JP2001148111A - Magnetic recording medium and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium having a magnetic layer, which consists of a coating ultra-thin film and is small in the fluctuations of the film thickness, and suitable for a reproducing system using a MR head, and to provide a method for manufacturing the magnetic recording medium. SOLUTION: This magnetic recording medium for the reproducing system using a MR head has a nonmagnetic layer, containing at least a binder resin on a nonmagnetic support and has the magnetic layer of 0.02-0.08 μm thickness on the nonmagnetic layer. The magnetic layer is formed on the nonmagnetic layer by applying a magnetic coating material after the nonmagnetic layer is formed by applying, drying and hardening. The magnetic layer contains a metal magnetic powder having 0.15 μm or shorter average major axis diameter and has a range of 50-250 for the product t×Br of residual magnetic flux density Br (in units of G) and layer thickness t (in units of μm) and 5 nm or smaller for surface roughness Ra.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a magnetic recording medium having a coating type ultra-thin magnetic layer, and more particularly to a magnetic recording medium used in an MR head reproducing system. The present invention also relates to a method for manufacturing the magnetic recording medium.

[0002]

2. Description of the Related Art In recent years, magnetic recording media such as VTR tapes, audio tapes, and computer tapes have been required to have high-density recording due to their high performance, long time recording, and small size and light weight. A magnetic recording system combining a head and a magnetic tape has been studied.

As the recording density increases, the thickness of the magnetic layer becomes thinner. For example, Japanese Patent Publication No. 5-59490 discloses 2 μm.
With respect to a method for producing a magnetic recording medium having a thin magnetic layer having a thickness of m or less, a method is disclosed in which a nonmagnetic undercoat layer and a magnetic layer are simultaneously coated on a nonmagnetic support. However, with such wet-on-wet coating, 0.08 μm
If an attempt is made to obtain a magnetic layer having the following thickness, the effect of output fluctuation due to uneven thickness of the magnetic layer increases.

[0004] Japanese Patent Application Laid-Open No. 10-312 discloses a system in which a combination with an MR head reproducing system is considered.
No. 525 discloses that a magnetic layer having a hexagonal ferrite powder has a saturation magnetic flux density of 300 to 1000 G and a coercive force of 20.
The magnetic layer containing at least 00 Oe or a ferromagnetic metal powder has a saturation magnetic flux density of 800 to 1500 G and a coercive force of 2
000 Oersted or more is disclosed.
Japanese Patent Application Laid-Open No. 10-302243 discloses that the height of the protrusion on the surface of the magnetic layer and the magnetization reversal volume are specified, and the coercive force of the magnetic layer is set to 2000 Oe or more.

[0005] However, neither of the above publications deeply describes the relationship between the magnetic characteristics and the thickness of the magnetic layer suitable for an MR head, and a sufficiently sensitive device suitable for an MR head has not yet been obtained. Also, according to the examples of both publications,
Although a magnetic layer having a thickness of 0.15 μm is formed by a simultaneous multilayer coating method, it is difficult to reduce the thickness to a more uniform thickness.

[0006]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, and to provide a magnetic recording medium having a coating-type ultra-thin magnetic layer having a small thickness variation, An object of the present invention is to provide a magnetic recording medium suitable for a head reproducing system. Another object of the present invention is to provide a method for manufacturing the magnetic recording medium.

[0007]

As a result of intensive studies, the present inventors have found that a magnetic recording medium suitable for a recording / reproducing system in which a recording signal is reproduced by an MR (Magneto-Resistive magnetoresistive) head can be obtained. Was.

According to the present invention, there is provided an MR device comprising: a nonmagnetic support; a nonmagnetic layer containing at least a binder resin; and a nonmagnetic layer having a thickness of 0.02 to 0.08 μm on the nonmagnetic layer.
A magnetic recording medium used in a recording / reproducing system in which a recording signal is reproduced by a head, wherein the magnetic layer is coated with the non-magnetic layer, dried, and cured, and then a magnetic paint is applied on the non-magnetic layer. The magnetic layer contains metal magnetic powder having an average major axis diameter of 0.15 μm or less,
The product t × Br of the residual magnetic flux density Br (unit G) of the magnetic layer and the thickness t (unit μm) of the magnetic layer is in the range of 50 to 250, and the surface roughness Ra of the magnetic layer is 5 nm or less. It is a magnetic recording medium. The binder resin contained in the nonmagnetic layer is preferably a radiation-curable binder resin.

As described above, the thickness of the magnetic layer is set to 0.02 to 0.02.
0.08 μm, and the residual magnetic flux density Br of the magnetic layer
By setting the product t × Br of (unit G) and the magnetic layer thickness t (unit μm) to 50 to 250, a coating type recording medium having excellent characteristics can be obtained in a system using an MR head. When the thickness of the magnetic layer is less than 0.02 μm or t
When xBr is less than 50, the magnetic flux density emitted from the magnetic layer becomes insufficient and the sensitivity decreases. The magnetic layer thickness is 0.08 μm
When the thickness becomes larger or t × Br exceeds 250, the MR head element becomes magnetically saturated. Also, noise can be reduced by setting the surface roughness Ra of the magnetic layer to 5 nm or less.

