JPH1173629A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain smooth surfaces of a lower nonmagnetic layer and an upper magnetic layer, to prevent electrification during travelling and to improve travelling durability by incorporating acicular iron oxide and carbon black into a nonmagnetic powder in the lower nonmagnetic layer, incorporating the acicular iron oxide by a specified amt. in the nonmagnetic powder, and further allowing the acicular iron oxide and carbon black to have acidic surfaces. SOLUTION: Lower nonmagnetic layers 2, 4 and upper magnetic layers 3, 5 are formed on a nonmagnetic support 1. The nonmagnetic powder in the lower nonmagnetic layers 2, 4 consists of acicular iron oxide and carbon black having acidic surfaces, and the acicular iron oxide is included by >=70 wt.% of the nonmagnetic powder. The carbon black is added as an antistatic agent. Thereby, surfaces of the lower nonmagnetic layers 2, 4 and upper magnetic layers 3, 5 are made smooth to obtain good electromagnetic conversion characteristics and to suppress problems such as sticking due to electrification. Since a lubricant added to the lower nonmagnetic layers 2, 4 easily moves to the upper magnetic layers 3, 5, good travelling durability can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-layer coated magnetic recording medium, and more particularly to a coated magnetic recording medium suitable for high-density recording and having excellent running durability.

[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). And a method of coating the surface of the non-magnetic oxide of the lower non-magnetic layer with an inorganic substance (Japanese Patent Laid-Open No. 5-1)
No. 82177), the lower non-magnetic layer has two different sizes.
Method using more than one kind of non-magnetic powder (JP-A-5-274)
No. 651), a method for regulating the standard deviation of the thickness of the upper magnetic layer to a specific range (Japanese Patent Application Laid-Open No. 5-298655), and a method for forming the upper magnetic layer from two or more magnetic layers (Japanese Patent Application Laid-Open No. 162485, JP-A-6-1624
No. 89) has been reported.

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.

The magnetic recording medium of the multi-layer coating type has a very smooth magnetic layer surface. If the surface is smooth, the electromagnetic conversion characteristics are advantageous, but the following disadvantages also occur.

First, when the surface of the magnetic layer is smooth, when it runs on the recording / reproducing apparatus, the contact area with various sliding members is inevitably increased, and the friction coefficient with respect to these sliding members becomes large. 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.

On the other hand, a method of adjusting the amount of lubricant in the upper layer (JP-A-1-224919, JP-A-5-18)
3178), a method of using a fluorine-based lubricant as a lubricant (JP-A-2-192018, JP-A-5-201918).
A method of reducing the friction coefficient of a medium by using a lubricant has been proposed.

Further, in the case of a multilayer coating type magnetic recording medium, there is also a problem of charging during running.

That is, the smoothness of the surface has a correlation with the dispersibility of the pigment, and a more dispersed coating film has a smoother surface as in the case of the multilayer coating type. However, a coating film in which pigments are well dispersed tends to have a high electrical resistance due to a small number of contact points between the pigments, and may cause discharge noise due to charging and a phenomenon of sticking to a drum or a guide pin. In the case of a tape-shaped medium, the above problem can be avoided to some extent by forming a conductive back coat layer on the side opposite to the side on which the magnetic layer is formed. However, in the case of a disk-shaped medium in which magnetic layers are formed on both sides of a non-magnetic support, other methods have to be used.

Techniques for preventing static electricity include a method in which a lower nonmagnetic layer contains an appropriate amount of carbon black as an antistatic agent (Japanese Patent Application No. 1-213822 and Japanese Patent Application Laid-Open No. 4-238111). Method of adding an appropriate amount of carbon black having a structure
Japanese Patent Application Laid-Open No. 177631, Japanese Patent Application Laid-Open No. 6-236542, and US Pat. No. 5,612,122) have been proposed.

[0019]

As described above, various studies have been made on magnetic recording media of the multilayer coating type. However, the dispersibility of the non-magnetic powder in the lower non-magnetic layer and the prevention of electrification on the medium surface are not yet sufficient.

In addition, there is a problem that even if a lubricant is used, the effect cannot be sufficiently obtained and it is difficult to improve the running durability. This is for the following reason.

That is, the lubricant improves the running property of the medium surface relative to the sliding member only when it is basically present on the medium surface. Therefore, in order to improve the running durability of the multilayer coating type magnetic recording medium, it is necessary to keep the lubricant on the surface of the upper magnetic layer. However, since the upper magnetic layer is very thin, even if the lubricant is held only in this layer, there is a limit to the amount of the lubricant to be added, and the lubricant is immediately lost by contact with the sliding member.

