JPH0991682A - Magnetic recording media - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To prevent the occurrence of so-called powder drop and tape pass shift of the back coat layer. According to a first aspect of the present invention, in a coating material for forming a back coat layer, at least carbon black having an average particle diameter of 0.01 to 0.05 μm and carbon black having an average particle diameter of 0.2 to 0.5 μm is used. An average particle size of 0.2 to 2 including two types and a sum of the two types of carbon black
The ratio of 0.5 μm carbon black is 4 to 1 by weight.
It is in the range of 2%. The second invention is such that the ten-point average roughness (SRz) of the surface of the back coat layer is in the range of 0.2 to 0.3 μm, and the friction coefficient (μ) is 0.13 to 0.18.
In the range.

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 magnetic layer and a back coat layer on opposite sides of a non-magnetic support.

[0002]

2. Description of the Related Art Conventionally, as a magnetic recording medium, a powder magnetic material such as an oxide magnetic powder or an alloy magnetic powder is used as a vinyl chloride-vinyl acetate copolymer, a polyester resin,
2. Description of the Related Art A coating type magnetic recording medium prepared by coating a magnetic coating material dispersed in an organic binder such as urethane resin or polyurethane resin on a non-magnetic support and drying it is widely used.

On the other hand, in the field of video tape recorders (VTRs) and the like, there is a strong demand for high-density magnetic recording in order to achieve high image quality, and as a magnetic recording medium corresponding thereto, Co-Ni alloy, C
Non-magnetic support of polyester film, polyamide, polyimide film, etc. for metal magnetic materials such as o-Cr alloys and Co-O based on plating or vacuum thin film forming technology (vacuum evaporation method, sputtering method, ion plating method, etc.) A so-called ferromagnetic metal thin film coating type magnetic recording medium, which is directly deposited on the body as a magnetic layer, has been proposed and has attracted attention.

In any of the above magnetic recording media, a back coat layer is formed in order to positively control the characteristics of the surface of the magnetic recording medium opposite to the magnetic layer. This back coat layer (1) lowers the electric resistance of the surface of the magnetic recording medium to prevent problems due to charging of the magnetic recording medium, and (2) prevents so-called fast-forwarding of the magnetic recording medium and disturbance of winding during rewinding. To prevent winding defects during long-term storage and transportation, (3) To increase the durability of the surface of the base film, which is a non-magnetic support, to prevent scratches during use, and to prevent dropouts. (4) The film is formed to reduce friction between magnetic recording media.

[0005]

By the way, the magnetic recording in recent years is in the direction of higher density, the magnetic recording medium is thinned to improve the electromagnetic conversion characteristics, and the surface of the magnetic recording medium is a mirror surface. There is a direction of change.

In the tendency of such a magnetic recording medium, the influence of spacing loss in sliding of the magnetic head is increasing. Therefore, the sliding of the magnetic head may cause so-called "powder drop". Such "powder drop" has a problem of causing a clog phenomenon (clogging) of the magnetic head.

On the other hand, in a path system of a video tape recorder (VTR), so-called "tape path shift" occurs in a VTR travel guide pin (also referred to as "guide pole") or a guide roll due to repeated pass travel. There is also a problem that the tape receives stress from the guide edge, and sometimes the magnetic tape rides on the guide pin or the like, which deteriorates the shape of the tape and prevents normal recording and reproduction.

[0008] These "powder drop" and "tape pass gap"
Such a problem is considered to be related to tape friction, but it is difficult to control the surface roughness and friction of the magnetic recording medium.

For thinning the magnetic tape,
Although studies have been made to reduce the thickness of the non-magnetic support, the mechanical strength becomes a problem, and the recent trend is that the ratio of the backcoat layer thickness to the magnetic tape thickness Is getting higher.

However, in a magnetic recording medium having a magnetic layer and a back coat layer on the opposite surfaces of a conventional non-magnetic support, it is not always sufficient to prevent powder from the back coat layer and tape misalignment. Was not what I could say.

Therefore, the present invention has been proposed in view of the above circumstances, and it is possible to prevent the so-called powder drop and the tape pass deviation of the back coat layer from occurring, so that the reliability is high and the running stability is high. It is an object of the present invention to provide an excellent magnetic recording medium.