The present invention also provides a method for producing the above magnetic recording medium, comprising applying a coating for forming a nonmagnetic layer containing at least a binder resin on a nonmagnetic support, drying and curing the coating. Next, an average major axis diameter of 0.1
A method for producing a magnetic recording medium, comprising applying a coating material for forming a magnetic layer containing at least a metal magnetic powder of 5 μm or less to form a magnetic coating film.

In this production method, it is preferable that the binder resin contained in the coating material for forming a nonmagnetic layer is a radiation-curable binder resin, and the radiation-curable binder resin is used.

Further, in this manufacturing method, the magnetic coating film is formed by applying the coating material for forming a magnetic layer in excess of a predetermined thickness of the magnetic layer on the hardened non-magnetic layer.
Thereafter, it is preferable to scrape off excess paint with a bar so as to obtain a magnetic layer having a predetermined dry film thickness in the range of 0.02 to 0.08 μm.

In this manufacturing method, it is preferable that the concentration of the solid content for forming the magnetic layer to be applied is 10% by weight or less, and redispersion is performed by an online disperser immediately before the application.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The magnetic recording medium of the present invention has a lower non-magnetic layer between a non-magnetic support and a magnetic layer as a coating composition. On the lower surface side of the non-magnetic support, a back coat layer is provided as necessary. Alternatively, magnetic layers may be provided on both surfaces of the non-magnetic support. In the present invention, a lubricant coating or various coatings for protecting the magnetic layer may be provided on the upper magnetic layer as necessary. On the surface of the non-magnetic support on which the magnetic layer is provided, an undercoat layer (easily adhesive layer) can be provided for the purpose of improving the adhesion between the coating film and the non-magnetic support.

[Lower Nonmagnetic Layer] The role of the lower nonmagnetic layer is as follows.
In the present invention, since the thickness of the magnetic layer is extremely thin, 0.02 to 0.08 μm, the amount of the lubricant in the coating film is insufficient with the magnetic layer alone. Therefore, a lubricant is included in the non-magnetic layer, and the lubricant is supplied from the lubricant to the magnetic layer. Also,
By interposing the lower non-magnetic layer, the surface roughness of the support film can be remarkably reduced, leading to an improvement in the surface properties of the magnetic layer.

For this purpose, it is desirable to include carbon black in the nonmagnetic layer. By including carbon black, a lubricant can be held. It also has the function of lowering the surface electric resistance of the magnetic layer.

The carbon black contained in the nonmagnetic layer is not particularly limited, but has an average particle diameter of 10 nm to 80 nm.
Carbon black of about nm is preferred. As such carbon black, known ones such as furnace carbon black, thermal carbon black, and acetylene black can be used. Further, a single system or a mixed system may be used. BET specific surface area of these carbon black 50 to 500 m ² / g, preferably 60 to 250 ²
/ G. Carbon black that can be used in the present invention is
For example, "Carbon Black Handbook", edited by Carbon Black Association can be referred to.

In the non-magnetic layer, other non-magnetic powder can be used in addition to carbon black. Such other non-magnetic powders include, but are not limited to, α-iron oxide, α-alumina, Cr ₂ O ₃ , SiO ₂ , ZnO, T
iO ₂ , ZrO ₂ , SnO _{2 and the} like. Among them, the combined use of acicular α-iron oxide having an average major axis diameter of 200 nm or less or granular α-iron oxide having an average particle diameter of 20 to 100 nm can reduce the thixotropy of a carbon black-only paint. Also, the coating film can be hardened. Further, the abrasive has an average particle diameter of 0.1 to 1.
When 0 μm α-alumina or Cr ₂ O ₃ is used in combination, the strength of the nonmagnetic layer is increased.

The mixing ratio of carbon black to other non-magnetic powder is preferably 100/0 to 75/25 by weight.
If the mixing ratio of the other non-magnetic powder exceeds 25 parts by weight, a problem occurs in the surface electric resistance.

As the resin used for the lower layer paint, a radiation curable resin is desirable. If a thermosetting resin is used, a thermosetting step is required after the application of the non-magnetic layer, so that the raw material is undesirably deformed due to the thermosetting. Further, when used without thermosetting, when a magnetic paint is applied thereon, the solvent permeates into the non-magnetic layer and swells, or the non-magnetic layer dissolves, thereby deteriorating the surface properties of the magnetic layer. In the present invention,
After the non-magnetic layer is cured, a magnetic paint is applied on the non-magnetic layer.

In the present invention, a radiation-curable binder resin is used as a binder resin for the lower non-magnetic layer, a coating for the lower non-magnetic layer is applied, dried, smoothed, irradiated with radiation, and irradiated with three-dimensional radiation. By cross-linking and then applying the upper magnetic layer coating thereon, favorable results could be obtained. According to this method, the lower nonmagnetic layer is not swelled by the organic solvent because the three-dimensional crosslink has already been formed at the time when the upper magnetic layer is provided. Therefore, the magnetic paint can be applied onto the lower non-magnetic layer immediately after the formation of the lower non-magnetic layer, so that the process can be continued and simplified.

The radiation-curable binder resin used in the present invention is a resin containing one or more unsaturated double bonds in the molecular chain, which generates a radical by radiation and cures by crosslinking or polymerization. Say.

Examples of the radiation-curable binder resin include a large number of resins such as vinyl chloride resins, polyurethane resins, polyester resins, epoxy resins, phenoxy resins, fiber resins, polyether resins, and polyvinyl alcohol resins. Can be Among them, vinyl chloride resin,
Polyurethane resins are typical, and it is preferable to use a mixture of both.