For this reason, in a multilayer coating type magnetic recording medium,
A method is also employed in which a lubricant is added to the lower non-magnetic layer, and the lubricant is transferred to the upper magnetic layer and replenished to the surface. This is because many fine holes are formed in the lower and upper layers formed by coating, and the lubricant added to the lower non-magnetic layer migrates from the lower non-magnetic layer to the upper magnetic layer through these fine holes. This is a technique that utilizes the phenomenon of migration to the surface.

However, even if a lubricant is actually added to the lower non-magnetic layer, the lubricant does not migrate to the surface of the upper magnetic layer as expected, and a sufficient lubricating effect cannot be obtained.

Accordingly, the present invention has been proposed in view of such a conventional situation. The lower non-magnetic layer and the upper magnetic layer have smooth surfaces, and are prevented from being charged during traveling. An object of the present invention is to provide a magnetic recording medium having excellent performance.

[0025]

In order to achieve the above object, the magnetic recording medium of the present invention comprises a non-magnetic support having a lower non-magnetic layer formed by dispersing a non-magnetic powder in a binder. An upper magnetic layer in which ferromagnetic powder is dispersed in a binder is formed on the lower non-magnetic layer. The non-magnetic powder contained in the lower non-magnetic layer is composed of acicular iron oxide and carbon black. And 70 of the total amount of the non-magnetic powder.
The weight percent or more is needle-shaped iron oxide, and the surface of the needle-shaped iron oxide and carbon black is acidic.

The non-magnetic powder of the lower non-magnetic layer contains needle-like iron oxide and carbon black, and 70% by weight of the non-magnetic powder.
When the above is made of acicular iron oxide, the surfaces of the lower non-magnetic layer and the upper magnetic layer can be formed smooth, and good electromagnetic conversion characteristics can be obtained. Also, electrification during running is prevented,
Various inconveniences such as sticking due to charging can be suppressed. Further, when the surfaces of the acicular iron oxide and the carbon black are acidic, the lubricant added to the lower non-magnetic layer easily moves to the upper magnetic layer, and the running durability of the medium is improved.

[0027]

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 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 having an upper magnetic layer formed by dispersing a powder in a binder.

According to the present invention, in such a multilayer coating type magnetic recording medium, as the nonmagnetic powder of the lower nonmagnetic layer, acicular iron oxide having an acidic surface and carbon black are used, and 70% by weight of the nonmagnetic powder is used. % Or more is composed of the acicular iron oxide.

The kind, content and liquid property of the surface of the non-magnetic powder are regulated in this manner by smoothing the surface of the lower non-magnetic layer and thereby forming the surface of the upper magnetic layer smoothly. In order to satisfy all of preventing the surface of the medium from being charged and easily transferring the lubricant added to the lower non-magnetic layer to the upper magnetic layer, the lubricant improves the running durability of the medium. To do that.

First, in order to improve the surface properties of the lower non-magnetic layer, there are various shapes of the needle-like iron oxide and carbon black. Among them, the surface roughness of the lower non-magnetic layer is increased. It is necessary to choose a shape that does not. It is known that there is a strong correlation between the surface roughness of the lower layer and the surface roughness of the upper layer, and in order to form a smooth upper layer surface,
It is essential to form a smooth underlayer.

The surface property of the lower non-magnetic layer is relatively strongly influenced by acicular iron oxide as a main component. Therefore, in order to improve the surface properties, it is important to appropriately select the shape of the acicular iron oxide.

From such a point, it is desirable that the needle-shaped iron oxide has a major axis length of 0.20 μm or less. Long axis length is 0.2
When the thickness is 0 μm or more, the surface roughness of the lower layer is increased, and the upper layer cannot have smooth surface properties that can withstand high-density recording.

The needle ratio of the needle-like iron oxide needs to be 5 or more and 20 or less. If the needle ratio is less than 5,
In the kneading and dispersing of the lower layer paint, it becomes difficult to apply shearing force,
Dispersion cannot be performed sufficiently. On the other hand, if the needle ratio exceeds 20, particles are easily broken during kneading and dispersion,
Since the broken particle fragments have strong cohesiveness, the agglomerates adversely affect the surface properties of the lower layer.

On the other hand, carbon black is added as an antistatic agent. When the surface electric resistance of the medium is high, the phenomenon of discharge noise is liable to occur, and the dust collection phenomenon due to static electricity also occurs. If dust adheres to the surface of the medium, it causes dropout, and the characteristics of the magnetic recording medium are impaired. Carbon black is for preventing such inconvenience due to charging.

