JPH09297912A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium which has the smoothness of a surface adequate for high-density recording and has high electromagnetic conversion characteristics as a wet-onwet coating type magnetic recording medium having a thin magnetic layer as extremely thin as <=0.5μm. SOLUTION: This magnetic recording medium is constituted by providing the surface of a nomnagnetic base 1 with a lower layer nonmagnetic layer 4 formed by dispersing nonmagnetic powder into a binder and forming an upper magnetic layer 2 formed by dispersing ferromagnetic power into a binder on this lower layer nonmagnetic layer 4. The binder of the lower nonmagnetic layer 4 contains a vinyl chloride copolymer which is mainly composed of vinyl chloride, contains an alkaline metal salt of a sulfonic acid group as a functional group and has a weight average mol.wt. of 8000 to 15000. The average thickness of the upper layer magnetic layer 2 is <=50μm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium, and more particularly to a very thin multilayer coating type magnetic recording medium in which a lower layer is a non-magnetic layer and an upper layer is 0.5 μm or less.

[0002]

2. Description of the Related Art As a magnetic recording medium, a magnetic coating is prepared by dispersing a ferromagnetic powder, a binder, and various additives together with an organic solvent, and then coating and drying it on a non-magnetic support to form a magnetic layer. A so-called coating type magnetic recording medium is known, and metal fine particles have been used as the above-mentioned ferromagnetic powder for the purpose of high density recording and shortening of wavelength.

A coating type magnetic recording medium using such metal fine particles is utilized as a recording medium for computers such as a magnetic tape for audio or video, a high density floppy disk, a data cartridge for backup, etc. In addition to becoming the mainstream of magnetic recording media in No. 6, there is a remarkable improvement in characteristics.

In order to realize high density recording of the coating type magnetic recording medium, thinning of the magnetic layer, improvement of the ferromagnetic powder, smoothing of the medium surface, improvement of calendering, etc. are mentioned. To be

First, thinning the magnetic layer is a method of improving electromagnetic conversion characteristics by reducing self-demagnetization loss during recording, and various coating methods have been proposed in recent years.

[0006] For example, when the thickness of the magnetic layer is made extremely thin such as about 2 μm or less, the surface shape of the non-magnetic support tends to be easily projected on the surface of the magnetic layer, and the surface of the magnetic layer becomes rough. . As a result, the electromagnetic conversion characteristics deteriorate due to the spacing loss, and dropout frequently occurs.

Therefore, a lower non-magnetic layer having a relatively large thickness is interposed between the magnetic layer and the non-magnetic support to make it difficult for the surface shape of the non-magnetic support to appear on the surface of the magnetic layer. Therefore, there is a so-called multi-layer coating method in which an upper magnetic layer and a lower non-magnetic layer are simultaneously coated on a non-magnetic support by a die coater. This multi-layer coating method is also effective as a method for improving the adhesiveness at the interface between the upper and lower magnetic layers, and is becoming the main coating method of recent multi-layer coating type magnetic recording media.

Next, as an improvement of the ferromagnetic powder, (1)
The use of a ferromagnetic alloy powder as the ferromagnetic powder, (2) miniaturization of the ferromagnetic powder, (3) increase of coercive force of the ferromagnetic powder and homogenization of coercive force distribution can be mentioned.

With respect to the above (1) and (2), as a result of positively promoting the improvement of magnetic materials, at present, a ferromagnetic powder having a saturation magnetization of more than 140 Am ² / kg and a long axis length of 0.1 μm.
Ferromagnetic powders of m or less have been developed. Regarding (3), a ferromagnetic powder having a coercive force of more than 160 kA / m 2 has also appeared, and the particle size distribution that reflects the coercive force distribution is extremely uniform, which is a remarkable development.

Further, smoothing of the medium surface has been studied for the purpose of minimizing spacing loss during recording and reproduction. In high density recording, since the recording wavelength used is short, it is easily affected by surface roughness, and it is particularly important to control the surface roughness.

As a method of smoothing the surface of the coating type magnetic recording medium, generally, a method of improving the dispersion of the powder to be contained or a method such as calendering has been carried out, and various studies have been conducted conventionally. It was For example, methods such as using a binder that strongly interacts with the powder to improve the dispersion of the powder, using various dispersants, and using a disperser having high dispersion efficiency have been put into practical use.

