JPH117625A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium capable of reducing surface electrical resistance while durability is maintained. SOLUTION: This medium comprises a first magnetic layer 3a which is provided as the uppermost layer and a second magnetic layer 3b which is provided adjacently to the first magnetic layer 3a and which contains a ferromagnetic powder, binder and carbon powder. In this case, the medium is such that the second magnetic layer contains 30-70 pts.wt. binder to 100 pts.wt. ferromagnetic powder; that the binder is constituted, to the 100 pts.wt. ferromagnetic powder, of 10-40 pts.wt. polyurethane binder whose glass transition point (Tg) is 50 deg.C or above; that the carbon powder, with the average grain size of the primary particle of not less than 10 nm but less that 30 nm, comprises 10-40 pts.wt. against the 100 pts.wt. ferromagnetic powder; and that the surface electrical resistance of the magnetic layer, when the medium is worked into the shape of a tape, is in the range of 10<6> to 10<9> Ω/square.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium, and more particularly to an improvement in a magnetic recording medium having at least two magnetic layers.

[0002]

2. Description of the Related Art As one of the methods for reducing dropout in a magnetic recording medium, a method of reducing surface electric resistance of a magnetic recording medium to suppress dust absorption is generally known. As a method generally used to reduce the surface electric resistance, a back coat layer provided on both or one of the upper and lower layers in a multilayer magnetic recording medium having two upper and lower magnetic layers, and further, on the side opposite to the magnetic layer There has been known a method of introducing carbon powder into the powder (for example, Japanese Patent Publication No. 6-54536). However, introducing carbon powder into the magnetic layer causes a reduction in dispersibility of various powders contained in the magnetic layer.
As a result, the filling rate (packing property) of the magnetic powder in the magnetic layer is reduced, so that the coating film rigidity is reduced and the durability is disadvantageous.

On the other hand, in order to improve the durability of a magnetic recording medium, it has been proposed to set the bending rigidity within a specific range (JP-A-4-61621). However, even with the use of the technology described in this publication, a magnetic recording medium that sufficiently satisfies the two opposing characteristics of reducing surface electric resistance and improving durability has not yet been obtained. is there.

Accordingly, it is an object of the present invention to provide a magnetic recording medium capable of realizing a dropout reduction by lowering the surface electric resistance while maintaining the durability. Another object of the present invention is to provide a magnetic recording medium having a high output and excellent running stability.

As a result of intensive studies, the present inventors have found that binders and carbon powders contained in a specific magnetic layer in a multi-layer coating type magnetic recording medium are of a specific type and are blended in a specific range. As a result, it has been found that a magnetic recording medium that can achieve the above object can be obtained.

The present invention has been made based on the above findings, and has a nonmagnetic support and a plurality of magnetic layers provided on the nonmagnetic support, and the plurality of magnetic layers are provided as uppermost layers. A magnetic recording medium including a first magnetic layer and a second magnetic layer provided adjacent to the first magnetic layer, wherein the first magnetic layer contains a ferromagnetic powder and a binder and has a thickness of Is 1.0 μm or less, and the second magnetic layer
A ferromagnetic powder, a binder and carbon powder, wherein the binder is used in an amount of 30 to 100 parts by weight of the ferromagnetic powder.
70 parts by weight of the binder and 10 to 40 parts by weight of a polyurethane binder having a glass transition point (Tg) of 50 ° C. or more with respect to 100 parts by weight of the ferromagnetic powder. Average particle size of 10
10 to 40 parts by weight with respect to 100 parts by weight of the ferromagnetic powder, and the surface electrical resistance of the magnetic layer side when processed into a tape shape is 10 ⁶ to
The above object has been attained by providing a magnetic recording medium characterized by a range of 10 ⁹ Ω / □.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the magnetic recording medium of the present invention will be described below with reference to the drawings. here,
FIG. 1 is a schematic sectional view showing the structure of a preferred embodiment of the magnetic recording medium of the present invention.

The magnetic recording medium 1 of the embodiment shown in FIG.
A first magnetic layer provided as an uppermost layer; a first magnetic layer provided as an uppermost layer; and a plurality of magnetic layers provided on the nonmagnetic support. First magnetic layer 3
and a second magnetic layer 3b provided adjacent to a.
A back coat layer 4 is provided on the back surface of the nonmagnetic support 2 as needed. Hereinafter, the nonmagnetic support and each layer constituting the magnetic recording medium 1 will be described.

[1] Non-Magnetic Support As the material constituting the non-magnetic support 2, a known material can be used without any particular limitation.
Specifically, flexible films and disks made of a polymer resin; non-magnetic metals such as Cu, Al and Zn, glass,
Films, disks and cards made of porcelain and ceramics such as ceramics can be used.