As the carbon black, it is desirable to use a carbon black having a structure structure because of its excellent conductivity. Here, the structure structure of carbon black refers to a chain connection or an aggregate structure of individual carbon black particles.
A measure of the degree of the development of the structure structure is the oil absorption of carbon black. The larger the oil absorption, the better the structure structure is developed and the conductivity tends to be excellent. The carbon black contained in the lower non-magnetic layer preferably has a structure and an oil absorption of at least 100 ml / 100 g.

However, if a large amount of carbon black having a structure structure is added, the surface property of the lower non-magnetic layer is impaired. From this point, in the present invention, the ratio of the acicular iron oxide in the nonmagnetic powder is regulated to 70% by weight or more based on the total amount of the nonmagnetic powder, and the ratio of the carbon black is controlled to the total amount of the nonmagnetic powder. On the other hand, it is controlled to be less than 30% by weight. With such an addition amount, a sufficient antistatic effect can be obtained while smoothing the surface of the lower nonmagnetic layer. In order to obtain a sufficient antistatic effect, the carbon black must be contained in an amount of 2% by weight or more. Therefore, it is more preferable that the acicular magnetic powder is 70% by weight or more and less than 98% by weight, and that the carbon black is 2% by weight. It is desirable to mix in a range of not less than 30 weight.

When the non-magnetic powder is used under the above-described conditions, the surface properties of the lower non-magnetic layer and the upper magnetic layer are improved and the medium is prevented from being charged. It also affects the behavior of the lubricant migrating from the magnetic layer to the upper magnetic layer. The transfer of the lubricant from the lower non-magnetic layer to the upper magnetic layer is performed by, for example, forming the upper magnetic layer as thin as 0.5 μm or less for the purpose of improving the electromagnetic conversion characteristics, and holding a sufficient amount of the lubricant in the upper magnetic layer. This is especially important when things are difficult. For this reason, it is necessary to select a nonmagnetic powder that can easily transfer the lubricant from the lower nonmagnetic layer to the upper magnetic layer.

First, the mechanism of the transfer of the lubricant from the lower non-magnetic layer to the upper magnetic layer in the multilayer coating type magnetic recording medium will be described.

A large number of fine holes are formed in the lower and upper layers formed by coating, and the lubricant added to the lower non-magnetic layer migrates to the upper magnetic layer through the holes. Then, the lubricant present in the upper magnetic layer also migrates to the surface through the holes, thereby contributing to the improvement of the running property between the medium surface and the sliding member.

Accordingly, the micropores formed in the lower layer and the upper layer have a very significant meaning in obtaining the effect of the lubricant.

The amount of micropores formed in the coating film is as follows:
It is controlled by the ratio of the powder component to the binder and the shape of the powder component. Among these ratios, the ratio of the acicular iron oxide as the main component and the binder is particularly important, and a sufficient amount of pores can be obtained when the weight ratio of the acicular iron oxide / binder is 4 or more. . However, if the ratio of needle-like iron oxide / binder exceeds 10, the ratio of the binder is too small, and the surface properties and the strength of the coating film are impaired. Thus, the needle-shaped iron oxide / binder is 4-10,
More preferably, it is set to 4 to 8.

Further, as for the shape of the powder component, it is preferable to use a material such as carbon black having a structural structure, which produces voids between particles. As a result, the amount of vacancies in the coating film can be increased.

Further, the transfer of the lubricant from the lower non-magnetic layer to the upper magnetic layer is greatly affected by the pH of the surface of the non-magnetic powder.

For example, when a fatty acid is used as a lubricant,
If the surface of the non-magnetic powder is basic, the fatty acid, which is an acidic compound, is strongly adsorbed on the surface, so that the transfer from the lower non-magnetic layer to the upper magnetic layer is inhibited.

On the other hand, if the surface of the non-magnetic powder is acidic, the fatty acid is unlikely to be adsorbed on the surface of the non-magnetic powder. Lubrication performance can be sufficiently exhibited.

In view of the above, in the present invention, as the non-magnetic powder, a needle-like magnetic powder having an acidic surface and carbon black are used.

When both the acicular iron oxide and the carbon black are acidic, it is advantageous for improving the dispersibility. In other words, if the surface of one of the non-magnetic powders is acidic and the surface of the other non-magnetic powder is basic, the two have different affinities for the binder, compared to non-magnetic powders having a large affinity. The dispersibility of the non-magnetic powder having low affinity is low. On the other hand, if both the acicular iron oxide and the carbon black are acidic, the affinity for the binder is substantially the same for both, so that a uniform dispersion state is obtained, and the surface of the lower non-magnetic layer and the upper magnetic layer Is smoothed.

The pH of the surface of the acicular iron oxide or carbon black is measured by a boiling method. In the boiling method, pure water is added to a non-magnetic powder sample to suspend it, and after boiling,
This is a method of measuring the pH of the supernatant. For the measurement of the pH, a pH meter using a glass electrode is used.

