JPH11134636A - Magnetic disk - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lessen warpage and to obtain stable output with a head floating type recording system by incorporating acicular magnetic powder of an acicular ratio within a specific range into nonmagnetic powder in forming magnetic layers contg. the magnetic powder on intermediate layers contg. the nonmagnetic powder. SOLUTION: The magnetic layers 4, 5 for higher density recording of the magnetic disk provided with the intermediate layers 2, 3 between a nonmagnetic base 1 and the magnetic layers 4 and 5 are formed thin and the intermediate layers 2, 3 are formed thick and, therefore, the conditions of the acicular powder in the intermediate layers 2, 3 affect the occurrence of the warpage of the magnetic disk. Then, the warpage of the disk is suppressed by specifying the acicular ratio of the non-management powder to 10 to 30 and generating the resistance force against the warpage. The nonmagnetic powder having the high acicular ratio of >=10 in the acicular ratio is capable of suppressing the warpage even if the nonmagnetic powder is aligned to some extent in a coating stage. At the acicular ratio above 30, the tendency to the flocculation of the nonmagnetic powder particles to each other increases and the resistance force against bending lowers.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic disk, and more particularly, to a magnetic disk suitable for use in a head flying type recording system.

[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.

Further, in order to realize high-density recording of a coating type magnetic recording medium, metal fine particles are used as a ferromagnetic powder, and the medium surface is super-smoothed to minimize spacing loss. It is also very effective to reduce the self-demagnetization loss by further reducing the thickness of the magnetic layer.

However, if 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, when the magnetic layer is thinned, a multi-layer coating structure in which a non-magnetic intermediate layer is interposed between the non-magnetic support and the magnetic layer is often adopted. By thus interposing the intermediate layer, the thickness is increased between the surface of the nonmagnetic support and the surface of the magnetic layer, and the surface shape of the nonmagnetic 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.

Further, not only the improvement on the medium side but also the improvement of the head-disk-interface greatly contribute to the development of the high-density recording technology.

For example, in a so-called floppy disk which uses a disk-shaped flexible support as a non-magnetic support among magnetic recording media, recording and reproduction are performed while a magnetic head is slid with respect to the disk surface. A head sliding recording method is used. On the other hand, recently, a flying type head in which a magnetic head is mounted on a slider is used, and the flying type head is floated at a minute distance from the disk surface by the rotating wind force of the disk, and recording / reproduction is performed in this state. Head floating type recording system for performing the following. This head floating type recording system has better access performance than the head sliding type recording system, and is suitable as a recording system for a high-density floppy disk.

[0008]

However, the recording method using the flying head has a problem in that the head flying characteristics fluctuate due to the warpage of the disk.

That is, in the case of the head sliding type recording system,
For example, for a single-sided magnetic disk in which a magnetic layer is formed only on one side, a magnetic head is arranged on the magnetic layer side and a felt-like pad is arranged on the back side, and the magnetic head is sandwiched between both. Recording and reproduction are performed. Further, for a double-sided magnetic disk having a magnetic layer formed on both sides, magnetic heads are arranged on both sides of the disk, and recording and reproduction are performed in a state of being sandwiched between both magnetic heads. That is, in any of the head sliding recording systems, recording and reproduction are performed in a state where the disk is strongly sandwiched from both sides, so that the warpage of the disk hardly causes a problem.

On the other hand, in the head flying type recording system in which the magnetic head flies above the disk surface, if the disk has a warp called curl, the head flying characteristics fluctuate, which greatly increases the electromagnetic conversion characteristics. Affect.

[0011] The warpage of the disk is roughly classified into one derived from a non-magnetic support and one derived from a coating step for forming a coating film.

The former warpage due to the non-magnetic support depends on the orientation direction of the polymer crystals in the step of preparing the polymer film for the support. However, since the orientation of the polymer crystal contributes to maintaining the physical properties and strength of the film, it cannot be completely eliminated.

