JPH07129947A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To enable high density recording as well as to keep the interface between a thinned magnetic layer and an intermediate layer in a stable state by forming the magnetic layer on the intermediate layer contg. spindle-shaped alpha-iron oxide by a simultaneous coating system. CONSTITUTION:In a magnetic recording medium with a substrate, an intermediate layer formed on the substrate and a magnetic layer formed on the intermediate layer, the layers are formed by a simultaneous coating system. The intermediate layer consists of spindle-shaped alpha-iron oxide having <=0.2mum average major axis size and a binder. The magnetic layer consists of iron-based acicular metal powder having <=0.2mum average major axis size and >=1,700Oe, preferably 1,700-2,000Oe coercive force and a binder or acicular cobalt stuck iron oxide having <=0.2mum average major axis size and >=1,600Oe, preferably 1,600-2,000Oe coercive force and a binder.

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 excellent in durability and recording / reproducing 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. In order to achieve this object, it is required to use a magnetic powder having a high coercive force and a saturation magnetization and a small particle size to make the magnetic layer as thin as possible.

If the magnetic layer is made thin, the durability of the magnetic recording medium deteriorates. Therefore, in order to solve this, it is generally practiced to provide a non-magnetic intermediate layer under the magnetic layer. In this case, if the method of forming the upper magnetic layer after coating and drying the lower layer (or the intermediate layer) is used, it is difficult to form the upper layer thinly and uniformly because the lower layer is dissolved. A method of forming the upper layer before the lower layer is dried, particularly simultaneous coating of two layers by die coating has been performed as a promising method (Japanese Patent Laid-Open Nos. 50-138036 and 63-88080, JP-A-2-265672, JP-A-2-251265, etc.).

However, when two layers are simultaneously formed,
Mixing of the upper layer and the lower layer may occur, and various methods have been proposed in order to form a thin and uniform magnetic layer. For example, in Japanese Unexamined Patent Publication (Kokai) No. 5-73883, the thickness variation at the interface is specified in detail to stabilize the thickness.

Further, a method of adding a pigment to the intermediate layer has been investigated (Japanese Patent Laid-Open No. 62-159337 and Japanese Laid-Open Patent Publication No. Sho 62-159337).
62-159338, JP-A-2-24822, etc.). Α- iron oxide as a pigment which can be added to the intermediate layer of the magnetic recording medium (α-Fe ₂ O ₃₎ is known. However, α-iron oxide has good dispersibility and is excellent as a pigment. However, when blended in the intermediate layer of a magnetic recording medium, the stability of the interface with the magnetic layer is not always sufficient, and a thin and uniform magnetic layer is obtained. Is difficult to form.

As described above, at present, it can be said that there is no general guiding principle for preventing the upper magnetic layer and the lower layer (intermediate layer) from being mixed with each other to form a thin and uniform upper magnetic layer.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a magnetic recording medium which is excellent in high frequency characteristics while achieving high density recording by thinning / uniformizing the magnetic layer of the magnetic recording medium. is there.

[0008]

Means for Solving the Problems As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a thin film can be formed by providing a magnetic layer on an intermediate layer containing spindle-shaped α-iron oxide by a multi-layer simultaneous coating method. The present inventors have completed the present invention by finding that the interface between the magnetic layer and the intermediate layer that has been made stable can be stably maintained, high-density recording can be performed, and a magnetic recording medium having further excellent high frequency characteristics can be obtained.

That is, the present invention is a magnetic recording medium having a base material, an intermediate layer formed on the base material, and a magnetic layer formed on the intermediate layer. The present invention provides a magnetic recording medium, characterized in that the layers are formed by a multi-layer simultaneous coating method, and the intermediate layer is spindle-shaped and comprises α-iron oxide having an average major axis length of 0.2 μm or less and a binder. .

The magnetic layer of the magnetic recording medium of the present invention will be described. In the present invention, the magnetic powder forming the magnetic layer may be any of those usually used in this field. Particularly preferable magnetic powders include acicular metal powders containing iron as a main component (metal powders), acicular cobalt-coated iron oxides and hexagonal ferromagnetic powders.

