JP2651270B2 - Magnetic recording media - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a magnetic recording medium comprising a magnetic layer of a non-magnetic support, and more particularly, to a magnetic recording medium having at least two magnetic layers. It is.

[Background of the Invention]

Magnetic recording media are widely used as recording tapes, video tapes, floppy disks, and the like. A magnetic recording medium is basically formed by laminating a magnetic layer in which ferromagnetic powder is dispersed in a binder (binder) on a non-magnetic support.

The magnetic recording medium is required to be at a high level in various characteristics such as electromagnetic conversion characteristics, running durability and running performance. That is, audio tapes for recording and reproducing music are required to have higher original sound reproduction capabilities. Also, video tapes are required to have excellent electromagnetic conversion characteristics such as excellent original image reproduction capability.

At the same time as having such excellent electromagnetic conversion characteristics, the magnetic recording medium is required to have good running durability as described above. In order to obtain running durability, the function of the abrasive usually plays an important role. That is, although the abrasive contained in the magnetic layer is distributed throughout the magnetic layer, a part thereof exists on the surface of the magnetic layer, and when the magnetic recording medium travels while contacting a traveling system member such as a head. The abrasive present on the surface of the magnetic layer at the contact surface between the surface of the magnetic layer and the head or the like exhibits a polishing effect. Therefore, it is possible to improve the running durability by including the abrasive in the magnetic recording medium. However, the abrasive present on the surface of the magnetic layer is only a small part of the amount actually added to the magnetic layer, and it has been difficult to obtain excellent running durability. For example, when the addition amount of the abrasive is increased to improve running durability, the content of the ferromagnetic powder is reduced, and when an abrasive having a large particle diameter is used,
Since the abrasive tends to excessively protrude from the surface of the magnetic layer, the above-mentioned electromagnetic conversion characteristics are deteriorated, which causes a problem. In addition, the use of an abrasive having a large particle size and an abrasive having a small particle size has also been performed, but sufficient electromagnetic conversion characteristics have not been obtained (see JP-A-57-162129, JP-A-57-162129). 58-8593
No. 1).

To solve this problem, the magnetic layer is provided with two layers, an upper layer and a lower layer. The upper layer is a magnetic layer with particularly excellent running durability, and the lower layer is a magnetic layer with particularly excellent electromagnetic conversion characteristics. It has been proposed to improve the running durability (Japanese Patent Application Laid-Open No. 58-200425). That is, the content of the ferromagnetic powder is increased without adding additives such as abrasives to the lower layer to improve the electromagnetic conversion characteristics, and the thickness of the upper layer is set to 0.5 to 1.5.
The use of a polishing material having a maximum particle size not exceeding the thickness of the upper layer as μm makes it possible to improve running durability without impairing the electromagnetic conversion characteristics.

However, by using two magnetic layers as described above, it is possible to obtain an electromagnetic recording medium having good electromagnetic conversion characteristics and running durability to some extent. I can't say that there is.

Therefore, in order to solve such a problem, in a magnetic recording medium in which a first magnetic layer and a second magnetic layer are provided in this order on the surface of a non-magnetic support, the second magnetic layer has an average Mohs hardness of 6 or more. There has been proposed a magnetic recording medium comprising an abrasive having a particle diameter of 0.45 μm or more and an abrasive having a Mohs hardness of 6 or more and an average particle diameter of 0.25 μm or less (Japanese Patent Laid-Open No. 220219/1990).

With this proposal, Y-S / N, still life and head contamination were improved, but Y-S / N was about 1.5 dB, and still life was also improved.
This is about 120 minutes, and there is still room for improvement in recent demands for high-density recording and high durability. Improvements have also been required in terms of head wear.

(Object of the Invention) An object of the present invention is to provide a magnetic recording medium such as an audio tape or a video tape which has excellent electromagnetic conversion characteristics, excellent running durability, and low head wear.

(Constitution of the Invention) That is, the object of the present invention is to provide a magnetic recording medium in which at least two magnetic layers containing a ferromagnetic powder and a binder are provided on the surface of a nonmagnetic support, wherein the second magnetic layer (upper layer) has an average It contains two types of abrasives having a particle size of R ₁ μm (abrasive A) and R ₂ μm (abrasive B) and carbon black having an average particle size of r ₂ μm, and the dry thickness of the second magnetic layer is d μm. And when the first magnetic layer contains carbon black having an average particle diameter of r ₁ μm, the following relational expression is satisfied.

