JPH03278313A - Magnetic recording medium and its production - Google Patents

Abstract

PURPOSE:To obtain a magnetic medium having low coefft. of friction against guide pins and excellent traveling property and to avoid noise due to transfer of a rugged pattern of a back coating layer by incorporating a specified nonmagnetic inorg. powder into the back coating layer and specifying the maximum surface roughness of the back coating layer to a specified value or lower. CONSTITUTION:The back coating layer contains CaCO3 powder of the average particle size gamma1 which satisfies the relation of 0.5l<=gamma1<=l+0.3mum wherein lis the film thickness of the back coating layer, or contains SiO2 powder compris ing spherical particles of average particle size gamma2 which satisfies the relation of 0.5l<=gamma2<=l+0.5mum. The maximum surface roughness of the back coating layer is specified to <=0.5mum. Thereby, the coefft. of friction against plastic guide pins can be significantly reduced, which gives good traveling stability and eliminates noise due to transferring of a rugged pattern on the surface of the magnetic recording layer.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a magnetic recording medium such as a magnetic tape having a magnetic layer on the front side of a non-magnetic support and a back coat layer on the back side, and a method for manufacturing the same. .

[Conventional technology]

In general, magnetic recording media such as magnetic tape have a magnetic layer containing magnetic powder and a binder formed on the surface of a non-magnetic support such as a polyester film, but with the recent increase in the density of magnetic recording, Therefore, there is a tendency to increase the residual magnetic flux density, improve output, and reduce noise by reducing the particle size of the magnetic powder used as much as possible to improve the surface smoothness of the magnetic layer.

However, there is a problem in that the running stability of the magnetic layer deteriorates as the surface smoothness of the magnetic layer increases.
As a means to cope with this problem and to prevent dust from adhering to the non-magnetic support due to charging, it has been common practice to provide a back coat layer on the back side of the non-magnetic support opposite to the magnetic layer.

This backcoat layer is usually made of AlzOs, Cr
z 03 , Tl 02 , α-Fez O), S
r Non-magnetic inorganic compound powder such as C1Ba5O,
It is made by binding fine particulate carbon black with a suitable binder, and while the above-mentioned inorganic compound powder imparts an appropriate surface roughness to the layer surface, the conductivity of carbon black prevents static electricity. . In addition, liquid or semi-solid lubricants such as fatty acids such as stearic acid and myristic acid, and esters thereof may be blended into the back coat layer in order to reduce the coefficient of friction.

[Problem to be solved by the invention]

However, in recent years, in order to prevent the irregularities on the surface of the backcoat layer from being transferred to the magnetic layer during winding and causing noise, the surface of the backcoat layer has also tended to be smoothed. A problem arises in that the coefficient of friction of the surface of the back coat layer with respect to the guide portion of the recycling equipment becomes large, and sufficient running stability is not achieved.

Also, when adding a lubricant to the back coat layer, if the amount is too large, the surface of the layer will become sticky and adhere to the guide section of recording/playback equipment, causing stains. There were limitations, and it was not possible to sufficiently reduce the coefficient of friction against the guide portion.

In particular, the guide bins of the guide parts of recording and reproducing devices that the back coat layer slides on are made of metal such as stainless steel or plastic such as polyacetal, but the effect of reducing the coefficient of friction a by the conventional ballast coat layer is Although it is quite effective for metal guide pins, it is completely inadequate for plastic guide bins, and creating a low coefficient of friction for guide bins made of these art materials has traditionally been a major challenge. It has become.

This invention was made in view of the situation mentioned in -F, and has a low coefficient of friction against the guide section of a recording/reproducing device, especially a low coefficient of friction against guide bins made of metal and plastic art materials, which improves running stability. It is an object of the present invention to provide a magnetic recording medium that is smooth and less likely to generate noise due to transfer of unevenness of a back coat layer, and a method for manufacturing the same.

