JP2002117528A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium having excellent electromagnetic transducing characteristics, running durability and preservability and to especially provide a magnetic recording medium having excellent preservability and capable of reproducing a signal which is not deteriorated even after long-term preservation. SOLUTION: The magnetic recording medium having a primary-coated layer provided on at least one surface of a non-magnetic substrate and at least a magnetic recording layer provided on the primary-coated layer by coating is characterized in that polyester resin manufactured from a raw material containing naphthalene dicarboxylic acid as a dicarboxylic acid component is applied to the primary-coated layer and the coated layer of the primary- coated layer side has >=0.01 μm and <=1 μm total thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a magnetic recording medium having an undercoat layer between a support and a magnetic layer, and more particularly to a magnetic recording medium having a stable undercoat layer and having excellent long-term storage characteristics.

[0002]

2. Description of the Related Art Gamma iron oxide, iron oxide containing Co, chromium oxide, ferromagnetic metal, etc. are used as tape-like magnetic recording media for audio, video, and computer use, and disk-like magnetic recording media such as floppy (registered trademark) disks. A magnetic recording medium having a magnetic layer in which a ferromagnetic fine powder such as a fine powder is dispersed in a binder is provided on a nonmagnetic support.
As a nonmagnetic support used for a magnetic recording medium, polyethylene terephthalate, polyethylene naphthalate, or the like is generally used. Since these supports are stretched and highly crystallized, they have high mechanical strength and excellent solvent resistance.

A magnetic layer obtained by applying a coating liquid in which ferromagnetic fine powder is dispersed in a binder to a nonmagnetic support has a high filling degree of the ferromagnetic fine powder, a small breaking elongation, and is brittle. The magnetic layer formed without the magnetic layer may be easily broken by applying a mechanical force and may be separated from the nonmagnetic support. Therefore, it has been practiced to provide an undercoat layer on the non-magnetic support and to strongly adhere the magnetic layer on the non-magnetic support.

For example, Japanese Patent Application Laid-Open No. 4-319514 describes a magnetic recording medium having a polar group-containing undercoat resin layer having a prescribed solubility in a mixed solvent of methyl ethyl ketone and cyclohexanone. It has been proposed to prevent the undercoat layer from being dissolved by the solvent of the coating solution for the magnetic layer when applying the magnetic layer using the undercoat resin with a reduced elution rate, and polyester resins and the like are mentioned. Although naphthalenedicarboxylic acid, terephthalic acid, isophthalic acid, and the like are exemplified as the constituent components of the polyester resin, it is not described which one is preferably used.

Japanese Patent Application Laid-Open No. 1-245421 proposes a magnetic recording medium using a polyester resin comprising ethylene glycol, diethylene glycol or the like and isophthalic acid or terephthalic acid as a polyester resin for an undercoat layer. It describes that the performance and running durability can be improved.

Further, in JP-A-56-87233, SO ₃ M magnetic recording medium has been proposed which uses the primer of group-containing polyester resin, by using SO ₃ M group-containing polyester resins, There has been proposed a magnetic recording medium in which the adhesion strength of a magnetic layer to a nonmagnetic support is increased.

On the other hand, a magnetic recording medium is required to have various characteristics according to the purpose of use, and not only is it possible to accurately record and reproduce recorded information, but also to have durability and long-term storage. There is a demand for a magnetic recording medium capable of reproducing high-quality recorded information. In particular, in the case of magnetic recording media for high-density recording such as S-VHS, 8 mm video, Hi-8, digital video, etc., the recording wavelength is reduced, and the performance required for the magnetic recording media becomes more severe. I have.

The magnetic recording medium having such an undercoat layer has excellent characteristics in terms of magnetic recording characteristics, running properties, running durability and the like. Sometimes occurred.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic recording medium having excellent electromagnetic characteristics, running durability, and preservability. It is an object of the present invention to provide a magnetic recording medium capable of reproducing a signal without deterioration even after long-term storage.

[0010]

The above object of the present invention is to provide:
The problem has been solved by the following means. (1) A magnetic recording medium in which an undercoat layer is provided on at least one surface of a nonmagnetic support and at least a magnetic recording layer is provided on the undercoat layer by coating, wherein the undercoat layer contains naphthalenedicarboxylic acid as a dicarboxylic acid component And (2) a total thickness of the magnetic recording medium of 6 μm, wherein the total thickness of the coating layer on the undercoat layer side is 0.01 μm or more and 1 μm or less. The following magnetic recording medium according to (1),
(3) The magnetic recording medium according to (1) or (2), wherein the nonmagnetic support is polyethylene naphthalate or aramid.
(4) Any one of (1) to (3) containing 2,6-naphthalenedicarboxylic acid as naphthalenedicarboxylic acid
(5) 2,6-naphthalenedicarboxylic acid in 5 parts by weight of dicarboxylic acid component in 100 parts by weight.
(4) The magnetic recording medium according to (4), comprising from 35 to 35 parts by weight.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, an increase in dropout in a magnetic recording medium after long-term storage is caused by a substance present in a surface layer of the magnetic recording medium after long-term storage, and particularly, it is used as an undercoat layer. It has been found that this is caused by the cyclic oligomer component contained in the polyester.

That is, in a conventional magnetic recording medium in which an undercoating layer using a polyester resin is provided between a support and a magnetic layer or a nonmagnetic layer, when the recording medium is stored for a long period of several years or more, the undercoating layer is not used. The cyclic oligomer, which is a trace component contained in the polyester, moves to the surface of the magnetic layer through the single-layer or multi-layer magnetic layer formed on the nonmagnetic support, and precipitates to form fine crystals. It has been found that such fine precipitates may cause dropout failure. Such a phenomenon has been found to be a serious problem, particularly in a magnetic recording medium for high-density recording having a short recording wavelength.

According to the present invention, a polyester resin having a very small amount of cyclic oligomers can be obtained by containing a naphthalenedicarboxylic acid as a dicarboxylic acid component used as a raw material for producing a polyester resin used as an undercoat layer. In this case, it has been found that even if the magnetic recording medium is stored for a long period of time, it is possible to suppress the generation of precipitates that cause dropout on the surface of the magnetic layer.