Formula 1.R ₁ >d> R ₂ Formula 2.1 / 2 × R ₂ ≧ r ₂ > 1/5 × R ₂ ≧ r ₁ > 1/20 × R ₂ More preferably, the above-mentioned object of the present invention is characterized in that said second magnetic The layer is characterized by containing 2/10 or less of a granular abrasive A having a mean particle diameter R ₁ μm and being close to a sphere, and containing 8/10 or more of a granular abrasive B having an average particle diameter R ₂ μm and having many corners. This can be achieved by a magnetic recording medium.

That is, the present invention remarkably improves the YS / N and the still life by defining the relationship between the layer thickness of the second magnetic layer (upper layer) and the average particle diameter of the two types of abrasives and carbon black, and further improves the head life. Wear is also improved.

This is because the abrasive A having a large average particle size protrudes from the surface of the magnetic layer and the surface of the magnetic layer is appropriately roughened to improve the still life and decrease the μ value.

By reducing the amount of this abrasive used, Y-S /
N does not deteriorate. The abrasive B having an average particle size smaller than the thickness of the second magnetic layer can maintain the smoothness of the surface of the magnetic layer and improve YS / N. In addition, since the amount of use is large, running durability can be ensured, and the edge portion can be prevented from being scraped. Further, since carbon black has an average particle diameter that fills the gap between the two abrasives, the smoothness of the magnetic layer surface is improved, the powder is densely packed, the running durability can be improved, and Y-S / N, Still life and head wear were remarkably improved at the same time, and head wear was also improved.

Conventionally, it is known to use two abrasives having an average particle diameter of 0.45 μm or more and an abrasive having an average particle diameter of 0.25 μm or less in the second magnetic layer. In this case, the thickness of the second magnetic layer is 0.5 μm.
The combination is such that the abrasive A: α-Al ₂ O _{3 has} an average particle diameter of 0.5 μm and the abrasive B: α-Al ₂ O _{3 has} an average particle diameter of 0.2 μm with respect to m. In such a combination, the abrasive A having a large average particle diameter is used.
Could not protrude to the required extent on the surface of the magnetic layer, and a gap was formed between the abrasive B having a small average particle diameter, and the μ value was reduced and the durability was improved. The present invention uses an abrasive having an average particle diameter larger than the thickness of the second magnetic layer so that the abrasive appropriately emerges on the surface of the magnetic layer, so that the contact area of the head and the like is reduced and the μ value is reduced. Along with the decrease, the abrasive B having a small average particle diameter contains carbon black having an average particle diameter of not more than 1/2 and larger than 1/5, so that a moderate gap is formed and a dense and smooth magnetic layer is obtained. As a result, YS / N, still life can be improved, and head wear can be improved. Since the carbon black used for the second magnetic layer is a carbon black having a slightly larger average particle diameter, it contributes to running durability.

On the other hand, since the first magnetic layer does not need to contain an abrasive, the filling degree of the ferromagnetic powder increases, and the electromagnetic conversion characteristics are improved.
Further, since the first magnetic layer contains carbon black having a small average particle diameter, an extremely smooth magnetic layer can be obtained, and the first magnetic layer has an antistatic effect, so that the S / N can be improved.

The carbon black of the first magnetic layer also has an effect of reducing the light transmittance that cannot be covered by the carbon black of the second magnetic layer.

The combination described above not only improved Y-S / N and head wear, but also exhibited an extremely excellent still life of 240 minutes.

The abrasive A having a large average particle diameter is preferably in the form of particles close to a sphere, and has a remarkable effect of improving still durability without causing head wear. On the other hand, it is preferable that the abrasive B having a small average particle diameter is formed into grains having many corners. By making the grains having many corners, the contact area is reduced, the μ value is reduced, and the running durability can be improved.

Preferred embodiments of the magnetic recording medium of the present invention are as follows.

1) The above magnetic recording medium, wherein the thickness of the second magnetic layer is 1.5 μm or less.

