JPH0729159A - Magnetic recording medium - Google Patents

Abstract

(57) [Summary] [Structure] A first magnetic layer containing at least two kinds of magnetic powder selected from ferromagnetic metal powder mainly composed of iron, cobalt-containing iron oxide powder and barium ferrite powder is formed on a substrate. A magnetic recording medium in which a second magnetic layer containing acicular cobalt-coated iron oxide having a coercive force of 1300 Oe or more is formed on the first magnetic layer. [Effect] Electromagnetic conversion characteristics such as output and noise, corrosion resistance,
Both physical properties such as runnability are improved in a well-balanced manner.

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 having a two-layer magnetic layer structure, and more particularly to a magnetic recording medium excellent in electromagnetic conversion characteristics and durability.

[0002]

2. Description of the Related Art A coating type magnetic recording medium is obtained by coating a magnetic coating material obtained by dispersing magnetic powder in a binder and an organic solvent on a substrate such as polyester and drying it. There is an increasing demand for higher density recording such as high quality video tapes and large capacity floppy disks.

Ferromagnetic metal powders composed mainly of iron are most often used as magnetic powders for achieving this purpose. Ferromagnetic metal powder composed mainly of iron has high saturation magnetization and coercive force, and has excellent output characteristics. However, since it is chemically unstable and easily rusts (is easily oxidized), the saturation magnetization is lowered by oxidation, and the noise characteristic is not always sufficient, and generally the noise level is higher than that of the oxide magnetic powder. Further, the ferromagnetic metal powder mainly composed of iron has a high cost and has some drawbacks in practical use.

A perpendicular magnetic recording method has been proposed as a method for realizing high-density recording, and hexagonal plate-shaped barium ferrite is used as a magnetic powder particularly suitable for this method. However, since barium ferrite has a low saturation magnetization, when used as a magnetic tape or a magnetic disk, the residual magnetic flux density becomes low and the output in the low range becomes insufficient. Further, when barium ferrite is used, there is a problem that the surface roughness of the formed magnetic layer becomes large.

In order to solve such a problem, it has been attempted to improve the output characteristics by making the magnetic layer having a single-layer structure, which has been a mainstream in the past, into a two-layer structure. For example, in JP-A-62-78718, an oxide having a coercive force equal to or higher than the coercive force of the metal magnetic powder in the lower layer is used by using a metal magnetic powder (iron, cobalt, nickel, etc. or an alloy thereof) in the lower layer. A magnetic recording medium has been disclosed in which magnetic powder (γ-Fe ₂ O ₃ , Fe ₃ O _4, etc.) is used as an upper layer and the magnetic layer has a two-layer structure. However, the magnetic recording medium described in this publication has an insufficient output in a high frequency region, and its S / N is low, so that the noise characteristic is also insufficient.

In addition to this, as a magnetic recording medium having a magnetic layer having a two-layer structure, Japanese Patent Publication Nos. 63-56608, 3-6575, 3-53689, and 3-5.
No. 3690, etc., but these are intended to improve either the magnetic characteristics or the corrosion resistance, and it has not been achieved to improve both of them.

[0007]

As described above, in order to further increase the recording density in the future, it is required to improve both magnetic characteristics and physical characteristics such as running property in a well-balanced manner. These points are not yet sufficient.
Therefore, the problem to be solved by the present invention is to further improve the S / N in a magnetic recording medium which enables high density recording, to lower the friction coefficient to improve the running property, and to improve the head performance. The aim is to eliminate clogging and to reduce the decrease in the saturation magnetic flux density during storage more than the ferromagnetic metal powder mainly composed of iron.

[0008]

As a result of intensive studies to solve the above-mentioned problems, the present invention has a magnetic layer having a two-layer structure of upper and lower layers.
By using at least two kinds of magnetic powders selected from ferromagnetic metal powders mainly composed of iron, cobalt-containing iron oxide powders and barium ferrite powders in the lower layer, and cobalt-coated iron oxide having a coercive force of 1300 Oe or more in the upper layer. The inventors have found that a magnetic recording medium having excellent characteristics can be obtained, and have completed the present invention.

That is, the present invention comprises a base material, a ferromagnetic metal powder (hereinafter abbreviated as metal powder) mainly composed of iron and formed on the base material, a cobalt-containing iron oxide powder and a barium ferrite powder. A first magnetic layer comprising at least two selected magnetic powders and a binder; and a second magnetic layer formed on the first magnetic layer and comprising a cobalt-coated iron oxide having a coercive force of 1300 Oe or more and a binder. And a magnetic recording medium having a layer.

The magnetic powder used in the first magnetic layer (lower layer) of the magnetic recording medium of the present invention will be described below.

Metal Powder The metal powder used in the first magnetic layer may be a conventionally known one mainly composed of iron. Iron powder or alloy powder of iron and cobalt, nickel, etc., and aluminum, chromium, Manganese, silicon, zinc, rare earth metal elements,
Examples thereof include alloy powders containing transition metal elements such as lanthanoids and actinoids. The remanent magnetization and coercive force of the metal powder are not limited, but for example, the coercive force is 800 to 1900 Oe.
Degree is general. Also, the particle size of the metal powder is similar,
Although not particularly limited, the average major axis length is 0.1 to 1.0 μm
Generally, the average minor axis length is about 0.01 to 0.1 μm. Also, the shape of the particles is not limited.

Cobalt-containing iron oxide powder (hereinafter referred to as Co--Fe
In the present invention, the cobalt-containing iron oxide includes Co surface-forming cobalt-coated iron oxide and Co containing Co therein.
-Doped type, Co-adsorption type cobalt-containing iron oxide, cobalt ferrite, or the like, and any of these may be used for the first magnetic layer. Especially in the first magnetic layer, Co-deposited Fe ₃ O ₄ , Co-deposited Fe ₂ O ₃ , Co-deposited
FeO _{x (x} = 1.33~1.50), or Fe ₃ O ₄ that the cobalt ferrite was deposited, Fe ₂ O _3, FeO _x is preferably used.

Further, the cobalt-containing iron oxide used in the first magnetic layer of the magnetic recording medium of the present invention is produced by a usual cobalt deposition method or the like using acicular iron oxide as a nucleus. The magnetic characteristics can be adjusted to predetermined values by controlling the amount of cobalt added and the amount of Fe ²⁺ in the manufacturing conditions. The amount of cobalt added is usually 1 to 20% by weight (based on the raw iron oxide),
It is preferably 2 to 15% by weight, and the amount of Fe ²⁺ is usually 0.1.
It is 5 to 30% by weight (ratio in the total cobalt-containing iron oxide), preferably 1 to 25% by weight.

The coercive force of Co--Fe used for the first magnetic layer is not particularly limited and may be appropriately determined depending on the desired performance of the magnetic recording medium, but is generally about 700 to 1800 Oe. When Co—Fe is used as a part of the magnetic powder of the first magnetic layer, the coercive force is preferably smaller than that of the cobalt-adhered iron oxide used in the second magnetic layer described later.

The grain size of Co--Fe is not particularly limited, but the average major axis length is generally 0.1 to 1.0 .mu.m, and the average minor axis length is generally 0.01 to 0.1 .mu.m. The shape of the particles is not limited, but needle-like particles are preferable.

Barium ferrite powder (hereinafter abbreviated as Ba) As Ba, particles having a hexagonal plate shape are used, and the particle diameter (diagonal length) is not particularly limited, but 0.02-1.
The thickness is generally about 0 μm, and the thickness is 0.001 to 0.1.
It is about μm. In addition, the coercive force of Ba is not particularly limited,
Although it is appropriately determined according to the desired performance of the magnetic recording medium, it is generally about 700 to 2000 Oe.

The magnetic powder used for the first magnetic layer may be selected in any combination, and all three types may be used. The weight ratio of the magnetic powder is arbitrary, but it is used for the first magnetic layer. It is preferable that each of the obtained magnetic powders accounts for 10% by weight or more in the total amount of the total magnetic powder used in the first magnetic layer.

When Co--Fe and metal powders are used among the above magnetic powders, the reduction of the saturation magnetic flux density during storage is reduced even when the metal powders are used alone. Further, since the surface properties of both the first and second magnetic layers are improved, the S / N is improved as compared with the case where one layer is made of metal powder or Ba alone.
Further, the cost is reduced and the economical efficiency is improved. In this case, Co−
The weight ratio of Fe to metal powder is Co-Fe: metal powder = 1.
The range of 0:90 to 90:10 is preferable.

When Co--Fe and Ba are used, the surface property of the first magnetic layer is better than when Ba is used alone.
The surface property of the second magnetic layer is also improved, and the S / N is improved.
When Ba and Co-Fe are used, low-frequency output is improved by selecting Co-Fe having a larger saturation magnetization than Ba. In addition, the lowering of the saturation magnetization amount is smaller than that in the case of using the metal powder alone, so that the low frequency output becomes stable for a long period of time. In this case, the weight ratio of Co-Fe and Ba is Co-Fe:
The range of Ba = 10: 90 to 90:10 is preferable.

If metal powder and Ba are used,
Since the surface properties of the first magnetic layer are better than when Ba is used alone, the surface properties of the second magnetic layer are also better and the S / N is improved. In addition, when using metal powder and Ba, Ba
The low-frequency output is improved by selecting a metal powder having a larger saturation magnetization than that. Further, the corrosion resistance is improved as compared with the case where the metal powder is used alone. In this case, the weight ratio of metal powder and Ba is as follows: metal powder: Ba = 10: 90
A range of 90:10 is preferred.

It is also possible to use all three kinds of Ba, Co--Fe and metal powder, and in this case, a magnetic recording medium having a higher output in addition to the above effects can be obtained. When all three types of magnetic powders are used, it is preferable to use each magnetic powder in an amount of at least 10% by weight based on the total amount of the magnetic powders. If the amount of each magnetic powder is less than 10% by weight, for example, if the metal powder is less than 10% by weight, the output decreases, and if Co-Fe or Ba is less than 10% by weight, the S / N becomes poor.

The first magnetic layer is composed of metal powder, Co--Fe and Ba.
At least two kinds of magnetic powders are selected from the above, preferably each magnetic powder is mixed in the above weight ratio, and a magnetic coating material containing them and a binder as a main component is applied onto a base material. It The weight ratio of the magnetic powder is not limited. The thickness of the first magnetic layer is 1 to 5 μm, preferably 2 to 4 μm.

Next, the cobalt-coated iron oxide used in the second magnetic layer of the magnetic recording medium of the present invention will be described.

The second magnetic layer (upper layer) of the magnetic recording medium of the present invention has a coercive force of 1300 Oe or more, preferably 130.
Cobalt-deposited iron oxide of 0-2000 Oe is used.
If the coercive force of the cobalt-deposited iron oxide is less than 1300 Oe, high output cannot be achieved. Specifically, as cobalt-deposited iron oxide, Co-deposited Fe ₃ O ₄ , Co-deposited Fe ₂ O ₃ , Co-deposited FeO _x
(1.34 ≦ x <1.50) is used, especially Co-deposited Fe
_Ox is preferred. Cobalt-doped iron oxide is not preferable because it has poor storage stability.

The cobalt-deposited iron oxide used in the second magnetic layer of the magnetic recording medium of the present invention is also produced by the usual method of depositing cobalt, using acicular iron oxide as a nucleus, as described above.
The magnetic characteristics can be adjusted to predetermined values by controlling the amount of cobalt added and the amount of Fe ²⁺ in the manufacturing conditions. The amount of cobalt added is usually 1 to 20% by weight (based on iron oxide as a raw material), preferably 2 to 15% by weight, and the amount of Fe ²⁺ is usually 0.5 to 30% by weight (cobalt deposition oxidation). The proportion of iron as a whole), preferably 1 to 25% by weight.

The cobalt-adhered iron oxide used in the second magnetic layer of the magnetic recording medium of the present invention is not particularly limited in particle size as long as it has a coercive force of 1300 Oe or more, but has an average major axis length of 0.1. -10 μm, average minor axis length 0.01-0.1 μm
The degree is general, and the shape is not limited, but needle-like ones are preferably used.

The second magnetic layer is formed by coating the above-described magnetic coating mainly composed of cobalt-coated iron oxide and a binder on the first magnetic layer. Sa is 0.1
-3 μm is preferred.

In the present invention, the coercive force (Hc ₁ ) of the first magnetic layer and the coercive force (Hc ₂ ) of the second magnetic layer preferably satisfy the following formula. | Hc ₁ −Hc ₂ | <400 Oe When Hc ₁ is larger than Hc ₂ by 400 Oe or more, recording on the first magnetic layer becomes difficult. Also, Hc ₁ is Hc ₂
If it is smaller than 400 Oe by more than 400 Oe, when the signal on the second magnetic layer side is recorded (high frequency signal), the first recorded signal is already recorded.
The signal on the magnetic layer side (low-frequency signal) is easily erased, resulting in poor S / N.

In the present invention, the method of forming (coating) the magnetic layers may be either a method of simultaneously forming the first magnetic layer and the second magnetic layer or a method of sequentially forming one layer at a time, and one layer at a time. In this case, calendar processing may be performed for each layer.

The magnetic coating material for forming the magnetic layer of the magnetic recording medium of the present invention uses the magnetic powder as described above for the first and second magnetic layers, and other components such as binder and organic solvent are common. Can be used.

The binder used in the present invention is a urethane resin, particularly a polyurethane containing polar groups such as sulfonic acid groups, metal sulfonate groups, sulfobetaine groups, carbobetaine groups, amino groups, hydroxyl groups, epoxy groups and the like. Resins, vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinylidene chloride copolymers, vinyl chloride-acrylonitrile copolymers, and other vinyl chloride-based copolymers, particularly sulfonic acid groups, sulfonic acid metal bases, amino Chloride copolymers containing polar groups such as groups, butadiene-acrylonitrile copolymers, polyamide resins, polyvinyl butyral, cellulose derivatives (cellulose acetate butyrate, cellulose propionate, nitrocellulose, etc.), styrene-butadiene copolymer Coalesced, polyester resin, various synthetic rubber, Nol resin, epoxy resin, urea resin, melamine resin, phenoxy resin, silicone resin, acrylic reaction resin, mixture of high molecular weight polyester resin and isocyanate prepolymer, mixture of polyester polyol and polyisocyanate, urea formaldehyde resin, low molecular weight glycol / Examples include high molecular weight diol / isocyanate mixtures, and mixtures thereof, and similar materials can be used for the first magnetic layer and the second magnetic layer. Usually, the binder is 3.0-
About 10.0% by weight is blended.

As the organic solvent, cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate, benzene, toluene, xylene, dimethylsulfoxide, tetrahydrofuran, dioxane and the like are suitable solvents for dissolving the binder resin used. There is no particular limitation and they may be used alone or in combination of two or more. Usually, the organic solvent is 20 to 80% by weight in the magnetic paint.
The degree is mixed.

In the magnetic coating composition, various commonly used additives such as dispersants, abrasives and lubricants may be appropriately added and used. As the dispersant, lecithin,
Nonionic surfactants, anionic surfactants, cationic surfactants and the like can be used. As an abrasive, α-
Fine powders having an average particle size of 0.05 to 1 μm, such as alumina, fused alumina, chromium oxide (Cr ₂ O ₃ ), iron oxide, silicon carbide, corundum, and diamond can be used, and usually 100 parts by weight of the binder as described above. 0.5 to 100 parts by weight is added. Examples of lubricants include silicone oils such as various polysiloxanes, inorganic powders such as graphite and molybdenum disulfide, fine plastic powders such as polyethylene and polytetrafluoroethylene, higher fatty acids, higher alcohols, higher fatty acid esters, and fluorocarbons. Is 0.1 to 100 parts by weight of the above-mentioned binder.
It is added in a proportion of 50 parts by weight.

The substrate used in the magnetic recording medium of the present invention includes synthetic resins (for example, polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyamides, polyolefins, cellulosics), non-magnetic metals, glass, ceramics and papers. Etc., and the form thereof is used in films, tapes, sheets, cards, disks and the like.

[0035]

According to the present invention, high density recording is possible,
A magnetic recording medium having a well-balanced improvement in electromagnetic conversion characteristics and physical characteristics can be obtained. In particular, the magnetic recording medium of the present invention is
The S / N ratio is improved, the friction coefficient is low, the running property is good, the head is not clogged, and the decrease in the saturation magnetic flux density is smaller than that of the magnetic recording medium using only metal powder.

[0036]

The present invention will be further described in the following examples, but the present invention is not limited to these examples.

Examples 1 to 6 and Comparative Examples 1 to 5 The following components were dispersed in a sand mill to prepare a magnetic coating material for the first magnetic layer or a magnetic coating material for the second magnetic layer. In addition,
The magnetic powders shown in Table 1 are used, and the first magnetic layer and the second magnetic layer are used.
The magnetic powder used for the magnetic layer is shown in Table 2. <Magnetic paint components> • Magnetic powder (A to H, see Table 1) 100 parts by weight • Vinyl chloride resin 10.x parts by weight (Zeon Corporation, MR 110) • Polyurethane resin 5.x parts by weight (Toyo Corporation) Spinning Co., Ltd., UR 8700) ・ Coronate L 3 x parts by weight [Curing agent from Nippon Polyurethane Industry Co., Ltd., added before coating] ・ Butyl stearate 2 parts by weight ・ Stearic acid 2 parts by weight ・ Carbon black (Average particle size 0.02 μm) 3 parts by weight Alumina (average particle size 0.3 μm) 7 parts by weight Methyl ethyl ketone 75 parts by weight Toluene 75 parts by weight Cyclohexanone 75 parts by weight In addition to the vinyl chloride resin, The “x” of the polyurethane resin is a value obtained by [specific surface area value of magnetic powder (see Table 1) / 45].

[0038]

[Table 1]

Note) Co-Fe: Cobalt-like cobalt-deposited FeO _x (1.34 ≦ x <1.
5) ... By using FeO _x as a raw material and manipulating the amount of cobalt and the amount of Fe ²⁺ , predetermined characteristics were obtained. Metal: Alloy containing 90% by weight of iron, 8% by weight of nickel and 2% by weight of aluminum Shape ratio: * (A to F) are needle-like ratios, ** (G, H) are plate-like ratios

[0040]

[Table 2]

The thickness after drying with a gravure roll is 3.0.
A magnetic coating material for the first magnetic layer was applied onto a polyethylene terephthalate (PET) film having a thickness of 7 μm so as to have a thickness of μm, and after longitudinal magnetic field orientation (8000 G), it was dried to form a first magnetic layer. Then, similarly, a magnetic coating material for the second magnetic layer is applied on the first magnetic layer so that the thickness after drying becomes 0.5 μm, and after longitudinal magnetic field orientation (8000 G),
It is dried to form the second magnetic layer and calendered (8
Then, aging was carried out at 50 ° C. for 24 hours.

Subsequently, the coating material for the back coat layer prepared by mixing the following components in a sand mill was applied to the surface of the film opposite to the surface provided with the magnetic layer so as to have a dry thickness of 0.5 μm, Then, it aged at 50 degreeC for 24 hours. <Backcoat layer coating material> Carbon black (average particle size 0.02 μm) 32 parts by weight Carbon black (average particle size 0.06 μm) 8 parts by weight Polyurethane resin 20 parts by weight (Nippon Polyurethane Co., Ltd.) , Nipollan 2301) -Nitrocellulose 20 parts by weight (viscosity indication by Hercules Powder CO. Is 1/2 second) -Stearic acid 1 part by weight-Methyl ethyl ketone 160 parts by weight-Toluene 80 parts by weight-Cyclohexanone 80 parts by weight.

As described above, the film on which the first and second magnetic layers were formed was cut into a tape having a width of 8 mm,
An 8 mm video cassette with a recording time of 120 minutes was manufactured by loading it in a cassette case. With respect to the obtained 8 mm video cassette, the friction coefficient, the saturation magnetic flux density decrease rate (ΔBs), the S / N ratio and the head clogging were evaluated by the following methods.
The results are shown in Table 3.

(1) Coefficient of Friction A tape running tester manufactured by Kyowa Tech Co., Ltd. was used, and the friction body was measured using a stainless steel guide pin for VTR having an outer diameter of 5 mm. (2) Saturation magnetic flux density reduction rate (ΔBs) 8mm video cassette 14 at 60 ° C, 90% RH
The decrease rate of the saturation magnetic flux density after storage for a day [ΔBs, the decrease amount is shown as a ratio (%) to the initial value. ] Was measured with a vibrating magnetometer. (3) The S / N 8 mm video cassette was set in a commercially available 8 mm VTR device to which a noise meter was connected, and the luminance S / N was measured by recording and reproducing a 50% white signal and measuring with a noise meter. The chroma S / N (AM, PM) was also measured according to this. (4) Clogged head Run 8mm video cassette for 120 minutes,
After repeated running tests, the magnetic head and the microscope were observed under a microscope to determine the stains attached to the head as follows. ◯: Almost no fouling Δ: Slight fouling X: Severe fouling In Table 3, the static magnetic properties (coercive force and residual magnetic flux density) of the first magnetic layer and the second magnetic layer are shown together. This is the value measured for each magnetic layer obtained by coating each of the magnetic coatings alone on a PET film (each having a dry film thickness of 2 μm), longitudinal magnetic field orientation, and calendering after drying.

[0045]

[Table 3]

Claims (3)

[Claims] 1. A base material, at least two kinds of magnetic powder selected from a ferromagnetic metal powder mainly composed of iron, a cobalt-containing iron oxide powder and a barium ferrite powder formed on the base material, and a binder. And a second magnetic layer formed on the first magnetic layer and formed of a cobalt-adhered iron oxide having a coercive force of 1300 Oe or more and a binder. 2. Each magnetic powder used in the first magnetic layer comprises:
All of them account for 10% by weight or more in the total amount of all magnetic powders used in the first magnetic layer.
The magnetic recording medium described. 3. The magnetic recording medium according to claim 1, wherein a coercive force (Hc ₁ ) of the first magnetic layer and a coercive force (Hc ₂ ) of the second magnetic layer satisfy the following formula. . | Hc ₁ −Hc ₂ | <400 Oe