On the other hand, in the calendering process, various improvements have been made such as high temperature treatment and the use of a calendering device composed only of steel rolls. It is also effective to use a binder with a low glass transition point to improve the moldability during calendering, but it also has a negative effect on practical properties such as impairing running durability. There is a tendency to use high point binders.

By the way, in order to realize a smooth surface for high density recording, it is necessary to further refine the powder contained in the coating layer to further improve the dispersion.

However, sufficient dispersion cannot be obtained only by the above various methods, and it has become difficult to form a smooth coated surface. In addition, even if the magnetic recording medium having such a coating surface is subjected to calendar treatment,
The reality is that the desired smooth surface cannot be obtained.

In particular, in the case of a multi-layer coating type magnetic recording medium composed of an extremely thin upper magnetic layer of 0.5 μm or less and a lower non-magnetic layer, the ferromagnetic powder contained in the upper magnetic layer is uniformly dispersed. , The surface shape of the lower non-magnetic layer is likely to appear on the surface of the upper magnetic layer due to the thin thickness of the upper magnetic layer, and the non-magnetic powder contained in the lower non-magnetic layer is uniformly dispersed to apply the lower non-magnetic layer alone. In such a case, it is important to keep the coated surface smooth.

As described above, the lower non-magnetic layer plays a large role in smoothing the surface of the multi-layer coating type magnetic recording medium, and a method for achieving uniform dispersion of the non-magnetic powder contained in the lower non-magnetic layer is established. Is desired.

Therefore, the present invention has a surface smoothness suitable for high density recording in a multilayer coating type magnetic recording medium having an extremely thin magnetic layer of 0.5 μm or less,
Another object of the present invention is to provide a magnetic recording medium having high electromagnetic conversion characteristics.

[0018]

In the magnetic recording medium of the present invention, a lower non-magnetic layer formed by dispersing non-magnetic powder in a binder is provided on a non-magnetic support, and a strong non-magnetic layer is formed on the lower non-magnetic layer. In a magnetic recording medium in which an upper magnetic layer is formed by dispersing magnetic powder in a binder, the binder of the lower non-magnetic layer is composed mainly of vinyl chloride, and an alkali metal salt of a sulfonic acid group is used as a functional group. Including weight average molecular weight 80
It is characterized by containing a vinyl chloride-based copolymer of 00 to 15000.

The magnetic recording medium of the present invention is characterized in that the average thickness of the upper magnetic layer is 0.5 μm or less.

Further, the present invention relates to a magnetic recording medium characterized in that the content of the vinyl chloride copolymer is 40% by weight or more based on the total binder contained in the lower non-magnetic layer.

The magnetic recording medium of the present invention comprises a vinyl chloride-based copolymer having a weight average molecular weight of 8000 to 15000, which is mainly composed of vinyl chloride as a binder of the lower non-magnetic layer and contains an alkali metal salt of a sulfonic acid group as a functional group. By containing the polymer, the dispersion of the non-magnetic powder contained in the lower non-magnetic layer is improved, and a smooth coated surface can be obtained when the lower non-magnetic layer is coated alone. Therefore, the surface smoothness of the medium having the multilayer structure in which the magnetic layer is provided on the lower non-magnetic layer is also improved, and the surface property after the calendar treatment is further improved.

Therefore, it is possible to provide a magnetic recording medium suitable for high density recording having an extremely thin magnetic layer of 0.5 μm or less.

The weight average molecular weight of the vinyl chloride copolymer in the present invention is 8,000 to 15,000, but if the molecular weight is lower than this range, the thickness of the binder adsorbed on the non-magnetic powder is insufficient and the inter-particle It is not suitable because aggregation easily occurs.

The amount of the alkali metal salt of sulfonic acid group contained in the above vinyl chloride copolymer is 10 ^-1 to 10 ^-8 m.
ol / g, preferably 10 ⁻² to 10 ⁻⁶ mol / g. If the amount of the functional group is too small, the effect on the dispersion of the non-magnetic powder will not be exhibited. On the other hand, if the amount of the functional group is too large, the viscosity of the coating will increase and the dispersion of the non-magnetic powder will decrease.