[0012]

DISCLOSURE OF THE INVENTION As a result of intensive studies for achieving the above-mentioned object, the inventors of the present invention have found that the problems of powder falling due to sliding of a magnetic head and tape pass deviation are related to tape friction. Therefore, the present invention has been completed.

That is, the magnetic recording medium of the first invention of the present application is a coating material for forming a back coat layer in a magnetic recording medium having a magnetic layer and a back coat layer on opposite surfaces of a non-magnetic support. , Average particle diameter 0.01 to 0.
05 μm carbon black and average particle size 0.2-0.5
containing at least two types of carbon black having a particle size of 0.2 μm, and the ratio of carbon black having an average particle size of 0.2 to 0.5 μm to the total of the two types of carbon black is in the range of 4 to 12% by weight. Is characterized by.

Here, the thickness of the back coat layer is 0.5 to
It is preferably in the range of 1.0 μm.

When the thickness of the back coat layer is out of the range of 0.5 to 1.0, running durability or powder falling (clogging) is deteriorated.

Similarly, the backcoat layer has a thickness of 0.5 to
Although it is in the range of 1.0, the running durability also when the ratio of the coarse particle carbon black is out of the range of 4 to 12%, or
The powder drop becomes worse.

Next, a magnetic recording medium according to the second invention of the present application,
In a magnetic recording medium having a magnetic layer and a back coat layer on opposite sides of a non-magnetic support, and the back coat layer containing an inorganic powder dispersed in a binder, ten points on the surface of the back coat layer. Average roughness (SRz) is 0.2 to 0.3
It is characterized in that it is in the range of μm and the friction coefficient (μ) represented by the following mathematical formula is in the range of 0.13 to 0.18.

Friction coefficient (μ) = (2 / π) × Ln (T2 / T1) Here, the surface of the back coat layer and the guide pin are 50 gr.
(T1) at a winding angle of 90 degrees, and the tension (T2) required to run the magnetic recording medium at a speed of 20 mm / sec under this condition.

According to the second invention, as in the first invention, the coating material for forming the back coat layer has an average particle diameter of 0.0
1 to 0.05 μm carbon black and average particle size 0.2
By including at least two kinds of carbon black of 0.5 μm to 0.5 μm, the ten-point average roughness (S
It may satisfy the conditions of Rz) and the friction coefficient (μ), but may not necessarily contain two kinds of carbon black as long as it is within the above range.

The friction coefficient (μ) of the stainless steel guide pin is preferably in the range of 0.13 to 0.18.

If the ten-point average roughness (SRz) is less than 0.2 and the friction coefficient (μ) is more than 0.18, the running durability will be poor.

If the ten-point average roughness (SRz) is 0.3 or more and the coefficient of friction (μ) is 0.13 or less, the reliability due to powder falling will be poor.

First, in the magnetic recording medium of the first invention of the present application, in the magnetic recording medium having the magnetic layer and the back coat layer on the surfaces opposite to each other with respect to the non-magnetic support, the average particle diameter is 0.01 to 0. Carbon black having an average particle size of 0.2 to 0.5 μm based on the total amount of the two types of carbon black, which contains at least two kinds of carbon black having an average particle size of 0.2 to 0.5 μm. The ratio is in the range of 4 to 12% by weight.

Thus, the average particle size is 0.01 to 0.05
Carbon black with an average particle size of 0.2-0.5μ
By mixing at least two kinds of carbon black of m, it is possible to prevent powder drop and tape pass deviation.

Next, in the magnetic recording medium of the second invention of the present application, the ten-point average roughness (SR) of the surface of the back coat layer is
Since z) is in the range of 0.2 to 0.3 μm and the friction coefficient (μ) is in the range of 0.13 to 0.18, powder drop and tape pass deviation are prevented.

The ten-point average roughness (S
Even when Rz) is out of the range of 0.2 to 0.3 μm, running durability or powder falling is deteriorated. Also, when the friction coefficient (μ) is out of the range of 0.13 to 0.18, running durability or powder falling is deteriorated.

In any of the above cases, when forming the backcoat layer, the dispersion and kneading for preparing the coating material for the backcoat layer are performed, for example, by a roll mill, a ball mill, a sand mill, an agitator, a kneader, an extruder, a homogenizer, An ultrasonic disperser or the like is used.