On the other hand, with respect to the warpage resulting from the latter coating step, measures have been taken to introduce a step for eliminating the warp, such as applying a heat treatment after performing a smoothing treatment on the coating film. I have. However, this alone cannot sufficiently eliminate the warpage of the disk.

As described above, the conventional magnetic disk is
There is a problem that it is difficult to suppress the warpage, and a stable output cannot be obtained when the head floating recording method is adopted.

Accordingly, the present invention has been proposed in view of such conventional circumstances, and it is an object of the present invention to provide a magnetic disk which has a small warp and can obtain a stable output when a head floating recording system is adopted. With the goal.

[0016]

In order to achieve the above-mentioned object, in the magnetic disk of the present invention, an intermediate layer containing a non-magnetic powder is formed on at least one principal surface of a flexible non-magnetic support. A magnetic layer containing a magnetic powder is formed thereon, and the non-magnetic powder contains a needle-like non-magnetic powder having a needle ratio of 10 to 30.

The nonmagnetic powder of the intermediate layer has a needle ratio of 10 to 10.
Use of 30 can reduce warpage. The magnetic disk in which the warpage is suppressed as described above has a small fluctuation in the head flying characteristics when performing recording / reproduction by the head flying recording method, and a stable output can be obtained.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described.

In the magnetic disk of the present invention, an intermediate layer containing a non-magnetic powder is formed on at least one principal surface of a flexible non-magnetic support, and a magnetic layer containing a magnetic powder is formed thereon. It is composed. These intermediate layers and magnetic layers
For example, the composition is formed by kneading and dispersing the composition of each layer together with an organic solvent to prepare a paint, and applying and drying the paint.

In the present invention, in such a magnetic disk, acicular nonmagnetic powder having an acicular ratio of 10 to 30 is used as the nonmagnetic powder contained in the intermediate layer. here,
This needle ratio is the ratio of the major axis length to the minor axis length of the nonmagnetic powder (major axis length / minor axis length). The major axis length and the minor axis length of the non-magnetic powder can be determined by randomly selecting 200 non-magnetic powders from transmission electron micrographs at a magnification of 50,000 to 100,000 and calculating the average value.

In order to suppress the warpage of the magnetic disk, it is important to regulate the needle ratio of the nonmagnetic powder contained in the intermediate layer.

That is, the deformation of the magnetic disk depends on the orientation of the polymer crystals constituting the nonmagnetic support, the composition of the coating film formed on the nonmagnetic support such as the intermediate layer and the magnetic layer, and the formation of these coating films. In particular, the composition of the coating film greatly affects the warpage of the magnetic disk.

For example, in the case of a magnetic disk of a type in which a coating film is formed on both surfaces of a non-magnetic support, warpage can be suppressed to some extent by controlling the composition so that the tension of the coating film is balanced on both surfaces. However, that alone is not enough. To reduce the warpage of the magnetic disk,
The placement of the powder in the coating is also a non-negligible factor.

That is, if the needle-shaped powder is oriented in a certain direction in the coating film, the powder tends to bend around the direction (fold). On the other hand, if the direction of each acicular powder is random, it acts as a resistance to bending in a certain direction, and warpage is suppressed. Therefore, in order to suppress the warpage of the disk, the orientation of the acicular nonmagnetic powder is preferably random.

On the other hand, in the manufacture of a magnetic disk,
A coating material for forming the intermediate layer and the magnetic layer is applied along the longitudinal direction of the long non-magnetic film, and thereafter, the raw material on which the coating film is formed is cut into a predetermined medium shape. Make a disk. In the coating step of the paint, a shear force is inevitably applied in the paint application direction, and this shear force acts to orient the acicular powder in the application direction. Therefore, the coating film thus formed is in a state where the acicular nonmagnetic powder is oriented in the longitudinal direction of the nonmagnetic film as it is.

