JP2002092847A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the magnetic recording medium of high capacity, especially, to provide a cassette tape for data backup. SOLUTION: In the magnetic recording medium forming at least one base coat layer and a magnetic layer on one side of a non-magnetic support in this order and having a back coat layer on the opposite side, the thickness of the magnetic layer is from 0.01 to 0.10 μm and the thickness non-uniformity of this magnetic layer is made from 1 to 15%. Thus, the magnetic recording medium of high capacity can be provided. Especially, when an MR head is used, reproducing-out and an output-to-noise ratio are high.

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 having a high recording capacity, an access speed, and a transfer speed, and more particularly to a magnetic recording for data backup using a reproducing head (hereinafter, MR head) using a magnetoresistive element. Regarding the medium.

[0002]

2. Description of the Related Art Magnetic tapes have various uses such as audio tapes, video tapes, and computer tapes. In the field of data backup tapes, in particular, with the increase in the capacity of a hard disk to be backed up, one tape is used.
With a storage capacity of several tens GB per winding, a large capacity backup tape is indispensable in order to cope with a further increase in the capacity of a hard disk in the future. In addition, in order to increase the access speed and the transfer speed, it is indispensable to increase the tape feeding speed and the relative speed between the tape and the head.

In order to increase the capacity per one roll of backup tape, the total thickness of the tape is reduced to increase the length of the tape per roll, and the thickness of the magnetic layer is reduced to reduce the thickness demagnetization. It is necessary to increase the recording density in the width direction by narrowing the track width while shortening the recording wavelength by shortening the recording wavelength. When the thickness of the magnetic layer is reduced, the entire magnetic layer is recorded in saturation, so that variations in the thickness of the magnetic layer are directly linked to variations in output. Also, when the recording density in the width direction is increased, the magnetic flux leaking from the tape is reduced. Therefore, it is necessary to use an MR head capable of obtaining a high output even with a small magnetic flux as the reproducing head. Furthermore, as a magnetic tape corresponding to the increase in the tape feeding speed and the relative speed between the tape and the head, a non-magnetic support, an undercoat layer, optimization of the mechanical properties of the magnetic layer, etc. There is a need for a means to improve the touch.

[0004] Known documents relating to magnetic recording media compatible with MR heads include, for example, JP-A-11-238225, JP-A-2000-40217, and JP-A-200200.
No. 0-40218. These patent publications disclose a magnetic recording medium in which the magnetic flux (product of the residual magnetic flux density and the thickness) of the magnetic recording medium is controlled to a specific value to prevent distortion of the output of the MR head, and a dent on the surface of the magnetic layer. A magnetic recording medium in which the thermal asperity of an MR head is reduced to a specific value or less is disclosed.

[0005]

However, the conventionally known magnetic recording medium has a problem in that the thickness of the magnetic layer is uneven and the MR layer has an MR layer.
There is a problem that the fluctuation of the reproduction output when reproducing with the head is large and the output-to-noise ratio (C / N) is small.

That is, when the thickness of the magnetic layer is extremely thin, such as 0.1 μm or less, the entire magnetic layer is magnetized, and when reproducing using a magnetoresistive head (hereinafter, MR head), the output is a magnetic flux (residual). (The product of the magnetic flux density and the thickness of the magnetic layer), the unevenness in the thickness of the magnetic layer directly changes the output. Therefore, when the thickness of the magnetic layer is as small as 0.1 μm, the thickness unevenness of the magnetic layer needs to be 15% or less, but the thickness unevenness of the magnetic layer obtained by the conventional method is about 20%. It was the limit.

[0007]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have conducted intensive studies. As a result, when the thickness of the magnetic layer is as thin as 0.1 μm or less, the thickness unevenness of the magnetic layer is reduced. In order to do this, it has been clarified that the undercoat layer and the magnetic layer should be dried simultaneously by far-infrared heating. That is, in the conventional heating and drying method, the heat is transmitted from the surface, so that the surface of the magnetic layer is dried first, and when the solvent coming out of the inside evaporates, the magnetic layer is disturbed, and this causes unevenness in the thickness of the magnetic layer. Cause. On the other hand, when dried by heating with far infrared rays, the carbon black of the undercoat layer and the magnetic powder of the magnetic layer are directly heated by far infrared rays. It has been found that even when the layer thickness is as thin as 0.1 μm or less, the thickness unevenness of the magnetic layer becomes 15% or less.

