JPH11288514A - Production of magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To permit high-density high-capacity recording with a magnetic recording medium of a double layer coating type by improving the electromagnetic conversion characteristics in a high-density recording region and making it possible to obtain good traveling durability even when the total thickness of the medium is reduced. SOLUTION: A coating material for an upper layer is prepd. by adding a polishing agent slurry formed by dispersing a polishing agent and a binder into a solvent to a magnetic coating material prepd. by kneading ferromagnetic powder and a binder together with a solvent. The coating material is applied on a nonmagnetic layer. At this time, the polishing agent incorporated into the polishing agent slurry is formed of inorg. powder having Mohs hardness of >=6 and an average primary grain size of <=0.10 μm. The central grain size of the polishing agent in the polishing agent slurry is specified to <0.20 μm just before the agent is added to the magnetic coating material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a multilayer coating type magnetic recording medium, and more particularly to an improvement in electromagnetic conversion characteristics and running durability in a high-density and large-capacity recording area.

[0002]

2. Description of the Related Art As a magnetic recording medium, a magnetic layer formed by dispersing a ferromagnetic powder, a binder, and various additives together with an organic solvent onto a non-magnetic support is dried to form a magnetic layer. A so-called coating type magnetic recording medium is known, and metal fine particles are used as the ferromagnetic powder for the purpose of high density recording.

A coating type magnetic recording medium using such metal fine particles is used as a recording medium for computers such as a magnetic tape for audio or video, a high-density floppy disk, a backup data cartridge, and the like. At the same time as becoming the mainstream of magnetic recording media, there has been a remarkable improvement in characteristics.

By the way, in order to realize high-density recording of a coating type magnetic recording medium, metal fine particles are used as ferromagnetic powder, the surface of the medium is super-smoothed, and a spacing loss is minimized. It is also important to reduce output loss due to recording demagnetization.

[0005] Techniques for achieving these objects include (1)
Increase of coercive force and saturation magnetization of ferromagnetic powder, (2) uniformity of coercive force distribution of ferromagnetic powder, (3) provision of perpendicular anisotropy,
(4) Thinning the magnetic layer and the like.

Among them, (1) and (2) are techniques for directly improving the output. In order to improve the coercive force and the saturation magnetization, the element composition of the ferromagnetic powder has been studied, and the fine metal particles having a coercive force exceeding 160 kA / m, and the fine metal particles having a saturation magnetization exceeding 140 Am ³ / kg have been studied. Are also being developed. Also, the coercive force distribution reflects the particle size distribution of the ferromagnetic powder,
By making the particle size uniform, the coercive force distribution is also significantly improved.

The method (3) of imparting perpendicular anisotropy is a technique for increasing the density by perpendicular magnetic recording. This is largely due to the control of the magnetic orientation of the ferromagnetic powder in the case of a coating type magnetic recording medium. For example, when using acicular particles, it has been attempted to perform a vertical alignment treatment or an oblique alignment treatment on the coating film. However, due to problems such as difficulty in controlling the orientation and disorder of the coating film surface due to the orientation, these orientation treatments have not yet become practical.

Next, the method (4) for reducing the thickness of the magnetic layer is considered to be very effective as a method for reducing self-demagnetization loss.

Here, when the thickness of the magnetic layer is simply reduced to, for example, 1 μm or less, the surface shape of the non-magnetic support tends to appear on the surface of the magnetic layer, and it becomes difficult to smooth the surface of the magnetic layer. For this reason, in the case where the magnetic layer is thinned, a multi-layer coating type configuration in which a non-magnetic coating layer is interposed between the non-magnetic support and the magnetic layer is often adopted. By interposing the non-magnetic layer in this way, a thickness is obtained between the surface of the non-magnetic support and the surface of the magnetic layer, and the surface shape of the non-magnetic support is less likely to appear on the surface of the magnetic layer. Therefore, a thin magnetic layer is formed with a smooth surface shape.

Various improvements have been proposed for the multilayer coating type magnetic recording medium. For example, a method in which the coating thickness of the lower non-magnetic layer is set to 0.5 μm to 3.5 μm (JP-A-63-187418). Japanese Patent Application Laid-Open No. Hei 4-2) and a method in which an appropriate amount of carbon black is contained in a lower nonmagnetic layer.
38111), a method of coating the surface of the non-magnetic oxide of the lower non-magnetic layer with an inorganic substance (JP-A-5-182177), and using two or more types of non-magnetic powders having different sizes in the lower non-magnetic layer. (Japanese Unexamined Patent Publication No. Hei 5-274651), a method for regulating the standard deviation of the thickness of the upper magnetic layer within a specific range (Japanese Unexamined Patent Publication No. Hei 5-298655), and a method in which the upper magnetic layer is composed of two or more magnetic layers. (Japanese Unexamined Patent Publication No. 6-1
62485, JP-A-6-162489) and the like.

A method for forming the lower non-magnetic layer and the upper magnetic layer has also been studied. A die head having two slits through which a non-magnetic paint and a magnetic paint are respectively extruded is used. A simultaneous multilayer coating method (wet-on-wet coating method) in which a nonmagnetic paint and a magnetic paint are simultaneously applied has been proposed. In this simultaneous multilayer coating method, a coating film having a uniform thickness with few coating defects and coating streaks can be formed. Therefore, a medium with excellent electromagnetic conversion characteristics and low noise can be obtained. Further, the formed lower layer and upper layer have high adhesion, and excellent durability can be obtained.

In this simultaneous multi-layer coating method, it is important to adjust the coating properties of the upper and lower layers. From such a viewpoint, a method of using a poor solvent for the binder as a solvent for forming the paint for the upper layer and the paint for the lower layer (Japanese Patent Application Laid-Open No.
No. 31028) and a method of matching the solubility parameters of the upper layer paint and the lower layer paint (JP-A-3-1195).
No. 18), a method for matching the Reynolds numbers of the paint for the upper layer and the paint for the lower layer (JP-A-4-271016),
A method for imparting the same or similar thixotropy to the upper layer paint and the lower layer paint (Japanese Patent Application Laid-Open No. 4-325917), a method for fitting the flow curve of the paint to a specific formula (Japanese Patent Application Laid-Open No. 5-128496), A method for defining an index (Japanese Patent Application Laid-Open No. 5-208165) and a method for specifying the amount of creep deformation of paint (Japanese Patent Application Laid-Open No.
JP-A-195690), a method of making the ratio of the maximum value and the minimum value of the loss elasticity term of a coating material constant (JP-A-5-26646).
No. 3) has been proposed.

Further, since the surface of the multi-layer coating type magnetic recording medium is formed very smooth, the running area on the recording / reproducing apparatus inevitably increases the contact area with various sliding members. The coefficient of friction with respect to the sliding member becomes a large value. For this reason, it is difficult to obtain running durability. In recent years, in the case of magnetic tapes, the total thickness of the medium has been reduced in order to increase the tape length that can be accommodated in a cassette and increase the recording capacity per cassette. This makes it more difficult to obtain durability.

For this reason, a method of adjusting the amount of the lubricant in the upper layer (JP-A-1-224919, JP-A-5-1831)
No. 78), a method of using a fluorine-based lubricant as a lubricant (JP-A-2-192018, JP-A-5-29).
No. 8679) has also been proposed.

[0015]

As described above, various improvements have been proposed for the magnetic recording medium of the multi-layer coating type. In particular, the electromagnetic conversion characteristics in the high-density recording area and the running when the thickness is reduced. Lack of knowledge on durability requires further study.

Accordingly, the present invention has been proposed in view of such a conventional situation, and a good electromagnetic conversion characteristic is obtained in a high-density recording area, and a good electromagnetic conversion characteristic is obtained even when the total thickness of the medium is reduced. It is an object of the present invention to provide a method for manufacturing a magnetic recording medium that can achieve high running durability and perform high-density, large-capacity recording.

