JP2642918B2 - Magnetic recording media - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having a very thin magnetic layer having a magnetic layer of 1.0 μm or less.
More particularly, the present invention relates to a magnetic recording medium having very excellent electromagnetic conversion characteristics and excellent production characteristics with good yield.

[0002]

2. Description of the Related Art Conventionally, as a magnetic recording medium such as a video tape, an audio tape, a magnetic disk, etc., a magnetic layer in which a ferromagnetic iron oxide, a Co-modified iron oxide, CrO ₂ , a ferromagnetic alloy powder or the like is dispersed in a binder is used. Are coated on a non-magnetic support. In recent years, the recording wavelength tends to be shortened with the increase in recording density, and the problem of thickness loss at the time of recording / reproducing, such as a decrease in output when the thickness of the magnetic layer is large, is increasing. For this reason, the thickness of the magnetic layer has been reduced. However, when the thickness of the magnetic layer is reduced to about 2 μm or less, the effect of the surface properties of the support on the surface of the magnetic layer tends to appear, and the electromagnetic conversion characteristics tend to deteriorate. . Therefore, by providing a nonmagnetic thick lower layer on the surface of the nonmagnetic support and then providing the magnetic layer on the upper layer, the problem due to the surface roughness of the support described above is eliminated and the magnetic layer is made a thin layer, Attempts to reduce thickness demagnetization and achieve high output have been proposed. For example, in Japanese Patent Application Laid-Open No. 62-154225, a conductive layer is provided between a magnetic layer and a substrate in order to reduce the thickness of the magnetic layer to 0.5 μm or less and to prevent the surface electric resistance of the magnetic layer from increasing. There has been proposed a magnetic recording medium provided with an undercoat layer having a thickness containing a powder equal to or greater than the thickness of the magnetic layer. JP-A-62-222427 discloses a support and an undercoat layer provided on the support and containing an abrasive having an average particle size of 0.5 to 3 μm, and an undercoat layer provided on the undercoat layer. A magnetic recording medium having a magnetic layer containing a magnetic powder and having a thickness of 1 μm or less has been proposed. However, since a part of the abrasive in the undercoat layer protrudes into the magnetic layer, a magnetic head of the magnetic recording medium has been proposed. The cleaning function is also provided. In this way, the magnetic layer is thinned to achieve high-density recording, and at the same time, carbon black is included in the lower non-magnetic layer to prevent charging, and an abrasive is added to improve cleaning characteristics and durability. doing. However, in the conventional technology, the lower magnetic layer is first applied to the non-magnetic support, and then dried and, if necessary, calendered, and then the upper magnetic layer is provided. There were the following problems. That is, in order to reduce the thickness of the magnetic layer, it is conceivable to reduce the amount of coating or to add a large amount of a solvent to the magnetic coating solution to reduce the concentration. In the former case, if the amount of application is reduced, there is not enough time for leveling after application, and drying starts, so that a problem such as an application defect, for example, a stripe or engraved pattern remains, and the yield is extremely deteriorated. If the latter method is adopted, if the concentration of the magnetic coating solution is low,
The resulting coating film has various harmful effects, such as that there are many voids and a sufficient degree of filling of the ferromagnetic powder cannot be obtained, and that there are many voids and the strength of the coating film is insufficient. One means for solving these problems is disclosed in JP-A-63-19 / 1988.
As described in Japanese Patent No. 1315, there has been proposed a method in which a non-magnetic layer is provided as a lower layer using a simultaneous multilayer coating method, and a high-concentration magnetic coating liquid is applied thinly. In the case of the simultaneous multilayer coating method or the sequential wet coating method, that is, in the case of the so-called wet-on-wet coating method in which the upper layer is simultaneously or sequentially coated while the lower layer is in a wet state, various studies have already been performed on the magnetic layer of the multilayer. Has been done. However, even when this technique was applied to the lower non-magnetic layer, similarly good results could not be obtained. In other words, Wet on We
When the lower non-magnetic layer and the upper magnetic layer were provided according to t, disturbance occurred at the interface between the two, causing pinholes and cissing of the magnetic layer. Then, the present inventors considered the composition of these lower nonmagnetic layers and the wet-on-wet.
As a result of intensive studies on the relationship between t and the coating method, it was found that there was a point in the rheological properties, and the present invention was reached.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a thin-layer magnetic recording medium which is excellent in output, C / N ratio and other electromagnetic conversion characteristics while securing a good yield and high production efficiency. Another object of the present invention is to provide a thin-layer magnetic recording medium having good head contact and good storage stability.

[0004]

[Prior art]

[0005]

The object of the present invention is to provide a lower non-recording layer in which a powder is dispersed in a binder on a support, and a ferromagnetic powder dispersed in the binder on the lower non-recording layer. In the magnetic recording medium provided with the upper recording layer, the dry thickness of the upper recording layer is 1.0 μm or less, the lower non-recording layer contains a magnetic powder and a non-magnetic material as the powder, and the lower non-recording layer This can be achieved by a magnetic recording medium characterized by having a saturation maximum magnetic flux density of 500 gauss or less. Preferred embodiments of the present invention are as follows. (1) The saturation maximum magnetic flux density of the lower non-recording layer is 30 to 5
A magnetic recording medium characterized by being 00 gauss. (2) A magnetic recording medium, wherein the dispersion of the lower non-recording layer in which the powder is dispersed in a binder has thixotropic properties. (3) The upper recording layer has an Hc of 1200 to 3000 Oe.
A magnetic recording medium characterized by the following. (4) The crystallite size of the ferromagnetic powder in the upper recording layer is 4
A magnetic recording medium having a thickness of 50 to 100 angstroms. (5) The ferromagnetic powder of the upper recording layer is a ferromagnetic alloy powder containing Fe, Ni or Co as a main component, and in addition to these atoms, Al, Si, S, Sc, Ti, V, Cr, Cu,
Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, B
a, Ta, W, Re, Au, Pb, Bi, La, Ce,
A magnetic recording medium comprising at least one atom selected from the group consisting of Pr, Nd, P, Co, Mn, Zn, Ni, Sr, and B. (6) The magnetic recording medium, wherein the powder of the lower non-recording layer contains a nonmagnetic inorganic powder having a Mohs hardness of 4 or more. (7) The powder of the lower non-recording layer has an α ratio of 90% or more.
-Alumina, β-alumina, γ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, silicon nitride, titanium carbide, titanium oxide, silicon dioxide, tungsten oxide, zirconium oxide, boron nitride, oxide At least one selected from zinc, calcium carbonate, calcium sulfate, barium sulfate, and molybdenum disulfide
A magnetic recording medium comprising a kind of non-magnetic inorganic powder. (8) The elastic modulus of the support is 100 to 2000 kg / m.
m ² . (9) The support is made of polyethylene terephthalate, polyethylene naphthalate polyesters, polyolefins, cellulose triacetate, polycarbonate,
A magnetic recording medium comprising at least one selected from polyamide, polyimide, polyamideimide, polysulfone, aramid, and aromatic polyamide.

