JPH08255335A - Magnetic recording medium - Google Patents

Abstract

PURPOSE: To provide a magnetic recording medium excellent in output characteristics and overwriting characteristics by forming 1st and 2nd magnetic layers on a substrate and incorporating platy magnetite powder into the 2nd magnetic layer. CONSTITUTION: This magnetic recording medium 1 has a substrate 2 and plural magnetic layers 3 formed on the substrate 2. The magnetic layers 3 are a 1st magnetic layer 3a as the top layer and a 2nd magnetic layer 3b adjacent to the top layer. A back coating layer 4 is disposed if necessary. The substrate 2 may be any of magnetic and nonmagnetic substrates but a nonmagnetic substrate is preferable and a flexible polymer film or disk or a substrate of a nonmagnetic metal such as Cu, Al or Zn, glass, porcelain, etc., is used. This magnetic recording medium has satisfactory surface smoothness and is excellent in output (C/N) characteristics and overwriting characteristics.

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 excellent in output (C / N) characteristics and overwrite characteristics.

[0002]

2. Description of the Related Art Conventionally, magnetic recording media have been widely used in the form of tapes, disks, drums or sheets. Such a magnetic recording medium is usually manufactured by coating a support with a magnetic coating material containing a magnetic powder and a binder as main components. In recent years, in particular, there has been a demand for miniaturization and high density recording of magnetic recording media. In order to meet such a demand, for example, it has been proposed to improve the coercive force Hc and the residual magnetic flux density Br of the magnetic layer and to reduce the thickness of the magnetic layer. A second layer between the magnetic layer and the support.
The so-called double coating method has been proposed in which the second layer and the magnetic layer are formed by multilayer coating.

However, the above proposal has a problem that the overwrite characteristic is deteriorated. Further, in the above proposal, it is necessary to make the flow characteristics of the coating material forming the magnetic layer and the coating material forming the second layer uniform, but they are not yet sufficiently aligned, and the flow characteristics of the magnetic layer and There is a problem that the interface with the second layer is disturbed and the surface smoothness of the layer is impaired to deteriorate the electromagnetic conversion characteristics.

Therefore, an object of the present invention is to provide a magnetic recording medium which has good surface smoothness and excellent output (C / N) characteristics and overwrite characteristics.

[0005]

As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a specific magnetic powder is formed adjacent to a magnetic layer provided as the uppermost layer of a magnetic recording medium. It has been found that a magnetic recording medium provided with a magnetic layer containing it can achieve the above object.

The present invention has been made based on the above findings, and has a support and a plurality of magnetic layers provided on the support, and the plurality of magnetic layers are provided as the uppermost layer. In the magnetic recording medium including a first magnetic layer and a second magnetic layer provided adjacent to the first magnetic layer, the above-mentioned second magnetic layer is provided.
The magnetic layer contains a plate-like magnetite powder to provide a magnetic recording medium.

The magnetic recording medium of the present invention will be described in detail below. First, with reference to FIG. 1, a preferred configuration of the magnetic recording medium of the present invention will be illustrated and described.

The magnetic recording medium 1 of the present invention shown in FIG. 1 has a support 2 and a plurality of magnetic layers 3 provided on the support 2, and the plurality of magnetic layers 3 are the uppermost layers. And a second magnetic layer 3b provided adjacent to the first magnetic layer 3a. A back coat layer 4 is provided on the back surface of the support 2 as needed.

In the magnetic recording medium of the present invention, in addition to the support, the first magnetic layer, the second magnetic layer and the back coat layer, a support and a second magnetic layer, or A primer layer provided between the back coat layer and another layer such as a third magnetic layer provided for recording a servo signal corresponding to a hard system using a long wavelength signal may be provided. .

As the support 2 used in the magnetic recording medium of the present invention, both a magnetic support and a non-magnetic support can be used, but a non-magnetic support is particularly preferably used. As the non-magnetic support,
Conventionally known materials can be used without particular limitation, but specifically, flexible films and disks made of polymer resin; non-magnetic metals such as Cu, Al, and Zn, glass, porcelain, and pottery. Film made of ceramics, etc.,
Disks, cards, etc. can be used.