[0017]

In order to achieve the above-mentioned object, a method for manufacturing a magnetic recording medium according to the present invention comprises a method of manufacturing a magnetic paint obtained by kneading a ferromagnetic powder and a binder together with a solvent, an abrasive and a binder. An upper layer paint is prepared by adding an abrasive slurry obtained by dispersing an agent in a solvent, and in a method for manufacturing a magnetic recording medium in which this is applied on a non-magnetic layer, the abrasive contained in the abrasive slurry is An inorganic powder having a Mohs hardness of 6 or more and an average primary particle size of less than 0.10 μm, and a center particle size of the abrasive in the abrasive slurry of less than 0.20 μm immediately before being added to the magnetic paint. It is a feature.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described below.

In the magnetic recording medium to which the present invention is applied, a lower non-magnetic layer in which a non-magnetic powder is dispersed in a binder is formed on a non-magnetic support, and a ferromagnetic layer is formed on the lower non-magnetic layer. This is a multilayer coating type magnetic recording medium which is formed by forming an upper magnetic layer in which a powder is dispersed in a binder.

In this multilayer coating type magnetic recording medium,
The upper magnetic layer and the lower non-magnetic layer are prepared by kneading and dispersing a ferromagnetic powder and a binder together with an organic solvent, and kneading an upper layer magnetic paint, a non-magnetic powder and a binder together with an organic solvent.
It is formed by applying and drying a non-magnetic paint for the lower layer prepared by dispersion.

In order to enable high-density, large-capacity recording in such a multilayer coating type magnetic recording medium, (a) fine metal powder is used as the magnetic powder, and the fine metal powder is placed in the upper magnetic layer. Disperse well and make high filling,
(B) The lower non-magnetic layer is designed to have a smooth surface, and (c) the thickness of the upper magnetic layer, the thickness of the lower magnetic layer, and the total thickness of the medium are reduced. is important.

In the present invention, from such a viewpoint, a metal magnetic powder is used as the magnetic powder for the upper magnetic layer.
In addition to regulating the major axis length, the type of binder, the hardness and particle size of the abrasive, the kneading conditions of the paint, and the surface roughness R
a, regulate the thickness. For the lower non-magnetic layer,
The major axis length, needle ratio (major axis length / minor axis length), type, and type of binder contained in the nonmagnetic powder are regulated. Further, the method of forming the upper magnetic layer and the lower nonmagnetic layer, and the thickness and Young's modulus of the nonmagnetic support are regulated.

Hereinafter, these will be described in detail.

First, the upper magnetic layer is selected from the viewpoint that the ferromagnetic powder is uniformly dispersed in the magnetic layer as described above, and the magnetic layer is highly filled.

First, as the ferromagnetic powder, Fe, Co,
Metals such as Ni, Fe-Co, Fe-Ni, Fe-Al,
Fe-Ni-Al, Fe-Al-P, Fe-Ni-Si
-Al, Fe-Ni-Si-Al-Mn, Fe-Mn-
Zn, Fe-Ni-Zn, Co-Ni, Co-P, Fe
-Co-Ni, Fe-Co-Ni-Cr, Fe-Co-
Ni-P, Fe-Co-B, Fe-Co-Cr-B, M
Powders composed of alloys such as n-Bi, Mn-Al, Fe-Co-V, iron nitride, iron carbide and the like are used alone or in combination of two or more. These ferromagnetic powders include Al, Si, and the like for the purpose of preventing sintering during reduction or maintaining the shape.
Light metal elements such as P and B may be contained in an appropriate amount. Among these, metal magnetic powders are preferable, and those in which at least one of Al and Si is added to Fe alone or an alloy of Fe-Co, Fe-Ni, and Fe-Co-Ni for the purpose of preventing sintering. General.

The specific surface area of these ferromagnetic powders is from 20 to 9
0 m ² / g, preferably it is said that it is desirable that 25~70m ² / g. This is because when the specific surface area is in this range, the ferromagnetic powder is often relatively fine particles, the noise characteristics are improved, and high-density recording becomes possible.

However, in the case of metal powder, it is difficult to say that the specific surface area directly reflects the particle size. This is for the following reason.

That is, assuming that the fine structure of the metal powder is cylindrical, the metal powder has a three-layer structure of a pure metal cylinder, an iron oxide shell, and a light metal shell from the inside. In a metal powder, the overall density (average density) is easily changed by slightly changing the ratio of these three layers. Therefore, the specific surface area expressed by the surface area per unit weight also changes depending on the ratio of these three layers regardless of the actual surface area.

Further, the metal particles have irregularities formed on the surface during the slow oxidation process. For this reason, the size of the unevenness increases the surface area irrespective of the size of the particles themselves. Because of the influence of the density and the influence of the surface irregularities, it is not possible to say that metal fine particles are fine particles merely because of their large specific surface area.

Therefore, when evaluating the particle size of metal particles, it is more practical to measure the particle size directly.

In the present invention, based on the above considerations, the average value L and the standard deviation σ of the major axis length 1 of the metal particles measured from the transmission electron micrograph are 0.01 μm <L ± 2σ <0.
It is regulated so as to satisfy the condition of 33 μm. By using metal particles satisfying such conditions for the upper magnetic layer, a magnetic recording medium having excellent electromagnetic conversion characteristics and noise characteristics can be obtained. L ± 2σ of metal particles is 0.01 μm
In the case of the following, dispersion becomes extremely difficult in the preparation stage of the magnetic paint and superparamagnetism is feared. When L ± 2σ is 0.33 μm or more, particles that do not contribute to recording and reproduction, so-called Particle Length Loss, increase, and the noise characteristics may be degraded.

Further, it is desirable that the metal particles are small as long as the acicular ratio maintains the shape anisotropy. Usually, an acicular ratio of 2 or more and 15 or less is selected, and more preferably an acicular ratio of 3 or more and less than 10. When the acicular ratio is less than 2, the orientation of the ferromagnetic powder decreases, and the output decreases. If the needle ratio exceeds 15, the short-wavelength signal output may be reduced.

One type of ferromagnetic powder may be used alone, or two or more types may be used in combination.

Next, the binder used in the upper magnetic layer will be described.

In general, when the particles dispersed in the binder become finer, the voids between the particles become finer as long as the geometrical arrangement does not change. In order to wet such a fine space between the particles and uniformly coat and disperse the particles with a binder,
It is necessary that the binder has a strong affinity for the ferromagnetic powder and has excellent fluidity.

Therefore, in the present invention, the average degree of polymerization is 180
In the following, a vinyl chloride copolymer containing a sulfonic acid metal salt as a polar group is used as a binder for the upper layer.
The vinyl chloride copolymer satisfies the requirements described above, and can uniformly disperse the ferromagnetic powder. The average degree of polymerization is optimized mainly from the viewpoint of the fluidity of the binder. When the average degree of polymerization is 180 or more, the fluidity of the binder deteriorates, and the dispersing ability for fine particles decreases. Further, a metal sulfonic acid salt is introduced to improve the affinity for the ferromagnetic powder. As the sulfonic acid metal salt, an alkali metal salt such as lithium, potassium, and sodium is selected.

The binder for the magnetic layer does not necessarily need to be composed of the vinyl chloride copolymer alone. If the vinyl chloride-based copolymer is contained in an amount of 50% by weight or more, another type of binder is used in combination, and the running properties, durability, contact with the head, coating strength, stiffness, and base of the magnetic recording medium are used. Practical characteristics such as adhesiveness may be improved.

As such a binder, known thermoplastic resins, thermosetting resins, reactive resins and the like conventionally used as a binder for magnetic recording media are used, and particularly, the number average molecular weight is 5,000 to 5,000. 100,000 is preferred.

Examples of the thermoplastic resin include vinyl chloride, vinyl acetate, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylate-acrylonitrile copolymer, Acrylate-vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylate-acrylonitrile copolymer,
Acrylate-vinylidene chloride copolymer, methacrylate-vinylidene chloride copolymer, methacrylate-vinyl chloride copolymer, methacrylate-ethylene copolymer, polyvinyl fluoride, vinylidene chloride-acrylonitrile copolymer Coalescence, acrylonitrile-butadiene copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose), styrene butadiene copolymer, polyurethane resin, polyester resin , Amino resin, synthetic rubber and the like.

