JP2614103B2 - Magnetic recording media - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a magnetic recording medium, and more particularly, to an improvement in a magnetic recording medium having at least two magnetic layers.

(Prior Art and Problems Thereof) In recent years, high-density recording has been required for magnetic recording media, and for this reason, many studies have been made on increasing the filling of magnetic materials. In order to achieve high packing, the study is narrowed down to two points, that is, fine particles of the magnetic substance and how to pack them into a high packing (improvement of packing properties).

Further, in order to improve the recording density, it is necessary to make the magnetic layer smooth as in the case of the high filling, and to make an effort to minimize the spacing loss.

Various means have been studied for the purpose of high filling, but one of the means is kneading in the process of compacting (also called consolidation) in the process of preparing the magnetic material itself and in the process of preparing the magnetic material coating solution. Methods for introducing processing are known.

Powder compaction (consolidation treatment) refers to the crushing of magnetic powder in a ball mill, colloid mill, etc., which removes air contained in the magnetic powder and reduces the bulk of the magnetic powder (ie, the bulk density). ) Is one means of improving the filling property.

The kneading treatment refers to kneading the magnetic material coating liquid with a roll mill, a kneader or the like before performing the dispersion treatment with a disperser such as a sand grinder, and is one means of improving the filling property.

It has long been known that these compacting and kneading processes are effective for improving high filling (that is, improving audio characteristics). For example, the compacting / kneading treatment is described in "Materials" Vol. 15, No. 150, pp. 160-165 (1966), and the kneading treatment is described in JP-A-59-165237 and JP-B-57-4288.
No. 8, etc.

However, in the conventional powder compaction and kneading processes, high filling for high-density recording can be achieved, but the magnetic properties of the magnetic recording medium have not always been improved. Particularly, in the case of a video tape having a multilayer structure, it is very difficult to achieve both excellent video characteristics and audio characteristics.

For example, if the consolidation process is performed strongly, the magnetic material is finely divided, but the needle-shaped magnetic material is broken, the needle-like shape is reduced, and when formed into a magnetic tape, the squareness ratio and the coercive force are reduced, and the video is reduced. The characteristics will be impaired.

Also, if the kneading process is performed strongly, the filling property can be improved, but if it is performed too strongly, the needle ratio of the needle-shaped magnetic material is impaired, the coercive force (Hc) decreases, and the squareness ratio decreases. Or degrade the video characteristics.

Therefore, even when using a magnetic material that has been subjected to consolidation processing or kneading processing for high filling, a magnetic recording medium having excellent magnetic characteristics, particularly a magnetic recording medium having a multilayer structure excellent in both video characteristics and audio characteristics, The development of was desired.

(Problems to be Solved by the Invention) An object of the present invention is to achieve both excellent video characteristics and audio characteristics in a magnetic recording medium having a multilayer structure.

(Means for Solving the Problems) The above object is achieved by a magnetic recording medium comprising a first magnetic layer and a second magnetic layer provided on a surface of a nonmagnetic support in that order.
The tap density of the magnetic material used for the first magnetic layer is 0.8 or more and 1.0 or less, and the tap density of the magnetic material used for the second magnetic layer is 0.
The tap density of the magnetic material of the first magnetic layer is greater than that of the magnetic material of the second magnetic layer.
The present invention has been found to be attained by a magnetic recording medium characterized by being 0.02 or more, and completed the present invention.

 Hereinafter, the present invention will be described in detail.

There are various means for increasing the strength of the kneading process, for example,
Examples include changing the amount of the solvent (including the solvent in the binder) added at the time of kneading, narrowing the gap between the rolls, and increasing the rotation speed so as to give a strong force to the kneaded material. Another method is to increase the tap density of the magnetic material. When the tap density is high, the air contained in the magnetic material is reduced, the bulk is reduced, and strong shearing can be applied to the kneaded material (magnetic material coating liquid).

However, if the consolidation treatment is performed strongly to increase the tap density, as described above, the acicular magnetic body is broken, the acicular ratio decreases, and when the magnetic tape is formed, the squareness ratio,
The coercive force is reduced and video characteristics are impaired.