2) The above magnetic recording medium, wherein the thickness of the second magnetic layer is 1.0 μm or less.

3) The magnetic recording medium as described above, wherein two types of abrasives having a Mohs' hardness of 6 or more contained in the second magnetic layer are used.

4) The above magnetic recording, wherein the content of the abrasive having a Mohs hardness of 6 or more contained in the second magnetic layer is 0.5 to 30 parts by weight based on 100 parts by weight of the ferromagnetic powder contained in the magnetic layer. Medium.

5) A magnetic recording medium characterized in that the abrasive B having a small average particle diameter is 0.01 to 0.5 μm, and the abrasive A having a large average particle diameter is 0.5 to 2.0 μm, preferably 0.5 to 1.5 μm.

The magnetic recording medium of the present invention basically has a configuration in which at least two magnetic layers containing ferromagnetic powder dispersed in a binder are provided on a nonmagnetic support.

Non-magnetic supports that can be used in the present invention include polyethylene terephthalate (PET), polyesters such as polyethylene naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, polyvinyl chloride, Film or sheet made of vinyl resin such as polyvinylidene chloride or the like, or synthetic resin such as polycarbonate, polyamide, polyamide imide or polyimide; Non-magnetic metal foil such as aluminum or copper; Metal foil such as stainless steel foil; From paper or ceramic sheet To be elected.

These supports have a thickness in the range of 2.5 to 100 μm, preferably in the range of 3 to 80 μm.

In the present invention, the first magnetic layer, which is the lower layer, is thicker than the upper layer (second magnetic layer), has a higher filling rate of ferromagnetic powder, and has excellent surface smoothness, from the viewpoint of maintaining the electromagnetic conversion characteristics. Magnetic layer is provided. In order to obtain such smoothness, it is preferable to provide a magnetic layer without adding an abrasive. On the first magnetic layer provided as described above, an extremely thin second magnetic layer containing the abrasive having a relatively large particle diameter and the abrasive having a small average particle diameter is provided.

Normally, an abrasive having a small average particle size does not cause a decrease in electromagnetic conversion characteristics, but has a small effect on improving running durability, and an abrasive having a large average particle size exhibits the opposite phenomenon.

However, when the magnetic layer has a multilayer structure including two magnetic layers as in the present invention, by making the first magnetic layer extremely excellent in smoothness as described above, the second magnetic layer is also A surface having remarkably excellent smoothness can be obtained. In order to obtain a thin and smooth surface of the second magnetic layer, it is preferable to use a simultaneous multi-layer coating method described later.

As described above, the magnetic layer of the present invention has extremely good smoothness. For this reason, even if the second magnetic layer is made of an abrasive having a slightly large average particle diameter which has a particularly large effect on running durability, the electromagnetic conversion characteristics are hardly reduced. Also, since the thickness of the second magnetic layer is extremely thin, even if an abrasive having a slightly larger average number of particles is used, the abrasive is present only in the vicinity of the entire surface of the magnetic layer and has almost no adverse effect on the electromagnetic conversion characteristics of the entire magnetic layer. Do not give.

That is, the magnetic recording medium of the present invention is a magnetic recording medium comprising at least two magnetic layers in which a first magnetic layer and a second magnetic layer are provided in this order, wherein the second magnetic layer has an average Mohs' hardness of 6 or more. An abrasive A having a particle diameter larger than the layer thickness of the second magnetic layer and an abrasive B having a Mohs hardness of 6 or more and an average particle diameter smaller than the layer thickness of the second magnetic layer, and having a large particle diameter are obtained. Less by weight than small abrasive B. And among the abrasives contained in the second magnetic layer, the shape of the abrasive whose average particle diameter is larger than the layer thickness of the second magnetic layer is round, and the shape of the abrasive whose average particle diameter is less than the second magnetic layer is It is preferably angular. It is considered that the abrasives of the above combination form fine irregularities on the surface of the magnetic layer while maintaining the smoothness of the surface of the magnetic layer to some extent. Such a magnetic layer surface is obtained because the second magnetic layer having a very small thickness is provided in the wet state of the first magnetic layer, and it can be said that the effect of the simultaneous multilayer coating is great.