The cyclic oligomer in the polyester resin is
Produced during the polymerization of polyester. Here, the cyclic oligomer refers to a cyclic dimer or cyclic quanter having a molecular weight of 1,000 or less. Among the aromatic dicarboxylic acids, cyclic dimer of isophthalic acid and ethylene glycol (molecular weight: 384), trimer (molecular weight: 576), tetramer (molecular weight: 960), cyclic dimer of terephthalic acid and ethylene glycol
A dimer (molecular weight: 576) and a tetramer (960) are easily formed when polymerizing a polyester.

Unlike these linear oligomers, these cyclic oligomers do not have terminal functional groups such as OH groups and COOH groups, and thus are less likely to react with or interact with other materials in the coating film, and thus are more likely to move. In particular, a dimer having a low molecular weight is easy to move, has high crystallinity, and easily crystallizes on the surface of the magnetic layer. The present invention reduces the amount of a substance that precipitates on the magnetic layer surface and causes dropout by reducing the content of the cyclic oligomer.

The polyester resin for the undercoat layer is obtained by, for example, condensation of a dicarboxylic acid component and a glycol component. The polyester resin of the present invention is characterized by containing naphthalenedicarboxylic acid as the dicarboxylic acid component.
Examples of the naphthalenedicarboxylic acid include 2,6-naphthalenedicarboxylic acid and 1,4-naphthalenedicarboxylic acid. Of these, 2,6-naphthalenedicarboxylic acid is preferable.

Preferably, the naphthalenedicarboxylic acid is contained in an amount of 15 mol% to 75 mol% based on the whole dicarboxylic acid component. More preferably, it is 20 mol% to 60 mol%. When the amount is less than this range, the adhesive force between the nonmagnetic support and the coating layer such as the nonmagnetic layer or the magnetic layer becomes small, and when the amount is too large, the solubility in the solvent becomes low and the coating solvent is limited to a special one. Therefore, it is not suitable for a commercial manufacturing process of a magnetic recording medium.

The dicarboxylic acid component used in combination with naphthalenedicarboxylic acid includes aliphatic dicarboxylic acids such as succinic acid, adipic acid and sebacic acid, and aromatic dicarboxylic acids such as orthophthalic acid, isophthalic acid and terephthalic acid. . Of these, isophthalic acid is not preferred because it easily forms a cyclic oligomer. Even when used, it is preferably at most 10 mol%.

Examples of the glycol component to be reacted with these dicarboxylic acid components include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, neopentyl glycol, hexanediol, bisphenol A, and alkylene oxide adducts of bisphenol A. Used.

The preferred molecular weight of the polyester resin for the undercoat layer of the present invention is from 10,000 to 200,000, more preferably from 20,000 to 50,000, by weight average molecular weight. The polyester resin used as the undercoat agent of the present invention preferably has a cyclic oligomer content of 1% by weight or less. More preferably, it is 0.5% by weight or less. The undercoat layer must not be affected by a solvent for coating the magnetic layer or the non-magnetic layer applied on the undercoat layer. For this purpose, the solubility in a solvent can be adjusted by introducing a polar group into the polyester molecule. Polar _{groups, - SO 3 M, -SO 4} M,
-OPO (OM) ₂ , -PO (OM) ₂ , -COOM and the like are used. Preferably from -SO ₃ Na. Here, M represents hydrogen or an alkali metal such as Na or K. The content of the polar group is preferably 1 × 10 ⁻⁶ eq / g to ⁴ × 10 ⁻⁴ eq / g.

Further, in order to prevent a blocking phenomenon in the production process of the undercoat layer, the glass transition temperature: Tg is preferably relatively high, and the glass transition temperature is preferably 50.
° C or higher, and more preferably 70 ° C or higher.

The polymer substance is hard and difficult to deform below the glass transition temperature Tg, but is flexible and easy to deform above the glass transition temperature Tg. In a magnetic recording medium, a polymer substance is used as a binder. The lower the Tg of the coating layer including the magnetic layer, the softer the film of the coating layer, and thus the better the head contact and the better the electromagnetic conversion characteristics. In the present invention, the Tg of the coating layer is 0 to 7
The temperature is preferably 0 ° C, particularly preferably from 40 ° C to 70 ° C. On the other hand, it was found that when the Tg of the coating layer was low, the low molecular component contained in the undercoat layer provided between the coating layer and the non-magnetic support moved to the surface of the coating layer, and was likely to precipitate and cause dropout. Therefore, it is necessary for the undercoat layer of the present invention to use a polyester resin produced from a raw material containing naphthalenedicarboxylic acid as a dicarboxylic acid component. Glass transition temperature Tg is DSC, DTA
It can be determined by thermal measurement or dynamic viscoelasticity measurement.

The non-magnetic support which can be used in the magnetic recording medium of the present invention includes biaxially stretched polyethylene naphthalate, polyethylene terephthalate, polyamide, polyimide, polyamide imide, aromatic polyamide, aromatic polyamide imide, and aromatic polyamide imide. Known compounds such as polyimides of group III and polybenzoxydazole can be used. Preferred are polyethylene naphthalate, aromatic polyamide, aromatic polyamideimide, and aromatic polyimide.

Particularly, polyethylene naphthalate is preferable because not only the strength is high but also the adhesive strength of the coating layer is increased by the polyester resin of the undercoat layer of the present invention. The smoothness of the non-magnetic support is preferably 10 nm or less in terms of center average roughness Ra.

A support made of aromatic polyamide, aromatic polyamideimide or aromatic polyimide is excellent in physical properties such as tensile strength. The aromatic polyamide preferably contains a repeating unit represented by the following formula (I) or (II). — (NH—Ar ¹ —NHCO—Ar ² —CO) — (I) — (NH—Ar ³ —CO) — (II) Ar ¹ , Ar ² , and Ar ³ each represent an aromatic ring (the aromatic ring is Condensed) or a group containing at least one aromatic ring. Ar ¹ , Ar ² and A
Examples of r ³ include the following.

[0026]

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Here, X and Y are -O- and -C, respectively.
H ₂ —, —CO—, —SO ₂ —, —S—, and —C (C
H ₃ ) ₂ — represents a group selected from The hydrogen atom of the aromatic ring may be substituted. Examples of the group or atom capable of substituting a hydrogen atom of an aromatic ring include a halogen atom (particularly, chlorine), a nitro group, an alkyl group having 1 to 3 carbon atoms (particularly, a methyl group), An alkoxy group can be mentioned. The hydrogen atom of the amide bond which is a constituent component of the polymer may be substituted.