The vinyl chloride copolymer may contain an epoxy group or a hydroxyl group for the purpose of improving running durability. The content of the vinyl chloride-based copolymer is 40% by weight or more with respect to the total amount of the binder. However, if the content is less than this, the effect of adding the binder is not sufficient, so that the dispersion of the non-magnetic powder is insufficient. Will be insufficient.

The molecular weight of the binder is measured by gel permeation chromatography (GPC) method.

In the present invention, the thickness of the upper magnetic layer is set to 0.
When the thickness is 5 μm or less, the self-demagnetization loss can be reduced, and the reproduction output at a short wavelength is improved. Moreover, since the magnetic layer is thin, the overwrite characteristic can be improved.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

As shown in FIG. 1, the magnetic recording medium to which the present invention is applied is constructed by forming two or more layers of a lower non-magnetic layer 4 and an upper magnetic layer 2 on a non-magnetic support 1. It is a multilayer coating type magnetic recording medium.

That is, on one surface of the non-magnetic support 1,
It has a magnetic layer 2 containing a ferromagnetic powder and a binder via a lower non-magnetic layer 4 containing a non-magnetic powder and a binder.

Further, on the surface (back surface) on which the above-mentioned magnetic layer is not provided on the non-magnetic support 1, a back coat layer 3 is provided for the purpose of improving the running property of the magnetic recording medium, preventing charging and preventing transfer. May be provided.

An undercoat layer may be provided between the lower non-magnetic layer 4 and the non-magnetic support 1 for the purpose of enhancing the adhesiveness between the coating film and the support 1. This subbing layer is
Needless to say, it is different from the lower non-magnetic layer in the present invention.

In this multilayer coating type magnetic recording medium,
The lower non-magnetic layer 4 and the upper magnetic layer 2 are prepared by kneading and dispersing a ferromagnetic powder and a binder with an organic solvent, and a magnetic paint for the upper layer prepared by kneading and dispersing the non-magnetic powder and a binder with an organic solvent. The magnetic coating material and the non-magnetic powder for the lower layer prepared as described above are applied and dried, respectively.

That is, the binder of the lower non-magnetic layer is composed mainly of vinyl chloride and contains, as a functional group, an alkali metal salt of a sulfonic acid group, a weight average molecular weight of 8000 to 1
It contains 5,000 vinyl chloride copolymers. Further, the content of the vinyl chloride-based copolymer is 40% by weight or more with respect to the total binder contained in the lower non-magnetic layer.

The average thickness of the upper magnetic layer is 0.5.
It is formed to be less than μm.

On the other hand, the other binder contained in the lower non-magnetic layer of the present invention is a known thermoplastic resin, thermosetting resin, reactive resin or the like which has been conventionally used as a binder for magnetic recording media. Can be used, in particular a weight average molecular weight of 1
Those of 5,000 to 200,000 are preferable. These binders are used for the purpose of imparting running durability, flexibility and toughness to the magnetic recording medium, and improving adhesion with the non-magnetic support.

Examples of thermoplastic resins include vinyl chloride,
Vinyl acetate, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylic ester-acrylonitrile copolymer, acrylic ester-vinyl chloride-vinylidene chloride copolymer , Vinyl chloride-acrylonitrile copolymer, acrylic ester-acrylonitrile copolymer, acrylic ester-vinylidene chloride copolymer, methacrylic acid ester-vinylidene chloride copolymer, methacrylic acid ester-vinyl chloride copolymer, methacrylic acid Ester-ethylene copolymer, polyvinyl fluoride, vinylidene chloride-allylonitrile copolymer, acrylonitrile-butadiene copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate butyrate, cellulose) Scan diacetate, cellulose triacetate, cellulose propionate, nitrocellulose), styrene-butadiene copolymer, polyurethane resin, polyester resin, amino resin, synthetic rubber, and the like.