[Means to solve the problem]

As a result of extensive studies to achieve the above object, the inventors have incorporated a specific non-magnetic inorganic compound powder into the back coat layer, and have set the maximum surface roughness of this back coat layer to a specific value. When the following settings are made, the coefficient of friction against guide bins made of metal and plastic art materials will be lower, and recording and reproducing equipment equipped with guide bins made of each of the above materials as well as equipment that coexists with guide bins made of banko art materials The present inventors have discovered that a magnetic recording medium exhibiting excellent running stability and low noise can be obtained by using any of the above methods, and this invention has been completed.

That is, the first aspect of the present invention is a magnetic recording medium comprising a magnetic layer on the front side of a non-magnetic support and a back coat layer on the back side, in which the back coat layer has an average particle diameter T.
, is 0.51≦γ1≦N for the layer thickness p of the back coat layer.
CaC○3 powder that satisfies the relationship of + 0.3 μm, or the average particle diameter γ2 is 0.5 for the same layer thickness l as above.
1≦T2≦l+0°5 μm Containing SiO2 powder consisting of spherical particles satisfying the relationship 1≦T2≦l+0°5 μm, and characterized in that the maximum surface roughness of the back coat layer is 0.5 μm or less. be.

In the magnetic recording medium according to the first invention, the above C
0.5-10% by weight of aC0+ powder in the back coat layer
A configuration in which the above-mentioned SiO2 powder is contained in the back coat layer in an amount of 0.1 to 30 M%, and a configuration in which the above-mentioned CaCO3 powder or SiO2 is contained in the back coat layer.
A particularly preferred embodiment is one in which carbon black is contained together with the powder.

Further, the second aspect of the present invention is that the above-mentioned C is contained in the back coat layer.
Regarding the manufacturing method of a magnetic recording medium containing carbon black together with aCO3 powder or SiO□ powder, when forming the back coat layer, CaC0, powder or Si
A coating solution containing dispersed O□ powder and a coating solution containing dispersed carbon black are separately prepared, and a back coat coating material made by mixing these two coating solutions is applied to the back side of the non-magnetic support.

The present invention relates to a method for manufacturing the magnetic recording medium described above, which is characterized by drying.

[Structure and operation of the invention]

In the back coat layer of the magnetic recording medium of the present invention, the CaCO3 powder or SiO2 powder contained therein has a particle size close to the layer thickness as described above, and the maximum surface roughness is set to be less than the above value. Therefore, the main part of the powder particles is embedded in the layer and a part of it protrudes from the layer surface, and the countless minute protrusions formed by the protruding parts of these particles prevent contact with the kissing part. It has a surface with a small area, and these minute convex parts have an appropriate hardness that is difficult to bite into the plastic surface due to the material CaCOs or SiO□, and in the case of 5iOz powder Since it is composed of spherical particles, it forms a smooth spherical surface and has good sliding properties.

Therefore, the magnetic recording medium of the present invention not only has a low friction coefficient μ(M) on the surface of the back coat layer against metal guide bins, but also has a low friction coefficient μCP) against plastic guide bins, which could not be sufficiently reduced in the past. It exhibits good running stability regardless of the type of recording and reproducing equipment used, and also reduces the noise generation and recording caused by the transfer of unevenness from the back coat layer to the magnetic layer surface and the falling of powder on the back coat layer side. It exhibits very low signal dropout and excellent electromagnetic conversion characteristics.

On the other hand, the average particle diameter T1 of the above Ca CO3 powder
Alternatively, if the average particle diameter T2 of the above SiO2 powder is less than 0.5 times the thickness l of the back coat layer, the ratio of particles buried under the surface of the back coat layer increases, and The coefficient of friction, particularly the coefficient of friction μCP for plastic guide bins, is not sufficiently reduced, making it impossible to achieve the object of the present invention.

In addition, the difference between the average particle diameter T of the CaCO3 powder and the thickness l of the back coat layer exceeds 0.3 μm, that is, (y
f) If it is >0.3 μm, the surface of the back coat layer will be roughened, and the retention strength of the CaCO3 particles by this layer will be insufficient, resulting in the transfer of unevenness from the back coat layer to the magnetic layer and the formation of powder in the back coat layer. The occurrence of noise due to dropout and the dropout of the recording signal will increase. Similarly, if the difference between the average particle diameter T2 of the SiO□ powder and the thickness l of the back coating layer exceeds 0.5 μm, that is, (
Even if γ2 Z)>0.5 μm, the same problem as above occurs.