Here, in the magnetic layer of the coating film formed by such coating, the magnetic powder is usually made non-oriented by a magnetic field treatment in order to stabilize the recording / reproducing output. According to the non-orientation process, the output of recording and reproduction is stabilized, and the secondary effect of suppressing the warpage of the magnetic disk by randomizing the direction of the magnetic powder can be obtained.

On the other hand, the nonmagnetic powder contained in the intermediate layer is
Non-orientation treatment by a magnetic field cannot be performed. But,
In a magnetic disk in which an intermediate layer is provided between the nonmagnetic support and the magnetic layer, the magnetic layer is thinner for high-density recording and the intermediate layer is thicker than that, so that the needle-like powder in the intermediate layer is used. Condition greatly affects the occurrence of warpage of the magnetic disk.

Therefore, in the present invention, the intermediate layer is not oriented, but the acicular ratio of the nonmagnetic powder is regulated to 10 to 30 so as to exhibit a resistance to warpage and suppress the warpage of the disk.

The resistance to warpage of the intermediate layer is largely affected by the acicular ratio of the nonmagnetic powder rather than the orientation of the nonmagnetic powder, and the acicular ratio of the nonmagnetic powder is 10 or more, ie, the acicular ratio is relatively high. If non-magnetic powder is used, even if the non-magnetic powder is oriented to some extent in the coating process, the warpage is sufficiently suppressed,
A stable output can be obtained when the head flying type recording method is used. This effect is obtained because, when the acicular ratio of the nonmagnetic powder is 10 or more, the total resistance to bending at the individual pigment particle level is expressed as the resistance of the coating film. it is conceivable that.

However, when the acicular ratio of the non-magnetic powder exceeds 30 and becomes large, the dispersibility of the non-magnetic powder in the coating material decreases, and the tendency of the particles of the non-magnetic powder to agglomerate increases. Agglomerates formed by agglomeration of particles behave as particles having an apparently small needle ratio, so that the resistance to bending is reduced. Therefore, the needle ratio of the nonmagnetic powder needs to be 10 to 30, and more preferably 10 to 20.

Specific examples of the acicular non-magnetic powder used at such an acicular ratio include goethite, α-Fe ₂ O ₃
And the like. These acicular nonmagnetic powders have an acicular ratio of 10 to 30 and a major axis length of 0.3.
It is desirable that it is not more than μm. If the major axis length of the acicular non-magnetic powder exceeds 0.3 μm, the surface properties of the intermediate layer are impaired, and the surface properties of the magnetic layer are also deteriorated to reflect this.

The acicular non-magnetic powder having an acicular ratio of 10 to 30 may be combined with another non-magnetic powder as long as the ratio does not fall below 10% by weight.

The non-magnetic powder to be combined includes 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,
Examples include β-alumina, γ-alumina, calcium sulfate, barium sulfate, molybdenum disulfide, magnesium carbonate, calcium carbonate, barium carbonate, strontium carbonate, and barium titanate. These nonmagnetic powders may be doped with an appropriate amount of impurities depending on the purpose.

However, when these nonmagnetic powders are used in combination, the average particle diameter is preferably 0.3 μm or less when the nonmagnetic powder is spherical particles, and the average major axis length is 0.3 μm or less when the nonmagnetic powders are acicular particles. More preferably, it is 0.2 μm or less. Also,
The specific surface area is preferably from 5 to 100 m ² / g,
More preferably, it is 20 to 70 m ² / g. Non-magnetic powder having an average particle diameter, an average major axis length, and a specific surface area in the above range,
Since the particles are fine particles, their particle shape hardly appears on the surface of the lower layer, which is advantageous for smoothing the surfaces of the intermediate layer and the magnetic layer.

In addition, carbon black may be used in combination as the nonmagnetic powder. Since carbon black has conductivity, it functions as an antistatic agent for preventing charging of a magnetic disk. When the magnetic disk is charged, discharge noise and sticking phenomenon of the head are induced, and dust adheres to the surface of the magnetic disk due to a dust collection phenomenon due to static electricity, which causes dropout. By using carbon black, these disadvantages due to charging can be prevented.