Further, in order to coat the back coat layer coating composition thus prepared on the back surface of the non-magnetic support, a gravure coater, knife coater, wire bar coater, doctor blade coater, reverse roll coater, dipping A coater, an air knife coater, a die coater, etc. are used.

In the magnetic recording medium of the present invention, for example, as a constituent material of the magnetic layer or the back coat layer, any conventionally known material can be used and is not limited at all.

[0030]

EXAMPLES The present invention will be described below with reference to specific examples, but it goes without saying that the present invention is not limited to these examples.

Example of the First Invention (Example 1) First, a magnetic layer was formed on a non-magnetic support made of a polyene ethylene terephthalate (PET) film having a thickness of 14 μm and a width of 1/2 inch. . This magnetic layer is composed of a composition containing Co-containing iron oxide dispersed in a vinyl chloride-vinyl acetate copolymer, a polyurethane resin, and a binder mainly composed of polyisocyanate, and a composition containing carbon black, an abrasive and a lubricant. Is a coating film having a thickness of 4 μm and the center line average roughness (SRa) of the surface is 0.05.
μm.

Next, the following composition excluding polyisocyanate and myristic acid was thoroughly kneaded and dispersed in a sand mill, and then polyisocyanate-myristic acid was added to the back surface of a magnetic recording tape having a magnetic layer on the surface thereof. It was applied and dried to form a back coat layer having a thickness of 0.6 to 1.0 μm.

Carbon black (trade name: MT-CI, manufactured by Columbian Carbon Co., average particle size: 0.35 μm) 4 parts by weight Carbon black (trade name: BP-L, average particle size: 0.02 μm manufactured by Cabot) 96 parts by weight Polyurethane (Brand name: MAU-4308HV, manufactured by Dainichiseika Co., Ltd.) 50 parts by weight Polyisocyanate (Brand name: Coronate L, manufactured by Nippon Polyurethane Company) 10 parts by weight Methyl ethyl ketone myristate 50 parts by weight Methyl isobutyl ketone 200 parts by weight Toluene 350 parts by weight Parts (Example 2) carbon black (trade name: MT-CI, manufactured by Columbian Carbon Co., average particle size 0.35 μm) 8 parts by weight carbon black (trade name: BP-L, average particle size 0.02 μm manufactured by Cabot) 92 parts by weight of carbon black Was changed to an amount shown in the to obtain a magnetic tape in the same manner as in Example 1.

Example 3 Carbon black (trade name: MT-CI, manufactured by Columbian Carbon Co., average particle size 0.35 μm) 8 parts by weight carbon black (trade name: BP-L, average particle size 0.02 μm manufactured by Cabot) ) 92 parts by weight A magnetic tape was obtained in the same manner as in Example 1 except that the amount of carbon black was changed to that shown above.

Example 4 Carbon black (trade name: MT-CI, manufactured by Columbia Carbon Co., average particle size: 0.35 μm) 12 parts by weight carbon black (trade name: BP-L, manufactured by Cabot, average particle size: 0.02 μ) m) 88 parts by weight A magnetic tape was obtained in the same manner as in Example 1 except that the amount of carbon black was changed to that shown above.

(Comparative Example 1) Carbon black (trade name: BP-L, Cabot, average particle size 0.02 μm) 100 parts by weight Example 1 except that the amount of carbon black was changed to that shown above. A magnetic tape was obtained in the same manner.

Comparative Example 2 Carbon black (trade name: MT-CI, manufactured by Columbia Carbon Co., average particle size: 0.35 μm) 2 parts by weight Carbon black (trade name: BP-L, manufactured by Cabot, average particle size: 0.02) μm) 98 parts by weight A magnetic tape was obtained in the same manner as in Example 1 except that the amount of carbon black was changed to that shown above.

COMPARATIVE EXAMPLE 3 Carbon black (trade name: MT-CI. Columbian Carbon, manufactured by average particle size: 0.35 μm) 15 parts by weight Carbon black (trade name: BP-L, manufactured by Cabot, average particle size: 0.02) μm) 85 parts by weight A magnetic tape was obtained in the same manner as in Example 1 except that the amount of carbon black was changed to that shown above.

Comparative Example 4 Carbon black (trade name: MT-CI, manufactured by CoIumbian Carbon, average diameter 0.35 μm) 20 parts by weight carbon black (trade name: BP-L, average particle diameter, Cabot Co. 0.02 μ) m) 80 parts by weight A magnetic tape was obtained in the same manner as in Example 1 except that the amount of carbon black was changed to the above amount and the thickness of the back coat layer was changed to 0.7 to 0.8 μm.