As the thermosetting resin, phenol resin, epoxy resin, polyurethane curable resin, urea resin, melamine resin, alkyd resin, silicone resin,
Polyamine resins, urea-formaldehyde resins and the like can also be used.

All of the above binders have -SO ₃ M, -OSO for the purpose of improving the dispersibility of the non-magnetic pigment.
₃ M, -COOM, P = O (OM) ₂ (where M represents a hydrogen atom or an alkali metal such as lithium, potassium, sodium, etc.), -NR ₁ R ₂ , -NR ₁ R ₂ R ₃ ⁺ X
^- represented by the side chain ^{_{amine,> NR 1 R 2 + X}} - main-chain amine represented by (wherein, wherein R _1, R _2, R ₃ represents a hydrogen atom or a hydrocarbon group, X ^- fluorine, chlorine, bromine,
It represents a halogen element ion such as iodine or an inorganic ion or an organic ion), and a polar functional group such as -OH, -SH, -CN, or an epoxy group may be introduced. The amount of these polar functional groups introduced into the binder is preferably from 10 ^-1 to 10 ^-8 mol / g, more preferably from 10 ^-2 to 10 ^-6 mol / g.

In order to make the dispersion of the ferromagnetic powder uniform, not only the type of binder but also the conditions for kneading the magnetic paint are important.

First, it is desirable that the magnetic coating material has a weight fraction of the non-volatile component in the kneading step of 80% or more and 90% or less. If the weight fraction of the non-volatile component (solid powder component) in the kneading step is less than 80%, sufficient pressure is not applied to the paste during kneading, and the surface of the ferromagnetic powder is completely covered with the binder. Is difficult. On the other hand, if the weight fraction of the non-volatile component exceeds 90%, the fluidity of the binder is lost, so that the surface of the ferromagnetic powder cannot be uniformly coated.

[0044] Solvents used for coating are acetone, methyl ethyl ketone, methyl isobutyl ketone,
Ketone solvents such as cyclohexanone, alcohol solvents such as methanol, ethanol and propanol, methyl acetate, ethyl acetate, butyl acetate, propyl acetate, ethyl lactate, ester solvents such as ethylene glycol acetate, diethylene glycol dimethyl ether, 2-ethoxyethanol, Ether solvents such as tetrahydrofuran and dioxane; aromatic hydrocarbon solvents such as benzene, toluene, and xylene; and halogenated hydrocarbon solvents such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, and chlorobenzene. Are appropriately mixed and used.

As the kneading apparatus, conventionally known kneading machines such as a continuous twin-screw kneader, a continuous twin-screw kneader capable of being diluted in multiple stages, a kneader, a pressure kneader and a roll kneader can be used, and there is no limitation. Not something.

As the dispersing device, any of a roll mill, a ball mill, a horizontal sand mill, a vertical sand mill, a spike mill, a pin mill, a tower mill, a DCP, an agitator, a homogenizer, and an ultrasonic disperser can be used.

The magnetic layer may contain various additives commonly used in magnetic recording media in addition to the ferromagnetic powder and the binder. Among the additives, the abrasive has a particle size of the upper magnetic layer. Strongly affects the surface roughness Ra. not to mention,
As the fine particles are used, the surface roughness Ra of the medium is improved.
Therefore, it is desirable to use an inorganic powder having a Mohs hardness of 6 or more and an average primary particle size of less than 0.10 μm as the abrasive. The average primary particle size can be measured by a transmission electron microscope photograph.

Here, when the abrasive is added, it may be added as a powder in the kneading step of the magnetic paint, but the abrasive is dispersed in a solvent together with a binder to prepare an abrasive slurry, which is then added to the magnetic paint. Is preferable at the kneading stage because dispersibility is improved. However, it is desirable that the abrasive slurry has a center particle diameter of the abrasive less than 0.20 μm immediately before being added. The center particle size of the abrasive in this slurry can be measured by a laser beam particle size measuring device.

Below the upper magnetic layer, a lower nonmagnetic layer in which nonmagnetic powder is dispersed in a binder is formed. It is known that a multilayer coating type magnetic recording medium has a strong correlation between the surface property of the lower layer and the surface property of the upper layer, that is, the surface roughness of the lower layer and the surface roughness of the upper layer. It is indispensable to form a smooth lower surface in order to form. Therefore, the material of the lower non-magnetic layer is selected in consideration of such surface smoothing.

First, as the non-magnetic powder, an Al compound,
Acicular hematite surface-modified with at least one of Si compounds (eg, oxides) is used. By modifying the surface of hematite with an Al compound or a Si compound, the dispersibility is improved, and the adsorption with a lubricant added as an additive is eliminated.

Here, the surface modification of hematite may be performed on either the Al compound or the Si compound, or on both.

The surface-modified hematite is made of Fe
(Ie, Al / Fe, Si /
(Fe or Al + Si / Fe) is desirably 0.5 to 10 atomic%. When this ratio is less than 0.5 atomic%, the effect of the surface modification is poor. Further, if this ratio exceeds 10 atomic%, the effect is not further increased in proportion to the added amount, but rather, the surface area of the particles is increased, which is not preferable. Hematite is an Al compound,
At the same time as the surface modification with the Si compound, the surface modification may be performed with a trace amount of metal oxide or light metal oxide, for example, phosphorus or boron.

To form a lower nonmagnetic layer having a smooth surface,
Hematite is surface-modified in this way, and the major axis length is less than 0.2 μm, and the acicular ratio (major axis length / minor axis length) is greater than the acicular ratio of the ferromagnetic powder used in the upper magnetic layer. It needs to be big.

When the major axis length of the acicular hematite is 0.2 μm or more, it is difficult to obtain a smooth surface that can withstand high recording density using a short wavelength. Hematite is non-magnetic and thus cannot be subjected to magnetic field orientation treatment. However, if the needle ratio is as described above, hematite is naturally oriented by the shearing force during coating, and a smooth surface can be obtained.

The lower non-magnetic layer contains needle-like hematite having such a surface modification as a main component of the non-magnetic powder. Other non-magnetic pigments are used in combination to improve dispersibility and improve conductivity. It is also possible to improve the color and the color tone.

Examples of nonmagnetic pigments used in combination with acicular hematite include goethite, rutile-type titanium oxide, anatase-type titanium oxide, carbon black, tin oxide, tungsten oxide, silicon oxide, zinc oxide, chromium oxide, and cerium oxide. , Titanium carbide, BN, α-alumina,
Examples include β-alumina, γ-alumina, calcium sulfate, barium sulfate, molybdenum disulfide, magnesium carbonate, calcium carbonate, barium carbonate, strontium carbonate, and barium titanate. These non-magnetic pigments can be doped with an appropriate amount of impurities according to the purpose.

The specific surface area of the nonmagnetic pigment is 5 to 100.
m ² / g, more preferably 20 to 70 m ² / g
Is more preferable. When the specific surface area of the non-magnetic pigment is in the above range, accompanied by fine particles of the shape of the non-magnetic pigment,
The lower non-magnetic layer is smoothed. As a result, the upper magnetic layer formed thereon is smoothed, the modulation noise characteristics and the like of the magnetic recording medium are improved, and the spacing loss is suppressed. When the specific surface area of the non-magnetic pigment is larger than this range,
If the specific surface area is too small, on the other hand, if the specific surface area is too small, the surface smoothness of the lower non-magnetic layer and the upper magnetic layer will be impaired, and the characteristics in the high-density recording area will be degraded.

In the lower non-magnetic layer, such a non-magnetic powder is used to form a smooth surface, but in a multilayer coating type magnetic recording medium, the uniformity of the interface between the upper and lower layers is also important. Among the factors that govern the state of the upper and lower interface,
Of particular importance are the paint properties of the upper magnetic paint and the lower non-magnetic paint, and more specifically, their viscosity properties. In other words, it is necessary to pay the greatest attention to the compatibility of the viscosity characteristics of the magnetic paint of the upper layer and the non-magnetic paint of the lower layer.