Furthermore, if the maximum surface roughness of the backcoat layer exceeds 0.5 μm, the transfer of unevenness from the backcoat layer to the magnetic layer becomes significant, resulting in increased noise generation. The maximum surface roughness of the rough layer here is
Maximum surface roughness measured by a stylus surface roughness meter (described later).

value].

In addition, the maximum surface roughness of the layer, the Ca used
C0, particle size and content of powder or S i 02 powder,
Particle size and content of other non-magnetic powders blended as necessary with CaC0, powder or 5lo2 powder, dispersion state of these powders in the back coat paint, surface treatment such as calendering after forming the back coat layer. It is determined by the applied pressure etc. in CaCO3 powder or Si
It largely depends on the content of O□ powder.

Therefore, in this invention, the above CaC0.

It is desirable to use the content of powder or 5iOz powder as a setting index for the maximum surface roughness. That is, when using CaCO3 powder, the content of this powder in the hackcoat upper layer is 10% by weight or less, and when using SiotM powder, the content of this powder in the back coat layer is 30% by weight or less. In this case, the above-mentioned maximum surface roughness can be easily set to 0.5 μm or less as specified in the present invention, without giving special consideration to other factors.

The lower limit of the content of CaCO5 powder or SiO2 powder in the pack coat layer is the friction coefficient μ(M), μ(P
), CaC0:+
The most preferable content of CaCO3 powder or SiO2 powder is C
It is 1 to 3% by weight for a CO3 powder and 0.3 to 1% by weight for S i O2 powder.

Note that the above CaC0, powder and Si used in this invention
As mentioned above, there are some differences between the O2 powder in the average particle diameter and the content in the back coat layer, but this is because the particle size distribution of the 540g powder is CaCO3.
This is due to the fact that it is extremely soapy compared to powder.

On the other hand, in this invention, the above CaCO5 powder and 5
40g powder can also be used in combination if necessary. In this case, the amount of each powder to be used can be selected within the same range as described above.

In this invention, the above-mentioned CaC is added to the back coat layer.
Along with O3 powder or SiO□ powder, conductive powder such as carbon black, the above CaCO3 powder or 5in2
Non-magnetic inorganic compound powder, lubricant, etc. other than powder may be blended as necessary. Among these, carbon black is commonly used to provide conductivity to prevent static electricity and light-shielding properties for end detection.
The average particle diameter is preferably about 10 to 35 mm,
In addition, the content in the back coat layer is usually 60 to 99% by weight.
It is good to have a certain degree.

Furthermore, as the above-mentioned inorganic compound powder, any of various non-magnetic powders that have been conventionally blended into the back coat layer of magnetic recording media can be used.
103, Crzo, , TiO7, (k' Fe
2 C1+ , BaSO4, BaCO3, MgC0
, , SiC, and the like. Also, the average particle diameter is 0.5I for the layer thickness l of the back coat layer! Fine particulate powder (aCO3 powder and SiO□ powder) which are less than 100% can also be used as other inorganic compound powders.

In particular, when the above-mentioned fine-grained CaCO3 powder is used in combination with the coarse 5i02 powder of this invention whose average particle diameter T satisfies the above relationship with respect to the layer thickness l of the back coat layer, the friction coefficient μCP) against the plastic guide bottle is reduced. It has been recognized that it has a further reducing effect. In this case, the amount of finely divided CaC0 powder in the back coat layer is preferably 1 to 70% by weight.

The formation of the back coat layer in the magnetic recording medium of the present invention is carried out by preparing a back coat paint consisting of each of the above-mentioned components, a binder, and an organic solvent, and applying this paint after forming a magnetic layer on the surface side. It can be applied to the back side of the previous non-magnetic support and dried.

Here, when carbon black is contained in the back coat layer together with the coarse CaCO3 powder or coarse SiO2 powder according to the present invention, a coating liquid containing the CaCO3 powder or SiOz powder dispersed therein and carbon A recommended method is to separately prepare a coating liquid containing dispersed black and then mix these coating liquids to form the above-mentioned backcoat coating.

The reason for this is that while carbon black generally has poor dispersibility, the large particle diameter Ca CO used in this invention
3 powder and SiOz powder have a property of being easily destroyed during dispersion due to their low hardness. Therefore, a raw material composition containing both powders is kneaded for a long time using a Sun I" mill etc. until the carbon black is sufficiently uniformly dispersed. If the above CaC
This is because the particle size of the 0:+ powder and the SiO2 powder decreases, making it difficult to obtain the magnetic recording medium of the present invention.

The thickness of the back coat layer formed in this way is usually 0.3 to 2 μm, preferably 0.4 to I μm.

The above-mentioned binders include cellulose resins such as nitrocellulose, vinyl chloride-vinyl acetate copolymers containing or not containing functional groups such as hydroxyl groups, carboxyl groups, sulfone groups, and phosphoric acid groups, and polyurethane-based copolymers. Various thermoplastic or thermosetting resins conventionally known as binders for backcoat layers, such as resins, phenolic resins, and amino resins, and radiation-sensitive resins with unsaturated double bonds that can be crosslinked and cured by radiation such as electron beams. Resin alone or two
A mixture of two or more types can be used, and a crosslinking agent component such as a polyisocyanate compound that is crosslinked and cured by reaction with a functional group-containing resin may be added.

In addition, the above organic solvents include methyl ethyl ketone,
Ketone solvents such as cyclohexanone, ester solvents such as ethyl acetate, alcohol solvents such as isopropyl alcohol, aromatic hydrocarbon solvents such as toluene, etc. can be used, and two or more of these may be used in combination.

Furthermore, as the non-magnetic support, in addition to those made of synthetic resin materials such as polyethylene terephthalate and polyamide, those made of non-magnetic metal materials can also be used.

The magnetic layer in the magnetic recording medium of the present invention is not particularly limited, and may include a coated magnetic layer containing magnetic powder and a binder;
The magnetic powder, which may be a magnetic layer made of a ferromagnetic metal thin film, is r-Fe.

Os, Fes Oa, Co-containing r-Fe, 03, C
Acicular oxide magnetic powder such as rO□, plate-shaped ferrite magnetic powder such as Ba, Sr, and pb ferrite, Fe, Ni, and C
Examples include metal magnetic powders such as o or alloys thereof.

Further, as the binder, the same binder as that for the back coat layer described above can be used. In addition to the magnetic powder and binder, various additives such as abrasives, fillers, antistatic agents, and lubricants may be added to the coated magnetic layer, if necessary.

〔Effect of the invention〕

According to this invention, the back coat layer is made of CaCO3 powder or 5in2 powder having a particle size in a specific ratio to its layer thickness.
Since it contains powder and has a maximum surface roughness below a specific value, it has a low coefficient of friction against guide bins made of both metal and plastic art materials, and is suitable for recording and reproducing equipment equipped with guide bins made of each of the above materials. It exhibits excellent running stability even when using any of the same devices equipped with a guide bin made of art material, and also prevents unevenness from being transferred from the back coat layer to the magnetic layer and from dusting off from the back coat layer. ＜A magnetic recording medium with low noise and little dropout of recording signals can be provided.

In addition, in this magnetic recording medium, the content of CaC0° powder in the back coat layer is 0.5 to 10% by weight, or the content of SiO□ powder in the back coat layer is 0.1 to 30% by weight. %, the maximum surface roughness of the back coat layer can be easily set to below the specified value,
Moreover, it becomes possible to fully exhibit the above-mentioned effects of CaC0z powder or SiO□ powder.

Furthermore, in this magnetic recording medium, if the back coating layer contains carbon black together with CaC0 powder or Sio2 powder, the antistatic effect based on the conductivity of carbon black will prevent dust and abrasion powder from forming on the magnetic layer. This has the advantage that it is less likely to adhere to the surface, and dropout of recording signals due to this adhesion is reduced.

On the other hand, this carbon black and CaCO3 powder or S
In forming the back coat layer containing io2 powder, C
A coating solution containing dispersed aC0, powder or srOf powder and a coating solution containing dispersed carbon black are separately prepared, and a back coat coating material obtained by mixing these two coating solutions is applied to the back side of the non-magnetic support. According to the coating and drying method of the present invention, it is possible to avoid a decrease in particle size due to destruction of CaC0z powder particles or SiO□ powder particles during the dispersion process, and therefore, the production of the magnetic recording medium of the present invention described above is extremely facilitated. It has the advantage of being

〔Example〕

Next, the present invention will be described in more detail based on examples. In addition, in the following, parts mean parts by weight.

Example 1 Polyurethane resin 7 parts myristic acid 3 parts cyclohexanone
1) (1 part toluene
1) 0 parts After sufficiently cross-dispersing the above composition using a sand grinder mill, a polyisocyanate compound (trade name: Coronate, manufactured by Nippon Polyurethane Co., Ltd.) was added.
A magnetic paint is prepared by adding 5 parts of
Coated on one side of a μm polyethylene terephthalate film to a dry thickness of 3.0 μm, dried,
A magnetic layer was formed by calendering.

Next, coating liquids A and B having the following compositions were prepared by cross-dispersion using a sand grinder mill.

<Paint liquid A> Cyclohexanone toluene <Paint liquid B> 150 parts 150 parts cyclohexanone 50 parts Toluene
Using 50 parts of the above coating liquids A and B, a back coat coating having the following composition was prepared by mixing and dispersing with a disper mill.

Paint liquid A 450 parts Paint liquid B
7.5 parts Polyurethane resin 50 parts (trade name UR, 8300 manufactured by Toyobo Co., Ltd.) Cyclohexanone 200 parts Toluene
200 parts of this back coat paint was applied to the surface of the polyethylene terephthalate film opposite to the magnetic layer on which the magnetic layer was formed, so that the thickness after drying was 0.8 μm, dried, and the back coat paint was applied. form a coating layer,
A videotape was produced by cutting it to a width of 1.265 cm.

Example 2 The amount of paint liquid B added to the paint for H-ZIK coating was 1
A videotape was produced in the same manner as in Example 1 except that the number of copies was changed to 5.

Example 3 CaC0 of coating liquid B, average particle size as powder is 0.6μ
A videotape was produced in the same manner as in Example 1, except that 100 copies of No. m were used.

Example 4 Paint liquid B (, aCC) + powder had an average particle diameter of 1.
A videotape was produced in the same manner as in Example except that 100 copies of 0 μm tape were used.

Example 5 CaCQ of coating liquid B, average particle size as powder is 0.4μ
A videotape was produced in the same manner as in Example I, except that 100 parts of Example 1 were used and the thickness of the back coat layer was 0.4 μm.

Comparative Example 1 A videotape was produced in the same manner as in Example 1, except that paint liquid B was not blended with the back coat paint.

Comparative Example 2 Ca CO3 powder of coating liquid B has an average particle diameter of 0.1
A videotape was produced in the same manner as in Example 1, except that 100 copies of 5 μm tape were used.

Comparative Example 3 CaCO3 powder of coating liquid B has an average particle size of 1.5μ
A videotape was produced in the same manner as in Example 1, except that 100 copies of No. m were used.

Comparative Example 4 A videotape was produced in the same manner as in Example 1, except that 100 parts of kaolin powder (average particle size 0.8 μm) was used in place of CaC0 powder in coating liquid B.

Regarding each videotape of Examples 1 to 5 and Comparative Examples 1 to 4 above, the coefficient of friction of the back coat layer with respect to the metal and plastic guide pins, the surface roughness of the same layer,
Video S/N ratio was measured.

The results are shown in Table 1 below. The measurement method for each item is as follows.