The parts by weight of carbon black and the thickness of the back coat layer of each material are summarized in Table 1.

[0041]

[Table 1]

The following tests were conducted on each material of the above magnetic recording tape, and the results shown in Table 2 were obtained.

(Test Method 1) The friction coefficient (μ) of the sample was evaluated by the following method.

The surface of the back coat layer of each sample was brought into contact with the guide pin at a tension (T1) of 50 gr and a winding angle of 90 degrees, and under this condition, the sample tape was required to run at a speed of 20 mm / sec. The tension (T2) was measured.

The coefficient of friction (μ) was determined by the following formula based on the measurement results. The coefficient of friction (μ) is 2
It was carried out under the environmental conditions of 2 ° C. and relative humidity of 65% RH.

Friction coefficient (μ) = (2 / π) × Ln (T2 / T1) Here, the surface of the back coat layer and the guide pin are 50 gr.
(T1) at a winding angle of 90 degrees, and the tension (T2) required to run the magnetic recording medium at a speed of 20 mm / sec under this condition.

The friction coefficient (μ) was measured in the initial state of the sample tape and VHS (Video Home System).
m) Measurement was performed after 100 passes on the video deck.

(Test Method 2) The running durability of the sample was evaluated by the following method.

After recording 50 IRE signals on a VHS VCR, and observing the video output signals at the time of continuous reproduction 100 times with an oscilloscope, further observing the damage conditions at the edges of the tape, the following criteria were used for evaluation. did.

"○": No tape damage was observed and the output waveform was normal.

"A": Light tape damage is observed, but output waveform is normal.

"X" A large tape damage was observed,
Something is wrong with the output waveform.

(Test Method 3) The powder drop of the sample was evaluated by the following method.

T-120 (12
(0 minute length) is continuously repeated 100 times, and whether or not the prerecorded signal is not displayed on the monitor due to the head message, and the powder falling state near the head tip after 100 times running It was observed and evaluated according to the following criteria.

"○" No head clogging, no head chip powder drop, or a very small amount.

"△" Head does not become clogged,
A lot of powder from the head tip is removed.

"X" A head clogging occurred.

The center line roughness of the surface of the magnetic layer and the surface of the back coat layer (SRa was measured by using a three-dimensional roughness meter "ET-30HK" manufactured by Kosaka Seisakusho Co., Ltd.
utoff) value.

[0059]

[Table 2]

As is clear from the results shown in Tables 1 and 2,
A combination of fine particle carbon black having an average particle size of 0.01 to 0.05 μm and coarse particle carbon black having an average particle size of 0.2 to 0.5 μm provides an average particle size of 0.2 to 0.5 μm in the carbon black. In Examples 1 to 5 in which the coarse particle ratio is in the range of 4 to 12%, all the tape characteristics are good, no tape damage is observed, the output waveform is not abnormal, and the head is clogged. There is no powder drop on the head chip. Therefore, it is excellent in running durability and reliability.

On the other hand, the average particle size is 0.2 to 0.5 μm.
Comparative Example 1 and Comparative Example 2 in which the ratio of coarse particles of m is less than 4%
Has a high friction coefficient (μ) and is inferior in running durability. Further, the ratio of coarse particles having an average particle diameter of 0.2 to 0.5 μm is 12%.
In Comparative Example 5 and Comparative Example 6 in which the value exceeds the range, reliability due to powder falling is poor.

As is clear from these results, the back coat layer contains at least two kinds of fine particle carbon black having an average particle size of 0.01 to 0.05 μm and coarse particle carbon black having an average particle size of 0.2 to 0.5 μm, Moreover, it can be seen that a magnetic recording medium having both excellent running durability and reliability can be obtained by setting the ratio of the coarse particle carbon black within the range of 4 to 12% by weight.

Next, the effect of the thickness of the back coat layer on the above results was tested. That is, this is to see the effect of the thickness of the back coat layer on powder falling and tape pass deviation.

First, the following examples and comparative examples were prepared.

