JPH0729152A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve the surface smoothness of a magnetic layer and further improve radio frequency characteristics by a method wherein hexagonal ferromagnetic powder having a high coercive force and fine magnetite powder are mixed with a specific weight ratio and applied to a substrate as magnetic coating material to form the magnetic layer. CONSTITUTION:Hexagonal ferromagnetic powder whose coercive force is not less than 1300Oe, low coercive force magnetite powder whose average particle diameter is not larger than 0.3mum and whose coercive force is not higher than 150Oe and binder and used as coating material which is applied to a substrate to form a magnetic layer. The magnetic layer is so formed as to have the weight ratio B/A of the magnetic powder B to the hexagonal ferromagnetic powder A within a rage of 0.03-0.3. With this constitution, stacking of the hexagonal ferromagnetic powder is suppressed and the surface roughness of the magnetic layer is improved, so that radio frequency characteristics can be further improved.

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 exhibiting excellent high frequency characteristics.

[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.

As a method for realizing high density recording,
A perpendicular magnetic recording system has been proposed, and in particular, hexagonal plate-shaped barium ferrite having a hexagonal crystal structure is used as magnetic powder suitable for this system.

[0004]

However, hexagonal plate-shaped barium ferrite has relatively good high frequency characteristics, but particles are likely to stack, the surface condition of the magnetic layer is deteriorated, and the surface roughness of the magnetic layer is reduced. There is a problem that it becomes very large, and in order to further improve the high frequency characteristics, it is required to improve the surface smoothness of the magnetic layer.

[0005]

As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a ferromagnetic powder having a hexagonal crystal structure with a coercive force of 1300 Oe or more, a low coercive force and a small amount. The inventors have found that the use of the magnetite powder in a specific weight ratio can improve the surface smoothness of the magnetic layer and can provide a magnetic recording medium having excellent high frequency characteristics, and completed the present invention.

That is, the present invention relates to a base material and a hexagonal ferromagnetic powder (A) formed on the base material and having a coercive force of 1300 Oe or more.
And a magnetic layer having an average particle size of 0.3 μm or less and a coercive force of 150 Oe or less of magnetite powder (B) and a binder, and the hexagonal ferromagnetic powder (A) in the magnetic layer. The weight ratio of the magnetite powder (B) is (B) / (A) = 0.03 ~
A magnetic recording medium of 0.3 is provided.

The magnetic recording medium of the present invention has a coercive force of 1300 O
e or more, preferably 1300 to 2000 Oe of hexagonal ferromagnetic powder is used. Here, the hexagonal ferromagnetic powder means a powder of a magnetic metal compound having a hexagonal crystal structure. If the coercive force of the hexagonal ferromagnetic powder is less than 1300 Oe, the high frequency characteristics will be insufficient.

As the hexagonal ferromagnetic powder used in the present invention, for example, barium ferrite, strontium ferrite, calcium ferrite, or some of iron atoms constituting these ferrites can be Co, Ti, Zr, Ni, In, Cu, G
Examples thereof include those substituted with at least one element selected from the group of e and Nb. It is desirable that such a hexagonal ferromagnetic powder have as uniform particle shape and particle size distribution as possible. Specifically, in the case of a hexagonal plate-shaped crystal,
Assuming that the length of the diagonal line is d and the thickness of the plate is t, the length (d) is 0.03 to 0.2 μm, and the plate thickness (d / t) with respect to the length of the diagonal line is 3 to 10. Is desirable. In particular, the hexagonal ferromagnetic powder having a large d / t has a flat plate shape, which facilitates stacking in the easy magnetization axis direction when the above-mentioned mixing gives an impact. If the particle shape and particle size distribution of the hexagonal ferromagnetic powder are not uniform, the small particles are sandwiched between the large particles, and the hexagonal ferromagnetic powder is laminated in the easy magnetization axis direction. It becomes difficult to make a unit aggregate.

In the present invention, the above hexagonal ferromagnetic powder
Along with (A), the average particle size is 0.3 μm or less and the coercive force is 150 O.
The magnetite powder (B) of e or less is used together. If the coercive force of the magnetite powder exceeds 150 Oe, the magnetic properties deteriorate. Also, if the average particle size of the magnetite powder exceeds 0.3 μm, the surface properties will deteriorate. As such magnetite powder, spherical, hexahedral, octahedral, or other shapes or irregular shapes are commercially available, and any of them can be used, or several kinds can be used in combination.

Further, in the present invention, the weight ratio of the hexagonal ferromagnetic powder (A) to the magnetite powder (B) in the magnetic layer must be (B) / (A) = 0.03 to 0.3. . When the weight ratio is less than 0.03, the effect of adding the magnetite powder (B) is small. Further, if the weight ratio exceeds 0.3, the magnetic properties deteriorate.

The hexagonal ferromagnetic powder (A) and the magnetite powder (B) may be surface-treated in order to further improve dispersibility. Such magnetic powder (A),
For the surface treatment of (B), refer to “Characterization of Powder Sur
faces ”; Academic Press can be referred to, for example, a method of coating the surface of the magnetic powder with an inorganic oxide. As the inorganic oxide used here, Al ₂ O
₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , ZnO and the like can be mentioned. These may be used in combination or may be used alone.

In the present invention, by using the hexagonal ferromagnetic powder (A) as described above and the magnetite powder (B) as described above in combination, stacking of the hexagonal ferromagnetic powder (A) is suppressed and the magnetic layer It is considered that a magnetic recording medium having improved surface roughness and excellent high frequency characteristics can be obtained.

The magnetic recording medium of the present invention comprises the above magnetic powder.
In addition to the magnetic layer containing (A) and (B), other magnetic layers and non-magnetic layers may be provided to provide a magnetic recording medium having a multilayer structure. In this case, the magnetic powder (A), ( A magnetic layer containing B) as the uppermost layer (on the surface of the base material on which the magnetic layer is formed,
The layer farthest from the substrate) is desirable. The nonmagnetic layer is, for example, an intermediate layer made of suitable nonmagnetic particles for the purpose of reinforcement. The thickness of the magnetic layer containing the magnetic powders (A) and (B) is 0.05 to 9 μm, preferably 0.1 to 5 μm. The thickness of the other magnetic layer and nonmagnetic layer is 0.1 to 5 μm, preferably 0.5 to 4 μm.

In the present invention, the method of forming (coating) the magnetic layer is a method of simultaneously forming a magnetic layer containing the magnetic powders (A) and (B) and another magnetic layer or non-magnetic layer (multi-layer simultaneous formation). Method) or a method of sequentially forming one layer at a time (sequential forming method). When forming one layer at a time, calendar processing may be performed for each layer.

The magnetic coating material forming the magnetic layer of the magnetic recording medium of the present invention comprises the above-mentioned magnetic powders (A) and (B), a binder, an organic solvent and the like.

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 and epoxy groups. 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, Nole 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 are high molecular weight diol / isocyanate mixtures, and mixtures thereof. Usually, the binder is 3.0-10.0 in the magnetic paint.
It is blended in a weight percentage of about.

Suitable organic solvents include cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate, benzene, toluene, xylene, dimethylsulfoxide, tetrahydrofuran, dioxane, etc., which are suitable 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 mixed in the magnetic coating material in an amount of about 20 to 80% by weight.

In the magnetic coating composition, various commonly used additives such as a dispersant, an abrasive and a lubricant 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-
A fine powder of alumina, fused alumina, chromium oxide (Cr ₂ O ₃ ), iron oxide, silicon carbide, corundum, diamond, etc. with an average particle size of 0.05 to 1 μm can be used, and usually 100 parts by weight of the binder as described above is used. 0.5 to 100 parts by weight is added. Examples of lubricants include silicone oils such as various polysiloxanes, graphite, inorganic powders such as molybdenum disulfide, fine plastic powders such as polyethylene and polytetrafluoroethylene, higher fatty acids, higher alcohols, higher fatty acid esters, and fluorocarbons. Etc. are added in a ratio of 0.1 to 50 parts by weight with respect to 100 parts by weight of the above-mentioned binder.

The base material 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, cellulose derivatives), non-magnetic metals, glass, ceramics and papers. Etc., and the form thereof is used in films, tapes, sheets, cards, disks and the like.

[0020]

According to the present invention, a magnetic recording medium in which the surface smoothness of the magnetic layer is improved and the high frequency characteristics are further improved can be obtained.

[0021]

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

Example 1 The following components were dispersed in a sand mill to prepare a magnetic paint. <Magnetic paint blend> 100 parts by weight of hexagonal ferromagnetic powder [Hexagonal barium ferrite, diagonal length 0.07 μm, plate ratio 4, coercive force 1340 Oe, saturation magnetization 63 emu / g] Magnetite powder 10 parts by weight [average Particle size 0.1 μm, sphere, coercive force 65 Oe, saturation magnetization 84 emu / g, remanent magnetization 5 emu / g] ・ Alumina (average particle size 0.3 μm) 6 parts by weight ・ Carbon black (average particle size 40-50 nm) 2 Parts by weight-Sulfonic acid group-containing vinyl chloride-vinyl acetate copolymer resin 8 parts by weight-sulfonic acid group-containing polyurethane resin 4 parts by weight-myristic acid 1 part by weight-oleic acid 1 part by weight-butyl stearate 2 parts by weight-poly Isocyanate [Takeda Pharmaceutical Co., Ltd. D-250 N] 3 parts by weight Solvent 180 parts by weight Polyethylene terephthalate (PET) with a thickness of 7 μm so that the thickness after drying with a gravure roll is 2.5 μm
The above magnetic coating material was applied onto the film, longitudinally oriented with a magnetic field strength of 5000 G, dried to form a magnetic layer, further calendered (80 ° C.), and then aged at 50 ° C. for 24 hours.

Then, the back coating layer coating material 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 that the dry thickness would be 0.5 μm, and then Aged at 50 ° C for 24 hours. <Coating ingredients for back coat layer> ・ 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 (Nipporan 2301 manufactured by Nippon Polyurethane Co., Ltd.) ) ・ Nitrocellulose 20 parts by weight (viscosity indication by Hercules Powder CO. Is 1/2 second) ・ Polyisocyanate [D-250 N manufactured by Takeda Pharmaceutical Co., Ltd.] 4 parts by weight ・ Stearic acid 1 part by weight ・ Solvent 240 parts by weight.

As described above, the film on which the magnetic layer and the back coat layer were formed was cut into a tape having a width of 8 mm, loaded into an 8 mm cassette case, and the recording time was 8/120.
mm video cassette was made.

Example 2 In the magnetic coating composition of Example 1, as hexagonal ferromagnetic powder, diagonal length 0.05 μm, plate ratio 6, coercive force 1520 Oe
, Barium ferrite hexagonal plate with saturation magnetization 58emu / g 1
00 parts by weight, and as magnetite powder, average particle diameter 0.13 μm, spherical shape, coercive force 92 Oe, saturation magnetization 83 em
u / g and residual magnetization of 5 emu / g were used in 13 parts by weight,
An 8 mm video cassette was manufactured in the same manner as in Example 1.

Example 3 An 8 mm video cassette having two magnetic layers was prepared by using the magnetic coating composition of Example 1 as the magnetic coating for the upper layer and the coating composition shown below as the magnetic coating for the lower layer. <Magnetic coating composition for lower layer> 100 parts by weight of iron oxide coated with acicular cobalt [average major axis length 0.13 μm, coercive force 850 Oe, saturation magnetization 84 emu / g] carbon black (average particle size 40-50 nm) 10 parts by weight Parts-Amino group-containing vinyl chloride-vinyl acetate copolymer resin 6 parts by weight-Amino group-containing polyurethane resin 6 parts by weight-2-Ethylhexyl stearate 4 parts by weight-Polyisocyanate [Takeda Chemical Industries D-250N] 2 parts by weight solvent 120 parts by weight Polyethylene terephthalate (PET) with a thickness of 7 μm so that the thickness after drying with a gravure roll is 2.5 μm
The above magnetic coating material for the lower layer was applied onto the film, longitudinally oriented at a magnetic field strength of 5000 G, dried, and further calendered (80 ° C.) to form a lower magnetic layer. Then, the magnetic coating material of Example 1 was applied on the lower magnetic layer so that the thickness after drying was 0.2 μm, longitudinally oriented at a magnetic field strength of 5000 G, and then dried to form the upper magnetic layer. A layer was formed, and after calendering (80 ° C), it was aged at 50 ° C for 24 hours.
After that, a back coat layer having a thickness of 0.5 μm was formed in the same manner as in Example 1, and thereafter, an 8 mm video cassette was produced in the same manner as in Example 1.

Example 4 A multilayer 8 mm video cassette was produced by using the magnetic coating composition of Example 2 as the upper layer magnetic coating composition and the following coating composition as the non-magnetic layer (intermediate layer) magnetic coating composition. <Intermediate layer coating formulation> · TiO ₂ (average particle size 30 nm) 90 parts Carbon black (average particle size 20 nm) 10 parts by weight Amino group-containing vinyl chloride - vinyl acetate copolymer resin 15 parts by weight hydroxyl group-containing Polyurethane resin 15 parts by weight ・ Polyisocyanate 2 parts by weight [Coronate HX, a curing agent manufactured by Nippon Polyurethane Industry Co., Ltd.] ・ 2-Ethylhexyl stearate 1 part by weight ・ Solvent 120 parts by weight The thickness after drying with a gravure roll Polyethylene terephthalate (PET) with a thickness of 7 μm to 2.5 μm
After applying the magnetic coating for the intermediate layer on the film,
It was dried to form an intermediate layer. Then, the magnetic coating material of Example 2 was applied onto the intermediate layer so that the thickness after drying was 0.2 μm, longitudinally oriented with a magnetic field strength of 5000 G, and then dried to form a magnetic layer. Formed and calendered (80
C.) and then aged at 50.degree. C. for 24 hours. After that, a back coat layer having a thickness of 0.5 μm was formed in the same manner as in Example 1, and thereafter, an 8 mm video cassette was produced in the same manner as in Example 1.

Comparative Example 1 An 8 mm video cassette was manufactured in the same manner as in Example 1 except that magnetite powder was not mixed in the magnetic coating composition of Example 1.

Comparative Example 2 An 8 mm video cassette was prepared in the same manner as in Example 1 except that magnetite powder was not mixed in the magnetic coating composition of Example 2.

Comparative Example 3 In the magnetic coating composition of Example 2, the hexagonal plate barium ferrite content was 70 parts by weight, and the magnetite powder content was 40 parts by weight. An 8 mm video cassette was manufactured in the same manner as in.

With respect to the 8 mm video cassettes obtained in Examples 1 to 4 and Comparative Examples 1 to 3, the surface roughness of the magnetic layer and the output at 7 Hz were measured by the following methods. The results are shown in Table 1. <Measurement Method of Surface Roughness of Magnetic Layer> Ra (nm) was measured by an optical interference type surface roughness meter (manufactured by ZYGO). <Measurement method of output at 7 Hz> An 8 mm video cassette was attached to a device obtained by modifying a commercially available Hi8VTR, and recording was performed at 7 MHz to measure a reproduction output. The values in Table 1 are
It is a relative value using Comparative Example 2 as a reference (0 dB).

[0032]

[Table 1]

Claims (2)

[Claims] 1. A base material and a coercive force formed on the base material,
Hexagonal ferromagnetic powder (A) of 1300 Oe or more and the average particle size
Magnetite powder with a coercive force of 150 Oe or less at 0.3 μm or less
(B) has a magnetic layer consisting of a binder, and the hexagonal ferromagnetic powder (A) and the magnetite powder in the magnetic layer
A magnetic recording medium in which the weight ratio of (B) is (B) / (A) = 0.03 to 0.3. 2. The magnetic recording medium according to claim 1, which has a multi-layer structure having the magnetic layer as an uppermost layer.