Therefore, the binder shown in the upper layer, that is, a vinyl chloride copolymer having an average degree of polymerization of 180 or less and containing a metal salt of sulfonic acid as a polar group, is added to the lower non-magnetic paint. Is preferably used so as to account for 50% by weight or more. If the vinyl chloride copolymer occupying 50% by weight or more of the binder is the same in the upper layer and the lower layer, the affinity between the magnetic paint in the upper layer and the non-magnetic paint in the lower layer is increased, and the upper layer and the lower layer have a lower affinity. Such an interface is obtained. By obtaining a uniform interface, coating defects such as color unevenness, coating unevenness, coating streaks, paint chipping phenomena, and chattering phenomena in the upper and lower layers can be eradicated.

In this lower non-magnetic layer, the binder does not necessarily need to be composed of a vinyl chloride copolymer having an average degree of polymerization of 180 or less and containing a metal salt of sulfonic acid as a polar group. Instead, other types of binders may be used in combination. As the binder used in combination with the vinyl chloride copolymer, any of the binders exemplified for the upper magnetic layer can be used.

As the solvent for forming the lower non-magnetic coating material, those exemplified in the upper layer can be used.

Further, as an apparatus for preparing the lower non-magnetic paint, roll mill, ball mill, sand mill, agitator, kneader, extruder, horizontal sand mill, vertical sand mill, spike mill, pin mill, tower mill, DCP, homogenizer, Ultrasonic disperser, etc.
Any conventionally known device can be used.

The upper magnetic coating and the lower non-magnetic coating prepared from the above-mentioned materials are coated on a non-magnetic support and dried to form an upper magnetic layer and a lower non-magnetic layer.

Here, as a method of applying the two kinds of prepared paints on a non-magnetic support, there is a method described in JP-A-6-2365.
No. 43, a so-called wet-on-dry coating method in which a lower-layer paint is applied and dried, and an upper-layer paint is applied and dried on the dried lower-layer coating film. There is a so-called wet-on-wet coating method (wet multilayer coating method) in which an upper layer coating film is applied over the lower layer coating film.

Among these, it is desirable to use a wet-on-wet coating method from the viewpoint of the uniformity of the coating film, the adhesiveness of the upper and lower interfaces, and the productivity. FIG. 1 shows an example of an application device for applying a paint by the wet-on-wet application method.

This coating apparatus has two slits (a slit 11 for a lower layer paint,
Die head 18 having slit 12) for upper paint
(4 lip type die head). That is, in this die head, the two slit portions 11, 1
A lower paint reservoir 13 and an upper paint reservoir 14 to which a lower paint and an upper paint are supplied, respectively, are formed on the back side of 2.
The lower layer paint and the upper layer paint supplied to the paint pools 13 and 14 are extruded through the slits 11 and 12 to the tip of the die head. On the other hand, the support 15 to which the paint is applied
Travels in the direction A in the figure from the slit 11 for the lower layer paint to the slit 12 for the upper layer paint along the tip surface of the die head.

The non-magnetic support 15 running as described above
First, when passing through the slit portion 11 for the lower layer paint, the lower layer paint extruded from the slit portion 11 is applied to the surface to form the lower layer coating film 16. Then, when passing through the slit portion 12 for the upper layer paint, the upper layer paint extruded from the slit portion 12 is applied on the lower layer coating film 16 in a wet state, and two layers of the coating films 16 and 17 are formed. . Then, the two-layer coating film in a wet state is dried and subjected to a surface smoothing treatment such as a calendering treatment as necessary, whereby a multilayer coating type magnetic recording medium is produced.

As the die head, besides the four-lip method, there are also a three-lip method and a two-lip method.

Since the lower layer and the upper layer formed by the wet-on-wet coating method are formed by applying the upper layer paint on the wet lower layer coating film, the surface of the lower layer, that is, The boundary between the lower layer and the upper layer is formed smoothly. Therefore, the surface properties of the upper layer are also very good, dropout is suppressed, high output,
It is suitable for high density recording where low noise is strictly required. Further, since the adhesion between the lower layer and the upper layer is high, film peeling hardly occurs and excellent durability can be obtained.

A clear boundary substantially exists between the lower layer and the upper layer formed by the wet-on-wet coating method, and a boundary in which the components of both layers are mixed with a certain thickness. There may be regions. In the present invention, when such a boundary region exists, a layer below the boundary region is defined as a lower layer and an upper layer is defined as an upper layer except for the boundary region.

On the other hand, when the upper layer and the lower layer are formed by a wet-on-dry coating method, the lower layer coating and the upper layer coating may be applied by a usual method such as a die head coating method, a gravure roll coating method, or a reverse roll coating method. An application method is adopted. However, in this case, the material of the lower layer needs to be selected so that the lower layer has sufficient solvent resistance to the upper layer paint.

The lower layer and the upper layer thus formed are
Since the material is selected so that the lower non-magnetic layer is formed with a smooth surface, the upper magnetic layer is also formed with a smooth surface reflecting this. That is, the upper magnetic layer has a surface roughness Ra measured by a non-contact optical surface roughness meter of 4 nm or less,
The non-magnetic support is formed to have a surface property such that the surface roughness Ra is smaller than the surface roughness Ra of the smaller surface. Therefore, it is suitable for high-density recording where dropout is suppressed and high output and low noise are strictly required.

The conditions for uniformly dispersing the ferromagnetic powder in the upper magnetic layer, filling the upper magnetic layer and filling the lower non-magnetic layer smoothly have been described above. It is necessary to regulate the thickness configuration and the characteristics of the nonmagnetic support. This is to reduce the thickness of the medium in order to increase the length of the tape that can be accommodated in the cassette, and to achieve sufficient durability even when the thickness is reduced.

First, the thickness of the upper magnetic layer should be 0.05 to
0.2 μm and not more than ５ of the total thickness of the lower non-magnetic layer and the upper magnetic layer, and
It is desirable that it is 20 or less. The thickness of the nonmagnetic support is desirably 5 μm or less. This thickness configuration
It is set in view of the balance of the thickness of each layer and the reduction of the total thickness of the medium.

That is, it is advantageous to make each layer as thin as possible in order to make the overall thickness of the medium thin. For example, when the thickness of the lower non-magnetic layer is smaller than this range, the roughness of the non-magnetic support is reduced. It is not possible to sufficiently mask the degree with this lower layer, and there is an adverse effect that the surface properties are deteriorated. Further, if the upper magnetic layer is too thin, the electromagnetic conversion characteristics are deteriorated. Therefore, the thickness of the upper and lower layers should not be reduced unnecessarily.Rather, the thickness of the non-magnetic support should be reduced to minimize the influence on the surface properties and hence the electromagnetic conversion characteristics while minimizing the effect on the entire medium. This is convenient because the thickness can be reduced. The film thickness configuration is set in consideration of such a thing.

However, simply reducing the thickness of the non-magnetic support lowers the tape stiffness, resulting in poor contact with the head as well as tape edge damage, wrinkles,
Problems such as poor shape of the wound figure occur. In order to avoid such a problem, the nonmagnetic support has a Young's modulus of 1000.
It is necessary to be at least kg / mm ² . Even if the thickness of the nonmagnetic support is less than 5 μm, the Young's modulus is 1000
If it is at least kg / mm ² , the strength of the upper layer and the lower layer described in detail above will be added, and sufficient practical characteristics can be secured.

Although the basic configuration of the magnetic recording medium of the present invention has been described above, the configuration of the magnetic recording medium is not limited to this. The characteristics may be improved by providing an additional configuration generally employed in a magnetic recording medium.

For example, the upper magnetic layer and the lower non-magnetic layer
If necessary, additives such as a lubricant and a surfactant may be added.