As the polymer resin forming the flexible film or the disc, polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polycyclohexylene dimethylene terephthalate, polyethylene bisphenoxycarboxylate, polyethylene, etc. , Polyolefins such as polypropylene, cellulose acetate butyrate,
Cellulose derivatives such as cellulose acetate propionate, polyvinyl resins such as polyvinyl chloride and polyvinylidene chloride, or polyamides, polyimides, polycarbonates, polysulfones, polyether ether ketones, polyurethanes, etc. may be used alone or Two or more kinds can be used in combination.

Further, in the magnetic recording medium of the present invention, the back coat layer, which is optionally provided on the back surface of the support, can be formed by using a known back coat paint without particular limitation.

In the magnetic recording medium of the present invention, the first magnetic layer provided on the support is the uppermost layer of the magnetic recording medium, that is, the layer provided on the surface of the magnetic recording medium. It is formed by applying the first magnetic paint on the second magnetic layer described later. The first of the above
As the magnetic paint, the paint containing magnetic powder, binder and solvent as main components is preferably used.

Examples of the magnetic powder used in the above-mentioned first magnetic coating material include ferromagnetic metal powder mainly composed of iron or hexagonal ferrite powder. The coercive force of the ferromagnetic metal powder is preferably 1600 to 2500 Oe, and more preferably 1700 to 2400 Oe.
The coercive force of the hexagonal ferrite powder is 140
It is preferably 0 to 2300 Oe. When the coercive force of each of the ferromagnetic metal powder and the hexagonal ferrite powder is less than the above lower limit, demagnetization is likely to occur, so that short-wave RF output is reduced, and when the above upper limit is exceeded, the head is increased. Since the magnetic field becomes insufficient, the writing ability becomes insufficient, and the overwrite characteristic deteriorates, the content is preferably within the above range. The saturation magnetization of the ferromagnetic metal powder is preferably 100 to 180 emu / g,
More preferably, it is 160 emu / g. The saturation magnetization of the hexagonal ferrite powder is 30 to 70 emu / g.
Is preferable, and 45 to 70 emu / g is more preferable. When the saturation magnetizations of the ferromagnetic metal powder and the hexagonal ferrite powder are less than the above lower limits, respectively, the magnetic flux density of the magnetic layer becomes low, the output decreases, and the above upper limit is exceeded. Is practically difficult to manufacture, and even when manufactured, the interaction between the magnetic powders is large, and as a result, the magnetic powders are in an agglomerated state and it is difficult to obtain a desired output. Therefore, it is preferably within the above range. Therefore, the coercive force of the first magnetic layer containing the ferromagnetic metal powder is preferably 1800 to
2400 Oe, more preferably 1800-2300 Oe
And the coercive force of the first magnetic layer containing the hexagonal ferrite powder is preferably 1600 to 2200 Oe.
Is. The saturation magnetic flux density of the first magnetic layer containing the ferromagnetic metal powder is preferably 3000 to 4500.
Gauss, more preferably 3200 to 4000 Gauss, and the saturation magnetic flux density of the first magnetic layer containing the hexagonal ferrite powder is preferably 1500 to 2500 Gauss, more preferably 1600 to 2500 Gauss.

The ferromagnetic metal powder has a metal content of 7
0 wt% or more, and 80 wt% or more of the metal content is Fe
The ferromagnetic metal powder which is Specific examples of the ferromagnetic metal powder include, for example, Fe-Co, Fe-Ni,
Fe-Al, Fe-Ni-Al, Fe-Co-Ni, F
Examples thereof include e-Ni-Al-Zn and Fe-Al-Si. The shape of the ferromagnetic metal powder is needle-like or spindle-like, and its major axis length is preferably 0.05 to 0.25 μm.
More preferably, it is 0.05 to 0.2 μm, the preferable acicular ratio is 3 to 20, and the preferable X-ray particle size is 130 to 2
50Å is desirable.

As the hexagonal ferrite powder, fine tabular barium ferrite powder and strontium ferrite powder, and some of their Fe atoms are replaced with atoms such as Ti, Co, Ni, Zn and V. Magnetic powder etc. are mentioned. The hexagonal ferrite powder has a plate diameter of 0.02 to 0.09 μm and a plate ratio of 2
It is preferably -7.

The magnetic powder may contain, if necessary,
A rare earth element or a transition metal element can be contained. In the present invention, the magnetic powder may be surface-treated in order to improve the dispersibility of the magnetic powder. The above surface treatment is "Characterization of Powder
Surfaces "; can be carried out by a method similar to the method described in Academic Press, for example, a method of coating the surface of the above magnetic powder with an inorganic oxide.
At this time, the inorganic oxides that can be used include Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , Sn.
O ₂ , Sb ₂ O ₃ , ZnO and the like can be mentioned, and when used, they can be used alone or in combination of two or more. The surface treatment may be performed by an organic treatment such as a silane coupling treatment, a titanium coupling treatment and an aluminum coupling treatment other than the above method.