That is, according to the present invention, by adopting the above structure, a dispersion liquid containing the powder used for the lower non-recording layer in a binder and the ferromagnetic powder used for the upper recording layer are dispersed in a binder. It has been found that the magnetic paints have the same or similar thixotropy as each other, and that when the two paints are applied in a wet-on-wet manner, the interface is not disturbed, pinholes and cissing do not occur, and the head contact can be improved.

The thixotropic property is also called thixotropy and refers to a phenomenon of softening of an object due to stress that involves recovery, but causes softening by increasing the strain of the object. Magnetic coatings exhibit typical thixotropic properties, for example, those which gradually break down and soften when given agitation to increase fluidity. When a magnetic paint is applied on a non-magnetic support and is oriented in a magnetic field, the ferromagnetic powder rotates, but at that time the internal structure is destroyed and the fluidity is considerably increased. Conventional dispersion liquids for the lower non-recording layer have been used which do not exhibit thixotropy, are very small, or exhibit extremely strong thixotropy, such as liquids containing carbon black. The present inventors have paid attention to the thixotropic properties of these two liquids, and surprisingly, no disturbance occurred at the interface when they were set to the same or similar values. That is, the thixotropy, that is, the viscoelasticity of the lower non-recording layer has not been noticed at all and has not been particularly controlled. However, by controlling this to a certain range, the effect of the present invention can be extremely effectively exerted. Here, the same or similar thixotropy means that the thixotropy of both liquids is such that when the dispersion of the lower non-recording layer and the magnetic paint of the upper recording layer are applied by the wet-on-wet method, mixing does not occur at the interface. It means that the sexes are similar.

In the present invention, the above configuration is adopted.
A lower layer dispersion obtained by dispersing the powder in a binder
Has thixotropic properties and a shear rate of 10^Four sec
^-1 Stress A10 at^Four And shearing speed 10 sec^-1 so
Ratio of shear stress A10 to A10 ^Four / A10 is 100 ≧
A10^Four / A10 ≧ 3.
A10 of the dispersion^Four / A10 ratio within a certain range
Thixotropy for upper magnetic coatings
With no disturbance at the interface, and as a result
No problems such as repelling and repelling occurred.

As described above, the basic idea of the present invention is that the lower non-recording layer has a maximum saturation magnetic flux density of 500 gauss or less so that the lower non-recording layer and the upper recording layer each have a specific rheological property. Liquid, in the lower non-recording layer, the dispersion, in the upper recording layer, the magnetic paint can be prepared to have the same or similar thixotropy at the time of coating, thereby eliminating coating defects, Yields can be improved, and electromagnetic conversion characteristics such as durability and output can be improved.

Here, the shear stress with respect to the shear rate is measured using a viscometer, for example, a Rotovisco viscometer RV-II manufactured by HAAKE.
The shear rate is determined by the diameter of the inner cylinder and the outer cylinder, the clearance, and the number of revolutions. The following relationship exists between the shear rate, apparent viscosity and shear stress.

A = η · D (A: shear stress, η: viscosity, D: shear rate) D = dv / dr (v: peripheral speed, r: radius) Therefore, A10 and A10 ⁴ are respectively A = 10η , A = 1
0 ⁴ refers to eta, A10 ⁴ / A10 ratio represents the degree of thixotropy, the smaller the thixotropy large,
The smaller the ratio, the greater the thixotropic property. Conventionally, nonmagnetic dispersions have only weak thixotropic properties, and A10 ⁴ / A10 is about 200 to 500. When A10 ⁴ / A10 is large, the dispersion liquid approaches the Newtonian fluid, and it is difficult to perform simultaneous multi-layer coating when the upper layer is a magnetic layer. If the viscosity is too low, the thixotropy is so strong that it is difficult to achieve both a viscosity that allows simultaneous multilayer coating and a viscosity that allows liquid feeding.

[0012] The magnetic recording medium according to the present invention, the saturation maximum magnetic flux density Bm of the lower non-recording layer 500 gauss or less, preferably but those to 30 to 500 Gauss, specifically A10 ⁴ / A10 a This is a dispersion in which thixotropic properties are adjusted by using magnetic powder that satisfies and does not adversely affect the upper recording layer. That is, in order to set the lower non-recording layer to 500 Gauss or less, preferably 30 to 500 Gauss, a magnetic powder having a low saturation magnetization (σ _S ) was used, or the filling ratio of the magnetic powder in the lower non-recording layer was reduced. This is achieved by preparing a dispersion.

In the present invention, as described above, the saturation magnetization (
It is preferable to use a magnetic powder having a low σ _s ) or to reduce the filling ratio of the magnetic powder in the lower non-recording layer. In this case, it is preferable to use a large amount of a nonmagnetic substance with respect to the magnetic powder. In the present invention, the nonmagnetic inorganic powder is a concept that does not include carbon black. The non-magnetic powder preferably contains a non-magnetic powder in addition to the binder, and more preferably a non-magnetic inorganic powder, carbon black and the like. As the magnetic powder, for example, Co-γ-Fe ₂ O ₃ magnetic powder is preferable, and its coercive force is not particularly limited, but is preferably about 1500 Oe or less, and more preferably about 400 Oe to 1500 Oe. Also, hexagonal barium ferrite can be used for the lower non-recording layer. There is no limitation on Hc at that time, but when Ba ferrite is used, it is 800 Oe or more and 450 Oe or more.
0 Oe or less is preferable. The Hc of the upper recording layer is 120
It is preferable that 0 Oe to 3000 Oe and Bm are 200 to 4500 Gauss.

When carbon black is used, the amount of the magnetic powder relative to the lower non-recording layer and the upper recording layer is 0.1.
It is preferable to use 1 to 30%. In addition to imparting thixotropy, carbon black has functions such as antistatic of the magnetic layer, reduction of friction coefficient, light-shielding property, and improvement of film strength.
These differ depending on the carbon black used. Therefore, these carbon blacks used in the present invention have different properties, such as particle size, oil absorption, conductivity, and pH, by changing the type, amount, and combination in the lower non-recording layer and the upper recording layer. It is, of course, possible to use them properly according to the purpose. For example, charging is prevented by using highly conductive carbon black for the lower non-recording layer, and the friction coefficient is reduced by using carbon black having a large particle diameter for the upper recording layer.

The physical properties of the non-magnetic inorganic powder are as follows. The particle diameter is preferably from 0.01 to 2 μm, but if necessary, various kinds of particles having different particle diameters can be combined, or even a single particle having a wide particle diameter distribution can have the same effect. The tap density is 0.05 to 2 g / cc, preferably 0.2 to 1.5 g / cc. The water content is 0.1-5%, preferably 0.2-3%. pH 2-11. Specific surface area is 1
100 m < ² > / g, preferably 5 to 50 m < ² > / g, more preferably 7 to 40 m < ² > / g. The crystallite size is preferably 0.01 μm to 2 μm. Oil absorption using DBP is 5 to 100 ml / 100 g, preferably 10 to 80 ml
/ 100g, more preferably 20-60ml / 100g
It is. The specific gravity is 1 to 12, preferably 2 to 8. The shape may be any of a needle shape, a spherical shape, and a dice shape, but a needle shape is preferable for improving magnetic properties, and a size and a shape close to the shape of the ferromagnetic powder are particularly preferable. The ratio of the major axis length of the acicular nonmagnetic inorganic powder to the length is particularly preferably in the range of 0.5 to 3. The above-mentioned nonmagnetic inorganic powder does not necessarily need to be 100% pure, and the surface may be treated with another compound according to the purpose. At this time, if the purity is 70% or more, the effect is not reduced.
For example, when using titanium oxide, it is generally used to treat the surface with alumina. Ignition loss is 20
% Is preferable. The Mohs hardness of the non-magnetic inorganic powder used in the present invention is preferably 4 or more.