Examples of the thermosetting resin or reactive resin include phenol resin, epoxy resin, polyurethane curable resin, urea resin, melamine resin, alkyd resin, silicone resin, polyamine resin, urea formaldehyde resin and the like. To be

All of the above-mentioned binders contain -SO ₃ M and -OSO ₃ for the purpose of improving the dispersibility of the non-magnetic pigment.
A polar functional group such as M, —COOM and P═O (OM) ₂ may be introduced. Here, M in the formula is a hydrogen atom or an alkali metal such as lithium, potassium, and sodium. Further, as the polar functional group, -NR ₁ R
₂ , there is a side chain type having an —NR ₁ R ₂ R ₃ ⁺ X ⁻ terminal group, and a main chain type> NR ₁ R ₂ ⁺ X ⁻ . However, in the formula, R ₁ , R ₂ , and R ₃ represent a hydrogen atom or a hydrocarbon group, and X ⁻ represents a halogen element ion such as fluorine, chlorine, bromine, or iodine, or an inorganic / organic ion. Also, -O
A polar functional group such as H, -SH, -CN, or an epoxy group may be introduced.

The amount of these polar functional groups introduced into the binder is
It is preferably from 10 ^-1 to 10 ^-8 mol / g, and more preferably from 10 ^-2 to 10 ^-6 mol / g.

These binders may be used alone or in combination of two or more. The amount of these binders used is 100 parts by weight of the non-magnetic powder,
1 to 10 in combination with the vinyl chloride binder in the present invention
It is 0 part by weight, preferably 10 to 50 parts by weight. When one kind of the vinyl chloride binder in the present invention is used alone, it is similarly 1 to 1 per 100 parts by weight of the non-magnetic powder.
The amount is preferably 00 parts by weight, preferably 10 to 50 parts by weight.

If the amount of the binder used is too large, plastic sliding of the coating film is likely to occur due to repeated sliding in the disk drive, and running durability is deteriorated. On the other hand, if the amount of the binder is too small, problems such as poor dispersion of the non-magnetic powder and a decrease in the mechanical strength of the coating film occur.

In the present invention, the non-magnetic powder used in the lower non-magnetic layer is, for example, α-F.
Non-magnetic iron oxide such as e ₂ O ₃ , 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,
There are molybdenum disulfide, magnesium carbonate, calcium carbonate, barium carbonate, strontium carbonate, barium titanate and the like, and these powders can be used alone or in combination of two or more.

The above non-magnetic powder can be doped with an appropriate amount of impurities according to the purpose, and for the purpose of improving dispersibility, imparting conductivity, improving color tone, etc., Al and S are added.
Surface treatment with a compound such as i, Ti, Sn, Sb, or Zr is also possible. The specific surface area of the non-magnetic powder is 30 to 80 m ² / g, preferably 40 to 70 m ² / g. Further, if necessary, a furnace for rubber, pyrolytic carbon, black for color, carbon black such as acetylene black may be contained. Specific surface area is 100
To 400 m ² / g, DBP oil absorption is 20 to 200 m
It is preferably 1/100 g.

When the specific surface areas of the non-magnetic powder and carbon black are in the above ranges, the lower non-magnetic layer is smoothed with the formation of fine particles, and as a result, the upper magnetic layer can be smoothed. It is possible to obtain a magnetic recording medium that has excellent modulation noise characteristics and is less affected by spacing loss. Although the non-magnetic powder has no magnetic cohesive force and is easier to disperse than the ferromagnetic powder, if the specific surface area is larger than the above range, the powder can be obtained by the method of the present invention. Becomes difficult to disperse. If the specific surface area is too small, the surface smoothness that can withstand high density recording cannot be ensured.

The ferromagnetic powder used in the upper magnetic layer of the present invention includes 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
Alloys such as —Co—V, iron nitride, iron carbide, etc. may be mentioned. Of course, even if a light metal element such as Al, Si, P, or B added for the purpose of preventing sintering or maintaining the shape during reduction is contained in an appropriate amount, the effect of the present invention is not hindered. Furthermore, γ-Fe ₂ O ₃ , Fe ₃ O ₄ , a beltride compound of γ-Fe ₂ O ₃ and Fe ₃ O _4, and Co-containing γ-Fe ₂
O ₃ , Co-containing Fe ₃ O ₄ , γ-Fe ₂ O ₃ containing Co
Compound of Fe and O ₃ and Fe ₃ O ₄ , CrO ₂ and one or more metal elements such as Te, Sb, Fe and B
There are oxides and the like containing the above.