The present invention has been completed based on the above findings. That is, the present invention provides a magnetic recording medium in which at least one undercoat layer and a magnetic layer are formed in this order on one surface of a non-magnetic support, and a back coat layer is formed on the opposite surface. Is 0.01 ~ 0.10μ
m, wherein the thickness variation of the magnetic layer is 1 to 15%, and the thickness of the magnetic layer (d
1) and the thickness ratio (d2 / d1) of the undercoat layer (d2) is 10
A magnetic recording medium characterized by being at least 100 and not more than 100; a magnetic recording medium in which a magnetic recording signal is reproduced by a reproducing head using a magnetoresistive element; A nonmagnetic support of 2 to 5 μm, a thickness of 0.01 to 0.1 μm, a coercive force of 160 to 320 kA / m,
Product of longitudinal residual magnetic flux density and magnetic layer thickness 0.0018μ
A magnetic layer having a thickness of not less than Tm and not more than 0.04 μTm;
a magnetic recording medium having a back coat layer having a thickness of 10 μm and a base coat layer comprising carbon black having a particle size of 10 to 100 nm, based on the weight of all inorganic powders in the base coat layer. The present invention relates to a magnetic recording medium containing 35 to 83% by weight of nonmagnetic iron oxide having a particle size of 0.05 to 0.20 μm and a particle size of 0.05 to 0.20 μm.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS At least one undercoat layer and a magnetic layer having a thickness of 0.01 to 0.10 μm are formed in this order on one surface of a nonmagnetic support, and a back coat layer is formed on the opposite surface. As a result of studying a method for reducing the thickness unevenness of the magnetic layer in a magnetic recording medium having a magnetic recording medium, a magnetic layer formed by forming an undercoat layer on a non-magnetic support, When dried, the thickness of the magnetic layer becomes uneven,
A very small magnetic recording medium is obtained. The smaller the unevenness in the thickness of the magnetic layer, the better, but it is difficult to realize the unevenness in the thickness of the magnetic layer of less than 1%. Is preferably 15% or less. 1-12% is more preferable, 1-10% is still more preferable. Magnetic layer thickness (d
The ratio (d2 / d1) of 1) to the thickness (d2) of the undercoat layer is 1
It is preferably from 0 to 100, more preferably from 20 to 100, even more preferably from 40 to 100. This range is preferable because if it is less than 10, the durability tends to be poor, and if it exceeds 100, the thickness of the entire magnetic recording medium becomes large, and the capacity per roll becomes small.

The thickness of the magnetic layer is preferably 0.01 to 0.10 μm, more preferably 0.02 to 0.10 μm, and more preferably 0.02 to 0.10 μm.
07 μm is more preferred. This range is preferred because
If the thickness of the magnetic layer is less than 0.01, it is difficult to reduce the thickness unevenness to less than 15%, and if it exceeds 0.10 μm, the output is reduced due to thickness loss. The coercive force of the magnetic layer is 1
60 to 320 kA / m is preferable, and 200 to 320 kA.
/ M is more preferred. This range is preferred for 160
If the recording wavelength is shorter than kA / m, the output decreases due to the demagnetizing field when the recording wavelength is shortened. If the recording wavelength exceeds 320 kA / m, it becomes difficult to perform recording by the magnetic head. The product of the residual magnetic flux density in the longitudinal direction and the thickness of the magnetic layer 0.0018μTm or more and 0.04μ
Tm or less, preferably 0.0036 μTm or more and 0.04 μT or less.
m or less, more preferably 0.004 μTm or more and 0.028 μm
Tm or less is more preferable. This range is preferred because
If it is less than 0.0018 μTm, the reproduction output by the MR head is small, and if it exceeds 0.04 μTm, the reproduction output by the MR head tends to be distorted. A magnetic recording medium comprising such a magnetic layer can shorten the recording wavelength,
This is preferable because the reproduction output when reproduced by the R head can be increased, and the distortion of the reproduction output can be reduced and the output-to-noise ratio can be increased.

The thickness of the back coat layer is 0.2 to 0.8 μm.
Is preferred. This range is preferable because when the thickness is less than 0.2 μm, the runnability of the magnetic recording medium deteriorates, and when the thickness exceeds 0.8 μm, the total thickness of the magnetic recording medium becomes thick and the tape length per roll becomes short. is there.