Also, the needle-like iron oxide and carbon black p
As a method of adjusting H, for example, a method of depositing an element on the powder surface or a method of adjusting conditions at the time of cleaning can be mentioned. In the case of the method of depositing an element on the powder surface,
For example, the deposition of Al changes the pH to the alkaline side,
Changes the pH to the acidic side. Therefore, by controlling the amount of these coatings, the surface of the nonmagnetic powder can be adjusted to a desired acidic pH value.

The above-mentioned acicular iron oxide and carbon black may be used in combination with other inorganic pigments as long as the ratio of the acicular iron oxide does not fall below 70% by weight.

Examples of such inorganic pigments include goethite, rutile type titanium oxide, anatase type titanium oxide, tin oxide, tungsten oxide, silicon oxide, zinc oxide, chromium oxide, cerium oxide, titanium carbide, BN, α-alumina. , Β-alumina, γ-alumina, calcium sulfate, barium sulfate, molybdenum disulfide, magnesium carbonate, calcium carbonate, barium carbonate, strontium carbonate, barium titanate and the like. These inorganic pigments may be doped with an appropriate amount of impurities.
However, when these inorganic pigments are used in combination, the average particle diameter is preferably 0.3 μm or less if the inorganic pigment is spherical particles, and the average major axis length is preferably 0.3 μm or less if the inorganic pigment is acicular particles.
It is more preferable that: The specific surface area of the inorganic pigment is preferably from 5 to 100 m ² / g, and preferably from 20 to 100 m ² / g.
More preferably, it is 70 m ² / g. Inorganic pigments having an average particle diameter, an average major axis length, and a specific surface area in the above-mentioned ranges are fine particles, so that their particle shape hardly appears on the surface of the lower layer, which is advantageous for smoothing the lower layer.

In the present invention, the above non-magnetic powder is used for the lower non-magnetic layer, but any other material which is usually used in a multi-layer coating type magnetic recording medium can be used.

First, the binder used in the lower non-magnetic layer is desirably selected from the following points.

That is, generally, when the particles dispersed in the binder become finer, if there is no change in the geometrical arrangement, the voids between the particles become finer. In order to wet such a fine space between the particles and uniformly coat and disperse the particles with the binder, the binder must have a strong affinity for the powder component and have excellent fluidity. is necessary.

Here, a vinyl chloride copolymer containing a metal salt of oxysulfonic acid as a polar group shows a strong affinity for acicular iron oxide or carbon black used as a nonmagnetic powder, and is suitable as a binder. It is.

This vinyl chloride copolymer preferably has an average degree of polymerization of 100 or more and 300 or less, more preferably 15 or more.
It is desirably 0 or more and less than 250. Average degree of polymerization is 3
If it exceeds 00, the fluidity of the binder becomes low, and the dispersibility in the non-magnetic powder becomes insufficient.

By using a vinyl chloride copolymer containing a metal oxysulfonate and another polymer material together,
The coating strength, stiffness, base adhesiveness and the like of the magnetic recording medium may be improved. However, even when other resin binders are used in combination, it is preferable that the vinyl chloride copolymer accounts for 30% by weight or more of the total binder amount.

The polymer material used in combination is a known thermoplastic resin conventionally used as a binder for a magnetic recording medium,
Thermosetting resins, reactive resins and the like are used, and those having a number average molecular weight of 5,000 to 100,000 are particularly preferable.

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 , 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, And synthetic rubber.

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.

On the other hand, the upper magnetic layer is mainly composed of a magnetic powder and a binder. Among them, the following are preferably used as the magnetic powder.

First, a metal such as Fe, Co, Ni, etc., Fe-
Co, Fe-Ni, Fe-Al, Fe-Ni-Al, F
e-Al-P, Fe-Ni-Si-Al, Fe-Ni-
Si-Al-Mn, Fe-Mn-Zn, Fe-Ni-Z
n, Co-Ni, Co-P, Fe-Co-Ni, Fe-
Co-Ni-Cr, Fe-Co-Ni-P, Fe-Co
-B, Fe-Co-Cr-B, Mn-Bi, Mn-A
1, ferromagnetic powders composed of alloys such as Fe-Co-V, iron nitride, iron carbide and the like are used alone or in combination of two or more. These ferromagnetic powders may contain an appropriate amount of a light metal element such as Al, Si, P, or B for the purpose of preventing sintering or maintaining the shape during reduction. Of these, Fe alone or Fe-Co, Fe-Ni, Fe-
It is preferable to use a ferromagnetic powder obtained by adding at least one of Al and Si to a Co-Ni alloy.

