JP2007004937A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium which has outstanding traveling durability and dispersibility of a magnetic substance into a binder, and in addition does not generate poisonous gas like chlorine gas and dioxin by burnup and incineration when discarding. <P>SOLUTION: In a magnetic recording medium which is formed by setting up a magnetism layer including ferromagnetic powder and a binder at least on one face of a non-magnetic support, the magnetic recording medium is characterized in that the magnetic layer contains 3-30 pts.mass of abrasive material of Mohs hardness 8-10 and 0.010-0.3 μm of mean particle diameter to 100 pts.mass of the ferromagnetic powder and the binder contains at least polyvinyl acetal resin, polyurethane resin and poly isocyanate. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a magnetic recording medium, and in particular, has excellent running durability and dispersibility with respect to a binder of a magnetic substance, and is less likely to generate chlorine gas or dioxin by combustion incineration at the time of disposal. The present invention relates to a magnetic recording medium.

  In recent years, means for transmitting terabyte-class information at a high speed have been remarkably developed, and images and data transfer with enormous information have become possible, while advanced techniques for recording, reproducing and storing them have been required. It has come to be. Examples of the recording / reproducing medium include a flexible disk, a magnetic drum, a hard disk, and a magnetic tape. In particular, a magnetic tape has a large recording capacity per volume, and plays a major role in data backup and the like.

  In general, a vinyl chloride resin is used as a binder in a magnetic recording medium such as a magnetic tape or a magnetic disk from the viewpoint of imparting excellent smoothness to the surface. On the other hand, however, it is known that incineration of vinyl chloride resin may cause the release of harmful gases such as chlorine gas and dioxin. Due to the recent increase in environmental conservation awareness, even in the binder of magnetic recording media, Assuming that no vinyl-based resin is used, the movement for exploration has been accelerated.

For example, Patent Document 1 includes a magnetic layer and a non-magnetic support for the purpose of providing a magnetic recording medium that improves electromagnetic conversion characteristics and running durability and does not generate harmful gases during disposal. The binder resin is a mixture of at least one selected from polyvinyl butyral, polyvinyl acetal, and acrylic and polyurethane, and 15 to 60 parts by weight of Mohs hardness 5 to 7 nonmagnetic inorganic powder with respect to 100 parts by weight of magnetic powder A magnetic recording medium containing a body is disclosed.
Patent Document 2 discloses a first magnetic layer containing cobalt-γ-iron oxide on a nonmagnetic support and a plate-shaped hexagonal ferrite magnetic powder for the same purpose as in Patent Document 1. Two magnetic layers are sequentially laminated, a polyurethane resin is used as a binder for the first magnetic layer, and at least one of polyvinyl butyral, polyvinyl acetal, phenoxy, and acrylic is used in addition to polyurethane as the binder for the second magnetic layer. A magnetic recording medium using the above is disclosed.

  However, the above-described conventional technology has inferior running durability and dispersibility of the magnetic substance with respect to the binder as compared with the conventional magnetic recording medium using a vinyl chloride resin as the binder, and there is room for improvement.

JP-A-6-290448 JP-A-7-57244

  Accordingly, an object of the present invention is to provide a magnetic recording medium that has excellent running durability and dispersibility with respect to a binder of a magnetic substance, and is less likely to generate harmful gases such as chlorine gas and dioxin by combustion incineration at the time of disposal. Is to provide.

The present invention is as follows.
1) In a magnetic recording medium in which a magnetic layer containing ferromagnetic powder and a binder is provided on at least one surface of a nonmagnetic support.
The magnetic layer contains 3 to 30 parts by mass of an abrasive having a Mohs hardness of 8 to 10 and an average particle size of 0.010 to 0.3 μm with respect to 100 parts by mass of the ferromagnetic powder.
The magnetic recording medium, wherein the binder contains at least a polyvinyl acetal resin, a polyurethane resin, and a polyisocyanate.
2) The magnetic recording medium as described in 1) above, wherein the polyvinyl acetal resin has a glass transition temperature (Tg) of 60 to 110 ° C.
3) The magnetic recording medium as described in 1) or 2) above, wherein the polyvinyl acetal resin has a bending strength of 700 to 1200 kg / cm ² (68.6 to 117.6 MPa).
4) The magnetic recording medium according to any one of 1) to 3) above, wherein the polyvinyl acetal resin has a tensile strength of 600 to 900 kg / cm ² (58.8 to 88.2 MPa).
5) The magnetism according to any one of 1) to 4) above, wherein a nonmagnetic layer containing a nonmagnetic powder and a binder is provided between the nonmagnetic support and the magnetic layer. recoding media.
6) The magnetic recording medium as described in any one of 1) to 5) above, wherein a back layer is provided on the other surface of the nonmagnetic support.

  According to the present invention, there is provided a magnetic recording medium that has excellent running durability and dispersibility with respect to a binder of a magnetic substance, and is less likely to generate harmful gases such as chlorine gas and dioxin by combustion incineration at the time of disposal. Can be provided.

Hereinafter, the present invention will be described in more detail.
[Non-magnetic support]
Examples of the nonmagnetic support in the present invention include a polyester support, an aromatic polyamide support, and an aromatic polyimide support.
Of these, a polyester support (hereinafter simply referred to as polyester) is preferred. Such a polyester is a polyester composed of a dicarboxylic acid and a diol, such as polyethylene terephthalate and polyethylene naphthalate.
The main constituent dicarboxylic acid components include terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, diphenylsulfone dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenylethanedicarboxylic acid, Examples thereof include cyclohexane dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl thioether dicarboxylic acid, diphenyl ketone dicarboxylic acid, and phenylindane dicarboxylic acid.
Examples of the diol component include ethylene glycol, propylene glycol, tetramethylene glycol, cyclohexanedimethanol, 2,2-bis (4-hydroxyphenyl) propane, 2,2-bis (4-hydroxyethoxyphenyl) propane, bis ( 4-Hydroxyphenyl) sulfone, bisphenol fluorene hydroxyethyl ether, diethylene glycol, neopentyl glycol, hydroquinone, cyclohexanediol and the like.
Among the polyesters having these as main constituents, terephthalic acid and / or 2,6-naphthalenedicarboxylic acid as a dicarboxylic acid component and ethylene glycol as a diol component from the viewpoint of transparency, mechanical strength, dimensional stability, etc. And / or a polyester mainly composed of 1,4-cyclohexanedimethanol is preferred.
Among them, a polyester mainly composed of polyethylene terephthalate or polyethylene-2,6-naphthalate, a copolymer polyester composed of terephthalic acid, 2,6-naphthalenedicarboxylic acid and ethylene glycol, and two or more kinds of these polyesters A polyester having a mixture as a main constituent is preferred. Particularly preferred is a polyester having polyethylene-2,6-naphthalate as a main constituent.
The polyester used in the present invention may be biaxially stretched or a laminate of two or more layers.
The polyester may be further copolymerized with other copolymerization components, or may be mixed with other polyesters. Examples of these include the dicarboxylic acid components and diol components mentioned above, or polyesters composed thereof.
In the polyester used in the present invention, an aromatic dicarboxylic acid having a sulfonate group or an ester-forming derivative thereof, a dicarboxylic acid having a polyoxyalkylene group or an ester-forming derivative thereof, in order to make it difficult for delamination during film formation. A diol having a polyoxyalkylene group may be copolymerized.
Of these, 5-sodium sulfoisophthalic acid, 2-sodium sulfoterephthalic acid, 4-sodium sulfophthalic acid, 4-sodium sulfo-2,6-naphthalenedicarboxylic acid, and the like in terms of polyester polymerization reactivity and film transparency. Compounds in which sodium is substituted with other metals (for example, potassium, lithium, etc.), ammonium salts, phosphonium salts, etc., or ester-forming derivatives thereof, polyethylene glycol, polytetramethylene glycol, polyethylene glycol-polypropylene glycol copolymers and their A compound obtained by oxidizing the hydroxy groups at both ends to form a carboxyl group is preferred. The proportion of copolymerization for this purpose is preferably 0.1 to 10 mol% based on the dicarboxylic acid constituting the polyester.
For the purpose of improving heat resistance, a bisphenol compound, a compound having a naphthalene ring or a cyclohexane ring can be copolymerized. The copolymerization ratio is preferably 1 to 20 mol% based on the dicarboxylic acid constituting the polyester.

In the present invention, the method for synthesizing the polyester is not particularly limited, and can be produced according to a conventionally known polyester production method. For example, a direct esterification method in which a dicarboxylic acid component is directly esterified with a diol component. First, a dialkyl ester is used as a dicarboxylic acid component, this is transesterified with the diol component, and this is heated under reduced pressure. A transesterification method of polymerizing by removing excess diol component can be used. At this time, if necessary, a transesterification catalyst or a polymerization reaction catalyst can be used, or a heat-resistant stabilizer can be added.
In addition, anti-coloring agents, antioxidants, crystal nucleating agents, slipping agents, stabilizers, anti-blocking agents, UV absorbers, viscosity modifiers, antifoaming and clearing agents, antistatic agents, pH adjustments in each process during synthesis One or more of various additives such as an agent, a dye, a pigment, and a reaction terminator may be added.