As the metal powder, conventionally known ones mainly composed of iron can be used. Iron powder, iron and cobalt,
Examples thereof include alloy powders of nickel and the like, and alloy powders containing aluminum, chromium, manganese, silicon, zinc, transition metal elements, rare earth metal elements and the like. The metal powder has an average major axis length of 0.2 μm or less and a coercive force of 1700 Oe.
Above all, 1700 to 2000 Oe is particularly preferable. The magnetic characteristics other than the coercive force are not particularly limited. Further, the metal powder used is needle-shaped, and the needle-shaped ratio is preferably 3 or more. When the acicular ratio is less than 3, the magnetic field orientation tends to decrease.

As the acicular cobalt-coated iron oxide, Co-coated
Fe ₃ O ₄ , Co-deposited Fe ₂ O ₃ , Co-deposited FeO _x (1.34 ≦ x <1.50) are used, and Co-deposited FeO _x is particularly preferable. Further, the cobalt-adhered iron oxide used in the present invention is produced by a conventional cobalt-adhesion method 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 material iron oxide), preferably 2 to 15% by weight, and the amount of Fe ²⁺ is usually 0.5 to 30% by weight.
(Percentage of cobalt-coated iron oxide as a whole), preferably 1 to 25% by weight. Further, the cobalt-coated iron oxide used in the present invention is preferably acicular, and the axial ratio is preferably 3-10. The cobalt-deposited iron oxide used in the present invention has an average major axis length of 0.2 μm or less and a coercive force of 160 μm.
It is preferably 0 Oe or more, particularly 1600 to 2000 Oe. The magnetic characteristics other than the coercive force are not particularly limited.

The hexagonal ferromagnetic powder may be barium ferrite, strontium ferrite, calcium ferrite, or some of the iron atoms constituting these ferrites.
Examples include those substituted with at least one element selected from the group consisting of Co, Ti, Zr, Ni, In, Cu, Ge, and Nb. Here, the hexagonal ferromagnetic powder means a powder of a magnetic metal compound having a hexagonal crystal structure. It is desirable that such a hexagonal ferromagnetic powder has 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 thickness of the plate relative to the length of the diagonal line ( Those having a d / t of 3 to 10 are usually available.
The hexagonal ferromagnetic powder used in the present invention has a coercive force
It is preferably 1400 Oe or more, particularly 1400 to 2000 Oe.
The magnetic characteristics other than the coercive force are not particularly limited.

The thickness of the magnetic layer containing the above magnetic powder (the thickness of the dry coating film) is preferably 1 μm or less, more preferably 0.5 μm or less.

Next, the intermediate layer of the magnetic recording medium of the present invention will be described. In the present invention, the intermediate layer on the substrate is formed of a spindle-shaped α-iron oxide having an average major axis length of 0.2 μm or less and a binder. Here, the "spindle shape" means that the central portion in the major axis direction is large (thick) and becomes thinner toward the end portion. When the average major axis length of α-iron oxide exceeds 0.2 μm, the surface property of the magnetic layer formed on the intermediate layer is impaired. The axial ratio of α-iron oxide is preferably 3-10.

In the present invention, the spindle-shaped α-
The reason why good magnetic properties are obtained by using iron oxide is not clear, but it is presumed as follows. That is, the flow characteristics of the paint containing the spindle-shaped α-iron oxide used in the present invention are different from those of the commonly available paint containing spherical or amorphous α-iron oxide, and the viscosity when a shear stress is applied. Has a strong so-called thixotropic property. On the other hand, the coating material for the magnetic layer also has strong thixotropic properties, and the magnetic coating material and the coating material containing the spindle-shaped α-iron oxide used for the lower layer (or the intermediate layer) have similar flow characteristics. It is considered that by matching the fluidity of the upper layer and the lower layer (or the intermediate layer) which are simultaneously coated in this way, a thin and uniform magnetic layer can be formed without disturbance of the interface.

The α-iron oxide (α-Fe ₂ O ₃ ) is usually spherical or amorphous, but the spindle-shaped α-FeOOH can be obtained by heating the spindle-shaped α-FeOOH. . The spindle-shaped α-FeOOH is synthesized, for example, by adding 1 to 5 times the specified amount of alkali carbonate to an aqueous solution of ferrous salt and performing air oxidation while maintaining the temperature at 30 to 60 ° C.