Examples of the lubricant include solid lubricants such as graphite, molybdenum disulfide and tungsten disulfide, silicone oil, fatty acids having 10 to 22 carbon atoms, and 10 to 22 carbon atoms.
And terpene-based compounds and oligomers thereof, fluorine-based lubricants, and the like. These lubricants may be added only to the upper layer, or may be added to both the upper layer and the lower layer. However, if it is added only to the upper layer,
Since the absolute amount of the lubricant is insufficient, it is preferable to add the lubricant to both the upper layer and the lower layer.

As the surfactant, any of nonionic, anionic, cationic and amphoteric surfactants can be used. These surfactants may be added only to either the upper layer or the lower layer, or may be added to both the upper layer and the lower layer. When a surfactant is added to both layers, the same type may be used for the upper layer and the lower layer, or different types may be used. Also, the amount of addition may be the same or different in the upper layer and the lower layer.

Further, a polyisocyanate for crosslinking and curing the binder may be used in combination. As the polyisocyanate, toluene diisocyanate and adduct thereof, alkylene diisocyanate and adduct thereof can be used. The amount of the polyisocyanate to be added is 5 to 80 parts by weight, preferably 10 to 50 parts by weight, based on 100 parts by weight of the binder. This polyisocyanate may be used in both the upper and lower layers, or may be used only in the upper layer. When used in both upper and lower layers, the amount may be equal in each layer or may be changed at an arbitrary ratio.

As shown in FIG. 2 (magnetic tape for data storage of a computer), the magnetic recording medium has a side on which the upper layer 2 and the lower layer 4 of the nonmagnetic support 1 are formed in addition to the upper and lower layers. The back coat layer 3 is formed on the opposite side to improve the running property, prevent static charge and prevent transfer.
May be provided. Also, between the lower layer and the non-magnetic support,
An undercoat layer may be provided for the purpose of enhancing the adhesion between the lower layer and the support. However, in order to achieve a large capacity, it is necessary to set the film thickness of each of these layers so that the total thickness of the medium does not become too large.

[0083]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described based on experimental results.

Examination of long axis length of ferromagnetic powder used in upper layer Fe-based ferromagnetic powder having the composition, magnetic properties, average long axis length, and standard deviation of long axis length shown in Table 1 (samples A-1 and B) -1, Sample C-1, Sample D-1, Sample A-2, Sample B-2, Sample C-2, Sample C-3, Sample D-2) were prepared. The alloy composition of the ferromagnetic powder was determined by X-ray fluorescence analysis (foundmenta).
l parameter method). The saturation magnetization s and the coercive force Hc were measured using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.). The external magnetic field during the measurement is 1.2M
A / m (15 kOe). The average value (average major axis length) L of the major axis length and the standard deviation σ of the major axis length were calculated from 200 samples randomly selected on a transmission microscope photograph.

Using this ferromagnetic powder, an upper layer paint was prepared based on the following composition. Coating is performed by mixing a ferromagnetic powder, a binder, an additive, and a solvent according to a conventional method, kneading with a kneader under the condition that the non-volatile component becomes 85% by weight, and further dispersing with a sand mill for 5 hours. went. However, Al ₂ O ₃ is slurried,
During the kneading stage, it was mixed with other compositions.

Upper layer coating composition Fe-based metal ferromagnetic powder 100 parts by weight Vinyl chloride-based copolymer 14 parts by weight (degree of polymerization 150, containing sodium sulfonate 5 × 10 ⁻⁵ mol / g as a polar functional group) 6 parts by weight of polyester polyurethane resin (containing 1 × 10 ⁻⁴ mol / g of sodium sulfonate as a polar functional group) Additive: 2 parts by weight of carbon 5 parts by weight of Al ₂ O ₃ (0.09 μm primary particle size, slurry) 1 7 parts by weight of stearic acid 1 part by weight of heptyl stearate 150 parts by weight of methyl ethyl ketone 150 parts by weight of cyclohexanone

Next, a non-magnetic powder based on the following composition:
After mixing the binder and the solvent, the nonvolatile component is 85% by weight.
The mixture was kneaded by a kneader under the following conditions, and further dispersed by a sand mill for 3 hours to prepare a lower layer paint.

Lower layer coating composition α-iron oxide (sample-f) 100 parts by weight (Si 3 atomic% treated, needle ratio 8, major axis length 0.18 μm) Vinyl chloride copolymer 14 parts by weight (degree of polymerization 150, containing 5 × 10 ⁻⁵ mol / g of sodium sulfonate as a polar functional group) 6 parts by weight of polyester polyurethane resin (containing 1 × 10 ⁻⁴ mol / g of sodium sulfonate as a polar functional group) Stearic acid 1 part by weight heptyl stearate 1 part by weight Methyl ethyl ketone 105 parts by weight Cyclohexanone 105 parts by weight

Then, 4 parts by weight and 2 parts by weight of polyisocyanate were added to the prepared upper layer paint and lower layer paint, respectively, and these paints were added to a 4.5 μm thick aramid using a 4-lip die coater. (Aromatic polyamide) film (surface roughness Ra: 5.5 nm in terms of good roughness)
(7.0 nm in terms of rough surface). Then, in an undried state, the upper coating film was subjected to an orientation treatment using a solenoid coil, and then dried, calendered, and cured to form an upper magnetic layer and a lower nonmagnetic layer. The coating thickness of each layer was set to 0.15 μm for the upper layer and 2.0 μm for the lower layer after drying.

On the other hand, a back coating was prepared based on the following composition.

Back coating composition Carbon black (trade name Asahi # 50) 100 parts by weight Polyester polyurethane 100 parts by weight (trade name N-2304 manufactured by Nipporan Co.) Methyl ethyl ketone 500 parts by weight Toluene 500 parts by weight

This back coating was applied to a surface opposite to the side on which the upper and lower layers of the non-magnetic support were formed in a coating thickness of 0.5 μm to form a back coat layer.

Then, the raw tape on which the upper layer, the lower layer, and the back coat layer were formed was formed into a tape by slitting it to a width of 8 mm.

The magnetic tape prepared as described above has magnetic properties, surface roughness Ra, electromagnetic conversion properties and C
The / N ratio was measured.

The magnetic characteristics are as follows: saturation magnetic flux density Bm, coercive force Hc, squareness ratio (= residual magnetic flux density / saturated magnetic flux density) R
For s, an external magnetic field of 0.
The measurement was performed under the condition of 8 MA / m (10 kOe).

The surface roughness Ra was measured with a non-contact optical surface roughness meter (microscope for laser interference measurement manufactured by ZYGO).

The electromagnetic conversion characteristics are obtained from Sony Hi
Using a measuring device modified from -8 deck, wavelength λ = 0.5
The evaluation was performed by measuring the output at two points of μm and λ = 0.33 μm. The C / N ratio was evaluated by measuring the level of noise 1 MHz away from the center frequency. Note that the output and the C / N ratio were standardized by setting the values in Experimental Example 5 to 0 dB.

The above measurement results are shown in Table 1 together with the characteristics of the ferromagnetic powder used in the upper magnetic layer.

[0099]

[Table 1]

In each of the combinations of Experimental Example 1, Experimental Example 5, Experimental Example 2, Experimental Example 6, Experimental Example 3, Experimental Example 7 and Experimental Example 8, Experimental Example 4 and Experimental Example 9, the upper magnetic layer was used. The average major axis length L of the used ferromagnetic powder is almost the same. However, the standard deviation?
The value is larger than that in Experimental Example 4, and in Experimental Examples 5 to 9, the relationship of 0.01 μm <L ± 2σ <0.33 μm is not satisfied.

When comparison is performed for each combination,
The magnetic tapes of Experimental Examples 5 to 9 had higher output and C / N compared to the corresponding magnetic tapes of Experimental Examples 1 to 4.
Both ratios are low. However, there is no remarkable difference in the magnetic properties and the surface roughness Ra. From this, the deterioration of the characteristics of the magnetic tapes of Experimental Examples 5 to 9 was caused by the deterioration of the particle size distribution of the ferromagnetic powder, that is, the increase in the maximum particle number of the particle
It seems to be due to an increase in ngth loss.