As the carbon black, it is desirable to use a carbon black having a structure structure because of its excellent antistatic effect. 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. It is desirable that the carbon black contained in the intermediate layer has a structure structure and the oil absorption is 100 ml / 100 g or more.

In the present invention, the above-mentioned non-magnetic powder is used for the intermediate layer, but any other material commonly used in a multilayer coating type magnetic disk can be used.

First, the intermediate layer is composed of at least a non-magnetic powder and a binder for bonding the non-magnetic powder to the non-magnetic support. This binder is desirably selected from the following points.

That is, 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 the binder, the binder must have a strong affinity for the powder component and have excellent fluidity. is necessary.

From such a point, it is desirable to use a vinyl chloride copolymer containing a polar group having an average degree of polymerization of 100 to 200 as the binder of the intermediate layer. When a vinyl chloride copolymer having an average degree of polymerization of more than 200 is used alone, the dispersibility of the non-magnetic powder decreases, and the non-magnetic powder behaves as agglomerates and apparently acts as particles having a small acicular ratio. Become. As a result, the amount of warpage of the disk increases. Further, when the dispersibility of the nonmagnetic powder is reduced, the surface property of the intermediate layer is impaired, and accordingly, the surface property of the magnetic layer is also deteriorated. However, even if the vinyl chloride copolymer has an average degree of polymerization of more than 200, the average degree of polymerization is 10
It may be used as long as it is mixed with a vinyl chloride copolymer of 0 to 200. In this case, the average degree of polymerization is 1
It is desirable that the ratio of the vinyl chloride copolymer of 00 to 200 is 10% by weight or more based on the total amount of the binder in the intermediate layer. In addition, the polar group introduced into these vinyl chloride copolymers contributes to improving the dispersibility of the nonmagnetic powder, and the like.
For example, metal oxysulfonate is introduced.

Further, a vinyl chloride copolymer having an average degree of polymerization of 100 to 200 and another kind of binder may be used in combination. The binder used in combination is a thermoplastic resin, a thermosetting resin, a reactive resin, or the like having a number average molecular weight of 5,000 to 100,000 shown below.
It is preferred to use

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.

The binder includes -SO ₃ M, -OSO ₃ M, -COO for the purpose of improving the dispersibility of the pigment powder.
M, P = O (OM) 2 ( where formula, M is a hydrogen atom or a lithium, potassium, an alkali metal such as sodium) _{or, -NR 1 R 2, -NR 1} R 2 R 3 + X - in A main chain amine represented by> NR ₁ R ₂ ⁺ X ⁻ (where R ₁ , R ₂ and R ₃ represent a hydrogen atom or a hydrocarbon group, and X ⁻ represents (Indicating halogen element ions such as fluorine, chlorine, bromine and iodine or inorganic ions and organic ions), and polar functional groups such as -OH, -SH, -CN and epoxy groups 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. These binders may be used alone or in combination of two or more.

The intermediate layer is composed of at least such a binder and a non-magnetic powder. The weight ratio of the non-magnetic powder to the binder (non-magnetic powder / binder) is preferably 2 to 10, It is more preferably 4 to 10. If this ratio is too small, the warpage of the disk cannot be sufficiently suppressed, while if it is too large, the strength of the coating film becomes small, and there is a possibility that peeling may occur.

On the other hand, the magnetic layer is mainly composed of a magnetic powder and a binder. Among them, it is desirable to use the following magnetic powder.

Metals such as Fe, Co and Ni, Fe--Co,
Fe-Ni, Fe-Al, Fe-Ni-Al, Fe-A
I-P, Fe-Ni-Si-Al, Fe-Ni-Si-
Al-Mn, Fe-Mn-Zn, Fe-Ni-Zn, C
o-Ni, Co-P, Fe-Co-Ni, Fe-Co-
Ni-Cr, Fe-Co-Ni-P, Fe-Co-B,
Fe-Co-Cr-B, Mn-Bi, Mn-Al, Fe
-Ferromagnetic powder composed of an alloy such as -Co-V, iron nitride, iron carbide or the like is 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-Co-N
It is preferable to use a ferromagnetic powder obtained by adding at least one of Al and Si to the alloy i.