The binder may be a thermoplastic resin, a thermosetting resin, a reactive resin, or the like, which may be used alone or as a mixture.
Specific examples of the binder include vinyl chloride resin,
Examples thereof include polyester, polyurethane, nitrocellulose, and epoxy resin, and in addition, JP-A-57-16
No. 2128, page 2, upper right column, line 19 to page 2, lower right column 1,
Resins and the like described in line 9 and the like can be mentioned. further,
The binder may contain a polar group for improving dispersibility and the like. The amount of the binder used is the magnetic powder 10
It is preferably about 5 to 100 parts by weight, more preferably 5 to 70 parts by weight, based on 0 parts by weight.

Examples of the solvent include ketone solvents, ester solvents, ether solvents, aromatic hydrocarbon solvents, chlorinated hydrocarbon solvents, and the like. The solvents described in JP-A-57-162128, page 3, lower right column, line 17 to page 4, lower left column, line 10 and the like can be used. The amount of the solvent used is preferably 80 to 500 parts by weight, more preferably 100 to 350 parts by weight, based on 100 parts by weight of the magnetic powder.

The above-mentioned first magnetic coating material is usually used as a magnetic recording medium such as a dispersant, a lubricant, an abrasive, an antistatic agent, a rust preventive, a fungicide and a curing agent. Additives can be added as needed. Specific examples of the additive include page 2, upper left column, line 6 to page 2, upper right column, line 10 and page 3, upper left column, line 6 to page 3, upper right column 18 of JP-A-57-162128. There may be mentioned various additives described in the publications.

To prepare the above-mentioned first magnetic coating material, for example, the above-mentioned magnetic powder and the above-mentioned binder are charged together with a part of the solvent into a Nauter mixer or the like and premixed to obtain a mixture, and the obtained mixture is obtained. Kneading with a continuous pressure kneader, etc., then diluting with a part of the solvent, dispersing using a sand mill, etc., mixing additives such as lubricants, filtering, and further curing polyisocyanate, etc. Examples include a method of mixing the agent and the remaining solvent.

The thickness of the first magnetic layer is 0.05 to
It is preferably 1.0 μm, and 0.05 to 0.8 μm
Is more preferable. When it is less than 0.05 μm,
It may be difficult to apply evenly, and durability may decrease.
If it exceeds 1.0 μm, the thickness loss becomes large and the overwrite characteristics may be significantly deteriorated. Therefore, it is preferable that the thickness be within the above range.

In the magnetic recording medium of the present invention, the second magnetic layer provided adjacent to the first magnetic layer is a layer containing plate-like magnetite powder, and the second magnetic layer is formed on the support. It is a layer formed by applying a paint. The second magnetic paint is a paint containing a plate-like magnetite powder, and specifically, a paint containing the plate-like magnetite powder, a binder and a solvent as main components can be preferably used.

In the magnetic recording medium of the present invention, the second magnetic layer contains the plate-like magnetite powder. The plate-shaped magnetite powder has an average plate diameter of preferably 0.01 to 0.2 μm, more preferably 0.01 to 0.12 μm. The average plate diameter is 0.
If it is less than 01 μm, it is practically difficult to manufacture, and even if it is manufactured, the saturation magnetization may be small.
When it exceeds m, the smoothness of the interface between the first magnetic layer and the second magnetic layer deteriorates, and as a result, the smoothness of the first magnetic layer in contact with the magnetic head deteriorates and the output decreases. Or the envelope characteristic may be deteriorated. In particular, when the average plate diameter of the plate-like magnetite powder is within the above preferable range and when the first magnetic layer contains a ferromagnetic metal powder, the coercive force and saturation of the first magnetic layer are The magnetic flux density is preferably in the above-mentioned preferred range, and when the first magnetic layer contains hexagonal ferrite powder,
It is preferable that the coercive force and the saturation magnetic flux density of the magnetic layer are within the above-mentioned preferable ranges. Further, at this time, the thickness of the first magnetic layer is within the above-described preferable range, and the first magnetic layer is applied and formed when the second magnetic layer is wet. preferable.