That is, in order to obtain a magnetic tape having excellent electromagnetic conversion characteristics, the particle size distribution of the ferromagnetic powder in the upper magnetic layer is important, and the ferromagnetic powder satisfying the relationship of 0.01 μm <L ± 2σ <0.33 is important. Was found to be preferable.

Incidentally, when the ferromagnetic powder is 0.01 μm <L ± 2
A comparison between Experimental Examples 1 to 4 satisfying the relationship of σ <0.33 shows that the output and the CN ratio tend to be improved as the average major axis length of the ferromagnetic powder is reduced. When the major axis length becomes very small (Experimental examples 3 and 4)
Output expansion is reduced. This is presumed to be due to the fact that when the ferromagnetic powder becomes too fine, the dispersing ability of the binder or the dispersing device is exceeded and complete dispersion is not performed.

Polarity of vinyl chloride copolymer used in upper layer
Examination of group content As the vinyl chloride copolymer used in the upper magnetic layer, a polymerization degree was 150, and the same as in Experimental Example 3 except that a copolymer containing sodium sulfonate in an amount shown in Table 2 was used. To make a magnetic tape.

The magnetic characteristics, surface roughness Ra, electromagnetic conversion characteristics, and C / N ratio of the magnetic tape thus prepared were measured. The measurement results are shown in Table 2 together with the content of the sodium sulfonic acid salt of the vinyl chloride copolymer.

[0106]

[Table 2]

As shown in Table 2, Experimental Examples 10 to 12 in which a vinyl chloride copolymer containing a sodium sulfonic acid salt was used in the upper magnetic layer were performed in vinyl chloride containing no sodium sulfonic acid salt. Good electromagnetic conversion characteristics are obtained as compared with Experimental Example 13 using a system copolymer. From this, it was found that the inclusion of a polar group such as sodium sulfonate in the vinyl chloride copolymer used in the upper magnetic layer improved the electromagnetic conversion characteristics of the magnetic tape.

The polarity of the vinyl chloride copolymer used in the upper layer
Investigation of base type As the vinyl chloride copolymer used in the upper magnetic layer, the degree of polymerization was 150, and the polar functional group shown in Table 2 was 5 × 10 ^−5.
A magnetic tape was prepared in the same manner as in Experimental Example 3, except that the magnetic tape was used in an amount of mol / g.

Then, the magnetic characteristics, surface roughness Ra, electromagnetic conversion characteristics, and C / N ratio of the magnetic tape thus prepared were measured. Table 3 shows the measurement results together with the polar functional group species introduced into the vinyl chloride copolymer.

[0110]

[Table 3]

As shown in Table 3, Experimental Example 14 using a vinyl chloride copolymer into which a potassium sulfonate group was introduced.
The magnetic tape of Example 3 has substantially the same characteristics as the magnetic tape of Experimental Example 3 using a vinyl chloride copolymer into which a sodium sulfonate group has been introduced. However, carboxylic acids,
When a vinyl chloride copolymer into which a tertiary amine or quaternary ammonium salt is introduced is used, the properties are deteriorated as compared with the magnetic tapes of Experimental Examples 3 and 14. From this, it was found that as the polar functional group to be introduced into the vinyl chloride copolymer used in the upper layer, a metal sulfonic acid salt was optimal.

Polymerization of vinyl chloride copolymer used for upper layer
As studied upper magnetic vinyl chloride copolymer used in layer polymer degrees, a sodium sulfonate salt containing at 5 × 10 ^-5 mol / g qs, the experimental examples but using degree of polymerization shown in Table 4 A magnetic tape was prepared in the same manner as in No. 3.

The magnetic properties of the magnetic tape thus prepared were
The surface roughness Ra, electromagnetic conversion characteristics, and C / N ratio were measured, and still durability, shuttle durability, and the number of dropouts were measured.

The still durability was evaluated by measuring the time required for the output to attenuate to -3 dB in the pause state. The shuttle durability was evaluated by performing shuttle running for 2 minutes at a time and measuring the time until the output decreased by 3 dB. The durability was measured under normal temperature and normal humidity (temperature: 25 ° C., relative humidity: 60%).

The number of dropouts was evaluated by measuring the number of output drops of 3 μsec and 10 dB or more in 3 minutes.

Table 4 shows the measurement results together with the degree of polymerization of the vinyl chloride copolymer.

[0117]

[Table 4]

As is clear from Table 4, when the polymerization degree of the vinyl chloride copolymer used in the upper layer is in the range of 180 or less (Experimental Examples 18 to 20), the degree of polymerization of the resin decreases as the degree of polymerization decreases. However, although the durability of the still and the durability of the shuttle slightly decrease, the durability and the electromagnetic conversion characteristics fall within the allowable ranges. However, when the degree of polymerization of the vinyl chloride copolymer is increased to 300 as in Experimental Example 21, although the durability is ensured, the electromagnetic conversion characteristics tend to be significantly deteriorated.

From this, it was found that the degree of polymerization of the vinyl chloride copolymer used in the upper layer is suitably 180 or less.

In the upper layer, the vinyl chloride copolymer and the polyether
Examination of the mixing ratio of the steal polyurethane resin A magnetic tape was prepared in the same manner as in Experimental Example 3 except that the mixing ratio of the vinyl chloride copolymer and the polyester polyurethane resin used in the upper magnetic layer was changed as shown in Table 5.

The magnetic characteristics,
The surface roughness Ra, electromagnetic conversion characteristics, and C / N ratio were measured, and still durability, shuttle durability, and the number of dropouts were measured. Table 5 shows the measurement results together with the mixing ratio of the binder.

[0122]

[Table 5]

As is clear from Table 5, when the blending ratio of the vinyl chloride copolymer in the binder in the upper layer was lowered,
The output, C / N ratio, still characteristics, and shuttle characteristics hardly change. However, dropouts show a remarkable increase especially in Experimental Examples 25 and 26 in which the blending ratio of the vinyl chloride copolymer was 6 parts by weight or 0 parts by weight. In these Experimental Examples 25 and 26, since the mixing ratio of the polyurethane resin was large, the affinity with the resin of the lower layer was lowered, and when performing simultaneous multilayer coating, coating unevenness was observed.

From this, it was found that the blending ratio of the vinyl chloride copolymer as the binder used in the upper layer had to be 50% by weight or more of the total binder.

In the upper layer, a vinyl chloride copolymer having a polymerization degree of 150 was used.
Compounding ratio of polymer and vinyl chloride copolymer having a degree of polymerization of 300
As a binder used in the upper magnetic layer, instead of a mixture of a vinyl chloride copolymer and a polyester polyurethane resin, the degree of polymerization was 150, and sodium sulfonate was used as a polar functional group at 5 × 10 ⁻⁵ mol / mol. g of a vinyl chloride copolymer and a degree of polymerization of 300, and 5 × 10 ⁻⁵ mol / g of sodium sulfonate as a polar functional group
A magnetic tape was prepared in the same manner as in Experimental Example 3 except that a mixture in which the vinyl chloride copolymer contained was mixed at the compounding ratio shown in Table 6 was used.

The magnetic characteristics of the prepared magnetic tape were
The surface roughness Ra, electromagnetic conversion characteristics, and C / N ratio were measured, and still durability, shuttle durability, and the number of dropouts were measured. Table 6 shows the measurement results together with the mixing ratio of the binder.

[0127]

[Table 6]

As is evident from Table 6, when the blending ratio of the vinyl chloride copolymer having a polymerization degree of 150 is reduced, the durability such as still characteristics, shuttle characteristics, and dropout hardly changes. However, regarding the electromagnetic conversion characteristics, particularly, the compounding ratio of the vinyl chloride copolymer having a polymerization degree of 150 was 6%.
In Experimental Examples 31 and 31 in which the weight% or 0% by weight was used, a remarkable decrease was observed.

Therefore, even when vinyl chloride copolymers having different degrees of polymerization are combined, even if a resin having an appropriate degree of polymerization, that is, a vinyl chloride copolymer having a degree of polymerization of 180 or less, the compounding ratio of the resin is not changed. It has been found that it is necessary to make it 50% by weight or more of the agent.