The aromatic polyamide used in the present invention is preferably a polymer having an aromatic ring bonded at the para position occupying 50% or more (more preferably 70% or more) of the total aromatic ring. The phrase “aromatic is in the para position” includes a condensed aromatic ring, for example, a 1,4-position or a 2,6-position of a naphthalene ring. In addition, from the viewpoint of reducing the hygroscopicity, it is preferable that the aromatic ring in which hydrogen atoms on the aromatic ring are substituted with halogen atoms (particularly chlorine atoms) accounts for 30% or more of the entire aromatic ring.

The aromatic polyamide used in the present invention contains 50 mol% of the repeating unit represented by the above (I) or (II).
As described above, the composition preferably contains 70 mol% or more, but as long as it satisfies the physical properties as a support, the repeating unit represented by the above formula (I) or (II), A polymer obtained by copolymerization or blending with a repeating unit of the above can be used. In the present invention, it is preferable to use aramid (a wholly aromatic polyamide). Representative examples of aramid products include Miktron (manufactured by Toray Industries, Inc.) and Aramica (manufactured by Asahi Kasei Kogyo Co., Ltd.)
Can be mentioned. The aromatic polyamide is described in JP-A-8-55327 or JP-A-8-55327.
-55328.

The aromatic polyimide is, for example, represented by the following formula (II)
It preferably contains a repeating unit represented by (I), (IV) or (V).

[0031]

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R ¹¹ , R ¹² and R ¹³ each represent an aromatic ring, a heterocyclic ring or a group containing at least one aromatic ring. The hydrogen atom of the above aromatic ring or heterocyclic ring may be substituted. Examples of the group or atom capable of substituting a hydrogen atom of an aromatic ring or a heterocyclic ring include a halogen atom (particularly chlorine), a nitro group, an alkyl group having 1 to 3 carbon atoms (particularly a methyl group), To 3 alkoxy groups. Examples of the above R ¹¹ , R ¹² and R ¹³ include the following.

[0033]

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The aromatic polyimide used in the present invention has a repeating unit represented by the above formula (III), (IV) or (V) of 5 units.
It is preferable that the support is composed of at least 0 mol%, preferably at least 70 mol%, but as long as it satisfies the physical properties as a support, it can be represented by the above formula (III), (IV) or (V). A polymer obtained by copolymerizing or blending the repeating unit with another repeating unit can be used.

The aromatic polyamideimide preferably contains, for example, a repeating unit represented by the following formula (VI), (VII) or (VIII).

[0036]

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The above Ar ⁴ and Ar ⁵ have the same meanings as the group represented by Ar ¹ , respectively. Aromatic polyamide,
The thickness of the support made of aromatic polyamideimide or aromatic polyimide is 2.0 to 4.7 μm (preferably 2.2 to 4.5 μm, more preferably 2.5 to 4.5 μm).
4.5 μm, particularly preferably in the range of 2.6 to 4.5 μm).

The support which can be preferably used in the present invention has at least one surface subjected to a corona discharge treatment. In the present invention, it is preferable from the viewpoint of manufacturing that at least the surface on the side on which the backcoat layer is provided is subjected to corona discharge treatment, and most preferably, both surfaces of the support are subjected to corona discharge treatment. The corona discharge treatment is generally performed by applying a high frequency and a high voltage between an electrode and a grounded counter electrode dielectric roll to generate a corona discharge and pass the support during the corona discharge. Conventionally, such a corona discharge treatment is performed by a spark gap method (high frequency), a vacuum tube method (high frequency),
And a solid state method (low frequency) are known, but in the present invention, any of these methods may be used.

When the above corona discharge treatment is performed, the support is charged by the generated radicals, and web handling becomes very difficult. For example, the web is meandering to the left and right on the transport roll, a problem such as the web is shifted to one side, or the web is wound to one side occurs, and between the counter electrode dielectric roll, various electrostatic obstacles, for example,
Electric shock, discharge, electrostatic sticking, and the like are likely to occur. For this reason, it is preferable to subject the support after the corona discharge treatment to a static elimination treatment in order to remove the charged static electricity. The static elimination process is performed by spraying an ion wind from an ion wind generator on the surface of the support after the corona discharge treatment. Such corona discharge treatment and, if necessary, charge removal treatment can be performed at the final stage of the production process of forming a support. In this case, the support that has been subjected to the corona discharge treatment and, if necessary, the charge removal treatment is once wound into a roll. The support wound up in the form of a roll retains the effect of the corona discharge treatment without deterioration even after being stored for about 4 months as it is.

These non-magnetic supports may be subjected to plasma treatment, heat treatment, etc. in addition to corona discharge in advance. The nonmagnetic support that can be used in the present invention has a surface having excellent smoothness such that the center line average surface roughness is in the range of 0.1 to 20 nm, preferably 1 to 10 nm at a cutoff value of 0.25 mm. Is preferred. It is preferable that these non-magnetic supports have not only a small center line average surface roughness but also no coarse protrusions of 1 μm or more.

The magnetic recording medium of the present invention has a single-layer structure using only a single magnetic layer, has a plurality of magnetic layers, and has a non-magnetic layer provided between a magnetic layer and an undercoat layer. And the like.

The effect of improving the storability of the present invention becomes more remarkable as the thickness of the nonmagnetic layer and the magnetic layer provided on the undercoat layer becomes smaller. That is, in the case of a magnetic recording medium in which the total thickness of the nonmagnetic layer and the magnetic layer formed on the support exceeds 3 μm, even with the conventional polyester resin, the precipitation of cyclic oligomers was relatively small and the problem was small. The total thickness of the coating layer selected from the layers is 0.01 μm to 1 μm
In a high-density magnetic recording medium having a thickness of m, the cyclic oligomer in the polyester of the undercoat layer tends to move to the surface of the magnetic layer.

The coating layer in the present invention refers to a layer containing a magnetic recording layer other than the undercoat layer applied on the undercoat layer of the present invention. More specifically, when a single magnetic recording layer or a plurality of magnetic recording layers are applied on the undercoat, the entire magnetic recording layer except the undercoat is referred to as a coating layer. When a non-magnetic layer is applied on the undercoat and a magnetic recording layer is applied thereon, the combined layer of the non-magnetic layer and the magnetic recording layer excluding the undercoat is referred to as a coating layer. When two magnetic recording layers are provided on the undercoat layer, it refers to the entire layer excluding the undercoat layer including the magnetic recording layer lower layer (magnetic lower layer) and the magnetic recording layer upper layer (magnetic upper layer). Non-magnetic layer, magnetic lower layer, or magnetic upper layer, non-magnetic powder,
It is formed by applying a composition in which a magnetic powder is dispersed in a binder.