Further, hexagonal plate-shaped ferrite can be used, and M type, W type, Y type, Z type barium ferrite, strontium ferrite, calcium ferrite, lead ferrite, and the coercive force of these are controlled. For the purpose, Co-Ti, Co-Ti-Zn, Co-Ti-N
b, Co-Ti-Zn-Nb, Cu-Zr, Ni-Ti
The addition of such as can also be used. These ferromagnetic powders can be used alone or in combination of two or more.

The specific surface area of the ferromagnetic powder used in the present invention is 30 to 80 m ² / g, preferably 40 to 70.
It is preferably m ² / g. When the specific surface area is in the above range, high-density recording can be performed with the formation of fine particles of the ferromagnetic powder, and a magnetic recording medium having excellent noise characteristics can be obtained.

Further, the ferromagnetic powder used in the present invention has a major axis length of 0.05 to 0.30 μm and an axial ratio of 2 to 15.
It is preferred that If the major axis length is less than 0.05 μm, it will be difficult to disperse it in the magnetic paint, and the major axis length will be 0.3.
If it exceeds 0 μm, the noise characteristics may deteriorate, which is not preferable. If the axial ratio is less than 2, the orientation property of the ferromagnetic powder is lowered and the output is lowered, and if the axial ratio exceeds 15, the short wavelength signal output may be lowered, which is not preferable.

In the case of plate ferrite, the plate diameter is 0.01 to
The thickness is preferably 0.5 μm and the plate thickness is about 0.001 to 0.2 μm. As the major axis length, the axial ratio, the plate diameter, and the plate thickness, an average value of 100 or more samples randomly selected from a transmission electron micrograph is adopted.

Further, as abrasive particles in the upper magnetic layer, aluminum oxide (α, β, γ), chromium oxide, silicon carbide, diamond, garnet, emery, boron nitride, titanium carbide, silicon carbide, titanium carbide, oxide is used. Titanium (rutile, anatase) etc. may be contained. The amount of these particles is 20 parts by weight, preferably 10 parts by weight or less, based on 100 parts by weight of non-magnetic powder, and the Mohs hardness is 4 or more, preferably 5 or more, and the specific gravity is 2 to 6, preferably Is in the range of 3 to 5, the average particle size is 0.5 μm or less,
It is preferably 0.3 μm or less.

Like the binder contained in the lower non-magnetic layer, the binder contained in the upper magnetic layer of the present invention is a known thermoplastic resin or thermosetting resin conventionally used as a binder for magnetic recording media. Resins and reactive resins can be used, and those having a weight average molecular weight of 15,000 to 200,000 are preferable.

These polar functional groups may be introduced into these binders for the purpose of improving the dispersibility of the pigment, and the introduction amount is 10 ^-1 to 10 ^-8 mol / g, preferably It is 10 ^-2 to 10 ^-6 mol / g. Also in the upper magnetic layer, when a vinyl chloride binder having a weight average molecular weight of 15,000 or less used in the present invention is blended, the dispersion of the ferromagnetic powder is further improved, which is effective.

In this case, the blending ratio of the vinyl chloride binder is preferably 40 to 80% by weight based on the total amount of the binder. When the content of the vinyl chloride-based binder is less than the above range, the effect on the dispersion of the magnetic powder does not appear, and when the content is too large, the coating film may drop due to repeated sliding with a drive, which is desirable. Absent.

In the present invention, it is possible to use a polyisocyanate for crosslinking and curing the above binder. Examples of the polyisocyanate include toluene diisocyanate and its adduct, alkylene diisocyanate, and its adduct. The mixing amount of these polyisocyanates with the binder is 100
5 to 80 parts by weight, preferably 10 parts by weight
To 50 parts by weight. These polyisocyanates can be used in both upper and lower layers, or can be used by limiting to only one layer. When used in both the upper and lower layers, the compounding amount can be added to each layer in the same amount, or can be changed in an arbitrary ratio.