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. Examples of the hexagonal plate-like ferrite include M-type, W-type, Y-type, and Z-type barium ferrite, strontium ferrite, calcium ferrite, and lead ferrite. For the purpose of controlling the coercive force of these hexagonal plate-like ferrites. Co-Ti, Co-Ti-Z
n, Co-Ti-Nb, Co-Ti-Zn-Nb, Cu
A material to which -Zr, Ni-Ti or the like is added may be used.

As the binder used for the magnetic layer, any of the binders exemplified for the intermediate layer can be used. These binders can be used alone or in combination of a plurality of binders. Absent.

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

Examples of the lubricant include silicone oil, fatty acids having 10 to 22 carbon atoms, fatty acid esters synthesized from fatty acids having 10 to 22 carbon atoms and alcohols having 2 to 26 carbon atoms, terpene compounds, oligomers thereof, and fluorine. And the like. Further, a solid lubricant such as graphite, molybdenum disulfide, and tungsten disulfide may be used in combination with these lubricants. Note that the lubricant may be added only to the magnetic layer because the lubricant exerts its lubricating performance on the surface of the magnetic layer, but especially when the thickness of the magnetic layer is reduced, the addition of only the magnetic layer is not necessary. Since the total amount of the lubricant is insufficient, it is desirable to add the lubricant to both the magnetic layer and the intermediate layer.

Non-magnetic reinforcing particles include α-alumina,
β-alumina, γ-alumina, chromium oxide, silicon carbide,
Examples include diamond, garnet, emery, boron nitride, titanium carbide, silicon carbide, titanium carbide, rutile-type titanium oxide, anatase-type titanium oxide, and the like. In the magnetic layer, the amount of the particles is 20 parts by weight or less, preferably 10 parts by weight or less, 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 3-5, and the average primary particle size is 1.0
μm or less, preferably 0.5 μm or less, 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 for both the magnetic layer and the intermediate layer,
It may be used only for the magnetic layer. When used for both the magnetic layer and the intermediate layer, the amount may be equal for each layer,
The ratio may be changed at an arbitrary ratio.

Examples of the nonmagnetic support on which the intermediate layer and the magnetic layer are formed include polyesters such as polyethylene-2,6-naphthalate, polyolefins such as polypropylene, and celluloses such as cellulose triacetate and cellulose diacetate. And a support formed of a polymer material such as vinyl resins, polyimides, and polycarbonates.

In a single-sided magnetic disk, an intermediate layer and a magnetic layer are formed only on one main surface of such a nonmagnetic support. In a double-sided magnetic disk, an intermediate layer and a magnetic layer are formed on main surfaces on both sides of a nonmagnetic support.

Next, a method for forming the intermediate layer and the magnetic layer will be described.

In order to form the intermediate layer and the magnetic layer, the respective compositions are kneaded and dispersed together with an organic solvent to prepare a coating for the intermediate layer (hereinafter referred to as a lower layer coating) and a coating for the magnetic layer (hereinafter referred to as an upper layer coating). Is prepared).

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 and propyl acetate. , Ethyl lactate, ester solvents such as ethylene glycol acetate, diethylene glycol dimethyl ether, ether solvents such as 2-ethoxyethanol, tetrahydrofuran, dioxane, aromatic hydrocarbon solvents such as benzene, toluene, xylene, methylene chloride, ethylene Halogenated hydrocarbon solvents such as chloride, carbon tetrachloride, 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 upper layer paint and lower layer paint are applied to a non-magnetic 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, the wet-on-wet coating method is desirably used as the coating method from the viewpoints 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.

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 in this manner 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 the 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, a reverse roll coating method, or the like. 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.