Therefore, it can be said that the magnetic recording medium of the present invention has excellent electromagnetic conversion characteristics and running durability.

The thickness of the first magnetic layer is preferably in the range of 1 to 4 μm, and the thickness of the second magnetic layer is preferably 1.5 μm or less, more preferably 1.0 μm or less.

The abrasive contained in the second magnetic layer of the present invention has a Mohs hardness of 6
For example, α-Al ₂ O ₃ (Mohs hardness 9), TiO ₂ (6.5), SiO ₂ (7), SnO ₂ (6.5),
Cr ₂ O ₃ (same 9), SiO (same 9) and TiO (same 9) can be mentioned. Preferably, they are α-Al ₂ O ₃ and Cr ₂ O ₃ .

AKP-5 manufactured by Sumitomo Chemical Co., Ltd.
0, AKP-30, AKP-20, AKP-12, and HIT-100, HIT-50, HIT-5 manufactured by Sumitomo Chemical Co., Ltd.
5, HIT-20 and others.

The content of the abrasive having a Mohs hardness of 6 or more contained in the second magnetic layer is preferably 0.5 to 30 parts by weight based on 100 parts by weight of the ferromagnetic powder contained in the magnetic layer.

In the magnetic recording medium of the present invention, since the upper second magnetic layer has a sufficient running durability, the first magnetic layer requires an amount of abrasive to obtain excellent electromagnetic conversion characteristics. It is preferably less, more preferably not containing.

The binder resin used for forming each magnetic layer of the present invention is not particularly limited. Examples of the resin for the binder include vinyl chloride copolymers (eg, vinyl chloride / vinyl acetate copolymer, vinyl chloride / vinyl acetate / acrylic acid copolymer, vinyl chloride / vinylidene chloride copolymer, vinyl chloride / acrylonitrile copolymer) Polymer, ethylene-vinyl acetate copolymer), cellulose derivatives such as nitrocellulose resin, acrylic resin, polyvinyl acetal resin, polyvinyl butyral resin,
Examples include epoxy resins, phenolic resins, and polyurethane resins (eg, polyester-based polyurethane resins, polyether-based polyurethane resins, and polycarbonate polyurethane resins). These resins may contain polar groups such as a hydroxyl group, a carboxyl group, an epoxy group, a metal sulfonate group, a phosphoric acid group and a phosphoric acid ester group.

These can be used alone or in combination.

When a curing agent is used, a polyisocyanate compound is usually used. The polyisocyanate compound is selected from those usually used as a curing agent component such as a polyurethane resin. Examples of the polyisocyanate compound include a reaction product of tolylene diisocyanate and 1 mol of trimethylolpropane (eg,
Decimodule L-75 (manufactured by Bayer AG)), a reaction product of 3 mol of isocyanate such as xylylene diisocyanate or hexamethylene diisocyanate with 1 mol of trimethylolpropane, a burette addition compound of 3 mol of hexane methylene diisocyanate, tolylene diisocyanate Polymers of 5 moles of isocyanurate compound, 3 moles of tolylene diisocyanate and 2 moles of hexamethylene diisocyanate isocyanate adduct, isophorone diisocyanate and diphenyl maytan diisocyanate can be mentioned.

When a curing treatment is performed by electron beam irradiation, a compound having a reactive double bond (eg, urethane acrylate) can be used.

The total weight of the resin component and the curing agent is 100
Usually, it is preferably in the range of 5 to 40 parts by weight, more preferably 10 to 20 parts by weight with respect to parts by weight.

Examples of the ferromagnetic powder used in the present invention include γ-Fe ₂ O ₃
Metal oxide-based ferromagnetic powders such as γ-Fe ₂ O ₃ containing other components such as cobalt and other metal-metal oxide-based ferromagnetic powders, and iron, cobalt or nickel Ferromagnetic metal fine powder containing a ferromagnetic metal can be mentioned.

When a ferromagnetic metal fine powder is used, it is a ferromagnetic metal fine powder containing iron, cobalt or nickel and has a specific surface area of at least 42 m ² / g (particularly preferably at least 45 m ² / g)
Is preferred.