Further, a filler may be added to the polyester. Examples of the filler include inorganic powders such as spherical silica, colloidal silica, titanium oxide, and alumina, and organic fillers such as crosslinked polystyrene and silicone resin.
Further, in order to increase the rigidity of the support, these materials can be highly stretched, or a metal, semimetal, or oxide layer thereof can be provided on the surface.

In the present invention, the thickness of the polyester as the nonmagnetic support is preferably 3 to 80 μm, more preferably 3 to 50 μm, and particularly preferably 3 to 10 μm. Further, the center line average roughness (Ra) of the support surface is 8 nm or less, more preferably 6 nm or less on the side where the magnetic layer is provided. This Ra was measured with TOPO-3D manufactured by WYKO.
Further, the Young's modulus in the longitudinal direction of the nonmagnetic support is preferably 6.0 GPa or more, and more preferably 7.0 GPa or more.

  The magnetic recording medium of the present invention comprises a magnetic layer containing ferromagnetic powder and a binder on at least one surface of the nonmagnetic support, and is substantially between the nonmagnetic support and the magnetic layer. It is preferable to provide a nonmagnetic layer (lower layer) that is nonmagnetic in nature.

[Magnetic layer]
The ferromagnetic powder contained in the magnetic layer preferably has a volume of (0.1 to 8) × 10 ⁻¹⁸ ml, and more preferably (0.5 to 5) × 10 ⁻¹⁸ ml. By setting it within this range, it is possible to effectively suppress a decrease in magnetic characteristics due to thermal fluctuations, and to obtain good C / N (S / N) while maintaining low noise. The ferromagnetic powder is not particularly limited, but ferromagnetic metal powder or hexagonal ferrite powder is preferable.
The volume of the acicular powder is obtained from the major axis length and minor axis length assuming that the shape is a cylinder.
In the case of plate-like powder, the volume is determined from the plate diameter and axial length (plate thickness) assuming that the shape is a prism (hexagonal prism in the case of hexagonal ferrite powder).
As for the size of the magnetic material, an appropriate amount of the magnetic layer is peeled off. N-Butylamine is added to 30 to 70 mg of the peeled magnetic layer, sealed in a glass tube, set in a thermal decomposition apparatus, and heated at 140 ° C. for about 1 day. After cooling, the contents are taken out from the glass tube and centrifuged to separate the liquid and solids. The separated solid is washed with acetone to obtain a powder sample for TEM. This sample is photographed with a Hitachi transmission electron microscope H-9000, and the particles are photographed at a photographing magnification of 100000 times and printed on a photographic paper to obtain a total magnification of 500,000 times to obtain a particle photograph. A target magnetic substance is selected from the particle photograph and placed on an image analyzer KS-400 digitizer manufactured by kontron, and the size of each particle is measured by tracing the outline of the powder. The size of 500 particles is measured, and the measured values are averaged to obtain an average size.

<Ferromagnetic metal powder>
The ferromagnetic metal powder used for the magnetic layer in the magnetic recording medium of the present invention is not particularly limited as long as it contains Fe as a main component (including an alloy), but is ferromagnetic with α-Fe as a main component. Alloy powder is preferred. These ferromagnetic powders include Al, Si, S, Sc, Ca, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, other than predetermined atoms. It may contain atoms such as W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, and B. It is preferable that at least one of Al, Si, Ca, Y, Ba, La, Nd, Co, Ni, and B is included in addition to α-Fe, and it is particularly preferable that Co, Al, and Y are included. More specifically, it is preferable that Co is contained in an amount of 10 to 40 atomic%, Fe is contained in an amount of 2 to 20 atomic%, and Y is contained in an amount of 0.1 to 15 atomic%.

The ferromagnetic metal powder may be previously treated with a dispersant, a lubricant, a surfactant, an antistatic agent or the like, which will be described later. The ferromagnetic metal powder may contain a small amount of water, hydroxide or oxide. The moisture content of the ferromagnetic metal powder is preferably 0.01-2%. It is preferable to optimize the moisture content of the ferromagnetic metal powder depending on the type of the binder. The pH of the ferromagnetic metal powder is preferably optimized depending on the combination with the binder used. The range is usually from 6 to 12, but preferably from 7 to 11. The ferromagnetic metal powder may contain inorganic ions such as soluble Na, Ca, Fe, Ni, Sr, NH ₄ , SO ₄ , Cl, NO ₂ and NO ₃ . These are preferably essentially absent. If the total of each ion is about 300 ppm or less, the characteristics are not affected. The ferromagnetic metal powder used in the present invention preferably has fewer vacancies, and its value is 20% by volume or less, more preferably 5% by volume or less.

  The average major axis length of the ferromagnetic metal powder is preferably 5 to 200 nm, more preferably 10 to 100 nm, and particularly preferably 30 to 80 nm. The crystallite size of the ferromagnetic metal powder is preferably 8 to 20 nm, more preferably 10 to 18 nm, and particularly preferably 12 to 16 nm. This crystallite size is an average value obtained by a Scherrer method from a half-value width of a diffraction peak using an X-ray diffractometer (RINT2000 series manufactured by Rigaku Corporation) under the conditions of a radiation source CuKα1, a tube voltage 50 kV, and a tube current 300 mA. .

The specific surface area by BET method of the ferromagnetic metal powder _{(S BET)} is preferably at least ^{50 m} 2 / g, more preferably from 55~100m ² / g, and most preferably 60~80m ² / g. Within this range, both good surface properties and low noise can be achieved. The pH of the ferromagnetic metal powder is preferably optimized depending on the combination with the binder used. The range is 4-12, preferably 7-10. The ferromagnetic metal powder may be surface-treated with Al, Si, P, or an oxide thereof as required. The amount thereof is 0.1 to 10% with respect to the ferromagnetic metal powder. When the surface treatment is performed, the adsorption of a lubricant such as a fatty acid is preferably 100 mg / m ² or less. The ferromagnetic metal powder may contain soluble inorganic ions such as Na, Ca, Fe, Ni, and Sr. However, if it is 200 ppm or less, it does not particularly affect the characteristics. Further, the ferromagnetic metal powder used in the present invention preferably has fewer vacancies, and its value is 20% by volume or less, more preferably 5% by volume or less.

The shape of the ferromagnetic metal powder may be needle-like, granular, rice-grained or plate-like as long as the particle volume shown above is satisfied, but it is particularly preferable to use a needle-like ferromagnetic powder. In the case of acicular ferromagnetic metal powder, the acicular ratio is preferably 4-12, more preferably 5-12. The coercive force (Hc) of the ferromagnetic metal powder is preferably 159.2 to 238.8 kA / m (2000 to 3000 Oe), more preferably 167.2 to 230.8 kA / m (2100 to 2900 Oe). . The saturation magnetic flux density is preferably 150 to 300 mT (1500 to 3000 G), and more preferably 160 to 290 mT. The saturation magnetization (σs) is preferably 140 to 170 A · m ² / kg (140 to 170 emu / g), and more preferably 145 to 160 A · m ² / kg. The SFD (switching field distribution) of the magnetic material itself is preferably small and is preferably 0.8 or less. When the SFD is 0.8 or less, the electromagnetic conversion characteristics are good, the output is high, the magnetization reversal is sharp, the peak shift is small, and it is suitable for high-density digital magnetic recording. In order to reduce the Hc distribution, there are methods such as improving the goethite particle size distribution in the ferromagnetic metal powder, using monodispersed αFe ₂ O ₃ , and preventing sintering between particles.

  As the ferromagnetic metal powder, those obtained by a known production method can be used, and the following methods can be mentioned. Reduction of hydrous iron oxide and iron oxide with anti-sintering treatment using iron or other reducing gas to obtain Fe or Fe-Co particles, reduction of complex organic acid salt (mainly oxalate) and hydrogen A method of reducing with a reactive gas, a method of thermally decomposing a metal carbonyl compound, a method of reducing a ferromagnetic metal aqueous solution by adding a reducing agent such as sodium borohydride, hypophosphite or hydrazine, a metal at a low pressure For example, the powder is obtained by evaporating in an inert gas. The ferromagnetic metal powder thus obtained is subjected to a known slow oxidation treatment. A method of reducing the amount of demagnetization by reducing the hydrous iron oxide and iron oxide with a reducing gas such as hydrogen and controlling the partial pressure, temperature and time of the oxygen-containing gas and inert gas to form an oxide film on the surface. preferable.

<Hexagonal ferrite powder>
Hexagonal ferrite powders include, for example, barium ferrite, strontium ferrite, lead ferrite, calcium ferrite, and their substitutes such as Co. More specifically, magnetoplumbite type barium ferrite and strontium ferrite, magnetoplumbite type ferrite whose particle surface is coated with spinel, and magnetoplumbite type barium ferrite and strontium ferrite partially containing a spinel phase, etc. Is mentioned. In addition to predetermined atoms, Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg , Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge, and Nb may be included. In general, elements such as Co—Zn, Co—Ti, Co—Ti—Zr, Co—Ti—Zn, Ni—Ti—Zn, Nb—Zn—Co, Sb—Zn—Co, and Nb—Zn are added. You can use things. Some raw materials and production methods contain specific impurities. Other preferable atoms and the content thereof are the same as those of the ferromagnetic metal powder.