The undercoat layer is composed of 15 to 35% by weight of carbon black having a particle size of 10 to 100 nm and 0.05 to 0.20 μm of a particle size of 10 to 100 nm, based on the weight of the whole inorganic powder in the undercoat layer. It is preferable to contain 35 to 83% by weight of non-magnetic iron oxide because the thickness unevenness of the magnetic layer formed thereon becomes small in wet-on-wet manner. Hereinafter, each component will be described in detail.

<Non-magnetic support> The thickness of the non-magnetic support is
Although different depending on the application, usually, those having a size of 2 to 5 μm are used. More preferably, it is 2.5 to 4.5 μm. When a non-magnetic support having a thickness in this range is used, if the thickness is less than 2 μm, it is difficult to form a film, and the tape strength is reduced, resulting in a 5 μm thickness.
If it exceeds m, the total thickness of the tape becomes thick, and the storage capacity per tape roll becomes small.

The Young's modulus of the non-magnetic support in the longitudinal direction (M
D) is preferably at least 0.13 GPa (1000 kg / mm ² ), more preferably at least 11.14 GPa (1100 kg / mm ² ). Young's modulus of non-magnetic support in longitudinal direction 10.13
The reason why GPa (1000 kg / mm ² ) or more is good is that if the Young's modulus in the longitudinal direction is less than 10.13 GPa (1000 kg / mm ² ), tape running becomes unstable. In the helical scan type, the specific ratio of the Young's modulus in the longitudinal direction / the Young's modulus in the width direction (MD / TD) is preferably 0.60 to 0.80. The ratio of the Young's modulus in the longitudinal direction / the Young's modulus in the width direction is more preferably 0.65 to 0.75. The specific range of the Young's modulus in the longitudinal direction / Young's modulus in the width direction is preferably 0.60 to 0.80. The mechanism is unknown at present if it is less than 0.60 or more than 0.80. This is because the output variation (flatness) between the entry side and the exit side of the track of the magnetic head increases. This variation is minimized when the Young's modulus in the longitudinal direction / Young's modulus in the width direction is around 0.70. Further, in the linear recording type, the Young's modulus in the longitudinal direction / Young's modulus in the width direction is 0.7.
0 to 1.30 is preferred. Non-magnetic supports satisfying such characteristics include a biaxially stretched aromatic polyamide base film and an aromatic polyimide film.

<Undercoat layer> The thickness of the undercoat layer is adjusted so that the ratio (d2 / d1) of the thickness (d1) of the magnetic layer to the thickness of the undercoat layer (d2) becomes 10 or more and 100 or less. It is good to set. 2
0 or more and 100 or less are more preferable, and 40 or more and 100 or less are still more preferable. This range is preferable because, if it is less than 10, the effect of reducing the unevenness of the thickness of the magnetic layer is small, and if it exceeds 100, the total thickness of the magnetic recording medium becomes too thick and the storage capacity per tape roll becomes small. .

To the undercoat layer, carbon black is added for the purpose of improving conductivity, and iron oxide is added for the purpose of controlling the rigidity of the tape. For the undercoat layer, carbon black having a particle size of 10 to 100 nm based on the weight of all the inorganic powders in the undercoat layer is added to the undercoat layer.
5 to 35% by weight, major axis length 0.05 to 0.20 μm, minor axis length 5
When 35 to 83% by weight of non-magnetic iron oxide of ~ 200 nm is contained, it is formed on wet-on-wet,
It is preferable because the thickness unevenness of the magnetic layer dried by far infrared rays is reduced. The non-magnetic iron oxide is usually acicular, but when a granular or amorphous non-magnetic iron oxide is used, the particle size is 5%.
Iron oxide of ~ 200 nm is preferred.