The physical properties of the carbon black that can be used are as follows. The specific surface area is 100 to 500, preferably 150 to 400 m ² / g, and the DBP oil absorption is 20 to 40.
0 ml / 100 g, preferably 30-200 ml / 10
0 g. The particle size is 5 μm to 80 μm, preferably 1 μm.
It is 0 to 50 μm, more preferably 10 to 40 μm. The pH is preferably 2 to 10, the water content is preferably 0.1 to 10%, and the tap density is preferably 0.1 to 1 g / cc.

In the present invention, other nonmagnetic powders that can be used include organic powders such as acrylstyrene resin powder, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment. Resin powder, polyamide resin powder, polyimide resin powder, and polyfluoroethylene resin are used. The production method is described in Japanese Patent Application Laid-Open No. 62-18564.
JP-A-60-255827 and JP-A-60-255
No. 827 can be used.

These non-magnetic powders are
It is used in the range of 0.1 to 10 in weight ratio and 10 to 0.2 in volume ratio.

The present invention has shown suitable means for adjusting A10 ⁴ / A10 to the above-mentioned range, and these have, for example, a relationship (including overlapping descriptions) with the following factors. , The desired A
It is possible to obtain a dispersion having a density of 10 ⁴ / A10 and a magnetic paint, and thus to produce a magnetic recording medium having desired characteristics.

As the factors, for example, regarding non-magnetic powder or magnetic powder to be dispersed, (1) particle size (specific surface area, average primary particle diameter, etc.), (2) structure (oil absorption, particle form, etc.) , (3) properties of the powder surface (pH, loss on heating, etc.), (4) suction force of particles (σ _S, etc.), etc., as for the binder, (1) molecular weight, (2) type of functional group, etc. Examples of the solvent include (1) type (polarity, etc.), (2) binder solubility, (3) solvent formulation, water content, and the like.

That is, the ratio of A10 ⁴ / A10 is, for example,
Various parameters such as the molecular weight of the binder, the structure, the affinity with the solvent, the water content are involved, but the largest contribution is to the powder to be filled, the surface properties of the dispersed powder, the particle size,
The amount of oil absorption, affinity with the solvent, etc. are related. The smaller the particle size, the stronger the thixotropy. In addition, carbon black having a large oil absorption and rich in structure has a very strong thixotropy. In addition, magnetic particles exhibit strong structural viscosity due to the magnetic attraction of the particles themselves, and usually have strong thixotropic properties. That is, the present invention utilizes such properties to adjust the thixotropic properties of the coating liquid for the lower non-recording layer and the magnetic paint for the upper recording layer to a predetermined range. Other specific means for having such thixotropic properties include the following embodiments. The magnetic recording medium of the present invention can make the thixotropic property of the dispersion liquid the same or similar to the thixotropic property of the magnetic paint by combining this means,
More specifically, the value can be in the range of A10 ⁴ / A10.

The powder of the lower non-recording layer contains at least carbon black and an inorganic powder having an average primary particle diameter smaller than the dry thickness of the lower non-recording layer, and the lower non-recording layer and the upper recording layer have a thermosetting polymer. A magnetic recording medium containing 10 to 70% by weight of an isocyanate in a binder.

That is, the magnetic recording medium described above uses a magnetic powder imparting a large thixotropy, a carbon black and a nonmagnetic inorganic powder imparting a small thixotropy as a powder to be dispersed, and a polyisocyanate. In this case, a dispersion prepared by the above method is used. As a result, the upper recording layer can be made thinner and the yield can be improved. By using polyisocyanate for the upper recording layer and the lower non-recording layer, the head contact can be maintained well, and the storage stability at high temperature and high humidity can be maintained. And a magnetic recording medium having appropriate rigidity and flexibility is obtained, and the durability is improved.

Hereinafter, general items common to the present invention will be described. Non-magnetic inorganic powder that can be used in the present invention, specifically, metal oxides, metal carbonates, metal sulfates,
It is an inorganic compound powder such as metal nitride, metal carbide, metal sulfide and the like. As the inorganic compound, for example, a pregelatinization rate of 90%
The above α-alumina, β-alumina, γ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, silicon nitride, titanium carbide, titanium oxide,
Silicon dioxide, tin oxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide and the like are used alone or in combination.

Specific examples of the non-magnetic inorganic powder used in the present invention include AKP-20 and AKP- manufactured by Sumitomo Chemical Co., Ltd.
30, AKP-50, HIT-50, HIT-100,
Nippon Kagaku Kogyo Co., Ltd .: G5, G7, S-1, Toda Kogyo Co., Ltd .: DPN-250BX, TF-100, TF-12
0, TF-140, manufactured by Ishihara Sangyo: TTO-55, FT-
1000, FT-2000, FTL-100, FTL-
200, M-1, S-1, SN-100, E-303,
E-304, manufactured by Titanium Industry: ECT-52, STT-4
D, STT-30, STT-65C, Y-LOP, Y-
Α-Hematite obtained from LOP, manufactured by Mitsubishi Metal Corporation: T-
1, Nippon Shokubai: NS-O, NS-3Y, NS-8Y and the like.

As the carbon black used in the present invention, furnace for rubber, thermal for rubber, black for color, acetylene black and the like can be used.
Specific examples of the carbon black used in the present invention include BLACKPEARLS 200 manufactured by Cabot Corporation.
0, 1300, 1000, 900, 800, 880, 7
00, VULCAN XC-72, manufactured by Mitsubishi Kasei Kogyo:
$ 3250B, $ 950, $ 650B, $ 970B, $
850B, manufactured by Columbian Carbon Co .: CONDUCT
EX SC, RAVEN 8800, 8000, 700
0,5750,5250,3500,2100,200
0, 1800, 1500, 1255, 1250, manufactured by Akzo Ketjen Black EC. Carbon black may be used after being surface-treated with a dispersant or the like or grafted with a resin, or may be one obtained by partially graphitizing the surface. Before adding the carbon black to the magnetic paint, the carbon black may be dispersed in a binder in advance. These carbon blacks can be used alone or in combination. The carbon black that can be used in the present invention can be referred to, for example, “Carbon Black Handbook” (edited by Carbon Black Association).