The particle size of the hexagonal ferrite powder is preferably a size that satisfies the above volume, but the average plate diameter is 10 to 100 nm, preferably 12 to 50 nm, and more preferably 20 to 40 nm.
The average plate ratio {average of (plate diameter / plate thickness)} is 1-15, and preferably 1-7. If the average plate ratio is 1 to 15, sufficient orientation can be obtained while maintaining high filling properties in the magnetic layer, and noise increase can be suppressed by stacking between particles. Further, the specific surface area (S _BET ) by the BET method within the above particle size range is preferably 40 m ² / g or more, more preferably 40 to 200 m ² / g, and 60 to 100 m ² / g. Is most preferred.

  The distribution of the particle plate diameter and plate thickness of the hexagonal ferrite powder is generally preferably as narrow as possible. Quantifying the particle plate diameter and plate thickness can be compared by randomly measuring 500 particles from a particle TEM photograph. In many cases, the distribution of the particle plate diameter and plate thickness is not a normal distribution, but when calculated and expressed as a standard deviation with respect to the average size, σ / average size = 0.1 to 2.0. In order to sharpen the particle size distribution, the particle generation reaction system is made as uniform as possible, and the generated particles are subjected to a distribution improvement process. For example, a method of selectively dissolving ultrafine particles in an acid solution is also known.

  The coercive force (Hc) of the hexagonal ferrite powder can be in the range of 159.2 to 238.8 kA / m (2000 to 3000 Oe), preferably 175.1 to 222.9 kA / m (2200 to 2800 Oe). More preferably, it is 183.1-214.9 kA / m (2300-2700 Oe). However, when the saturation magnetization (σs) of the head exceeds 1.4T, it is preferably 159.2 kA / m or less. The coercive force (Hc) can be controlled by the particle size (plate diameter / plate thickness), the type and amount of the contained element, the substitution site of the element, the particle generation reaction conditions, and the like.

The saturation magnetization (σs) of the hexagonal ferrite powder is 40 to 80 A · m ² / kg (emu / g). Higher saturation magnetization (σs) is preferable, but tends to be smaller as the particles become finer. In order to improve the saturation magnetization (σs), it is well known to combine spinel ferrite with magnetoplumbite ferrite, and to select the type and amount of contained elements. It is also possible to use W-type hexagonal ferrite. When the magnetic material is dispersed, the surface of the magnetic material particles is also treated with a material suitable for the dispersion medium and the polymer. As the surface treatment agent, inorganic compounds and organic compounds are used. Typical examples of the main compound include oxides or hydroxides such as Si, Al, and P, various silane coupling agents, and various titanium coupling agents. The addition amount is 0.1 to 10% by mass with respect to the mass of the magnetic substance. The pH of the magnetic material is also important for dispersion. Usually, about 4 to 12 has optimum values depending on the dispersion medium and polymer, but about 6 to 11 is selected from the chemical stability and storage stability of the medium. Water contained in the magnetic material also affects the dispersion. Although there is an optimum value depending on the dispersion medium and the polymer, 0.01 to 2.0% is usually selected.

The hexagonal ferrite powder is manufactured by mixing (1) barium oxide / iron oxide / metal oxide replacing iron and boron oxide as a glass forming substance so as to have a desired ferrite composition, and then melting and quenching. After being reheated, washed and ground to obtain barium ferrite crystal powder, glass crystallization method, (2) neutralizing barium ferrite composition metal salt solution with alkali, by-product Hydrothermal reaction method to obtain barium ferrite crystal powder by liquid phase heating at 100 ° C. or higher after removal of water, (3) neutralization of barium ferrite composition metal salt solution with alkali, by-product There is a coprecipitation method in which the product is removed, dried, treated at 1100 ° C. or lower, and pulverized to obtain a barium ferrite crystal powder, but the present invention does not select a production method. The hexagonal ferrite powder may be subjected to surface treatment with Al, Si, P, or an oxide thereof as required. The amount thereof is 0.1 to 10% with respect to the ferromagnetic powder, and the surface treatment is preferable because the adsorption of a lubricant such as a fatty acid is 100 mg / m ² or less. The ferromagnetic powder may contain inorganic ions such as soluble Na, Ca, Fe, Ni, and Sr. Although it is preferable that these are essentially not present, if they are 200 ppm or less, they do not particularly affect the characteristics.

<Binder>
The binder (binder) used in the magnetic layer of the present invention is characterized by containing at least a polyvinyl acetal resin, a polyurethane resin and a polyisocyanate.

  As is well known, the polyvinyl acetal resin is a polymer obtained by reacting polyvinyl alcohol with an aldehyde to acetalize part or most of the hydroxyl group, and has the following structural units.

  In the above formula, R represents an alkyl group, preferably R is a butyl group. n represents the degree of polymerization.

  In the production of polyvinyl acetal resin, not all hydroxyl groups of polyvinyl alcohol can be acetalized. For example, in the case of polyvinyl butyral, acetyl groups and hydroxyl groups remain. By controlling the ratio of the acetyl group and the hydroxyl group, desired physical and chemical characteristics can be obtained. The acetyl group remaining in such a polyvinyl acetal resin is 0.1 to 10 mol%, preferably 0.5 to 5 mol%, more preferably 1 to 3 mol%. A hydroxyl group is 15-50 mol%, Preferably it is 20-40 mol%, More preferably, it is 20-38 mol%.

A polyvinyl acetal resin having the following characteristics is preferable because the running durability of the obtained magnetic recording medium and the dispersibility of the magnetic substance are further improved.
(1) Glass transition temperature (Tg) is 60-110 degreeC, Preferably it is 60-90 degreeC, More preferably, it is 60-75 degreeC.
(2) The bending strength is 700 to 1200 kg / cm ² (68.6 to 117.6 MPa).
(3) tensile strength 600~1200kg ^/ cm 2 (58.8~117.6MPa), preferably 650~1000kg ^/ cm 2 (63.7~98.0MPa), more preferably 700~800kg ^/ cm 2 (68 .6 to 78.4 MPa).
The Tg is a value measured by DSC measurement according to JIS K7203 for bending strength and JIS K7113 for tensile strength.
The degree of polymerization n is 100 to 1000, preferably 150 to 800, and more preferably 200 to 500.

  What is marketed can be utilized for the polyvinyl acetal resin in this invention, for example, Sekisui Chemical Co., Ltd. make, brand name S REC B series, and K series are mentioned.

  The polyurethane resin used in the present invention is not particularly limited, but the following resins are preferable. That is, a cyclic structure such as hydrogenated bisphenol A and a polypropylene oxide adduct of hydrogenated bisphenol A, a polyol having an alkylene oxide chain having a molecular weight of 500 to 5000, and a polyol having a cyclic structure as a chain extender and having a molecular weight of 200 to 500 And an organic diisocyanate and a polar resin introduced polyurethane resin, or an aliphatic dibasic acid such as succinic acid, adipic acid or sebacic acid, and 2,2-dimethyl-1,3-propanediol, A polyester polyol composed of an aliphatic diol having no cyclic structure having an alkyl branched side chain, such as ethyl-2-butyl-1,3-propanediol and 2,2-diethyl-1,3-propanediol; 2-ethyl-2-butyl-1,3-propanediol as an agent A polyurethane resin or dimer in which an aliphatic diol having a branched alkyl side chain having 3 or more carbon atoms such as 2,2-diethyl-1,3-propanediol is reacted with an organic diisocyanate compound and a polar group is introduced It is preferable to use a polyurethane resin in which a polar structure such as a diol, a polyol compound having a long alkyl chain, and an organic diisocyanate are reacted and a polar group is introduced.

  The average molecular weight of the polar group-containing polyurethane resin used in the present invention is preferably 5,000 to 100,000, and more preferably 10,000 to 50,000. If the average molecular weight is 5,000 or more, it is preferable because the obtained magnetic coating film is not brittle and the physical strength is not lowered, and the durability of the magnetic recording medium is not affected. Further, when the molecular weight is 100,000 or less, the solubility in the solvent does not decrease, and the dispersibility is also good. Further, since the viscosity of the paint at a predetermined concentration does not increase, the workability is good and the handling is easy.

Examples of the polar group contained in the polyurethane resin, for _{example, -COOM, -SO 3 M, -OSO} 3 M, -P = O (OM) 2, -O-P = O (OM) per ₂ (or more, M is a hydrogen atom or an alkali metal base), —OH, —NR ₂ , —N ⁺ R ₃ (R is a hydrocarbon group), an epoxy group, —SH, —CN, etc., and at least one of these polar groups One in which two or more are introduced by copolymerization or addition reaction can be used. Moreover, when this polar group-containing polyurethane-based resin has an OH group, it preferably has a branched OH group from the viewpoint of curability and durability, and preferably has 2 to 40 branched OH groups per molecule. It is more preferable to have 3 to 20 molecules per molecule. The amount of such polar group is from ¹⁰ -1 ^{to 10 -8} mol / g, preferably ¹⁰ -2 ^{to 10 -6} mol / g.