As the polymer resin forming the flexible film and the disc, polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polycyclohexylene dimethylene terephthalate and polyethylene bisphenoxycarboxylate; polyethylene and Polyolefins such as polypropylene; Cellulose derivatives such as cellulose acetate butyrate and cellulose acetate propionate; Vinyl resins such as polyvinyl chloride and polyvinylidene chloride; Polyamide; Polyimide; Polycarbonate; Polysulfone; And the like. When using,
These can be used alone or in combination of two or more.

The thickness of the non-magnetic support 2 is not particularly limited, and can be appropriately selected according to the use and form of the magnetic recording medium.
To 100 µm, more preferably 2 to 30 µm.

[2] Magnetic Layer The magnetic layer 3 provided on the non-magnetic support 1 is, as shown in FIG. 1, a top layer of the magnetic recording medium, that is, a first layer located on the surface side of the magnetic recording medium. And a second magnetic layer 3b provided adjacent to the magnetic layer 3a.

[2-a] First Magnetic Layer First, the first magnetic layer 3a will be described. The first magnetic layer 3a is a layer containing a ferromagnetic powder and a binder and having a thickness of 1.0 μm or less.

The ferromagnetic powder is preferably a ferromagnetic powder composed of a needle-like metal or metal oxide. The major axis length of the acicular ferromagnetic powder is preferably 0.04 to 0.1.
8 μm, more preferably 0.04 to 0.12 μm
It is. Further, the needle ratio is preferably 3 to 12,
More preferably, it is 5-10. Examples of the ferromagnetic powder include a metal content of 70% by weight or more,
A ferromagnetic metal powder containing 0% by weight or more of Fe, FeOx
A ferromagnetic iron oxide represented by (1.3 ≦ x ≦ 1.5), a material obtained by adding Co or the like to FeOx, or the like can be used, but is not limited thereto. Specific examples of these include, for example, Fe-Co, Fe-Ni, Fe
-Al, Fe-Ni-Al, Fe-Co-Ni, Fe-
Ni-Al-Zn, Fe-Al-Si and the like can be mentioned.
The ferromagnetic powder may contain a rare earth element or a transition metal element as needed.

The coercive force of the ferromagnetic powder can be used to prevent a decrease in reproduction output during recording in a short wavelength region, to prevent insufficient writing ability due to an insufficient head magnetic field, and to prevent a decrease in overwrite characteristics. From the point, 1500-
2500 Oe, preferably 1700 to 230
More preferably, it is 0 Oe. In addition, the saturation magnetization of the ferromagnetic powder is set at 110 to prevent the magnetic powder from being aggregated to obtain a desired output.
~ 150 emu / g, preferably from 125 to 1
More preferably, it is 40 emu / g.

In the present invention, the above ferromagnetic powder
In order to improve the dispersibility, etc., the ferromagnetic powder is subjected to a surface treatment.
May be applied. The above-mentioned surface treatment is referred to as “Characterization
 of Powder Surfaces "; described in the Academic Press
Can be performed in the same manner as
A method of coating the surface of the ferromagnetic powder with an inorganic oxide
No. At this time, the inorganic acid which can be used
The compound is Al_TwoO_Three, SiO_Two, TiO_Two, Zr
O _Two, SnO_Two, Sb_TwoO_Three, ZnO, etc.
When using, use alone or as a mixture of two or more.
Can be. The above-mentioned surface treatment may be performed in addition to the above-described method.
Coupling treatment, titanium coupling treatment and aluminum coupling treatment
It can be performed by organic treatment such as mina coupling treatment.
Wear.

The coercive force of the first magnetic layer 3a is 150
It is preferably 0 to 2500 Oe. The coercive force is 1
If it is less than 500 Oe, a short wavelength region (for example, λ =
At 0.5 μm or less),
If it exceeds 2500 Oe, the overwrite characteristics may be poor. The coercive force depends on the characteristics of the magnetic head, but is preferably 1800 to 2500 Oe, more preferably 1900 to 2200 Oe.

The saturation magnetic flux density of the first magnetic layer 3a is:
It is preferably 2500 gauss or more. If the saturation magnetic flux density is less than 2500 Gauss, there is a possibility that the reproduction output in recording in both the long and short wavelength regions will be reduced. The saturation magnetic flux density is more preferably 3000 Gauss or more, and even more preferably 3500 Gauss or more.

The thickness of the first magnetic layer 3a is 1.
0 μm or less. When the film thickness exceeds 1.0 μm, the reproduction output in recording in a short wavelength region decreases due to self-demagnetization loss and film thickness loss, and it becomes difficult to increase the recording density. It is assumed that. The lower limit can be as thin as possible for actual recording, but is preferably 0.1 μm at a long wavelength, specifically, at a recording wavelength of λ = 2 μm or more, for the purpose of maintaining the output at a high level. The thickness is preferably 0.1 to 0.5 μm,
More preferably, it is 0.1 to 0.3 μm.