Example 5 Carbon black (trade name: MT-CI, manufactured by Columbia Carbon Co., average particle size: 0.35 μm) 4 parts by weight carbon black (trade name: BP-L, manufactured by Cabot, average particle size: 0.02 μ) m) 96 parts by weight Polyurethane (trade name: MAU-4308HV, manufactured by Dainichiseika) 50 parts by weight Polyisocyanate (trade name: Coronate L, manufactured by Nippon Polyurethane Co., Ltd.) 10 parts by weight Methyl ethyl ketone myristate 50 parts by weight Methyl isobutyl ketone 200 Parts by weight Toluene 350 parts by weight The thickness of the back coat layer is 0.5 to 0.6 μm.
It is.

(Example 6) The thickness of the back coat layer was adjusted to 0.
A magnetic tape was obtained in the same manner as in Example 5 except that the thickness was 8 to 0.9 μm.

(Example 7) Carbon black (trade name: MT-CI, manufactured by Columbia Carbon Co., average particle size 0.35 μm) 8 parts by weight carbon black (trade name: BP-L, manufactured by Cabot, average particle size 0.02 μ) m) 92 parts by weight A magnetic tape was obtained in the same manner as in Example 5 except that the amount of carbon black was changed to the amount shown above and the thickness of the back coat layer was changed to 0.6 to 0.7 μm.

Example 8 Carbon black (trade name: MT-CI, manufactured by Columbia Carbon Co., average particle size 0.35 μm) 8 parts by weight carbon black (trade name: BP-L, average particle size 0.02 μm manufactured by Cabot) ) 92 parts by weight A magnetic tape was obtained in the same manner as in Example 5, except that the amount of carbon black was changed to the amount shown above and the thickness of the back coat layer was changed to 0.9 to 1.0 μm.

(Example 9) Carbon black (trade name: MT-CI, manufactured by Columbia Carbon Co., average particle size 0.35 μm) 12 parts by weight carbon black (trade name: BP-L, manufactured by Cabot, average particle size 0.02 μ) m) 88 parts by weight A magnetic tape was obtained in the same manner as in Example 5, except that the amount of carbon black was changed to the amount shown above and the thickness of the back coat layer was changed to 0.9 to 1.0 μm.

(Comparative Example 5) Carbon black (trade name: BP-L, made by Cabot, average particle size 0.02 μm) 100 parts by weight Carbon black was changed to the amount shown above, and the thickness of the back coat layer was changed. A magnetic tape was obtained in the same manner as in Example 5, except that the thickness was 0.7 to 0.8 μm.

(Comparative Example 6) Carbon black (trade name: MT-CI, manufactured by Columbia Carbon, average particle size: 0.35 μm) 4 parts by weight carbon black (trade name: BP-L, manufactured by Cabot, average particle size: 0.02) μm) 96 parts by weight A magnetic tape was obtained in the same manner as in Example 5 except that the amount of carbon black was changed to the amount shown above and the thickness of the back coat layer was changed to 0.3 to 0.4 μm.

(Comparative Example 7) Carbon black (trade name: MT-CI. Columbian Carbon, manufactured by average particle size 0.35 μm) 4 parts by weight Carbon black (trade name: BP-L, manufactured by Cabot, average particle size 0.02) μm) 96 parts by weight A magnetic tape was obtained in the same manner as in Example 5, except that the amount of carbon black was changed to the amount shown above and the thickness of the back coat layer was changed to 1.5 to 1.6 μm.

(Comparative Example 8) Carbon black (trade name: MT-CI, manufactured by Columbia Carbon, average particle size: 0.35 μm) 15 parts by weight carbon black (trade name: manufactured by BP-Cabot, average particle size: 0.02 μm) 85 Parts by Weight A magnetic tape was obtained in the same manner as in Example 5 except that the amount of carbon black was changed to the above amount and the thickness of the back coat layer was 0.4 to 0.5 μm.

Comparative Example 9 Carbon black (trade name: MT-CI, manufactured by CoIumbian Carbon, average diameter 0.35 μm) 20 parts by weight Carbon black (trade name: BP-L, manufactured by Cabot, average particle diameter 0.02 μ) m) 80 parts by weight A magnetic tape was obtained in the same manner as in Example 5 except that the amount of carbon black was changed to the amount shown above and the thickness of the back coat layer was changed to 0.7 to 0.8 μm.

The parts by weight of carbon black and the thickness of the back coat layer of each material are summarized in Table 3.