  Examples of the polyisocyanate used in the present invention include tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, tri Examples thereof include isocyanates such as phenylmethane triisocyanate, products of these isocyanates and polyalcohols, and polyisocyanates generated by condensation of isocyanates. Commercially available product names of these isocyanates include: Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR Millionate MTL, Takeda Pharmaceutical Taketake D-102, Takenate D-110N, Takenate D-200, Takenate D-202, Sumika Bayer's Death Module L, Death Module IL, Death Module N, Death Module HL, etc., these may be used alone or two or more using the difference in curing reactivity It can be used in combination of the above.

  In the binder, the polyvinyl acetal resin, the polyurethane resin and the polyisocyanate are used in an amount of 1 to 40% by mass, preferably 3 to 25% by mass, more preferably 5 to 20% by mass, and 40 to 40% polyurethane resin. 90% by weight, preferably 40-85% by weight, more preferably 50-80% by weight, and polyisocyanate 3-40% by weight, preferably 5-30% by weight, more preferably 5-20% by weight. .

  In addition, according to the present invention, binders other than those described above can be used as required, but from the gist of the present invention, it is desirable that vinyl chloride resin is not used or is used in a very small amount. Other binders that can be used in combination include conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof. As the thermoplastic resin, for example, a polymer or copolymer containing vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic ester, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl ether or the like as a structural unit. Examples thereof include polymers and various rubber resins. Examples of the thermosetting resin or reactive resin include phenol resin, epoxy resin, urea resin, melamine resin, alkyd resin, acrylic reaction resin, formaldehyde resin, silicone resin, and epoxy-polyamide resin. it can. The thermoplastic resin, thermosetting resin and reactive resin are all described in detail in “Plastic Handbook” issued by Asakura Shoten. Further, an electron beam curable resin may be used for the magnetic layer. Examples of electron beam curable resins and methods for producing the same are described in detail in JP-A No. 62-256219.

  Specific examples of binders that can be used in combination (including vinyl chloride resin) include, for example, VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, manufactured by Union Carbide Corporation. PKHJ, PKHC, PKFE, Nissin Chemical Industry MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, MPR-TAO, Denki Kagaku 1000W, DX80, DX81, DX82, DX83, 100FD, Nippon Zeon's MR-104, MR-105, MR110, MR100, MR555, 400X-110A, Nippon Polyurethane Nipporan N2301, N2302, N2304, Dainippon Ink's Pandex T-5105, TR 080, T-5201, Barnock D-400, D-210-80, Crisbon 6109, 7209, Byron UR8200, UR8300, UR-8700, RV530, RV280, Toyobo Co., Ltd. Daiferamin 4020, 5020, 5100, manufactured by Dainichi Seika 5300, 9020, 9022, 7020, MX5004 manufactured by Mitsubishi Chemical Corporation, Samprene SP-150 manufactured by Sanyo Chemical Co., Ltd., Saran F310, F210 manufactured by Asahi Kasei Corporation, and the like.

  The amount of the binder used in the magnetic layer of the present invention is in the range of 5 to 50% by mass, preferably in the range of 10 to 30% by mass with respect to the mass of the ferromagnetic metal powder.

  When the magnetic recording medium used in the present invention is, for example, a flexible disk, it can be composed of two or more layers on both sides of a nonmagnetic support. Therefore, the amount of binder, the amount of resin in the binder, the molecular weight of each resin forming the magnetic layer, the amount of polar groups, or the physical properties of the resin described above, etc., can be determined as necessary. Of course, it is possible to change in each layer. Rather, it should be optimized in each layer, and a known technique relating to a multilayer magnetic layer can be applied. For example, when changing the amount of binder in each layer, it is effective to increase the amount of binder in the magnetic layer in order to reduce scratches on the surface of the magnetic layer. The amount of binder in the magnetic layer can be increased to provide flexibility.

Additives can be added to the magnetic layer in the present invention as necessary. Examples of the additive include an abrasive, a lubricant, a dispersant / dispersion aid, an antifungal agent, an antistatic agent, an antioxidant, a solvent, and carbon black. Examples of these additives include molybdenum disulfide, tungsten disulfide, graphite, boron nitride, fluorinated graphite, silicone oil, silicone having a polar group, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing ester, Polyolefin, polyglycol, polyphenyl ether, phenylphosphonic acid, benzylphosphonic acid, phenethylphosphonic acid, α-methylbenzylphosphonic acid, 1-methyl-1-phenethylphosphonic acid, diphenylmethylphosphonic acid, biphenylphosphonic acid, benzylphenylphosphonic acid , Α-cumylphosphonic acid, toluylphosphonic acid, xylylphosphonic acid, ethylphenylphosphonic acid, cumenylphosphonic acid, propylphenylphosphonic acid, butylphenylphosphonic acid, heptylph Aromatic ring-containing organic phosphonic acids such as enylphosphonic acid, octylphenylphosphonic acid, nonylphenylphosphonic acid and their alkali metal salts, octylphosphonic acid, 2-ethylhexylphosphonic acid, isooctylphosphonic acid, isononylphosphonic acid, isodecylphosphonic acid Acids, isoundecyl phosphonic acid, isododecyl phosphonic acid, isohexadecyl phosphonic acid, isooctadecyl phosphonic acid, isoeicosyl phosphonic acid, alkylphosphonic acid and alkali metal salts thereof, phenyl phosphate, benzyl phosphate, phosphoric acid Phenethyl, α-methylbenzyl phosphate, 1-methyl-1-phenethyl phosphate, diphenylmethyl phosphate, biphenyl phosphate, benzylphenyl phosphate, α-cumyl phosphate, toluyl phosphate, xylyl phosphate, ethyl phosphate Phenyl, Li Aromatic phosphates such as cumenyl phosphate, propylphenyl phosphate, butylphenyl phosphate, heptylphenyl phosphate, octylphenyl phosphate, nonylphenyl phosphate, and alkali metal salts thereof, octyl phosphate, 2-ethylhexyl phosphate , Alkyl phosphates such as isooctyl phosphate, isononyl phosphate, isodecyl phosphate, isoundecyl phosphate, isododecyl phosphate, isohexadecyl phosphate, isooctadecyl phosphate, isoeicosyl phosphate, and alkali metal salts thereof, alkylsulfonic acid Esters and alkali metal salts thereof, fluorine-containing alkyl sulfates and alkali metal salts thereof, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, butyl stearate, oleic acid, linoleic acid, linolenic acid, Eleiji Monobasic fatty acids that may contain or be branched, such as acids and erucic acids, and their metal salts, or butyl stearate, octyl stearate, amyl stearate, isooctyl stearate Monobasic, which may contain or be branched, containing an unsaturated bond having 10 to 24 carbon atoms, such as octyl myristate, butyl laurate, butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan tristearate Fatty acid and 1-6 hexahydric alcohol which may contain or be branched including an unsaturated bond having 2 to 22 carbon atoms, alkoxy alcohol or alkylene oxide which may contain or be branched an unsaturated bond having 12 to 22 carbon atoms Mono-fatty acid ester comprising any one of polymer monoalkyl ethers Di-fatty acid esters or polyvalent fatty acid esters, fatty acid amides having 2 to 22 carbon atoms, and aliphatic amines having 8 to 22 carbon atoms can be used. In addition to the above hydrocarbon groups, nitro groups and alkyl groups, aryl groups, and aralkyl groups in which groups other than hydrocarbon groups such as halogen-containing hydrocarbons such as F, Cl, Br, CF ₃ , CCl ₃ , and CBr ₃ are substituted. You may have something.
In the present invention, it is necessary to contain an abrasive that satisfies the following conditions in the magnetic layer.
(1) The Mohs hardness of the abrasive is 8 to 10.
(2) The average particle size of the abrasive is 0.010 to 0.3 μm.
(3) The addition amount of the abrasive is 3 to 30 parts by mass with respect to 100 parts by mass of the ferromagnetic powder.
Examples of the abrasive having a Mohs hardness of 8 to 10 include α-alumina, chromium oxide, SiC, ZrO ₂ , Si ₃ N ₄ , B ₄ C, cBN, diamond, and AlN. The average particle size of the abrasive is more preferably from 0.01 to 0.15 μm, particularly preferably from 0.02 to 0.15 μm. The addition amount of the abrasive is more preferably 5 to 30 parts by mass and particularly preferably 5 to 20 parts by mass with respect to 100 parts by mass of the ferromagnetic powder.
By satisfying the above conditions, the present invention reduces the concern about the generation of harmful gases related to chlorine, even if these catalytic activities are manifested in the presence of metal powders and fine particle abrasives. The effect of giving can be produced.

  In addition, nonionic surfactants such as alkylene oxide, glycerin, glycidol, and alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocyclics, phosphonium or sulfoniums, etc. Amphoteric interfaces such as cationic surfactants, anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, and sulfate ester groups, amino acids, aminosulfonic acids, sulfuric or phosphate esters of aminoalcohols, and alkylbetaines An activator or the like can also be used. These surfactants are described in detail in “Surfactant Handbook” (published by Sangyo Tosho Co., Ltd.).