As the binder, polyurethane resin, polyester resin, polyamide resin, vinyl chloride resin,
Acrylic resin copolymerized with styrene, acrylonitrile, methyl methacrylate, etc., cellulose resin such as nitrocellulose, epoxy resin, phenoxy resin,
A single or a mixture of a plurality of resins such as polyvinyl alkylal resins such as polyvinyl acetal and polyvinyl butyral can be used. Preferred among these are polyurethane resins, PVC resins, and acrylic resins.

The binder preferably has a functional group (polar group) adsorbed on the surface of the magnetic substance and the non-magnetic powder in order to improve the dispersibility of the powder. -SO ₃ M is preferred functional _{group, -SO 4 M, -PO (OM} ) 2, -OP
_{O (OM) 2, -COOM,} > NSO 3 M,> NRSO 3
_{M, -NR 1 R 2, -N} + R 1 R 2 R 3 X - , and the like. Here, M is hydrogen or an alkali metal such as Na or K, R is an alkylene group, R ₁ , R ₂ , R ₃ is an alkyl group or a hydroxyalkyl group or hydrogen, and X is a halogen such as Cl or Br. The amount of the functional group in the binder is 10 μeq / g to 200 μ
eq / g is preferable, and further 30 eq / g to 120 μm
eq / g is preferred. Even if it exceeds or falls short of this range, the dispersibility decreases.

The binder preferably has a functional group having an active hydrogen such as an --OH group in order to form a crosslinked structure by reacting with an isocyanate curing agent and to improve the strength of the coating film, in addition to the adsorption functional group. . The preferred amount is 0.1 meq / g
２2 meq / g. The molecular weight of the binder is preferably 10,000 to 200,000, more preferably 20,000 to 100,000 in weight average molecular weight. If it is smaller than this range, the strength of the coating film is insufficient and the durability is reduced. If it is large, the dispersibility decreases.

A polyurethane resin which is a preferable binder is described in, for example, "Polyurethane Resin Handbook" (Keiji Iwata)
Ed., 1986, Nikkan Kogyo Shimbun), which is usually obtained by addition polymerization of a long-chain diol, a short-chain diol (sometimes called a chain extender) and a diisocyanate compound. Long-chain diols have a molecular weight of 500-
5,000 polyester diols, polyether diols, polyether ester diols, polycarbonate diols, polyolefin diols and the like are used. Depending on the type of the long-chain polyol, it is called polyester urethane, polyether urethane, polyether ester urethane, polycarbonate urethane, or the like.

The polyester diol is obtained by condensation polymerization of an aliphatic dibasic acid such as adipic acid, sebacic acid or azelaic acid, or an aromatic dibasic acid such as isophthalic acid, orthophthalic acid, terephthalic acid or naphthalenedicarboxylic acid with glycol. Can be As the glycol component, ethylene glycol, 1,2-propylene glycol, 1,
3-propanediol, 1,4-butanediol, 1,
5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 2,2-dimethyl-1,3-propanediol, 1,8-octanediol, 1,9-nonanediol, Examples include cyclohexanediol, cyclohexanedimethanol, hydrogenated bisphenol A, and the like. Polyester diols such as polycaprolactone diol and polyvalerolactone diol obtained by ring-opening polymerization of a lactone such as ε-caprolactone and γ-valerolactone can also be used. As the polyester diol, those having a branched side chain and those obtained from aromatic or alicyclic raw materials are preferable from the viewpoint of hydrolysis resistance.

Examples of polyether diols include aromatic glycols such as polyethylene glycol, polypropylene glycol, polytetramethylene glycol, bisphenol A, bisphenol S, bisphenol P, and hydrogenated bisphenol A, and alicyclic diols such as ethylene oxide and propylene oxide. Examples include those obtained by addition polymerization of an alkylene oxide.

These long-chain diols may be used in combination of two or more kinds. The short-chain diol can be selected from the same compounds as those exemplified for the glycol component of the polyester diol.
When a trifunctional or higher polyhydric alcohol such as trimethylolethane, trimethylolpropane, or pentaerythritol is used in a small amount, a polyurethane resin having a branched structure can be obtained to lower the solution viscosity or to increase the number of OH groups at the end of the polyurethane to obtain isocyanate. Curability with a system curing agent can be enhanced.

As the diisocyanate compound, MDI
(Diphenylmethane diisocyanate), 2,4-TD
I (tolylene diisocyanate), 2,6-TDI,
1,5-NDI (naphthalene diisocyanate), TO
Aromatic diisocyanates such as DI (trizine diisocyanate), p-phenylene diisocyanate, XDI (xylylene diisocyanate), transcyclohexane-1,4-diisocyanate, HDI (hexamethylene diisocyanate), IPDI (isophorone diisocyanate), H6XDI (hydrogenated xylylene An aliphatic or alicyclic diisocyanate such as H12MDI (hydrogenated diphenylmethane diisocyanate) or H12MDI (hydrogenated diphenylmethane diisocyanate) is used.

The preferred composition of the long-chain diol / short-chain diol / diisocyanate in the polyurethane resin is (80 to
15% by weight) / (5 to 40% by weight) / (15 to 50% by weight). Urethane group concentration of polyurethane resin is 1m
eq / g to 5 meq / g is preferred. Furthermore, 1.5-
4.5. If it is less than this range, the mechanical strength is small, and if it is too large, the solution viscosity is high and the dispersibility is reduced.

The glass transition temperature of the polyurethane resin is 0 ° C.
To 200 ° C, more preferably 40 ° C to 160 ° C. If it is lower than this range, the durability decreases, and if it is too high, calender moldability decreases and the electromagnetic conversion characteristics deteriorate.

As a method for introducing the above-mentioned adsorptive functional group (polar group) into the polyurethane resin, a method in which a functional group is used as a part of a monomer of a long-chain diol, a method in which a functional group is used as a part of a short-chain diol, or a method in which polyurethane is polymerized. Later, there is a method of introducing a polar group by a polymer reaction.