[0076]

[Table 3]

The above test methods 1 to 3 were performed on each material of the magnetic recording tape, and the results shown in Table 4 were obtained.

[0078]

[Table 4]

Next, the ratio of coarse particle carbon black is 4
The thickness of the back coat layer is 0.
In Comparative Example 6 and Comparative Example 7, which are out of the range of 5 to 1.0, running durability or powder falling (clogging) is deteriorated.

Similarly, the thickness of the back coat layer is 0.5 to
In Comparative Example 8 and Comparative Example 9 in which the ratio of the coarse particle carbon black is out of the range of 4 to 12% in the range of 1.0, running durability or powder falling (clogging) is deteriorated.

As is clear from these results, the back coat layer contains at least two kinds of fine particle carbon black having an average particle diameter of 0.01 to 0.05 μm and coarse particle carbon black having an average particle diameter of 0.2 to 0.5 μm, Moreover, by setting the ratio of the coarse particle carbon black in the range of 4 to 12% by weight and the thickness of the back coat layer in the range of 0.5 to 1.0, excellent running durability, It can be seen that it is possible to obtain a magnetic recording medium having both reliability.

Example of the Second Invention (Example 10) Carbon black (trade name: MT-CI, Columbia Carbon Co., manufacture average particle size 0.35 μm) 8 parts by weight carbon black (trade name: BP-L, Cabot Co.) Made average particle size 0.02 μm) 92 parts by weight Polyurethane (trade name: MAU-4308HV, manufactured by Dainichiseika Co., Ltd.) 50 parts by weight Polyisocyanate (trade name: Coronate L, manufactured by Nippon Polyurethane Co., Ltd.) 10 parts by weight Methyl ethyl ketone ketone 50 parts by weight Methyl isobutyl ketone 200 parts by weight Toluene 350 parts by weight About the thickness of the back coat layer, 0.5 to 0.6 μm
It is.

Example 11 In Example 10, carbon black (trade name: MT-CI, manufactured by Columbia Carbon Co., Ltd., average particle size: 0.35 μm) 8 parts by weight carbon black (trade name: BP-L, manufactured by Cabot Co., Ltd.) A magnetic tape was obtained in the same manner as in Example 10 except that the charging ratio was changed to 92 parts by weight with an average particle diameter of 0.02 μm).

Example 12 In Example 10, carbon black (trade name: MT-CI, manufactured by Columbian Carbon Co., average particle size: 0.35 μm) 10 parts by weight carbon black (trade name: BP-L, average manufactured by Cabot Co., Ltd.) A magnetic tape was obtained in the same manner as in Example 6 except that the charging ratio was changed to 90 parts by weight of particle size 0.02 μm).

(Example 13) In Example 10, except that the thickness of the back coat layer was 0.4 to 0.5 µm.
A magnetic tape was obtained in the same manner as in Example 10.

(Comparative Example 10) A magnetic tape was obtained in the same manner as in Example 10 except that the carbon black in Example 10 was changed to 100 parts by weight of carbon black (trade name: BP-L, manufactured by Cabot). .

Comparative Example 11 A magnetic tape was obtained under the same conditions as in Example 10, except that the thickness of the back coat layer was 1.3 to 1.5 μm. Test Method 4 (Comparative Example 12) Carbon black (trade name: MT-CI, manufactured by Columbia Carbon Co., average particle size 0.35 μm) in Example 10 was used in an amount of 2 parts by weight of carbon black (trade name: BP-L, Cabot). 98 parts by weight (average particle size 0.02 μm, manufactured by the company) (Comparative Example 13) In Example 10, the carbon black was replaced with carbon black (trade name: MT-CI, manufactured by Columbia Carbon Co., Ltd. average particle size 0.35 μm) 15 parts by weight carbon A magnetic tape was obtained in the same manner as in Example 10 except that the charging ratio was changed to 85 parts by weight of black (trade name: BP-L, average particle size 0.02 μm manufactured by Cabot).

Comparative Example 14 In Example 10, 20 parts by weight of carbon black (trade name: MT-CI, manufactured by Columbia Carbon Co., average particle size: 0.35 μm) was used. Carbon black (trade name: BP-L, Cabot) A magnetic tape was obtained in the same manner as in Example 10 except that the charging ratio was changed to 80 parts by weight (average particle size manufactured by the company: 0.02 μm).