The binder used in the upper magnetic layer is as follows:
A magnetic tape using a mixture of a vinyl chloride copolymer having a polymerization degree of 150, a vinyl chloride copolymer having a polymerization degree of 300, and a polyurethane resin in a mixing ratio of 10 parts by weight: 5 parts by weight: 5 parts by weight is also used. The characteristics were evaluated in the same manner. As a result, coercive force tHc = 175 kA / m, saturation magnetic flux density Bm = 440 mT, squareness ratio Rs = 0.86, surface roughness Ra = 2.6 nm, output = + 5.9 dB, C / N =
+4.4 dB, still durability> 120 minutes, shuttle durability> 150 times, dropout = 36 pieces, and good values were obtained. This shows that, even when three types of binders are used in combination, it is appropriate that the blending ratio of the vinyl chloride copolymer having an appropriate degree of polymerization be 50% by weight or more of the total binders. Was.

Examination of the amount of non-volatile components during kneading of the upper layer paint The same procedure as in Experimental Example 22 or 3 was carried out except that the amount of non-volatile components during kneading of the upper layer paint was changed as shown in Tables 7 and 8. To make a magnetic tape.

The magnetic characteristics of the magnetic tape thus prepared were
The surface roughness Ra, electromagnetic conversion characteristics, C / N ratio, and number of dropouts were measured. The measurement results are shown in Tables 7 and 8 together with the amount of nonvolatile components at the time of kneading the upper layer paint. Table 7 shows the measurement results of the magnetic tape prepared according to Experimental Example 22, and Table 8 shows the measurement results of the magnetic tape prepared according to Experimental Example 3.

[0133]

[Table 7]

[0134]

[Table 8]

Table 7 shows a system using a vinyl chloride copolymer having a polymerization degree of 150 alone as a binder, and Table 8 shows a system using a mixture of polyvinyl chloride having a polymerization degree of 150 and a polyurethane resin. However, it can be seen that in any of the systems, the electromagnetic conversion characteristics and the dropout characteristics are improved by setting the non-volatile solid content at the time of kneading the upper layer paint to 80% by weight or more and 90% by weight or less.

In Experimental Example 32, the dispersibility of the coating material was low, and application unevenness was observed when the upper coating material and the lower coating material were simultaneously coated. In Experimental Example 37, since the solid content during kneading was too low, almost no kneading effect was observed.

Examination of Particle Size of Abrasive Used in Upper Layer Experimental examples except that the primary particle diameter and the center particle diameter in the slurry shown in Table 9 were used as Al ₂ O ₃ used in the upper magnetic layer. A magnetic tape was prepared in the same manner as in No. 3. However, in Experimental Examples 45 and 49, Al ₂ O ₃ was added as a powder in the kneading step. In addition, Al ₂ in the slurry
The central particle size of O ₃ was controlled by changing the dispersion time.

The magnetic properties of the prepared magnetic tape were
The surface roughness Ra, electromagnetic conversion characteristics, C / N ratio and still durability, shuttle durability, and dropout number were measured. Table 9 shows the measurement results together with the primary particle size of Al ₂ O ₃ and the center particle size in the slurry.

[0139]

[Table 9]

As can be seen from Table 9, in Experimental Examples 45 and 49 in which Al ₂ O ₃ was supplied as a powder during kneading, not only the surface properties deteriorated, but also the dropout increased. On the other hand, when Al ₂ O ₃ is slurried and charged, Al ₂ O ₃ is highly dispersed in the magnetic layer, and a magnetic tape having excellent surface properties and electromagnetic conversion characteristics can be obtained. In particular, when Al ₂ O ₃ is dispersed in the slurry until the center particle diameter becomes 0.17 μm or less, a magnetic tape having extremely excellent electromagnetic conversion characteristics and durability can be realized.

Examination of the thickness constitution of the magnetic tape A magnetic tape was prepared in the same manner as in Experimental Example 3 except that the thicknesses of the upper layer, the lower layer, the base film and the back coat layer were changed as shown in Table 10.

The magnetic conversion characteristics, C / N ratio and overwrite characteristics of the prepared magnetic tape were measured.

Note that the overwrite characteristic is a wavelength of 4 μm
Was evaluated by measuring the output of the original signal remaining after erasing the above signal with a signal having a wavelength of 1 μm. The output is experimental example 3.
Was standardized as 0 dB.

Table 10 shows the measurement results together with the film thickness composition.

[0145]

[Table 10]

As can be seen from Table 10, the thickness of the upper layer is not more than 0.
All magnetic tapes satisfying the condition that the upper layer thickness is 2 μm or less and the upper layer thickness is 1/5 or less of the total thickness of the upper and lower layers and 1/20 or less of the total tape thickness are 5.5 dB.
The above output is obtained, and the overwrite characteristics are excellent. The thickness of the base film does not affect the output and overwrite characteristics, but is preferably 5 μm or less from the viewpoint of making the tape thinner.

Examination of Surface Modification of Nonmagnetic Powder Used in Lower Layer As the nonmagnetic powder of the lower nonmagnetic layer, S
i, A magnetic tape was prepared in the same manner as in Experimental Example 3 except that hematite (α iron oxide) coated with Al was used.

The coating of Si and Al on hematite was performed as follows.

After hematite (the hematite before Si coating used in Experimental Example 1) was sufficiently dispersed in water, the dispersion of hematite was added to an aqueous solution of this hematite under an alkaline atmosphere in an amount corresponding to the target coating amount. A solution of a soluble salt containing Al was added and stirred. As a result, hematite is uniformly coated with Si and Al. The hematite coated with Si and Al was subjected to decantation washing, granulated, dried until the water content became 1% or less, and pulverized.

With respect to the magnetic tape thus prepared, the surface roughness Ra when the lower layer was formed as a single layer, the surface roughness Ra when the lower layer was formed as the upper layer, and the electromagnetic conversion characteristics were measured.
The surface roughness Ra of the lower single layer was measured by applying a coating for the lower layer only and drying the same to form a sample section on which the nonmagnetic layer was formed.

Table 11 shows the measurement results together with the Si coating amount and Al coating amount on hematite.

[0152]

[Table 11]

As apparent from Table 11, the Si compound,
When hematite whose surface is coated with an Al compound is used as the nonmagnetic powder of the lower nonmagnetic layer, Si / F of hematite is used.
If e, Al / Fe or Si + Al / Fe is within the range of 0.5 to 10 atomic%, the surface property of the lower non-magnetic layer becomes good, and the upper magnetic layer also has a good surface reflecting the fact. Formed by the nature. The magnetic tape on which the upper magnetic layer is formed with good surface properties can obtain a much higher output than a magnetic tape using hematite not coated with Si or Al.

When the amount of hematite covered with Si and Al exceeds 10 atomic%, the surface property of the lower non-magnetic layer is deteriorated because the size of the powder itself becomes large, and the shape of the powder becomes large. Is greatly affected.

Examination of the shape of the non-magnetic powder used in the lower layer Experimental example 2 or experiment was conducted except that the non-magnetic powder having a major axis length and a needle ratio shown in Table 12 was used as the non-magnetic powder in the lower non-magnetic layer. A magnetic tape was prepared in the same manner as in Example 3.

With respect to the prepared magnetic tape, the surface roughness Ra when the lower layer was formed as a single layer, the surface roughness Ra when the lower layer was formed as the upper layer, and the electromagnetic conversion characteristics were measured.
The measurement results are shown in Tables 12 and 13 together with the major axis length and needle ratio of the lower nonmagnetic powder. Table 12 shows the measurement results of magnetic tapes prepared according to Experimental Example 2.
3 is a measurement result of a magnetic tape prepared according to Experimental Example 3.