<Friction coefficient> 5US with a diameter of 4vm and a surface roughness of 0.23
A cylinder made of 304 and a cylinder made of polyacetal with a diameter of 4 mm and finished with a surface roughness of 0.53 are each supported horizontally.
9 with the bottom layer side of the videotape as the contact surface.
Find the stress (T) when applying a 30g load to one end of the tape and pulling it at a speed of 141/sec with the other end horizontal, and calculate this stress (T). The coefficient of friction (μ) was determined by applying the following formula.

2 T Here, the friction coefficient μ [M] for the cylinder made of 5US304 with respect to the metal guide pin, and the friction coefficient μ [P] for the cylinder made of polyacetal against the plastic guide pin, respectively.

<Surface roughness> Use a stylus type surface roughness meter, stylus speed 0.06 w/sec,
Under the condition of cutoff 0.013n, surface average roughness (RA
The maximum surface roughness (R, . . . value) was measured.

<Video S/N> The signal-to-noise ratio during playback was measured using a video color noise meter, and the reference tape in Example 1 without a back coat layer (other configurations were the same as the tape in Example 1) was measured. It is expressed as a relative value when the value of

As is clear from the results in Table 1 above, the videotapes of the present invention (Examples 1 to 5) have friction coefficients μCM],
It can be seen that μ[P) is low in both cases, and even if a recording/reproducing device having either guide pin is used, good running stability is exhibited, and furthermore, noise is low.

On the other hand, a videotape using other non-magnetic inorganic compound powder instead of CaCO3 powder (Comparative Example 4) had little effect in reducing the coefficient of friction, especially when using a recording/playback device with a plastic guide bin. It is clear that the running stability is completely inferior in this case.

In addition, in the video tape (comparative example 3) where the particle size of the CaCO3 powder used is too large compared to the thickness of the back coat layer, the friction coefficient μ [M], μ
(P) is low, but the noise increases significantly, and conversely, the videotape with the above-mentioned particle size that is too small (Comparative Example 2) has little effect of reducing the friction coefficient, especially the friction coefficient μCP) against plastic guide bins. I understand that there is something.

Example 6 <Coating liquid C> Cyclohexanone 50 parts Toluene 50 parts Coating liquid C having the above composition
After sufficiently cross-dispersing the same coating liquid A as in Example 1 using a sand grinder mill, using these coating liquids A and C, a back coat coating having the following composition was mixed and dispersed using a disper mill. Prepared.

Paint liquid A 450 parts Paint liquid 0
1 part cellulose resin
210 parts (product name Seltsuba BT manufactured by Asahi Kasei Co., Ltd.
H-1) 200 parts of cyclohexanone 200 parts of toluene A magnetic layer of 3.0 μm thick was formed using this back coat paint in exactly the same manner as in Example 1. A back coat layer was formed by coating and drying the film to a thickness of 0.8 μm after drying on the side opposite to the layer, and the videotape was cut into the same width as in Example 1. Created.

Example 7 A videotape was produced in the same manner as in Example 6, except that the amount of paint liquid C added to the hack coat paint was changed to 2 parts.

Example 8 As spherical Si○2 powder of coating liquid C, the average particle diameter is 1.2
A videotape was produced in the same manner as in Example 6, except that 100 copies of μm tape were used.

Example 9 As spherical SiO□ powder of 2H liquid C, the average particle diameter is 0.6
A videotape was produced in the same manner as in Example 6, except that 100 copies of μm tape were used.

Example 10 Average particle size of spherical SiO2 powder of coating liquid C is 0.5
A videotape was produced in the same manner as in Example 6, except that 100 parts of a .mu.m thick film was used and the thickness of the back coat layer was 0.5 .mu.m.

Comparative Example 5 A videotape was produced in the same manner as in Example 6, except that paint liquid C was not blended with the back coat paint.

Comparative Example 6 As spherical S r Oz powder of coating liquid C, the average particle diameter is 1
．． A videotape was produced in the same manner as in Example 6, except that 100 copies of 5 μm tape were used.

Comparative Example 7 As spherical SiO2 powder of coating liquid C, the average particle size was 0.
．． Example 6 except that 100 parts of 36 μm was used.
A videotape was made in the same manner.

Example 1) Same as Example 6 except that the amount of carbon black used in the paint WIA was changed to 70 parts and 30 parts of finely divided CaC0° powder (average particle size 0.04 μm) was added. A videotape was made.

Example 12 A videotape was produced in the same manner as in Example 1), except that the amount of carbon black used in coating liquid A was changed to 40 parts, and the amount of finely particulate CaCO3 powder used was changed to 60 parts. .

Comparative Example 8 A videotape was produced in the same manner as in Example 1) except that paint liquid C was not blended into the back coat paint.

Comparative Example 9 A videotape was produced in the same manner as in Example 6, except that 100 parts of silicone resin powder (average particle size: 0.8 μm) was used in place of the spherical SiO□ powder in coating liquid C.

For each of the video tapes of Examples 6 to 12 and Comparative Examples 5 to 9, the coefficient of friction of the back coat layer with respect to the metal and plastic guide bins, the surface roughness of the same layer, and the video S/N ratio were determined as described above. It was measured in the same manner as. The results are shown in Table 2 below. In addition, the video S/N is
The values are shown as relative values when the value of the reference tape in Example 6 without a back coat layer (other configurations are the same as the tape in Example 6) is expressed as OdB.

From the results in Table 2 above, the videotape of the present invention (Examples 6 to 12) has low friction coefficients μ [M] and μ [P] with respect to metal guide bins and plastic guide bins. It can be seen that even when using a recording/reproducing device having the following characteristics, it shows good running stability and has low noise. In addition, spherical 5iOz in the back coat layer
Embodiment in which fine particulate Ca COz powder is included together with the powder (Example 1). 12), it can be seen that the coefficient of friction against the plastic guide bin is further reduced.

In contrast, a videotape using other non-magnetic inorganic compound powder instead of spherical S s Oz powder (Comparative Example 8)
．． 9) has little effect in reducing the coefficient of friction, and it is clear that the running stability is completely inferior, especially when a recording/reproducing device having a plastic guide bin is used.

In addition, in the videotab (comparative example 6) in which the particle size of the spherical S s Oz powder used is too large compared to the thickness of the henk coat layer, the friction coefficient μ [M
], μ(P) is low, but the noise increases significantly, and conversely, in the videotape where the particle size is too small (Comparative Example 7),
It can be seen that the effect of reducing the coefficient of friction is small, and in particular, the coefficient of friction μCP) for plastic guide bins is not sufficiently reduced.

Claims (5)

[Claims] (1) In a magnetic recording medium comprising a magnetic layer on the front side of a non-magnetic support and a back coat layer on the back side, in the back coat layer, the average particle diameter γ is equal to the layer thickness l of the back coat layer. CaCO_3 powder that satisfies the relationship 0.5l≦γ_1≦l+0.3μm or average particle size γ_2
is 0.5l≦γ_2≦l+0. for the same layer thickness l as above.
Contains SiO_2 powder consisting of spherical particles satisfying the relationship of 5 μm, and the maximum surface roughness of the back coat layer is 0.
．． A magnetic recording medium characterized by having a diameter of 5 μm or less. (2) CaCO_3 powder in the back coat layer is 0.5~
The magnetic recording medium according to claim 1, containing 10% by weight. (3) SiO_2 powder is 0.1 to 3 in the back coat layer.
The magnetic recording medium according to claim 1, wherein the magnetic recording medium contains 0% by weight. (4) CaCO_3 powder or Si in the back coat layer
The magnetic recording medium according to any one of claims (1) to (3), wherein carbon black is contained together with O_2 powder. (5) To form a back coat layer containing carbon black together with CaCO_3 powder or SiO_2 powder, separately prepare a coating liquid containing dispersed CaCO_3 powder or SiO_2 powder and a coating liquid containing dispersed carbon black, and then prepare both coating liquids separately. 5. The method for producing a magnetic recording medium according to claim 4, further comprising applying a mixed back coat paint to the back side of the non-magnetic support and drying the mixture.