher in recording density. Therefore, the recording wavelength is shorter, the recording track width is narrower, the recording medium thickness is thinner, and the minimum The direction of decreasing the recording unit. Correspondingly, the magnetic layer has been made thinner, and as the magnetic powder, ferromagnetic metal powder and hexagonal ferrite magnetic powder which are fine particles and have a large magnetic energy have been used.

  However, the finer the magnetic powder or the higher the magnetic energy, the stronger the cohesive force of the individual particles. In order to obtain a high reproduction output of short wavelength recording and a good S / N ratio (SNR). There has been a problem that sufficient dispersibility of the magnetic powder in the required magnetic paint cannot be obtained. Since the dispersibility of the magnetic powder directly affects the surface properties of the magnetic layer, it has been demanded to solve this problem by increasing the dispersibility and realizing good surface properties.

As a technique for improving the surface property of the magnetic layer, for example, Patent Document 1 discloses an abrasive having a height of 15 nm or more within a predetermined area of the surface of the magnetic layer, measured using an atomic force microscope (AFM). There is disclosed a magnetic recording medium that realizes high output and high density recording and excellent running durability by defining the proportion of protrusions within a predetermined range. In Patent Document 2, the ratio of the surface protrusion area of 10 nm or more from the reference surface measured by AFM on the surface of the magnetic layer is defined within a predetermined range, and at least one of the magnetic layer and the lower non-magnetic layer contains fatty acid and fatty acid. By including at least one of the esters and defining the surface lubricant index within a predetermined range, both high-density recording characteristics, excellent electromagnetic conversion characteristics such as durability, and good handling characteristics in the manufacturing process are realized. A recording medium is disclosed.
Japanese Patent Laid-Open No. 2002-312920 (Claims etc.) JP 2001-331924 A (Claims etc.)

  As described above, with the recent demand for higher recording density for magnetic recording media, even if the magnetic powder is finely divided, good dispersibility, and consequently surface properties, can be obtained. There has been a demand for realizing a magnetic recording medium that has both electromagnetic conversion characteristics such as an S / N ratio and medium characteristics such as an error rate.

  Therefore, the object of the present invention is to achieve high recording density and electromagnetic conversion characteristics and error by improving the dispersibility and realizing good surface properties, particularly when a ferromagnetic metal powder is used as the magnetic powder. It is an object of the present invention to provide a magnetic recording medium with an improved rate.

In order to solve the above problems, a magnetic recording medium of the present invention comprises a nonmagnetic layer containing a nonmagnetic powder and a binder resin, a ferromagnetic metal powder and a binder resin on one surface of a nonmagnetic support. Including a magnetic layer,
The average major axis length of the ferromagnetic metal powder is 80 nm or less,
The three-dimensional center plane average roughness in the 100 μm ² region measured by an atomic force microscope on the surface of the magnetic layer is 3.0 nm or less, and
In the 100 μm ² region measured by an atomic force microscope on the surface of the magnetic layer, the occupying area of the concavo-convex portions of ± 5.0 nm or more from the average height surface is 15% or less.

  The magnetic recording medium of the present invention is particularly suitable for a magnetic recording / reproducing system for reproducing with a magnetoresistive (MR) head. The binder resin for the nonmagnetic layer is preferably one having an electron beam functional group. Furthermore, the thickness of the magnetic layer is preferably 300 nm or less.

  As described above, a technique for defining the surface property of the magnetic layer using the AFM, that is, the uneven state of the surface is known. However, the technique described in the above-mentioned Patent Document 1 describes the running characteristics such as head wear characteristics and head dirt. The object is to improve the durability, and the object is different from that of the present invention relating to the improvement of electromagnetic conversion characteristics. In particular, considering the case of application to a magnetic recording / reproducing system using an MR head, a protrusion having a height of 15 nm or more is too high, which is insufficient as a surface property of an MR head-compatible magnetic recording medium. is there. The technique described in Patent Document 2 is different from the present invention in that the surface property of the magnetic layer is defined only with respect to the protrusion area from the reference surface, and the description regarding the recess is not seen.

Furthermore, as described above, the magnetic recording medium of the present invention has a three-dimensional center plane average roughness of 3.0 nm or less and an average height surface in the 100 μm ² region measured by AFM with respect to the magnetic layer surface. The occupying area of the concavo-convex portion of ± 5.0 nm or more is 15% or less. When this is measured by the method described in Patent Document 2, for example, the AFM on the surface of the magnetic layer is used. The surface protrusion area of 10 nm or more from the reference surface measured in step 1 is about one digit smaller than 33 μm ^{2 of} Comparative Example 3, which is the minimum value in the document.

  According to the present invention, magnetic recording that achieves high recording density and improved electromagnetic conversion characteristics and error rate by increasing dispersibility in a magnetic layer and realizing good surface properties even with finely divided magnetic powder. A medium can be realized.