The specific surface area of these ferromagnetic powders is 20 to 9
Is preferably from 0 m ² / g, and more preferably 25~70m ² / g. Since the ferromagnetic powder having a specific surface area in this range is relatively fine, the noise characteristics of the medium are improved, and high-density recording becomes possible.

Further, hexagonal plate-like ferrite can be used as the magnetic powder. M as hexagonal ferrite
Type, W type, Y type, and Z type barium ferrite, strontium ferrite, calcium ferrite, and lead ferrite. Co-Ti, Co-Ti-Zn, Co- Ti
-Nb, Co-Ti-Zn-Nb, Cu-Zr, Ni-
A material to which Ti or the like is added may be used.

As the binder used for the upper magnetic layer, any of the binders exemplified for the lower non-magnetic layer can be used. Even if these binders are used alone, a plurality of binders may be used in combination. It does not matter. However, if the same binder is used in the lower non-magnetic layer and the upper magnetic layer, the affinity between the lower coating and the upper coating is increased, and the coating can be performed uniformly.

The lower non-magnetic layer and the upper magnetic layer are basically composed of the above-mentioned materials, but if necessary, additives such as a lubricant and non-magnetic reinforcing particles may be added.

As the lubricant, particularly when added to the lower non-magnetic layer, one having one carbon atom such as myristic acid or oleic acid may be used.
It is desirable to use 0-22 fatty acids. As mentioned above,
In this magnetic recording medium, the surface of the needle-like iron oxide and carbon black contained in the lower non-magnetic layer is made acidic, so that when an acidic lubricant such as a fatty acid is added to the lower non-magnetic layer, these powder components The lubricant is hardly adsorbed to the surface, easily migrates to the upper magnetic layer, and is supplied to the surface. Therefore,
The effect of the lubricant is sufficiently exhibited, and the running durability of the magnetic recording medium is improved.

The fatty acid may be used in combination with another lubricant. Examples of the lubricant to be combined include solid lubricants such as graphite, molybdenum disulfide, and tungsten disulfide; silicone oil;
Fatty acid esters, terpene-based compounds and oligomers thereof, and fluorine-based lubricants synthesized with a fatty acid of 2 and an alcohol having 2 to 26 carbon atoms.

As the lubricant to be added to the upper magnetic layer, fatty acids or other lubricants may be used alone or in combination.

As the non-magnetic reinforcing particles, aluminum oxide (α, β, γ), chromium oxide, silicon oxide, diamond, garnet, emery, boron nitride, titanium carbide, silicon carbide, titanium carbide, titanium oxide (rutile, Anatase). In the upper magnetic layer, the amount of the particles is 20 parts by weight or less, preferably 10 parts by weight or less, and more preferably 5 parts by weight or less based on 100 parts by weight of the ferromagnetic powder. The Mohs hardness is 4 or more, preferably 5 or more,
More preferably 6 or more, specific gravity 2-6, preferably 3
The average primary particle size is 1.0 μm or less, preferably 0.5 μm or less, and more preferably 0.3 μm or less.

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 the non-magnetic support on which the lower non-magnetic layer and the upper magnetic layer are formed, polyethylene-
Polyesters such as 2,6-naphthalate, polyolefins such as polypropylene, celluloses such as cellulose triacetate and cellulose diacetate, polymer materials represented by vinyl resins, polyimides, and polycarbonates, or metals and glass A support or the like formed of ceramics or the like is used.

Next, a method for forming the lower non-magnetic layer and the upper magnetic layer will be described.

In order to form the lower non-magnetic layer and the upper magnetic layer, the respective compositions are kneaded and dispersed together with an organic solvent to prepare a lower coating and an upper coating.

Organic solvents used for coating include ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, alcohol solvents such as methanol, ethanol and propanol, methyl acetate, ethyl acetate, butyl acetate, propyl acetate, and the like. Ester solvents such as ethyl lactate and ethylene glycol acetate; ether solvents such as diethylene glycol dimethyl ether, 2-ethoxyethanol, tetrahydrofuran, and dioxane; aromatic hydrocarbon solvents such as benzene, toluene, and xylene; methylene chloride, ethylene chloride; Halogenated hydrocarbon solvents such as carbon chloride, chloroform and chlorobenzene are used alone or as a mixture.

As the kneading device, a conventionally known kneading machine such as a continuous twin-screw kneader, a continuous twin-screw kneader capable of multi-stage dilution, a kneader, a pressure kneader, a roll kneader, an extruder, etc. can be used. It is not limited.

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.

Then, the prepared lower layer paint and upper layer paint are applied to a nonmagnetic support.

As the coating method, there is a so-called wet-on-dry coating method in which a lower layer coating is applied and dried, and an upper layer coating is applied on the dried lower layer coating and dried. 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.

Of these, it is preferable 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 slit portions (a slit portion 11 for lower layer paint,
Die head 18 having slit 12) for upper paint
(4 lip type die head). On the back side of the two slit portions 11 and 12, a lower paint reservoir 13 and an upper paint reservoir 14 to which a lower paint and an upper paint are supplied are formed, and the paint reservoirs 13 and 14 are formed.
The lower layer paint and the upper layer paint supplied to the slits 11 and 12
Through 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 lower paint slit 11 to the upper paint slit 12 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 wet two-layer coating film is dried.

The die head includes a three-lip method, a two-lip method, and the like in addition to the four-lip method.

Since the lower layer and the upper layer formed by the wet-on-wet coating method in this manner are formed by applying the upper layer paint on the wet lower layer coating film, the lower layer surface, 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 where 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 paint and the upper layer paint are 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.

After the lower layer and the upper layer are formed in this manner, the surface is smoothed, if necessary, by calendering or the like, and cut into a desired medium shape. For example, in the case of a tape-shaped medium, the slit may be slit in the longitudinal direction so as to have a desired tape width.

The above is the basic structure of the magnetic recording medium of the present invention. The magnetic recording medium may have an additional structure which is usually used in a multi-layer coating type.

For example, in a tape-shaped medium, a lower non-magnetic layer,
A back coat layer for improving running properties may be provided on the side opposite to the side on which the upper magnetic layer is formed. The back coat layer is
The coating layer is mainly composed of a non-magnetic powder such as carbon black and a binder, and any of these materials can be used. In a disk-shaped medium, a lower nonmagnetic layer and an upper magnetic layer may be provided on both sides of a nonmagnetic support.

Further, the case where each of the upper non-magnetic layer and the lower magnetic layer is constituted by a single layer has been described in detail.
The lower non-magnetic layer and the upper magnetic layer may be composed of a plurality of layers having different compositions as long as the conditions regulated by the present invention are not deviated.

[0092]

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

Manufacturing of Magnetic Recording Medium First, the manufacturing process of the sample disk manufactured in the following experimental example will be described. As shown in FIG. 2, the manufactured sample disk has a configuration in which lower non-magnetic layers 2 and 4 and upper magnetic layers 3 and 5 are formed on both sides of a non-magnetic support 1, respectively.

In order to manufacture this sample disk,
First, an upper layer paint was prepared based on the following composition. Coating was performed by mixing a ferromagnetic powder, a binder, an additive, and a solvent according to a conventional method, kneading with a continuous kneader, and further dispersing with a sand mill for 5 hours. However, A
l ₂ O ₃ was slurried and mixed with another composition after the dispersion was completed.

Upper layer coating composition Magnetic powder: 100 parts by weight of Fe-based metal ferromagnetic powder (average major axis length: 0.2 μm, needle ratio: 9, saturation magnetization: 130 Am ² / kg, coercive force: 135 kA / m Binder: vinyl chloride copolymer 14 parts by weight (degree of polymerization 150, containing 5 × 10 ⁻⁵ mol / g of sodium oxysulfonate as a polar functional group) 6 parts by weight of polyester polyurethane resin (oxy as a polar functional group) 1 × 10 ^-4 sodium sulfonate Additives: Al ₂ O ₃ 5 parts by weight (primary particle size 0.09 μm, center particle size in slurry 0.17 μm) Stearic acid 1 part by weight Heptyl stearate 1 part by weight Solvent: methyl ethyl ketone 150 Next, a non-magnetic powder, a binder, an additive, and a solvent are mixed based on the following composition, and then kneaded by a continuous kneader under the condition that the nonvolatile component becomes 85% by weight. For 3 hours to prepare a lower layer paint.

Lower layer coating composition Non-magnetic powder: α iron oxide 100 parts by weight (average major axis length: 0.15 μm, needle ratio: 12, pH 5.7) Carbon black 11 parts by weight (pH 5.7) Binder: 25 parts by weight of vinyl chloride copolymer (degree of polymerization 150, containing 5 × 10 ⁻⁵ mol / g of potassium oxysulfonate as a polar functional group) Additive: 1 part by weight of stearic acid 1 part by weight of heptyl stearate Solvent: Methyl ethyl ketone 105 parts by weight Cyclohexanone 105 parts by weight After adding 4 parts by weight and 2 parts by weight of polyisocyanate to the prepared upper layer paint and lower layer paint, respectively, these paints were applied using a 4-lip type die coater. Coating was performed simultaneously on a 62 μm-thick polyethylene terephthalate film (surface roughness Ra: 8 nm). Then, in an undried state, the upper layer coating film was subjected to a non-orientation treatment using a solenoid coil, and then dried. Subsequently, the upper layer paint and the lower layer paint were simultaneously applied on the back surface of the film in the same manner as above, subjected to non-orientation treatment, and then dried. The coating thickness of each layer was 0.25 μm for the upper layer and 2.0 μm for the lower layer after drying.
m. Then, the surface of the upper layer coating film was smoothed by performing a calendering process, and punched into a 3.5-inch diameter to produce a sample disk.

The above is the basic manufacturing process of the sample disk. In each of the following experimental examples, the pH of α-iron oxide and carbon black, the ratio of α-iron oxide and carbon black, the ratio of α-iron oxide and binder, and the type of binder were changed from the basic composition of the lower layer paint. The appropriate conditions were examined.

Experimental Examples Regarding pH of Iron Oxide Various sample disks were prepared in the same manner as described above, except that the pH of α-iron oxide and carbon black contained in the lower layer were changed as shown in Table 1.

With respect to the manufactured sample disk, the surface electric resistance, the surface roughness Ra, the electromagnetic conversion characteristics (RF output) and the running durability were examined. These measuring methods are shown below.

Surface electric resistance: Measured with an insulation resistance meter (manufactured by Yokogawa Hewlett-Packard Company).

Surface roughness Ra: Center line average roughness Ra defined by JIS B 0601, measured by a non-contact optical surface roughness meter (manufactured by ZYGO).

Electromagnetic conversion characteristics: A floppy disk drive (manufactured by Sony Corporation, trade name: MPF-42B) was used, the head was run on the outermost track of the disk at a disk rotation speed of 3600 rpm, and 35 MHz reproduced at that time.
Was evaluated by measuring the output component. The head is a thin film head having a gap length of 0.2 μm. The measurement data was recorded as a relative value when the value on the magnetic disk of Example 1 was set to 0 dB.

Running durability: Using a micro floppy disk drive, the head was brought into contact with the disk and slid. At that time, the torque applied to the head was detected as a current value, and evaluation was made by measuring the number of passes until the current value showed an abnormality.

The above measurement results are shown in Table 1 together with the pH of α-iron oxide and carbon black used in the lower layer.

[0105]

[Table 1]

As shown in Table 1, the sample disk 1 using α-iron oxide and carbon black having an acidic surface was the same as the sample disk 2 and the sample disk 3 using α-iron oxide and carbon black having a basic surface. As compared with this, the surface roughness Ra is low, and good electromagnetic conversion characteristics can be obtained. Further, the lubricant is sufficiently supplied to the surface of the upper magnetic layer, and good running durability is obtained.

Regarding the ratio of iron oxide to carbon black
Experimental Examples Various sample disks were produced in the same manner as described above, except that the weight ratio between α-iron oxide and carbon black contained in the lower layer was changed as shown in Table 2.

With respect to the manufactured sample disk, the surface electric resistance, the surface roughness Ra, the electromagnetic conversion characteristics (RF output) and the running durability were examined. Table 2 shows the results of these measurements together with the weight ratio of α-iron oxide to carbon black.

[0109]

[Table 2]

As shown in Table 2, sample disks 4 to 6 containing α iron oxide in an amount of 70% by weight or more with respect to the total amount of the nonmagnetic powder were sample disks having a lower α iron oxide content. 7, Sample disk 8
, The surface roughness Ra is low, good electromagnetic conversion characteristics are obtained, and the running durability is excellent.

From this, it was found that the amount of α-iron oxide contained in the lower layer should preferably be 70% by weight or more based on the total amount of the nonmagnetic powder.

However, in the sample disk 4 to which no carbon black is added, the surface electric resistance is high, and there is a possibility that discharge noise due to charging, a sticking phenomenon, and a dust collecting phenomenon may occur.

Therefore, it is desirable that at least 2% by weight of the total amount of the non-magnetic powder is composed of carbon black.

Experimental Example Regarding Ratio of Iron Oxide to Binder Various sample disks were prepared in the same manner as described above except that the α-iron oxide / binder (weight ratio) in the lower layer was changed as shown in Table 3.

With respect to the manufactured sample disks, the surface electric resistance, the surface roughness Ra, the electromagnetic conversion characteristics (RF output) and the running durability were examined. Table 3 shows the results of these measurements together with α iron oxide / binder (weight ratio).

[0116]

[Table 3]

As shown in Table 3, α iron oxide / binder
Sample disk 9 to sample disk 11 as described above
Provides better running durability than the sample disk 12 in which α iron oxide / binder is smaller than 4.

From this, it was found that the ratio of the α-iron oxide and the binder contained in the lower layer should preferably be 4 or more.

Experimental Examples Regarding the Kinds of Binders Various sample disks were prepared in the same manner as described above except that the binder shown in Table 4 was used as the binder contained in the lower layer. The types of binders used are as follows.

Resin A (150): A vinyl chloride copolymer having a polymerization degree of 150 containing 5 × 10 ⁻⁵ mol / g of potassium ^oxysulfonate Resin B: 1 sodium oxysulfonate
Polyester polyurethane resin containing × 10 ⁻⁴ mol / g The prepared sample disk was examined for surface electric resistance, surface roughness Ra, electromagnetic conversion characteristics (RF output), and running durability. Table 4 shows the measurement results together with the composition of the binder.

[0121]

[Table 4]

As shown in Table 4, the sample disks 13 and 14 in which the vinyl chloride copolymer containing the metal oxysulfonate was contained in the lower binder were used.
Has a lower surface roughness than the sample disk 15 in which the vinyl chloride copolymer is not contained in the lower layer binder, has good electromagnetic conversion characteristics, and has excellent running durability.

From this, it was found that a vinyl chloride copolymer was more suitable as a binder contained in the lower layer than a polyester polyurethane resin.

Experimental Examples Regarding Degree of Polymerization of Binder Various sample disks were prepared in the same manner as described above except that the binder shown in Table 5 was used as the binder contained in the lower layer. The types of binders used are as follows.

Resin A (150): Vinyl chloride copolymer having a polymerization degree of 150 containing 5 × 10 ⁻⁵ mol / g of potassium oxysulfonate Resin A (300): 5 × potassium oxysulfonate
Vinyl chloride copolymer having a polymerization degree of 300 containing 10 ^-5 mol / g Resin A (400): 5 × potassium oxysulfonate
A vinyl chloride copolymer having a polymerization degree of 400 containing 10 ^-5 mol / g was examined for surface electrical resistance, surface roughness Ra, electromagnetic conversion characteristics (RF output), and running durability. Table 5 shows the measurement results together with the composition of the binder.

[0126]

[Table 5]

As shown in Table 5, sample disks 16 to 1 in which the binder in the lower layer contained at least 30% by weight of a vinyl chloride copolymer having a degree of polymerization of 300 or less.
Sample No. 8 has a lower surface roughness than that of the sample disk 19 using a vinyl chloride copolymer having a degree of polymerization of 400 alone as a lower layer binder, and provides good electromagnetic conversion characteristics.

For this reason, it is not preferable to use a vinyl chloride copolymer having a high degree of polymerization alone as the lower layer binder.
It turned out that it is desirable to contain it by more than weight%.

[0129]

As is clear from the above description, the magnetic recording medium of the present invention has a multi-layer coating type structure, in which the non-magnetic powder of the lower non-magnetic layer contains acicular iron oxide and carbon black. At the same time, 70% by weight or more of the total amount of the nonmagnetic powder is composed of acicular iron oxide, and since the surfaces of the acicular iron oxide and carbon black are acidic, the surfaces of the lower nonmagnetic layer and the upper magnetic layer are smoothed. And good electromagnetic conversion characteristics can be obtained. Further, charging during traveling is prevented, and various inconveniences such as sticking due to charging are suppressed. Further, since the lubricant added to the lower non-magnetic layer is easily transferred to the upper magnetic layer, good running durability can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an application device for applying a lower layer paint and an upper layer paint.

FIG. 2 is a schematic sectional view showing an example of a magnetic recording medium to which the present invention is applied.

[Explanation of symbols]

1 Non-magnetic support, 2, 4 Lower non-magnetic layer, 3, 5 Upper magnetic layer

Claims (5)

[Claims] 1. 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 powder is dispersed in the binder on the lower non-magnetic layer. An upper magnetic layer is formed, and the nonmagnetic powder contained in the lower nonmagnetic layer contains acicular iron oxide and carbon black, and 70% by weight or more of the total amount of the nonmagnetic powder is acicular iron oxide. The magnetic recording medium is characterized in that the surface of the acicular iron oxide and the carbon black is acidic. 2. The magnetic recording according to claim 1, wherein the ratio of the needle-like iron oxide and the binder in the lower non-magnetic layer (needle-like iron oxide / binder) is 4 or more by weight. Medium. 3. The magnetic recording medium according to claim 1, wherein the lower non-magnetic layer contains a fatty acid. 4. The binder contained in the lower non-magnetic layer,
2. The magnetic recording medium according to claim 1, comprising a vinyl chloride copolymer having an oxysulfonic acid metal salt in an amount of 30% by weight or more. 5. The binder contained in the lower non-magnetic layer,
2. The magnetic recording medium according to claim 1, wherein the vinyl chloride copolymer having a degree of polymerization of 300 or less is contained in an amount of 30% by weight or more.