As an example of the ferromagnetic metal fine powder, the metal content in the ferromagnetic metal fine powder is 75% by weight or more, and
At least one ferromagnetic metal or alloy of at least one weight% (eg, Fe, Co, Ni, Fe-Co, Fe-Ni, Co-Ni, Co-Ni
-Fe), and other components (eg, Al, Si, S, Sc, Ti, V, Cr, Mn, Cu, Zn,
Y, Mo, Rh, Pd, Ag, Sn, Sb, B, Ba, Ta, W, Re, A
u, Hg, Pb, P, La, Ce, Pr, Nd, Te, Bi). Further, the ferromagnetic metal component may contain a small amount of water, hydroxide or oxide.

Methods for producing these ferromagnetic powders are known, and the ferromagnetic powder used in the present invention can also be produced according to a known method.

The shape of the ferromagnetic powder is not particularly limited, but is usually acicular,
Small, dice-shaped, rice-grained and plate-shaped ones are used. In particular, it is preferable to use acicular ferromagnetic powder.

The above resin components, hardeners and ferromagnetic powders, solvents usually used for preparing magnetic paints (eg, methyl ethyl ketone, dioxane, cyclohexane, ethyl acetate)
And kneaded and dispersed to form a magnetic paint. The kneading and dispersion can be performed according to a usual method.

In addition, in the magnetic paint, in addition to the above components, commonly used additives or fillers such as an antistatic agent (eg, carbon black), a lubricant (eg, fatty acid, fatty acid ester, silicone oil), and a dispersant ( Of course) may be included.

Next, a method for manufacturing the magnetic recording medium of the present invention will be described. The coating is performed by applying the magnetic paint prepared using the above-mentioned materials on a non-magnetic support by the following method. First, a coating solution for the first magnetic layer is prepared by kneading and dispersing a resin component for the first magnetic layer, a ferromagnetic powder, and a solvent for a magnetic layer forming component such as a curing agent that is optionally blended. Then, for the second magnetic layer, two kinds of abrasives having a Mohs hardness of 6 or more and different average particle systems are added as described above in addition to the above-mentioned magnetic layer forming components, and the coating liquid for the second magnetic layer is similarly adjusted. I do.

The method for producing a magnetic recording medium of the present invention includes, for example, applying a coating solution for a first magnetic layer to the surface of a nonmagnetic support under running, and while the coating solution is in a wet state, continuously applying the coating solution on the coating layer. And the thickness of the second magnetic layer after drying the second magnetic layer
The coating is to be 1.5 μm or less (preferably 1.0 μm or less). The method of continuous application of the two layers, for example, when an extrusion coat is used as a coating machine, two sets of extrusion coats may be continuously installed so as to sandwich the non-magnetic support under running, and may be applied. Alternatively, two first magnetic layers may be provided at intervals so as to maintain a wet state (that is, a state in which the coating layer still contains a solvent and exhibits tackiness).

As a coating machine for applying the magnetic paint, air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat,
Reverse roll coat, transfer roll coat,
Gravure coat, kiss coat, cast coat, spray coat, spin coat, etc. can be used. In the present invention, a simultaneous multilayer coating extrusion coating having two slots as described in JP-A-62-246331 is desirable.

As described above, in the magnetic layer of the present invention, the thickness of the second magnetic layer is preferably 1.5 μm or less (more preferably 1.0 μm or less). Usually, when two magnetic layers are provided, they are formed by applying and drying the first magnetic layer coating solution and then applying the second magnetic layer coating solution. However, when the second magnetic layer is applied after drying the first magnetic layer, 1.5 μm
It is difficult to obtain a thin layer having a thickness of not more than m as a uniform layer. In order to obtain such a thin film having a thickness of 1.5 μm or less, the coating liquid for the first magnetic layer is applied as described above, and then the coating liquid for the second magnetic layer is continuously applied while the coating layer is in a wet state. It is preferable to form the magnetic layer by applying the magnetic layer, whereby a thin second magnetic layer having a thickness of 1.5 μm or less can be obtained with a uniform thickness.

The magnetic layer obtained by using the above manufacturing method can be applied with a uniform layer thickness and a smooth surface even if the magnetic layer has an extremely thin layer thickness of 1.5 μm or less. . As a result, it is possible to manufacture a magnetic recording medium having excellent running durability according to the present invention and not impairing the electromagnetic conversion characteristics.