Hereinafter, embodiments of the present invention will be described in detail.
The present invention is a magnetic recording medium having, on one surface of a nonmagnetic support, a nonmagnetic layer containing a nonmagnetic powder and a binder resin, and a magnetic layer containing a ferromagnetic metal powder and a binder resin, It is characterized in that the ferromagnetic metal powder contained in the magnetic layer is made into fine particles and two parameters relating to the surface properties of the magnetic layer measured by an atomic force microscope (AFM) are set within a predetermined range.

Specifically, in the present invention, a fine ferromagnetic metal powder having an average major axis length of 80 nm or less, preferably 20 to 80 nm, more preferably 20 to 75 nm is used. When the average major axis length of the ferromagnetic metal powder exceeds 80 nm, the electromagnetic conversion characteristics are adversely affected. Such ferromagnetic metal powder has a coercive force Hc of 119.4 to 238.7 kA / m (1500 to 3000 Oe), a saturation magnetization σs of 100 to 160 Am ² / kg (100 to 160 emu / g), and an average short It is preferable to use one having an axial length of 10 to 20 nm and an aspect ratio of about 1.2 to 8.0. Further, Co, Ni, Al, Si, Y, and other rare earth metals may be added as additive elements according to the purpose.

  In addition, such a ferromagnetic metal powder should just be contained about 70 to 90 weight% in a magnetic layer composition. If the content of the ferromagnetic metal powder is too large, the content of the binder resin decreases, so that the surface smoothness due to calendering tends to be deteriorated. On the other hand, if the content is too small, a high reproduction output is difficult to obtain.

In the present invention, the three-dimensional center plane average roughness in the 100 μm ² region measured by AFM on the surface of the magnetic layer is 3.0 nm or less, preferably 2.5 nm or less. When the three-dimensional center plane average roughness exceeds 3.0 nm, the electromagnetic conversion characteristics are adversely affected. Further, in the 100 μm ² region measured by AFM on the surface of the magnetic layer, the occupied area of the uneven portions of ± 5.0 nm or more from the average height surface is 15% or less, preferably 10% or less. Even when the occupied area exceeds 15%, the electromagnetic conversion characteristics are adversely affected.

  In the present invention, in order to obtain good surface properties of the magnetic layer, as described above, the ferromagnetic metal powder finely divided in the magnetic layer is used, and the surface irregularity state is defined by parameters determined by AFM measurement. Is. Therefore, as long as these conditions can be satisfied, the constituent material of the magnetic layer and the constituent material of the nonmagnetic layer other than the ferromagnetic metal powder are not particularly limited. It is preferable to appropriately select the constituent materials and further improve the dispersibility of both the magnetic layer and the nonmagnetic layer. That is, in the present invention, in particular, each configuration of ferromagnetic metal powder, abrasive, carbon black, etc. contained in the magnetic layer, and nonmagnetic powder, abrasive, carbon black, etc. contained in the nonmagnetic layer It is preferable to make the material finer. Further, such fine particles tend to cause deterioration of dispersibility in magnetic paints and non-magnetic paints. In addition, appropriate binder resins and dispersants are used for each of the magnetic layer and the non-magnetic layer. It is preferable to select.

  As the binder resin for the magnetic layer, conventionally known thermoplastic resins, thermosetting resins, various radiation curable resins and mixtures thereof can be suitably used, and are not particularly limited.

  As the thermoplastic resin, those having a softening temperature of 150 ° C. or less, a number average molecular weight of 500 to 200,000, and a degree of polymerization of about 50 to 2000 can be used. Those having the same number average molecular weight and degree of polymerization and having an infinite molecular weight due to a crosslinking reaction by heating and / or electron beam irradiation after coating, drying and calendering are used. .

  Among these, a combination of a vinyl chloride copolymer and a polyurethane resin as shown below is preferably used.

  The vinyl chloride copolymer preferably has a vinyl chloride content of 60 to 95% by weight, particularly 60 to 90% by weight, and the average degree of polymerization is preferably about 100 to 500%.

Such vinyl chloride copolymers include vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-hydroxyalkyl (meth) acrylate copolymer, vinyl chloride-vinyl acetate-maleic acid copolymer, chloride Vinyl-vinyl acetate-vinyl alcohol-maleic acid copolymer, vinyl chloride-vinyl acetate-hydroxyalkyl (meth) acrylate copolymer, vinyl chloride-vinyl acetate-hydroxyalkyl (meth) acrylate-maleic acid copolymer, chloride There are copolymers such as vinyl-vinyl acetate-vinyl alcohol-glycidyl (meth) acrylate copolymer and vinyl chloride-hydroxyalkyl (meth) acrylate-glycidyl (meth) acrylate copolymer. Especially, vinyl chloride and epoxy A monomer containing a (glycidyl) group; Copolymers are preferred. Such vinyl chloride copolymer, a sulfuric acid group and / or sulfonic acid polar group (hereinafter, referred to as "S-containing polar group") which contained as is preferred, S-containing polar group (-SO ₄ Y, —SO ₃ Y), Y may be either H or an alkali metal, but it is particularly preferred that Y = K, —SO ₄ K, —SO ₃ K, and these S-containing polar groups May be either one or both, and when both are included, the ratio is arbitrary. Further, these S-containing polar groups are preferably contained in the molecule in an amount of 0.01 to 10% by weight, particularly 0.1 to 5% by weight as S atoms.