As the ferromagnetic powder used in the upper recording layer of the present invention, γ-FeOx (x = 1.33 to 1.5), Co
Known ferromagnetic powders such as modified γ-FeOx (x = 1.33 to 1.5), ferromagnetic alloy fine powder containing Fe or Ni or Co as a main component (75% or more), barium ferrite, and strontium ferrite are used. it can. These ferromagnetic powders include Al, Si, S, Sc,
Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag,
Sn, Sb, Te, Ba, Ta, W, Re, Au, H
g, Pb, Bi, La, Ce, Pr, Nd, P, Co,
It may contain atoms such as Mn, Zn, Ni, Sr, and B. These ferromagnetic fine powders may be preliminarily treated with a dispersant, a lubricant, a surfactant, an antistatic agent, and the like before dispersion before dispersion. Specifically, JP-B-44-14090, JP-B-45-18372, JP-B-47-22062, JP-B-47-22513,
JP-B-46-28466, JP-B-46-38755
No., JP-B-47-4286, JP-B-47-12422
No., JP-B-47-17284, JP-B-47-1850
No. 9, JP-B-47-18573, JP-B-39-103
07, JP-B-48-39639, and U.S. Pat.
No. 6215, No. 3031341, No. 3100194
Nos. 3,242,005 and 3,389,014.

Among the above ferromagnetic powders, the ferromagnetic alloy fine powder may contain a small amount of hydroxide or oxide. A ferromagnetic alloy fine powder obtained by a known production method can be used, and the following method can be used. A method of reducing a complex organic acid salt (mainly oxalate) with a reducing gas such as hydrogen, a method of reducing iron oxide with a reducing gas such as hydrogen to obtain Fe or Fe—Co particles, Thermal decomposition method, reduction method by adding reducing agent such as sodium borohydride, hypophosphite or hydrazine to aqueous solution of ferromagnetic metal, reduction of fine powder by evaporating metal in low pressure inert gas And how to get it. The ferromagnetic alloy powder thus obtained is subjected to a known slow oxidation treatment, that is, a method of immersing in an organic solvent and then drying, and immersing in an organic solvent and then feeding an oxygen-containing gas to form an oxide film on the surface and drying. Any of the methods of forming an oxide film on the surface by adjusting the partial pressure of oxygen gas and inert gas without using an organic solvent can be used.

The ferromagnetic powder of the upper recording layer of the present invention was BET
When expressed in terms of specific surface area by the method, it is 25 to 80 m ² / g,
Preferably it is 35-60 m < ² > / g. If it is 25 m ² / g or less, noise increases, and if it is 80 or more, it is difficult to obtain surface properties, which is not preferable. The crystallite size of the ferromagnetic powder of the upper recording layer of the present invention is 450 to 100 Å,
Preferably it is 350 to 150 angstroms. The σ _{S of the} iron oxide magnetic powder is 50 emu / g or more, preferably 70 emu / g or more, and in the case of the ferromagnetic metal fine powder, it is preferably 100 emu / g or more.

The r1500 of the ferromagnetic powder is preferably 1.5 or less. More preferably, r1500 is 1.
0 or less. r1500 indicates the% of the amount of magnetization remaining without being inverted when a magnetic field of 1500 Oe is applied in the opposite direction after the magnetic recording medium is saturated.

The ferromagnetic powder preferably has a water content of 0.01 to 2%. It is preferable to optimize the water content of the ferromagnetic powder depending on the type of the binder. The tap density of γ iron oxide is preferably 0.5 g / cc or more, more preferably 0.8 g / cc or more.

When γ iron oxide is used, the ratio of divalent iron to trivalent iron is preferably 0 to 20%, and more preferably 5 to 10%. The amount of cobalt atoms relative to iron atoms is 0 to 15%, preferably 2 to 8%.

It is preferable that the pH of the ferromagnetic powder be optimized depending on the combination with the binder used. The range is 4-1
2, but preferably 6 to 10. The ferromagnetic powder may be subjected to a surface treatment with Al, Si, P or an oxide thereof, if necessary. The amount thereof is 0.1 to 10% with respect to the ferromagnetic powder, and it is preferable that the surface treatment is performed so that the adsorption of a lubricant such as a fatty acid becomes 100 mg / m ² or less. Ferromagnetic powder contains soluble Na, Ca, Fe, Ni,
May contain inorganic ions such as Sr.
If it is less than m, the characteristics are not particularly affected.

The ferromagnetic powder used in the present invention preferably has a small number of vacancies, and the value is preferably 20% by volume or less, more preferably 5% by volume or less. The shape may be any of acicular, granular, rice granular, and plate-like shapes as long as the above-mentioned characteristics regarding the particle size are satisfied. In order to achieve the SFD of the ferromagnetic powder of 0.6 or less, it is necessary to reduce the distribution of Hc in the ferromagnetic powder. For this purpose, there are a method of improving the particle size distribution of goethite, a method of preventing sintering of γ-hematite, and a method of lowering the deposition rate of cobalt with respect to cobalt-modified iron oxide.

The present invention also provides barium ferrite, strontium ferrite as platelet hexagonal ferrite,
Hexagonal Co powder can be used. When barium ferrite is used, its particle size is 0.001 to 1 μm in diameter and 1/2 to 1/20 of its diameter. Specific gravity is 4-6g
/ Cc, and the specific surface area is 1 to 60 m ² / g.

As the binder used in the present invention, conventionally known thermoplastic resins, thermosetting resins, reactive resins and mixtures thereof are used. As the thermoplastic resin, the glass transition temperature is −100 to 150 ° C., and the number average molecular weight is 100.
0 to 200,000, preferably 10,000 to 10,000
0, having a degree of polymerization of about 50 to 1,000. Such examples include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylate ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal, There are polymers or copolymers containing vinyl ether, etc. as constituent units, polyurethane resins, and various rubber resins. Also,
Thermosetting resins or reactive resins include phenolic resins, epoxy resins, polyurethane curable resins, urea resins, melamine resins, alkyd resins, acrylic reactive resins, formaldehyde resins, silicone resins, epoxy-polyamide resins, polyester resins and isocyanates Examples include a mixture of a prepolymer, a mixture of a polyester polyol and a polyisocyanate, and a mixture of a polyurethane and a polyisocyanate.

These resins are described in detail in "Plastic Handbook" published by Asakura Shoten. Also, a known electron beam-curable resin can be used for the first layer or the second layer. These examples and the method for producing them are described in detail in JP-A-62-256219. The above resins can be used alone or in combination, but preferred are at least one selected from the group consisting of vinyl chloride resin, vinyl chloride vinyl acetate resin, vinyl chloride vinyl acetate vinyl alcohol resin, vinyl chloride vinyl acetate maleic anhydride copolymer. Examples include a combination of a seed and a polyurethane resin, or a combination of these with a polyisocyanate.

As the structure of the polyurethane resin, known materials such as polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, and polycaprolactone polyurethane can be used. For all of the binders shown here, COO is required to obtain better dispersibility and durability.
M, SO ₃ M, OSO ₃ M, P = O (OM) ₂ , OP
ＯO (OM) ₂ (where M is a hydrogen atom or an alkali metal base), OH, NR ₂ , N ⁺ R ₃ , R is a hydrocarbon group), epoxy group, SH, CN, etc. It is preferable to use one obtained by introducing one or more polar groups by copolymerization or addition reaction. The amount of such a polar group is 10 ^-1 to 10 ^-8 mol / g, preferably ¹ to 10 ^-8 mol / g.
0 ^-2 to 10 ^-6 mol / g.