As the vinyl chloride resin, a resin obtained by copolymerizing a vinyl chloride monomer with various monomers is used.
As copolymerized monomers, vinyl acetate, fatty acid vinyl esters such as vinyl propionate, methyl (meth) acrylate, ethyl (meth) acrylate, isopropyl (meth) acrylate, butyl (meth) acrylate,
Acrylates such as benzyl (meth) acrylate, methacrylates, allyl methyl ether, allyl ethyl ether, allyl propyl ether, alkyl allyl ether such as allyl butyl ether, and other styrenes;
α-methylstyrene, vinylidene chloride, acrylonitrile, ethylene, butadiene, acrylamide, and vinyl alcohol as a copolymerizable monomer having a functional group, 2-
Hydroxyethyl (meth) acrylate, polyethylene glycol (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 3-hydroxypropyl (meth) acrylate, polypropylene glycol (meth)
Acrylate, 2-hydroxyethyl allyl ether,
2-hydroxypropyl allyl ether, 3-hydroxypropyl allyl ether, p-vinylphenol, maleic acid, maleic anhydride, acrylic acid, methacrylic acid, glycidyl (meth) acrylate, allyl glycidyl ether, phosphoethyl (meth) acrylate, sulfoethyl ( (Meth) acrylate, p-styrenesulfonic acid, and Na salts and K salts thereof are used. In addition, (meth) acrylate means one containing at least one of acrylate and methacrylate.

The composition of the vinyl chloride monomer in the vinyl chloride resin is preferably from 60 to 95% by weight. If less than this, the mechanical strength decreases, and if too large, the solvent solubility decreases,
Solution viscosity is high and dispersibility decreases. The preferable amount of the functional group for improving the curability with the adsorption functional group (polar group) and the polyisocyanate-based curing agent is as described above. As a method for introducing these functional groups, the functional group-containing monomer may be copolymerized, or the functional groups may be introduced by polymer reaction after copolymerizing a vinyl chloride resin. The preferred degree of polymerization is 200 to 600, more preferably 240 to 450.
It is. If it is smaller than this range, the mechanical strength decreases, and if it is too high, the solution viscosity increases and the dispersibility decreases.

A curing agent can be used to crosslink and cure the binder of the present invention to increase the mechanical strength and heat resistance of the coating film. Preferred curing agents include polyisocyanate compounds. The polyisocyanate compound is preferably a trifunctional or higher polyisocyanate. Specifically, a compound obtained by adding 3 mol of TDI (tolylene diisocyanate) to trimethylolpropane (TMP), a compound obtained by adding 3 mol of HDI (hexamethylene diisocyanate) to TMP,
IPDI (isophorone diisocyanate) in TMP
Adduct-type polyisocyanate compound such as a compound obtained by adding 3 moles of XDI (xylylene diisocyanate) to TMP, a condensed isocyanurate trimer of TDI, a condensed isocyanurate pentamer of TDI, a condensed isocyanate of TDI Nurate heptamers, and mixtures thereof. HDI isocyanurate condensate, IPD
An isocyanurate-type condensate of I. Further Crude MDI
and so on. Of these, TMP is preferably TDI
, And isocyanurate-type trimers of TDI.

In addition to the isocyanate-based curing agents, radiation-curable curing agents such as electron beams or ultraviolet rays may be used. In this case, a curing agent having two or more, preferably three or more acryloyl groups or methacryloyl groups in the molecule as the radiation-curable functional group can be used. For example, T
MP (trimethylolpropane) triacrylate,
Pentaerythritol tetraacrylate, urethane acrylate oligomer and the like. In this case, it is preferable to introduce a (meth) acryloyl group into the binder in addition to the curing agent. In the case of ultraviolet curing, a photosensitizer is additionally used. The curing agent is 0 based on 100 parts by weight of the binder.
It is preferable to add ~ 80 parts by weight. If it is too large, the dispersibility decreases.

The ferromagnetic powder used in the magnetic recording medium of the present invention is a ferromagnetic iron oxide, a cobalt-containing ferromagnetic iron oxide or a ferromagnetic alloy powder having an S _BET specific surface area of 40 to 80 m ² / g.
, Preferably 50 to 70 m ² / g. The crystallite size is between 12 and 25 nm, preferably between 13 and 22 nm, particularly preferably between 14 and 20 nm. The major axis length is 0.05 to 0.25 μm, preferably 0.07 to
0.2 μm, particularly preferably 0.08 to 0.15
μm. Fe, Ni, Fe
-Co, Fe-Ni, Co-Ni, Co-Ni-Fe and the like, and aluminum, silicon, sulfur, scandium, titanium, vanadium, chromium, manganese, copper within a range of not more than 20% by weight of the metal component. , Zinc, yttrium, molybdenum, rhodium, palladium, gold, tin, antimony, boron, barium, tantalum, tungsten,
Examples include rhenium, silver, lead, phosphorus, lanthanum, cerium, praseodymium, neodymium, tellurium, and alloys containing bismuth. Further, the ferromagnetic metal powder may contain a small amount of water, hydroxide or oxide. The method for producing these ferromagnetic powders is already known, and the ferromagnetic powder used in the present invention can also be produced according to a known method. There is no particular limitation on the shape of the ferromagnetic powder,
Usually, needles, granules, dice, rice granules, and plate shapes are used. It is particularly preferable to use acicular ferromagnetic powder.

The above-mentioned resin component, curing agent and ferromagnetic powder are kneaded and dispersed together with a solvent such as methyl ethyl ketone, dioxane, cyclohexanone and ethyl acetate which are usually used in preparing a coating solution for a magnetic layer to form a magnetic paint. . The kneading and dispersion can be performed according to a usual method. The magnetic recording medium of the present invention has a nonmagnetic undercoat layer composed of a nonmagnetic powder or a magnetic powder and a binder containing a polyurethane resin used for the magnetic layer described above on a nonmagnetic support, and a magnetic undercoat layer. Is also good. The nonmagnetic powder can be selected from inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, and the like. Examples of the inorganic compound include α-alumina, β-alumina, γ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, silicon nitride, titanium carbide, and titanium oxide having an α conversion of 90 to 100%. , Silicon dioxide, tin oxide, magnesium oxide, tungsten oxide,
Zirconium oxide, boron nitride, zinc oxide, calcium oxide, calcium sulfate, barium sulfate, molybdenum disulfide and the like can be used alone or in combination. Particularly preferred are titanium dioxide, zinc oxide, iron oxide and barium sulfate, and more preferred is titanium dioxide. The average particle size of these non-magnetic powders is preferably 0.005 to 2 μm. However, if necessary, non-magnetic powders having different average particle sizes may be combined, or even a single non-magnetic powder may have a wide particle size distribution to achieve the same effect. Can also be provided. Particularly preferably, the average particle size of the nonmagnetic powder is 0.01 to 0.2 μm. The pH of the nonmagnetic powder is particularly preferably between 6 and 9. The specific surface area of the nonmagnetic powder is 1 to 100 m ² / g, preferably 5 to 5 m ² / g.
0 m ² / g, more preferably from 7~40m ² / g.
The crystallite size of the nonmagnetic powder is preferably 0.01 μm to 2 μm. Oil absorption using DBP is 5-100ml / 10
0 g, preferably 10 to 80 ml / 100 g, and more preferably 20 to 60 ml / 100 g. Specific gravity is 1 to 1
2, preferably 3 to 6. The shape may be any of a needle shape, a spherical shape, a polyhedral shape, and a plate shape.