As the polar group, if necessary, in addition to the S-containing polar group, -OPO ₂ Y group, -PO ₃ Y group, -COOY group (Y is H or an alkali metal), -N ⁺ R ₃ , —NR ₂ (R is H, an alkyl group or a hydroxyalkyl group) and the like.

  Polyurethane resins used in combination with such vinyl chloride resins are particularly effective in terms of good wear resistance and adhesion to the support, and these have polar groups, hydroxyl groups, etc. in the side chains. In particular, those containing a polar group containing a sulfur atom (S), a phosphorus atom (P) and / or a nitrogen atom (N) are preferred.

  These polyurethane resins are generic terms for resins obtained by the reaction of a hydroxy group-containing resin such as polyester polyol and / or polyether polyol and a polyisocyanate-containing compound, and the synthetic raw materials described in detail below are expressed in terms of number average molecular weight. It is polymerized to about 500 to 100,000, and its Q value (weight average molecular weight / number average molecular weight) is about 1.5 to 4.

  In addition, these polyurethane resins include at least two types of binders having different glass transition temperatures in the range of −20 ° C. ≦ Tg ≦ 100 ° C., and the total amount is 10% of the total binder. When used in an amount of ˜90% by weight, the inclusion of these plural polyurethane resins is preferable in that a balance between running stability under high temperature environment, calendar workability, and electromagnetic conversion characteristics can be obtained.

  Further, the vinyl chloride copolymer and the S, P and / or N-containing polar group-containing polyurethane resin are preferably mixed and used so that the weight mixing ratio thereof is 10:90 to 90:10. In addition to these resins, various known resins may be contained within a range of 20% by weight or less of the whole.

  Among the raw materials for polyurethane resins, the hydroxyl group-containing resins include polyalkylene glycols such as polyethylene glycol, polybutylene glycol and polypropylene glycol, alkylene oxide adducts such as bisphenol A, various glycols and hydroxyl groups at the molecular chain terminals. A polyester polyol etc. are mentioned.

  Examples of the carboxylic acid component of the polyester polyol include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid and 1,5-naphthalic acid, and aromatic oxy such as p-oxybenzoic acid and p- (hydroxyethoxy) benzoic acid. Fatty acid dicarboxylic acid such as carboxylic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, dodecane dicarboxylic acid, unsaturated fatty acid such as fumaric acid, maleic acid, itaconic acid, tetrahydrophthalic acid, hexahydrophthalic acid and alicyclic Examples include tricarboxylic and tetracarboxylic acids such as dicarboxylic acid, trimellitic acid, trimesic acid, and pyromellitic acid.

  The glycol component of the polyester polyol includes ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, Diethylene glycol, 2,2,4-trimethyl-1,3-pentanediol, 1,4-cyclohexanedimethanol, ethylene oxide adducts such as bisphenol A and propylene oxide adducts, hydrogenated bisphenol A ethylene oxide and propylene There are oxide adducts, polyethylene glycol, polypropylene glycol, polytetramethylene glycol and the like. Moreover, you may use together tri and tetraols, such as a trimethylol ethane, a trimethylol propane, glycerol, a pentaerythritol.

  Other examples of the polyester polyol include lactone-based polyester diol chains obtained by ring-opening polymerization of lactones such as caprolactone.

  The polyisocyanates used are tolylene diisocyanate, phenylene diisocyanate, diphenylmethane diisocyanate, hexamethylene diisocyanate, tetramethylene diisocyanate, naphthalene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, diisocyanate methylcyclohexane, diisocyanate cyclohexylmethane, dimethoxybiphenylene diisocyanate, diisocyanate. Examples of the diisocyanate compound such as diphenyl ether, or a triisocyanate compound such as a trimer of tolylene diisocyanate of 7 mol% or less and a trimer of hexanemethylene diisocyanate among all isocyanate groups.

  Other thermoplastic resins include, for example, (meth) acrylic resin, polyester resin, acrylonitrile-butadiene copolymer, polyamide resin, polyvinyl butyral, nitrocellulose, styrene-butadiene copolymer, polyvinyl alcohol resin, and acetal resin. And polyfunctional polyethers such as epoxy resins, phenoxy resins, polyether resins and polycaprolactone, polyamide resins, polyimide resins, phenol resins and the like.

  Thermosetting resins include phenol resins that undergo condensation polymerization, epoxy resins, polyurethane curable resins, urea resins, butyral resins, formal resins, melanin resins, alkyd resins, silicone resins, acrylic reaction resins, polyamide resins, and epoxy resins. -Polyamide resins, saturated polyester resins, urea formaldehyde resins and the like.