Further, even in the strong kneading process, as described above, the needle-shaped magnetic material is frequently broken, the needle ratio of the magnetic material is reduced, the coercive force of the magnetic recording medium, the squareness ratio is reduced, and the video characteristics are impaired. Will be.

Therefore, it is necessary to perform the above-mentioned consolidation processing and kneading processing appropriately.

In particular, in the case of a multi-layered video tape, the video signal is mainly recorded in the second magnetic layer, so that the video signal is reduced due to a decrease in the squareness and coercive force due to a decrease in the needle ratio of the magnetic material.
The S / N ratio and the frequency characteristics (f characteristics) of the RF signal are reduced, and various adverse effects (especially, reduced video characteristics) are caused. For this reason, it is necessary to prevent the magnetic substance from breaking when performing the consolidation processing or the kneading processing of the magnetic substance coating liquid. For this purpose, it is not appropriate to make the consolidation treatment or kneading treatment stronger than necessary. However, if the consolidation processing and the kneading processing are omitted, the density of the magnetic layer cannot be increased (that is, the improvement of the audio characteristics). Therefore, it is important to appropriately perform the consolidation processing and the kneading processing.

Generally, the video characteristics are more strongly affected by the needle ratio and coercive force distribution of the magnetic material than the audio characteristics. In this sense, since the first magnetic layer contributes less to the video signal than the second magnetic layer, the first magnetic layer has a smaller tap density than the second magnetic layer magnetic material in order to improve the audio characteristics, that is, to improve the filling property. By increasing the tap density of the magnetic material, it is possible to knead strongly.

As a result of the investigations by the present inventors, it has been found that, by adjusting the tap density of the magnetic material used for the first magnetic layer and the second magnetic layer to a specific range by consolidation, high filling is achieved, and the video characteristics are improved. It was found that both audio characteristics and audio characteristics could be satisfied.

As described above, as a scale for estimating the degree of the consolidation processing, the tap density specified below is appropriate.

To measure tap density, measure powder into a graduated cylinder.
40g is weighed and dropped 100 times together with the measuring cylinder from the height of 30mm, and the volume of the powder after falling 100 times (that is, after tapping 100 times) is measured. calculate.

If the consolidation is performed strongly, the tap density increases. If the tap density is increased more than necessary, the needle-shaped magnetic material is crushed. In addition, when the tap density is high, excessive shear is applied during the kneading process, and the acicular ratio of the acicular magnetic body is reduced. That is, an appropriate consolidation process and an appropriate kneading process can be achieved by keeping the tap density of the magnetic material after the consolidation process within an appropriate range.

As an appropriate range of tap density, the tap density of the magnetic material of the first magnetic layer is 0.8 to 1.0, preferably 0.9 to 0.9.
5 The tap density of the magnetic material of the second magnetic layer is 0.7
0.90.9, preferably 0.7550.85.

The tap density of the magnetic substance of the first magnetic layer is preferably higher than the tap density of the magnetic substance of the second magnetic layer because the filling degree can be increased. However, if the tap density is too high, the fracture of the magnetic substance proceeds. Also decrease. Therefore, the difference between the tap density of the magnetic material of the first magnetic layer and the tap density of the magnetic material of the second magnetic layer is 0.02 or more, and preferably the difference is 0.15 or less.

 Preferred embodiments of the present invention are as follows.

(I) The specific surface area of the magnetic material of the first magnetic layer by the BET method is 45 m.
² / g or less and a crystallite size of not less than 290 Å, a magnetic recording medium and a crystallite size BET specific surface area of the magnetic body 30 m ² / g or more second magnetic layer is equal to or less than 400Å .

(Ii) the first magnetic layer contains 1 to 20 parts by weight of carbon black having an average particle diameter of less than 20 mμ based on 100 parts by weight of the magnetic substance;
A magnetic recording medium characterized in that the second magnetic layer contains 0.1 to 10 parts by weight of carbon black having an average particle diameter of 40 to 80 μm based on 100 parts by weight of the magnetic substance.

(Iii) A magnetic recording medium wherein the thickness of the first magnetic layer is 2.5 μm or more and the thickness of the second magnetic layer is 1.5 μm or less.

(Iv) the first magnetic layer contains an abrasive having a Mohs hardness of less than 7,
A magnetic recording medium, wherein the second magnetic layer contains an abrasive having a Mohs hardness of 7 or more.

(V) The coercive force of the magnetic material of the first magnetic layer is 600 Oe or more, the coercive force of the magnetic material of the second magnetic layer is 1000 Oe or less, and the magnetic material of the first magnetic layer and the second magnetic layer is cobalt-modified γ. −FeO
_x (x = 1.33 to 1.50).

As the magnetic material used in the present invention, γ-Fe ₂ O ₃ , Co-containing (adhered, denatured, doped) γ-Fe ₂ O ₃ , Fe ₃ O ₄ , Co-containing (adhered, denatured, doped) Fe ₃ O ₄ , γ-FeOx, Co-containing (adhered, modified, doped) γ-FeOx (x = 1.33 to 1.50)
For example, a known magnetic material can be used.

These magnetic materials may have a tap density that satisfies the above-mentioned rules. The magnetic material generally has a particle size of about 0.005 to 1
Micron in length, the ratio of the axial length / axis-width is about 1 / 1-50 / 1, and a specific surface area is 1m ² / g~70m ² / g approximately.

As the magnetic material used in the present invention, plate-like hexagonal barium ferrite can also be used. Also for the barium ferrite, the tap density may satisfy the above-mentioned rule. Generally, the particle size of these barium ferrites is about 0.001 to 1 micron in diameter and 1/2 to 1/20 of the diameter in thickness. The specific gravity of barium ferrite is 4-6 g / cc, a specific surface area is 1m ² / g~70m ² / g.

The surfaces of these magnetic materials may be impregnated with a dispersant, a lubricant, an antistatic agent, and the like described below in a solvent prior to dispersion and adsorbed thereon for each purpose.

In particular, as described above, the magnetic material of the first magnetic layer is S _BET.
The crystallite size is 290 mm or more at 45 m ² / g or less, and the magnetic material of the second magnetic layer has a S _BET of 30 m ² / g or more and a crystallite size of 400
Å Particularly preferred is Further, as described above, Co-modified γ-FeOx (x = 1.33 to 1.50) is particularly preferable.

As the binder used for the magnetic layer and the back layer of the present invention, conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof are used.

The thermoplastic resin has a softening temperature of 150 ° C. or less, an average molecular weight of 10,000 to 30,000, and a degree of polymerization of about 50 to 2,000.For example, vinyl chloride-vinyl acetate copolymer, vinyl chloride copolymer, vinyl chloride chloride Vinylidene copolymer, vinyl acrylonitrile copolymer, acrylate acrylonitrile copolymer, acrylate vinylidene chloride copolymer, acrylate styrene copolymer, methacrylate acrylonitrile copolymer,
Methacrylate vinylidene chloride copolymer, methacrylate styrene copolymer, urethane elastomer, nylon-silicone resin, nitrocellulose-polyamide resin, polyvinyl fluoride, vinylidene chloride acrylonitrile copolymer, butadiene acrylonitrile copolymer Coalesce, polyamide resin, polyvinyl butyral, cellulose derivatives (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose, ethylcellulose, methylcellulose, propylcellulose, methylethylcellulose, carboxymethylcellulose, acetylcellulose, etc.), styrene Butadiene copolymer, polyester resin, chlorovinyl ether acrylate copolymer, Bruno resins, various thermoplastic resins and mixtures of these synthetic rubber type or the like is used.

The thermosetting resin or the reactive resin has a molecular weight of 200,000 or less in the state of a coating liquid, and the molecular weight becomes infinite due to reactions such as condensation and addition by heating after coating and drying. Among these resins, those which do not soften or melt before the resin is thermally decomposed are preferred. Specifically, for example, phenol resin, phenoxy resin, epoxy resin, polyurethane curable resin, urea resin, melamine resin, alkyd resin, silicone resin, acrylic-based reaction resin, epoxy-polyamide resin, nitrocellulose melamine resin, high molecular weight polyester resin Mixture of methacrylate copolymer and diisocyanate prepolymer, mixture of polyester polyol and polyisocyanate, urea formaldehyde resin, low molecular weight glycol /
High molecular weight diol / triphenylmethane triisocyanate mixture, polyamine resin, polyimine resin and mixtures thereof.

These binders may be used alone or in combination, and other additives may be added. The mixing ratio of the magnetic material and the binder in the magnetic layer is in the range of 5 to 300 parts by weight of the binder with respect to 100 parts by weight of the magnetic material. The mixing ratio of the fine powder and the binder in the back layer is in the range of 30 to 300 parts by weight of the binder with respect to 100 parts by weight of the fine powder. Additives include dispersants, lubricants, abrasives, antistatic agents, antioxidants, solvents and the like.

These thermoplastic and thermosetting resins and reactive resins have a carboxylic acid, a sulfinic acid, a sulfonic acid, a phosphoric acid, a sulfuric acid, a sulfuric ester group, a phosphoric ester group, a functional group other than the main functional group, and an alkyl ester group thereof. Acid groups, amino acids; aminosulfonic acids, sulfuric acid or phosphoric acid esters of amino alcohols, amphoteric groups such as alkyl betaine, amino groups, imino groups, imide groups, amide groups, etc .; hydroxyl groups, alkosyl groups, thiol groups , Halogen group,
Usually contains at least one silyl group and at least six siloxane groups,
Preferably, each functional group contains 1 × 10 ⁻⁶ eq to 1 × 10 ⁻² eq per gram of resin.

As the polyisocyanate used in the present invention, tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, triphenylmethane triisocyanate And isocyanates such as isophorone diisocyanate, a product of the isocyanate and a polyalcohol, and a di- to 10-mer polyisocyanate formed by condensation of the isocyanate. The average molecular weight of these polyisocyanates is 100 to 20,000
Are preferred. Commercially available trade names of these polyisocyanates include Coronate L, Coronate
HL, Coronate 2030, Coronate 2031, Millionate M
R, Millionate MTL (manufactured by Nippon Polyurethane Co., Ltd.), Takenate D-102, Takenate D-110N, Takenate D
-200, Takenate D-202, Takenate 300S, Takenate 500 (manufactured by Takeda Pharmaceutical Co., Ltd.), Sumidur T-80, Sumidur 44S, Sumidur PF, Sumidur L, Sumidur N, Death Module L, Death Module IL,
Death Module N, Death Module HL, Death Module
T65, death module 15, death module R, death module RF, death module SL, death module Z4273
(Manufactured by Sumitomo Bayer), and these can be used alone or in combination of two or more utilizing the difference in curing reactivity. For the purpose of accelerating the curing reaction, a compound having a hydroxyl group (such as butanediol or hexanediol) or an amino group (such as monomethylamine, dimethylamine or trimethylamine) or a metal oxide catalyst can be used in combination. It is desirable that these compounds having a hydroxyl group or an amino group are polyfunctional.

As the carbon black used in the magnetic layer and the back layer of the present invention, furnace for rubber, thermal for rubber, black for color, acetylene black and the like can be used. Specific examples of abbreviations in the United States of these carbon blacks include ASF, ISAF, IISAF, T, HAF, SPF, FF, FEF, HM
F, GPF, APF, SRF, MPF, ECF, SCF, CF, FT, MT, HCC,
There are HCF, MCF, LFF, RCF, etc.
Those classified into 65-82a can be used. The average particle size of these carbon blacks used in the present invention is 5 to 1000 millimicrons (electron microscope),
Nitrogen adsorption method Specific surface area is 1-800 m ² / g, pH is 4-11 (JIS standard K-6221-1982 method), dibutyl phthalate (DBP) oil absorption is 10-800 ml / 100 g (JIS standard K-6221-1982). Law). The size of the carbon black used in the present invention is 5 to 1000 millimicron carbon black for the purpose of reducing the surface electric resistance of the coating film, and 50 to 1000 millimicron carbon black for controlling the strength of the coating film. With In order to control the surface roughness of the coating film, fine carbon black (100 mm or less) is used for smoothing to reduce spacing loss, and coarse carbon is used for roughening the surface to reduce the friction coefficient. Uses black (50 mm or more). As described above, the type and the amount of carbon black to be used are properly used depending on the purpose required for the magnetic recording medium.

In addition, these carbon blacks may be surface-treated with a dispersant described below or the like, or may be graphed with a resin for use. Further, a carbon black produced by treating the furnace at a temperature of 2000 ° C. or higher at a temperature of 2000 ° C. or more and partially graphitizing the surface can also be used. Also, hollow carbon black can be used as a special carbon black.

These carbon blacks are 100
It is desirable to use 0.1 to 20 parts by weight based on parts by weight. The carbon black that can be used in the present invention can be referred to, for example, "Carbon Black Handbook", edited by The Carbon Black Association (issued in 1971).

In particular, as described above, the first magnetic layer has an average particle diameter of 20 m.
Carbon black less than μ per 100 parts by weight of magnetic material
Preferably, the second magnetic layer contains 0.1 to 10 parts by weight of carbon black having an average particle diameter of 40 to 80 μm based on 100 parts by weight of the magnetic material.

The abrasive used in the magnetic layer and the back layer of the present invention is a material having a polishing action or polishing action which is generally used, and is α-alumina, γ-alumina, α-γ-alumina, fused alumina, silicon carbide, chromium oxide, oxide cerium,
Corundum, artificial diamond, α-iron oxide, garnet, emery (main component: corundum and magnetite), garnet, silica, silicon nitride, boron nitride, molybdenum carbide, boron carbide, tungsten carbide, titanium carbide,
Materials such as tripoly, diatomaceous earth, and dolomite, each having a Mohs hardness of 6 or more are used in combinations of 1 to 4 types. These abrasives have an average particle size of 0.005 to 5 microns, and particularly preferably 0.01 to 5 μm.
~ 2 microns. These abrasives are added in the range of 0.01 to 20 parts by weight based on 100 parts by weight of the binder.

In particular, as described above, it is preferable to use an abrasive having a Mohs hardness of less than 7 for the first magnetic layer and to use an abrasive having a Mohs hardness of 7 or more for the second magnetic layer.

Lubricants used for the magnetic layer and the back layer of the present invention include molybdenum disulfide, boron nitride, graphite fluoride, calcium carbonate, barium sulfate, silicon oxide, titanium oxide, zinc oxide, tin oxide, tungsten disulfide and the like. Inorganic fine powder, acrylic styrene resin fine powder, benzoguanamine resin fine powder, melamine resin fine powder, polyolefin resin fine powder, polyester resin fine powder, polyamide resin fine powder, polyimide resin fine powder, polyfluorinated Resin fine powders such as finely powdered ethylene resin, silicon oil, fatty acid-modified silicone oil, graphite, fluorine alcohol,
Polyolefin (eg, polyethylene wax), polyglycol (eg, polyethylene oxide wax), tetrafluoroethylene oxide wax, polytetrafluoroglycol, perfluorofatty acid, perfluorofatty acid ester, perfluoroalkyl sulfate, perfluoroalkyl phosphate, alkyl phosphate ,
Polyphenyl ether, monobasic fatty acid having 10 to 20 carbon atoms and monohydric alcohol having 3 to 12 carbon atoms or any one of dihydric alcohol, trihydric alcohol, tetrahydric alcohol, and hexahydric alcohol Or a fatty acid ester comprising two or more fatty acids, a monobasic fatty acid having 10 or more carbon atoms and a fatty acid comprising a monohydric to hexahydric alcohol having 11 to 28 carbon atoms in total with the carbon number of the fatty acid Organic compound lubricants such as esters can be used. In addition, fatty acids having 8 to 22 carbon atoms, fatty acid amides, and aliphatic alcohols can also be used. Specific examples of these organic compound lubricants include:
Butyl caprylate, octyl caprylate, ethyl laurate, butyl laurate, octyl laurate, butyl myristate, octyl myristate, ethyl palmitate, butyl palmitate, octyl palmitate, ethyl stearate, butyl stearate, stearic acid Octyl, amyl stearate, anhydrosorbitan monostearate, anhydrosorbitan distearate, anhydrosorbitan tristearate, anhydrosorbitan tetrastearate, oleyl oleate,
There are oleyl alcohol, lauryl alcohol and the like, which can be used alone or in combination. Further, as a lubricant used in the present invention, a so-called lubricating oil additive can be used alone or in combination, and an antioxidant (such as an alkylphenol),
Rust killing agents (naphthenic acid, alkenyl succinic acid, dilauryl phosphate, etc.), oil agents (rapeseed oil, lauryl alcohol, etc.), extreme pressure agents (dibenzyl sulfide, tricresyl phosphate, tributyl phosphite, etc.),
There are cleaning dispersants, viscosity index improvers, pour point depressants, foaming agents and the like. These lubricants are based on 100 parts by weight of binder.
It is added in the range of 0.05 to 20 parts by weight.

As the dispersant used in the present invention, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, elaidic acid, linoleic acid,
Fatty acids having 10 to 22 carbon atoms (R ₁ COOH, R ₁ is an alkyl group having 9 to 21 carbon atoms) such as linolenic acid and stearolic acid, and alkali metals (Li, Na, K, NH ₄ ⁺ ) of the fatty acids Etc.) or alkaline earth metals (Mg, Ca, Na, etc.), metal soaps composed of Cu, Pb, etc., fatty acid amides, lecithin, etc. are used. In addition, higher alcohols having 4 or more carbon atoms (butanol, octyl alcohol, myristyl alcohol, stearyl alcohol) and their sulfates, phosphates,
Amine compounds and the like can also be used. Further, polyethylene oxide, sulfosuccinic acid, sulfosuccinate, and the like can also be used. These dispersants are usually used in one or more types, and one type of dispersant is used in an amount of 0.0 to 100 parts by weight of the binder.
It is added in the range of 5 to 20 parts by weight. Regarding the method of using these dispersants, the dispersants may be adsorbed on the surface of the magnetic substance or the non-magnetic fine powder in advance, or may be added during dispersion.

As the antistatic agent used in the present invention, graphite, carbon black, carbon black graft polymer,
Conductive powders such as tin oxide-antimony oxide, tin oxide, titanium oxide-tin oxide-antimony oxide; natural surfactants such as saponin; alkylene oxide-based, glycerin-based;
Nonionic surfactants such as glycidol, polyhydric alcohols, polyhydric alcohol esters, and alkylphenol EO adducts; higher alkylamines, cyclic amines, hydantoin derivatives, amidoamines, esteramides, quaternary ammonium salts, pyridine and other heterocycles , A surfactant such as a phosphonium or sulfonium, etc .;
Anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, phosphoric acid, sulfate group, and phosphate group; amino acids; amphoteric surfactants such as aminosulfonic acids, sulfuric acid or phosphate esters of amino alcohols, and alkyl betaine type Agent is used.

These surfactants may be added alone or as a mixture. The amount of these surfactants used in the magnetic recording medium is 0.01 to 10 parts by weight per 100 parts by weight of the magnetic substance. These are used as antistatic agents, but are sometimes applied for other purposes such as dispersion, improvement of magnetic properties, improvement of lubricity, and application aids.

Dispersion of the present invention, kneading, as the organic solvent used in the application, acetone, methyl ethyl ketone in any ratio,
Ketones such as methyl isobutyl ketone, cyclohexanone, isophorone and tetrahydrofuran; alcohols such as methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol and methyl cyclohexanol; methyl acetate, ethyl acetate;
Ester systems such as butyl acetate, isobutyl acetate, isopropyl acetate, lactate ester and glycol monoethyl ether; ether systems such as diethyl ether, tetrahydrofuran, glycol dimethyl ether, glycol monoethyl ether and dioxane; benzene, toluene, xylene, cresol, chlorobenzene , Styrene and other tar-based (aromatic hydrocarbons); methylene chloride, ethylene chloride, carbon tetrachloride, chloroform,
Chlorinated hydrocarbons such as ethylene chlorohydrin and dichlorobenzene, N, N-dimethylformaldehyde, hexane and the like can be used.

The formation of the magnetic layer is performed by dissolving in an organic solvent any combination of the above compositions and the like, and coating and drying the coating solution on a support. When used as a tape, the thickness of the support is 2.5 to 1
It is preferably about 00 microns, preferably about 3 to 70 microns. Materials include polyesters such as polyethylene terephthalate and polyethylene naphthalate; polyolefins such as polypropylene; cellulose derivatives such as cellulose triacetate and cellulose diacetate; vinyl resins such as polyvinyl chloride; and plastics such as polycarbonate, polyamide and polysulfone. Metals such as aluminum and copper, and ceramics such as glass can also be used. Prior to coating, these supports may be subjected to corona discharge treatment, plasma treatment, undercoating, heat treatment, dust removal treatment, metal deposition treatment, and alkali treatment.

The method of kneading is not particularly limited, and the order of addition of each component can be appropriately set. For the preparation of the magnetic paint and the back layer paint, a usual kneading machine, for example, a two-roll mill, a three-roll mill, a ball mill, a pebble mill, a tron mill, a sand grinder, a Zegbari (Szegvar)
i) Attritor, high-speed impeller, disperser, high-speed stone mill, high-speed impact mill, disper, kneader, high-speed mixer, ribbon blender, co-kneader, intensive mixer, tumbler, blender, disperser, homogenizer, single screw extruder, A twin screw extruder, an ultrasonic disperser, or the like can be used. See TCPA
By TTON (TC Patton) “Paint Flow and Pig”
ment Dispersion "(Paint Flow and Pigment Dispersion), published in 1964 by John Wiley & Sons, Inc. (John Wiley and Sons), Shinichi Tanaka, Industrial Materials, Vol. 25, 37 (1977) and other references cited in the book. There is also a description in the specifications such as U.S. Patent Nos. 2,581,414 and 2,855,156.In the present invention, kneading and dispersing are performed according to the method described in the above-mentioned book or the cited reference of the book. A magnetic paint and a back layer paint can be prepared.

As a method of applying the magnetic layer and the back layer on the support, air doctor coating, blade coating,
Air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure coat, kiss coat, cast coat, spray coat, etc. can be used, other methods are also possible, and specific explanations of these are published by Asakura Shoten 『Coating Engineering』 25
It is described in detail on pages 3 to 277 (issued on March 30, 1970).

According to such a method, the magnetic layer coated on the support is subjected to a treatment for orienting the magnetic material in the layer immediately if necessary, and then the formed magnetic layer is dried. At this time, the transport speed of the support is usually 10 m / min to 900 m / min, and the drying temperature is controlled at 20 ° C. to 130 ° C. If necessary, the magnetic recording medium of the present invention is manufactured by performing a surface smoothing process or cutting into a desired shape.

Non-magnetic powders, binders, and additives used in the present invention (eg, lubricants, dispersants, antistatic agents, surface treatment agents, carbon black, abrasives, light shielding agents, antioxidants, antifungal agents, etc.) The solvent, the support and the substrate (which may have an undercoat layer, a back layer, and a back undercoat layer) or a method for producing a magnetic recording medium can be referred to those described in JP-B-56-26890.

(Examples) Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. It will be readily understood by those skilled in the art that the components, ratios, operation sequences, and the like shown herein can be changed without departing from the spirit of the present invention. Therefore, the present invention should not be limited to the following examples. “Department” in Examples and Comparative Examples
Indicates "parts by weight" unless otherwise specified.

The magnetic materials A to C used in Examples and Comparative Examples are as follows.

Magnetic substance A (Co-containing γ-Fe ₂ O ₃ ): Hc; 700 Oe Tap density (TD); 0.65 BET specific surface area; 40 m ² / g Magnetic substance B (Co-containing γ-Fe ₂ O ₃ ): Hc; 650 Oe tap density (TD); 0.75 BET specific surface area; 30 m ² / g Magnetic substance C (Co-containing γ-Fe ₂ O ₃ ): Hc; 710 Oe tap density (TD); 0.72 BET specific surface area; 48 m ² / g Using the above magnetic substances A to C, a consolidation treatment was performed under the following consolidation conditions. Table 1 shows the tap density of the obtained magnetic material.

Consolidation conditions: Machine used; Sand mill MPUV05 (manufactured by Matsumoto Iron Works) Charge amount: 1 kg Rotational speed: 20 rpm Processing pressure: described in Table 1 Processing time; described in Table 1 A magnetic material having the tap density described in Table 1 was used. A magnetic substance coating solution was prepared using the following formulation.

Coating solution for the first magnetic layer: Co-containing γ-Fe ₂ O ₃ (described in Table 1) 100 parts Vinyl chloride / vinyl acetate / maleic acid copolymer (86:13:
1 part by weight, degree of polymerization 400) 11 parts Polyester polyurethane resin 6 parts Carbon black (average particle diameter 18 mμ) 10 parts α-Fe ₂ O ₃ (average particle diameter 0.5 μm, Mohs hardness 5) 10 parts Butyl stearate 1 part Stearin Acid 1 part Butyl acetate 200 parts Cyclohexanone 40 parts Coating solution for the second magnetic layer: Co-containing γ-Fe ₂ O ₃ (described in Table 1) 100 parts Vinyl chloride / vinyl acetate / maleic acid copolymer (86:13 :
1 part by weight, degree of polymerization 400) 11 parts Polyester polyurethane resin 8 parts Carbon black (average particle diameter 80 mμ) 5 parts α-Al ₂ O ₃ (average particle diameter 0.3 μm, Mohs hardness 9) 8 parts Butyl stearate 15 parts Stearin 3 parts of acid 200 parts of butyl acetate 60 parts of methyl ethyl ketone For each of the above two paints, each component was kneaded using an open kneader manufactured by Moriyama Seisakusho (kneading load peak power is described in Table 1), and then a sand grinder was used. And dispersed.

To the obtained dispersion, 6 parts of polyisocyanate and 40 parts of butyl acetate were added, and the mixture was filtered using a filter having an average pore diameter of 1 μm to obtain a coating solution for forming the first magnetic layer and the second magnetic layer. Each was prepared.

After adjusting the viscosity of the magnetic coating material, a single layer coating was applied on polyethylene terephthalate of 15 μm of a non-magnetic support so that the dry film thickness was 3.5 μm for the first magnetic layer and 0.6 μm for the second magnetic layer (see below). For measuring the squareness ratio and saturation magnetic flux density) or multi-layer coating (for measuring the video S / N ratio and audio sensitivity below), orienting the magnetic field with a 3,000 gauss magnet, drying, and calendering the magnetic recording Created media. Thereafter, the tape was slit to a width of 1/2 inch to produce a video tape. The properties of the obtained tape were evaluated by the following methods and are shown in Table 1.

[Evaluation method] Video S / N ratio The video S / N ratio was measured using a noise meter (925C, manufactured by Shibasoku Co., Ltd.), and the standard tape (super AG T-120, manufactured by Fuji Photo Film Co., Ltd.) was measured. The difference from the video S / N ratio was determined. High pass filter 10kHz, low pass filter 4MH
The noise level was measured at z. The used VTR is AG-6800
(Matsushita Electric Industrial Co., Ltd.).

Audio sensitivity A 1kHz sine wave signal was recorded with a specified bias at a specified input level. The reproduction output level was measured with a level meter, and the difference from the specified output level was expressed in dB.

Squareness ratio (SR) Using a vibrating sample magnetometer (manufactured by Toei Kogyo), Hm was determined at 5 kOe.

Saturation magnetic flux density (Bm) Measurement method is the same as squareness ratio (SR).

As is clear from the examples shown in Table 1, the magnetic recording medium of the present invention shows excellent performance in multilayer video S / N, multilayer video playback output, and multilayer audio playback output.

In the case of the comparative example, whether one or both of the tap density of the magnetic material of the first magnetic layer and the tap density of the magnetic material of the second magnetic layer is out of the range of the tap density according to the present invention (Comparative Example 1). 5 and 7), or the tap density of the magnetic material of the first magnetic layer is smaller than that of the second magnetic layer (Comparative Examples 6 and 8), and the multilayer video S / N, multilayer video playback output, and multilayer audio playback output are low. It shows inferior performance.

Claims (2)

(57) [Claims] A first magnetic layer and a second magnetic layer are provided on a surface of a nonmagnetic support.
In the magnetic recording medium having the magnetic layers provided in this order, the tap density of the magnetic material used for the first magnetic layer is 0.8 or more and 1.0 or less, and the tap density of the magnetic material used for the second magnetic layer is 0.7 or less.
0.9 or less, and the tap density of the magnetic material of the first magnetic layer is larger than that of the magnetic material of the second magnetic layer.
A magnetic recording medium, characterized by having 0.02 or more. 2. The magnetic recording medium according to claim 1, wherein
The magnetic material of the magnetic layer has a specific surface area by BET method of 45 m ² / g or less and a crystallite size of 290 ° or more, and the magnetic material of the second magnetic layer has a specific surface area of 30 m ² / g or more and crystallite size by BET method. Is 400 mm or less.