Next, regarding the surface property of the surface of the back coat layer of the magnetic tape, a ten-point average roughness (SR) was measured using a three-dimensional roughness meter "ET-30HK" manufactured by Kosaka Seisakusho KK
z) was measured. The cutoff value at that time was 80 μm. The guide pin was made of stainless steel.

[0090]

[Table 5]

As is clear from the results of Table 5, the ten-point average roughness (SRz) is in the range of 0.2 to 0.3 μm, and the friction coefficient (μ) is 0.13 to 0.18. All of the characteristics of Examples 10 to 13 in between are good,
It can be seen that it has excellent running durability and reliability.

On the other hand, Comparative Examples 10 to 12
Has a ten-point average roughness (SRz) of less than 0.2 and a coefficient of friction (μ) of more than 0.18, and the durability due to running is poor.

Further, in Comparative Examples 10 and 11, the ten-point average roughness (SRz) is 0.3 or more and the friction coefficient (μ) is 0.13 or less, and the reliability due to powder falling may be poor. I understand.

As is clear from these results, the ten-point average roughness (SRz) of the surface of the backcoat layer was 0.2 to 10.
Since the friction coefficient (μ) with respect to the stainless guide pin is within the range of 0.3 μm to 0.18 in the range of 0.3 μm, the tape is not damaged and the head chip is not dusted. I understand.

In the above-mentioned second embodiment of the present invention, the coating material for forming the back coat layer has an average particle diameter of 0.01 to
0.05 μm carbon black and average particle size 0.2-
By including at least two kinds of 0.5 μm carbon black, the ten-point average roughness (SR
z) and the coefficient of friction (μ) have been described, but if the ten-point average roughness (SRz) and the coefficient of friction (μ) are within the above ranges, two types of carbon black are necessarily included. It goes without saying that it is not necessary.

[0096]

As described above, the magnetic recording medium according to the first invention can be used as a coating material for forming a back coat layer,
It contains at least two kinds of carbon black having an average particle size of 0.01 to 0.05 μm and carbon black having an average particle size of 0.2 to 0.5 μm, and has an average particle size of 0. Since the ratio of carbon black of 2 to 0.5 μm is in the range of 4 to 12% by weight, so-called powder falling of the back coat layer and tape pass deviation are prevented.

On the other hand, in the magnetic recording medium according to the second invention, the ten-point average roughness (SRz) of the surface of the back coat layer is in the range of 0.2 to 0.3 μm, and the friction coefficient (μ). Is in the range of 0.13 to 0.18,
The so-called powder falling of the back coat layer and the occurrence of tape pass deviation are prevented.

Therefore, the magnetic recording media of any of the inventions have high reliability and excellent running stability.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hideaki Takahashi 1-6-3 Nihombashi Muromachi, Chuo-ku, Tokyo Sony Chemical Co., Ltd.

Claims (4)

[Claims] 1. In a magnetic recording medium having a magnetic layer and a back coat layer on opposite sides of a non-magnetic support, a paint for forming the back coat layer has an average particle size of 0.
01-0.05 μm carbon black and an average particle size of 0.1.
2 to 0.5 μm of at least two types of carbon black, and the ratio of carbon black having an average particle size of 0.2 to 0.5 μm to the total of the two types of carbon black is 4 to 12% by weight. A magnetic recording medium characterized by a range. 2. The thickness of the back coat layer is 0.5 to 1.0.
The magnetic recording medium according to claim 1, wherein the magnetic recording medium is in the range of μm. 3. A magnetic recording medium having a magnetic layer and a back coat layer on opposite sides of a non-magnetic support, wherein the back coat layer contains an inorganic powder dispersed in a binder. Surface roughness of 10 points (SRz) is 0.
A magnetic recording medium having a range of 2 to 0.3 μm and a friction coefficient (μ) represented by the following mathematical formula of 0.13 to 0.18. Friction coefficient (μ) = (2 / π) × Ln (T2 / T1) Here, the surface of the back coat layer and the guide pin are 50 gr.
(T1) at a winding angle of 90 degrees, and the tension (T2) required to run the magnetic recording medium at a speed of 20 mm / sec under this condition. 4. The magnetic recording medium according to claim 3, wherein the friction coefficient (μ) of the stainless steel with respect to the guide pin is in the range of 0.13 to 0.18.