  Among the above-mentioned copolymers, those having a hydroxyl group at the terminal and / or side chain are preferable as a reactive resin because crosslinking using an isocyanate or electron beam cross-linking modification can be easily used. And may contain a polar group containing a sulfur atom (S), a phosphorus atom (P), and / or a nitrogen atom (N) as a polar group in the side chain. Is preferred. These may be used individually by 1 type, or may be used in combination of 2 or more types.

  The content of the binder resin used in the magnetic layer is preferably 5 to 40 parts by weight, particularly 10 to 30 parts by weight with respect to 100 parts by weight of the ferromagnetic metal powder. If the content of the binder resin is too small, the strength of the magnetic layer is lowered, and the running durability is likely to deteriorate. On the other hand, if the amount is too large, the content of the ferromagnetic metal powder is lowered, so that the electromagnetic conversion characteristics are lowered.

  Examples of the crosslinking agent for curing the binder resin include various known polyisocyanates in the case of a thermosetting resin, and the content of the crosslinking agent is 100 parts by weight of the binder resin. 10 to 30 parts by weight. Moreover, you may add dispersing agents, such as an abrasive | polishing material and surfactant, a higher fatty acid, and other various additives as needed in a magnetic layer.

As such an abrasive, various inorganic powders having a Mohs hardness of 9 or more are preferable, and specific examples include α-alumina (α-Al ₂ O ₃ ), carbon silicon (SiC), chromium oxide (Cr ₂ O ₃ ), and the like. be able to. Of these, α-alumina (α-Al ₂ O ₃ ) is preferable. The average particle diameter of the abrasive is 0.01 to 0.5 μm, preferably 0.02 to 0.4 μm. When the average particle size is larger than 0.5 μm, the height of protrusion from the surface of the magnetic layer becomes high, and the surface property according to the present application cannot be obtained. On the other hand, if it is smaller than 0.01 μm, the polishing ability required for a magnetic recording medium cannot be obtained, which causes head clogging during recording and reproduction. The addition amount of the abrasive is 3 to 25 parts by weight, preferably 5 to 20 parts by weight with respect to 100 parts by weight of the ferromagnetic metal powder. When the added amount exceeds 25 important parts, the content ratio of the ferromagnetic powder in the magnetic layer is lowered, so that the magnetic characteristics are lowered and the electromagnetic conversion characteristics are lowered. On the other hand, if the amount is less than 3 parts by weight, the polishing ability required for the magnetic recording medium cannot be obtained, which causes head clogs and the like during recording and reproduction.

  The coating material for forming the magnetic layer is prepared by adding an organic solvent to the above components. There is no restriction | limiting in particular in the organic solvent to be used, 1 type or 2 or more types of various solvents, such as ketone solvents, such as methyl ethyl ketone (MEK), methyl isobutyl ketone, cyclohexanone, and aromatic solvents, such as toluene, are used suitably. That's fine. The addition amount of the organic solvent may be about 100 to 900 parts by weight with respect to 100 parts by weight of the total amount of solids (ferromagnetic metal powder, abrasive, etc.) and binder resin.

  The thickness of the magnetic layer is preferably 300 nm or less, more preferably 10 to 300 nm, and still more preferably 20 to 250 nm from the viewpoint of high density recording. If the magnetic layer is thicker than 300 nm, the self-demagnetization loss and the thickness loss are increased, and the electromagnetic conversion characteristics and the error rate are deteriorated.

The nonmagnetic layer according to the present invention contains at least a nonmagnetic powder and a binder resin. As the nonmagnetic powder, various inorganic powders and carbon black can be used. As the inorganic powder, for example, acicular nonmagnetic iron oxide (α-Fe ₂ O ₃ ) can be used. In addition, calcium carbonate (CaCO ₃ ), titanium oxide (TiO ₂ ), barium sulfate (BaSO ₄ ), α-alumina (α-Al ₂ O ₃ ), and the like may be used.

Further, as carbon black, furnace black for rubber, thermal black for rubber, black for color, acetylene black and the like can be used. As such carbon black, those having a BET specific surface area of 5 to 600 m ² / g, DBP oil absorption of 30 to 400 ml / 100 g, and average particle diameter of 10 to 100 nm can be suitably used. The carbon black that can be used can be specifically selected with reference to, for example, “Carbon Black Handbook” (edited by Carbon Black Association).

  The blending ratio of the inorganic powder and carbon black is preferably 90/10 to 10/90 by weight. When the blending ratio of the inorganic powder exceeds 90, a problem occurs in the surface electrical resistance. On the other hand, when the blending ratio is less than 10, the coating strength is lowered.

  As the binder resin for the non-magnetic layer, conventionally known thermoplastic resins, thermosetting resins, radiation curable resins and mixtures thereof can be used as well as the magnetic layer. Among these, radiation curable resins can be used. Most preferably, is used. The reason will be described below.

  For the magnetic layer / non-magnetic layer coating, after applying the non-magnetic paint on the non-magnetic support, the manufacturing method (1) in which the magnetic paint is applied while the paint is wet, and the non-magnetic paint is applied. Then, after drying, calendering and curing, there are two methods: a production method (2) in which a magnetic paint is applied. Comparing the two manufacturing methods, the manufacturing method (2) is superior in terms of the flatness and uniformity of the magnetic layer / nonmagnetic layer interface. In the production method (1), the interface between the magnetic layer and the nonmagnetic layer becomes non-uniform, which may cause output fluctuation.