Specific examples of these binders used in the present invention include those manufactured by Union Carbide: VAGH, V
YHH, VMCH, VAGF, VAGD, VROH, V
YES, VYNC, VMCC, XYHL, XYSG, P
KHH, PKHJ, PKHC, PKFE, manufactured by Nissin Chemical Industries: MPR-TA, MPR-TA5, MPR-TA
L, MPR-TSN, MPR-TMF, MPR-TS,
MPR-TM, manufactured by Denki Kagaku: 1000 W, DX80,
DX81, DX82, DX83, manufactured by Zeon Corporation: MR
110, MR100, 400X110A, manufactured by Nippon Polyurethane Co .: Nipporan N2301, N2302, N23
04, manufactured by Dainippon Ink Co., Ltd .: Pandex T-5105,
T-R3080, T-5201, Barnock D-40
0, D-210-80, Crisbon 6109, 720
9. Toyobo: Byron UR8200, UR830
0, RV530, RV280, manufactured by Dainichi Seika Co., Ltd .: Daifu eramine 4020, 5020, 5100, 5300, 90
20,9022,7020, manufactured by Mitsubishi Kasei: MX500
4. Sanyo Kasei Co., Ltd .: Samprene SP-150; Asahi Kasei Co., Ltd .: Saran F310, F210.

The binder used in the upper recording layer of the present invention is used in an amount of 5 to 50% by weight, preferably 10 to 30% by weight, based on the ferromagnetic powder. It is preferable to use 5 to 30% by weight when using a vinyl chloride resin, 2 to 20% by weight when using a polyurethane resin combination, and 2 to 20% by weight for polyisocyanate in combination.

In the present invention, when a polyurethane resin is used, the glass transition temperature is −50 to 100 ° C., the breaking elongation is 100 to 2000%, and the breaking stress is 0.05 to 10 kg.
/ Cm ² , and the yield point is preferably 0.05 to 10 kg / cm ² . The magnetic recording medium of the present invention has two layers. Therefore,
Amount of binder, amount of vinyl chloride resin, polyurethane resin, polyisocyanate or other resin in binder, molecular weight of each resin forming magnetic layer, amount of polar group, or physical properties of resin described above It is of course possible to change the characteristics and the like between the lower non-recording layer and the upper recording layer as needed.

The polyisocyanate used in the present invention includes tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,
5-diisocyanate, o-toluidiisocyanate,
Isophorone diisocyanate, isocyanates such as triphenylmethane triisocyanate, and products of these isocyanates and polyalcohol,
Polyisocyanates produced by condensation of isocyanates can be used. Commercially available trade names of these isocyanates include: Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR, Millionate MTL, manufactured by Nippon Polyurethane Co., Ltd., and Takenate D-10, manufactured by Takeda Pharmaceutical Co., Ltd.
2, Takenate D-110N, Takenate D-200,
Takenate D-202, manufactured by Sumitomo Bayer: Desmodur L, Desmodur IL, Desmodur N, Desmodur HL, and the like. These may be used alone or in combination of two or more using the difference in curing reactivity. Both the lower non-recording layer and the upper recording layer can be used.

As the carbon black used in the upper recording layer, ie, the magnetic layer of the present invention, furnace black for rubber, thermal for rubber, black for color, acetylene black and the like can be used. Specific surface area is 5 to 500 m ² / g, D
BP oil absorption is 10-400ml / 100g, particle size is 5
mμ ~ 300mμ, pH 2 ~ 10, water content 0.1 ~
The tap density is preferably 10 to 1 g / cc.
Specific examples of the carbon black used in the present invention include BLACKPEARLS200 manufactured by Cabot Corporation.
0, 1300, 1000, 900, 800, 700, V
ULCAN XC-72, manufactured by Asahi Carbon: {80,}
60, $ 55, $ 50, $ 35, manufactured by Mitsubishi Kasei Kogyo:
2400B, $ 2300, $ 900, $ 1000, $ 3
0, $ 40, $ 10B, manufactured by Columbian Carbon: C
ONDUCTEX SC, RAVEN 150, 50,
40, 15 and the like. Carbon black may be used after being surface-treated with a dispersant or the like or grafted with a resin, or may be one obtained by graphitizing a part of the surface. Before adding the carbon black to the magnetic paint, the carbon black may be dispersed in a binder in advance.
These carbon blacks can be used alone or in combination. When carbon black is used, it is preferably used in an amount of 0.1 to 30% of the amount based on the ferromagnetic powder. Carbon black has the functions of preventing static charge of the magnetic layer, reducing the coefficient of friction, imparting light-shielding properties, improving film strength, and the like.
These differ depending on the carbon black used. Therefore, these carbon blacks used in the present invention, the first layer, ie, the lower non-recording layer, the second layer, that is, the type, amount, change the combination in the upper recording layer, particle size, oil absorption, conductivity, Of course, it is possible to use differently according to the purpose based on the above-mentioned various properties such as pH. For example, charging is prevented by using highly conductive carbon black for the lower non-recording layer, and the friction coefficient is reduced by using carbon black having a large particle diameter for the upper recording layer.
The carbon black that can be used in the upper recording layer of the present invention can be referred to, for example, “Carbon Black Handbook” (edited by Carbon Black Association).

Examples of the abrasive used in the upper recording layer of the present invention include α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, and the like. Known materials mainly having a Mohs hardness of 6 or more, such as silicon nitride, silicon carbide, titanium carbide, titanium oxide, silicon dioxide, and boron nitride, are used alone or in combination. Further, a composite of these abrasives (abrasive whose surface is treated with another abrasive) may be used. These abrasives may contain compounds or elements other than the main component, but the main component is 9
If it is 0% or more, the effect is not changed. The particle size of these abrasives is preferably from 0.01 to 2 μm. However, if necessary, abrasives having different particle sizes can be combined, or even a single abrasive can have the same effect by widening the particle size distribution. . Tap density is 0.3-2 g / cc, water content is 0.1-5%, pH is 2-11, specific surface area is 1-30 m
² / g is preferred. The shape of the abrasive used in the present invention may be any of a needle shape, a spherical shape, and a dice shape, but a shape having a part of a corner is preferable because of high abrasiveness.

Specific examples of the abrasive used in the present invention include AKP-20, AKP-30, and Sumitomo Chemical Co., Ltd.
AKP-50, HIT-50, manufactured by Nippon Chemical Industries: G
5, G7, S-1, manufactured by Toda Kogyo KK: 100ED, 140
ED, and the like. The abrasive used in the present invention can be of different types, amounts and combinations for the first layer and the second layer, and can be used properly according to the purpose. These abrasives may be added to the magnetic paint after being subjected to dispersion treatment with a binder in advance. The abrasive present on the surface of the upper recording layer and the end face of the upper recording layer of the magnetic recording medium of the present invention is preferably 5/100 μm ² or more.