  The lubricants, antistatic agents, and the like are not necessarily pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposition products, and oxides in addition to the main components. These impurities are preferably 30% by mass or less, more preferably 10% by mass or less.

  Specific examples of these additives include, for example, NAF-102, castor oil hardened fatty acid, NAA-42, cation SA, Naimine L-201, Nonion E-208, Anon BF, Anon LG, Takemoto, manufactured by NOF Corporation. Oil and fat: FAL-205, FAL-123, Shin Nippon Chemical Co., Ltd .: NJ Lube OL, Shin-Etsu Chemical Co., Ltd .: TA-3, Lion Corporation: Armido P, Lion Corporation: Duomin TDO, Nisshin Oilio Co., Ltd .: BA-41G, Sanyo Kasei Co., Ltd .: Profan 2012E, New Pole PE61, Ionette MS-400, etc. are mentioned.

In addition, carbon black can be added to the magnetic layer in the present invention as necessary. Examples of carbon black that can be used in the magnetic layer include rubber furnace, rubber thermal, color black, and acetylene black. Specific surface area is 5 to 500 m ² / g, DBP oil absorption is 10 to 400 ml / 100 g, particle diameter is 5 to 300 nm, pH is 2 to 10, water content is 0.1 to 10%, tap density is 0.1 to 1 g / ml is preferred.

  Specific examples of carbon black used in the present invention include Cabot's BLACKPEARLS 2000, 1300, 1000, 900, 905, 800, 700, VULCAN XC-72, Asahi Carbon Co., Ltd. # 80, # 60, # 55. # 50, # 35, Mitsubishi Chemical Corporation # 2400B, # 2300, # 900, # 1000, # 30, # 40, # 10B, Columbian Carbon Corporation CONDUCTEX SC, RAVEN150, 50, 40, 15, RAVEN- MT-P, Ketjen Black International Ketjen Black EC, etc. are mentioned. Carbon black may be surface-treated with a dispersant, or may be used after being grafted with a resin, or may be obtained by graphitizing a part of the surface. Carbon black may be dispersed with a binder in advance before being added to the magnetic coating. These carbon blacks can be used alone or in combination. When using carbon black, it is preferable to use 0.1-30 mass% with respect to the mass of a magnetic body. Carbon black functions to prevent the magnetic layer from being charged, reduce the friction coefficient, impart light-shielding properties, and improve the film strength. These differ depending on the carbon black used. Therefore, these carbon blacks used in the present invention have different types, amounts, and combinations in the magnetic layer and the nonmagnetic layer, and are based on the above-mentioned characteristics such as particle size, oil absorption, conductivity, pH, etc. Of course, it is possible to use them properly according to the purpose, but rather they should be optimized in each layer. For the carbon black that can be used in the magnetic layer of the present invention, for example, “Carbon Black Handbook” edited by Carbon Black Association can be referred to.

  Known organic solvents can be used in the present invention. The organic solvent used in the present invention is an arbitrary ratio of ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, tetrahydrofuran, methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, methyl Alcohols such as cyclohexanol, esters such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycol acetate, glycol ethers such as glycol dimethyl ether, glycol monoethyl ether, dioxane, benzene, toluene, xylene, Aromatic hydrocarbons such as cresol and chlorobenzene, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethyl Nkuroruhidorin, chlorinated hydrocarbons such as dichlorobenzene, N, N- dimethylformamide, may be used hexane.

  These organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposition products, oxides, and moisture in addition to the main components. These impurities are preferably 30% or less, more preferably 10% or less. The organic solvent used in the present invention is preferably the same in the magnetic layer and the nonmagnetic layer. The amount added may be changed. Use a solvent with high surface tension (such as cyclohexanone or dioxane) in the nonmagnetic layer to increase the coating stability. Specifically, the arithmetic average value of the upper layer solvent composition may not fall below the arithmetic average value of the nonmagnetic layer solvent composition. It is essential. In order to improve dispersibility, it is preferable that the polarity is somewhat strong, and it is preferable that 50% or more of a solvent having a dielectric constant of 15 or more is included in the solvent composition. Moreover, it is preferable that a solubility parameter is 8-11.

  These dispersants, lubricants, and surfactants used in the present invention can be properly used in the magnetic layer and further in the nonmagnetic layer described later as needed. For example, of course, the dispersant is not limited to the example shown here, but the dispersant has a property of adsorbing or binding with a polar group, and in the magnetic layer, mainly on the surface of the ferromagnetic metal powder and nonmagnetic. In the layer, it is presumed that the organic phosphorus compound adsorbed or bonded mainly to the surface of the non-magnetic powder with the above-mentioned polar group, for example, is difficult to desorb from the surface of metal or metal compound. Accordingly, the surface of the ferromagnetic metal powder or the nonmagnetic powder of the present invention is in a state of being coated with an alkyl group, an aromatic group, etc., so that the binder resin component of the ferromagnetic metal powder or nonmagnetic powder is used. The affinity is improved and the dispersion stability of the ferromagnetic metal powder or nonmagnetic powder is also improved. In addition, since the lubricant exists in a free state, fatty acids with different melting points are used in the nonmagnetic layer and magnetic layer to control the bleeding to the surface, and leaching to the surface using esters with different boiling points and polarities. It is conceivable to improve the coating stability by controlling the amount of the surfactant, to improve the lubrication effect by increasing the additive amount of the lubricant in the nonmagnetic layer. All or part of the additives used in the present invention may be added in any step during the production of the coating solution for the magnetic layer or nonmagnetic layer. For example, when mixing with a ferromagnetic powder before the kneading step, when adding at a kneading step with a ferromagnetic powder, a binder and a solvent, when adding at a dispersing step, when adding after dispersing, when adding just before coating, etc. There is.

[Nonmagnetic layer]
Next, detailed contents regarding the nonmagnetic layer will be described. The magnetic recording medium of the present invention can have a nonmagnetic layer containing a nonmagnetic powder and a binder on a nonmagnetic support. The nonmagnetic powder that can be used in the nonmagnetic layer may be an inorganic substance or an organic substance. Carbon black or the like can also be used. Examples of the inorganic substance include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides.

Specifically, titanium oxide such as titanium dioxide, cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO ₂ , SiO ₂ , Cr ₂ O ₃ , α-alumina, β-alumina having an α conversion of 90 to 100%, γ-alumina, α-iron oxide, goethite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide, copper oxide, MgCO ₃ , CaCO ₃ , BaCO ₃ , SrCO ₃ , BaSO ₄ , silicon carbide Titanium carbide or the like is used alone or in combination of two or more. Preferable are α-iron oxide and titanium oxide.

  The shape of the nonmagnetic powder may be any of acicular, spherical, polyhedral and plate shapes. The crystallite size of the nonmagnetic powder is preferably 4 nm to 1 μm, and more preferably 40 to 100 nm. A crystallite size in the range of 4 nm to 1 μm is preferred because it does not become difficult to disperse and has a suitable surface roughness. The average particle size of these non-magnetic powders is preferably 5 nm 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. It can also have an effect. The average particle size of the particularly preferred nonmagnetic powder is 10 to 200 nm. The range of 5 nm to 2 μm is preferable because the dispersion is good and the surface roughness is suitable.

The specific surface area of the nonmagnetic powder is 1 to 100 m ² / g, preferably 5 to 70 m ² / g, and more preferably 10 to 65 m ² / g. A specific surface area in the range of 1 to 100 m ² / g is preferred because it has a suitable surface roughness and can be dispersed with a desired amount of binder. The oil absorption using dibutyl phthalate (DBP) is 5 to 100 ml / 100 g, preferably 10 to 80 ml / 100 g, and more preferably 20 to 60 ml / 100 g. The specific gravity is 1 to 12, preferably 3 to 6. The tap density is 0.05 to 2 g / ml, preferably 0.2 to 1.5 g / ml. When the tap density is in the range of 0.05 to 2 g / ml, there are few particles to be scattered, the operation is easy, and there is a tendency that it is difficult to adhere to the apparatus. The pH of the nonmagnetic powder is preferably 2 to 11, but the pH is particularly preferably between 6 and 9. When the pH is in the range of 2 to 11, the friction coefficient does not increase due to high temperature, high humidity, or liberation of fatty acids. The moisture content of the nonmagnetic powder is 0.1 to 5% by mass, preferably 0.2 to 3% by mass, and more preferably 0.3 to 1.5% by mass. A water content in the range of 0.1 to 5% by mass is preferable because the dispersion is good and the viscosity of the paint after dispersion is stable. The ignition loss is preferably 20% by mass or less, and the ignition loss is preferably small.