As the plate-like magnetite powder, an alkaline suspension containing ferric hydroxide or goethite is hydrothermally treated using an autoclave to form plate-like hematite particles from the aqueous solution, and the plate-like hematite particles are produced. Plate-like magnetite powder obtained by the method of heat-reducing in a reducing gas, plate-like magnetite powder obtained by the method of rapidly oxidizing an alkaline suspension containing ferrous hydroxide with a strong oxidant, from an aqueous solution Examples of the plate-like magnetite powder obtained by a method for directly producing a plate-like magnetite powder, the plate-like magnetite powder obtained by the method is a non-sintered and non-porous plate-like magnetite powder. It is preferably used. Here, the above-mentioned non-sintered and non-porous plate-like magnetite powder is a plate-like magnetite powder in which pores lacking water molecules generated during the reduction reaction do not exist because no reducing gas is used. Say.

Details of the method for producing the above-mentioned non-sintered and non-porous plate-like magnetite powder are described in JP-A-63-20.
1019, JP-A-1-176233, and JP-A-3-752.
28, etc., specific examples of the manufacturing method include the following manufacturing method and the like. Fe produced by reacting an aqueous solution of ferrous salt with an aqueous solution of alkali carbonate
A production method in which CO ₃ ²⁻ and Fe ²⁺ are brought to desired concentrations when oxygen is passed through an aqueous solution containing CO ₃ to oxidize it.

The plate-shaped ratio (plate diameter / plate thickness) of the plate-shaped magnetite powder is preferably 2 to 50, more preferably 3
~ 15. The specific surface area of the plate-like magnetite powder is preferably 5 to 100 m ² / g, more preferably 10 to 90 m ² / g. The saturation magnetization of the plate-like magnetite powder is preferably 20 to 90 emu / g,
More preferably, it is 30 to 85 emu / g. The squareness ratio of the plate-like magnetite powder is preferably 0.05 to
0.3, and more preferably 0.08 to 0.25. The coercive force of the plate-like magnetite powder is preferably 10 to 200 Oe, more preferably 20 to 150 Oe.

The binder and the solvent used in the second magnetic coating material may be the same as the binder and the solvent used in the first magnetic coating material. The blending ratio of the binder in the second magnetic coating material is preferably 5 to 500 parts by weight with respect to 100 parts by weight of the plate-like magnetite powder,
5 to 200 parts by weight is more preferable. Further, the mixing ratio of the solvent in the second magnetic coating material is preferably 80 to 1000 parts by weight, and more preferably 100 to 800 parts by weight, based on 100 parts by weight of the plate-like magnetite powder.

The second magnetic paint contains a dispersant,
Lubricants, abrasives, antistatic agents, rust preventives, antifungal agents, hardeners and other additives usually used in magnetic recording media,
It can be added if necessary. Specific examples of the additive include page 2, upper left column, line 6 to page 2, upper right column, line 10 and page 3, upper left column, line 6 to page 3, upper right column 18 of JP-A-57-162128. There may be mentioned various additives described in the publications.

Further, a non-magnetic powder may be added to the above second magnetic paint. The non-magnetic powder is not particularly limited as long as it is non-magnetic, but carbon black, graphite, titanium oxide, barium sulfate, zinc sulfide, magnesium carbonate, calcium carbonate, zinc oxide,
Calcium oxide, magnesium oxide, tungsten disulfide, molybdenum disulfide, boron nitride, tin dioxide, silicon dioxide, non-magnetic chromium oxide, alumina, silicon carbide, cerium oxide, corundum, artificial diamond, non-magnetic iron oxide, garnet , Garnet, silica stone, silicon nitride, molybdenum carbide, boron carbide, tungsten carbide, titanium carbide, diatomaceous earth, dolomite, resinous powder, and the like. Among them, carbon black, titanium oxide, barium sulfate, calcium carbonate, alumina. Non-magnetic iron oxide and the like are preferably used. The non-magnetic powder may be subjected to the same surface treatment as the non-magnetic powder in order to improve the dispersibility of the non-magnetic powder. .

The mixing ratio of the non-magnetic powder used as needed is preferably 0.5 to 1000 parts by weight, and 1 to 500 parts by weight, based on 100 parts by weight of the plate-like magnetite powder. Is more preferable.