As the additives used in 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 with polar groups, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing ester, polyolefin, polyglycol, alkylphosphoric acid Esters and alkali metal salts thereof, alkyl sulfates and alkali metal salts thereof,
Polyphenyl ethers, fluorine-containing alkyl sulfates and alkali metal salts thereof, monobasic fatty acids having 10 to 24 carbon atoms (which may contain an unsaturated bond or may be branched), and metal salts thereof ( Li, N
a, K, Cu etc.) or monovalent having 12 to 22 carbon atoms,
Divalent, trivalent, tetravalent, pentavalent, hexavalent alcohols (which may contain an unsaturated bond or may be branched), alkoxy alcohols having 12 to 22 carbon atoms, 10 to 24 carbon atoms
A monobasic fatty acid (which may contain an unsaturated bond or may be branched) and a monovalent, divalent, 2 to 12 carbon atom,
One of trihydric, tetrahydric, pentahydric, and hexahydric alcohols (may contain unsaturated bonds or be branched)
A monofatty acid ester or difatty acid ester or trifatty acid ester, a fatty acid ester of a monoalkyl ether of an alkylene oxide polymer,
22 fatty acid amides, aliphatic amines having 8 to 22 carbon atoms,
Etc. can be used. 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, octyl myristate, butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan distearate, anhydrosorbitan tristearate, oleyl alcohol , Lauryl alcohol.

Nonionic surfactants such as alkylene oxides, glycerins, glycidols, alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocycles, phosphoniums and Cationic surfactants such as sulfoniums, anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, phosphoric acid, sulfate group, phosphate group, and the like, amino acids, aminosulfonic acids, sulfuric acid of amino alcohol or Ampholytic surfactants such as phosphates and alkylbedynes can also be used. These surfactants are described in detail in "Surfactant Handbook" (published by Sangyo Tosho Co., Ltd.). These lubricants, antistatic agents and the like are not necessarily 100% pure, and may contain impurities such as isomers, unreacted materials, by-products, decomposition products, oxides, etc. in addition to the main components. These impurities are preferably 30% or less, more preferably 10% or less.

These lubricants and surfactants used in the present invention can be selectively used in the lower non-recording layer and the upper recording layer, as required, in the type and amount thereof. For example, the lower non-recording layer and the upper recording layer control bleeding to the surface using fatty acids having different melting points, control bleeding to the surface using esters with different boiling points and polarities, and adjust the amount of surfactant. By doing so, it is conceivable to improve the stability of coating, to increase the amount of lubricant added in the lower non-recording layer to improve the lubricating effect, and of course, the present invention is not limited to the examples shown here.

All or a part of the additives used in the present invention may be added at any step in the production of the magnetic paint. For example, when the additives are mixed with the ferromagnetic powder before the kneading step, the ferromagnetic powder may be added. In a kneading step using a solvent and a binder and a solvent, there is a case where it is added in a dispersion step, a case where it is added after dispersion, and a case where it is added just before coating. Examples of commercial products of these lubricants used in the present invention include NAA-102, NAA-415, and NAA-31 manufactured by NOF Corporation.
2, NAA-160, NAA-180, NAA-17
4, NAA-175, NAA-222, NAA-34,
NAA-35, NAA-171, NAA-122, NA
A-142, NAA-160, NAA-173K, castor-hardened fatty acid, NAA-42, NAA-44, cation SA, cation MA, cation AB, cation BB, Nymeen L-201, Nymeen L-202, Nymeen S-202 , Nonion E-208, Nonion P-20
8, Nonion S-207, Nonion K-204, Nonion NS-202, Nonion NS-210, Nonion HS
-206, nonion L-2, nonion S-2, nonion S-4, nonion O-2, nonion LP-20R, nonion PP-40R, nonion SP-60R, nonion O
P-80R, Nonion OP-85R, Nonion LT-2
21, Nonion ST-221, Nonion OT-221,
Monogly MB, Nonion DS-60, Anone BF, Anone LG, butyl stearate, butyl laurate, erucic acid, manufactured by Kanto Kagaku: oleic acid, manufactured by Takemoto Yushi: FA
L-205, FAL-123, manufactured by Shin Nippon Rika Co., Ltd .: Engelbu LO, Engelbu IPM, Sansocizer E4
030, manufactured by Shin-Etsu Chemical Co., Ltd .: TA-3, KF-96, KF-
96L, KF-96H, KF410, KF420, KF
965, KF54, KF50, KF56, KF-90
7, KF-851, X-22-819, X-22-82
2, KF-905, KF-700, KF-393, KF
-857, KF-860, KF-865, X-22-9
80, KF-101, KF-102, KF-103, X
-22-3710, X-22-3715, KF-91
0, KF-3935, manufactured by Lion Armor: Armide P, Armide C, Armoslip CP, manufactured by Lion Yushi: Duomin TDO, manufactured by Nisshin Oil Co., Ltd .: BA-41
G, manufactured by Sanyo Kasei Co., Ltd .: Profan 2012E, Newpole PE61, Ionnet MS-400, Ionnet MO
-200, IONET DL-200, IONET DS-
300, IONET DS-1000, IONET DO-
200 and the like.

The organic solvent used in the present invention may be any ratio of ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone and tetrahydrofuran, methanol, ethanol, propanol, butanol, isobutyl alcohol and isopropyl alcohol. Alcohols such as methylcyclohexanol, esters such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, and glycol acetate; glycol ethers such as glycol dimethyl ether, glycol monoethyl ether, and dioxane; benzene; Aromatic hydrocarbons such as toluene, xylene, cresol, chlorobenzene, methylene chloride, ethylene chloride, carbon tetrachloride, Chloroform, ethylene chlorohydrin, dichlorobenzene, chlorinated hydrocarbons and the like, N,
N-dimethylformamide, hexane and the like can be used. These organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted products, by-products, decomposed products, oxides, and moisture in addition to the main components. These impurities are preferably 30% or less, more preferably 10% or less. The kind and amount of the organic solvent used in the present invention may be changed between the upper recording layer and the lower non-recording layer if necessary. Improve the surface properties by using a highly volatile solvent for the lower non-recording layer, improve the coating stability by using a solvent with high surface tension (cyclohexanone, dioxane, etc.) for the upper recording layer, and adjust the solubility parameter of the upper recording layer. Examples thereof include increasing the degree of filling using a high solvent, but it is a matter of course that the present invention is not limited to these examples.

The thickness of the magnetic recording medium of the present invention is such that the nonmagnetic support is 1 to 100 μm, preferably 4 to 80 μm, and the lower non-recording layer is 0.5 to 10 μm, preferably 1 to 5 μm.
μm, the upper recording layer is preferably 0.05 μm or more and 1.0
μm or less, more preferably 0.05 μm or more and 0.6 μm
m or less, particularly preferably 0.05 μm or more, 0.3 μm
It is as follows. The total thickness of the upper recording layer and the lower non-recording layer is 1/100 to 2 times the thickness of the nonmagnetic support. Further, an undercoat layer may be provided between the non-magnetic support and the lower non-recording layer to improve adhesion. Their thickness is 0.01-2 μm, preferably 0.05-0.
5 μm. Further, a back coat layer may be provided on the non-magnetic support on the side opposite to the magnetic layer side. This thickness is 0.1 to 2 μm, preferably 0.3 to 1.0 μm. Known undercoating layers and backcoat layers can be used.