  Therefore, when the multilayer coating film is formed by the above production method (2), the crosslinking reaction of the binder resin contained in the layer is completed in the nonmagnetic layer in order to avoid swelling of the nonmagnetic layer when applying the magnetic paint. Need to be. To that end, usable binder resins are limited to radiation curable binder resins. That is, in the conventionally used thermoplastic resins and thermosetting resins, in order to obtain sufficient coating film properties, the non-magnetic layer-coated raw roll is placed in an oven for a long time (for example, 70 ° C., 2 to 2 ° C.). 48 hours), it is necessary to put and cure, and this causes problems in the deformation of the non-magnetic layer coating film due to tightening and a decrease in the smoothness of the surface of the non-magnetic layer, in addition to troublesome manufacturing processes. .

  In order to eliminate such drawbacks, when a nonmagnetic layer is provided, a radiation curable binder resin is used as the binder resin, the nonmagnetic layer is applied, dried, calendered, When a method of applying a magnetic layer thereon after applying three-dimensional cross-linking by irradiation and generating a three-dimensional cross-linking by radiation, a suitable result can be obtained. According to this method, since the three-dimensional cross-link is already formed at the time when the magnetic layer is provided, the magnetic coating can be immediately applied as it is without being swollen by the organic solvent, and the process can be continued. Simplification can be achieved.

  The radiation curable binder resin that can be preferably used in the present invention is an electron that contains one or more unsaturated double bonds in a molecular chain that generates radicals by radiation and is cured by crosslinking or polymerization. It is a resin having a linear functional group. Resins containing electron beam functional groups include vinyl chloride copolymers and polyurethane resins, (meth) acrylic resins, polyester resins, acrylonitrile-butadiene copolymers, polyamide resins, polyvinyl butyral, nitrocellulose, styrene- Butadiene copolymer, polyvinyl alcohol resin, acetal resin, epoxy resin, phenoxy resin, polyether resin, polyfunctional polyethers such as polycaprolactone, polyamide resin, polyimide resin, phenol resin, polybutadiene elastomer, chlorinated rubber In addition, a thermoplastic resin such as acrylic rubber, isoprene rubber, and epoxy-modified rubber that has been subjected to electron beam sensitive modification by introducing a (meth) acrylic double bond by a known method can be used.

Further, the content of the electron beam functional group is 1 to 40 mol%, preferably 10 to 30 mol%, particularly in the vinyl chloride copolymer, from the viewpoint of stability during production, electron beam curability and the like. In this case, when the monomer is reacted so as to have 1 to 20, preferably 2 to 10 functional groups per molecule, an electron beam curable resin having excellent dispersibility and curability can be obtained. . Furthermore, as a polar group, —COOH, —SO ₃ M, —OSO ₃ M, —OPO ₃ M, —PO ₃ M, —PO ₂ M, —N ⁺ R ₃ Cl ⁻ , —NR ₂ are attached to the terminal or side chain. (Wherein M is H or an alkali metal, R is H, a methyl group or an ethyl group) and the like, and preferably contains an acidic polar group or a basic polar group. Suitable for improving dispersibility.

  The irradiation amount of the electron beam is indicated by the absorbed dose of the nonmagnetic layer, and the larger the amount, the more hardened. The irradiation amount is preferably 2.5 to 15 Mrad, more preferably 3.5 to 10 Mrad, still more preferably 4 to 10 Mrad. If this irradiation amount is too small, the curing is insufficient and the nonmagnetic layer may be attacked by the solvent in the magnetic paint. On the other hand, if it is too large, the structure of the resin and the nonmagnetic support may be destroyed, leading to a decrease in reliability.

  The nonmagnetic paint can be produced using the same amount of organic solvent as that of the magnetic layer. The addition amount of the organic solvent may be about 100 to 900 parts by weight with respect to 100 parts by weight of the total amount of the solid content (carbon black, inorganic powder, etc.) and the binder resin.

  In the nonmagnetic layer in the present invention, a lubricant, a dispersant such as a surfactant, and other various additives may be further added as desired.

  The film thickness of the nonmagnetic layer is preferably 2.5 μm or less, more preferably 0.1 to 2.3 μm, and still more preferably 0.2 to 2.1 μm. Even if this thickness is made larger than 2.5 μm, further improvement in performance cannot be expected. On the other hand, the thickness tends to be non-uniform when the coating film is provided, the coating conditions become severe, and the surface smoothness tends to deteriorate. Become.

  Non-magnetic supports include various flexible materials such as polyester resins such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), known resin films such as polyamide resins or aromatic polyamide resins, or laminated resins thereof. The thickness can be appropriately selected from the film, and the thickness and the like are within a known range and should not be particularly limited.