In the present invention, a lubricant and a surface active agent may be contained in the magnetic layer and the non-magnetic layer, if necessary. Examples of the lubricant include graphite, molybdenum disulfide, tungsten disulfide, silicone oil, fatty acids having 10 to 22 carbon atoms, and fatty acid esters of alcohols having 2 to 26 carbon atoms, terpene compounds, and There are these oligomers and the like. These lubricants can be added only to the upper magnetic layer, or can be added to both upper and lower layers.

The above-mentioned surfactants include nonionic, anionic, cationic and amphoteric surfactants. It is possible to use these surfactants for each of the upper and lower layers depending on the type and amount, or to use only one layer.

Examples of the non-magnetic support include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polyolefins such as polypropylene, celluloses such as cellulose triacetate and cellulose diacetate, vinyl resins, polyimides and polycarbonates. Examples of the material include a polymer material typified by a class, a support formed of metal, glass, ceramics, or the like.

To form a coating film on the above-mentioned non-magnetic support, the above-mentioned upper and lower non-magnetic layer forming materials are applied and dried as a paint, and the solvent used for forming this paint is acetone or methyl ethyl ketone. , Methyl isobutyl ketone, ketone solvents such as cyclohexanone, methanol,
Alcohol solvents such as ethanol and propanol, ester solvents such as methyl acetate, ethyl acetate, butyl acetate, propyl acetate, ethyl lactate and ethylene glycol acetate, diethylene glycol dimethyl ether, ether solvents such as 2-ethoxyethanol, tetrahydrofuran and dioxane, Aromatic hydrocarbon solvents such as benzene, toluene and xylene, halogenated hydrocarbon solvents such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform and chlorobenzene are used.

The above-mentioned paint is prepared by the kneading step, the mixing step, and the dispersing step. For dispersion and kneading,
A roll mill, a ball mill, a sand mill, an agitator, a kneader, an extruder, a homogenizer, an ultrasonic disperser or the like is used.

When the lower non-magnetic layer 4 and the magnetic layer 2 (upper magnetic layer) are formed on the non-magnetic support 1, a method of coating and drying one layer at a time (so-called wet-on-dry coating method) and a drying method There is a so-called wet-on-wet coating method (wet multi-layer coating method) in which a magnetic layer is overlaid and coated on a lower non-magnetic layer which is not wet.

Among these, as the coating method, it is desirable to use the wet-on-wet coating method from the viewpoints of the homogeneity of the coating film, the adhesiveness of the upper and lower interfaces, and the productivity. FIG. 2 shows an example of a coating device that coats by this wet-on-wet coating method.

This coating apparatus has a die head 18 having two slits for extruding the coating material at the tip, that is, a slit portion 11 for the lower coating material and a slit portion 12 for the upper coating material. That is, in the die head 18, the lower layer coating liquid reservoir 13 and the upper layer coating liquid reservoir 14 to which the lower layer coating material 6 and the upper layer coating material 7 are respectively supplied are formed on the back sides of the two slit portions 11 and 12, and the coating liquid The lower layer coating material 6 and the upper layer coating material 7 supplied to the reservoirs 13 and 14 are extruded to the front end of the die head through the slits 11 and 12. The die head 18
Is a 4-lip system, but there are also a 3-lip system, a 2-lip system, and the like.

On the other hand, the support 15 to which the coating material is applied runs in the direction of arrow A from the slit portion 11 for the lower layer coating material to the slit portion 12 for the upper layer coating material along the tip surface of the die head 18.

In the non-magnetic support 15 which runs in this manner, first, when passing through the slit portion 11 for the lower layer paint,
The lower layer coating material 6 extruded from the slit portion 11 is applied to the surface to form a lower layer coating film 16. When passing through the slit portion 12 for the upper layer coating material, the upper layer coating material 7 extruded from the slit portion 12 is applied on the lower layer coating film 16 in a wet state to form two-layer coating films 16 and 17. It

The lower nonmagnetic layer and the upper magnetic layer thus formed by the wet-on-wet coating method are formed by applying the upper coating material 7 on the lower coating film in a wet state. The surface of the lower layer, that is, the boundary surface between the lower non-magnetic layer 4 and the upper magnetic layer 2 is formed smoothly (see FIG. 1). Therefore, the surface property of the upper magnetic layer is 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.

As a result, the performance required as a magnetic recording medium, which is required to have high output and low noise for high density recording, is satisfied, and peeling of the film is eliminated to improve the film strength. Also, dropout can be reduced and reliability is improved.

The upper and lower non-magnetic layers formed by the wet-on-wet multi-layer coating method are mixed with components of both layers with a certain thickness except when there is substantially a clear boundary. There may be a border area. In the present invention, when such a boundary region exists, the layer below the boundary region except the boundary region is defined as the lower non-magnetic layer and the upper layer is defined as the magnetic layer.

On the other hand, for example, Japanese Patent Laid-Open No. 6-2365
When the lower non-magnetic layer 4 and the upper magnetic layer 2 are formed by the wet-on-dry method as in the case of Japanese Patent No. 43, the die head coating method and the gravure roll coating method are used to apply the lower layer coating material and the upper layer coating material. Ordinary coating methods such as reverse roll coating method are adopted.

However, in this case, the material of the lower layer must be selected so that the lower layer has sufficient solvent resistance to the upper coating material. Also, unless a solvent resistant one is selected, the smoothness of the surface is impaired, and as a result, the electromagnetic conversion characteristics are impaired.

After the above-mentioned multi-layer coating, it is introduced into a drier, and if necessary, it is introduced into a calender device and wound on a winding roll. Further, after a back coat layer is applied to the opposite surface of the multilayer coating layer, the tape is slit into, for example, 8 mm width to prepare a magnetic tape, and the magnetic tape is accommodated in a cassette to manufacture a tape cassette.

[0072]

EXAMPLES Hereinafter, specific examples and comparative examples of the present invention will be described, but it goes without saying that the present invention is not limited to these examples. Example 1 The upper magnetic layer and the lower non-magnetic layer were made into a coating material with the following composition. In order to form a paint, a pigment, a binder, an additive and a solvent were mixed according to a conventional method, and the mixture was kneaded with an extruder and then dispersed by a sand mill for 4 hours.

<Upper layer coating composition> 100 parts by weight of Fe-based metal ferromagnetic powder (coercive force = 131 kA / m 2, saturation magnetization = 125 Am ² / kg, specific surface area = 51 m ² / g, major axis length = 0.25 μm, Needle ratio = 10) Vinyl chloride copolymer 14 parts by weight (Nippon Zeon's trade name MR-110) Polyester polyurethane resin 3 parts by weight (Toyobo Co., Ltd. weight average molecular weight 35000) α-Al ₂ O ₃ 5 parts by weight Stearin Acid 1 part by weight Heptyl stearate 1 part by weight Methyl ethyl ketone 150 parts by weight Cyclohexanone 150 parts by weight <Lower layer coating composition> α-Fe ₂ O ₃ 100 parts by weight (specific surface area = 53 m ² / g, major axis length = 0.15 μm, needle) Jo ratio = 5) vinyl-based copolymer 16 parts by weight chloride (weight average molecular weight 10000, functional group _{[-SO 3 K] = 7 ×} 10 -5 mol / g) Incidentally, Examples 2-6, the types and blending ratio of the binder used in Comparative Examples 1-5 are set forth in Table 1. The functional group of the vinyl chloride copolymer is --SO ₃ in any of the examples and comparative examples.
K, and the content is the same as in Example 1.

Stearic acid 1 part by weight Heptyl stearate 1 part by weight Methyl ethyl ketone 105 parts by weight Cyclohexanone 105 parts by weight Then, immediately before coating, polyisocyanate is added to the upper layer coating in an amount of 3 parts.
By adding 3 parts by weight to the lower layer coating material, each coating material is coated on a 7 μm thick PET (polyethylene terephthalate) film using a 4-lip type die coater to form an upper magnetic layer of 0.2.
Simultaneous multi-layer coating was performed so as to have a thickness of .mu.m and a lower non-magnetic layer of 2 .mu.m, and orientation treatment was performed with a solenoid coil, followed by drying, calendering, and curing.

Further, a back coating material for forming a back coating layer having the following composition is applied to the surface opposite to the above-mentioned application surface, and 8 m
Slit into m width to make a tape.

<Back coating composition> Carbon black (trade name: Asahi # 50) 100 parts by weight Polyester polyurethane (trade name: Nipolan N-2304) 100 parts by weight Methyl ethyl ketone 500 parts by weight Toluene 500 parts by weight The center line average roughness (Ra) specified by JISB0601 was measured using a contact surface roughness meter.

The electromagnetic conversion characteristics were measured using a fixed head type electric characteristic measuring machine. This measuring machine is composed of a rotating drum and a head in contact therewith, and a magnetic tape is wound around the drum. In the actual measurement, first, a rectangular wave signal of 10 MHz was recorded at the optimum recording current of each tape,
A spectrum analyzer detects an output level of 10 MHz.

The relative speed between the magnetic tape and the magnetic head was 3.33 m / s, and a tape obtained by applying a single layer of the above magnetic paint to a thickness of 2 μm was used as a reference (0 dB) ( Ra of the surface is 5.3 nm). Table 1 shows the results.

[0079]

[Table 1]

From Table 1, in Examples 1 to 6 of the present invention, vinyl chloride having a weight average molecular weight of 8000 to 15000, which is mainly composed of vinyl chloride in the lower non-magnetic layer and contains an alkali metal salt of a sulfonic acid group as a functional group. It contains a system copolymer, the surface of the medium is smooth, and the reproduction output at 10 MHz is high due to the effect of thinning the magnetic layer.

On the other hand, in Comparative Examples 1, 2 and 5, since the vinyl chloride polymer contained in the lower non-magnetic layer had a large molecular weight and the non-magnetic powder was not sufficiently dispersed, the surface of the medium was smooth. The performance is inferior to that of the embodiment, and the reproduction output is low due to the spacing loss caused by this. In Comparative Example 3, since the vinyl chloride polymer contained in the lower non-magnetic layer has too small a molecular weight, a stable dispersion system of the non-magnetic powder cannot be obtained, smoothness is impaired and reproduction output is low. Comparative Example 4 is an example in which the vinyl chloride polymer of the present invention is incorporated in a small amount.
As is clear from the above, the smoothness of the surface is insufficient.

From the above, the magnetic recording medium of the present invention is composed mainly of vinyl chloride in the lower non-magnetic layer, and contains a vinyl chloride-based copolymer of a weight average molecular weight of 8000 to 15000 containing an alkali metal salt of a sulfonic acid group as a functional group. It can be seen that the inclusion of the coalescence improves the dispersion of the non-magnetic powder contained in the lower non-magnetic layer and makes it possible to obtain a smooth coated surface when the lower non-magnetic layer is coated alone. .

[0083]

The magnetic recording medium of the present invention is composed mainly of vinyl chloride in the lower non-magnetic layer and has a weight average molecular weight of 800 including an alkali metal salt of a sulfonic acid group as a functional group.
Since the vinyl chloride copolymer of 0 to 15000 is contained, the surface smoothness of the medium having the multilayer structure in which the magnetic layer is provided on the lower non-magnetic layer is also improved, and the surface property after the calendar treatment is further improved. . Therefore, it is a multilayer coating type magnetic recording medium suitable for high density recording having an extremely thin magnetic layer of 0.5 μm or less.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a magnetic recording medium according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of a die coater of a coating apparatus for manufacturing the magnetic recording medium of the present embodiment.

[Explanation of symbols]

1 non-magnetic support, 2 upper magnetic layer, 3 back coat layer, 4 lower non-magnetic layer

Claims (3)

[Claims] 1. A lower non-magnetic layer formed by dispersing non-magnetic powder in a binder on a non-magnetic support, and an upper layer comprising ferromagnetic powder dispersed in the binder on the lower non-magnetic layer. In a magnetic recording medium having a magnetic layer formed, a vinyl chloride-based copolymer having a weight average molecular weight of 8000 to 15000, which is mainly composed of vinyl chloride as a binder of a lower non-magnetic layer and contains an alkali metal salt of a sulfonic acid group as a functional group. A magnetic recording medium containing a polymer. 2. The average thickness of the upper magnetic layer is 0.5 μm.
2. The magnetic recording medium according to claim 1, wherein: 3. The magnetic recording medium according to claim 1, wherein the content of the vinyl chloride-based copolymer is 40% by weight or more based on the total binder contained in the lower non-magnetic layer.