The center line surface roughness Ra of the first magnetic layer 3a is preferably 10 nm or less from the viewpoint of improving the electromagnetic conversion characteristics in a high-density magnetic recording medium.
The smaller the center line surface roughness is, the more effective it is for improving the electromagnetic conversion characteristics. The center line surface roughness is preferably 8 nm or less, more preferably 4 nm or less. Means for keeping the center line surface roughness Ra within the above range include, for example, improving the dispersibility of a magnetic paint described later,
To slow down the drying conditions of the coating film, control the calendering conditions, use a binder with a low glass transition point in the magnetic coating composition, apply a high-concentration (solid content of 35% or more) low-viscosity magnetic coating, etc. There are means. The center line surface roughness was measured using a laser light interference type surface roughness meter (Laser manufactured by Zygo).
Interferometric Microscope Maxim 3D Model 5700) was measured under the following conditions. Lens used: Fizeau 40 × Remove: Cylinder Filter: off Sampling length: 180 μm Sampling number: 260

The first magnetic layer 3a contains the above-mentioned ferromagnetic powder, binder and solvent as main components, and further comprises an abrasive,
It is formed by applying a first magnetic paint containing carbon powder, a lubricant, a curing agent, and the like. Hereinafter, these components constituting the magnetic paint will be described.

<Binder> As the binder,
Examples thereof include a thermoplastic resin, a thermosetting resin, and a reactive resin. When used, they can be used alone or as a mixture. Specific examples of the binder include vinyl chloride resin, polyester, polyurethane, nitrocellulose, epoxy resin and the like.
Resins and the like described in page 7, upper right column, line 19 to page 2, lower right column, line 19 of JP-A-7-162128 are exemplified.
Further, as the binder, it is also preferable to use a polyurethane binder having a glass transition point of 50 ° C. or higher contained in the second magnetic layer described later. Further, the binder may contain a polar group for improving the dispersibility and the like of the ferromagnetic powder. The binder comprises the ferromagnetic powder 1
The amount is preferably 10 to 40 parts by weight, more preferably 12 to 35 parts by weight, per 100 parts by weight.

<Abrasive> The abrasive is a powder of a substance having a Mohs hardness of 7 or more, for example, alumina, silica, T
Powder of iO ₂ , ZrO ₂ , Cr ₂ O _{3 or} the like is used.
The average particle size of the primary particles of the abrasive is preferably from 0.03 to 0.2 in terms of a reduction in friction coefficient during running and an improvement in running durability.
0 μm, preferably 0.05 to 0.15 μm
Is more preferable. The abrasive is made of the above ferromagnetic powder 100 from the viewpoint of improving running stability and electromagnetic conversion characteristics.
The amount is preferably 2 to 15 parts by weight, more preferably 5 to 10 parts by weight based on parts by weight.

<Carbon Powder> The carbon powder is used as an antistatic agent or a solid lubricant for a magnetic recording medium. As the carbon powder, it is preferable to use carbon black having an average primary particle size of 10 to 30 nm (particularly, 20 to 30 nm). Further, as the carbon powder, two or more kinds of carbon blacks having different average particle diameters can be used in combination. The carbon powder is preferably blended in an amount of 10 to 40 parts by weight, more preferably 20 to 40 parts by weight, based on 100 parts by weight of the ferromagnetic powder.
30 parts by weight are blended.

<Lubricant> Fatty acids and fatty acid esters are generally used as the lubricant. Examples of the fatty acid include caproic acid, caprylic acid, capric acid,
Lauric acid, myristic acid, palmitic acid, stearic acid, isostearic acid, linolenic acid, oleic acid, elaidic acid, behenic acid, malonic acid, succinic acid, maleic acid, glutaric acid, adipic acid, pimelic acid, azelaic acid, sebacic acid , 1,12-dodecanedicarboxylic acid, octanedicarboxylic acid and the like. On the other hand, examples of the above fatty acid ester include alkyl esters of the above fatty acids, and those having a total carbon number of 12 to 36 are preferable. The fatty acid and the fatty acid ester are preferably added in an amount of preferably 1 to 10 parts by weight, more preferably 4 to 6 parts by weight, based on 100 parts by weight of the ferromagnetic powder.
It is blended by weight.

<Curing Agent> As the above-mentioned curing agent, generally,
Isocyanate-based curing agents and amine-based curing agents represented by Coronate L (trade name) manufactured by Nippon Polyurethane Industry Co., Ltd. are used.

<Solvent> Examples of the solvent include ketone solvents, ester solvents, ether solvents, aromatic hydrocarbon solvents and chlorinated hydrocarbon solvents. Solvents described in JP-A-57-162128, page 3, lower right column, line 17 to page 4, upper left column, line 10 can be used. The amount of the solvent is
100-1000 with respect to 100 parts by weight of the ferromagnetic powder
The parts by weight are preferable, and 200 to 600 parts by weight are more preferable.

In the first magnetic paint, in addition to the above-mentioned components, additives such as a dispersant, a rust preventive, and a fungicide which are usually used for magnetic recording media may be added. it can. Specific examples of the additive include those described in
Various additives described in page 162128, 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, line 18 can be mentioned. .