On the surface of these nonmagnetic powders, Al
₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , S
It is preferable to perform a surface treatment with b ₂ O ₃ and ZnO. Particularly preferred for dispersibility are Al ₂ O ₃ , SiO ₂ , TiO ₂ , Z
rO ₂ , more preferably Al ₂ O ₃ , Si
O ₂ and ZrO ₂ . These may be used in combination or may be used alone. Further, a co-precipitated surface treatment layer may be used according to the purpose, or a method of first treating with alumina and then treating the surface layer with silica, or vice versa, may be employed. Although the surface treatment layer may be a porous layer depending on the purpose, it is generally preferable that the surface treatment layer be homogeneous and dense.

Magnetic powders that can be used in the lower coating layer include γ-Fe ₂ O ₃ , Co-modified γ-Fe ₂ O ₃ ,
An alloy containing Fe as a main component, CrO ₂ or the like is used. Particularly, Co-modified γ-Fe ₂ O ₃ is preferable. The ferromagnetic powder used in the lower layer of the present invention preferably has the same composition and performance as the ferromagnetic powder used in the upper magnetic layer. However, it is known that the performance is changed between the upper and lower layers according to the purpose. For example, in order to improve long-wavelength recording characteristics, it is desirable that Hc of the lower magnetic layer be set lower than that of the upper magnetic layer, and that Br of the lower magnetic layer be higher than that of the upper magnetic layer. It is valid. In addition to the above, advantages obtained by adopting a known multilayer structure can be provided.

As other additives used in the magnetic layer or the lower coating layer of the present invention, those having a lubricating effect, an antistatic effect, a dispersing effect, a plasticizing effect and the like are used. Molybdenum disulfide, tungsten disulfide, graphite, boron nitride, graphite fluoride, silicone oil, silicone having polar groups, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing ester, polyolefin, polyglycol, alkyl phosphorus Acid esters and their alkali metal salts, alkyl sulfates and their alkali metal salts, polyphenyl ethers, fluorine-containing alkyl sulfates and their alkali metal salts, containing unsaturated bonds having 10 to 24 carbon atoms, and also branched Also good monobasic fatty acids, and
These metal salts (Li, Na, K, Cu, etc.) or
Monovalent, divalent, trivalent, tetravalent, pentavalent, hexavalent alcohols containing an unsaturated bond having 12 to 22 carbon atoms, which may be branched, including an unsaturated bond having 12 to 22 carbon atoms But also an alkoxy alcohol which may be branched,
It may contain 0 to 24 unsaturated bonds, may contain a monobasic fatty acid which may be branched, and may contain an unsaturated bond having 2 to 12 carbon atoms, or may contain a monovalent, divalent or trivalent which may be branched. Mono-, di-, penta-, hexa-hydric alcohols, mono- or di- or tri-fatty acid esters, fatty acid esters of monoalkyl ethers of alkylene oxide polymers, fatty acid amides having 2 to 22 carbon atoms And an aliphatic amine having 8 to 22 carbon atoms. Specific examples of these include lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, butyl stearate, oleic acid, linoleic acid, linolenic acid, elaidic acid, octyl stearate, amyl stearate, isooctyl stearate, myristin Octyl acid, butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan distearate, anhydrosorbitan tristearate, oleyl alcohol, lauryl alcohol.

Further, nonionic surfactants such as alkylene oxides, glycerins, glycidols, alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocycles, phosphonium or Cationic surfactants such as sulfoniums, carboxylic acid, sulfonic acid, phosphoric acid, sulfate group, phosphate group,
Anionic surfactants containing acidic groups such as amino acids,
Aminosulfonic acids, sulfuric acid or phosphoric acid esters of amino alcohols, alkylbedine-type amphoteric surfactants, and the like can also be used. For these surfactants,
It is described in detail in "Surfactant Handbook" (published by Sangyo Tosho Co., Ltd.). These lubricants, antistatic agents and the like are not necessarily pure, and may contain impurities such as isomers, unreacted substances, by-products, decomposition products, oxides, etc. in addition to the main components. These impurities are preferably 30% by weight or less,
More preferably, it is at most 10% by weight.

The types and amounts of these lubricants and surfactants used in the present invention can be properly selected in the non-magnetic layer and the magnetic layer as required. For example, to control bleeding to the surface using fatty acids having different melting points in the non-magnetic layer and magnetic layer, to control bleeding to the surface using esters having different boiling points and polarities, and to adjust the amount of surfactant. It can be considered to improve the stability of coating and improve the lubricating effect by increasing the amount of the lubricant added to the non-magnetic layer. Of course, the present invention is not limited to the examples shown here. Further, all or a part of the additives used in the present invention may be added in any step of producing a coating solution for the magnetic layer or the lower layer. For example, when mixing with a ferromagnetic powder before a kneading step, when adding in a kneading step with a ferromagnetic powder, a binder and a solvent, when adding in a dispersing step, when adding after dispersing, when adding just before coating, and the like. There is.