  In the present invention, a backcoat layer may be provided on the surface of the nonmagnetic support opposite to the surface on which the nonmagnetic layer and the magnetic layer are formed. The binder resin used for the backcoat layer is the same as the magnetic layer and the nonmagnetic layer, and various thermoplastic resins, thermosetting or reactive resins, radiation-sensitive modified resins, etc. are used. What is necessary is just to select suitably according to the characteristic and process conditions of a magnetic recording medium.

  In the backcoat layer, if necessary, nonmagnetic inorganic powders such as various abrasives used in the magnetic layer, carbon black, a dispersant such as a surfactant, a lubricant such as higher fatty acid, fatty acid ester, silicone oil, Various other additives may be added.

  The film thickness of the back coat layer (after calendering) is 0.1 to 1.0 μm, preferably 0.2 to 0.8 μm. If this thickness exceeds 1.0 μm, the friction with the medium sliding contact path becomes too large, and the running stability tends to decrease. On the other hand, if the thickness is less than 0.1 μm, the coating film of the backcoat layer tends to be scraped when the medium is running.

  In addition, in order to realize the uneven state of the surface according to the present invention, in particular, in addition to the selection of the above constituent materials, etc., kneading conditions, dispersion conditions and filtration conditions in the paint manufacturing process, coating drying conditions in the coating process, and It is preferable to select an appropriate calendar condition. Especially, it is thought that the coating material dispersion | distribution conditions in a coating-material manufacturing process have a big influence on the unevenness | corrugation of a surface.

  Conventionally used glass beads, metal beads, alumina beads, titania beads, zirconia beads, etc. can be used for the dispersion media used in the dispersion process in the paint manufacturing process. Among them, zirconia beads, titania beads, etc. The ceramic type beads are preferred. As the diameter of the dispersion medium is reduced, the unevenness of the surface can be reduced, and is preferably 2.0 mm or less, more preferably 1.5 mm or less, and most preferably 0.05 to 1.25 mm. If the diameter of the dispersion medium is larger than 2.0 mm, it is difficult to obtain the desired surface property according to the present invention.

  Dispersers used in the dispersion process include agitator mills LMJ and LMZ manufactured by Ashizawa Finetech Co., Ltd. and ultra-fine crusher AMC, Pure Mill, Nanomill and Picomill manufactured by Asada Tekko Co., Ltd., Kotobuki Giken Kogyo Co., Ltd. It is possible to use a super apex mill manufactured by Co., Ltd., an SC mill manufactured by Mitsui Mining Co., Ltd., or the like.

  Since the magnetic recording medium of the present invention has a surface property that is particularly suitable for MR heads, it can be suitably applied to a magnetic recording / reproducing system for reproducing with an MR head.

Hereinafter, the present invention will be described in detail by way of examples.
[Magnetic paint A]
Preparation of binder solution The following composition was put into a hypermixer, mixed and stirred to obtain a binder solution.
Vinyl chloride resin (manufactured by Nippon Zeon Co., Ltd .: MR-110) 10 parts by weight Polyester polyurethane resin (Toyobo Co., Ltd .: UR-8300) 7 parts by weight Methyl ethyl ketone 21 parts by weight Toluene 21 parts by weight Cyclohexanone 21 parts by weight

Kneading The following composition was put into a pressure kneader and kneaded for 2 hours.
α-Fe magnetic powder (a) 100 parts by weight (Hc = 167 kA / m (2100 Oe), σs = 130 Am ² / kg (130 emu / g), BET specific surface area = 58 m ² / g, average major axis length = 75 nm)
α-Al ₂ O ₃ 10 parts by weight (manufactured by Sumitomo Chemical Co., Ltd .: HIT-60A, average particle size = 0.20 μm)
40 parts by weight of the binder solution

The following composition was added to the slurry after kneading to adjust the viscosity to be optimal for the dispersion treatment.
40 parts by weight of the binder solution 15 parts by weight of methyl ethyl ketone 15 parts by weight of toluene 15 parts by weight of cyclohexanone

Dispersion Ashizawa Finetech Co., Ltd. Agitator mill LMJ was filled with 85% of φ = 0.8 mm zirconia beads, and the slurry was dispersed using this mill. The dispersion residence time was 60 minutes.

Viscosity adjusting liquid The following composition was put into a hypermixer and mixed and stirred for 1 hour to obtain a viscosity adjusting liquid.
Stearic acid 0.5 parts by weight Myristic acid 0.5 parts by weight Butyl stearate 0.5 parts by weight Methyl ethyl ketone 210 parts by weight Toluene 210 parts by weight Cyclohexanone 210 parts by weight

After mixing and stirring the above viscosity adjusting liquid into the finished dispersion and dispersion slurry, 85% of zirconia beads having the bead diameters (media diameters) shown in Tables 1 and 2 below are added to Agitator Mill LMZ manufactured by Ashizawa Finetech Co., Ltd. After filling, this mill was used for finish dispersion treatment to obtain a paint. The dispersion residence time was as shown in Tables 1 and 2 below. This paint was subjected to circulation filtration using a depth filter with 95% cut filtration accuracy = 1.2 μm.