When the nonmagnetic powder is an inorganic powder, the Mohs hardness is preferably 4-10. If the Mohs hardness is in the range of 4 to 10, durability can be ensured. The nonmagnetic powder has a stearic acid adsorption amount of 1 to 20 μmol / m ² , more preferably ² to 15 μmol / m ² . The heat of wetting of the nonmagnetic powder into water at 25 ° C. is preferably in the range of 200 to 600 erg / cm ² (200 to 600 mJ / m ² ). Moreover, the solvent which exists in the range of this heat of wetting can be used. The amount of water molecules on the surface at 100 to 400 ° C. is suitably 1 to 10 / 100Å. The pH of the isoelectric point in water is preferably between 3 and 9. It is preferable that Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , and ZnO are present on the surface of these nonmagnetic powders by surface treatment. Particularly preferred for dispersibility are Al ₂ O ₃ , SiO ₂ , TiO ₂ , and ZrO ₂ , but more preferred are Al ₂ O ₃ , SiO ₂ , and ZrO ₂ . These may be used in combination or may be used alone. Further, a surface-treated layer co-precipitated according to the purpose may be used, or a method of treating the surface layer with silica after first treating with alumina, or vice versa may be employed. The surface treatment layer may be a porous layer depending on the purpose, but it is generally preferable that the surface treatment layer is homogeneous and dense.

  Specific examples of the non-magnetic powder used in the non-magnetic layer of the present invention include, for example, Showa Denko Nanotite, Sumitomo Chemical HIT-100, ZA-G1, Toda Kogyo DPN-250, DPN-250BX, DPN-245, DPN-270BX, DPB-550BX, DPN-550RX, Ishihara Sangyo Titanium oxide TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100, MJ -7, α-iron oxide E270, E271, E300, STT-4D, STT-30D, STT-30, STT-65C manufactured by Titanium Industry, MT-100S, MT-100T, MT-150W, MT-500B manufactured by Teika T-600B, T-100F, T-500HD, etc. are mentioned. FINEX-25, BF-1, BF-10, BF-20, ST-M, Dowa Mining DEFIC-Y, DEFIC-R, Nippon Aerosil AS2BM, TiO2P25, Ube Industries 100A, 500A, Titanium Industry Y-LOP manufactured and what baked it are mentioned. Particularly preferred nonmagnetic powders are titanium dioxide and α-iron oxide.

Carbon black can be mixed in the nonmagnetic layer together with nonmagnetic powder to lower the surface electrical resistance, reduce the light transmittance, and obtain a desired micro Vickers hardness. The micro-Vickers hardness of the nonmagnetic layer is generally 25~60kg ^/ mm 2 (245~588MPa), preferably in order to adjust the head ^contact, a 30~50kg / mm 2 (294~490MPa), thin film hardness meter ( Using a HMA-400 manufactured by NEC, measurement can be performed using a diamond triangular pyramid needle having a ridge angle of 80 degrees and a tip radius of 0.1 μm at the tip of the indenter. It is standardized that the light transmittance is generally 3% or less for absorption of infrared rays having a wavelength of about 900 nm, for example, 0.8% or less for a VHS magnetic tape. For this purpose, rubber furnace, rubber thermal, color black, acetylene black and the like can be used.

Non specific surface area of the carbon black used in the magnetic layer is 100 to 500 m ² / g, preferably 150~400m ² / g, DBP oil absorption of the present invention are 20 to 400 ml / 100 g, preferably 30 to 200 ml / 100 g . The particle size of carbon black is 5 to 80 nm, preferably 10 to 50 nm, and more preferably 10 to 40 nm. Carbon black preferably has a pH of 2 to 10, a water content of 0.1 to 10%, and a tap density of 0.1 to 1 g / ml.

  Specific examples of carbon black that can be used in the non-magnetic layer of the present invention include BLACKPEARLS 2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72, manufactured by Cabot Corporation, # 3050B, manufactured by Cabot Corporation. # 3150B, # 3250B, # 3750B, # 3950B, # 950B, # 650B, # 970B, # 850B, MA-600, CONDUCTEX SC, Raven8800, 8000, 7000, 5750, 5250, 3500, 2100, Columbia Carbon Co., Ltd. 2000, 1800, 1500, 1255, 1250, and Ketjen Black EC manufactured by Ketjen Black International.

  Carbon black may be surface-treated with a dispersant, or may be grafted with a resin, or may be obtained by graphitizing a part of the surface. Moreover, before adding carbon black to a coating material, you may disperse | distribute with a binder beforehand. These carbon blacks can be used in a range not exceeding 50% by mass with respect to the inorganic powder and in a range not exceeding 40% of the total mass of the nonmagnetic layer. These carbon blacks can be used alone or in combination. The carbon black that can be used in the nonmagnetic layer of the present invention can be referred to, for example, “Carbon Black Handbook” edited by Carbon Black Association.

  Further, an organic powder can be added to the nonmagnetic layer according to the purpose. Examples of such organic powder include acrylic styrene resin powder, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment, but polyolefin resin powder, polyester resin powder, polyamide resin powder, polyimide resin, and the like. Resin powder and polyfluorinated ethylene resin can also be used. As the production method, those described in JP-A Nos. 62-18564 and 60-255827 can be used.

  As the binder resin, lubricant, dispersant, additive, solvent, dispersion method and the like of the nonmagnetic layer, those of the magnetic layer can be applied. In particular, with respect to the binder resin amount, type, additive, and dispersant addition amount and type, known techniques relating to the magnetic layer can be applied.

  The magnetic recording medium of the present invention may be provided with an undercoat layer. By providing the undercoat layer, the adhesive force between the support and the magnetic layer or the nonmagnetic layer can be improved. As the undercoat layer, a solvent-soluble polyester resin is used.

[Layer structure]
As for the thickness structure of the magnetic recording medium used in the present invention, the thickness of the non-magnetic support is 3 to 80 μm, more preferably 3 to 50 μm, and particularly preferably 3 to 10 μm as described above. When an undercoat layer is provided between the nonmagnetic support and the nonmagnetic layer or magnetic layer, the thickness of the undercoat layer is 0.01 to 0.8 μm, preferably 0.02 to 0.6 μm.

  The thickness of the magnetic layer is optimized according to the saturation magnetization amount, head gap length, and recording signal band of the magnetic head to be used, but is generally 10 to 150 nm, preferably 20 to 120 nm, and more preferably. Is 30 to 100 nm, particularly preferably 30 to 80 nm. Further, the thickness variation rate of the magnetic layer is preferably within ± 50%, and more preferably within ± 40%. There may be at least one magnetic layer, and the magnetic layer may be separated into two or more layers having different magnetic characteristics, and a configuration related to a known multilayer magnetic layer can be applied.

  The thickness of the nonmagnetic layer of the present invention is 0.5 to 2.0 μm, preferably 0.8 to 1.5 μm, and more preferably 0.8 to 1.2 μm. The non-magnetic layer of the magnetic recording medium of the present invention exhibits its effect if it is substantially non-magnetic. For example, even if it contains a small amount of magnetic material as an impurity or intentionally, This shows the effect of the invention and can be regarded as substantially the same configuration as the magnetic recording medium of the invention. Note that “substantially the same” means that the residual magnetic flux density of the nonmagnetic layer is 10 mT or less or the coercive force is 7.96 kA / m (100 Oe) or less, and preferably has no residual magnetic flux density and coercive force. Means.

[Back layer]
In the magnetic recording medium of the present invention, a back layer is preferably provided on the other surface of the nonmagnetic support. The back layer preferably contains carbon black and inorganic powder. For the binder and various additives, the formulation of the magnetic layer and the nonmagnetic layer is applied. The thickness of the back layer is preferably 0.9 μm or less, and more preferably 0.1 to 0.7 μm.

[Production method]
The step of producing the magnetic layer coating liquid for the magnetic recording medium used in the present invention comprises at least a kneading step, a dispersing step, and a mixing step provided before and after these steps. Each process may be divided into two or more stages. All raw materials such as ferromagnetic metal powder, non-magnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant, and solvent used in the present invention may be added at the beginning or middle of any step. In addition, individual raw materials may be added in two or more steps. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion. In order to achieve the object of the present invention, a conventional known manufacturing technique can be used as a partial process. In the kneading step, it is preferable to use a kneading force such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Further, glass beads can be used to disperse the liquid for the magnetic layer and the liquid for the nonmagnetic layer. Such glass beads are preferably zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media. The particle diameter and filling rate of these dispersion media are optimized. A well-known thing can be used for a disperser.

  In the method for producing a magnetic recording medium of the present invention, for example, a magnetic layer is applied to the surface of a support under running so as to have a predetermined film thickness. Here, a plurality of magnetic layer coating solutions may be applied sequentially or simultaneously, and a nonmagnetic layer coating solution and a magnetic layer coating solution may be applied sequentially or simultaneously. The coating machine for applying the above magnetic coating solution or non-magnetic layer coating solution includes 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, etc. can be used. As for these, for example, “Latest Coating Technology” (May 31, 1983) issued by General Technology Center Co., Ltd. can be referred to.

  In the case of a magnetic tape, the magnetic layer coating solution is subjected to magnetic field orientation treatment in the longitudinal direction using a cobalt magnet or a solenoid on the ferromagnetic metal powder contained in the magnetic layer coating solution. In the case of a disk, a sufficiently isotropic orientation may be obtained even without non-orientation without using an orientation device, but known random methods such as alternately arranging cobalt magnets obliquely and applying an alternating magnetic field with a solenoid. It is preferable to use an alignment device. In the case of a ferromagnetic metal powder, the isotropic orientation is generally preferably in-plane two-dimensional random, but can also be three-dimensional random with a vertical component. Further, isotropic magnetic characteristics can be imparted in the circumferential direction by using a well-known method such as a counter-polarized magnet and making it vertically oriented. In particular, when performing high density recording, vertical alignment is preferable. Moreover, circumferential orientation can also be achieved using spin coating.