Specific examples of these lubricants used in the present invention include NAA-102, castor oil-hardened fatty acid, NAA-42, cation SA, Nymeen L-201, and Nonion E-manufactured by NOF Corporation. 208, Anon BF,
Anone LG, butyl stearate, butyl laurate,
Erucic acid, manufactured by Kanto Chemical Co., Ltd .: Oleic acid, manufactured by Takemoto Yushi Co., Ltd .: FAL-205, FAL-123, manufactured by Shin Nippon Rika Co., Ltd .: Nujierubu OL, Shin-Etsu Chemical Co., Ltd.
Manufactured by TA-3, manufactured by Lion Armor: Armide P,
Lion Corporation, Duomin TDO, Nisshin Oil Co., Ltd .:
BA-41G, manufactured by Sanyo Chemical Industry Co., Ltd .: PROFAN 20
12E, Newpole PE61, Ionet MS-40
0 and the like.

The coating solution prepared from the above materials is coated on a non-magnetic support to form a lower coating layer or a magnetic layer. There is a direction toward increasing the volume recording density. To increase the volume recording density, it is necessary to reduce the total thickness of the magnetic recording medium. Therefore, the total thickness in the present invention is preferably 2 μm or more and 6 μm or less (2 to 6 μm, the same applies to the present invention). To reduce the total thickness, there is a method of reducing the coating thickness or the thickness of the support. However, if the thickness of the support is reduced, the strength of the magnetic recording medium is reduced. Therefore, it is desirable to reduce the coating thickness. 0.01 μm as coating thickness
To 1 μm, more preferably 0.05 μm to 1 μm.
μm. Here, a plurality of magnetic layer coating solutions may be sequentially or simultaneously multi-layer coated, or a lower layer coating solution and a magnetic layer coating solution may be sequentially or simultaneously multi-layer coated. As a coating machine for applying the magnetic coating liquid or the lower layer coating liquid,
Air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure coat, kiss coat, cast coat, spray coat, spin coat and the like can be used. These are described, for example, in “Latest Coating Technology” published by Sogo Gijutsu Center (May 31, 1983)
Day) can be referred to.

When the present invention is applied to a magnetic recording medium having two or more layers, the following can be proposed as an example of a coating apparatus and method. (1) The lower layer is first applied by a coating device such as gravure, roll, blade, or extrusion which is generally applied in the application of the magnetic layer coating solution. No., JP-A-60-238
No. 179, JP-A-2-265672, etc., the upper layer is applied by a support pressurized extrusion coating apparatus. (2) JP-A-63-88080, JP-A-2-17
No. 971 and Japanese Unexamined Patent Publication No. 2-265672 disclose the upper and lower layers almost simultaneously by one coating head having two coating liquid passage slits. (3) The upper and lower layers are coated almost simultaneously by an extrusion coating device with a backup roll as disclosed in JP-A-2-174965.

A backcoat layer (backing layer) is formed on the surface of the non-magnetic support used in the present invention on which the magnetic paint is not applied.
May be provided. The back coat layer is provided by applying a back coat layer forming paint in which a particulate component such as an abrasive and an antistatic agent and a binder are dispersed in an organic solvent on a surface of the non-magnetic support on which the magnetic paint is not applied. Layer. Various inorganic pigments and carbon black can be used as the particulate component, and resins such as nitrocellulose, phenoxy resin, vinyl chloride resin, and polyurethane can be used alone or as a mixture of these as a binder. . The durability can be further improved by using the polyurethane resin of the present invention as the polyurethane resin used for the back coat layer. Note that an adhesive layer may be provided on the surface of the non-magnetic support on which the magnetic paint and the back coat layer forming paint are applied.

The coating layer of the magnetic layer coating solution is dried after subjecting the ferromagnetic powder contained in the coating layer of the magnetic layer coating solution to a magnetic field orientation treatment. After being dried in this manner, the coating layer is subjected to a surface smoothing treatment. For the surface smoothing treatment, for example, a super calender roll or the like is used. By performing the surface smoothing treatment, voids generated by the removal of the solvent during drying disappear, and the filling rate of the ferromagnetic powder in the magnetic layer is improved, so that a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. it can. As the calendering roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, or polyamideimide is used. Moreover, it can also process with a metal roll.

The magnetic recording medium of the present invention has a center line average roughness of 0.1 to 0.1 at a cutoff value of 0.25 mm.
It is preferable for the magnetic recording medium for high-density recording to have a surface having extremely excellent smoothness of 5 nm, preferably 1 to 4 nm. As a method for this, for example, as described above, a magnetic layer formed by selecting a specific ferromagnetic powder and a binder is subjected to the above-described calendering treatment. The calendering conditions are such that the temperature of the calender roll is in the range of 60 to 100 ° C, preferably 70 to 100 ° C.
° C, particularly preferably in the range of 80 to 100 ° C, the pressure in the range of 980 to 4,900 N / cm,
It is preferably in the range of 1,960 to 4,410 N / cm, particularly preferably in the range of 2,940 to 3,920 N / cm. The obtained magnetic recording medium can be used after being cut into a desired size using a cutting machine or the like.

[0072]

The present invention will be described below with reference to examples. The indication of "parts" in the examples indicates "parts by weight". Production of Samples 1 to 11 (Production of polyester resin for undercoat layer) Table 1 was prepared in a reaction vessel equipped with a thermometer, a stirrer, and a partial reflux condenser. A total of 800 g of each dimethyl ester of the dicarboxylic acid component and the glycol component described in 1 mol% was added, and 0.1 g of antimony trioxide and 0.15 g of zinc acetate were further added.
To 220 ° C., and a transesterification reaction was carried out for 3 hours. The pressure was further reduced to 1 mmHg over 1 hour,
It was condensed by reducing the pressure to 0.2 mmHg over time to obtain a copolymer of the polyester resins A to C for the undercoat layer.

The ratio of the cyclic oligomer component having a weight average molecular weight of 1,000 or less of the obtained polyester resin was defined as “Mw
≦ 1000 ”, and the ratio of the cyclic oligomer dimer composed of isophthalic acid and ethylene glycol was expressed as“ IPA / EG dimer ”in weight%. Table 1 shows the glass transition temperature and weight average molecular weight of the obtained polyester resin in terms of Tg (° C.) and Mw. The weight average molecular weight is a value measured by gel permeation chromatography (GPC) based on standard polystyrene.

Measurement Method of Tg The viscoelasticity of the tape was measured at a frequency of 110 Hz using a dynamic viscoelasticity measuring device (Leo Vibron manufactured by Toyo Baldwin). Tg is the peak temperature of E ″. This method applies vibration from one end of the tape and measures the vibration propagating to the other end.