0.8 parts by weight of an isocyanate compound (manufactured by Nippon Polyurethane Industry Co., Ltd .: Coronate L) is added to 100 parts by weight of the paint after final paint filtration, and the mixture is stirred and mixed. A depth filter with 95% cut filtration accuracy = 1.2 μm Was used as a final coating A for the magnetic layer.

[Magnetic paint B]
α-Fe magnetic powder (b) instead of α-Fe magnetic powder (a) (Hc = 179 kA / m (2250 Oe), σs = 138 Am ² / kg (138 emu / g), BET specific surface area = 59 m ² / g The coating material produced in the same manner as above except that the average major axis length = 60 nm was used as magnetic coating material B.

[Magnetic paint C]
α-Fe magnetic powder (c) instead of α-Fe magnetic powder (a) (Hc = 167 kA / m (2100 Oe), σs = 110 Am ² / kg (110 emu / g), BET specific surface area = 70 m ² / g The coating material produced in the same manner as described above except that the average major axis length was 40 nm was used as the magnetic coating material C.

[Magnetic paint D]
α-Fe magnetic powder (d) instead of α-Fe magnetic powder (a) (Hc = 161 kA / m (2025 Oe), σs = 127 Am ² / kg (127 emu / g), BET specific surface area = 59 m ² / g The coating material prepared in the same manner as above except that the average major axis length was 95 nm was used as the magnetic coating material D (comparative sample).

[Non-magnetic paint]
Preparation of binder solution The following composition was charged into a hypermixer and stirred to obtain a binder solution.
10 parts by weight of an electron beam curable vinyl chloride resin (manufactured by Toyobo Co., Ltd .: TB-4246, 2-isocyanatoethyl methacrylate, acrylic-modified MR-110 manufactured by Nippon Zeon Co., Ltd.) = 300, polar group: -OSO ₃ K = 1.5 / molecule, acrylic content = 6 / molecule)
7 parts by weight of an electron beam curable polyester polyurethane resin (Toyobo Co., Ltd .: TB-1216, number average molecular weight = 20000, Tg = 10 ° C., acrylic content = 6 / molecule)
Methyl ethyl ketone 21 parts by weight Toluene 21 parts by weight Cyclohexanone 21 parts by weight

Kneading The following composition was put into a pressure kneader and kneaded for 2 hours.
85 parts by weight of acicular α-Fe ₂ O ₃ (manufactured by Toda Kogyo Co., Ltd .: DAN-855BX, average major axis length = 70 nm, BET specific surface area = 58 m ² / g)
15 parts by weight of carbon black (manufactured by Mitsubishi Chemical Corporation: # 950B, average particle size = 16 nm, BET specific surface area = 260 m ² / g, DPB oil absorption = 74 ml / 100 g)
40 parts by weight of the binder solution

The following composition was added to the slurry after kneading to adjust the viscosity to be optimal for the dispersion treatment.
40 parts by weight of the binder solution 15 parts by weight of methyl ethyl ketone 15 parts by weight of toluene 15 parts by weight of cyclohexanone

An agitator mill LMJ manufactured by Dispersing Ashizawa Finetech Co., Ltd. was filled with 85% φ = 0.8 mm zirconia beads, and the slurry was dispersed using this mill. The dispersion residence time was 60 minutes.

Viscosity adjusting liquid The following composition was charged into a hypermixer and stirred to obtain a viscosity adjusting liquid.
Stearic acid 0.5 parts by weight Myristic acid 0.5 parts by weight Butyl stearate 0.5 parts by weight Methyl ethyl ketone 100 parts by weight Toluene 100 parts by weight Cyclohexanone 100 parts by weight

After mixing and stirring the above viscosity adjusting liquid in the final dispersion and final paint dispersion slurry, 85% of φ = 0.1mm zirconia beads are filled in Agitator Mill LMZ manufactured by Ashizawa Finetech Co., Ltd., and this mill is used. Finished and dispersed to obtain a paint. The dispersion residence time was 10 minutes. This paint was subjected to circulation filtration using a depth filter with 95% cut filtration accuracy = 1.2 μm to obtain a final paint for the nonmagnetic layer.

[Back coat paint]
Preparation of binder solution The following composition was charged into a hypermixer and stirred to obtain a binder solution.
Nitrocellulose resin (Asahi Kasei Co., Ltd .: BTH-1 / 2) 50 parts by weight Polyester polyurethane resin (Toyobo Co., Ltd .: UR-8300) 50 parts by weight Methyl ethyl ketone 260 parts by weight Toluene 260 parts by weight Cyclohexanone 260 parts by weight

Dispersion The following composition was placed in a ball mill and dispersed for 24 hours.
80 parts by weight of carbon black (manufactured by Columbian Carbon Co., Ltd .: Conductex SC, average particle size = 20 nm, BET specific surface area = 220 m ² / g)
1 part by weight of carbon black (manufactured by Colombian Carbon Co., Ltd .: Sevacarb MT, average particle size = 350 nm, BET specific surface area = 8 m ² / g)
α-Fe ₂ O ₃ (manufactured by Toda Kogyo Co., Ltd .: TF100, average particle size = 0.1 μm) 1 part by weight The above binder solution 880 parts by weight