  It is preferable that the drying position of the coating film can be controlled by controlling the temperature, air volume, and coating speed of the drying air, the coating speed is preferably 20 m / min to 1000 m / min, and the temperature of the drying air is preferably 60 ° C. or higher. Also, moderate pre-drying can be performed before entering the magnet zone.

After drying, the coating layer is usually subjected to a surface smoothing treatment. For the surface smoothing process, for example, a super calendar 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 metal powder in the magnetic layer is improved, so that a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. Can do. As the calendering roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, polyamideimide or the like is used. Moreover, it can also process with a metal roll.
The magnetic recording medium of the present invention has a surface having extremely excellent smoothness with a center surface average roughness of 0.1 to 4 nm, preferably 1 to 3 nm at a cutoff value of 0.25 mm. preferable. As the method, for example, as described above, a magnetic layer formed by selecting a specific ferromagnetic metal powder and a binder is subjected to the calendar treatment. As calendering conditions, the temperature of the calendar roll is in the range of 60 to 100 ° C., preferably in the range of 70 to 100 ° C., particularly preferably in the range of 80 to 100 ° C., and the pressure is 100 to 500 kg / cm (98 to 490 kN). / M), preferably 200 to 450 kg / cm (196 to 441 kN / m), particularly preferably 300 to 400 kg / cm (294 to 392 kN / m). Is preferably carried out by

  The obtained magnetic recording medium can be cut into a desired size using a cutting machine or the like. The cutting machine is not particularly limited, but is preferably provided with a plurality of pairs of rotating upper blades (male blades) and lower blades (female blades). The slitting speed, the engagement depth, and the upper blade (male blade) are appropriately selected. The peripheral speed ratio (upper blade peripheral speed / lower blade peripheral speed) between the blade and the lower blade (female blade), the continuous use time of the slit blade, and the like are selected.

[Physical properties]
The saturation magnetic flux density of the magnetic layer of the magnetic recording medium used in the present invention is preferably 100 to 300 mT. The coercive force (Hr) of the magnetic layer is preferably 143.3 to 318.4 kA / m (1800 to 4000 Oe), more preferably 159.2 to 278.6 kA / m (2000 to 3500 Oe). The coercive force distribution is preferably narrow, and SFD and SFDr are 0.6 or less, more preferably 0.2 or less.

The friction coefficient with respect to the head of the magnetic recording medium used in the present invention is 0.50 or less, preferably 0.3 or less in the range of temperature -10 to 40 ° C. and humidity 0 to 95%. The surface resistivity is preferably 10 ⁴ to 10 ¹² Ω / sq of the magnetic surface, and the charging position is preferably within −500 V to +500 V. The elastic modulus at 0.5% elongation of the magnetic layer is preferably 0.98 to 19.6 GPa (100 to 2000 kg / mm ² ) in each in-plane direction, and the breaking strength is preferably 98 to 686 MPa (10 to 70 kg). / Mm ² ), the elastic modulus of the magnetic recording medium is preferably 0.98 to 14.7 GPa (100 to 1500 kg / mm ² ) in each in-plane direction, and the residual spread is preferably 0.5% or less, 100 ° C. The thermal shrinkage at any of the following temperatures is preferably 1% or less, more preferably 0.5% or less, and most preferably 0.1% or less.

The glass transition temperature of the magnetic layer (maximum point of loss elastic modulus measured by dynamic viscoelasticity measured at 110 Hz) is preferably 50 to 180 ° C, and that of the nonmagnetic layer is preferably 0 to 180 ° C. The loss elastic modulus is preferably in the range of 1 × 10 ⁷ to 8 × 10 ⁸ Pa (1 × 10 ⁸ to 8 × 10 ⁹ dyne / cm ² ), and the loss tangent is preferably 0.2 or less. If the loss tangent is too large, adhesion failure is likely to occur. These thermal characteristics and mechanical characteristics are preferably almost equal within 10% in each in-plane direction of the medium.

The residual solvent contained in the magnetic layer is preferably 100 mg / m ² or less, more preferably 10 mg / m ² or less. The porosity of the coating layer is preferably 30% by volume or less, more preferably 20% by volume or less for both the nonmagnetic layer and the magnetic layer. The porosity is preferably small in order to achieve high output, but it may be better to ensure a certain value depending on the purpose. For example, in the case of a disk medium in which repeated use is important, a larger void ratio is often preferable for running durability.

The maximum height SR _max of the magnetic layer is 0.5 μm or less, the ten-point average roughness SRz is 0.3 μm or less, the center plane peak height SRp is 0.3 μm or less, and the center plane valley depth SRv is 0.3 μm or less. The center surface area ratio SSr is preferably 20 to 80%, and the average wavelength Sλa is preferably 5 to 300 μm. These can be easily controlled by controlling the surface properties with the filler of the support or the surface of the calendered roll. The curl is preferably within ± 3 mm.

  When the magnetic recording medium of the present invention is composed of a nonmagnetic layer and a magnetic layer, these physical characteristics can be changed between the nonmagnetic layer and the magnetic layer according to the purpose. For example, the elastic modulus of the magnetic layer can be increased to improve running durability, and at the same time, the elastic modulus of the nonmagnetic layer can be made lower than that of the magnetic layer to improve the contact of the magnetic recording medium with the head.

EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to the following example.
In addition, the display of "part" in an Example shows "mass part".

Example 1
《Nonmagnetic layer paint component》
(1)
Iron oxide powder (average particle diameter: 0.06 × 0.01 μm) 72 parts α-alumina (average particle diameter: 0.05 μm) 8 parts carbon black 20 parts (average particle diameter: 20 nm, DBP oil absorption: 200 ml / 100 g )
Stearic acid / palmitic acid (1/1) 5 parts vinyl chloride copolymer (containing -SO ₃ Na group: 1.2 × 10 ⁻⁴ equivalent / g) 12 parts polyvinyl acetal 12 parts (BL1 manufactured by Sekisui Chemical Co., Ltd.) Tg 67 ° C, molecular weight 19000)
Polyester polyurethane resin 5 parts (Tg: 40 ° C., contained —SO ₃ Na group: 1 × 10 ⁻⁴ equivalent / g)
Cyclohexanone 30 parts Methyl ethyl ketone 60 parts Toluene 5 parts (2)
Isooctyl stearate 3 parts Oleate oleate 2 parts Cyclohexanone 40 parts Methyl ethyl ketone 80 parts Toluene 20 parts (3)
Polyisocyanate (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) 5 parts Cyclohexanone 8 parts Methyl ethyl ketone 18 parts Toluene 5 parts

<Coating component for magnetic layer>
(1)
100 parts of ferromagnetic iron-based metal powder (Co / Fe: 21 at%, Y / (Fe + Co): 8 at%, Al / (Fe + Co): 5 wt%, Ca / Fe: 0 wt%, σs: 155 A · m ² / kg, (Hc: 149.6 kA / m, pH: 9.4, average long axis length: 0.10 μm)
5 parts of polyvinyl butyral (BL1: Tg 67 ° C., molecular weight 19000, manufactured by Sekisui Chemical Co., Ltd.)
5 parts Polyester polyurethane resin (containing _-SO 3 Na group: 1.0 × ^{10 -4} eq / g)
α-alumina (average particle size: 0.12 μm) 8 parts α-alumina (average particle size: 0.07 μm) 2 parts carbon black 1 part (average particle size: 75 nm, DBP oil absorption: 72 ml / 100 g)
Methyl acid phosphate 2 parts Palmitic acid amide 1.5 parts Isooctyl stearate 2 parts Stearic acid 0.5 parts Diethylene glycol 1 part Tetrahydrofuran 65 parts Methyl ethyl ketone 245 parts Toluene 85 parts (2)
Polyisocyanate (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) 2 parts Cyclohexanone 167 parts

  The following methyl acid phosphate was used.

  In the above formula, n is 1 or 2, m. w: 119, SG: 1.43, acid value: KOH mg / g 650 <, phosphorus: 26.1%.