When manufacturing a magnetic disk having an intermediate layer and a magnetic layer only on one side of a nonmagnetic support,
After the lower layer coating film and the upper layer coating film are formed in this manner, a magnetic disk is manufactured by performing a surface smoothing treatment such as a calendering treatment as necessary and punching out a disk having a predetermined diameter.

When a double-sided magnetic disk having an intermediate layer and a magnetic layer on both sides of a non-magnetic support is manufactured, a lower layer coating and an upper layer coating are formed on one side of the non-magnetic support. Similarly, a lower layer coating and an upper layer coating are formed on the opposite side of the nonmagnetic support, and the coating on both sides is subjected to a surface smoothing treatment. Then, a magnetic disk is manufactured by punching a raw material having a lower layer coating film and an upper layer coating film formed on both surfaces into a disk shape having a predetermined diameter.

Further, in the case of a double-sided magnetic disk, a lower-layer coating film and an upper-layer coating film are formed on one side of a non-magnetic support, and then the coating on this surface is subjected to a surface smoothing treatment. On the opposite side, a lower layer coating film and an upper layer coating film may be formed and a surface smoothing treatment may be performed, followed by punching into a disk shape.

[0074]

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

The acicular ratio of the nonmagnetic powder contained in the intermediate layer
Experiment concerning First, the manufacturing process of the sample disc was prepared in the following experimental examples. As shown in FIG. 2, the prepared sample disks include an intermediate layer 2 and a
3 and the magnetic layers 4 and 5 are formed.

To produce this sample disk,
First, a coating material for a magnetic layer was prepared based on the following composition. The coating was performed by mixing a ferromagnetic powder, a binder, an additive, and a solvent, kneading with a continuous kneader, and further dispersing the mixture with a sand mill for 5 hours. However, Al ₂ O ₃ is
After the slurry was formed and dispersed, the mixture was mixed with another composition.

Coating composition for magnetic layer Magnetic powder: 100 parts by weight of Al, Si-coated Fe metal ferromagnetic powder (average major axis length: 0.2 μm, needle ratio: 9, saturation magnetization: 130 Am ² / kg, Magnetic force: 135 kA / m) Binder: vinyl chloride-based copolymer 14 parts by weight (degree of polymerization 150, containing potassium oxysulfonate 5 × 10 ⁻⁵ mol / g as a polar functional group) 6 parts by weight of polyester polyurethane resin ( 1 × 10 ^-4 sodium oxysulfonate as a polar functional group mol / g) Additive: Al ₂ O ₃ 5 parts by weight (primary particle size 0.20 μm) Stearic acid 3 parts by weight Heptyl stearate 3 parts by weight Solvent: methyl ethyl ketone 150 parts by weight Cyclohexanone 150 parts by weight After mixing the non-magnetic powder, the binder, the additives, and the solvent based on the composition, the mixture was kneaded by a continuous kneader, and further dispersed by a sand mill for 3 hours to prepare a coating for an intermediate layer. However, as the nonmagnetic powder, various α iron oxides having different needle ratios and major axis lengths as shown in Table 1 were used.

Coating composition for intermediate layer Non-magnetic powder: 100 parts by weight of α-Fe ₂ O ₃ Binder: 20 parts by weight of vinyl chloride copolymer (degree of polymerization 150, potassium oxysulfonate 5 × as polar functional group 10 ^-5 mol / g-containing) additives: stearic acid 3 parts by weight heptyl stearate 3 parts by weight solvent: Methyl ethyl ketone 150 parts by weight cyclohexanone 150 parts by weight the thus prepared magnetic layer coating material, the intermediate layer coating material, 4 parts by weight and 2 parts by weight of polyisocyanate were added. Then, the magnetic layer paint was used as an upper layer paint and the intermediate layer paint was used as a lower layer paint, and was simultaneously overlaid on a 62 μm-thick polyethylene terephthalate film (surface roughness Ra: 8 nm) using a 4-lip die coater.
Next, in the undried state, the magnetic coating film was subjected to a non-orientation treatment using a solenoid coil, and then dried. Next, the coating material for the magnetic layer and the coating material for the intermediate layer were similarly applied on the back surface of the film in a multi-layer manner, subjected to a non-orientation treatment, and then dried. The coating thickness of each layer was set to 0.25 μm for the magnetic layer and 2.0 μm for the intermediate layer after drying. Then, the surface of the magnetic coating film is smoothed by calendering, punched into a 3.5-inch diameter, and then left at a temperature of 60 ° C. for 48 hours to remove the distortion of the non-magnetic support. Sample disks 1 to 8) were prepared.