[0157]

[Table 12]

[0158]

[Table 13]

In Tables 12 and 13, examples of experiments using hematite having substantially the same acicular ratio as the nonmagnetic powder (for example, Examples 80, 83, 85, 86 or 9)
0, 93, 95, 96), the shorter the major axis of the non-magnetic powder, the better the surface properties of the lower non-magnetic layer,
In particular, when the major axis length is less than 0.2 μm, it can be seen that extremely good surface properties can be realized.

On the other hand, for the surface properties of the upper magnetic layer, it is important to match not only the major axis length but also the acicular ratio of the pigment powder used in the lower layer and the pigment powder used in the upper layer. Unless a lower pigment (ie, hematite) having a higher needle ratio than that of the upper pigment (ie, metal powder) is used in the lower layer, it is difficult to obtain a smooth surface in the upper layer. The reason for this has not been completely elucidated, but is presumed as follows.

That is, when the needle ratio of the lower layer hematite is smaller than the needle ratio of the upper layer metal powder, the stability of the upper layer metal powder at the lower layer interface becomes poor. For this reason, it is assumed that the fluctuation of the paint occurs, and as a result, the surface roughness deteriorates.

As described above, as the nonmagnetic powder used in the lower nonmagnetic layer, the major axis length is less than 0.2 μm, and the acicular ratio is larger than the acicular ratio of the ferromagnetic powder used in the upper magnetic layer. By using hematite, the surface properties and electromagnetic characteristics of the magnetic tape are improved.

Examination of Vinyl Chloride Copolymer Used in Lower Layer As a binder for the lower nonmagnetic layer, a vinyl chloride copolymer having a degree of polymerization shown in Table 14 and containing a polar functional group of the kind and amount shown in the same table was used. A magnetic tape was prepared in the same manner as in Experimental Example 3 except that the copolymer was used alone.

With respect to the prepared magnetic tape, the surface roughness Ra when the lower layer was formed as a single layer, the surface roughness Ra when the lower layer was formed as an upper layer, and the electromagnetic conversion characteristics were measured.
The measurement results are shown in Table 14 together with the degree of polymerization, polar group type and polar group content of the vinyl chloride copolymer used in the lower nonmagnetic layer.

[0165]

[Table 14]

As is apparent from Table 14, when a vinyl chloride copolymer containing a polar group other than the metal sulfonic acid salt was used as the lower layer binder (Experimental Examples 101 to 10)
In 4), the surface of the lower single layer and the surface of the lower upper two layers both have high surface roughness. In addition, when a vinyl chloride copolymer having a high degree of polymerization is used as in Experimental Example 106, the dispersibility of the lower layer decreases, and in this case, the surface of the lower single layer and the surface of the lower upper layer 2 also decrease. Both have high surface roughness.

From this, it was found that a vinyl chloride copolymer having a degree of polymerization of 180 or less and containing a metal salt of sulfonic acid as a polar group is suitable as a binder used in the lower layer.

In the lower layer, vinyl chloride copolymer and polyester
Examination of Mixing Ratio of Ter Polyurethane Resin A magnetic tape was prepared in the same manner as in Example 3 except that the mixing ratio of the vinyl chloride copolymer and the polyester polyurethane resin used in the upper magnetic layer was changed as shown in Table 15.

With respect to the magnetic tape thus prepared, the surface roughness Ra when the lower layer was formed as a single layer, the surface roughness Ra when the lower upper layer was formed as two layers, and the electromagnetic conversion characteristics were measured, and the coated state was evaluated. . The coating state was evaluated by visually observing the state of coating unevenness on the coating surface.
Table 15 shows the measurement results together with the mixing ratio of the vinyl chloride copolymer and the polyester polyurethane resin in the lower nonmagnetic layer.

[0170]

[Table 15]

As can be seen from Table 15, in the lower layer, a vinyl chloride copolymer having a degree of polymerization of 180 or less and containing a metal salt of sulfonic acid as a polar group was used in an amount of 50% by weight or more of the total binder. When it is contained, a good coated surface with low surface roughness Ra and no coating unevenness can be obtained. For this reason, it is preferable that the lower layer also has a degree of polymerization of 180 or less and uses a vinyl chloride copolymer containing a metal sulfonate as a polar group in a proportion of 50% by weight or more of the total binder. all right.

Inspection of the combination of the upper binder and the lower binder
As 討 lower non-magnetic layer by using vinyl chloride copolymer, a degree of polymerization shown in Table 16, except for using those containing type, the amount of polar functional groups shown in the same table in the same manner as in Example 3 To make a magnetic tape.

With respect to the magnetic tape thus prepared, the surface roughness Ra when the lower layer was formed as a single layer, the surface roughness Ra when the lower upper layer was formed as two layers, and the electromagnetic conversion characteristics were measured, and the coated state was evaluated. . The measurement results are shown in Table 16 together with the degree of polymerization of the vinyl chloride-based copolymer in the lower non-magnetic layer, the type of the polar functional group, and the content of the polar functional group.

[0174]

[Table 16]

As shown in Table 16, in Experimental Examples 113 to 115 in which a vinyl chloride copolymer different from the vinyl chloride copolymer used in the upper layer at a compounding ratio of 50% by weight or more was used, Unevenness is observed on the coated surface, and the surface roughness Ra is also a high value.

From the above, it is understood that the vinyl chloride copolymer used in the upper layer in a compounding ratio of 50% by weight or more and the lower layer in 50% by weight.
It has been found that it is desirable to use the same type as the vinyl chloride copolymer used in the above mixing ratio.

Examination of Base Film Example 3 was repeated except that a film having the thickness, material and Young's modulus shown in Table 17 was used as the base film.
A magnetic tape was prepared in the same manner as described above. The Young's modulus is a value in the longitudinal direction.

The coated state of the prepared magnetic tape was evaluated. Table 17 shows the results.

[0179]

[Table 17]

As shown in Table 17, the Young's modulus was 1000
Experimental example 11 using a base film of less than kg / mm ²
7, with the magnetic tape of Experimental Example 118, coating unevenness is observed. This uneven coating is caused by insufficient strength of the base.
This is because the running became unstable during the application. From this, the Young's modulus of the base film is 100
It was found that it was necessary to be 0 kg / mm ² or more.

Examination of Coating Method When applying the upper layer paint and the lower layer paint, the lower layer paint is applied and dried, and then the upper layer paint is applied and dried, that is, the lower layer paint is applied by a wet-on-dry method. A magnetic tape was prepared in the same manner as in Example 3 except that a paint and a paint for an upper layer were applied.

As a result, since the set thickness of the upper layer is small,
The upper layer coating did not spread evenly on the lower layer and could not be taped.

[0183]

As is apparent from the above description, in the method for manufacturing a magnetic recording medium of the present invention, the hardness and the particle size of the abrasive,
Because the kneading conditions are regulated, good electromagnetic characteristics can be obtained in the high-density recording area, and good running durability can be obtained even when the total thickness of the medium is reduced. A recording medium can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a configuration example of a magnetic recording medium to which the present invention is applied.

FIG. 2 is a schematic diagram showing an application device for applying a lower layer paint and an upper layer paint.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Non-magnetic support 2 Upper magnetic layer 3 Back coat layer 4 Lower non-magnetic layer

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Taro Omura 6-7-35 Kita-Shinagawa, Shinagawa-ku, Tokyo Inside Sony Corporation

Claims (2)

[Claims] An upper layer paint is prepared by adding an abrasive and a slurry obtained by dispersing a binder and a solvent to a magnetic paint obtained by kneading a ferromagnetic powder and a binder together with a solvent. In a method for producing a magnetic recording medium, wherein the abrasive contained in the abrasive slurry is an inorganic powder having a Mohs hardness of 6 or more and an average primary particle size of less than 0.10 μm. A method for producing a magnetic recording medium, wherein the median particle diameter of the abrasive in the step (a) is less than 0.20 μm immediately before being added to the magnetic paint. 2. A method for applying a lower layer coating on a nonmagnetic support to form a lower layer coating, and then applying the upper layer coating on the lower layer coating while the lower layer coating is wet. The method for manufacturing a magnetic recording medium according to claim 1, wherein: