JPH04318321A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve HiFi characteristics, etc., by using magnetic powder having prescribed characteristics in the lower layer of double magnetic layers and incorporating barium ferrite and polycarbonate polyurethane having prescribed characteristics into the upper layer. CONSTITUTION:This magnetic recording medium is constituted by providing the plural magnetic layers dispersed with ferromagnetic powder and binders on a nonmagnetic base. The lower magnetic layer near the nonmagnetic base contains cobalt-contg. iron oxide powder having 20/100 to 50/100 molar ratio of Fe<2+>/Fe<3+>, and 750 to 15000e coercive force as the ferromagnetic powder. The uppermost magnetic layer contains hexagonal planar barium ferrite having <=4 planer ratio, <=0.1mum average grain size and 500 to 1500Oe coercive force as the ferromagnetic powder. The binder of the uppermost magnetic layer contains a polycarbonate polyurethane resin. The reproduced output is improved over a wide wavelength region from a low-frequency side of a long wavelength to a high-frequency side of a short wavelength in this way. In addition, the recording medium having excellent sound signals and good traveling durability is obtd.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a magnetic recording medium having a plurality of magnetic layers, which has a high reproduction output over a wide wavelength range from long wavelengths to short wavelengths, has excellent durability, and has a small increase in DO. .

0002

[Prior Art] Conventionally, for magnetic recording and reproduction, a magnetic coating liquid in which ferromagnetic powder consisting of needle-like crystals of Co-containing iron oxide, CrO2, etc. is dispersed in a binder is applied to a non-magnetic support. Recording media have been widely used. but,
Recently, there has been a strong demand for improved recording density, and conventional C
O-containing magnetic iron oxides are also being made into finer particles. However, it is still insufficient to improve the reproduction output and C/N frequency characteristics in the short wavelength high frequency region, and although a magnetic layer using ferromagnetic alloy powder can improve considerably, it is simply a matter of improving the reproduction output and C/N frequency characteristics. A magnetic layer using magnetic alloy powder alone has the disadvantage that running durability is difficult to obtain.

Recently, plate-shaped hexagonal ferrite-based ferromagnetic powder has been developed, but magnetic recording media using this ferromagnetic powder have high output at short wavelengths and high frequencies, but low output at long wavelengths and low frequencies. The disadvantage was that the output was low. Therefore, in order to eliminate these drawbacks, instead of creating a single-layer magnetic recording medium using only ferromagnetic alloy powder or barium ferrite powder, we developed a magnetic recording medium with a lower (first) magnetic layer containing ferromagnetic alloy powder on a non-magnetic support. A two-layer magnetic recording medium (JP-A-60-2223018) in which an upper layer (second layer) magnetic layer containing a hexagonal barium ferrite magnetic material is provided thereon, and a lower layer (first layer) A magnetic recording medium (JP-A-58-119610
No. 62-212924, Japanese Patent Application Laid-Open No. 1983
JP-A-129935, JP-A-64079-930, JP-A-1-128228, etc. have been proposed.

Specifically, in JP-A-62-212924, a first coated magnetic layer containing acicular magnetic powder is provided on a non-magnetic support, and a hexagonal magnetic layer is formed on the first coated magnetic layer. In a magnetic recording medium provided with a second coated magnetic layer containing crystalline ferrite magnetic powder, the coercive force (coercive force) of the acicular magnetic powder
A magnetic recording medium was proposed in which the linear audio characteristics of conventional products were set at approximately 450 to 720 oersteds. The MOL and C/N are improved by considering the coercive force Hc of the acicular magnetic powder in the layer.

However, although the characteristics of such magnetic recording media in the short wavelength region have been improved to some extent, the running durability is not sufficient and needs to be improved. Further, although the above-mentioned magnetic recording medium uses barium ferrite ferromagnetic powder in the upper layer, barium ferrite ferromagnetic powder is fine particles and has poor dispersibility, resulting in deterioration of surface properties and deterioration of S/N. In addition, due to poor dispersibility, ferromagnetic powders tend to aggregate, and the aggregates tend to be scraped off when the tape is running.This causes an increase in DO, clogs the head, and prevents the screen from appearing. There was a need for improvement. In addition, in recent VTRs, not only video but also audio have become important characteristics, and conventionally known magnetic recording media do not have optimal Hc in the upper and lower layers, resulting in long-wavelength audio characteristics (HiFi In addition, the dispersibility of barium ferrite is poor, surface properties cannot be obtained, and video characteristics in short wavelength recording are poor, making it impossible to achieve both short wavelength characteristics and long wavelength characteristics.

[0006] In order to solve the above-mentioned drawbacks, the inventors of the present invention have intensively investigated combinations of barium ferrite ferrite ferromagnetic powder and various ferromagnetic powders. By using barium ferrite powder for the uppermost layer and selecting polycarbonate polyurethane as the binder for the uppermost magnetic layer, characteristics in the entire wavelength range, running durability, and D
It was discovered that a magnetic recording medium having excellent durability such as an increase in O can be obtained, leading to the present invention.

[0007]

SUMMARY OF THE INVENTION That is, an object of the present invention is to improve the reproduction output over a wide wavelength range from the long wavelength, low frequency side to the short wavelength, high frequency side, and to provide excellent audio signal characteristics. The object of the present invention is to provide a magnetic recording medium with good running durability.

[0008]

[Means for Solving the Problems] That is, the above-mentioned object of the present invention is to provide a magnetic recording medium having a plurality of magnetic layers formed by dispersing ferromagnetic powder and a binder on a non-magnetic support. The lower magnetic layer close to the body has a Fe2+/Fe3+ molar ratio of 20/100 to 50/1 as the ferromagnetic powder.
00, containing cobalt-containing iron oxide powder with a coercive force of 750 to 1500 Oe, and the uppermost magnetic layer is the ferromagnetic powder with a plate ratio of 4 or less and an average particle size of 0.1 μm.
The following can be achieved by a magnetic recording medium characterized in that it contains hexagonal plate-shaped barium ferrite having a coercive force of 500 to 1500 Oe, and in which the binder of the uppermost magnetic layer contains a polycarbonate polyurethane resin.

The above-mentioned object of the present invention is to provide a magnetic recording medium having a plurality of magnetic layers formed by dispersing ferromagnetic powder and a binder on a non-magnetic support, in which the lower magnetic layer near the non-magnetic support is The ferromagnetic powder includes a cobalt-containing iron oxide powder with a Fe2+/Fe3+ molar ratio of 20/100 to 50/100, and the thickness of the lower magnetic layer is 2.
A non-magnetic intermediate layer having a size of .mu.m or less contains carbon black, and an uppermost magnetic layer provided on the non-magnetic intermediate layer contains hexagonal plate-shaped barium ferrite as the ferromagnetic powder,
This can be achieved by a magnetic recording medium characterized in that the binder of the uppermost magnetic layer contains a polycarbonate polyurethane resin.

That is, in the present invention, the molar ratio of Fe2+/Fe3+ is 20/100 to 5 as the ferromagnetic powder of the lower magnetic layer.
By using 0/100 cobalt-containing iron oxide powder, the saturation magnetization (σs) is improved, the Bm of the tape is increased,
Improved output and linear audio characteristics. In addition, the coercive force of the ferromagnetic powder of the lower magnetic layer is set to 750 to 1500 Oe.
By doing so, the HiFi characteristics and linear audio characteristics are improved. In addition, the upper magnetic layer has a plate ratio of 4 or less, a particle size of 1.0 μm or less, and a coercive force of 500 to 150.
By using barium ferrite of 0 Oe in combination with polycarbonate polyurethane, dispersibility can be significantly improved and noise can also be reduced. Since agglomeration of barium ferrite can also be prevented, the surface properties are improved, and the electromagnetic conversion characteristics in the short wavelength range can be improved, and the noise reduction effect becomes more pronounced as the wavelength becomes shorter. Further, in the present invention, by providing a non-magnetic intermediate layer containing carbon black between the uppermost magnetic layer and the lower magnetic layer and removing carbon black from the uppermost magnetic layer, the filling degree of the uppermost magnetic layer can be improved. On the other hand, the necessary antistatic properties can be obtained, and the electromagnetic conversion properties and running durability can be further improved.

The magnetic recording medium of the present invention has at least two magnetic layers mainly containing ferromagnetic powder and a binder on a nonmagnetic support. The magnetic layer may have a two-layer structure, or may have a three-layer structure or more. That is, in the magnetic recording medium of the present invention, among the plurality of magnetic layers, the magnetic layer furthest from the support (hereinafter referred to as "top magnetic layer", "upper magnetic layer", or "upper layer") ) and a magnetic layer that is in contact with the uppermost layer and is closer to the support than the uppermost magnetic layer (hereinafter referred to as "lower magnetic layer" or "lower layer"). There may be another magnetic layer between the magnetic layer and the supports. In the present invention, the lowermost magnetic layer is made of ferromagnetic powder and has a Fe2+/Fe3+ molar ratio of 20/100 to 50/
100, preferably 25/100 to 35/100 cobalt-containing iron oxide powder. When the Fe2+/Fe3+ molar ratio is less than 20/100, the saturation magnetization (δ
s) is lowered, the Bm of the tape is lowered, the output is also lowered, and the linear audio characteristics are also deteriorated, which are undesirable. Furthermore, if the molar ratio of Fe2+/Fe3+ exceeds 50/100, the transfer characteristics will be greatly deteriorated, which is not preferable.

[0012] Furthermore, the molar ratio of Fe2+/Fe3+ is 50/1.
If it exceeds 00, it becomes out of stoichiometry and cannot be synthesized. (
When the Fe2+/Fe3+ molar ratio is 50/100, magnetite Fe2 O3 = FeO・Fe2 O3 )
Further, the coercive force of the cobalt-containing iron oxide powder used for the lower magnetic layer is 750 to 1500 Oe, preferably 75 Oe.
It is 0 to 950 Oe. HiF when coercive force is less than 750
i-characteristics are poor. Furthermore, if the coercive force exceeds 1500 Oe, the linear audio characteristics will be greatly degraded. When the magnetic recording medium of the present invention is used as a video magnetic recording medium (for example, a magnetic recording medium using a system such as VHS),
The upper magnetic layer mainly controls the video characteristics, but Fe2+
By using cobalt-containing iron oxide with a /Fe3+ molar ratio of 20/100 to 50/100 and a coercive force of 750 to 1500 Oe in the lower layer, chroma reproduction output, HiFi characteristics,
Linear audio characteristics are improved.

Further, the upper magnetic layer of the present invention contains barium ferrite powder as a ferromagnetic powder, which has a plate ratio of 4 or less, a particle size of 1.0 μ or less, and a coercive force of 500 to 1500 Oe. . When the platelet ratio (diameter/thickness) of barium ferrite powder exceeds 4, the dispersibility deteriorates rapidly, particles aggregate with each other, and surface properties deteriorate, resulting in poor electromagnetic conversion characteristics in the short wavelength region. . Further, if the particle size is 1.0 μm or more, the noise becomes large, and this tendency becomes particularly noticeable as the wavelength becomes shorter. The coercive force is preferably 500 to 1000 Oe, more preferably 550 to 1000 Oe. If the coercive force is less than 500 Oe, the high frequency output will decrease. Moreover, when it exceeds 1500 Oe, the low frequency output decreases significantly.

[0014] Also, in the present invention, barium ferrite in the form of fine particles and in the form of a plate is used in the uppermost layer, but unlike the needle-shaped ferromagnetic powder commonly used in the past, the barium ferrite in the form of a plate has poor dispersibility. However, the running durability was poor and improvements were needed. For this reason, as a result of intensive research on the binder that is most compatible with barium ferrite, it was found that by using a binder containing polycarbonate polyurethane as the binder, dispersibility and running durability could be improved.

The binder for the magnetic layer of the magnetic recording medium of the present invention is polycarbonate polyurethane, preferably 30 to 9
It is a binder containing 0% by weight. The polycarbonate polyurethane is a polymer obtained by condensing (1) a polyol, (2) a polyisocyanate, and (2) a chain extender if necessary. The above ■polyol has the general formula (1
) [Formula 1] A polyhydric alcohol having a molecular weight of 300 to 200,000 and a hydroxyl value of 20 to 1000, synthesized by condensation or transesterification with phosgene, chloroformic acid ester, dialkyl carbonate, or diallyl carbonate.
or the polycarbonate polyol and the general formula (2) HOOC-R1 -COOH
(2) [In the formula, R1 represents an alkylene group having 3 to 6 carbon atoms, a phenylene group, or a cyclohexylene group. ] is a polycarbonate polyester polyol having a molecular weight of 400 to 200,000 and a hydroxyl value of 5 to 1,000, obtained by condensation with a dihydric carboxylic acid having the following.

[0016]

[Chemical formula 1]

[0017] In addition to the above polyol, other polyols,
For example, polyether polyol, polyester ether polyol, or polyester, 90% of the above polyol is used.
They may be mixed up to % by weight and used in combination. There are no particular restrictions on the above polyisocyanate, and commonly used polyisocyanates can be used. For example, hexamethylene diisocyanate, tolidine diisocyanate, isophorone diisocyanate, xylylene diisocyanate, cyclohexane diisocyanate, toluidine diisocyanate, 2,4-tolylene diisocyanate, 2,6-
Examples include tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, and 1,5-naphthylene diisocyanate.

As the above chain extender, substances known per se, such as the above-mentioned polyhydric alcohols, aliphatic polyamines, alicyclic polyamines, aromatic polyamines, etc. can be used. The polycarbonate polyurethane obtained from the above ■ and ■, and in some cases ■, has a number average molecular weight of 1.
It is preferably in the range of 0,000 to 250,000. If a polycarbonate polyurethane with a molecular weight smaller than the above range is used, the running durability of the tape will be poor, and if the molecular weight is larger than the above range, the solubility of the binder will decrease or the dispersibility will be poor. Unfavorable from an industrial standpoint.

[0019] Furthermore, polycarbonate polyurethane is
-SO3M, -COOM, -NH2, -SH, -O
It may have at least one functional group selected from the group consisting of H, a phosphoric acid group, and a phosphoric ester group (wherein M represents a hydrogen atom or an alkali metal atom). Polycarbonate polyurethane having such functional groups can be produced by a method known per se. The above-mentioned polycarbonate polyurethane is, for example, polycarbonate polyurethane 900 manufactured by Dainichiseika Kagyo Co., Ltd.
It can be easily obtained as 0 series, 7000 series, 5000 series, Goodrich Estane 5700 series, etc.

The binder in the upper magnetic layer of the magnetic recording medium of the present invention is a mixture of 30 to 90% by weight, preferably 35 to 60% by weight of the above polycarbonate polyurethane and the remainder other binders. If the content of the polycarbonate polyurethane in the magnetic layer exceeds the above range, the reproduction output of the magnetic recording medium will decrease, and still reproduction and full-length running clogging will be poor. Furthermore, if the content of the polycarbonate polyurethane is less than the above range, the weather resistance and running durability of the magnetic recording medium will be poor. There are no particular restrictions on the other binders used in combination with the polycarbonate polyurethane.

The thickness of the magnetic layer of the present invention is 0.1 to 1 for the upper layer.
．． The thickness is preferably 5 μm or less, more preferably 0.1 to 0.
More preferably, the thickness is 5 μm. Further, the thickness of the lower magnetic layer is preferably 2.0 to 6.0 μm, more preferably 2.5 to 5.0 μm. In particular, it is desirable that the thickness of the lower layer be 2.0 μm or more, preferably 2.5 μm or more, since this improves the transfer characteristics. In the present invention, in order to improve video playback output,
Barium ferrite is used for the upper layer, but the coercive force of the vertical component of barium ferrite is larger than that of acicular particles. Therefore, when recorded magnetic tapes are wound and stored, the magnetic layer between the overlapping tapes is A leakage magnetic field causes a phenomenon in which signals recorded on other magnetic layers overlap (generally referred to as transfer). If the transfer is large, the transferred signal generates noise when reproduced, which deteriorates the quality of the reproduced signal. For this reason, as a result of investigating methods for improving the transfer, it was found that the improvement could be made to a level where no problem would arise if the thickness of the lower magnetic layer was increased to 2.0 μm or more. This means that there is a distance of 2.0 μm between the two upper magnetic layers between the overlapping tapes when the magnetic tape is wound.
It is presumed that this is because the distance between the two upper magnetic layers increases due to the inclusion of the thicker lower layer.

The present invention is also characterized in that it does not contain carbon black in order to further improve the filling degree of the lower layer. In the method disclosed in JP-A No. 62-212924, the light transmittance of barium ferrite itself is very high, and it is necessary to add carbon black or the like to the upper or lower layer. This has the disadvantage that the degree of filling decreases and the electromagnetic conversion characteristics deteriorate. In the present invention, since a ferromagnetic powder with a Fe2+/Fe3+ molar ratio of 20/100 to 50/100 is used as the lower layer ferromagnetic powder, there is no need to add carbon black, and the filling degree of the ferromagnetic powder can be improved. , electromagnetic conversion characteristics, especially chroma sensitivity, can be improved.

In the present invention, it is preferable to provide a nonmagnetic intermediate layer between the uppermost magnetic layer and the lower magnetic layer. The role of the nonmagnetic intermediate layer is to improve the electromagnetic conversion characteristics of the uppermost magnetic layer. It is necessary to remove non-magnetic material from the uppermost magnetic layer to increase the degree of filling. By removing the carbon black from the uppermost magnetic layer, it becomes more highly filled, but the conductivity decreases. In order to compensate for this, further improvements were made by providing an intermediate conductive layer containing non-magnetic carbon black. Carbon black, tin oxide, etc. can be used as the conductive substance. The binder used can be the same as that used for the magnetic layer.

Fe2 used in the lower magnetic layer of the present invention
A method for producing cobalt-containing iron oxide powder having a +/Fe3+ molar ratio within the above range is already known, and is carried out, for example, as follows. First, γ-Fe2 O3 is dispersed in water, CoSO4 and FeSO4 are added thereto, and NaOH is further added to make it alkaline and heated. Then, the obtained surface-treated γ-Fe2O3 was washed with water and heated and dried in nitrogen gas to remove Co and Fe.
Obtain metamorphosed γ-Fe2O3. During the above process, F
The e2+/Fe3+ molar ratio is the above CoSO4 and Fe
This can be done by changing the ratio and amount with SO4. The magnetic recording medium of the present invention is manufactured, for example, as follows.

Specific magnetic supports used in the present invention include, for example, polyesters such as polyethylene terephthalate (PET) and polyethylene naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, and polychlorinated Films or sheets made of vinyl, vinyl resins such as polyvinylidene chloride, synthetic resins such as polycarbonate, polyamide, polyamideimide, polyimide, etc. Non-magnetic metal foils such as aluminum and copper foils Metal foils such as stainless steel foils Paper, ceramic sheets Selected from etc. There is also no particular limit to the thickness of the non-magnetic support, but generally 2.
It is 5 to 100 μm, preferably 3 to 80 μm.

The lower magnetic layer in the magnetic recording medium of the present invention is a layer in which ferromagnetic powder is dispersed in a binder resin. The ferromagnetic powder used in the present invention is a cobalt-containing iron oxide powder, but other ferromagnetic powders may be used in combination if desired. Examples of other ferromagnetic powders include γ-Fe2O3, Fe3O4, Co-containing F
e3 O4, CrO2, Co-Ni-P alloy, Fe
-Co-Ni alloy, and other known ferromagnetic powders may be used.

The binder solution for producing the lower layer magnetic coating used in the present invention is a binder solution containing a resin component, a solvent, and, if desired, a lubricant and an abrasive. As the resin component, conventionally known thermoplastic resins, thermosetting resins, radiation curable resins, reactive resins, or mixtures thereof are used. Examples of resin components include vinyl chloride copolymers (e.g., vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinyl acetate-vinyl alcohol copolymers, vinyl chloride-vinyl acetate-acrylic acid copolymers, Vinyl-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, ethylene-vinyl acetate copolymer, -SO3 Na or -OSO2 Na, -P
(vinyl chloride copolymers into which polar groups such as O3 Na2, -OPO3 Na2 and epoxy groups have been introduced), cellulose derivatives such as nitrocellulose resins, acrylic resins, polyvinyl acetal resins, polyvinyl butyral resins, epoxy resins, phenoxy resins, Polyurethane resin (e.g. polyester polyurethane resin, -SO
Examples include polyurethane resins and polycarbonate polyurethane resins into which polar groups such as 3 Na or -SO2 Na are introduced.

Further, when a curing agent is used, a polyisocyanate compound is usually used. The polyisocyanate compound is selected from those commonly used as curing agent components for polyurethane resins and the like. Further, when performing curing treatment by electron beam irradiation, a compound having a reactive double bond (for example, urethane acrylate) can be used.

Examples of solvents used in the production of magnetic paints include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, ethyl isobutyl ketone, and cyclohexanone; methyl acetate, ethyl acetate, butyl acetate,
Esters such as acetic acid glycol monoethyl ether; glycol ethers such as ether, glycol dimethyl ether, dioxane; aromatic hydrocarbons such as benzene, toluene, xylene; methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chloride Examples include chlorinated hydrocarbons such as hydrin and dichlorobenzene. These solvents can be used alone or in combination. Particularly preferred are polar solvents such as ketones or solvents containing polar solvents.

[0030] The ferromagnetic powder is uniformly kneaded and dispersed together with the binder solution. This kneading and dispersion is carried out using a two-roll mill,
Generally, a method is used in which pre-dispersion is performed using a three-roll mill, open kneader, pressure kneader, continuous kneader, twin-screw type continuous kneading mixer, etc., and then post-dispersion is performed using a sand grinder, ball mill, etc.

Since it is necessary to obtain a high residual magnetic flux density in the magnetic paint used in the present invention, a twin-screw continuous kneading mixer is particularly preferred as the device used for kneading the cobalt-containing iron oxide. By using this, the ferromagnetic powder can be kneaded (pre-dispersed) while applying a high shearing force. Furthermore, when diluting this kneaded material, it is preferable to perform a fine dispersion treatment and then perform the above-mentioned dispersion treatment. Of course, any one of various additives such as abrasives, lubricants, dispersants, and antistatic agents may be added to the magnetic paint according to known techniques depending on the purpose.

Next, a method for manufacturing the magnetic recording medium of the present invention will be described. To apply the magnetic layer, a magnetic coating material prepared by using the above-mentioned materials and prepared by the above-mentioned kneading and dispersing method is applied onto a non-magnetic support by the following method. First, the magnetic layer forming components such as the specific cobalt-containing iron oxide powder as the resin component and ferromagnetic powder for the lower magnetic layer and a hardening agent blended as desired are kneaded and dispersed together with a solvent as described above to form the lower layer. A magnetic layer coating liquid (first magnetic layer coating liquid) is prepared. Then, for the uppermost magnetic layer, a coating liquid for the uppermost magnetic layer (coating liquid for the second magnetic layer) is prepared in the same manner as described above.

The method for producing a magnetic recording medium of the present invention involves coating the surface of a non-magnetic support while it is running with a coating solution for the lower magnetic layer, and applying a coating solution on the coating layer while the coating layer is still wet. It is preferable to apply a coating solution for the uppermost magnetic layer so that the thickness of the uppermost magnetic layer after drying is within the range of 0.1 to 1.5 μm. The method of continuously coating the two layers, the lower layer and the top layer, is, for example, when using a reverse roller as a coating machine, two reverse rollers are installed in succession so as to sandwich the non-magnetic support running under it. Alternatively, two coaters may be installed with an interval between them so that the lower magnetic layer can be kept in a wet state (that is, the coated layer still contains solvent and exhibits tackiness).

[0034] In addition to reverse roller coating, methods for applying magnetic paint include air doctor coating, blade coating, rod coating, extrusion coating, air knife coating,
squeeze coat, impregnation coat, transfer roll coat, gravure coat, kiss coat, cast coat,
Methods such as spray coating and spin coating can be used.

The coated layer of the magnetic paint is coated with the dry thickness of the total magnetic layer of the obtained magnetic recording medium (the uppermost magnetic layer, the lower magnetic layer, and if there is another magnetic layer, the thickness of that magnetic layer). The total dry film thickness of all magnetic layers (including dry film thickness) is usually 2.1 to 10
The coating is applied within the range of μm. A backing layer may be provided on the surface of the nonmagnetic support used in the present invention that is not coated with the magnetic paint. Normally, the back layer is formed by applying a back layer forming paint made of particulate components such as abrasives and antistatic agents and a binder dispersed in an organic solvent to the surface of the non-magnetic support that is not coated with the magnetic paint. It is a layer provided. Incidentally, an adhesive layer may be attached to the surface of the nonmagnetic support on which the magnetic paint and the back layer forming paint are applied.

Usually, the coated layer of magnetic paint is dried after being subjected to a treatment for orienting the ferromagnetic powder contained in the coated layer of magnetic paint, that is, a magnetic field orientation treatment.

After being dried in this manner, the magnetic coating layer is subjected to surface smoothing treatment. For example, a super calender roll is used for the surface smoothing treatment. By performing the surface smoothing treatment, the pores generated by the removal of the solvent during drying disappear and the filling rate of the ferromagnetic powder in the magnetic layer increases, making it possible to obtain a magnetic recording medium with high electromagnetic conversion characteristics. can. After surface smoothing treatment as above,
A hardening treatment is performed by appropriately applying radiation treatment or heat treatment. The thus cured laminate is then cut into a desired shape. The cutting can be performed under normal conditions using a normal cutting machine such as a slitter.

[0038]

[Examples] Next, the present invention will be specifically explained with reference to Examples, but the present invention is not limited thereto. Note that "parts" in the examples indicate "parts by weight."

Example 1 Coating liquid for upper magnetic layer (liquid A) Barium ferrite magnetic material

100 parts (plate ratio, coercive force, and particle size are listed in Table 1) Vinyl chloride/vinyl acetate copolymer

10 parts (sulfonic acid group 0.25
(wt% content) Polycarbonate polyurethane (9000 manufactured by Dainichiseika Chemical Co., Ltd.) 5 parts Polyisocyanate (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) 6 parts Stearic acid (industrial use)

1 part Butyl stearate (industrial use)

1 part α-alumina (average particle size: 0.1 μm)

10 parts carbon black (average particle size: 70 nm)

1 part Solvent (methyl ethyl ketone/cyclohexanone = 7/3 weight ratio) 100 parts

Coating liquid for lower magnetic layer (liquid B) Co-γ-FeOx

100 copies (x is Fe2+/Fe3
+ molar ratio. (Fe2+/Fe3+ molar ratio and coercive force are listed in Table 1) Vinyl chloride/vinyl acetate copolymer

11 parts (contains 0.25% by weight of sulfonic acid groups)
Polyester urethane (sulfonic acid group 0.1% by weight)
Contains) 4 parts Polyisocyanate (Coronate L manufactured by Nippon Polyurethane Co., Ltd.)
6 parts Stearic acid (industrial use)

1 part Butyl stearate (industrial use)
1 part α-alumina (average particle size: 0.1 μm)
10
part solvent (methyl ethyl ketone/cyclohexanone=
7/3 weight ratio) 100 parts

The above compositions were kneaded and dispersed using a kneader to obtain coating solutions A and B. The obtained coating liquid was applied to the upper layer (A
A multilayer coating was applied to the surface of a polyethylene terephthalate support with a thickness of 14 μm so that the layer (liquid) had a thickness of 0.5 μm and the lower layer (liquid B) had a thickness of 3.5 μm, followed by magnetic field alignment treatment, drying,
and super calender treatment to prepare sample No. 1
~17 were created.

Coating liquid for upper magnetic layer of comparative example (liquid C) Barium ferrite magnetic material

100 parts (plate ratio, coercive force, and particle size are listed in Table 1) Vinyl chloride/vinyl acetate copolymer

10 parts (sulfonic acid group 0.25
Polyester urethane (contains 0.1% by weight of sulfonic acid groups) 4 parts Polyisocyanate (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) 6 parts Stearic acid (
industrial)
1 part Butyl stearate (industrial use)

1 part α-alumina (average particle size: 0.1 μm
)
10 parts carbon black (average particle size: 70
nm)
1 part Solvent (methyl ethyl ketone/cyclohexanone = 7/3 weight ratio) 100 parts A sample was prepared in the same manner as in Example 1 using the upper layer solution "C" and combining it with the lower layer "B". Sample No. 18. The magnetic recording medium thus obtained was slit, and the following performance evaluation was performed.

Example 2 Using the upper magnetic layer liquid and lower magnetic layer liquid of Example 1, a tape was prepared in which the intermediate nonmagnetic layer shown below was coated between the upper layer and the lower layer. (Sample No. 19) Vinyl chloride/vinyl acetate copolymer
25 copies
(Contains 0.25% by weight of sulfonic acid groups) Polyurethane resin

15 parts Polyisocyanate (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) 40 parts Stearic acid (industrial use)

2 parts carbon black (average particle size: 20 nm)

25 parts Solvent (methyl ethyl ketone)

300 parts, and the coating thickness of the intermediate layer was 1.0 μm. The magnetic recording medium thus obtained was slit and the following performance evaluation was performed.

Table 1 Upper layer magnetic material Upper layer magnetic material Upper layer magnetic material Lower layer magnetic material Lower layer magnetic material Upper layer poly
Sample No. Coercive force Plate ratio Particle size Coercive force Fe2/Fe3 Urethane

molar ratio
1 450 3
0.05 800 3
0/100 A 2
1700 3
0.05 800 30/100
A 3 *
900 3 0.05
800 30/100
A4 * 600
3 0.05
800 30/100 A
5 * 1400
3 0.05 800
30/100A
6 900 3
0.05 800 3
0/100 A 7
900 3
0.05 800 30/100
A 8 *
900 3 0.10
800 30/100
A 9 900
3 0.15
800 30/100 A
10 900
3 0.05 720
30/100A
11 900 3
0.05 600 30/
100 A 12
900 3 0.0
5 1700 30/100
A 13 900
3 0.05
800 10/100 A
14 * 900
3 0.05 800
20/100A
15 * 900 3
0.05 800 40
/100 A 16 *
900 3 0.
05 800 50/100
A 17 90
0 3 0.05
800 60/100 A
18 900
3 0.05 800
30/100B
19 * 900 3
0.05 800 3
0/100A
*Example of the present invention A: Polycarbonate polyurethane B: Polyester polyurethane

Table 2 7MHz Video Chroma
HiFi Linear O Driving durability DO increase Sample No. Output S/N Sensitivity Characteristics Dio characteristics (Number of times) (Number of pieces/ (dB) (dB
) (dB) (dB) (dB)
1 minute)
1 -2.0 -2.5
0.8 -1.5 1.0
300 50 2
3.0 2.2 -2.0
2.0 1.0 3
00 30 3 *
2.5 2.1 1.5 1.
5 2.0 300
20 4 * 2.7
2.0 1.4 1.2
1.6 300
20 5 * 2.5
2.3 1.6 1.8
2.0 300 20
6 1.0 0.0
1.0 1.8 2.0
150 150
7 0.5 -1.0
1.7 1.3 2.0
50 200
8 * 2.0 2.0 1.8
1.5 2.0
300 38 9
1.0 -0.9 1.5 1
．． 8 2.0 70
200 10 1.2
0.9 1.5 0.3
2.0 300
50 11 0.5
0.5 1.8 -0.2 2
．． 0 300 50
12 1.7 0.6
-0.2 2.2 -0.5
300 50
13 1.8 0.8 -1.
7 1.8 -0.9
300 50 14 *
1.0 1.9 1.8 1
．． 8 1.0 300
50 15 * 1.8
1.9 2.0 1.8
1.8 300
50 16 * 1.8
1.9 2.2 1.8 1
．． 8 300 50
17 1.0 0.9
0.5 0.2 0.1
300 50
18 1.8 1.9 1.
8 1.8 1.8
50 300 19 *
3.0 2.7 2.5 1
．． 8 2.5 300
5 *
Examples of the invention

Performance Evaluation ■ A 7 MHz output VHS type deck was used, a reference signal was recorded, and the reproduced RF output at 7 MHz was measured. The reference tape was Super XG (T-120) manufactured by Fuji Photo Film Co., Ltd., and the playback output was 0 dB. ■Running durability A T-120 videotape was run on a VHS-type VTR at 40°C and 80% RH, and repeated tests were carried out to run the entire length. Shown in number of trips. ■Video S/N Using a noise meter (925C) made by Shibasoku, the reference tape was
0), and the difference in S/N ratio was determined. Noise was measured using a high-pass filter at 10 KHz and a low-pass filter at 4 MHz. The VTR used is Matsushita NV8300. ■Chroma sensitivity image signal The video signal of 501RE was recorded at the standard recording current. This was reproduced, and the reproduced output signal was measured through a high-pass filter with a high frequency of 50 KHz. The reference tape was similarly measured, and the difference between the obtained reference value and the measured value for the sample was expressed in dB. (2) HiFi characteristic standard recording current was recorded, this was played back, and the playback output at 1.7 MHz was measured. The reference tape was measured in the same way, and the difference between the obtained reference value and the measured value for the sample was d.
Represented by B. (2) The number of dropouts after repeatedly running the T-120 tape with increased DO for 100 passes is shown. Dropout is determined by the dropout counter for a period of 15 x 10-6 seconds or more.
The number of pieces whose playback output decreased by 16 dB or more per minute was displayed. ■Linear audio frequency characteristics Sine wave signals of 1 KHz and 10 KHz were recorded at a level 10 dB lower than the specified input level with the specified bias. This was reproduced, and the reproduction output levels at 1 KHz and 10 KHz were measured using a level meter. The deviation of the 10 KHz playback output level from the 1KHz playback output level was determined, and the S/N with respect to the reference tape (sample No. 36) was expressed in dB.

According to the results in Table 1, sample N falls within the scope of the present invention.
o. 3, 4, 5, 8, 14, 15, 16, 19 are 7M
It can be seen that not only the electromagnetic conversion characteristics of Hz output, video S/N, chroma sensitivity, HiFi characteristics, and linear audio characteristics are excellent, but also the running durability and DO increase are significantly improved. On the other hand, sample No. which is a comparative example. 1, 2, 6,
Nos. 7, 9, 10, 11, 12, 13, 17, and 18 had any of the characteristics deteriorated and did not exhibit the effects of the present invention.

[0048]

Effects of the Invention The present invention provides a magnetic recording medium having a plurality of magnetic layers, in which the lower magnetic layer contains Fe2+/Fe3+ and cobalt-containing iron oxide with a constant coercive force, and the uppermost layer has a plate-like ratio. By using a combination of barium ferrite and polycarbonate polyurethane with an average particle diameter and coercive force of 4 or less, electromagnetic conversion characteristics such as 7MHz output, video S/N, chroma sensitivity, HiFi characteristics, linear audio characteristics, etc., and running durability are achieved. , the DO increase is significantly improved. In addition, further improvement can be achieved by not using carbon black in the uppermost magnetic layer and by including carbon black in the intermediate layer.

Claims (2)

[Claims] 1. A magnetic recording medium comprising a plurality of magnetic layers formed by dispersing ferromagnetic powder and a binder on a non-magnetic support, wherein a lower magnetic layer close to the non-magnetic support has the ferromagnetic powder as the ferromagnetic powder. The molar ratio of Fe2+/Fe3+ is 20/100
~50/100, containing a cobalt-containing iron oxide powder with a coercive force of 750 to 1500 Oe, and the uppermost magnetic layer as the ferromagnetic powder has a plate ratio of 4 or less and an average particle size of 0.1 μm or less, A magnetic recording medium comprising hexagonal plate-shaped barium ferrite having a coercive force of 500 to 1500 Oe, and wherein the binder of the uppermost magnetic layer comprises a polycarbonate polyurethane resin. 2. A magnetic recording medium comprising a plurality of magnetic layers formed by dispersing ferromagnetic powder and a binder on a non-magnetic support, wherein a lower magnetic layer close to the non-magnetic support has the ferromagnetic powder as the ferromagnetic powder. The molar ratio of Fe2+/Fe3+ is 20/100
~50/100 cobalt-containing iron oxide powder,
A non-magnetic intermediate layer with a thickness of 2 μm or less provided on the lower magnetic layer contains carbon black, and an uppermost magnetic layer provided on the non-magnetic intermediate layer contains hexagonal plate-shaped ferromagnetic powder. A magnetic recording medium comprising barium ferrite, and wherein the binder of the uppermost magnetic layer comprises polycarbonate polyurethane resin.