[0075]

[Table 1]

(Preparation of Undercoating Coating Solution) Undercoating polyester resin 20 parts Cyclohexanone 80 parts

(Preparation of Magnetic Coating for Upper Layer) Ferromagnetic metal fine powder 100 parts Composition Fe / Co = 70/30 Hc 189.1 × 10 ³ A / m Specific surface area by BET method 45 m ² / g Crystallite size 170A Surface Treatment agent Al ₂ O ₃ , Y ₂ O ₃ Particle size (major axis diameter) 0.09 μm Needle ratio 8

Vinyl chloride copolymer 10 parts MR-110 polyurethane resin manufactured by Zeon Corporation (Toyobo Co., Ltd.) 6 parts Molecular weight: 40,000, Tg: 38 ° C. Chemical composition mol neopentyl alcohol 2.5 Hydroxycaproic acid 3.1 phthalic acid 2.7 sodium bis (2-hydroxyethyl) sulfoisophthalate 0.1 diphenylmethane diisocyanate 1.0 -SO ₃ Na group 6.5 × 10 ^-5 eq / g α- Al ₂ O ₃ (average particle size 0.15 μm) 5 parts Carbon black (average particle size 0.08 μm) 0.5 part Butyl stearate 1 part Stearic acid 5 parts Methyl ethyl ketone 90 parts Cyclohexanone 30 parts Toluene 60 parts

The above components were kneaded with an open kneader, and then dispersed using a sand mill.
To the resulting dispersion, 5 parts of a polyisocyanate (Coronate L, manufactured by Nippon Polyurethane Industry Co., Ltd.) was added, and 40 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone was added to each, and the mixture was filtered using a filter having an average pore diameter of 1 μm. A coating solution was prepared.

Lower coating layer (non-magnetic) Non-magnetic powder (hematite) 80 parts pH 9.0 Tap density 0.8 DBP oil absorption 27-38 / g / 100 g Surface treatment agent Al ₂ O ₃ , SiO ₂ carbon black 20 parts Average primary particle size 16 nm DBP oil absorption 120 ml / 100 g pH 8.0 Specific surface area by BET method 250 m ² / g Volatile content 1.5%

Polyvinyl chloride copolymer 16 parts MR-104 manufactured by Zeon Corporation 16 parts Polyurethane resin (Toyobo) 6 parts Chemical composition mol neopentyl alcohol 2.5 hydroxycaproic acid 3.1 Phthalic acid 2.7 Sodium bis (2-hydroxyethyl) sulfoisophthalate 0.1 diphenylmethane diisocyanate 1-SO ₃ Na group 6.5 × 10 ^-5 eq / g α-Al ₂ O ₃ (average particle size 0.2 μm) 1 part Ethyl stearate 1 part Stearic acid 1 part Ethyl ethyl ketone 100 parts Cyclohexanone 50 parts Toluene 50 parts

Each of the above-mentioned paints was kneaded with an open kneader, and then dispersed using a sand mill. To the resulting lower dispersion, 5 parts of a polyisocyanate (Coronate L, manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to the lower coating layer, and 40 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone were added to each of the lower layer and an average pore diameter of 1 μm. Then, the mixture was filtered using a filter having the following to prepare a coating solution for forming a lower coating layer and a magnetic layer. An undercoat layer was applied on the support shown in Table 2 so that the applied amount of the solid content was 50 mg / m ² . The lower layer coating solution was applied thereon, and immediately thereafter, the magnetic layer was applied thereon. While both layers were still wet, they were oriented and dried by a cobalt magnet having a magnetic force of 5000 G and a solenoid having a magnetic force of 4000 G, and then a 0.3 μm thick back layer was applied. Table 2 shows the thickness of the magnetic layer and the lower coating layer after drying. Then, at a temperature of 90 ° C. and a speed of 200 minutes per minute using a seven-stage calendar composed of a metal roll and an epoxy resin roll.
Processing was performed at m / min. Thereafter, the sample was slit to a width of 8 mm to prepare a sample.

[0083]

[Table 2]

(Measurement Method) (1) Evaluation of Output Using an 8 mm video deck FUJIX-M860HK, a 7 MHz single frequency sine wave was recorded at the optimum recording current.
The spectrum was measured using a spectrum analyzer manufactured by Shibasoku, and the ratio of the carrier output at 7 MHz was determined. (2) Dropout increase by storage at 50 ° C. and 90% RH for one week Using a 8 mm video deck FUJIX-M860HK, a 7 MHz single frequency sine wave was recorded and reproduced at an optimum recording current. The number of dropouts whose output decreased more than −10 dB for a time of 15 μs or more was examined. Then
After the sample tape was stored in a 60 ° C. 90% RH atmosphere for one week, it was regenerated in the same manner as before storage, and the number of dropouts was examined in the same manner. (3) Observation of precipitates on the surface of the magnetic layer by storage at 60 ° C. and 90% RH for one week The surface of the sample tape was observed with an optical microscope to check for the presence of precipitates. (4) Adhesive force (adhesive force) One adhesive surface of double-sided adhesive tape (SPLICING TAPE manufactured by Sumitomo 3M Ltd.) is adhered on a glass plate, and one end of sample tape 10 mm is adhered to the other adhesive surface. And the load at the time of peeling in the direction of 180 ° was measured and defined as the adhesive force (N). (5) Suitability for long winding It was examined whether 210 m of the tape could be wound on a cassette (hub diameter: 16 mm) for an 8 mm camcorder.

[0085]

As the polyester resin for the undercoat layer is a polyester resin made from a dicarboxylic acid component containing naphthalenedicarboxylic acid, the storage stability can be maintained even when the coating layer is made thin in order to keep the suitability for long winding. A magnetic recording medium with high dropout after storage was provided.

Further, after storage, a magnetic recording medium was obtained in which precipitates on the surface of the magnetic layer were hardly generated and the adhesion between the support and the magnetic layer was excellent.

Claims (1)

[Claims] 1. A magnetic recording medium wherein an undercoat layer is provided on at least one surface of a nonmagnetic support, and at least a magnetic recording layer is provided on the undercoat layer by applying naphthalenedicarboxylic acid as a dicarboxylic acid component to the undercoat layer. A magnetic recording medium coated with a polyester resin produced from raw materials containing the undercoat layer, and the total thickness of the coating layer on the undercoat layer side is 0.01 μm or more and 1 μm or less.