The support used in the present invention is non-magnetic
Preferably, polyethylene terephthalate, polyether
Polyesters such as styrene naphthalate, polyolefin
Ins, cellulose triacetate, polycarbonate
, Polyamide, polyimide, polyamideimide, poly
Known sulfon, aramid, aromatic polyamide and the like
Film can be used. These supports
Corona discharge treatment, plasma treatment, easy adhesion treatment, heat treatment,
For example, dust removal processing may be performed. Achieved the purpose of the present invention
To achieve this, the centerline average surface roughness of the non-magnetic support
0.03 μm or less, preferably 0.02 μm or less,
It is necessary to use a thing of 0.01 μm or less
is there. In addition, these non-magnetic supports are simply referred to as the center line average table.
Not only is the surface roughness small, but coarse protrusions of 1 μm or more
Preferably not. Also, the surface roughness shape may be
Free depending on the size and amount of filler added to the support
Is controlled by With these fillers
For example, oxides such as Ca, Si, Ti, etc.
In addition to acid salts, organic fine powders such as acrylics can be used.
F of the non-magnetic support used in the present invention in the tape running direction
The -5 value is preferably 5 to 50 kg / mm.^Two , Tape width
The F-5 value in the direction is preferably 3 to 30 kg / mm ^Two In
F-5 value in the longitudinal direction of the tape is F-5 in the tape width direction.
Generally, the strength is higher than the
This is not the case when it is necessary to do so.

The heat shrinkage of the nonmagnetic support in the tape running direction and the width direction at 100 ° C. for 30 minutes is preferably 3
%, More preferably 1.5% or less, and the heat shrinkage at 80 ° C. for 30 minutes is preferably 1% or less, more preferably 0.5% or less. Breaking strength 5-100 in both directions
Kg / mm ² , elastic modulus is 100-2000 Kg / mm ²
Is preferred.

The step of producing the magnetic paint of the magnetic recording medium of the present invention comprises at least a kneading step, a dispersion step, and a mixing step provided before and after these steps as necessary. Each step may be divided into two or more steps. All the raw materials such as the ferromagnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant, and solvent used in the present invention may be added at the beginning or during any step. Further, individual raw materials may be added in two or more steps in a divided manner. For example, polyurethane may be divided and supplied 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, it is a matter of course that a conventional known manufacturing technique can be used as a part of the process, but in the kneading step, a strong kneading force such as a continuous kneader or a pressure kneader is used. By using the magnetic recording medium, it is possible to obtain a high Br of the magnetic recording medium of the present invention. When a continuous kneader or a pressure kneader is used, all or a part of the ferromagnetic powder and the binder (preferably 30% or more of the total binder) and 15 to 500 parts of kneading with 100 parts of the ferromagnetic powder are kneaded. It is processed. For details of these kneading processes, see JP-A-1-106338,
It is described in JP-A-64-79274.

In the present invention, more efficient production can be achieved by using a simultaneous multilayer coating method as disclosed in JP-A-62-212933. The following configuration can be proposed as an example of an apparatus and method for applying a magnetic recording medium having a multilayer configuration as in the present invention. 1. First, a lower non-recording layer is applied by a gravure coating, roll coating, blade coating, extrusion coating device or the like generally used in the application of a magnetic paint, and the lower non-recording layer is placed in a wet state while the coating is performed. No. and JP-A-60-238
No. 179 and JP-A-2-265672, the upper recording layer is coated by a support pressure type extrusion coating apparatus. 2. The upper recording layer is coated almost simultaneously by one coating head having two built-in coating liquid passage slits as disclosed in JP-A-63-88080, JP-A-2-17921 and JP-A-2-265672. I do. 3. The upper recording layer is coated almost simultaneously by an extrusion coating device with a backup roll disclosed in JP-A-2-174965. Incidentally, in order to prevent the electromagnetic conversion characteristics of the magnetic recording medium from deteriorating due to the aggregation of the ferromagnetic powder, Japanese Patent Application Laid-Open Nos. Sho 62-95174 and Hei 1-2
It is desirable to apply shear to the coating liquid inside the coating head by a method as disclosed in US Pat. No. 36968.

In order to obtain the medium of the present invention, it is necessary to perform strong orientation. 1000G or more solenoid and 200
It is preferable to use a cobalt magnet of 0 G or more in combination, and it is preferable to provide an appropriate drying step before orientation so that the orientation after drying is the highest.

Further, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, or polyimide amide is used as the calendering roll. Further, the treatment can be performed between metal rolls. The processing temperature is preferably 70 ° C. or higher, more preferably 80 ° C. or higher. The linear pressure is preferably 200 kg / cm, more preferably 300 kg / cm or more, and the speed is 20 m / cm.
/ Min to 700 m / min.

The friction coefficient of the upper recording layer surface of the magnetic recording medium of the present invention and the opposite surface to SUS420J is preferably 0.5 or less, more preferably 0.3 or less, and the surface resistivity is preferably 10 ^-5 to 10 ^-12 ohm. / Sq, the elastic modulus at 0.5% elongation of the upper recording layer is preferably 100 to 2000 kg / mm ^{2 in} the running direction and the width direction, the breaking strength is preferably 1 to 30 kg / cm ² , and the elastic modulus of the magnetic recording medium. Is preferably 100 to 1500 K in both the running direction and the longitudinal direction.
g / mm ² , and the residual growth is preferably 0.5% or less.
Thermal shrinkage at any temperature below 0 ° C. is preferably 1%
Or less, more preferably 0.5% or less, most preferably 0.1% or less.

The residual solvent contained in the upper recording layer is preferably 100 mg / m ² or less, more preferably 10 mg / m ² or less.
/ M ² or less, and it is preferable that the residual solvent contained in the upper recording layer is smaller than the residual solvent contained in the lower non-recording layer.

The magnetic characteristics of the magnetic recording medium of the present invention are as follows.
When measured by KOe, the squareness ratio in the tape running direction is 0.1.
70 or more, preferably 0.80 or more, and more preferably 0.90 or more. The squareness ratio in two directions perpendicular to the tape running direction is preferably 80% or less of the squareness ratio in the running direction. The SFD of the magnetic layer is preferably 0.6 or less.

Although the magnetic recording medium of the present invention has an upper recording layer and a lower non-recording layer, it is easily presumed that these physical characteristics can be changed between the lower non-recording layer and the upper recording layer according to the purpose. That is. For example, the elastic modulus of the upper recording layer is increased to improve running durability, and at the same time, the elastic modulus of the lower non-recording layer is made lower than that of the upper recording layer to improve the contact of the magnetic recording medium with the head.

[0063]

Hereinafter, specific examples of the present invention will be described.
The present invention is not limited by this. still,
“Parts” indicates parts by weight.