The radiation-curable vinyl chloride resin is synthesized by radiation-sensitive modification of a vinyl chloride resin as a raw material. The raw material vinyl chloride resin has a vinyl chloride content of 60
-100% by weight, particularly preferably 60-95% by weight. Such vinyl chloride resins include vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-hydroxyalkyl (meth) acrylate copolymer,
Vinyl chloride-vinyl acetate-maleic acid copolymer, vinyl chloride-vinyl acetate-vinyl alcohol-maleic acid copolymer, vinyl chloride-hydroxyalkyl (meth) acrylate-maleic acid copolymer, vinyl chloride-vinyl acetate-vinyl Alcohol-glycidyl (meth) acrylate copolymer, vinyl chloride-hydroxyalkyl (meth) acrylate-glycidyl (meth) acrylate copolymer, vinyl chloride-hydroxyalkyl (meth) acrylate-allyl glycidyl ether copolymer, vinyl chloride Examples include a vinyl acetate-vinyl alcohol-allyl glycidyl ether copolymer, and a copolymer of vinyl chloride and a monomer containing an epoxy (glycidyl) group is particularly preferable. And the average degree of polymerization is 100-900
Is more preferable, and more preferably 100 to 600.

Further, in order to enhance the dispersibility, -SO ₄ Y, -SO ₃ Y, -POY, -PO ₂ Y,-
PO ₃ Y, —COOY (Y is hydrogen or an alkali metal),
_{-SR, -NR 2, -NR 3 Cl} (R is a hydrogen or a hydrocarbon group), phosphobetaine, sulfobetaine, phosphamine, it is also preferable that the polar group of sulphamic etc. introduced in any manner. In order to enhance thermal stability, it is also preferable to introduce an epoxy group.

As a method for radiation-modifying the above vinyl chloride resin, a resin having a hydroxyl group or a carboxylic acid group is modified by reacting a compound having a (meth) acryl group with a carboxylic anhydride or a dicarboxylic acid. A method of reacting and modifying a reaction product (adduct) of TDI and 2-HEMA, having at least one ethylenically unsaturated double bond and one isocyanate group in one molecule,
A typical method is to react a monomer having no urethane bond in the molecule. Of these, the third method is excellent in the easiness of modification and the dispersibility and physical properties after the modification, and it is preferable that the modification is performed by the third method. Examples of such a monomer include 2-isocyanatoethyl (meth) acrylate. Acrylic or methacrylic groups in the molecule of the binder have an average of 1-2 in the molecule.
More preferably, there are two to fifteen.

The radiation-curable polyurethane resin is a urethane resin having at least one acrylic bond in the molecule, and is a polyurethane acrylate compound bonded to an acrylic double bond-containing compound via a urethane bond.

The term "acrylic double bond" as used herein means a residue (acryloyl group or methacryloyl group) such as acrylic acid, acrylic ester, acrylamide, methacrylic acid, methacrylic ester, and methacrylamide.

Examples of the acrylic double bond-containing compound (A) include mono (meth) glycols such as ethylene glycol, diethylene glycol, and hexamethylene glycol.
Acrylate, trimethylolpropane, glycerin,
Mono (meth) acrylates of triol compounds such as trimethylolethane and mono (meth) acrylates of divalent or higher polyols such as di (meth) acrylate, pentaerythritol and dipentaerythritol mono (meth) acrylate and di (meth) acrylate
Hydroxy group-containing acrylic compounds such as acrylate and tri (meth) acrylate, glycerin monoallyl ether, and glycerin diallyl ether are preferable. These acrylic double bonds must be present in the binder molecule in at least one or more, preferably 2 to 20.

The polyurethane acrylate resin is generally obtained by reacting a hydroxyl group-containing resin (B '), a hydroxyl group-containing acrylic compound (A') and a polyisocyanate-containing compound (C '). Examples of the hydroxy group-containing resin include polyalkylene glycols such as polyethylene glycol, polybutylene glycol, and polypropylene glycol, and bisphenol A.
Alkylene oxide adducts, various glycols, and polyester polyols (B ') having a hydroxyl group at the terminal of the molecular chain. Among these, a polyurethane acrylate resin obtained by using the polyester polyol (B ′) as one component is preferable.

The carboxylic acid component of the polyester polyol (B ') includes aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid and 1,5-naphthalic acid, p-oxybenzoic acid, and p- (hydroxyethoxy). Aromatic oxycarboxylic acids such as benzoic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, aliphatic dicarboxylic acids such as dodecanedicarboxylic acid, fumaric acid, maleic acid, itaconic acid, tetrahydrophthalic acid, hexahydrophthalic acid, etc. And tri- or tetracarboxylic acids such as alicyclic dicarboxylic acid, trimellitic acid, trimesic acid and pyromellitic acid.

The glycol component of the polyester polyol (B ') includes ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, Ethylene oxide adducts and propylene oxide adducts such as pentyl glycol, diethylene glycol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, 1,4-cyclohexanedimethanol, bisphenol A, and hydrogenated bisphenol A Ethylene oxide and propylene oxide adducts, polyethylene glycol, polypropylene glycol, polytetramethylene glycol and the like. Further, tri- or tetraol such as trimethylolethane, trimethylolpropane, glycerin, and pentaerythritol may be used in combination. In addition to the above, examples of the polyester polyol include a lactone-based polyester diol chain obtained by ring-opening polymerization of a lactone such as caprolactone.