Table 1 shows the curl amount of the sample disk produced as described above, together with the needle ratio and long axis length of the nonmagnetic powder used in the intermediate layer.

Here, the major axis length and the minor axis length of the non-magnetic powder are calculated by randomly selecting 200 non-magnetic powders from a transmission electron micrograph at a magnification of 50,000 to 100,000, and calculating the average value. I asked by that.

The amount of curl of the sample disk is the difference between the highest and lowest parts of the warp of the disk, and this difference was measured by observing the sample disk with a microscope.

[0082]

[Table 1]

As can be seen from Table 1, the curls of Sample Disks 2 to 5 using α-Fe ₂ O ₃ having a needle ratio of 10 or more were suppressed to less than 1.0 mm.
It has a good disk shape. On the other hand, the sample disks 6 to 8 using α-Fe 2 O 3 having a needle ratio of less than 10 have a curl amount of more than 1.0 mm, which causes trouble when the head floating recording method is used. Probability is high.

However, the sample disk 2 using α-Fe ₂ O ₃ having a needle ratio of 30 has a smaller needle ratio of α-Fe ₂ O _3.
-Fe ₂ O ₃ sample disk 3 using, curl amount in comparison with the sample disk 4 is large. Further, a sample disk 1 using α-Fe ₂ O ₃ having a needle ratio of 38 was used.
In this case, at the same time as the curling amount increased, the dispersion was significantly deteriorated, and the surface properties were deteriorated to the extent that they could be visually confirmed. This is presumed to be because α-Fe ₂ O ₃ having an excessively high acicular ratio becomes insufficiently dispersed to form an agglomerate and behaves as a component having an apparently low acicular ratio.

Therefore, in order to suppress the warpage of the magnetic disk, the needle ratio of the nonmagnetic powder used in the intermediate layer should be 10%.
It turned out that it is necessary to use a thing of ~ 30.

As a nonmagnetic powder contained in the intermediate layer,
Of non-magnetic powder having a needle-like ratio of other non-magnetic powder
As a non-magnetic powder of the coating material for the intermediate layer, α-Fe ₂ O ₃ (needle ratio: 10, major axis length: 0.15 μm) and spherical carbon black were mixed at the ratio shown in Table 2. Powder or α-
Except that a mixed powder obtained by mixing Fe ₂ O ₃ (needle ratio: 10, major axis length: 0.15 μm) and needle titanium oxide (needle ratio: 5) at the ratio shown in Table 3 was used, Similarly, sample disks (sample disks 9 to 16) were prepared, and the curl amount was measured. The results are shown in Tables 2 and 3.

[0087]

[Table 2]

[0088]

[Table 3]

As can be seen from Tables 2 and 3, the sample disks 12 and 16 using spherical carbon black or needle-like titanium oxide alone have extremely large warpage. On the other hand, sample disks 5 and sample disks 9 to 11, sample disks 13 to 13 in which α-Fe ₂ O ₃ having a needle ratio of 10 was mixed with spherical carbon black or needle-like titanium oxide at a ratio of 10% by weight or more.
The sample disk 15 has a small warpage.

Thus, when a non-magnetic powder having an acicular ratio outside the predetermined range is used in combination, the non-magnetic powder having an acicular ratio of 10 to 30 is used in such an amount that the proportion of the non-magnetic powder does not become less than 10% by weight. It turned out to be desirable.