As carbon black (hereinafter also referred to as CB) to be added to the undercoat layer, acetylene black, furnace black, thermal black and the like can be used.
Particles having a particle size of 5 nm to 200 nm are used, and particles having a particle size of 10 to 100 nm are preferable. This range is preferable because, because the carbon black has a structure, it is difficult to disperse CB when the particle size is 10 nm or less, and the smoothness is deteriorated when the particle size is 100 nm or more. The amount of CB to be added depends on the particle size of CB.
5-35% by weight is preferred. This range is preferred because
If it is less than 15% by weight, the effect of improving conductivity is poor, and if it exceeds 35% by weight, the effect is saturated. Particle size 15nm
More preferably, CB having a particle size of 20 to 50 nm is used in an amount of 20 to 30%.
More preferably, it is used by weight. By adding carbon black having such a particle size and amount, the electric resistance is reduced, the occurrence of electrostatic noise and the unevenness in tape running are reduced, and the thickness unevenness of the far-infrared dried magnetic layer is also reduced.

As the non-magnetic iron oxide to be added to the undercoat layer, in the case of a needle shape, the major axis length is 0.05 to 0.20 μm and the minor axis length is 5 μm.
The particle size is preferably from 200 to 200 nm. In the case of granular or amorphous particles, the particle size is preferably from 5 to 200 nm. Needle-like ones are more preferable because the orientation of the magnetic layer is improved. The addition amount is preferably 35 to 83% by weight. The particle size in this range (short axis length in the case of needles) is preferable because if the particle size is less than 5 nm, uniform dispersion is difficult, and if it exceeds 200 nm, unevenness at the interface between the undercoat layer and the magnetic layer increases. . The addition amount in this range is preferable because the effect of improving the strength of the coating film is small when the amount is less than 35% by weight, and the strength of the coating film decreases when the amount exceeds 83% by weight.

Further, as a result of examining the Young's modulus of the coating layer comprising the undercoat layer and the magnetic layer, the Young's modulus of the coating layer has an optimum range, and the Young's modulus of the coating layer is different from the longitudinal direction of the nonmagnetic support. When the average value of the Young's modulus in the width direction is in the range of 40 to 100%, the durability of the tape is large, the touch between the tape and the head is improved, and the output of the magnetic head from the track entry side to the output side is improved. It has been found that variation (flatness) is reduced. The range is more preferably from 50 to 100%, even more preferably from 60 to 90%. This range is preferable because if it is less than 40%, the durability of the coating film becomes small, and if it exceeds 100%, the touch between the tape and the head becomes poor. Note that a control method based on calendar conditions is used to control the Young's modulus of the coating layer including the undercoat layer and the magnetic layer.

The Young's modulus of the undercoat layer is preferably 80 to 99% of the Young's modulus of the magnetic layer. The reason why the undercoat layer Young's modulus is preferably lower than that of the magnetic layer is that the undercoat layer acts as a kind of cushion.

<Lubricant> Lubricants having different roles are used in the coating layer composed of the undercoat layer and the magnetic layer. The undercoat layer contains 0.5 to 4.0% by weight of higher fatty acid based on the whole powder,
It is preferable to contain 0.2 to 3.0% by weight of an ester of a higher fatty acid because the coefficient of friction between the tape and the rotary cylinder is reduced. The higher fatty acid addition in this range is preferable because if it is less than 0.5% by weight, the effect of reducing the friction coefficient is small,
If the amount exceeds 4.0% by weight, the undercoat layer is plasticized and the toughness is lost. Also, the addition of the higher fatty acid ester in this range is preferable if the amount is less than 0.5% by weight, the effect of reducing the coefficient of friction is small, and if it exceeds 3.0% by weight, the amount transferred to the magnetic layer is too large. This is because there are side effects such as sticking of the rotating cylinder.

The magnetic layer has a content of 0.5 to 3.0 with respect to the ferromagnetic powder.
It is preferable to contain the fatty acid amide in an amount of 0.2% by weight and the ester of a higher fatty acid in an amount of 0.2 to 3.0% by weight because the coefficient of friction between the tape and the rotary cylinder is reduced. Fatty acid amides in this range are preferably used at less than 0.2% by weight because direct contact at the interface between the head and the magnetic layer is apt to occur and the effect of preventing seizure is small. When it exceeds 3.0% by weight, bleed-out and drop-out occur. This is because defects such as the above occur. Amides such as palmitic acid and stearic acid can be used as the fatty acid amide. Further, the addition of the higher fatty acid ester in the above range is preferable because if it is less than 0.2% by weight, the effect of reducing the friction coefficient is small, and if it exceeds 3.0% by weight, there are side effects such as sticking of the tape and the rotary cylinder. It is. Note that this does not exclude mutual movement between the lubricant of the magnetic layer and the lubricant of the undercoat layer.

<Magnetic Layer> The thickness of the magnetic layer is as described above.
It is preferably from 0.01 μm to 0.1 μm, more preferably from 0.02 μm to 0.1 μm. This range is preferable because, when the thickness is less than 0.01 μm, the reproduction output by the MR head becomes small due to the small magnetic flux of the magnetic layer, and when it exceeds 0.1 μm, the reproduction output of the MR head becomes easily distorted. As described above, the coercive force of the magnetic layer in the longitudinal direction is 16
0-320 kA / m is preferable, and 200-320 kA / m
m is more preferred. This range is preferred for 160k
If the recording wavelength is shorter than A / m, the output will decrease due to the demagnetizing field. If the recording wavelength exceeds 320 kA / m, it becomes difficult to perform recording with a magnetic head. The product of the residual magnetic flux density in the longitudinal direction and the thickness of the magnetic layer is 0.0018 μTm or more and 0.04 μT.
m or less, preferably from 0.0036 μTm to 0.04 μTm.
The following is more preferable, and 0.004 μTm or more and 0.028 μT
m or less is more preferable. This range is preferably 0.
If it is less than 0018 μTm, the reproduction output by the MR head is small, and if it exceeds 0.04 μTm, the reproduction output by the MR head is easily distorted. A magnetic recording medium comprising such a magnetic layer can shorten the recording wavelength, and furthermore, the MR
This is preferable because the reproduction output when reproducing with a head can be increased, and the distortion of the reproduction output can be reduced and the output-to-noise ratio can be increased.

As the magnetic powder to be added to the magnetic layer, ferromagnetic iron-based metal powder and hexagonal barium ferrite powder are used. The coercive force of the ferromagnetic iron-based metal powder and the hexagonal barium ferrite powder is preferably 160 to 320 kA / m, and the saturation magnetization is 120 to 200 for the ferromagnetic iron-based metal powder.
^{A · m 2 / kg (120~200emu} / g) are preferable,
130-180A · m ² / kg (130-180 emu /
g) is more preferred. For the hexagonal barium ferrite powder, 50 to 70 A · m ² / kg (50 to 70 emu / g) is preferable. The magnetic properties of this magnetic layer and the magnetic properties of the ferromagnetic powder were measured using an external magnetic field of 1.
It refers to a measured value at 28 MA / m (16 kOe).

The average major axis length of the ferromagnetic iron-based metal powder of the present invention is preferably 0.03 to 0.2 μm, more preferably 0.03 to 0.2 μm.
0.18 μm is more preferable, and 0.04 to 0.15 μm is further preferable. This range is preferable because the average major axis length is
When it is less than 0.03 μm, the cohesive force of the magnetic powder increases, and it becomes difficult to disperse the magnetic powder in the coating. When it is more than 0.2 μm, the coercive force decreases, and the particle noise based on the particle size increases. Because it becomes. In the case of hexagonal barium ferrite powder, the plate diameter is 5 to 200 for the same reason.
nm is preferred. The average major axis length and the particle diameter are determined by measuring the particle size of a photograph taken with a scanning electron microscope (SEM) and averaging 100 particles. Further, BET specific surface area of the ferromagnetic iron-based metal powder is preferably at least 35m ² / g, more preferably at least 40 m ² / g, most preferably at least 50 m ² / g. The BET specific surface area of the hexagonal barium ferrite powder is 1 to 100 m ^2.
/ G or more is preferably used.

For the undercoat layer and the magnetic layer, vinyl chloride resin, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-acetic acid There is a combination of at least one selected from a vinyl-maleic anhydride copolymer, a vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer, and nitrocellulose with a polyurethane resin. Among them, it is preferable to use a vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer resin and a polyurethane resin in combination. Polyurethane resins include polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, and the like.

COOH, SO ₃ M, OSO
₂ M, P = O (OM) ₃ , OP = O (OM) ₂ , [M
Is a hydrogen atom, an alkali metal base or an amine salt], OH,
NR'R ", N ⁺ R"'R""R'"" [R ', R ",
R ′ ″, R ″ ″, R ′ ″ ″ represent a hydrogen or hydrocarbon group],
A binder such as a urethane resin made of a polymer having an epoxy group is used. The use of such a binder is
This is because the dispersibility of the magnetic powder and the like is improved as described above.
When used in combination of two or more resins are preferably match the polarity of the functional groups, among them the combination of each other -SO ₃ M group.

These binders are used in an amount of 7 to 50 parts by weight, preferably 10 to 35 parts by weight, based on 100 parts by weight of the ferromagnetic powder. Particularly, as a binder, 5 to 30 parts by weight of a vinyl chloride resin and 2-2 to a polyurethane resin
It is most preferable to use 0 parts by weight in combination.

It is desirable to use together with these binders a thermosetting crosslinking agent that bonds to a functional group or the like contained in the binder to crosslink. Examples of the crosslinking agent include tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, a reaction product of these isocyanates with a compound having a plurality of hydroxyl groups such as trimethylolpropane, and a condensation product of the above isocyanates. Various polyisocyanates are preferred. These crosslinking agents are generally used in a proportion of 10 to 50 parts by weight based on 100 parts by weight of the binder. More preferably, it is 15 to 35 parts by weight.

A conventionally known abrasive can be added to the magnetic layer. As these abrasives, α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond,
Silicon nitride, silicon carbide, titanium carbide, titanium oxide,
Silicon dioxide, boron nitride, and the like having a Moh's hardness of 6 or more can be used alone or in combination. Of these, alumina is particularly preferable because of its high hardness and excellent head cleaning effect with a small amount of addition. As for the particle size of the abrasive, for a magnetic layer as thin as 0.01 to 0.1 μm, the average particle size is usually preferably 0.002 to 0.15 μm, more preferably 0.005 to 0.10 μm. preferable.
The addition amount is preferably 5 to 20% by weight based on the ferromagnetic powder. More preferably, it is 8 to 18% by weight.

Further, conventionally known carbon black (CB) can be added to the magnetic layer of the present invention for the purpose of improving conductivity and surface lubricity. Examples of such CB include acetylene black and furnace black. And thermal black. Particle size of 5 nm to 200
nm is used, and the particle size is 10 nm to 100 nm.
Are preferred. This range is preferred when the particle size is 5
If it is less than nm, it is difficult to disperse CB, and if it is more than 200 nm, it is necessary to add a large amount of CB, and in any case, the surface becomes coarse and the output is reduced. The addition amount is preferably 0.2 to 5% by weight based on the ferromagnetic powder. More preferably, it is 0.5 to 4% by weight.

<Backcoat layer> For the backcoat layer of the present invention, a conventionally known backcoat having a thickness of 0.2 to 0.8 µm can be used for the purpose of improving running properties. The reason why this range is good is that if the thickness is less than 0.2 μm, the effect of improving the running property is insufficient, and
If the thickness exceeds 8 μm, the total thickness of the tape becomes large, and the storage capacity per roll becomes small. As carbon black (CB), acetylene black, furnace black, thermal black and the like can be used. Usually, small particle size carbon and large particle size carbon are used. As the small-diameter carbon, one having a particle diameter of 5 nm to 200 nm is used, and one having a particle diameter of 10 nm to 100 nm is more preferable. This range is more preferable because when the particle size is 10 nm or less, it is difficult to disperse CB, and when the particle size is 100 nm or more, it is necessary to add a large amount of CB. This is because it causes set-off (embossing). When a large particle size carbon having a particle size of 300 to 400 nm and 5 to 15% by weight of the small particle size carbon is used as the large particle size carbon, the surface is not roughened, and the effect of improving the running property is increased. The addition amount of the small particle size carbon and the large particle size carbon is preferably 60 to 98% by weight, more preferably 70 to 95% by weight based on the weight of the inorganic powder. The surface roughness Ra is preferably from 3 to 8 nm, more preferably from 4 to 7 nm.

Further, iron oxide having a particle diameter of 0.1 μm to 0.6 μm is preferably added to the back coat layer for the purpose of improving strength, more preferably 0.2 μm to 0.5 μm. The addition amount is preferably 2 to 40% by weight, more preferably 5 to 30% by weight based on the weight of the inorganic powder.

A cassette tape incorporating the above-described tape has a large capacity per one roll, a high reproduction output and an output-to-noise ratio when an MR reproducing head is used, and has a high reliability as a backup tape for a hard disk drive. High, especially excellent.

[0035]

The present invention will be described below in detail with reference to examples, but the present invention is not limited to these examples. In addition, the part of an Example and a comparative example shows a weight part.

Example 1 << Coating Component for Undercoat Layer >> (1) Iron oxide powder (particle size: 0.11 × 0.02 μm) 68 parts Alumina (gelatinization ratio: 50%, particle size: 0.07 μm) 8 parts Carbon black (particle size: 25 nm) 24 parts Stearic acid 2.0 parts Vinyl chloride copolymer 8.8 parts (contained -SO ₃ Na group: 0.7 × 10 ^-4 equivalents / g) Polyester polyurethane resin 4 .4 parts (Tg: 40 ° C., containing -SO ₃ Na group: 1 × 10 ⁻⁴ equivalent / g) cyclohexanone 25 parts methyl ethyl ketone 40 parts toluene 10 parts (2) butyl stearate 1 part cyclohexanone 70 parts methyl ethyl ketone 50 parts toluene 20 Part (3) polyisocyanate 1.4 parts cyclohexanone 10 parts methyl ethyl ketone 15 parts toluene 10 parts

<< Coating Component for Magnetic Layer >> (1) Ferromagnetic iron-based metal powder (Co / Fe: 30% by weight, Y / (Fe + Co): 3% by weight, Al / (Fe + Co): 5% by weight, Ca / Fe: 0% by weight, σs: 155 A · m ² / kg, Hc: 188.2 kA / m, pH: 9.4, major axis length: 0.10 μm) 100 parts Vinyl chloride-hydroxypropyl acrylate copolymer 12. 3 parts (content -SO ₃ Na group: 0.7 × 10 ^-4 equivalents / g) Polyester polyurethane resin 5.5 parts (content -SO ₃ Na group: 1.0 × 10 ^-4 equivalents / g) α-alumina (Average particle diameter: 0.2 μm) 8 parts α-alumina (Average particle diameter: 0.07 μm) 2 parts Carbon black 2.0 parts (Average particle diameter: 75 nm, DBP oil absorption: 72 cc / 100 g) Methyl acid phosphate 2 Part palmitic acid amide 1.5 part n-butyl stearate 1.0 65 parts Methyl ethyl ketone 245 parts 85 parts of toluene tetrahydrofuran (2) Polyisocyanate 2.0 parts Cyclohexanone 167 parts

After kneading (1) in the above undercoat layer coating component with a kneader, adding (2), stirring, and performing a dispersion treatment with a sand mill with a residence time of 60 minutes, and adding (3) to this. After stirring and filtration, the mixture was used as a coating for an undercoat layer.
Separately, the above-mentioned magnetic layer coating component (1) is kneaded with a kneader, dispersed by a sand mill with a residence time of 45 minutes, and the magnetic layer coating component (2) is added thereto, followed by stirring and filtration. It was a magnetic paint. The above undercoat paint was coated with an aromatic polyamide film (thickness: 3.9 μm, MD = 11GP).
a, MD / TD = 0.70, trade name: Mictron, manufactured by Toray Industries Co., Ltd.)
The magnetic coating material is further applied on the undercoat layer by wet-on-wet such that the magnetic layer after magnetic field orientation treatment, drying and calendering treatment has a thickness of 0.08 μm. After the magnetic field orientation treatment, the resultant was dried using a dryer and far-infrared rays to obtain a magnetic sheet. The magnetic field orientation treatment is performed by installing an N-N counter magnet (5 kG) before the dryer, and in the dryer, on the near side of the finger coating dry position of the coating film 75.
Two NN counter magnets (5 kG) were installed at 50 cm intervals from cm. The coating speed was 100 m / min.

<< Coating composition for back coat layer >> Carbon black (particle size: 25 nm) 80 parts Carbon black (particle size: 370 nm) 10 parts Iron oxide (particle size: 0.4 μm) 10 parts Nitrocellulose 45 parts Polyurethane resin ( (Containing SO ₃ Na group) 30 parts Cyclohexanone 260 parts Toluene 260 parts Methyl ethyl ketone 525 parts

After the above-mentioned coating composition for a backcoat layer was dispersed in a sand mill with a residence time of 45 minutes, 15 parts of polyisocyanate was added to prepare a coating for the backcoat layer, followed by filtration. On the other side, apply so that the thickness after drying and calendering becomes 0.5 μm,
Dried.

The magnetic sheet thus obtained was heated at a temperature of 100.degree.
After being mirror-finished at 50 kg / cm and aged at 70 ° C. for 72 hours with the magnetic sheet wound on a core,
The surface of the magnetic layer was subjected to post-processing such as lapping tape polishing, blade polishing and surface wiping while running at 200 m / min to produce a magnetic tape. At this time, K10000 was used for the wrapping tape, a carbide blade was used for the blade, and Toraysee was used for wiping the surface, and the treatment was performed with a running tension of 30 g. The magnetic tape obtained as described above was assembled in a cartridge to produce a computer tape.

Examples 2 to 8 Example 1 was repeated except that some of the conditions were changed to those shown in Table 1.
In the same manner as in Example 1, computer tapes of Examples 2 to 8 were produced.

Comparative Examples 1 to 4 Example 1 was repeated except that some of the conditions were changed to those shown in Table 1.
In the same manner as in the above, computer tapes of Comparative Examples 1 to 4 were produced.

The evaluation was performed as follows. Table 1
Shows the evaluation results. <Magnetic Layer Thickness (d1) and Undercoat Layer Thickness (d2)> The thickness was measured by filling a magnetic tape with a resin, cutting it out with a diamond cutter, and observing the cross section with a transmission electron microscope to measure the thickness. From the average value, the magnetic layer thickness (d
1) and the thickness of the undercoat layer (d2) were determined.

<Magnetic Layer Thickness Unevenness> The magnetic layer thickness unevenness is determined by calculating the maximum thickness M (ma) when calculating the magnetic layer thickness (d1).
x), the minimum value is defined as M (min), and is defined as follows. Magnetic layer thickness unevenness (%) = | M (max) −M (min) | / d1
× 100

<Surface Roughness of Magnetic Surface> The surface roughness of the magnetic surface was measured using an optical interference three-dimensional surface roughness meter.

<Reproduction Output and Output-to-Noise> The reproduction output and output-to-noise were determined by recording (recording wavelength 0.37 μm) and reproducing using an LTO drive modified for thin tape. The reproduction output and the output-to-noise are values when the tape of Comparative Example 1 is set to 0 dB.

[0048]

[Table 1]

[0049]

Examples 1 to 8 and Comparative Examples 1 to 4 in Table 1
As is clear from FIG. 7, the thickness of the magnetic layer is 0.01 to 0.10 μm.
m, the magnetic recording medium of the present invention in which the thickness unevenness of the magnetic layer is 1 to 15% is an excellent magnetic recording medium having a high storage capacity. In particular, the reproduction output and the output-to-noise ratio when an MR head is used are high, and it is excellent as a backup tape for a hard disk or the like.

Claims (5)

[Claims] 1. A magnetic recording medium having at least one undercoat layer and a magnetic layer formed on one surface of a non-magnetic support in this order, and having a back coat layer on the other surface. Is 0.01 to 0.10 μm, and the thickness unevenness of the magnetic layer is 1 to 15%. 2. The thickness (d1) of the magnetic layer and the undercoat layer (d2)
The magnetic recording medium according to claim 1, wherein a ratio (d2 / d1) to a thickness of the magnetic recording medium is 10 or more and 100 or less. 3. The magnetic recording medium according to claim 1, wherein a magnetic recording signal is reproduced by a reproducing head using a magnetoresistive element. 4. A nonmagnetic support having a thickness of 2 to 5 μm, a magnetic layer having a coercive force of 160 to 320 kA / m, and a product of a residual magnetic flux density in the longitudinal direction and a thickness of 0.0018 μTm or more and 0.04 μTm or less. 4. The magnetic recording medium according to claim 1, further comprising a backcoat layer having a thickness of 0.2 to 0.8 [mu] m. 5. The undercoat layer is 15 to 35% by weight of carbon black having a particle size of 10 to 100 nm, based on the weight of all inorganic powders in the undercoat layer, and has a particle size of 0.05 to 0.5%. The magnetic recording medium according to any one of claims 1 to 4, wherein the magnetic recording medium contains 35 to 83% by weight of nonmagnetic iron oxide of 20 µm.