The coating layer of the magnetic paint is applied so that the thickness of the magnetic layer (the total thickness of the first magnetic layer and the second magnetic layer) of the obtained magnetic recording medium is usually in the range of 0.5 to 10 μm. You.

A back layer (backing layer) may be provided on the surface of the non-magnetic support used in the present invention on which the magnetic paint is not applied. Normally, the back layer is formed by applying a back layer forming paint in which a particulate component such as an abrasive and an antistatic agent and a binder are dispersed in an organic solvent, on the surface of the non-magnetic support on which the magnetic paint is not applied. It is a layer provided.

An adhesive layer may be provided on the surface of the non-magnetic support on which the magnetic paint and the back layer forming paint are applied.

Usually, the applied layer of the applied magnetic paint is subjected to a process for orienting the ferromagnetic powder contained in the applied layer of the magnetic paint, that is, a magnetic field orientation process, and then dried.

After being dried in this manner, the coating layer is subjected to a surface smoothing treatment. For the surface smoothing treatment, for example, a super calender roll or the like is used. By performing the surface smoothing treatment, voids generated by removing the solvent during drying disappear, and the filling rate of the ferromagnetic powder in the magnetic layer is improved, so that a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. it can.

The laminate thus cured is then cut into a desired shape.

Cutting can be performed under normal conditions using a normal cutting machine such as a slitter.

Although the magnetic recording medium of the present invention has been described with reference to the upper and lower two-layer system, the magnetic recording medium may have three or more layers as long as it includes two magnetic layers having the above-specified properties.

(Effects of the Invention) The present invention has a constant abrasive material A having an average particle diameter larger than the layer thickness of the second magnetic layer (upper layer), an abrasive material B having an average particle diameter smaller than the layer thickness, and an abrasive material A. By distributing the carbon black having the average particle diameter in the relationship (formula 2) to the upper and lower layers, the upper layer can be smoothed and moderately roughened, the μ value can be reduced, and the abrasive and the carbon black can be densely filled. Y
-S / N, still life could be improved, and head wear was also significantly improved.

The first magnetic layer contained fine carbon black particles compared to the abrasive B, so that the filling degree of the ferromagnetic powder could be improved, and the S / N could be further improved.

(Embodiment) Although the magnetic recording medium of the present invention has been described with reference to the upper and lower two-layer system, the magnetic recording medium may have three or more layers as long as it retains the above-specified properties and includes two magnetic layers.

Next, the present invention will be described more specifically with reference to Examples and Comparative Examples. In each example, “parts” means “parts by weight” unless otherwise specified.

[Example 1] Coating solution for first magnetic layer Co-γ-Fe ₂ O ₃ [Hc: 650 Oe, SBET specific surface area: 35 m ² / g] 100
Parts vinyl chloride / vinyl acetate / maleic anhydride copolymer (composition ratio 86: 13: 1, degree of polymerization 400) 12 parts polyester polyurethane resin 6 parts carbo black (average particle system: 0.02 μm) 3 parts butyl stearate 1 part Stearic acid 2 parts Butyl acetate 200 parts Coating solution for second magnetic layer Co-γ-Fe ₂ O ₃ [Hc: 700 Oe, S BET specific surface area: 40 m ² / g] 100
Part vinyl chloride / vinyl acetate / maleic anhydride copolymer (composition ratio 86: 13: 1, degree of polymerization 400) 12 parts polyester polyurethane resin 6 parts carbon black 2 (average particle diameter: 0.08 μm) 3 parts abrasive A: α-Al ₂ O ₃ (average particle diameter: 0.9 μm, shape: granular with many corners) 8 parts Abrasive B: α-Al ₂ O ₃ (average particle diameter: 0.2 μm, shape: granular with many corners) butyl steer Rate 1 part Stearic acid 2 parts Butyl acetate 200 parts For each of the above two paints, each component was kneaded and dispersed using a sand mill. To the obtained dispersion, 6 parts of polyisocyanate and 40 parts of butyl acetate were added, and the mixture was filtered using a filter having an average pore diameter of 1 μm, and the coating liquid for forming the first magnetic layer and the coating liquid for forming the second magnetic layer were respectively added. It was adjusted.

The application of the first and second magnetic layer forming coating solutions was performed as follows using a simultaneous multilayer coating having a first magnetic layer coating slot and a second magnetic layer coating slot.

The resulting coating solution for the first magnetic layer has a thickness of 3.0 after drying.
The polyethylene terephthalate support having a thickness of 14 μm was run at a speed of 60 m / min so as to have a thickness of 1 μm, and was applied to the surface of the support using an extrusion coat having a throttle for coating the first magnetic layer. The thickness after drying the coating solution for the second magnetic layer while the first magnetic layer is wet) is 0.5μ.
m is applied using an extrusion coat having a slot for coating the second magnetic layer so that the magnetic layer is oriented by the magnet while the magnetic layer is in a wet state. To make a videotape.

Example 2 A video tape was manufactured in the same manner as in Example 1, except that the shape of the abrasive A in the second magnetic layer coating solution was changed to a particle shape close to a sphere.

Example 3 In Example 1, the amount of the abrasive A in the second magnetic layer coating solution was changed from 2 parts to 1 part, and the amount of the abrasive B was changed from 8 parts to 9 parts.
A video tape was manufactured in the same manner as in Example 1 except that the parts were changed to parts.

Example 4 A video tape was manufactured in the same manner as in Example 3, except that the shape of the abrasive A in the coating solution for the second magnetic layer was changed from a granular shape having a large angle to a granular shape close to a sphere. .

Example 5 A video tape was manufactured in the same manner as in Example 4, except that the average particle diameter of carbon black 2 in the coating solution for the second magnetic layer was changed from 0.8 μm to 0.06 μm.

Example 6 A video tape was manufactured in the same manner as in Example 1, except that the average particle size of the abrasive A in the coating solution for the second magnetic layer was changed from 0.9 μm to 0.7 μm.

Example 7 In Example 6, the amount of the abrasive A in the second magnetic layer coating solution was changed from 2 parts to 1 part, and the amount of the abrasive B was changed from 8 parts to 9 parts.
A video tape was manufactured in the same manner as in Example 6 except that the parts were changed to parts.

Example 8 A video tape was manufactured in the same manner as in Example 7, except that the shape of the abrasive A in the coating solution for the second magnetic layer was changed to a granular shape close to a sphere.

Example 9 A video tape was manufactured in the same manner as in Example 8, except that the average particle diameter of the carbon black 2 in the coating solution for the second magnetic layer was changed from 0.08 μm to 0.06 μm.

Example 10 A video tape was manufactured in the same manner as in Example 8, except that the shape of the abrasive B in the coating solution for the second magnetic layer was changed to a granular shape close to a sphere.

Example 11 In Example 7, the average particle diameter of the abrasive B in the second magnetic layer coating solution was changed from 0.2 μm to 0.1 μm, and the average particle diameter of the carbon black 2 was changed from 0.08 μm to 0.05 μm. A video tape was manufactured in the same manner as in Example 7, except that the average particle size of carbon black 1 in the coating solution for the first magnetic layer was changed from 0.020 μm to 0.015 μm.

Example 12 A video tape was manufactured in the same manner as in Example 11, except that the shape of the abrasive A in the coating solution for the second magnetic layer was changed to a granular shape close to a sphere.

Example 13 A video tape was manufactured in the same manner as in Example 12, except that the average particle diameter of the carbon black 2 in the coating solution for the second magnetic layer was changed from 0.05 μm to 0.03 μm.

Example 14 A video tape was manufactured in the same manner as in Example 12, except that the shape of the abrasive B in the coating solution for the second magnetic layer was changed to a granular shape close to a sphere.

Comparative Example 1 In Example 1, the amount of the abrasive A in the second magnetic layer coating solution was changed from 2 parts to 10 parts, and the average particle diameter of the abrasive A was changed from 0.9 μm to 0.5 μm. A video tape was manufactured in the same manner as in Example 1 except that the abrasive B was not used.

[Comparative Example 2] In the same manner as in Example 1, except that the amount of the abrasive A in the coating solution for the second magnetic layer was changed from 2 parts to 10 parts, and the abrasive B was not used. Video tapes.