The thickness of the second magnetic layer is 0.
The thickness is preferably from 2 to 5 μm, more preferably from 0.5 to 4 μm, most preferably from 0.5 to 2.5 μm. If it is less than 0.2 μm, the strength of the obtained magnetic recording medium becomes weak, and if it exceeds 5 μm, the overwrite property is deteriorated. Therefore, it is preferably within the above range.

The magnetic recording medium of the present invention is for audio,
It is suitable as a magnetic tape for video or computer, but can also be applied as another magnetic recording medium such as a floppy disk.

Next, the outline of the method for producing the magnetic recording medium of the present invention will be described. First, the above-mentioned first magnetic coating material and the above-mentioned second magnetic coating material are wet-on so that the dry thicknesses of the first magnetic layer and the second magnetic layer are the above-mentioned thicknesses, respectively. Simultaneous multilayer coating is performed by a wet method to form coating films for the first and second magnetic layers. That is, it is preferable that the first magnetic layer is applied and formed when the second magnetic layer is wet. Next, the coating film is subjected to a magnetic field orientation treatment, followed by a drying treatment and winding. After that, calendering is performed if necessary, and then a back coat layer is further formed if necessary. Then, if necessary, for example, when a magnetic tape is to be obtained, it is aged at 40 to 70 ° C. for 6 to 72 hours and slit to a desired width.

The above-mentioned simultaneous multilayer coating method is disclosed in JP-A-5-73.
No. 42, line 31 to column 43, line 31 and the like of Japanese Patent No. 883 publication, the first magnetic layer is formed before the second magnetic coating material forming the second magnetic layer is dried. A method of applying the above-mentioned first magnetic coating material to be formed, wherein the interface between the above-mentioned first magnetic layer and the above-mentioned second magnetic layer is smoothed and the surface of the above-mentioned first magnetic layer is formed. Since the magnetic recording medium has good properties, it is possible to obtain a magnetic recording medium with less dropouts, high density recording, and excellent durability of the coating film (first magnetic layer and second magnetic layer).

The magnetic field orientation treatment is performed before the first and second magnetic paints are dried. For example, when the magnetic recording medium of the present invention is a magnetic tape, the first magnetic paint is used. 500 Oe or more in the direction parallel to the coated surface of
Preferably, it can be carried out by a method of applying a magnetic field of about 1000 to 10000 Oe, a method of passing a magnetic field of 1000 to 10000 Oe while the first and second magnetic paints are in a wet state.

The drying treatment is carried out, for example, at 30 to 120 ° C.
The heating can be performed by supplying the gas heated to the above temperature. At this time, the drying degree of the coating film can be controlled by controlling the temperature of the gas and the supply amount thereof.

The calendering treatment can be carried out by a super calendering method in which two rolls such as a metal roll and a cotton roll or a synthetic resin roll, a metal roll and a metal roll are passed. Also,
The conditions for the calendar treatment are 60 to 140 ° C. and 100
It can be up to 500 kg / cm.

The backcoat layer, which is provided if necessary, is provided on the back surface of the support (the surface on the side where the first and second magnetic layers are not provided).
It is obtained by coating the above-mentioned support with a backcoat coating material which is usually used for forming a backcoat layer.

When manufacturing the magnetic recording medium of the present invention, finishing steps such as polishing and cleaning of the surface of the magnetic layer may be carried out, if necessary. The first and second magnetic paints can also be applied by a generally known sequential multi-layer coating method.

In general, a magnetic recording device having an optical sensing mechanism (eg, audio compact cassette, VTR such as VHS or 8 mm video, QIC) is used.
A magnetic recording medium for a data cartridge such as the above) may be provided with a standard value (upper limit value) of light transmittance. In the case of a magnetic recording medium such as a floppy disk or magnetic tape, the ID of the recorded track or sector
Differences in light transmittance are often used to control the running of the medium such as recognition and recognition of the beginning and end of the tape. When utilizing the difference in transmittance as described above, the light transmittance of the magnetic recording layer (coating layer) is made low (light does not pass through), and light such as an uncoated portion or a punched portion is exposed. A method of providing a portion with high transmittance is adopted, but when performing this method,
The light transmittance of the coating layer is preferably low, and in some cases, an additive such as carbon black may be added to reduce the light transmittance. In particular, in the case of using a magnetic recording medium containing a magnetic powder and using an oxide magnetic powder as the magnetic powder or thinning the magnetic layer, it may be necessary to reduce the light transmittance as described above. is there. In short, in the magnetic recording medium, it may be required that the magnetic layer has a low light transmittance. However, since the magnetic recording medium of the present invention adopts the above-mentioned constitution, It satisfies the requirement for transmittance.