As the polyisocyanate (C ') used, 2,4-tolylene diisocyanate, 2,6-
Tolylene diisocyanate, p-phenylene diisocyanate, biphenylmethane diisocyanate, m-phenylene diisocyanate, hexamethylene diisocyanate, tetramethylene diisocyanate, 3,3′-dimethoxy 4,4′-biphenylene diisocyanate,
2,4-naphthalenediisocyanate, 3,3′-dimethyl-4,4′-biphenylenediisocyanate, 4,
4'-diphenylene diisocyanate, 4,4'-diisocyanate-diphenyl ether, 1,5-naphthalene diisocyanate, p-xylylene diisocyanate, m-xylylene diisocyanate, 1,3-diisocyanatomethylcyclohexane, 1,4-isocyanatomethylcyclohexane Diisocyanate compounds such as 4,4'-diisocyanate dicyclohexane, 4,4'-diisocyanate cyclohexylmethane, isophorone diisocyanate, or a trimer of 2,4-tolylene diisocyanate of 7 mol% or less of all isocyanate groups; A triisocyanate compound such as a trimer of methylene diisocyanate is exemplified.

Further, in order to enhance dispersibility, -SO ₄ Y, -SO ₃ Y, -POY, -PO ₂ Y,-
PO ₃ Y, —COOY (Y is hydrogen or an alkali metal),
_{-SR, -NR 2, -NR 3 Cl} (R is a hydrogen or a hydrocarbon group), phosphobetaine, sulfobetaine, phosphamine, it is also preferable that the polar group of sulphamic etc. introduced in any manner. In order to enhance thermal stability, it is also preferable to introduce an epoxy group.

On the other hand, apart from the above radiation-curable urethane synthesis method, radiation-sensitive modification may be carried out using a thermosetting polyurethane resin as a raw material in the same manner as the vinyl chloride resin.

If necessary, a radiation-curable monomer or oligomer may be used, and the use thereof can increase the degree of crosslinking of the coating film. The addition amount is preferably 3 to 30 parts by weight, more preferably 5 to 20 parts by weight, based on the resin contained in the coating material for the lower non-magnetic layer. When the amount is less than 3 parts by weight, the effect of curing the coating film due to the addition is small. The radiation-curable monomer or oligomer may be added at any time after the preparation of the paint and at the time of dispersion.

The content of the radiation-curable binder in the lower non-magnetic layer was 10 times in total of carbon black and non-magnetic powder.
10 to 70 parts by weight, preferably 20 parts by weight,
-40 parts by weight is more preferred. If the content of the binder is too small, the coating film becomes brittle, and if the content of the binder is too large, the ability to retain the lubricant is reduced.

The radiation used in the present invention is an electron beam, γ-ray, β-ray, ultraviolet ray, etc., preferably an electron beam. The dose is preferably 1 to 10 Mrad.
~ 7 Mrad is more preferred. The irradiation energy (acceleration voltage) is preferably 100 Kv or more. Irradiation with radiation is preferably performed before winding after coating and drying, but may be performed after winding.

The lower non-magnetic layer of the present invention preferably contains a lubricant as required. As a lubricant,
Known fatty acid, higher fatty acid ester, paraffin, fatty acid amide and the like can be used.

The paint for forming the lower non-magnetic layer is prepared by adding an organic solvent to the above components. The organic solvent used is not particularly limited, and one or two or more of various solvents such as ketone solvents such as methyl ethyl ketone (MEK), methyl isobutyl ketone and cyclohexanone, and aromatic solvents such as toluene are appropriately selected. It may be used. The amount of the organic solvent to be added may be about 100 to 900 parts by weight based on 100 parts by weight of the total of the solid content (carbon black, various inorganic particles, etc.) and the binder.

The thickness of the lower nonmagnetic layer is 0.5 to 3.0 μm.
m is preferred, and 1.0 to 3.0 μm is particularly preferred. 0.
When the thickness is less than 5 μm, the magnetic layer is easily affected by the surface properties of the base. For this reason, the surface properties of the magnetic recording medium tend to deteriorate, which in turn affects the electromagnetic conversion characteristics. Furthermore, when the nonmagnetic layer becomes thinner, the amount of the lubricant tends to be insufficient, and the durability tends to deteriorate. On the other hand, when the thickness is 3 μm or more, the coating property at the time of coating is deteriorated, which is not preferable in production.

[Magnetic Layer] The magnetic layer is provided on such a non-magnetic layer and contains at least a magnetic powder and a binder. As the magnetic powder, metals such as Fe, Co, and Ni, or alloy fine powders thereof, and iron carbide are preferable. Considering the characteristics of the MR head, Hc of the magnetic powder is 1400 Oe.
~ 2600 Oe is preferred. The size of the magnetic powder is preferably 0.15 μm or less, more preferably 0.12 μm or less in average long axis diameter in order to reduce noise.

The content of the magnetic powder in the magnetic layer is 50 to 85% by weight, preferably 55 to 75% by weight of the whole magnetic layer. If necessary, the magnetic layer may contain abrasives, non-magnetic particles such as carbon black, and various additives such as lubricants.

As the binder in the magnetic layer, a thermoplastic resin, a thermosetting or reactive resin, an electron beam-sensitive modified resin, or the like is used, and the combination thereof is appropriately selected according to the characteristics of the medium, process conditions and the like. Used.

Examples of the thermoplastic resin include (meth) acrylic resin, polyester resin, polyurethane resin, vinyl chloride copolymer, acrylonitrile-butadiene copolymer, polyamide resin, polyvinyl butyral, nitrocellulose, styrene-butadiene copolymer. Polymer, polyvinyl alcohol resin, acetal resin, epoxy resin,
Examples include polyfunctional polyethers such as phenoxy resins, polyether resins, and polycaprolactone, polyamide resins, polyimide resins, phenol resins, polybutadiene elastomers, chlorinated rubbers, acrylic rubbers, isoprene rubbers, and epoxy-modified rubbers.

Examples of the thermosetting resin include a phenol resin, an epoxy resin, a polyurethane curable resin, a urea resin, a butyral resin, a polymeric resin, a melanin resin, an alkyd resin, a silicone resin, an acrylic reaction resin, a polyamide resin, Epoxy-polyamide resins, saturated polyester resins, urea-formaldehyde resins, and the like.

Among the above resins, those having a hydroxyl group at the terminal and / or the side chain are preferred because they can be easily used as a reactive resin, such as cross-linking using polyisocyanate or electron beam cross-linking modification. Further as a polar group at a terminal or side _{chain, -COOH, -SO 3 M, -OSO} 3 M, -
OPO ₃ X, -PO ₃ X, -PO ₂ X, -N ⁺ R ₃ Cl
^- , -NR ₂ and other acidic polar groups, basic polar groups, betaines, etc., which are suitable for improving dispersibility. These may be used alone or in combination of two or more.

Of these, those preferably used are the following combinations of a vinyl chloride copolymer and a polyurethane resin. Examples of the resin used in the present invention include a vinyl chloride-based copolymer, and more specifically, a vinyl chloride content of 60 to 95 wt%,
In particular, it is preferably 60 to 90 wt%, and the average degree of polymerization is preferably about 100 to 500. The polyurethane resin used in combination with such a vinyl chloride resin is particularly effective in that it has good wear resistance and good adhesion to a support. In addition, in addition to these resins, various known resins may be contained.

Further, it is also possible to use a resin obtained by introducing a (meth) acrylic double bond into the above-mentioned resin and subjecting it to electron beam-sensitive modification. The content of the electron beam functional group is 1 to 40% by mole, preferably 10 to 30% by mole in the hydroxyl group component in view of stability at the time of production, electron beam curability, etc., especially in the case of a vinyl chloride copolymer. When a monomer is reacted so as to have 1 to 20, preferably 2 to 10 functional groups per molecule, an electron beam curable resin excellent in both dispersibility and curability can be obtained.

The coating material for forming the magnetic layer is prepared by adding an organic solvent to each of the above components. The organic solvent used is not particularly limited, and the same organic solvents as those used for the lower non-magnetic layer can be used.

Center line average roughness (Ra) of the surface of the magnetic layer
Is 5.0 nm or less, preferably 1.0 to 4.5.
nm. When Ra is less than 1.0 nm, the surface is too smooth, running stability is deteriorated, and trouble during running is likely to occur. On the other hand, if it exceeds 5.0 nm, the surface of the magnetic layer becomes rough, and in a reproducing system using an MR type head, if there is a protrusion on the surface of the upper magnetic layer, the noise tends to increase suddenly or intermittently due to the influence. is there.

[Backcoat layer] The backcoat layer is provided for improving running stability and preventing the magnetic layer from being charged. The back coat layer preferably contains 30 to 80% by weight of carbon black. If the content of carbon black is too small, the antistatic effect tends to decrease, and the running stability tends to decrease. Also, since the light transmittance is likely to be high, there is a problem in a method of detecting the end of the tape by a change in the light transmittance. On the other hand, if the content of carbon black is too large, the strength of the back coat layer is reduced, and running durability is likely to be deteriorated. Any carbon black may be used as long as it is commonly used, and the average particle size is preferably about 5 to 500 nm. The average particle size is usually measured by a transmission electron microscope.

The back coat layer may contain, in addition to the carbon black, non-magnetic inorganic powders such as various abrasives mentioned in the description of the magnetic layer in order to increase the mechanical strength. The content of the nonmagnetic inorganic powder is preferably 0.1 to 5 parts by weight, based on 100 parts by weight of carbon black,
More preferably, it is 0.5 to 2 parts by weight. The average particle size of the non-magnetic inorganic powder is preferably 0.1 to 0.5 μm. If the content of such a nonmagnetic inorganic powder is too small, the mechanical strength of the back coat layer tends to be insufficient, and if it is too large, the amount of abrasion of the guide in the tape sliding path tends to increase.

In addition, if necessary, a dispersant such as a surfactant, a lubricant such as a higher fatty acid, a fatty acid ester, or silicone oil, and other various additives may be added.

A binder, a cross-linking agent,
The solvent and the like may be the same as those used in the above-mentioned coating for the magnetic layer. The content of the binder is preferably 15 to 200 parts by weight, more preferably 50 to 180 parts by weight, based on 100 parts by weight of the total of the solid content. If the content of the binder is too large, the friction with the medium sliding contact path becomes too large, the running stability is reduced, and a running accident is likely to occur.
In addition, problems such as blocking with the magnetic layer occur. When the content of the binder is too small, the strength of the back coat layer is reduced, and the running durability is easily reduced.

The thickness (after calendering) of the back coat layer is 1.0 μm or less, preferably 0.1 to 1.0 μm.
m, more preferably 0.2 to 0.8 μm. If the back coat layer is too thick, the friction between the back coat layer and the medium sliding contact path becomes too large, and the running stability tends to decrease. On the other hand, if it is too thin, the surface properties of the backcoat layer will decrease due to the influence of the surface properties of the nonmagnetic support. For this reason, when the back coat is thermally cured, the roughness of the back coat layer surface is transferred to the magnetic layer surface, resulting in a decrease in high-frequency output, S / N, and C / N. On the other hand, if the back coat layer is too thin, the back coat layer will be shaved during running of the medium.

[Non-magnetic support] The material used as the non-magnetic support is not particularly limited, and may be selected from various flexible materials and various rigid materials according to the purpose. The shape and the size may be set. For example, as a flexible material, polyethylene terephthalate, polyesters such as polyethylene naphthalate, polyolefins such as polypropylene, polyamide, polyimide,
Various resins such as polycarbonate are exemplified.

[Production Method] In the magnetic recording medium of the present invention, a coating material for the lower non-magnetic layer and a coating material for the magnetic layer are respectively prepared using the above-mentioned materials, and the coating material for the lower non-magnetic layer is formed on the non-magnetic support. Is applied, dried, and preferably cured by irradiation, and then the lower layer nonmagnetic layer is coated with the coating material for the magnetic layer, dried and smoothed. In the present invention, wet-on-dry coating is performed.

As a method of applying the nonmagnetic layer, various known coating means such as gravure coating, reverse roll coating, and die nozzle coating can be used.

Regarding the method of applying the coating material for the magnetic layer, it is preferable to apply the coating in two stages. That is, first, the target magnetic layer wet film thickness (the solvent is hardly evaporated) using a known various coating means such as a gravure coat, a reverse roll coat, and a die nozzle coat on the coated raw material of the nonmagnetic layer. The magnetic paint is supplied in excess of the thickness of the coating liquid in the state. The supply amount at this time varies depending on the deformation state of the non-magnetic layer-coated raw material, the hardness of the base used, and the like. For example, the supply amount is about 2 to 20 times the target magnetic layer thickness formation amount. And Die nozzle coating is suitable as it can easily change the supply amount. Next, the excessively supplied magnetic paint is scraped off to a required thickness with a bar (rod), and the excess paint is removed to form a magnetic coating film. By adopting the two-stage coating method as described above, it is possible to minimize the unevenness in the thickness of the ultrathin magnetic layer.

Regarding the solid content concentration of the magnetic coating material to be applied, since a wire bar or a non-wire coater bar having a groove in a rod is used, if the solid content concentration is high, the groove eyes become coated. It is necessary to reduce the yield value of the coating material to 10% by weight or less in consideration of the fact that it will remain, and to improve the leveling property. At this time, since the magnetic paint becomes a very dilute solution, aggregation tends to occur. For this reason, it is preferable to perform re-dispersion by an online disperser immediately before coating. As a method of redispersion, a known method can be used, and it is particularly preferable to use an ultrasonic disperser.

In the present invention, after the magnetic layer is provided, it is preferable to apply a magnetic field to orient the magnetic particles in the layer. Depending on the purpose, the orientation direction may be a random direction in the case of a disk medium, a parallel direction in the case of a tape medium, a vertical direction, or an oblique direction with respect to the running direction of the medium. Is also good. In order to direct in a predetermined direction, it is preferable to apply a magnetic field of 1000 G or more with a permanent magnet such as a ferrite magnet or a rare earth magnet, an electromagnet, a solenoid, or the like, or to use a plurality of these magnetic field generating means. Furthermore, an orientation may be performed by providing an appropriate drying step before the orientation or performing drying at the same time as the orientation so that the orientation after the drying is the highest.

After being coated in this manner, the coating film subjected to the orientation treatment is usually coated with known drying and evaporating means such as hot air, far infrared rays, an electric heater, and a vacuum device provided inside a drying oven. Dry and fixed. The drying temperature may be appropriately selected from the range of room temperature to about 300 ° C. depending on the heat resistance of the nonmagnetic support, the kind of the solvent, the concentration, and the like, or a temperature gradient may be provided in the drying furnace. Further, as the gas atmosphere in the drying furnace, general air or an inert gas may be used.

After the magnetic layer is dried as described above, a calendar process is performed as a surface smoothing process as needed.
As the calendering roll, a heat-resistant plastic roll such as epoxy, polyester, nylon, polyimide, polyamide, or polyimide amide (carbon,
It is preferable to use a combination of metal and other inorganic compounds) and a metal roll (combination of three to seven steps). Further, the treatment can be performed between metal rolls. The processing temperature is preferably 70 ° C. or higher, more preferably 90 ° C. or higher. The linear pressure is preferably 200 kg / cm or more, more preferably 25 kg / cm.
0 kg / cm or more, processing speed is 20 m / min to 900 m
/ Min range. In the present invention, the effect can be further enhanced at a temperature of 100 ° C. or more and a linear pressure of 250 kg / cm or more.

[0065]

EXAMPLES The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. [Example 1] <Lower layer paint> 55 parts by weight of granular α-Fe ₂ O ₃ (FRO-3 manufactured by Sakai Chemical Industry Co., Ltd.) (average particle size = 30 nm, BET = 45 m ² / g) Carbon black (Mitsubishi) 30 parts by weight (Average particle size = 24 nm, BET = 125 m ² / g, DBP oil absorption = 47 ml / 100 g) α-Al ₂ O ₃ (manufactured by Sumitomo Chemical Co., Ltd.) (AKP50) 15 parts by weight (average particle size = 0.20 μm, BET = 8 m ² / g) Electron beam-curable vinyl chloride copolymer (Toyobo Co., Ltd .: TB0246, 30% solution) (degree of polymerization = 300 , Polar group: —OSO ₃ K = 1.5 / molecule) 12 parts by weight Electron beam-curable polyurethane resin (manufactured by Toyobo Co., Ltd .: TB0242, 35% solution) (Mn = 25000, polar group: phosphorus compound = 1 piece) / 1 molecule) 4 parts by weight Trifunctional acrylic monomer (TA505, manufactured by Sanyo Chemical Industry Co., Ltd.) 2 parts by weight Isocetyl stearate 10 parts by weight Butyl stearate 4 parts by weight MEK 126 parts Parts by weight Toluene 38 parts by weight Cyclohexanone 38 parts by weight After kneading the above composition, it was dispersed by a sand grinder mill to prepare a lower layer coating material.

<Upper layer magnetic paint> Metal magnetic powder 100 parts by weight (Hc = 2400 Oe, σs = 143 emu / g, BET = 51 m ² / g, PH = 10, average major axis diameter = 0.10 μm, Fe / Co = 100 / 30 (weight ratio, including Al and Y as additive elements) 14 parts by weight of vinyl chloride copolymer (manufactured by Nippon Zeon Co., Ltd .: MR110) (degree of polymerization = 300, polar group: —OSO ₃ K = 1.5) / Molecule) Phosphobetaine-containing polyurethane resin 4 parts by weight (Mw = 40,000, polar group concentration = 1.5 units / molecule) -OSO ₃ Na-containing polyurethane resin 2 parts by weight (Mn = 25000, polar group concentration = 1 unit / 1 molecule) Abrasive α-Al ₂ O ₃ (AKP100, manufactured by Sumitomo Chemical Co., Ltd.) 10 parts by weight (average particle size = 0.06 μm, BET = 28 m ² / g) Carbon black (manufactured by Mitsubishi Chemical Corporation: CF- 9) 3 parts by weight (Average particle size = 40 nm, BET = 60 m ² / g, DBP oil absorption = 64 ml / 100 g) Sorbitan monostearate 3 parts by weight Isocetyls 3 parts by weight of tearate 2 parts by weight of butyl stearate 380 parts by weight of MEK 130 parts by weight of toluene 130 parts by weight of cyclohexanone After kneading the above composition, it was dispersed by a sand grinder mill to prepare an upper layer magnetic paint.

First, the lower layer paint was coated with a surface roughness (Ra) of 9 nm, 62 μm.
Extruded on a m-thick PET film by extrusion die nozzle method to a dry thickness of 1.5 μm, and dried at a drying temperature of 100 ° C.
Electron beam irradiation (5 Mrad) was performed. Next, a lower-layer coating film was formed on the other surface in the same procedure to form a roll of a double-sided undercoat layer.

Next, 4 parts by weight of Coronate L (manufactured by Nippon Polyurethane Industry Co., Ltd.), 580 parts by weight of MEK, 200 parts by weight of toluene and 200 parts by weight of cyclohexanone were added to the upper layer paint, and the paint was subjected to ultrasonic dispersion. While redispersing, apply to the roll coated with the lower layer by extrusion die nozzle method so as to have a thickness of 0.5 μm in terms of dry film thickness, and then use a wire bar so that the dry film thickness becomes 0.05 μm. Excess paint was scraped off. Then, non-oriented by a non-oriented magnet, dried at a drying temperature of 100 ℃, a linear pressure of 30
A calender treatment was performed at 0 kg / cm and a temperature of 90 ° C. to finish a coating film on one side. Next, a magnetic coating film was formed on the other surface by the same procedure, and a raw roll of a double-sided magnetic layer was prepared. Finally, punch this material roll into a disc,
Thereafter, heat curing was performed at 70 ° C. for 24 hours to produce a disk.

[Comparative Example 1] A lower layer paint was coated with a surface roughness (Ra) of 9
Extrusion onto a PET film having a thickness of 62 μm and a thickness of 1.5 μm by extrusion die nozzle method. While it is still wet, 4 parts by weight of Coronate L (manufactured by Nippon Polyurethane Industry Co., Ltd.) and MEK A paint containing 580 parts by weight, 200 parts by weight of toluene and cyclohexanone each added thereto was applied thereon by an extrusion die nozzle method while being redispersed by an ultrasonic disperser so that the dry film thickness became 0.05 μm. Then, it is non-oriented by a non-oriented magnet, dried at a drying temperature of 100 ° C, calendered at a linear pressure of 300 kg / cm and a temperature of 90 ° C, and irradiated with an electron beam (5 Mrad) to coat one side. Finished the membrane. Next, a coating film was formed on the other surface by the same procedure, and a raw roll of a double-sided magnetic layer was prepared. Finally, the raw roll was punched out into a disk shape, and then thermally cured at 70 ° C. for 24 hours to produce a disk.

<Evaluation of Uneven Thickness> In order to evaluate uneven thickness, the samples of Example 1 and Comparative Example 1
The modulation was evaluated using an IP drive. The modulation is a value represented by [(AB) / (A + B)] × 100 (%) where A is the maximum output and B is the minimum output in one round of the disk. In the sample of Example 1, the modulation was 6%, whereas in the sample of Comparative Example 1, the modulation was 20%. Thus, in Example 1, the thickness unevenness was very small. A sample with very little output fluctuation was obtained.

Example 2 A disk was prepared in the same manner as in Example 1 except that the thickness of the magnetic layer was changed to 0.07 μm. [Embodiment 3] In the embodiment 1, the thickness of the magnetic layer was set to 0.03.
A disk was prepared in the same manner as in Example 1 except that the disk size was changed to μm. [Example 4] In Example 1, the magnetic powder was mixed with the average major axis diameter:
A disc was prepared in the same manner as in Example 1, except that the disc was changed to 0.15 μm. [Example 5] In Example 1, the magnetic powder was mixed with the average major axis diameter:
A disc was prepared in the same manner as in Example 1 except that the disc was changed to 0.07 μm.

Comparative Example 2 A disc was prepared in the same manner as in Example 1, except that the thickness of the magnetic layer was changed to 0.15 μm. [Comparative Example 3] In Example 1, the thickness of the magnetic layer was changed to 0.01.
A disk was prepared in the same manner as in Example 1 except that the disk size was changed to μm. [Comparative Example 4] In Example 1, the calender temperature was set to 50 ° C.
And a disk was prepared in the same manner as in Example 1 except that the surface property was lowered. [Comparative Example 5] In Example 1, the magnetic powder was replaced with an average major axis diameter:
A disk was prepared in the same manner as in Example 1, except that the disk was changed to 0.20 μm.

<Evaluation of Disk Samples>
5. The surface roughness, magnetic characteristics, and electromagnetic conversion characteristics (SN ratio) of the disk samples of Comparative Examples 2 to 5 were measured as follows. (1) Centerline surface roughness Ra Using a Talystep system manufactured by Taylor Hobson Co., Ltd., filter 0.18 to 9 Hz, stylus 0.1 × 2.5 μm special stylus, stylus pressure 2 mg, measuring speed 0.03 m
The measurement was performed under the conditions of m / sec and a measurement length of 500 μm. (2) Magnetic properties 10 kOe using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.)
Was measured. (3) Electromagnetic conversion characteristics (SN ratio) An MR head made of TDK (write side: M
IG head, gap 0.15 μm, reproduction side: Ni-F
e element MR head) was measured.

[0074]

[Table 1]

From Table 1, it can be seen that the disk samples of Examples 1 to 5 all have an SN of 20 dB or more and are suitable for reproduction with an MR head. According to the evaluation with an actual MR head, if the SN value is 20 dB or more, it is suitable for reproduction with the MR head.

[0076]

According to the present invention, there is provided a thin film magnetic recording medium having good characteristics when an MR head is used. Further, according to the manufacturing method of the present invention, a coating-type thin-film magnetic recording medium having a small thickness variation is provided.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01F 1/047 H01F 1/08 H 1/06 1/06 J 1/08 L

Claims (6)

[Claims] 1. A non-magnetic support, comprising a non-magnetic layer containing at least a binder resin on a non-magnetic support,
A magnetic recording medium having a magnetic layer having a thickness of 0.08 μm and used for a recording / reproducing system in which a recording signal is reproduced by an MR head, wherein the magnetic layer is formed by coating, drying and curing the non-magnetic layer. ,
The magnetic layer is formed by applying a magnetic paint on the non-magnetic layer. The magnetic layer contains metal magnetic powder having an average major axis diameter of 0.15 μm or less, and the residual magnetic flux density Br (unit: G ) And the thickness t (unit: μm) of the magnetic layer are in the range of 50 to 250, and the surface roughness Ra of the magnetic layer is 5
a magnetic recording medium having a diameter of not more than nm. 2. The magnetic recording medium according to claim 1, wherein the binder resin contained in the non-magnetic layer is a radiation-curable binder resin. 3. The method for producing a magnetic recording medium according to claim 1, wherein a coating material for forming a nonmagnetic layer containing at least a binder resin is applied on a nonmagnetic support, followed by drying and curing. Next, applying a magnetic layer forming paint containing at least a metal magnetic powder having an average major axis diameter of 0.15 μm or less on the non-magnetic layer to form a magnetic coating film. Manufacturing method. 4. The method for producing a magnetic recording medium according to claim 3, wherein the binder resin contained in the coating material for forming a nonmagnetic layer is a radiation-curable binder resin, and is subjected to radiation curing. 5. A method for forming a magnetic coating film, comprising: applying a coating material for forming a magnetic layer on the cured non-magnetic layer in excess of a predetermined thickness of a magnetic layer;
5. A method for producing a magnetic recording medium according to claim 3, wherein an excess paint is scraped off with a bar so that a magnetic layer having a predetermined dry film thickness in the range of 0.02 to 0.08 [mu] m is obtained. Method. 6. The method according to claim 3, wherein the concentration of the solid content for forming the magnetic layer to be applied is 10% by weight or less, and redispersion is performed by an online disperser immediately before the application. The manufacturing method of the magnetic recording medium according to the above.