Viscosity adjusting liquid The following composition was charged into a hypermixer and stirred to obtain a viscosity adjusting liquid.
Stearic acid 1 part by weight Myristic acid 1 part by weight Butyl stearate 2 parts by weight Methyl ethyl ketone 210 parts by weight Toluene 210 parts by weight Cyclohexanone 210 parts by weight

The above solution was mixed and stirred in the viscosity-adjusted and dispersed slurry, and then dispersed again with a ball mill for 3 hours. This paint was subjected to circulation filtration using a depth filter with 95% cut filtration accuracy = 1.2 μm.

1 part by weight of an isocyanate compound (manufactured by Nippon Polyurethane Industry Co., Ltd .: Coronate-L) is added to 100 parts by weight of the paint after the final paint filtration, and the mixture is stirred and mixed to obtain a 95% cut filtration accuracy = 1.2 μm depth filter. Was used to circulate and filter to obtain a back coat paint.

[Production of magnetic tape]
A nonmagnetic paint was applied to the surface of a nonmagnetic support (PEN film having a thickness of 6.2 μm), dried, calendered, and cured by irradiation with an electron beam in a nitrogen gas atmosphere. On this nonmagnetic layer, a magnetic coating was applied, oriented and dried, and then calendered. The orientation direction was the in-plane longitudinal direction. The thickness of the magnetic layer / nonmagnetic layer after calendaring was 0.15 / 2.0 μm. Furthermore, the coating material for the back coat layer was applied to the opposite surface of the nonmagnetic support, dried, and calendered. The film thickness of the back coat layer after calendering was 0.5 μm for all samples.

  The obtained raw fabric roll was allowed to stand at room temperature for 24 hours, then cured in a heating oven at 60 ° C. for 24 hours, then cut to a 1/2 inch width, incorporated into an LTO Ultrium cartridge, and a magnetic tape sample and did.

[AFM measurement method]
Using an atomic force microscope (Auto Probe M5) manufactured by TM Microscope, the three-dimensional center plane average roughness of a 10 μm × 10 μm (100 μm ² ) region was measured under the following conditions.
Probe: Si single crystal probe, tip radius of curvature = 2 nm, half cone angle = 10 degrees Measurement area: 10 μm × 10 μm
・ Scanning speed: 0.6Hz
Number of pixels: 512 x 512 data points

From the image data obtained under the above analysis conditions, the three-dimensional center plane average roughness was determined according to the following conditions.
• Use the entire image for correction calculations. (Do not select a specific area in the image data)
-Perform both horizontal and vertical quadratic tilt correction on the entire image data.
-The average roughness of the three-dimensional center plane is obtained for the image data subjected to the tilt correction.

Further, an average height surface is obtained from the image data of all pixels subjected to inclination correction, and an area of a height surface of 5.0 nm or more and an area of a depth surface of 5.0 nm or less with respect to the average height surface, respectively. The ratio occupied in the 100 μm ² region was determined by the following formula.
(Ratio of unevenness of ± 5.0 nm or more from the center surface) (%) = [(area of height surface of 5.0 nm or more with respect to average height surface) (μm ² ) + (5 with respect to average height surface] ... 0 nm depth area) (μm ² )] / 100 (μm ² ) × 100

[Electromagnetic conversion characteristics]
Measurement was carried out using Small Format Tape Evaluation System for LTO manufactured by Measurement Analysis Corporation. The head used was an LTO Ultrium 1 drive mounted head. The results are relative values when Comparative Example 1 is 0 dB, and are shown for Signal Amplitude and Broadband SNR.

[Bit error rate]
The bit error rate was measured using an Hewlett Packard LTO Ultra 1 drive. The results are shown in logarithmic form as relative values when Comparative Example 1 is 0.
These results are also shown in Tables 1 and 2 below.

Claims (4)

On one surface of the nonmagnetic support, it has a nonmagnetic layer containing nonmagnetic powder and a binder resin, and a magnetic layer containing ferromagnetic metal powder and a binder resin,
The average major axis length of the ferromagnetic metal powder is 80 nm or less,
The three-dimensional center plane average roughness in the 100 μm ² region measured by an atomic force microscope on the surface of the magnetic layer is 3.0 nm or less, and
A magnetic recording medium characterized in that in the 100 μm ² region measured by an atomic force microscope on the surface of the magnetic layer, the occupying area of concavo-convex portions of ± 5.0 nm or more from the average height surface is 15% or less.   2. The magnetic recording medium according to claim 1, which is used in a magnetic recording / reproducing system for reproducing with a magnetoresistive head.   The magnetic recording medium according to claim 1, wherein the binder resin of the nonmagnetic layer has an electron beam functional group. The magnetic recording medium according to claim 1, wherein the magnetic layer has a thickness of 300 nm or less.