In order to prepare the first magnetic paint, for example, the ferromagnetic powder and the binder are put into a Nauter mixer or the like together with a part of the solvent and premixed to obtain a mixture. Is kneaded with a continuous pressure kneader or the like, then diluted with a part of the above solvent, and subjected to a dispersion treatment using a sand mill or the like, and then mixed with an additive such as a lubricant, filtered, and further filtered. And a method of mixing the remainder of the solvent.

[2-b] Second Magnetic Layer Next, the second magnetic layer 3b will be described. The second magnetic layer 3b contains a ferromagnetic powder, a binder and a carbon powder. Hereinafter, these components will be described.

As the ferromagnetic powder, a ferromagnetic powder composed of a hexagonal ferromagnetic oxide is preferably used, but is not limited to this. Examples of the hexagonal ferromagnetic oxide include hexagonal ferrites, for example, barium ferrite and strontium ferrite in the form of microplates, and a part of Fe atoms thereof are replaced with atoms such as Ti, Co, Ni, Zn or V. It is preferable to use a barium ferrite having a small flat plate shape. In this case, the plate diameter is preferably from 0.02 to 0.08 μm, and the plate ratio is preferably from 2 to 6.

The coercive force of the ferromagnetic powder comprising the hexagonal ferromagnetic oxide is preferably from 1500 to 2500 Oe, and the saturation magnetization is preferably from 40 to 70 emu / g.

The ferromagnetic powder contained in the second magnetic layer 3b may contain a rare earth element or a transition metal element, if necessary, similarly to the ferromagnetic powder contained in the first magnetic layer. It can be contained. Furthermore, the above-mentioned surface treatment may be applied to the above-mentioned ferromagnetic powder.

In the present invention, in particular, the second magnetic layer 3
The magnetic recording is carried out by setting the blending amount of the binder contained in b in a specific range and using a specific type and using the carbon powder in a specific range and using a specific type (particle size). The balance between improving the durability of the medium and reducing the surface electric resistance is achieved. More specifically, by specifying the type and amount of the binder, the filling property of the ferromagnetic powder is increased, and the durability is improved by increasing the rigidity of the coating film. In addition, by specifying the type and amount of the carbon powder, the surface electric resistance is reduced. As described above, in the present invention, the surface electrical resistance is reduced while the durability is improved by balancing the respective amounts of the specific binder and the carbon powder.

As the binder, the same binder as that used for the first magnetic layer 3a can be used. As described above, the balance between the improvement of the durability of the magnetic recording medium and the reduction of the surface electric resistance is obtained. Therefore, it is contained in an amount of 30 to 70 parts by weight based on 100 parts by weight of the ferromagnetic powder (the ferromagnetic powder contained in the second magnetic layer 3b). However, in the case of the binder, a polyurethane binder having a glass transition point (hereinafter, referred to as “Tg”) of 50 ° C. or higher is also used from the viewpoint of the balance between the improvement of the durability of the magnetic recording medium and the reduction of the surface electric resistance. 10 to 40 parts by weight based on 100 parts by weight of the magnetic powder (ferromagnetic powder contained in the second magnetic layer 3b)
Occupies part by weight.

If the total amount of the binder in the second magnetic layer 3b is less than 30 parts by weight with respect to 100 parts by weight of the ferromagnetic powder, the stiffness (flexural rigidity) of the entire magnetic recording medium is weakened to form a tape. In such a case, breakage or bending during running is likely to occur, resulting in poor running.
If the amount exceeds 0 parts by weight, a decrease in the filling factor of the ferromagnetic powder and a decrease in output due to deterioration of the surface properties are caused. Second magnetic layer 3
b, the total amount of the binder is 10%.
The amount is preferably 50 to 70 parts by weight, more preferably 50 to 60 parts by weight, based on 0 parts by weight. If the amount of the polyurethane binder is less than 10 parts by weight based on 100 parts by weight of the ferromagnetic powder, the output decreases due to the decrease in the filling rate of the ferromagnetic powder due to poor dispersibility, and the stiffness ( Flexural stiffness) is weakened, and if it exceeds 40 parts by weight, there is no room for containing another binder mainly for the purpose of preventing the coating film from being damaged. The amount of the polyurethane binder is more preferably 15 to 35 parts by weight based on 100 parts by weight of the ferromagnetic powder.

The polyurethane binder is described in detail. The Tg of the polyurethane binder is 50 ° C. or higher as described above. By using a polyurethane binder having a Tg of 50 ° C. or higher, particularly a polyurethane binder having a Tg of 50 ° C. or higher and an adsorption amount described later in a specific range, high filling properties of a ferromagnetic powder and durability of a magnetic recording medium at high temperature environment can be obtained. It has been found by the inventors that they can be improved at the same time. If a polyurethane binder having a Tg of 50 ° C. or higher is not added at all, it becomes impossible to mix carbon powder in a well-balanced manner so as to optimize filling properties. The Tg of the polyurethane binder is preferably 70 ° C. or higher. The method of measuring Tg will be described in detail in Examples described later.

Examples of the polyurethane binder include T
If g is 50 ° C. or more, it can be used without any particular limitation. For example, those obtained by a known polyurethane production method using a polyol, a polyisocyanate and a chain extender can be used. In particular, as the polyurethane binder, the amount of adsorption to the hexagonal ferromagnetic oxide is 2.5 to 4.0 mg / m ² , particularly 3.0 to 4.0 mg / m ² .
The use of 3.5 mg / m ² is preferable because the balance between the improvement of the durability of the magnetic recording medium and the reduction of the surface electric resistance is further improved. The method for measuring the amount of adsorption will be described in detail in Examples described later. In the second magnetic layer 3b, as long as the polyurethane binder having a Tg of 50 ° C. or more is contained in an amount of 10 to 40 parts with respect to 100 parts by weight of the ferromagnetic powder, a polyurethane binder having a Tg of less than 50 ° C. Can be used at the same time.

As the carbon powder, as described above,
From the viewpoint of the balance between the improvement of the durability of the magnetic recording medium and the reduction of the surface electric resistance, the average primary particle size is 10 nm or more and 30 or more.
10 to 40 parts by weight based on 100 parts by weight of the above ferromagnetic powder (ferromagnetic powder contained in the second magnetic layer 3b).
Used by weight. If the average particle size of the primary particles of the carbon powder is less than 10 nm, sufficient solid lubricity cannot be maintained, and dispersibility cannot be ensured.
If it is more than nm, the surface roughness at the time of forming a coating film will be impaired. If the amount of the carbon powder is less than 10 parts by weight, the surface electric resistance increases to the order of 10 ¹⁰ Ω / □ or more, and if it exceeds 40 parts by weight, the dispersibility cannot be secured and the output is reduced. I will. The average particle size of the primary particles of the carbon powder is preferably 15 to 25 nm. The amount of the carbon powder is preferably 15 to 25 parts by weight based on 100 parts by weight of the ferromagnetic powder. The average particle size of the primary particles of the carbon powder can be measured by an electron microscope (TEM).

The second magnetic layer 3b contains the ferromagnetic powder, the binder and the carbon powder,
It is formed by applying a second magnetic paint containing an abrasive, a lubricant, a hardener, a solvent, and the like. Second
As the abrasives, lubricants, hardeners, solvents, and the like contained in the magnetic paint, those similar to those contained in the first magnetic paint can be used. The method for preparing the second magnetic paint is the same as the method for preparing the first magnetic paint.

The second magnetic paint may contain non-magnetic powder other than carbon powder and abrasive. The non-magnetic powder is not particularly limited as long as it is non-magnetic,
Barium sulfate, zinc sulfide, magnesium carbonate, calcium carbonate, zinc oxide, calcium oxide, magnesium oxide, tungsten disulfide, molybdenum disulfide, boron nitride, tin dioxide, nonmagnetic chromium oxide, silicon carbide, cerium oxide, corundum, artificial Diamond, non-magnetic iron oxide (bengala), garnet, silica, silicon nitride, molybdenum carbide, boron carbide, tungsten carbide, titanium carbide, diatomaceous earth, dolomite, resinous powders, and among others, barium sulfate , Calcium carbonate, non-magnetic iron oxide (Bengara) and the like are preferably used. The non-magnetic powder may be subjected to the above-described surface treatment in order to improve the dispersibility of the non-magnetic powder. The compounding amount of the non-magnetic powder is
It is preferably from 50 to 200 parts by weight, more preferably from 120 to 180 parts by weight, based on 100 parts by weight of the ferromagnetic powder.

The thickness of the second magnetic layer 3b formed from the second magnetic paint is 0.5 to 3.0 μm from the viewpoint of securing the rigidity of the magnetic recording medium and preventing a decrease in overwrite characteristics.
Is preferably, and more preferably, 1.0 to 2.5 μm. The coercive force and saturation magnetic flux density of the second magnetic layer are 1000 to 2000 Oe and 4 respectively.
Preferably from 800 to 800 gauss, from 1200 to 800 gauss
More preferably, it is 1900 Oe and 450 to 650 Gauss.

In the magnetic recording medium 1 shown in FIG. 1, as described above, specific types are used as the binder and carbon powder contained in the second magnetic layer 3b, and these are blended in a specific range. Thus, the balance between the improvement of the durability of the magnetic recording medium and the reduction of the surface electric resistance is achieved. As a result, in the magnetic recording medium 1, the surface electric resistance on the magnetic layer 3 side when processed into a tape shape becomes 1
0 is limited to ^{6 ~10 9 Ω} / □ low range of. Therefore, dust absorption and the like are prevented, and the dropout of the magnetic recording medium is reduced without lowering the output, as is apparent from the embodiments described later.

Regarding the rigidity related to the durability of the magnetic recording medium, regarding the bending rigidity when the magnetic recording medium 1 is processed into a tape shape, the bending stress per unit width of the magnetic recording medium is changed in the longitudinal direction. (MD), preferably 5.8 to 8.0 mg / mm (particularly 7.0 to 8.0 mg / mm).
mm), preferably 3.8 to 6.0 m in the width direction (TD).
g / mm (particularly 5.0 to 6.0 mg / mm), and as will be clear from the examples described later, the magnetic recording medium has sufficient durability. The method for measuring the bending stress will be described in detail in Examples described later.

[4] Backcoat Layer In the magnetic recording medium 1 shown in FIG. 1, the backcoat layer 4 provided on the non-magnetic support 2 on the side opposite to the side on which the magnetic layer 3 is provided, It can be formed using a known backcoat paint containing carbon black and a binder without any particular limitation.

Although the magnetic recording medium of the present invention has been described based on the preferred embodiments, the present invention is not limited to the above embodiments, and various changes can be made without departing from the gist of the present invention. . For example, in the magnetic recording medium 1 of the embodiment shown in FIG. 1, a primer layer is further provided between the nonmagnetic support 2 and the second magnetic layer 3b or the back coat layer 4, or a long-wavelength signal Other magnetic layers and other layers for recording servo signals and the like may be provided in accordance with a hardware system using. In addition, the magnetic recording medium of the present invention may be an 8 mm video tape, a DAT tape,
It is suitable as a magnetic tape such as an LT tape or a DVC tape, but can also be applied as another magnetic recording medium such as a magnetic disk such as a flexible disk.

Next, an outline of a preferred method of manufacturing the magnetic recording medium 1 of the embodiment shown in FIG. 1 will be described. First, the second magnetic paint and the first magnetic paint are placed on the non-magnetic support 2 so that the dry thicknesses of the second magnetic layer 3b and the first magnetic layer 3a are each the above-described preferable thickness. Wet
Simultaneous multi-layer coating is performed by an on-wet method to form coating films of 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, these coating films are subjected to a magnetic field orientation treatment, and then subjected to a drying treatment and wound up. Thereafter, a calendering process is performed to further form the back coat layer 4. Alternatively, the magnetic layer 3 may be formed after the back coat layer 4 is formed. Then, if necessary, for example,
When obtaining a magnetic tape, 6 to 6
Aging treatment is performed for 100 hours, and slit to a desired width.

The above simultaneous multi-layer coating method is described in JP-A-5-73.
No. 883, column 42, line 31 to column 43, line 31, etc., which is a method of applying the first magnetic paint before the second magnetic paint dries, Since the boundary surface between the first magnetic layer 3b and the first magnetic layer 3a is smooth and the surface property of the first magnetic layer 3a is good, the dropout is small, the high-density recording can be performed, and the coating film (first and third layers) can be formed. A magnetic recording medium having excellent durability of the second magnetic layer) can be obtained.

Further, the magnetic field orientation treatment is performed in the first and second magnetic fields.
This is performed before the magnetic paint is dried. For example, when the magnetic recording medium of the present invention is a magnetic tape, about 500 Oe or more, preferably about 1000 to 10000 Oe, in a direction parallel to the coating surface of the magnetic paint. How to apply a magnetic field,
1000 to 10000 while the magnetic paint is wet
It can be performed by a method of passing through an Oe solenoid or the like.

The drying treatment can be performed, for example, by supplying a heated gas. At this time, the degree of drying of the coating film can be controlled by controlling the temperature of the gas and the amount of the supplied gas.

The calendering treatment can be performed by a super calendering method in which a metal roll and a cotton roll or a synthetic resin roll, a metal roll and a metal roll are passed between two rolls.

[0052]

The following examples illustrate the effectiveness of the present invention. However, the scope of the present invention is not limited to such an embodiment. In the following examples, “parts” means “parts by weight” unless otherwise specified.

[Example 1] The following components (excluding the curing agent) were kneaded with a kneader, dispersed by a stirrer, finely dispersed by a sand mill, and filtered by a 1 µm filter. The curing agent was added last to prepare the first and second magnetic paints and the backcoat paint, respectively.

<First magnetic paint> 100 parts of ferromagnetic powder (acicular ferromagnetic metal powder, coercive force: 1820 Oe, saturation magnetization: 131 emu / g, average major axis length: 0.11 μm, needle ratio; 8) ・ 11 parts of vinyl chloride binder [MR110 (trade name) manufactured by Zeon Corporation] ・ 7 parts of polyurethane binder A [UR-D (trade name) manufactured by Toyobo, Tg = 33 ° C.] ・ 12 parts of abrasive (α -Alumina, average particle size of primary particles: 0.3 μm) ・ Carbon black (average particle size of primary particles: 20 nm) 0.3 part ・ Lubricant A (myristic acid) 2 parts ・ Lubricant B (butyl stearate) 2 parts ・ Curing agent (Coronate L) 4 parts ・ Catalyst (dibutyltin dilaurate) 1 part ・ Solvent A (methyl ethyl ketone) 100 parts ・ Solvent B (toluene) 50 parts ・ Solvent C (cyclohexanone) 00 parts

<Second Magnetic Coating> 100 parts of ferromagnetic powder (fine flat barium ferrite, coercive force: 1680 Oe, saturation magnetization: 57 e mu / g, plate diameter: 0.06 μm) Non-magnetic powder (Bengara) 125 parts) Vinyl chloride binder 32 parts [Zeon MR110 (trade name) manufactured by Zeon] 4 parts of polyurethane binder A [UR-D (trade name) manufactured by Toyobo, Tg = 33 ° C.] 16 parts of polyurethane binder B [UR-F (trade name) manufactured by Toyobo, Tg = 70 ° C., adsorption amount to barium ferrite = 3.18 mg / m ² ] 17 parts of abrasive (α-alumina, average particle size of primary particles: 0.3 μm) ・ 25 parts of carbon black (average particle diameter of primary particles: 20 nm) ・ 4 parts of lubricant A (myristic acid) ・ 3 parts of lubricant B (butyl stearate) ・ Hard Agent (Coronate L) 10 parts Catalyst (dibutyltin dilaurate) 1 part Solvent A (methyl ethyl ketone) 100 parts Solvent B (toluene) 50 parts Solvent C (cyclohexanone) 100 parts

<Backcoat paint>-38.5 parts of carbon black (average particle diameter of primary particles: 18 nm)-1.5 parts of carbon black (average particle diameter of primary particles: 75 nm)-50 parts of polyurethane binder [Nippon Polyurethane Nipporan 2301 (trade name) manufactured by Kogyo Co., Ltd.)-28.6 parts of nitrocellulose binder [Celnova BTH 1/2 (trade name) manufactured by Asahi Kasei Kogyo Co., Ltd.]-4 parts of hardener [Takeda Pharmaceutical Co., Ltd. ) -Made polyisocyanate, D-250N (trade name)]-Copper phthalocyanine 5 parts-Lubricant (stearic acid) 1 part-Solvent A (methyl ethyl ketone) 140 parts-Solvent B (toluene) 140 parts-Solvent C (cyclohexanone) 140 copies

Each of the first and second magnetic paints was coated on a polyethylene terephthalate support having a thickness of 7 μm.
Has a magnetic layer thickness of 2.5 μm and a first magnetic layer thickness of 0.2
Simultaneous multilayer coating was performed by a die coater so as to have a thickness of μm. Next, while these magnetic layers were changed from a wet state to a dry state, a magnetic field orientation treatment was performed using a 5000 Oe solenoid. Further, in a drying oven, hot air of 80 ° C. was blown for 10 minutes.
The coating film was dried by spraying at a speed of m / min. After drying, calendering is applied, and the above-mentioned back coat paint is applied on the surface on the opposite side of the above-mentioned support so that the dry thickness becomes 0.5 μm, and dried at 90 ° C. to form a back coat layer did. Finally, the magnetic tape was slit to a 1/2 inch width to produce a magnetic tape having the structure shown in FIG.

Examples 2 to 4 and Comparative Examples 1 to 6 Magnetic tapes were obtained in the same manner as in Example 1 except that the second magnetic paint was used as shown in Table 1.

[0059]

[Table 1]

The Tg of the polyurethane binder used in Examples and Comparative Examples and the adsorption amount of the above-mentioned barium ferrite, which is a hexagonal ferromagnetic oxide, were measured by the following methods.

<Method of measuring Tg> Rheometrics
Using a dynamic viscoelasticity measuring device RSA-2 (trade name) manufactured by Co., Ltd., the temperature corresponding to the peak value of the loss elastic modulus (E ″) when the elastic modulus-temperature characteristic at a frequency of 110 Hz is plotted is measured. This is defined as Tg.

<Measurement Method of Adsorption Amount> A magnetic paint for measurement with a known binder charge concentration (note that this magnetic paint does not contain any powder other than hexagonal ferromagnetic oxide powder) is subjected to an ultracentrifuge. (HITACHI, HIMAC CP85
After the treatment for 3 hours at 30,000 rpm using B), the binder concentration of the supernatant is measured. [(1−concentration after centrifugation treatment / concentration before centrifugation treatment) × 100] (%) is calculated as a binder adsorption rate (%) to hexagonal ferromagnetic oxide powder
Then, the adsorption amount (mg / m ² ) is calculated from the following equation (1). In the following formula (1), the specific surface area (m ² / g) of the hexagonal ferromagnetic oxide powder is a value measured by the BET method.

[0063]

(Equation 1)

For the magnetic tapes obtained in Examples and Comparative Examples, the bending stress and the center line surface roughness Ra on the magnetic layer side were measured.
In addition to measuring the surface electric resistance, the output and durability (head contamination) were measured. Table 2 shows the results. The bending stress, output and durability were measured by the following methods.

<Measurement Method of Bending Stress> Apparatus 10 shown in FIG.
The measurement was performed using. The apparatus 10 shown in FIG. 2 includes upper and lower sample fixing portions 11 and 12, a micrometer 13 connected to the upper sample fixing portion 11, and a balance 14 connected to the lower sample fixing portion 12. I have. The upper sample fixing unit 11 is movable in the vertical direction, and the moving distance is measured by the micrometer 13. From the magnetic tape having a width of 1/2 inch obtained in each of the examples and the comparative examples, a 1/2 inch (1
2.65 mm) A sample 15 on each side is cut out, and both ends in the longitudinal direction and the width direction of the sample are fixed to the upper and lower sample fixing portions 11.
12 is fixed by an adhesive tape 16 in an extended state. From this state, the upper sample fixing part 11 is lowered to
And the distance between the upper and lower sample fixing parts 11 and 12 is 5 m.
m, the load displayed on the balance 14 is read in this state, and the value is divided by 12.65 mm to obtain the bending stress in the longitudinal direction and the width direction. The bending is performed so that the magnetic layer is bent inward and outward, respectively, and the average value is defined as bending stress.

<Method of Measuring Output> A 1800 ft total length tape was subjected to rectangular wave recording at a recording frequency of 1 MHz and 2 MHz at a tape feed speed of 100 inch / sec using a fixed MIG head, and the output was read. . As a reference, the upper layer has a thickness of 0.25 μm.
A magnetic tape having a magnetic / non-magnetic multilayer structure in which a m ferromagnetic layer and a lower layer were a non-magnetic layer having a thickness of 1.95 μm was used.

<Durability (Head Contamination) Measuring Method>
After running continuously for 300 hours in the cycle environment specifically shown below, the determination was made based on the level of dirt (wisker) stuck around the magnetic head. <Cycle environment> Low temperature → high temperature ((44 ° C., 80% RH) → [48.9
℃, 30% RH) → low temperature → high temperature [(28.9 ° C, 20
% RH) → (35 ° C., 80% RH)].

[0068]

[Table 2]

As is clear from the results shown in Table 2, the second
The magnetic tape of the embodiment (the product of the present invention) using specific types of binder and carbon powder contained in the magnetic layer of (1) and blending them in a specific range has low surface electric resistance and low output power. It is understood that the bending stress value is high and the durability is further excellent. On the other hand, although the magnetic tape of the comparative example has low surface electric resistance, it has a low value of bending stress and is inferior in durability.

[0070]

As described above, according to the present invention,
A magnetic recording medium capable of reducing surface electric resistance while maintaining durability can be obtained. Further, according to the present invention, a magnetic recording medium having a high output and excellent running stability can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing the structure of a preferred embodiment of a magnetic recording medium according to the present invention.

FIG. 2 is a diagram showing an apparatus for measuring a bending stress of a magnetic tape.

[Explanation of symbols]

 Reference Signs List 1 magnetic recording medium 2 support 3 magnetic layer 3a first magnetic layer 3b second magnetic layer 4 back coat layer

Claims (4)

[Claims] 1. A non-magnetic support comprising: a first magnetic layer provided as an uppermost layer; and a plurality of magnetic layers provided on the non-magnetic support. A magnetic recording medium including a second magnetic layer provided adjacent to the magnetic layer, wherein the first magnetic layer contains a ferromagnetic powder and a binder and has a thickness of 1.0 μm or less; 2 comprises a ferromagnetic powder, a binder and a carbon powder, the binder comprising
30 to 70 parts by weight based on 100 parts by weight of a polyurethane binder having a glass transition point (Tg) of at least 50 ° C.
0 to 40 parts by weight, the carbon powder has an average primary particle diameter of 10 nm or more and less than 30 nm, and 10 to 40 parts by weight with respect to 100 parts by weight of the ferromagnetic powder. A magnetic recording medium characterized in that the surface electrical resistance of the magnetic layer side when processed into a range of 10 ⁶ to 10 ⁹ Ω / □. 2. The bending stiffness when processed into a tape shape has a bending stress per unit width of the magnetic recording medium of 5.8 to 8.0 mg / mm in the longitudinal direction and 3.8 to 3.8 mg / mm in the width direction.
2. The magnetic recording medium according to claim 1, wherein the amount is 6.0 mg / mm. 3. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in the second magnetic layer comprises a hexagonal ferromagnetic oxide. 4. The method according to claim 3, wherein the polyurethane binder contained in the second magnetic layer has an adsorption amount of 2.5 to 3.5 mg / m ² with respect to the hexagonal ferromagnetic oxide. The magnetic recording medium according to the above.