The intermediate layer can also be formed by using the above-mentioned α-iron oxide in combination with other inorganic powder or organic powder. Examples of the inorganic powder include carbon black, graphite carbon, alumina, chromium oxide, titanium oxide, barium sulfate, zinc oxide, calcium carbonate and the like.
Further, as the organic powder, polystyrene beads, nylon powder, benzoguanamine resin beads, polyacrylic acid-based polymer beads, polyvinyl chloride resin beads, and other polymer beads can be used. The average particle size of these is preferably 0.2 μm or less. In addition, the weight ratio of α-iron oxide and other powders is α-iron oxide: other powder = 100:
0 to 10:90 is preferable. If the content of other powders exceeds 90% by weight, the high frequency characteristics of the magnetic recording medium deteriorate.

In the present invention, the magnetic powder and α-
These may be surface-treated in order to improve the dispersibility of the iron oxide. The surface treatment of such magnetic powder and α-iron oxide is described in "Characterization of Powder Surfaces";
Academic Press can be referred to, and examples thereof include a method of coating the surface of magnetic powder or α-iron oxide with an inorganic oxide. The inorganic oxides used here include Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ and ZnO.
Etc. These may be used in combination or may be used alone. Further, the surface of the magnetic powder or α-iron oxide can be coated with an organic substance. For example, a method of heat treatment in an inert organic solvent with a specific phosphoric acid ester and a silane coupling agent (Japanese Patent Publication No. 3-34841), a specific compound having an isocyanate group, and a specific phosphoric acid ester A method of coating with a reaction product (Japanese Patent Publication No. 3-8017), a method of coating with a specific phosphoric acid ester and a specific silane coupling agent (Japanese Patent Laid-Open No. 1-149220), and a specific method having an isocyanate group. A method of coating with a reaction product of a compound and one or more compounds selected from a specific compound group having a functional group capable of reacting with an isocyanate group (JP-A-59-139135). In addition, treatment with a dispersant such as other fatty acids, oligomers, and surfactants can be performed.

The magnetic layer and the intermediate layer of the magnetic recording medium of the present invention are non-magnetic by a so-called multi-layer simultaneous coating method in which the magnetic coating material is coated while the coating liquid for the intermediate layer coated on the non-magnetic support is wet. It is formed on a support. In the present invention, the "multi-layer simultaneous coating method" means that the intermediate layer is first coated on the non-magnetic support and then the magnetic layer is coated on the intermediate layer as quickly as possible while it is in a wet state, so-called. It refers to a sequential coating method, a method of simultaneously coating multiple layers by an extrusion method, and the like. Specifically, JP-A-61-13
The coating method described in No. 9929 can be adopted. Also,
As the multi-layer simultaneous coating method, for example, a two-layer simultaneous coating method by ordinary die coating, a simultaneous multi-layer coating method shown in JP-A-62-212933, JP-A-63-88080, JP-A-2-88080.
No. 17921, JP-A-2-265672, a method of coating an intermediate layer (lower layer) and a magnetic layer (upper layer) at substantially the same time by a coating head having two slits for passing the coating liquid therein. There is a method of coating the intermediate layer and the magnetic layer almost at the same time with an extrusion coating apparatus with backup as disclosed in JP-A-174965. In the case of the sequential multi-layer system, the intermediate layer coating solution is first coated on the non-magnetic support by a gravure roll, dried to form an intermediate layer, and then the intermediate layer is magnetically coated by a gravure roll. The intermediate layer and the magnetic layer can be formed by a method of applying a paint and drying the same to form the magnetic layer, but the purpose of forming a thinner magnetic layer (upper layer) is not sufficiently achieved. .

In the present invention, a desired layer can be formed between the intermediate layer and the base material. In this case,
The method of applying such a layer is not limited. Further, the back coat layer can be formed by a known method.

In the present invention, the mixed coating material for the intermediate layer and the magnetic coating material for the magnetic layer are composed of α-iron oxide or magnetic powder, a binder, an organic solvent, etc. Also, a common binder or the like can be used.

As the binder used in the present invention, a polyurethane resin, particularly a sulfonic acid group, a sulfonic acid metal base, a sulfobetaine group, a carbobetaine group, an amino group,
Polyurethane resin containing polar groups such as hydroxyl group and epoxy group, vinyl chloride-vinyl acetate copolymer, vinyl chloride-
Vinylidene chloride copolymers, vinyl chloride-based copolymers such as vinyl chloride-acrylonitrile copolymers, in particular sulfonic acid groups, metal sulfonate groups, vinyl chloride copolymers containing polar groups such as amino groups, Butadiene-acrylonitrile copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate butyrate, cellulose propionate, nitrocellulose, etc.), styrene-butadiene copolymer, polyester resin, various synthetic rubbers, phenolic 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 Mixture of molecular weight glycol / high molecular weight diol / isocyanate, and mixtures thereof and the like can be exemplified. Normally, the binder is 3.0 to 10.0 in the magnetic paint.
It is blended in the amount of about wt%. Further, the compounding paint for the intermediate layer contains 10 to 500 parts by weight, based on 100 parts by weight of the above-mentioned α-iron oxide and other powders optionally mixed.
It is preferably used in an amount of 15 to 200 parts by weight.

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 compounded in the magnetic paint or the compounded paint for the intermediate layer 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-
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 are used. 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. And the like are added in a ratio of 0.1 to 50 parts by weight to 100 parts by weight of the above-mentioned binder. In addition, such a lubricant may be blended in the paint for the intermediate layer.

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

[0027]

According to the present invention, it is possible to obtain a magnetic recording medium having a multi-layer structure in which the interface state between the magnetic layer and the intermediate layer is stable even if the thickness of the magnetic layer is reduced, and high density recording is possible. In addition, it is possible to obtain a magnetic recording medium having a better high frequency output.

[0028]

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

Examples 1 to 5 and Comparative Examples 1 to 5 (1) Intermediate Layer Coating Material and Magnetic Coating Material Compound coating materials A to D for intermediate layer (hereinafter referred to as intermediate layer coating material) and magnetic layer coating material The magnetic paints I to III of Table 1 are shown in Table 1.
An 8 mm video cassette was produced by using the combination shown in.

[0030] <Intermediate layer coating A> · average major axis length of 0.12 .mu.m, spindle of the thickness of the central portion is 0.02μm α-Fe ₂ O ₃ 100 parts by weight ^{_{· -N + (CH 3) 3}} Cl - and Containing vinyl chloride / vinyl acetate copolymer 10 parts by weight Polyester polyurethane resin 3 parts by weight Butyl stearate 1 part by weight Oleic acid 0.5 parts by weight Solvent 250 parts by weight (methyl ethyl ketone / toluene / cyclohexanone = 3/1/2) ).

<Intermediate Layer Coating Material B> In the composition of the intermediate layer coating material A, the spindle-type α-Fe ₂ O ₃ is blended in an amount of 80 parts by weight, and further carbon black (average particle diameter 20 nm) is blended in 20 parts by weight. Made paint.

<Intermediate Layer Coating C> As spindle-shaped α-Fe ₂ O _{3 in the} composition of the intermediate layer coating A, 1% of γ-aminopropyltriethoxysilane and Gafac RE 610 (Toho Chemical Co., Ltd.) were previously prepared in toluene. (2) Phosphoric acid ester manufactured by (1)) and then toluene was distilled off to obtain spindle-shaped α-Fe ₂ O ₃ (average major axis length 0.12 μm, central thickness 0.02 μm).

<Intermediate Layer Coating D> In the composition of the intermediate layer coating A, the average particle diameter is 0.1 μm in place of the spindle-shaped α-Fe ₂ O _3.
A paint using amorphous α-Fe ₂ O ₃ of.

<Magnetic paint I> 100 parts by weight of needle-shaped metal powder mainly composed of iron [Fe: Al: Ca: Si: Ni: Co = 92: 2: 1: 1: 2: 2, coercive force = 1730 Oe, saturation magnetization 132emu / g, average major axis length = 0.13 μm] ・ Alumina (average particle size 0.3 μm) 7 parts by weight ・ Carbon black (average particle size 20 nm) 3 parts by weight ・ Sulfonic acid group-containing vinyl chloride-acrylic acid Copolymer resin 8 parts by weight Sulfonic acid group-containing polyurethane resin 6 parts by weight Stearic acid 1 part by weight 2-Ethylhexyl myristate 2 parts by weight Polyisocyanate 4 parts by weight [Coronate L, manufactured by Nippon Polyurethane Industry Co., Ltd. Curing agent] -Solvent 230 parts by weight (methyl ethyl ketone / toluene / cyclohexanone = 2/1/2).

<Magnetic Paint II> In the composition of the magnetic paint I, in place of the acicular metal powder mainly composed of iron, cobalt-coated iron oxide [Co-γ-FeO _x (1.34 ≦ x <1.50), coercive force = 1650 Oe, saturation magnetization 81 emu / g, average major axis length = 0.15μ
m] is a magnetic paint.

<Magnetic Paint III> In the composition of Magnetic Paint I, hexagonal plate-shaped barium ferrite powder [plate diameter = 0.05 μm, plate ratio = instead of iron-like needle-shaped metal powder]
5, magnetic coercive force = 1450 Oe, saturation magnetization = 55 emu / g]

(2) Manufacture of 8 mm video cassette (i) In the case of Examples 1 to 5 and Comparative Examples 1, 3 and 5, the above intermediate layer coating material and magnetic coating material were used in the combination shown in Table 1 and extrusion coating. Depending on the equipment, the thickness after drying and calendering was 2.5 μm for the intermediate layer and
Simultaneous coating on a polyethylene terephthalate (PET) film with a thickness of 7 μm so that it becomes 0.5 μm,
While both layers were in a wet state, they were oriented and dried by a 5000 G solenoid magnet, and subsequently calendered at 80 ° C.

Further, a backcoat 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 4 parts by weight [Coronate HX, a curing agent manufactured by Nippon Polyurethane Industry Co., Ltd.] ・ Stearic acid 1 part by weight -320 parts by weight of solvent.

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

(Ii) Cases of Comparative Examples 2 and 4 In the above (i), the formation of the intermediate layer was omitted, the thickness of the magnetic layer was set to 2.9 μm, and otherwise the 8 mm video cassette was manufactured.

(3) Performance evaluation of 8 mm video cassette With respect to the 8 mm video cassette obtained as described above, the output at 7 MHz was measured by the following method. The results are shown in Table 1.

<7MHz output> 8mm video cassette,
The commercially available Hi8VTR was mounted on a modified device, the recording and reproducing characteristics at 7 MHz were measured, and the reproducing output was shown as a relative value with respect to a reference. The numerical values in Table 1 are relative values with Examples 1 to 3 and Comparative Example 2 using Comparative Example 1 as a reference (0 dB),
Example 4 and Comparative Example 4 are relative values based on Comparative Example 3, and Example 5 is a relative value based on Comparative Example 5 (0 dB).

[0043]

[Table 1]

Claims (4)

[Claims] 1. A magnetic recording medium having a substrate, an intermediate layer formed on the substrate, and a magnetic layer formed on the intermediate layer, wherein the intermediate layer and the magnetic layer are multi-layered. A magnetic recording medium formed by a simultaneous coating method, wherein the intermediate layer is spindle-shaped and comprises α-iron oxide having an average major axis length of 0.2 μm or less and a binder. 2. The magnetic recording medium according to claim 1, wherein the magnetic layer comprises an acicular metal powder mainly composed of iron having an average major axis length of 0.2 μm or less and a coercive force of 1700 Oe or more, and a binder. 3. The magnetic recording medium according to claim 1, wherein the magnetic layer comprises an acicular cobalt-coated iron oxide having an average major axis length of 0.2 μm or less and a coercive force of 1600 Oe or more, and a binder. 4. The magnetic recording medium according to claim 1, wherein the magnetic layer comprises a hexagonal ferromagnetic powder having a coercive force of 1400 Oe or more and a binder.