Hereinafter, specific examples of the present invention will be described. Examples 1-1 to 1-6, Comparative Examples 1-1 to 1-3 Lower non-recording layer Cobalt-modified iron oxide 0 or 100 parts Hc 950 Oe, specific surface area 58 m ² / g Crystallite size 250Å Particle size (major axis diameter) ) 0.20, 8 vinyl chloride acicular ratio - vinyl acetate - vinyl alcohol copolymer 13 parts ^{_{-N (CH 3) 3 + Cl}} - polar groups of 5 × 10 ^-6 eq / g containing polyester polyurethane resin 11 parts Neopentyl glycol / caprolactone polyol / MDI = 0.9 / 2.6 / 1-SO ₃ Na group 1 × 10 ⁻⁴ eq / g containing α-iron oxide needles See Tables 1 and 2, major axis length 0.3 μm, the ratio by BET surface area of 45m ² / g pH3.5 α- alumina (particle size 0.3 [mu] m) 5 parts carbon black 100 or 500 parts average primary particle size 16Emumyu DBP oil absorption of 80 ml / 100 g pH 8.0 BET method A specific surface area of 250 meters ² / g Volatiles 1.5% Butyl stearate 1 part Stearic acid 2 parts Methyl ethyl ketone 400 parts upper recording layer ferromagnetic metal powder composition Fe / Zn / Ni = 92/ 4/4 100 parts Hc 1600 Oe, BET Specific surface area by method 60 m ² / g Crystallite size 195Å Particle size (major axis diameter) 0.20 μ, needle ratio 10 σ _S : 130 emu / g Vinyl chloride copolymer 12 parts -SO ₃ Na content: 1 × 10 ^-4 eq / g polymerization degree 300 polyester polyurethane resin 3 parts neopentyl glycol / caprolactone polyol / MDI = 0.9 / 2.6 / 1-SO ₃ Na group 1 × 10 ^-4 eq / g containing α-alumina ( Particle size 0.3 μm) 2 parts Carbon black (particle size 0.10 μm) 0.5 parts Butyl stearate 1 part Stearic acid 2 parts Methyl ethyl Ketone 200 parts For each of the above two paints, each component was kneaded with a continuous kneader, and then dispersed using a sand mill.
To the resulting dispersion, 6 parts of polyisocyanate was added to the coating solution for the lower non-recording layer, 3 parts to the coating solution for the upper recording layer, 40 parts of butyl acetate was added, and a filter having an average pore diameter of 1 μm was added. Then, a coating solution for forming a lower non-recording layer and a coating solution for forming an upper recording layer were respectively prepared.
The A10 ⁴ / A10 ratio of the coating solution for the lower non-recording layer is also shown.

The obtained coating solution for the lower non-recording layer is coated so that the thickness after drying becomes 2 μm, and immediately thereafter, the thickness of the upper recording layer is further reduced to 0.3 μm. 7μ
m and a centerline surface roughness of 0.01μ. Polyethylene terephthalate support is coated simultaneously, and while both layers are still wet, they are oriented by a cobalt magnet having a magnetic force of 3000G and a solenoid having a magnetic force of 1500G. After drying, the film was treated at a temperature of 90 ° C. with a seven-stage calender composed of only metal rolls, slit to a width of 8 mm, and produced an 8 mm video tape.

Example 1-7 In the above example, the ferromagnetic powder of the upper recording layer was replaced with the following C
Samples were prepared under the conditions shown in Table 2 in the same manner as described above except that o-modified γ-Fe ₂ O ₃ was used.

Co-modified γ-Fe ₂ O ₃ : Hc 1400 Oe, specific surface area by BET method 45 m ²
/ G Crystallite size 290 ° Particle size (major axis diameter) 0.3 μm, needle ratio 10 σ _S : 75 emu / g The above samples were evaluated as follows, and the results were shown in Tables 1 and 2.
Shown in Evaluation method Electromagnetic conversion characteristics 7 MHz output A 7 MHz signal was recorded using a FUJIX8, 8 mm video deck manufactured by Fuji Photo Film Co., Ltd., and a 7 MHz signal reproduction output when this signal was reproduced was measured with an oscilloscope. Yield The percentage of non-defective products after slitting was completed. Usually, 97% or more is required.

[0068]

[Table 1]

[0069]

[Table 2]

As is clear from the results of Tables 1 and 2, Examples 1-1 to 1-6 of the present invention were improved in both output and yield at 7 MHz, and Comparative Example 1 out of the scope of the present invention. In Examples 1 to 3, the output at 7 MHz or the sufficient yield could not be obtained.

[0071]

According to the present invention, the thixotropy of the coating solution for the lower non-recording layer and the coating solution for the upper recording layer are determined, particularly the physical properties and composition of the non-magnetic or magnetic powder of the coating solution for the lower non-recording layer. By doing so, it is possible to easily provide a magnetic recording medium having excellent yield, durability and excellent electromagnetic conversion characteristics, which can be thinned by a simultaneous multi-layer or sequential wet coating method.

 ──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-4-238111 (JP, A) JP-A-4-283416 (JP, A) JP-A-4-321924 (JP, A) JP-A-4-321924 325915 (JP, A) JP-A-4-325917 (JP, A) JP-A-5-12650 (JP, A) JP-A-62-212922 (JP, A)

Claims (10)

(57) [Claims] A magnetic recording medium comprising: a lower non-recording layer in which a powder is dispersed in a binder on a support; and an upper recording layer in which a ferromagnetic powder is dispersed in a binder. The dry thickness of the upper recording layer is 1.0 μm or less, the lower non-recording layer contains a magnetic powder and a non-magnetic material as the powder,
And a saturation maximum magnetic flux density of the lower non-recording layer is 500 gauss or less. 2. The method according to claim 1, wherein said lower non-recording layer has a maximum saturation magnetic flux density of 3
2. The magnetic recording medium according to claim 1, wherein the magnetic recording medium has a value of 0 to 500 Gauss. 3. The magnetic recording medium according to claim 1, wherein the dispersion of the lower non-recording layer in which the powder is dispersed in a binder has thixotropic properties. 4. The Hc of the upper recording layer is from 1200 to 300.
2. The magnetic recording medium according to claim 1, wherein the magnetic recording medium is 0 Oe. 5. The magnetic recording medium according to claim 1, wherein a crystallite size of the ferromagnetic powder in the upper recording layer is 450 to 100 Å. 6. The ferromagnetic powder of the upper recording layer is a ferromagnetic alloy powder containing Fe, Ni or Co as a main component, and in addition to these atoms, Al, Si, S, Sc, Ti, V, C
r, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb,
Te, Ba, Ta, W, Re, Au, Pb, Bi, L
a, Ce, Pr, Nd, P, Co, Mn, Zn, Ni,
2. The magnetic recording medium according to claim 1, comprising at least one kind of atom selected from Sr and B. 7. The powder of the lower non-recording layer has a Mohs hardness of 4.
2. The method according to claim 1, wherein said non-magnetic inorganic powder is contained.
3. The magnetic recording medium according to claim 1. 8. The powder of the lower non-recording layer has an α conversion of 90%.
The above α-alumina, β-alumina, γ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, silicon nitride, titanium carbide, titanium oxide,
2. The method according to claim 1, further comprising at least one non-magnetic inorganic powder selected from silicon dioxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, and molybdenum sulfide. The magnetic recording medium according to the above. 9. The elastic modulus of the support is 100 to 2000.
^2. The magnetic recording medium according to claim 1, wherein Kg / mm ² . 10. The support comprises polyethylene terephthalate, polyethylene naphthalate polyesters, polyolefins, cellulostriacetate, polycarbonate.
2. The magnetic recording medium according to claim 1, wherein the magnetic recording medium is at least one selected from the group consisting of boron, polyamide, polyimide, polyamideimide, polysulfone, aramid, and aromatic polyamide.