<Back layer paint component>
Carbon black (average particle diameter: 19 nm) 90 parts Carbon black (average particle diameter: 350 nm) 5 parts Iron oxide (average major axis length: 0.1 μm, axial ratio: about 10) 5 parts Nitrocellulose 40 parts Polyurethane resin (SO ₃ Na-containing) 30 parts cyclohexanone 250 parts toluene 220 parts methyl ethyl ketone 600 parts

After kneading (1) with the kneader in the above-mentioned coating composition for the nonmagnetic layer, add (2), and after stirring, disperse with a sand mill with a residence time of 90 minutes. After filtration, a nonmagnetic paint was obtained. Separately, after kneading the above-mentioned magnetic layer coating component (1) with a kneader, dispersing it with a sand mill with a residence time of 60 minutes, adding the magnetic layer coating component (2) to this, stirring and filtering, A magnetic paint was used. Thickness after drying the above-mentioned non-magnetic paint into a polyethylene terephthalate film (thickness 6.0 μm, MD Young's modulus = 7.0 GPa, MD Young's modulus / TD Young's modulus = 1.3) Was applied so as to be 0.8 μm.
On the nonmagnetic layer, the magnetic coating material was applied so that the thickness after drying was 0.2 μm, subjected to magnetic field orientation treatment, and dried to obtain a magnetic sheet. In the magnetic field orientation treatment, an NN counter magnet 500 mT (5 kG) is installed in front of the dryer, and an NN counter magnet 500 mT (5 kG) 2 is applied from the front side 75 cm of the dry coating position of the coating film in the dryer. The installation was performed at intervals of 50 cm. The coating speed of the magnetic paint was 300 m / min.
Subsequently, after dispersing the coating component for the back layer in a sand mill with a residence time of 60 minutes, 15 parts of polyisocyanate was added to prepare a coating for the back layer and filtered, and the opposite of the magnetic layer of the magnetic sheet produced above The surface was coated so that the thickness after drying and calendering was 0.3 μm and dried.
The magnetic sheet thus obtained was mirror-finished on a seven-stage calendar composed of metal rolls at a temperature of 110 ° C. and a linear pressure of 330 kg / cm (323 kN / m), and then cut to a width of 12.65 mm to obtain a magnetic sheet. I got a tape.

The obtained magnetic tape was sequentially subjected to the following LDWT (wrapping / diamond wheel / tissue) treatment.
(1) Lapping treatment: Abrasive tape (wrapping tape KX20000S manufactured by Fuji Photo Film Co., Ltd.) is moved at a speed of 0.2 cm / sec in the opposite direction to the tape feed (7 m / sec) by a rotating roll, and from the top by a guide block The tape is brought into contact with the surface of the magnetic layer by pressing. The magnetic tape was subjected to a polishing treatment with a magnetic tape unwinding tension of 100 g and a wrapping tape tension of 200 g.
(2) Rotary diamond wheel processing: 1 inch (25.4 mm) wide, 60 mm diameter, 2 mm wide grooved wheel for air venting (groove angle 45 degrees) and magnetic layer in the opposite direction to the tape at a contact angle of 90 degrees The magnetic tape was processed at a rotational speed of 1200 rpm.
(3) Tissue treatment: Toray Co., Ltd. ultra-fine fiber Toraysee is fed with a rotating rod to the back layer and the magnetic layer surface at a speed of 0.2 mm / sec in the opposite direction to the tape feed, and the magnetic tape front and back surfaces are cleaned. It was.

  Using a servo writer, write a magnetic servo signal to the magnetic layer of the magnetic tape at a speed of 4 m / sec (4000 mm / sec), wind this magnetic tape around a reel, and incorporate it into the case body, so that it can be used for computers. A magnetic tape cartridge was prepared.

With respect to such a magnetic tape, the center line surface roughness Ra of the magnetic layer and the error rate after repeated 1000 passes were examined. Ra was measured by using a 3D millo manufactured by WYKO, and an error rate was measured by using a model ultrium 203i manufactured by HP.
The results are shown in Table 1.

(Examples 2-3)
In Example 1, except that the amount of polyvinyl butyral (BL1 manufactured by Sekisui Chemical Co., Ltd.) used for the coating component for the magnetic layer was changed to 10 parts (Example 2) or 20 parts (Example 3). Example 1 was repeated.

Example 4
In Example 1, polyvinyl butyral (BL1 manufactured by Sekisui Chemical Co., Ltd.) used for the coating component for the magnetic layer was replaced with polyvinyl butyral (BM2 manufactured by Sekisui Chemical Co., Ltd., Tg 68 ° C., molecular weight 52000, bending strength 740 kg / cm ² (72. 5 MPa) and tensile strength 750 kg / cm ² (73.5 MPa)). Example 1 was repeated.

(Examples 5-6)
In Example 4, Example 1 was repeated except that the amount of polyvinyl butyral (BM2 manufactured by Sekisui Chemical Co., Ltd.) was changed to 10 parts (Example 5) or 20 parts (Example 6).

(Example 7)
In Example 1, polyvinyl butyral (BL1 manufactured by Sekisui Chemical Co., Ltd.) used for the coating component for the magnetic layer was replaced with polyvinyl butyral (BH3 manufactured by Sekisui Chemical Co., Ltd., Tg 70 ° C., molecular weight 110000, bending strength 760 kg / cm ² (74. 5 MPa) and tensile strength 800 kg / cm ² (78.4 MPa)). Example 1 was repeated.

(Examples 8 to 9)
In Example 7, Example 1 was repeated except that the amount of polyvinyl butyral (BH3 manufactured by Sekisui Chemical Co., Ltd.) was changed to 10 parts (Example 8) or 20 parts (Example 9).

(Reference Example 1)
In Example 1, polyvinyl butyral (BL1 manufactured by Sekisui Chemical Co., Ltd.) used as the coating component for the magnetic layer was converted into a vinyl chloride-hydroxypropyl acrylate copolymer (containing -SO ₃ Na group: 0.7 × 10 ⁻⁴ equivalent / g). Example 1 was repeated except that it was changed to).

(Reference Examples 2-3)
In Reference Example 1, Example 1 was repeated except that the amount of vinyl chloride-hydroxypropyl acrylate copolymer used was changed to 10 parts (Reference Example 2) or 20 parts (Reference Example 3).

(Comparative Example 1)
In Example 1, except that polyvinyl butyral (BL1 manufactured by Sekisui Chemical Co., Ltd.) used for the magnetic layer coating component was changed to polyurethane (trade name UR8300 manufactured by Toyobo Co., Ltd.), and the amount used was changed to 10 parts. Example 1 was repeated.
(Comparative Example 2)
Example 1 except that in Example 1, α-alumina (average particle size: 0.12 μm) used for the coating component for the magnetic layer was changed to 8 parts α-alumina having an average particle size of 0.4 μm. Was repeated.
(Comparative Example 3)
In Comparative Example 2, Comparative Example 2 was repeated except that the amount of α-alumina having an average particle size of 0.4 μm was changed to 3 parts.
(Comparative Example 4)
In Comparative Example 2, Comparative Example 2 was repeated except that the amount of α-alumina having an average particle size of 0.4 μm was changed to 1 part.
(Comparative Example 5)
In Example 1, except that α-alumina (average particle size: 0.12 μm) used for the coating component for the magnetic layer was changed to 28 parts α-alumina having an average particle size of 0.005 μm. Was repeated.
(Comparative Example 6)
In Comparative Example 5, Comparative Example 5 was repeated except that the amount of α-alumina having an average particle size of 0.005 μm was changed to 18 parts.
(Comparative Example 7)
In Comparative Example 5, Comparative Example 5 was repeated except that the amount of α-alumina having an average particle size of 0.005 μm was changed to 8 parts.
(Comparative Example 8)
In Example 1, Example 1 was repeated except that the addition amount of α-alumina (average particle size: 0.12 μm) used for the coating component for the magnetic layer was changed from 8 parts to 48 parts.
(Comparative Example 9)
In Example 1, Example 1 was repeated except that the addition amount of α-alumina (average particle size: 0.12 μm) used for the coating component for the magnetic layer was changed from 8 parts to 38 parts.
(Comparative Example 10)
In Example 1, Example 1 was repeated except that the addition amount of α-alumina (average particle size: 0.12 μm) used for the coating component for the magnetic layer was changed from 8 parts to 0 part.
The results are shown in Table 1. For reference, the chlorine content (% by mass) of the binder resin is also shown.

  From the results shown in Table 1, it can be seen that the magnetic recording medium of the present invention showed running durability comparable to that of a conventional magnetic recording medium using a vinyl chloride resin as a binder for the magnetic layer. It can also be seen that the running durability is excellent even when compared with a magnetic recording medium using polyurethane as a binder for the magnetic layer.

Claims (6)

In a magnetic recording medium comprising a magnetic layer containing a ferromagnetic powder and a binder on at least one surface of a nonmagnetic support,
The magnetic layer contains 3 to 30 parts by mass of an abrasive having a Mohs hardness of 8 to 10 and an average particle size of 0.010 to 0.3 μm with respect to 100 parts by mass of the ferromagnetic powder.
The magnetic recording medium, wherein the binder contains at least a polyvinyl acetal resin, a polyurethane resin, and a polyisocyanate.   The magnetic recording medium according to claim 1, wherein the polyvinyl acetal resin has a glass transition temperature (Tg) of 60 to 110 ° C. The magnetic recording medium according to claim 1, wherein the polyvinyl acetal resin has a bending strength of 700 to 1200 kg / cm ² (68.6 to 117.6 MPa). The magnetic recording medium according to claim 1, wherein the polyvinyl acetal resin has a tensile strength of 600 to 900 kg / cm ² (58.8 to 88.2 MPa).   The magnetic recording medium according to claim 1, wherein a nonmagnetic layer containing a nonmagnetic powder and a binder is provided between the nonmagnetic support and the magnetic layer.   The magnetic recording medium according to claim 1, wherein a back layer is provided on the other surface of the nonmagnetic support.