As the binder contained in the intermediate layer,
Experimental example in which different binders were mixed and used α-Fe ₂ O ₃ (needle ratio: 10, major axis length: 0.15 μm) was used alone as a nonmagnetic powder of the coating material for the intermediate layer, and was used as a binder. Vinyl chloride copolymer having a degree of polymerization of 150 (5 × 10 5 potassium oxysulfonate as a polar functional group)
^-5 mol / g) and a vinyl chloride copolymer having a degree of polymerization of 300 (containing potassium oxysulfonate as a polar functional group in an amount of 5 × 10 ^-5 mol / g) in a ratio shown in Table 4. Sample disks (sample disk 17 to sample disk 19) were prepared in the same manner as described above except that was used.

Then, the curl amount and the surface roughness Ra of the manufactured sample disk were measured.

The surface roughness Ra is JIS B 06
It is a center line average roughness (Ra) defined in No. 01 and measured by a non-contact optical surface roughness meter (manufactured by ZYGO).

[0094]

[Table 4]

In Table 4, first, a sample disk 5 using a polyvinyl chloride resin having a polymerization degree of 150 alone,
Comparing the sample disk 19 using the polyvinyl chloride resin having a degree of polymerization of 300 alone, the sample disk 1
9 shows that the surface roughness Ra is larger and the curl amount is larger than that of the sample disk 5. The reason why the curl amount is large in the sample disk 19 is that when the binder having a high degree of polymerization is used alone, the dispersion of the nonmagnetic powder becomes insufficient and the nonmagnetic powder becomes agglomerates, and the apparent needle-like ratio is reduced. Presumed to behave as a low component. Also,
The reason why the surface roughness Ra is high is that the surface property of the intermediate layer is impaired due to insufficient dispersion of the nonmagnetic powder, and the surface property of the magnetic layer is impaired to reflect the impairment. Therefore, it is not preferable to use a binder having a polymerization degree of more than 200 alone as the binder of the intermediate layer from the viewpoint of surface properties and warpage.

However, even if a vinyl chloride copolymer having a degree of polymerization of 300 was used, the sample disk 1 was mixed with a vinyl chloride copolymer having a degree of polymerization of 150.
7. In the sample disk 18, the curl amount is 1.0 mm
And the surface roughness is also a low value.

Therefore, when a resin having a degree of polymerization of more than 200 is used as a binder for the intermediate layer, it is desirable to add a resin having a degree of polymerization of 200 or less in an amount of 10% by weight or more. I understood.

[0098]

As is apparent from the above description, in the magnetic disk of the present invention, an intermediate layer containing nonmagnetic powder is formed on at least one side of a flexible nonmagnetic support.
A magnetic layer containing a magnetic powder is formed thereon, and the nonmagnetic powder contains a nonmagnetic powder having a needle ratio of 10 to 30, so that the warpage of the disk can be suppressed to a small value.
Therefore, when recording / reproducing is performed by the head flying type recording method, the fluctuation of the head flying characteristics is small, and a stable output 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, 2,3 intermediate layer, 4,5 magnetic layer

Claims (4)

[Claims] An intermediate layer containing a non-magnetic powder is formed on at least one principal surface of a flexible non-magnetic support, and a magnetic layer containing a magnetic powder is formed thereon. A magnetic disk, wherein the magnetic powder contains an acicular nonmagnetic powder having an acicular ratio of 10 to 30. 2. The magnetic material according to claim 1, wherein the content of the acicular nonmagnetic powder having an acicular ratio of 10 to 30 is at least 10% by weight of the total amount of the nonmagnetic powder contained in the intermediate layer. disk. 3. The magnetic disk according to claim 1, wherein the intermediate layer contains carbon black. 4. The intermediate layer comprises at least a nonmagnetic powder and a binder, and the binder contains at least 10% by weight of a vinyl chloride copolymer containing a polar group having a degree of polymerization of 100 to 200. 2. The magnetic disk according to claim 1, wherein: