JP3370212B2 - Manufacturing method of magnetic recording medium - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having a high C / N ratio in a high frequency region and having characteristics suitable for a digital recording medium.

[0002]

BACKGROUND OF THE INVENTION In recent years, development of thin-layer metal media using ferromagnetic metal powder as a tape for a digital VTR or a high-density data tape or as a medium for a high-density floppy disk has been actively pursued. By the way, in order to improve the C / N ratio for enabling high density recording as a medium for digital recording and to improve the overwrite characteristic, it is necessary to thin the magnetic layer to a thickness of about 0.1 μm. However, it was difficult to achieve this thickness by single-layer coating. Therefore, although efforts have been made to realize by using the simultaneous multilayer coating, even if the simultaneous multilayer coating is used, the film thickness of the uppermost magnetic layer is reduced to 0.3 μm or less, particularly to about 0.1 μm. In this case, the coating property was deteriorated and a lot of streaks were generated, or the surface of the multi-layered surface was roughened, and the conventional methods (JP-A-4-325917 and JP-A-6-195690) were not sufficient. For example, in order to apply a thin film to the uppermost magnetic layer, it is conceivable to reduce the amount of liquid fed during coating or dilute the coating liquid with a solvent to reduce the solid content concentration. However, the former method deteriorates the coating property and also lowers the yield, and the latter method causes shock when diluted with a solvent and deteriorates the dispersibility, and roughening of the multi-layer surface occurs during coating. In some cases, satisfactory characteristics could not be obtained.

The present inventor diligently studied a method of obtaining a satisfactory medium even if the uppermost magnetic layer is thinned to about 0.1 μm by simultaneous multilayer coating, and as a result, 1) the uppermost magnetic layer,
Setting the specific gravity of the coating liquid of the underlayer to an appropriate range, and 2) setting the specific gravity of the uppermost magnetic layer paint to 0.9 to
The average major axis length (La) satisfies a specific condition in order to obtain a stable dispersion that does not cause shock even when diluted with a large amount of solvent, so that the above-mentioned problems can be solved. The inventors have found that this is a technical point for enabling thin film coating with a thickness of 0.3 μm or less, particularly around 0.1 μm, and reached the present invention.

[0004]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention 1) It is possible to perform multi-layer coating which has good coatability even when the thickness of the uppermost magnetic layer is as thin as 0.05 to 0.3 μm and does not cause surface roughness. , 2) a high C / N ratio, and 3) a method of manufacturing a magnetic recording medium having excellent overwrite characteristics.

[0005]

Invention 1: A plurality of layers are formed on a support and the thickness of the uppermost magnetic layer is from 0.05 to
In the manufacturing method of performing multilayer coating while the underlying layer is 0.3 μm in a wet state, the specific gravity of the coating solution of the uppermost magnetic layer at 20 ° C. is 0.9 to 1.0, and It is smaller than the specific gravity of the coating liquid of the layer underlying the upper magnetic layer, and the average major axis length (La) of the ferromagnetic metal powder contained in the uppermost magnetic layer is smaller than the film thickness of the uppermost magnetic layer. A method of manufacturing a magnetic recording medium satisfying ≦ La ≦ 110 (nm).

Invention 2: Saturation magnetization (σ of ferromagnetic metal powder)
The method for producing a magnetic recording medium according to claim 1, wherein s) is 130 to 170 emu / g.

[0007]

The reason why the thin film coating of the uppermost magnetic layer becomes possible by adopting the constitution of the present invention is not necessarily clear, but it is presumed as follows.

1) First, 20 of the coating liquid for the uppermost magnetic layer
The specific gravity at 0 ° C. is set to 0.9 to 1.0. Usually, when the method of diluting with a solvent is adopted, it is 1.
Greater than 05 [2 at diluted solids (NV) concentration
Range greater than 0%]. In order to realize thin layer coating with a film thickness of 0.05 to 0.3 μm this time, the specific gravity of the coating liquid was lowered to 0.9 to 1.0, and was made smaller than the specific gravity of the coating liquid of the underlying layer. It is considered that the results of the present invention are affected.

2) Secondly, in order to set the specific gravity of the paint of the uppermost magnetic layer to 0.9 to 1.0, it is necessary to dilute it with a large amount of solvent. Metal powder (average major axis length 120 to 200 nm, σs is 120 to 12)
5 emu / g) dispersion liquid causes let-down shock when diluted with a large amount of solvent, resulting in poor dispersibility. In addition, the creep deformation amount is small from the rheological aspect of the paint, and roughening of the multi-layer surface occurs. There was a problem that a good multi-layer coating property could not be obtained. Therefore, as a result of repeated experiments to make a paint having good dispersibility and large creep deformation even when diluted with a large amount of solvent, it was found that the average major axis length (La) of the ferromagnetic metal powder was the film thickness of the magnetic layer. Smaller, 30-11
0 nm, and σs is preferably 130 to 170e
It has been found that the intended coating material can be obtained when the one having a mu / g is used.

According to the conventional knowledge, the average major axis length of 120-
Σs of ferromagnetic metal powder of 200 nm which is usually used is 1
It has been known that when the number is increased to 30 or more, the dispersibility is deteriorated due to agglomeration between magnetic particles. Here, the average major axis length of the ferromagnetic metal powder is smaller than the film thickness of the magnetic layer, and
At the same time, the particle size is set to 110 nm, which is smaller than that of the conventional one.
In particular, when a ferromagnetic metal powder having σs of 130 to 170 emu / g is used, surprisingly, the coating solution is diluted with a large amount of solvent and the specific gravity of the coating solution is set to 0.9 to 1.0 without causing shock. It was possible to obtain a coating liquid having good rheological properties suitable for multi-layer coating, which maintains good dispersibility and has a large amount of creep deformation. Here, the specific gravity of the coating liquid for the uppermost magnetic layer is preferably 0.92 to 1.0, and more preferably 0.95 to 1.0. When the coating solution is diluted with a solvent, the solvent is usually added little by little while stirring with a dissolver to dilute. Further, it is more preferable to dilute while sending the solvent using a sand mill.

By setting the average major axis length of the ferromagnetic metal powder to be smaller than the film thickness of the uppermost magnetic layer and set to 30 to 110 nm in the fine particle region as compared with the conventional case, even if the coating solution is diluted with a large amount of solvent, shock is generated. The reason why good dispersibility is maintained without causing the above is not clear, but it can be considered as follows by paying attention to the interparticle distance between the magnetic powders after dilution. That is, when the conventional magnetic powder having a relatively large particle size with an average major axis length of more than 110 nm is used, the interparticle distance between the magnetic powders becomes large when diluted with a large amount of solvent.
The mutual interaction becomes small, the paint cannot form an appropriate structure, the amount of creep deformation becomes small, and a coating solution having rheological properties suitable for multilayer coating cannot be obtained. On the other hand, when the average major axis length of the magnetic powder is smaller than the film thickness of the uppermost magnetic layer and is 30 to 110 nm, and fine particle powder is used conventionally, even if the coating solution is diluted with a large amount of solvent, the mutual magnetic powder It is possible to keep the distance between the fine particles to a certain degree small. Therefore, since an appropriate interaction works between the magnetic powders, the structure of the coating material can be maintained, the amount of creep deformation is large, and a coating liquid having a rheological property suitable for multilayer coating can be obtained. Here, by setting the saturation magnetization (σs) of the ferromagnetic metal powder to 130 to 170 emu / g, the saturation magnetization per one magnetic powder can be maintained even when the particles are made fine, and the magnetic powders also have a suitable magnetic interaction. It works and is preferable from the viewpoint of the rheology of the paint.

[0012]

DETAILED DESCRIPTION OF THE INVENTION The magnetic recording medium of the present invention will be described in detail below. —Structure of Magnetic Recording Medium— The magnetic recording medium of the present invention comprises a non-magnetic support (1),
At least one non-magnetic layer between the non-magnetic support and the magnetic layer as an underlying layer in the uppermost magnetic layer (2) containing a ferromagnetic powder, preferably a ferromagnetic metal powder. (3) is provided. Further, a back coat layer (4) is provided if necessary.

(1) Non-magnetic support As the material for forming the non-magnetic support, for example, polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polyolefins such as polypropylene, cellulose triacetate, cellulose die, etc. Cellulose derivatives such as acetate, polyamide,
Examples include plastics such as aramid resin and polycarbonate.

The form of the non-magnetic support is not limited, and there are mainly tape, film, sheet, card, disk, drum and the like. The thickness of the non-magnetic support is not particularly limited, but for example, in the case of a film or sheet, it is usually 2 to 100 μm, preferably 3 to 50 μm.
m, and in the case of a disk or card, 30 μm to 10
mm, and in the case of a drum shape, it is appropriately selected according to the recorder or the like.

The non-magnetic support may have a single-layer structure or a multi-layer structure. The non-magnetic support may be subjected to surface treatment such as corona discharge treatment. A back coat layer is preferably provided on the surface (back surface) of the non-magnetic support, on which the magnetic layer is not provided, for the purpose of improving the running property of the magnetic recording medium, preventing electrification, and preventing transfer. If desired, an overcoat layer may be provided on the uppermost magnetic layer.

(2) Uppermost magnetic layer The uppermost magnetic layer contains a ferromagnetic metal powder having an average major axis length (La) of 30 to 110 nm observed by a transmission electron microscope. Here, La is preferably 40 to 11
It is 0 nm, more preferably 50 to 100 nm. La
By setting in this range, even if diluted with a large amount of solvent, shock is not caused, good dispersibility is maintained, and a large amount of creep deformation, a coating solution with suitable rheological properties for multilayer coating is obtained. To be

At this time, the crystallite size (Sa) by X-ray diffraction is preferably 5 ≦ Sa ≦ 17 (nm), and more preferably 6 ≦ Sa ≦ 15 (nm). When the size of the crystallite is in this range, sufficient recording can be obtained, and when the size of the crystallite is less than 5 nm, the magnetic powder is likely to exhibit paramagnetism and magnetic recording may not be performed. Further, it may contain a binder and, if necessary, other components.

In this specification, the average major axis length is an average value of 500 major axis lengths of ferromagnetic powder or nonmagnetic powder measured by a transmission electron micrograph. The axial ratio was determined by measuring the average major axis length and the average minor axis length of 500 particles using an electron micrograph (average major axis length / average minor axis length). The crystallite size was measured by the Scherrer method using Si powder as a reference, using the integral width of the (110) diffraction line of Fe with an X-ray diffractometer.

The shape and axial ratio of the magnetic powder can be controlled by combining known methods such as selection of a starting material as a starting material, selection of oxidation-reduction conditions, and selection of a sintering inhibitor. it can.

The thickness of the uppermost magnetic layer is 0.05 to
0.3 μm, preferably 0.05 to 0.2 μm
And more preferably 0.07 to 0.15 μm. If the dry thickness of the uppermost magnetic layer is less than 0.05 μm, the recording may not be sufficiently performed, and thus output may not be obtained during reproduction, while 0.3 μm for a high-density digital recording medium. When it exceeds, the sufficient reproduction output may not be obtained due to the film thickness loss and the overwrite characteristic may be deteriorated. Furthermore, the coercive force Hc of the magnetic recording medium
Is preferably 1700 to 3000 Oe, more preferably 2000 to 2500 Oe. In this range, sufficient recording can be obtained.

(2-1) Magnetic Powder In the present invention, the uppermost layer which is a magnetic layer contains a specific ferromagnetic metal powder described later. As the ferromagnetic metal powder, any ferromagnetic metal powder can be used, and at least one selected from the group consisting of Fe, Al, Co, and Sm, Nd, Y, Pr, and La as its constituent elements. It is preferable that the rare earth element is included.

In the ferromagnetic metal powder of the present invention, the abundance ratio of Fe, Al, Co and at least one rare earth element selected from the group consisting of Sm, Nd, Y, Pr and La in the overall composition is: Al atoms are 2 to 20 parts by weight and Co atoms are 10 to 100 parts by weight of Fe atoms.
To 60 parts by weight, and the amount of one or more rare earth elements selected from the group consisting of Sm, Nd, Y, Pr and La is 1 to 1
It is preferably 6 parts by weight. Further, Fe, Al, Co and Sm, Nd, Y, Pr and La on the surface thereof
The abundance ratio of one or more rare earth elements selected from the group consisting of is 100 for Fe atoms and 70 for Al atoms.
The number of atoms of one or more rare earth elements selected from the group consisting of Sm, Nd, Y, Pr and La is 0.
It is preferably 5 to 50.

Further, the ferromagnetic metal powder further contains Na and Ca as its constituent elements, and Ca, Ba, Sr, M
The total abundance ratio of one or more elements selected from g is F
It is preferably 0.001 to 20 parts by weight with respect to 100 parts by weight of e atom. Further, the ferromagnetic metal powder further contains Ni and Si as constituent elements, and Fe atom 10
Ni atom is 2 to 20 parts by weight with respect to 0 parts by weight,
The Si atom content is preferably 0.01 to 5 parts by weight. It is preferable to select the above-mentioned element as a constituent element of the ferromagnetic metal powder because it has an excellent effect of preventing sintering, the magnetic powder is made into fine particles, and the saturation magnetization (σs) is increased.

The content of the ferromagnetic metal powder is usually 60 to 95% by weight based on the total solid content in the layer.
And preferably 70 to 90% by weight.

The ferromagnetic metal powder uses iron oxyhydroxide as a starting stock solution, which is obtained by blowing an oxidizing gas into an aqueous suspension prepared by mixing a ferrous salt and an alkali. Such iron oxyhydroxide may be obtained by reacting an aqueous ferrous salt solution with an alkaline aqueous solution to obtain an aqueous solution containing ferrous hydroxide, and oxidizing the aqueous solution by air oxidation or the like. preferable. The type of iron oxyhydroxide is α-F
eOOH is preferable, and as a method for producing the same, a ferrous salt is neutralized with an alkali hydroxide to obtain a water suspension of Fe (OH) ₂ , and an oxidizing gas is blown into the suspension to form needle-shaped α-OH. F
There is a first production method using eOOH. Here, the needle-like shape refers to a material having an average major axis length / average minor axis length of 2 to 20.
On the other hand, there is a second manufacturing method in which a ferrous salt is neutralized with an alkali carbonate to form a FeCO ₃ aqueous suspension, and an oxidizing gas is blown into the suspension to form a spindle-shaped α-FeOOH. For the spindle-shaped α-FeOOH, see, for example, JP-A-6-1808.
Those disclosed in No. 37 can be used. At this time, Ni salt, Ca salt, Ba salt, S
Alkaline earth element salts such as r salts, Cr salts, Zn salts and the like may be present together, and the particle shape and the like can be controlled by appropriately selecting and using such salts.

As the ferrous salt, ferrous chloride and ferric sulfate
Iron or the like is preferable. Also, as alkali, NaOH, NH
₄ OH, (NH ₄ ) ₂ CO ₃ , Na ₂ CO _{3 and the} like are preferable. As the Ni salt, nickel chloride or the like, Ca salt, B
As the a salt and the Sr salt, chlorides such as calcium chloride, barium chloride and strontium chloride are preferable.

Using the iron oxide hydroxide as a starting material,
The following operation is performed using this slurry. Further, when introducing Co into the magnetic powder, it is before introducing Al and / or Si and a rare earth element. Specifically, a Co compound such as cobalt sulfate and cobalt chloride is used, and this aqueous solution is mixed with By stirring and mixing with a slurry of iron hydroxide.

Next, Al and / or Si is introduced, which is performed as follows. That is, preferably, after preparing a slurry of iron oxyhydroxide containing Co, an aqueous solution containing an Al compound and / or a Si compound and an aqueous solution containing a compound of a rare earth element are respectively added to the slurry and stirred. Just mix.

The concentration of the aqueous solution containing the Al compound and the Si compound may be 0.5 to 1.5 M, may contain only one of the compounds, or may contain both compounds. In this case, the total amount should be within the above range. As the Al compound used, sodium aluminate,
There are sodium metaaluminate and the like, and Si compounds include sodium silicate and the like.

Examples of the rare earth element introduced in the present invention include Sm, Nd, Y, Pr, La and the like.

Examples of the compound of the rare earth element used for preparing the above aqueous solution include chlorides such as neodymium chloride, samarium chloride, praseodymium chloride, lanthanum chloride and yttrium chloride, and nitrates such as neodymium nitrate. Further, two or more kinds of rare earth elements may be used together.

In order to introduce Al, Si and a rare earth, it is preferable to separately prepare an aqueous solution containing Al and / or Si and an aqueous solution containing a rare earth element as described above and add them. May be added after preparing an aqueous solution containing Al and / or Si and a rare earth element.

In the case of separately preparing and adding, both solutions may be added at the same time, or one solution may be added and then the other solution may be added.
Alternatively, it is preferable to first add the aqueous solution containing Si and then add the aqueous solution containing the rare earth element.

In order to introduce Al, Si and rare earths, alkaline earth elements, Al rare earth elements, C
An iron oxyhydroxide containing o is obtained.

This is thoroughly washed with water, dried, and heat-treated at a temperature of 300 to 800 ° C. in a non-reducing atmosphere. When the heat treatment temperature is 300 ° C. or less, the α-Fe ₂ O ₃ particles produced by dehydration of α-FeOOH have many pores, and as a result, the properties of the ferromagnetic metal powder after reduction are deteriorated. When the heat treatment temperature exceeds 800 ° C, α
The melting of the —Fe ₂ O ₃ particles begins, the shape of the particles changes, or sintering progresses, and the properties of the resulting ferromagnetic metal powder deteriorate. Next, the ferromagnetic metal powder after the heat treatment is reduced under a hydrogen gas stream at a temperature of 300 to 600 ° C., and an oxide film is formed on the surface by a known method to obtain a ferromagnetic metal powder.

(2-2) Binder Typical binders contained in the uppermost magnetic layer are, for example, polyurethane, polyester and vinyl chloride copolymers, and these resins are --SO.
It is preferable to have at least one polar group selected from ₃ M, —OSO ₃ M, —COOM, PO (OM ¹ ) ₂ and sulfobetaine groups. However, in the polar group, M represents a hydrogen atom or an alkali metal such as Na, K and Li, and M ¹ represents a hydrogen atom, an alkali metal such as Na, K and Li or an alkyl group.

The above polar group has the function of improving the dispersibility of the magnetic powder, and the content in each resin is 0.1 to 8.0 mol%, preferably 0.2 to 6.0 mol%. is there. When the content is less than 0.1 mol%, the dispersibility of the magnetic powder is deteriorated, and when the content is more than 8.0 mol%, the magnetic coating tends to gel. The weight average molecular weight of each resin is preferably in the range of 10,000 to 100,000.

The content of the binder is the ferromagnetic metal powder 1
It is usually 8 to 25 parts by weight, preferably 10 to 20 parts by weight, relative to 00 parts by weight. The binder is not limited to one kind alone, and two or more kinds may be used in combination. In this case, the ratio of polyurethane and / or polyester to the vinyl chloride copolymer is usually 90:10 to 1 by weight.
It is 0:90, preferably 70:30 to 30:70.

Regarding the technique for introducing a polar group into a vinyl chloride-based copolymer, JP-A-57-44227 and JP-A-5-44227 have been used.
8-108052, 59-8127, 60-1
No. 01161, No. 60-235814, No. 60-23
No. 8306, No. 60-238371, No. 62-121
No. 923, No. 62-146432, No. 62-1464.
No. 33 and the like are described, and these can be used in the present invention.

Next, the polyester used in the present invention is obtained by reacting a polyol with a polybasic acid. Note that other polar group-introduced polyesters can also be synthesized by a known method. Polyurethanes are also obtained from the reaction of polyols with polyisocyanates. As the polyol, a polyester polyol obtained by reacting a polyol with a polybasic acid is generally used.

Therefore, when a polyester polyol having a polar group is used as a raw material, a polyurethane having a polar group can be synthesized. As a technique for introducing a polar group into polyurethane, Japanese Patent Publication No. 58-415
No. 65, JP-A Nos. 57-92422 and 57-9242.
No. 3, No. 59-8127, No. 59-5423, No. 5
No. 9-5424, No. 62-121923, etc., and these can be used in the present invention.

(2-3) Other components In the present invention, in order to improve the quality of the uppermost magnetic layer, an abrasive, a lubricant, a curing agent, a dispersant, an antistatic agent and a conductive (fine) agent. Additives such as powder can be contained as other components.

Specific examples of the abrasive include α-alumina,
Examples thereof include chromium oxide, titanium oxide, α-iron oxide, silicon oxide, silicon nitride, tungsten carbide, molybdenum carbide, boron carbide, corundum, zinc oxide, cerium oxide, magnesium oxide and boron nitride. The number average particle size of the abrasive contained in the magnetic layer is preferably 0.03 ≦ Za ≦ 0.2 μm, where Za (μm) is the number average particle size of the abrasive having a Mohs hardness of 6 or more. More preferably, 0.05 ≦ Za ≦ 0.2 μm, and particularly preferably 0.05 ≦ Za ≦ 0.15 μm. When the number average particle diameter of the abrasive in the nonmagnetic layer is Zb (μm), it is preferably 0.10 ≦ Zb ≦ 0.50, more preferably 0.
15 ≦ Zb ≦ 0.40, particularly preferably 0.15 ≦ Zb
It is ≦ 0.30, and a relationship of Za <Zb is particularly preferable for improving the dropout characteristics. The content of the abrasive in the uppermost magnetic layer is usually 0.5 to 20 parts by weight, preferably 1 to 10 parts by weight, and particularly preferably 1 to 5 parts by weight. By setting the content of the abrasive within this range, the electromagnetic conversion characteristics can be improved.

Fatty acids and / or fatty acid esters can be used as the lubricant. In this case, the amount of fatty acid added is preferably 0.2 to 10% by weight, and particularly preferably 0.5 to 5% by weight, based on the magnetic powder. If the addition amount is less than 0.2% by weight, the running property is likely to decrease, and if the addition amount is more than 10% by weight, the fatty acid tends to exude to the surface of the uppermost magnetic layer or the output tends to be reduced.

The amount of fatty acid ester added is preferably 0.2 to 10% by weight, more preferably 0.5 to 5% by weight, based on the magnetic powder. If the addition amount is less than 0.2% by weight, the still durability is apt to deteriorate, and if it exceeds 10% by weight, the fatty acid ester exudes to the surface of the uppermost magnetic layer or the output is lowered. It will be easier. Fatty acid and fatty acid ester are in a weight ratio of 10:90 to 90: 1.
0 is preferred. The fatty acid may be a monobasic acid or a dibasic acid, and preferably has 6 to 30 carbon atoms, and 1
The range of 2 to 22 is more preferable. Specific examples of fatty acids include caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, isostearic acid, linolenic acid, oleic acid, elaidic acid, behenic acid, malonic acid, succinic acid, maleic acid. Acid, glutaric acid, adipic acid, pimelic acid, azelaic acid, sebacic acid, 1,12-dodecanedicarboxylic acid, octanedicarboxylic acid and the like can be mentioned.

Specific examples of the fatty acid ester include oleyl oleate, isocetyl stearate, dioleyl malate, butyl stearate, butyl palmitate, butyl myristate, octyl myristate, octyl palmitate, pentyl stearate, pentyl palmiate. Tate, isobutyl oleate, stearyl stearate,
Lauryl oleate, octyl oleate, isobutyl oleate, ethyl oleate, isotridecyl oleate, 2-ethylhexyl stearate, 2-ethylhexyl palmitate, isopropyl palmitate, isopropyl myristate, butyl laurate, cetyl-2.
-Ethyl hexalate, dioleyl adipate, diethyl adipate, diisobutyl adipate, diisodecyl adipate, oleyl stearate, 2-ethylhexyl myristate, isopentyl palmitate, isopentyl stearate, diethylene glycol-mono-butyl ether palmitate, diethylene glycol mono- Butyl ether palmitate and the like.

Various substances can be used as lubricants other than the above fatty acids and fatty acid esters, for example, silicone oil, fluorocarbon, fatty acid amide, α-
Olefin oxide and the like can be used.

Examples of the curing agent include polyisocyanates. Examples of the polyisocyanates include aromatic polyisocyanates such as adducts of tolylene diisocyanate (TDI) and active hydrogen compounds, and hexamethylene diisocyanate ( There are aliphatic polyisocyanates such as adducts of HMDI) with active hydrogen compounds. The weight average molecular weight of the polyisocyanate is preferably in the range of 100 to 3,000 in order to improve the durability of the medium.

Examples of the dispersant include polyalkylene oxide alkyl ester; lecithin; trialkyl polyolefinoxy quaternary ammonium salt: azo compounds having a carboxyl group and a sulfonic acid group. These dispersants are usually used in the range of 0.5 to 5% by weight based on the ferromagnetic powder.

Examples of the antistatic agent include conductive powders such as carbon black and graphite; cationic surfactants such as quaternary amines; and acid groups such as sulfonic acid, sulfuric acid, phosphoric acid, phosphoric acid ester and carboxylic acid. Examples thereof include anionic surfactants containing; amphoteric surfactants such as aminosulfonic acid; natural surfactants such as saponin. The above-mentioned surfactant is usually added in the range of 0.01 to 40% by weight with respect to the binder.

The number average particle diameter (Ya) of the conductive powder contained in the uppermost magnetic layer is usually 5 to 350 nm, preferably 30 to 300 nm, and more preferably 40 to 200 nm. The content of the conductive fine powder is 0.1 to 100 parts by weight of the magnetic powder.
It is 3 parts by weight, preferably 0.1 to 1 part by weight.
And the number average particle diameter (Yb) of the conductive powder contained in the undercoat layer is usually 10 to 40 nm, preferably 10 to 35 nm, more preferably 10 to 30 nm, especially running durability (particularly Head clogging suppression)
Is preferable in that the above can be improved.

(3) Underlying layer The underlying layer is composed of at least one layer, and is formed with a single layer or a plurality of layers between the non-magnetic support and the uppermost magnetic layer. The underlayer may be formed of one type of layer or a combination of two or more types of layers without any particular limitation. In the present invention, the undercoat layer described later is preferable, and the undercoat layer containing needle-shaped non-magnetic powder is particularly preferable.

The term “underlying layer” as used herein means a layer which is substantially non-magnetic (saturation magnetic flux density Bm is 0) and a layer which is substantially non-magnetic (slightly magnetic layer). In addition, Bm of 0.01 to 29 Gauss) is also included. In particular, when acicular α-Fe ₂ O ₃ is used as the filler for the lower layer, the Bm of the layer is usually about 0.01 to 29 Gauss, but in this case also, it is referred to as the undercoat layer in the present invention. I will.

The thickness of the underlying layer is usually 0.2 to
It is 2.5 μm, preferably 0.5 to 2.0 μm.

The underlying layer contains non-magnetic powder. Further, it contains a binder and, if necessary, other components. Hereinafter, this point will be described.

(3-1-1) Non-Magnetic Powder In the present invention, the following non-magnetic powder can be used. Examples of the non-magnetic powder include carbon black, graphite, TiO ₂ , BaSO ₄ , and Zn.
S, MgCO ₃ , CaCO ₃ , ZnO, CaO, W
S ₂ , MoS ₂ , BN, MgO, SnO ₂ , SiO ₂ ,
_{Cr 2 O 3, α-Al} 2 O 3, α-Fe 2 O 3, α-F
eOOH, SiC, CeO, artificial diamond, BN,
Examples thereof include MoC, WC, TiC and the like.

Of these, preferred are carbon black, CaCO ₃ , TiO ₂ , BaSO ₄ , and α-Al _2.
It is an inorganic powder such as O ₃ , α-Fe ₂ O ₃ , α-FeOOH, and Cr ₂ O ₃ , and α-Fe ₂ O ₃ is particularly preferable.

Such iron oxyhydroxide is obtained by reacting an aqueous ferrous salt solution with an alkaline aqueous solution to obtain an aqueous solution containing ferrous hydroxide, and oxidizing this by air oxidation or the like. Preferably there is.

In the present invention, it is preferable to use α-Fe ₂ O ₃ powder having a needle-like powder shape. When the acicular α-Fe ₂ O ₃ powder is used, the smoothness of the surface of the underlying layer can be improved, and the smoothness of the surface of the uppermost magnetic layer laminated thereon can be improved at the same time. It is preferable because it can be obtained. Further, if the saturation magnetic flux density (Bm) is in the range of 0.01 to 29 Gauss, the α-Fe ₂ O ₃ powder contains 0.01 to 5% by weight of γ-Fe ₂ O ₃ or Fe ₃ O ₄ . It is also possible to add at least one kind. The coercive force of the added magnetic powder may be 200 to 600 Oe.

The acicular α-Fe ₂ O ₃ powder in the present invention uses iron oxyhydroxide obtained by blowing an oxidizing gas into a water suspension prepared by mixing a ferrous salt and an alkali as a starting stock solution. . As the type of iron oxyhydroxide,
α-FeOOH is preferable, and as a method for producing the same, a ferrous salt is neutralized with an alkali hydroxide to form a water suspension of Fe (OH) ₂ , and an oxidizing gas is blown into the suspension to form needle-like particles. α
There is a manufacturing method of using -FeOOH. On the other hand, the ferrous salt is neutralized with an alkali carbonate to form an aqueous suspension of FeCO ₃ , and an oxidizing gas is blown into the suspension to form a spindle-shaped α-FeOOH.
There is a manufacturing method to Here, the spindle shape refers to a spheroidal shape in which the central part of the particle is thick and both ends are thin. For the spindle-shaped α-FeOOH, see, for example, Japanese Patent Laid-Open No.
The one disclosed in No. 180837 can be used. The latter method is suitable for the present invention. On this occasion,
Ni salt, alkaline earth element salts such as Ca salt, Ba salt, Sr salt, Cr salt, Zn salt, phosphorus compound and the like may coexist in the ferrous iron aqueous solution, and such salt is appropriately selected. The particle shape (acicular ratio) and the like can be controlled by using the same.

As the ferrous salt, ferrous chloride, ferrous sulfate, ferrous nitrate and the like are preferable. Further, as the alkali, NaOH, NH ₄ OH, (NH ₄ ) ₂ CO ₃ , Na ₂ C
O _{3 and the} like are preferable. Also, nickel salt or the like as Ni salt, Ca salt, Ba salt, Sr salt, Mg salt as calcium chloride, barium chloride, strontium chloride, magnesium chloride, Cr salt, Zn salt as chromium salt, zinc chloride respectively. Chlorides are preferred.

Next, introduction of Al and Si will be described. The first manufacturing method is a manufacturing method in which an aqueous solution containing an Al compound and / or a Si compound is added to the slurry and mixed by stirring. This slurry is thoroughly filtered, washed with water, dried, and heat-treated at a temperature of 300 to 800 ° C. in a non-reducing atmosphere to obtain α-Fe ₂ O ₃ powder. Processing temperature is 30
Below 0 ° C, α-F generated by dehydration of α-FeOOH
The voids in the e ₂ O ₃ powder are increased, and as a result, α-Fe
The dispersibility of ₂ O ₃ deteriorates. Also, the processing temperature is 800 ℃
In the above, the melting of α-Fe ₂ O ₃ started, the particle shape changed, or the sintering proceeded.
The characteristics of —Fe ₂ O ₃ deteriorate. The second method is to wash the slurry thoroughly with water and dry it,
This is a manufacturing method in which heat treatment is performed at a temperature of 0 to 800 ° C. to obtain α-Fe ₂ O ₃ particles, which are dispersed in an aqueous solution to form an aqueous suspension, and an Al compound and / or a Si compound are added and stirred. . Filtration Thereafter, washing with water, drying, grinding, _α-Fe 2 _{O 3} by the deaeration is obtained. Examples of Al compounds include sodium aluminate and sodium metaaluminate, and examples of Si compounds include sodium silicate.

The amount of Al, Si, P, alkaline earth elements, and Na added to the α-Fe ₂ O ₃ powder is adjusted by using such a manufacturing method.

In the present invention, a non-magnetic powder having a needle-like powder shape can be preferably used. The use of the needle-shaped non-magnetic powder can improve the surface smoothness of the underlying layer, and can also improve the surface smoothness of the upper layer composed of the uppermost magnetic layer laminated thereon. it can.

The major axis length (Lb) of the acicular non-magnetic powder is 80≤Lb≤280, preferably 100.
≦ Lb ≦ 250, particularly preferably 120 ≦ Lb ≦
200. The axial ratio (Xb) of the acicular non-magnetic powder is preferably 5 to 12, and particularly preferably 6
~ 9. The axial ratio mentioned here means a ratio of the major axis length to the minor axis length (major axis length / minor axis length).

When the non-magnetic powder having the major axis length, the axial ratio and the specific surface area within the above ranges is used, the surface property of the non-magnetic layer can be improved, and the surface property of the upper layer which is the magnetic layer is also improved. It is preferable in that it can be brought into a state.

The content of the non-magnetic powder in the non-magnetic layer which is in contact with the non-magnetic powder is usually 50 to 99% by weight, preferably 6 based on the total amount of all components constituting the non-magnetic layer.
It is 0 to 95% by weight, and particularly preferably 70 to 95% by weight. When the content of the non-magnetic powder is within the above range,
The surface state of the uppermost magnetic layer and the underlying non-magnetic layer can be improved.

(3-1-2) Binder As the binder contained in the non-magnetic layer under the binder, the resin exemplified in the above-mentioned magnetic layer (2-2) can be used.

(3-1-3) Other Components As the other components contained in the undercoating nonmagnetic layer, the compounds exemplified in the magnetic layer (2-3) can be used.

(4) Backcoat Layer In the present invention, various known backcoat layers can be used without particular limitation. In particular, the backcoat layer has at least 2 having a number average particle size of 10 to 30 nm and 35 to 350 nm. It is preferable that a kind of conductive powder is used and the center line average surface roughness (Ra) of the back coat layer is 3 to 20 nm.

FIG. 1 shows a cross-sectional view of the recording medium of the present invention configured as described above. In FIG. 1, 1 is a support, 2 is a non-magnetic layer which is in contact with the base, 3 is an uppermost magnetic layer, and 4 is a back coat layer.

-Manufacture of Magnetic Recording Medium- In the magnetic recording medium of the present invention, the uppermost magnetic layer is coated by a so-called wet-on-wet system in which the underlying layer is in a wet state. Is preferred. As the wet-on-wet method, a method used for manufacturing a known multi-layer structure type magnetic recording medium can be appropriately adopted. For example, generally, a magnetic powder, a binder, a dispersant, a lubricant, an abrasive, an antistatic agent, and the like are kneaded with a solvent to prepare a high-concentration magnetic coating, and then the high-concentration magnetic coating is diluted to produce a magnetic coating. After the paint is prepared, this magnetic paint is applied to the surface of the non-magnetic support through the non-magnetic layer which is in contact therewith.
Examples of the solvent include acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIB).
K), ketones such as cyclohexanone; alcohols such as methanol, ethanol, propanol; esters such as methyl acetate, ethyl acetate, butyl acetate; cyclic ethers such as tetrahydrofuran; methylene chloride,
Halogenated carbon hydrogen such as ethylene chloride, carbon tetrachloride, chloroform and dichlorobenzene can be used.

In kneading and dispersing the magnetic layer forming components,
Various kneading dispersers can be used. Examples of the kneading / dispersing machine include a two-roll mill, a three-roll mill, a ball mill, a pebble mill, a cobol mill, a tron mill, a sand mill, a sand grinder, and a Sqegvar.
i Attritor, high speed impeller disperser, high speed stone mill, high speed impact mill, disper, high speed mixer, homogenizer, ultrasonic disperser, open kneader, continuous kneader, pressure kneader and the like. Among the above kneading dispersers, the kneading dispersers capable of providing a power consumption load of 0.05 to 0.5 KW (per 1 Kg of magnetic powder) are a pressure kneader, an open kneader, a continuous kneader, and a two-roll mill. .

In order to coat the uppermost magnetic layer and the underlying layer on the non-magnetic support, specifically, as shown in FIG. 2, first, as shown in FIG. 1, the extrusion type extrusion coater 41,
After applying the uppermost magnetic layer coating material and the lower layer coating material in multiple layers by a wet-on-wet method using 42, it passes through the orientation magnet or the vertical orientation magnet 33, and the drier 34.
Then, hot air is blown from the nozzles arranged above and below to dry it. Next, the dried non-magnetic support 1 with a coating layer is introduced into a super calendar device 37 composed of a combination of calendar rolls 38, and after being calendered here,
It is wound up on a winding roll 39. The magnetic film thus obtained can be cut into a tape having a desired width to produce a magnetic recording tape for 8 mm video, for example.

In the above method, each paint is extruded through an in-line mixer (not shown) and the coaters 41, 42 are extruded.
May be supplied to. It should be noted that in the figure, the arrow indicates the conveying direction of the non-magnetic support. Extrusion coaters 41 and 42 are respectively provided with liquid retention portions 43 and 44, and the coating materials from the respective coaters are superposed in a wet-on-wet system. That is, the upper layer coating material is applied in multiple layers immediately after the application of the coating material for the undercoat layer (when in a wet state).

The extrusion coater is shown in FIG.
In addition to the two extruding coaters shown in FIG. 1, extruding coaters of the types shown in FIGS. The lower coat material and the upper coat material are co-extruded by an extrusion coater to apply multiple layers. As the solvent to be blended in the paint or a diluent solvent at the time of applying the paint, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone: alcohols such as methanol, ethanol, propanol and butanol:
Esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, ethylene glycol cenoacetate: Ethers such as glycol dimethyl ether, glycol monoethyl ether, dioxane, tetrahydrofuran:
Aromatic hydrocarbons such as benzene, toluene, xylene: methylene chloride, ethylene chloride, carbon tetrachloride,
It is possible to use halogenated hydrocarbons such as chloroform and dichlorobenzene. These various solvents may be used alone or in combination of two or more. The magnetic field in the orienting magnet or the vertically orienting magnet is about 20 to 10,000 gauss, the drying temperature by the dryer is about 60 to 120 ° C., and the drying time is about 0.1 to 1 minute. .

In the wet-on-wet system, a combination of a reverse roll and an extrusion coater, a combination of a gravure roll and an extrusion coater, etc. can also be used. Furthermore, air doctor coater, blade coater, air knife coater, squeeze coater, impregnation coater,
It is also possible to combine a transfer roll coater, a kiss coater, a cast coater, a spray coater and the like. In the multi-layer coating in this wet-on-wet method, since the upper layer is coated while the lower layer under the upper layer is in a wet state, the surface of the lower layer (that is, the boundary surface with the upper layer) is smooth. As a result, the surface properties of the upper layer are improved, and the adhesiveness between the upper and lower layers is also improved. As a result,
In particular, a film that meets the requirements for high output and low noise for high-density recording, for example, the performance required as a magnetic tape for digital recording and high durability is required. Eliminates peeling, improves film strength,
Sufficient durability. Further, the wet-on-wet multi-layer coating method can also reduce dropout and improve reliability.

-Smoothing of Surface- In the present invention, it is also possible to carry out a surface smoothing treatment by calendering. After that, a burnishing treatment or a blade treatment is performed as necessary to perform slitting. In the surface smoothing treatment, temperature, linear pressure, calendar speed and the like can be mentioned as calendar conditions. In the present invention, the temperature is usually 50 to 1
It is preferable to maintain the temperature at 40 ° C., the linear pressure at 50 to 1000 kg / cm, and the calendar speed at 20 to 1,000 m / min.

Ra of the magnetic layer in the present invention is 0.5 to 6.
Although the thickness is preferably nm, Ra can be controlled to 0.5 to 6 nm by, for example, setting the calender conditions in the above manufacturing process and controlling the surface smoothness of the magnetic layer.

<< Measurement of Various Elements of Magnetic Metal Powder >><OverallComposition>: F in the overall composition of the ferromagnetic metal powder
e, Al, Co and Sm, Nd, Y, Pr and La groups,
Regarding the abundance ratio of each element of Na, Ca, Ba, Sr, Mg, Ni and Si, a wavelength dispersive X-ray fluorescence analyzer (WD
X) was used to measure the fluorescent X-ray intensity of each element in the sample, and then the intensity was calculated according to the fundamental parameter method (hereinafter referred to as the FP method).

<Size of crystallite> F by X-ray diffraction method
Using the integral width of the (110) diffraction line of e, it was determined by the Scherrer method using Si powder as a reference. For how to obtain
By the method described in the X-ray Diffraction Guide (Rigaku Denki Co., Ltd.), the correction of spread due to double lines was obtained by the correction by A: Jones (integral width) described on page 77.

[0082]

EXAMPLES Next, the constitution and effects of the present invention will be specifically described by way of examples. The components, proportions, and operation order shown below are
Various modifications can be made without departing from the scope of the present invention. In the following examples, "parts" are "parts by weight" unless otherwise specified. Table 1 shows the characteristics of the uppermost magnetic layer and the underlying layer according to the present invention. The magnetic layer coating material and the non-magnetic layer coating material having the following compositions were kneaded and dispersed using a kneader and a sand mill to prepare a magnetic layer coating material and a non-magnetic layer coating material, respectively.

{Uppermost magnetic layer coating material} Fe-Al based ferromagnetic metal powder 100 parts (magnetic powder A-1) Fe: Al: Co: Y = 100: 1
0: 40: 8 (weight ratio) Hc: 2200 Oe, BET: 60 m ² / g Average major axis length: 70 nm σs: 145 emu / g, crystallite size: 12 nm, axial ratio 5.5

(Magnetic powder B-1) Fe: Al: Co:
Y: Si = 100: 5: 20: 5: 1 (weight ratio) Hc: 2000 Oe, BET: 72 m ² / g σs: 140 emu / g, crystallite size: 10 nm, axial ratio 2.4 average major axis length 25 nm

(Magnetic powder B-2) Fe: Al: Co:
Y: Si = 100: 5: 20: 5: 1 (weight ratio) Hc: 2000 Oe, BET: 54 m ² / g σs: 140 emu / g, crystallite size: 18 nm, axial ratio 6.0 average major axis length 120 nm

(Magnetic powder A-2) Fe: Al: Co: Y
= 100: 10: 40: 8 (weight ratio) Hc: 2200 Oe, BET: 70 m ² / g σs: 145 emu / g, crystallite size: 10 nm, axial ratio 2.5 average major axis length 30 nm

(Magnetic powder A-3) Fe: Al: Co: Y
= 100: 10: 40: 8 (weight ratio) Hc: 2200 Oe, BET: 65 m ² / g σs: 145 emu / g, crystallite size: 12 nm, axial ratio 4.0 average major axis length 50 nm

(Magnetic powder A-4) Fe: Al: Co: Y
= 100: 10: 40: 8 (weight ratio) Hc: 2200 Oe, BET: 58 m ² / g σs: 145 emu / g, crystallite size: 12 nm, axial ratio 6.5 average major axis length 100 nm

[0089]   Vinyl chloride resin containing potassium sulfonate group 10 parts   (MR-110 manufactured by Nippon Zeon Co., Ltd.)   Sodium sulfonate group-containing polyurethane resin 10 parts   (Toyobo Co., Ltd. UR-8700)   Carbon black (number average particle size 90 nm) 0.5 part   α-alumina (number average particle size 0.15 μm) 3 parts   Stearic acid 1 part   Butyl stearate 1 part   Cyclohexanone 100 parts   100 parts of methyl ethyl ketone   100 parts of toluene

{Paint of Layer Underlying Layer} Nonmagnetic powder a [α-Fe ₂ O ₃ average major axis length: 160 nm, crystallite size: 18 nm, axial ratio: 8. (Containing 0.2 wt% of Si relative to α-Fe ₂ O ₃ and 0.5 wt% of Al relative to α-Fe ₂ O ₃ )] 100 parts Potassium sulfonate group-containing vinyl chloride resin 12 Parts (Nippon Zeon Co., Ltd. MR-110) Sodium sulfonate group-containing polyurethane resin 8 parts (Toyobo Co., Ltd. UR-8700) α-alumina (number average particle size 0.2 μm) 5 parts Carbon black (number Average particle size 15 nm) 10 parts Stearic acid 1 part Butyl stearate 1 part Cyclohexanone 100 parts Methyl ethyl ketone 100 parts Toluene 100 parts

5 parts of a polyisocyanate compound (Coronate L, manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to each of the obtained uppermost magnetic layer paint and the undercoating layer paint, and methyl ethyl ketone / toluene (weight: (Ratio 50:50) Add the mixed solvent with stirring with a dissolver (so that NV after dilution is in Table 1 and Table 2),
The specific gravity of the paint at a temperature of 20 ° C. was readjusted so as to be the values shown in Tables 1 and 2.

Examples 1 to 9 and Comparative Examples 1 to 6 Wet-on using the coating material for the uppermost magnetic layer containing the ferromagnetic metal powder and the non-coating material for the underlying layer containing the non-magnetic powder. -After being applied on a polyethylene-2,6-naphthalate film having a thickness of 5 µm by a wet method, a magnetic field orientation treatment is performed while the coating film is undried, and subsequently, a temperature of 70 ° C, 80 ° C, 90 ° C. After hot-air drying in order, a surface smoothing treatment was performed with a calendar to form a constituent layer having various characteristics shown in Tables 1 and 2. The calendar conditions are a temperature of 85 ° C and a linear pressure of 300 kg.
However, in Comparative Example 6, the temperature was 50 ° C. and the linear pressure was changed to 150 Kg / cm. The tension of the web on the coating line was 0.01 kg / mm.

Further, a coating I having the following composition was applied to the surface (rear surface) of the polyethylene-2,6-naphthalate film on the side opposite to the magnetic layer, the coating film was dried, and the above-mentioned calendar was used. By calendering according to the conditions, a back coat layer having a thickness of 0.8 μm was formed to obtain a wide original anti-magnetic tape. Thereafter, at a temperature of 60 ° C., the samples of Examples 1 to 5, 7 to 9 and Comparative Examples 1 to 6 were heat cured for 48 hours, and the sample of Example 6 was heat cured for 72 hours.

[0094] {Backcoat paint}   Carbon black 1 (number average particle size 23 nm) 45 parts   Carbon black 2 (number average particle size 50 nm) 5 parts   Barium sulfate (average particle size 200 nm) 1 part   Nitrocellulose 25 parts   Polyurethane resin 25 parts   (N-2301 manufactured by Nippon Polyurethane Co., Ltd.)   Polyisocyanate compound 10 parts   (Nippon Polyurethane Co., Ltd., Coronate L)   Cyclohexanone 400 parts   250 parts of methyl ethyl ketone   250 parts of toluene

The original magnetic tape thus obtained was slit to prepare a magnetic recording medium for video. The following evaluations were performed on this magnetic recording medium. The results are shown in Tables 1 and 2.

Electromagnetic conversion characteristics: With an 8 mm video camera CCDV-900 manufactured by Sony Corporation, the CN ratio of 7 MHz is the output difference (dB) between 7 MHz and 6 MHz, and the CN ratio of 9 MHz is the output difference between 9 MHz and 8 MHz. (DB) was measured.

{Center line average surface roughness (Ra)} JIS-
It was measured (cutoff is 0.25 mm) with a three-dimensional surface roughness meter (SE-3FK) manufactured by Kosaka Laboratory by the method described in Section 5 of B0601.

{Multilayer coating characteristics} The multilayer coating characteristics were visually evaluated. A: Good coatability B: Lauan eye was generated in part C: Lauan eye was generated on the entire surface D: Sample could not be prepared because there were no conditions for good multilayer coating

[0099] Record the {overwrite characteristics} 2MHz signal at saturation level was determined residual output level then 9MH _Z signal (overwriting) of the 2MHz signal when recorded. The lower the residual output level, the better the overwrite characteristics.

[0100]

[Table 1]

[0101]

[Table 2]

[0102]

According to the present invention, it has a high C / N ratio in a high frequency band, is excellent in electrical characteristics, is excellent in coating properties and does not cause surface roughness, which is particularly suitable as a digital recording medium. It is possible to provide a magnetic recording medium having excellent overwrite characteristics.

[Brief description of drawings]

FIG. 1 is a sectional view of a recording medium of the present invention.

FIG. 2 is a diagram for explaining multi-layer coating of magnetic layers by a wet-on-wet coating method.

3 (a), (b) and (c) are views showing an example of an extruding coater for magnetic paint.

[Explanation of symbols]

1 Non-magnetic support 2 Layers in contact 3 Top magnetic layer 4 Back coat layer 41 Extrusion coater 42 extrusion coater 43 Liquid pool 44 Liquid pool 32 supply rolls 33 Orientation magnet or vertical orientation magnet 34 dryer 37 Super calendar device 38 Calendar Roll 39 winding roll

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yasunobu Kobayashi 1 Sakura-cho, Hino-shi, Tokyo Konica Co., Ltd. (72) Inventor Akira Saito 1-13-1, Nihonbashi, Chuo-ku, Tokyo TDK Corporation (56) References JP-A-5-109063 (JP, A) JP-A-5-285443 (JP, A) JP-A-4-325916 (JP, A) (58) Fields investigated (Int. Cl. ⁷⁾ , DB name) G11B 5/62-5/858 B05D 1/26 B05D 7/24

Claims (2)

(57) [Claims] 1. Multi-layer coating is carried out while the underlying layer is in a wet state so that a plurality of layers are formed on a support and the thickness of the uppermost magnetic layer is 0.05 to 0.3 μm. In the manufacturing method to be performed, the specific gravity of the coating liquid for the uppermost magnetic layer at 20 ° C. is 0.9 to 1.0, which is smaller than the specific gravity of the coating liquid for the layer in contact with the uppermost magnetic layer, and The average major axis length (La) of the ferromagnetic metal powder contained in the magnetic layer is smaller than the film thickness of the uppermost magnetic layer, and 30 ≦ La ≦ 110 (n
m) A method of manufacturing a magnetic recording medium. 2. The saturation magnetization (σs) of the ferromagnetic metal powder is 13
The method for producing a magnetic recording medium according to claim 1, wherein the magnetic recording medium has a density of 0 to 170 emu / g.