[0042]

EXAMPLES Next, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

[Examples 1 and 2 and Comparative Examples 1 and 2] First magnetic coating materials A and B having the following composition and second magnetic coating materials (a), (b) and (c) having the following composition were prepared. Using the combination shown in [Table 1] and using the backcoat paint having the following composition as the backcoat paint, magnetic tapes were manufactured according to the following [Method for manufacturing magnetic recording medium]. A magnetic tape as a magnetic recording medium was obtained in which the first and second magnetic layers were formed from the first and second magnetic paints shown. The first magnetic layer formed by using the first magnetic coating material A or B (dry film thickness of A; 0.5 μm, B
When the coercive force and the saturation magnetic flux density of the dried film thickness (0.3 μm) are measured according to the [Measuring method] described below, the coercive force of the first magnetic layer formed using the first magnetic coating material A is , 18
It was 40 Oe, and the saturation magnetic flux density was 3620 gauss. Further, the coercive force of the first magnetic layer formed using the first magnetic coating material B was 1650 Oe, and the saturation magnetic flux density was 1920 gauss.

First magnetic paint A. Needle-shaped ferromagnetic metal powder mainly composed of iron 100 parts by weight Fe: Al: Ba: Si: Ni: Co: Nd (weight ratio) = 82: 2: 1: 1 : 3: 10: 3 coercive force; 1760 Oe, saturation magnetization; 130 emu / g average major axis length; 0.11 μm, specific surface area; 64 m ² / g crystallite size; 165Å, axial ratio; 10 ・ alumina (average particle size 0 .3 μm) 8 parts by weight Carbon black (average primary particle size 50 nm) 0.5 parts by weight “MR-104” 10 parts by weight [trade name, manufactured by Nippon Zeon Co., Ltd., sulfonic acid group-containing vinyl chloride polymer] ] "UR-8200" 7 parts by weight [trade name, Toyobo Co., Ltd., sulfonic acid group-containing polyurethane resin] 2-ethylhexyl stearate 2 parts by weight Palmitic acid 2 parts by weight "Coronate L" 3 parts by weight Department Product name, Nippon Polyurethane Industry Co., Ltd., a polyisocyanate compound] Methyl ethyl ketone 120 parts by weight Toluene 80 parts by weight of cyclohexanone, 40 parts by weight

First magnetic paint B. Hexagonal ferrite powder 100 parts by weight (Hexagonal plate-shaped Co-Ti substituted barium ferrite powder) Coercive force; 1580 Oe, Saturation magnetization; 58 emu / g Average plate diameter; 0.06 μm, plate 5: Alumina (average particle size 0.2 μm) 5 parts by weight Carbon black (average primary particle size 20 nm) 2 parts by weight “MR-104” 6 parts by weight [trade name, manufactured by Nippon Zeon Co., Ltd., Sulfonic acid group-containing vinyl chloride polymer] "UR-8200" 4 parts by weight [trade name, Toyobo Co., Ltd., sulfonic acid group-containing polyurethane resin] 2-Ethylhexyl stearate 1.5 parts by weight Palmitin Acid 1.5 parts by weight "Coronate L" 3 parts by weight (trade name, manufactured by Nippon Polyurethane Industry Co., Ltd., polyisocyanate compound) -Methyl ethyl ketone 90 parts by weight Parts Toluene 60 parts Cyclohexanone 30 parts by weight

Second magnetic coating material (a) Plate-shaped magnetite powder (non-sintered, non-porous type) 100 parts by weight [plate diameter 0.05 μm, plate ratio (plate diameter / plate thickness) 3 to 5, Specific surface area 35 m ² / g, saturation magnetization 76 emu / g, squareness ratio 0.12, coercive force 83 Oe] -Carbon black (average primary particle size 0.023 μm) 3.5 parts by weight-Alumina (average particle size 0.2 μm ) 3 parts by weight "MR-104" 12 parts by weight [trade name, manufactured by Nippon Zeon Co., Ltd., sulfonic acid group-containing vinyl chloride polymer]-"UR-8200" 6 parts by weight [trade name, Toyobo ( Co., Ltd., sulfonic acid group-containing polyurethane resin]-"Coronate L" 3.5 parts by weight [trade name, manufactured by Nippon Polyurethane Industry Co., Ltd., polyisocyanate] -Oleyl oleate 2 parts by weight-Myristic acid 1 part by weight-Methyl ethyl keto 90 parts by weight Toluene 60 parts by weight Cyclohexanone 30 parts by weight

Instead of the second magnetic coating material (b) plate-shaped magnetite powder, spherical (amorphous) magnetite powder [particle diameter 0.05 μm, specific surface area 17 m ²
/ G, saturation magnetization 78emu / g, squareness ratio 0.16, coercive force 1
10 Oe], but the same as the above second magnetic paint (ii).

Second magnetic coating material (c) Instead of plate magnetite powder, needle magnetite powder [long axis length 0.17 μm, needle ratio 10, specific surface area 37 m ²
/ G, saturation magnetization 83emu / g, squareness ratio 0.46, coercive force 3
90 Oe], except that the above second magnetic paint (ii) was used.

(Blending of backcoat paint) Carbon black (average primary particle size 0.028 μm) 32 parts by weight Carbon black (average primary particle size 0.052 μm) 8 parts by weight “Nipporan 2301” 20 parts by weight [Product Name, Polyurethane manufactured by Nippon Polyurethane Industry Co., Ltd.]-Nitrocellulose 20 parts by weight (Viscosity display made by Hercules Powder Co. of 1/2 second)-"D-250N" 4 parts by weight [Trade name, Takeda Pharmaceutical Co., Ltd. Polyisocyanate manufactured by Co., Ltd.] Copper phthalocyanine 5 parts by weight Stearic acid 1 part by weight Methyl ethyl ketone 120 parts by weight Toluene 120 parts by weight Cyclohexanone 120 parts by weight

[Production of magnetic recording medium] The above-mentioned first magnetic coating material and the above-mentioned second magnetic coating material were each dried on the surface of a polyethylene naphthalate film having a thickness of 6.5 μm to have a dry thickness as shown in Table 1 below. Simultaneous multi-layer coating by wet-on-wet method to obtain the thickness shown in
A coating film for the magnetic layer and the second magnetic layer was formed. Then
While the coating film was in a turbid state, it was passed through a solenoid of 5000 Oe for magnetic field orientation treatment, dried at 60 to 100 ° C., and then wound. Then, 85 ℃, 30
After calendering under the conditions of 0 kg / cm to form the first and second magnetic layers, a back coat coating composition was applied on the back surface of the support so that the dry thickness was 0.7 μm, and the temperature was 90 ° C. After being dried in, it was wound up. Then, it is aged at 50 ° C. for 16 hours and then 3.8.
A magnetic tape having a width of 3.81 mm was obtained by slitting to a width of 1 mm. The obtained magnetic tape was evaluated for surface roughness, output (C / N) characteristics (4.7 MHz), overwrite characteristics and running durability as described below. The results are shown in [Table 1].

With respect to the obtained magnetic tape as a magnetic recording medium, the surface roughness, output (C / N) characteristics, and
The overwrite characteristics and the light transmittance were evaluated. The results are shown in [Table 1].

[Measurement Method] ◎ Coercive Force and Saturation Magnetic Flux Density The support and the second magnetic layer were coated on the support and the second magnetic layer using an adhesive tape.
Only the first magnetic layer is peeled from the magnetic layer, and the first magnetic layer is punched into a predetermined size and shape, and a vibrating magnetometer is used to apply a coercive force and a saturation magnetic flux density at an applied magnetic field of 10 kOe. Each was measured. ◎ Surface Roughness of Magnetic Recording Medium Surface Roughness Ra The magnetic recording medium obtained was manufactured by Tokyo Seimitsu Co., Ltd.
Using a surface roughness shape measuring instrument, trade name "Surfcom 553A", measurement was performed under the conditions of a needle radius of 2 µm, a load of 30 mg, a magnification of 200,000 and a cutoff of 0.08 mm. still,
Ra (center line average roughness) is obtained by extracting a portion of the measurement length L from the roughness curve in the direction of the center line, the center line of the extracted portion as the X axis, and the vertical magnification direction as the Y axis. Y
= F (x), the value obtained by the following equation is expressed in [nm].

[0053]

[Equation 1]

Output (C / N) characteristics (4.7 MHz) The obtained 3.81 mm wide magnetic tape was loaded into a DAT cassette to obtain a DAT tape cassette for testing. The obtained test DAT tape cassette is a Media equipped with a DDS-1 drive (data recording DAT drive).
Product name "Tape Evaluator" manufactured by Logic
Model 4500 ”was loaded, a signal of 4.7 MHz was recorded on the magnetic tape, and the output (reproduction output) when the signal was reproduced was measured. At the same time, measure the noise level (N) at 3.7 MHz, and output at 4.7 MHz (C)
And the noise level ratio of 3.7 MHz to C of 4.7 MHz
/ N. The recording wavelength of 4.7 MHz is 0.67μ.
It was m. The output value of Comparative Example 5 is used as a reference (0 dB)
And ◎ Overwrite characteristics DDS-1 drive (DAT drive for data recording)
Was used to record a 1.2 MHz or 130 KHz signal, and then a 4.7 MHz or 1.5 MHz signal was overwritten, and the output level of the remaining 1.2 MHz or 130 KHz signal was measured. The comparative example 5 was used as a reference.
Also, the smaller the residual output level, the better the overwrite characteristic. Light transmittance The light transmittance of 900 nm was measured by a usual method.

[Comparative Example 3] Using the above first magnetic coating material A, the first magnetic layer was formed so that the dry film thickness was 1.8 μm, and the second magnetic layer was not formed. A magnetic tape was prepared in the same manner as in Example 1, and the obtained magnetic tape was tested and evaluated in the same manner as in Example 1. The results are shown in [Table 1].

[Comparative Example 4] A magnetic tape was obtained in the same manner as in Example 1 by using the above-mentioned first magnetic coating material B and the following non-magnetic coating material (d). The same evaluation as was done. The results are shown in [Table 1]. Non-magnetic paint (d) Needle-shaped α-Fe ₂ O ₃ powder [long axis length 0.13 μm, needle-shaped ratio 10, specific surface area 52 m, instead of plate-shaped magnetite powder]
² / g] (non-magnetic powder), the same as the second magnetic paint (a) above.

[Comparative Example 5] A magnetic tape was obtained in the same manner as in Example 1 using the above-mentioned first magnetic coating material A and the above-mentioned non-magnetic coating material (d). The same evaluation as was done. The results are shown in [Table 1].

[Comparative Example 6] A magnetic tape was obtained in the same manner as in Comparative Example 3 except that the above-mentioned first magnetic coating material A was replaced with the above-mentioned first magnetic coating material B.
The same evaluation as in Example 1 was performed. The results are shown in [Table 1].

[0059]

[Table 1]

As is clear from the results shown in [Table 1],
In the magnetic recording medium of the present invention, since the second magnetic layer contains the plate-like magnetite powder, it is found that the output (C / N) characteristic and the overwrite characteristic are excellent and the surface smoothness is good.

[0061]

The magnetic recording medium of the present invention has good surface smoothness and excellent output (C / N) characteristics and overwrite characteristics.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of a magnetic recording medium of the present invention.

Claims (6)

[Claims] 1. A support, and a plurality of magnetic layers provided on the support, the plurality of magnetic layers comprising a first magnetic layer provided as an uppermost layer and the first magnetic layer. A magnetic recording medium including a second magnetic layer provided adjacent to the magnetic layer, wherein the second magnetic layer contains a plate-like magnetite powder. 2. The plate-like magnetite powder contained in the second magnetic layer has an average plate diameter of 0.01 to 0.2.
The magnetic recording medium according to claim 1, wherein the magnetic recording medium has a thickness of μm. 3. The first magnetic layer contains a ferromagnetic metal powder and has a coercive force of 1800 to 8000.
2400 Oe and saturation magnetic flux density of 3000 to 450
The magnetic recording medium according to claim 2, wherein the magnetic recording medium is 0 gauss. 4. The first magnetic layer contains hexagonal ferrite powder, and the coercive force of the first magnetic layer is 1 or less.
It is 600-2200 Oe, and the saturation magnetic flux density is 1500.
3. The magnetic recording medium according to claim 2, wherein the magnetic recording medium has a density of 2,500 Gauss. 5. The first magnetic layer has a thickness of 0.
It is 05-1.0 micrometers, and the said 1st magnetic layer is apply | coated and formed at the time of the said 2nd magnetic layer wet, The claim | item 3 or 4 characterized by the above-mentioned. Magnetic recording medium. 6. The plate-like magnetite powder contained in the second magnetic layer is a plate-like magnetite powder which is non-sintered and non-porous. The magnetic recording medium described.