Comparative Example 3 In Example 1, the amount of the abrasive A in the coating solution for the second magnetic layer was changed from 2 parts to 10 parts, and the average particle diameter of the abrasive A was changed from 0.9 μm to 0.7 μm. A video tape was manufactured in the same manner as in Example 1 except that the abrasive B was not used.

[Comparative Example 4] In Example 1, the amount of the abrasive B in the coating solution for the second magnetic layer was changed from 8 parts to 10 parts, and the same procedure was performed as in Example 1 except that the abrasive A was not used. Video tapes.

Comparative Example 5 In Example 1, the amount of the abrasive B in the coating solution for the second magnetic layer was changed from 8 parts to 10 parts, and the average particle diameter of the abrasive B was changed from 0.2 μm to 0.1 μm. A video tape was manufactured in the same manner as in Example 1 except that abrasive A was not used.

Comparative Example 6 In Example 6, except that the shape of the abrasive A in the second magnetic layer coating liquid was changed to a particle shape close to a sphere, and the average particle diameter of the carbon black 2 was changed from 0.08 μm to 0.20 μm. A video tape was manufactured in the same manner as in Example 6.

[Comparative Example 7] In Example 6, except that the shape of the abrasive A in the second magnetic layer coating liquid was changed to a particle shape close to a sphere, and the average particle diameter of the carbon black 2 was changed from 0.08 µm to 0.30 µm. A video tape was manufactured in the same manner as in Example 6.

Comparative Example 8 In Example 6, the shape of the abrasive A in the second magnetic layer coating solution was changed to a particle shape close to a sphere, and the average particle diameter of the carbon black 1 in the first magnetic layer coating solution was 0.020 μm. From 0.050μ
A videotape was produced in the same manner as in Example 6, except that m was changed to m.

[Comparative Example 9] In Example 6, the shape of the abrasive A in the second magnetic layer coating liquid was changed to a particle shape close to a sphere, and the average particle diameter of the carbon black 1 in the first magnetic layer coating liquid was 0.020 µm. From 0.100μ
A videotape was produced in the same manner as in Example 6, except that m was changed to m.

[Comparative Example 10] In Example 10, the average particle diameter of the abrasive A was changed from 0.7, 1 part to 0.5 μm, to 3 parts, 9 parts of the abrasive B to 3 parts, and the average particle of the carbon black 2 was changed. A video tape was manufactured in the same manner as in Example 10 except that the diameter was changed to 0.02 μm.

For the video tapes obtained in the above Examples and Comparative Examples, the abrasive used for the second magnetic layer, its average particle size,
The amount used, the layer thickness, and the physical properties of each tape measured by the following measurement methods are shown in the table.

Measurement method (1) Y-S / N 7 when the output level of the video tape of the comparative example was set to 0 dB
The C / N of the luminance signal of MHz was measured.

(2) Still life Each video tape is played in the still mode, and the S / N of the screen is displayed.
The time until the value decreased by 6 dB was measured.

(3) Head Wear The degree of wear of the head when each video tape was reproduced for 120 minutes × 20 times at 5 ° C. and 80% RH was measured.

Of the present invention, the first magnetic layer comprises a carbon average particle径径is r _1, an average particle diameter of two different abrasive in the second magnetic layer Mohs hardness of 6 or more (average particle diameter of each of R ₁ , R ₂ ,
And R ₁ > R ₂ ) and carbon having an average particle size of r ₂ , and when the thickness of the second magnetic layer is d, R ₁ >d> R ₂ , 1/2 × R ₂ ≧ As is clear from the above table, a video tape in which the relationship of r ₂ > 1/5 × R ₂ ≧ r ₁ > 1/20 × R ₂ is satisfied has no decrease in Y−S / N and still life Since it is long, it can be said that the magnetic recording medium has improved running durability without impairing the electromagnetic conversion characteristics.

On the other hand, in Comparative Examples 1 to 5 in which only one type of abrasive for the second magnetic layer was used, the average particle diameter of carbon in the second magnetic layer was smaller in the two types of abrasives having different average particle diameters. of
Comparative rows 6 to 7 using ones larger than 1/2, and the average particle diameter of carbon in the first magnetic layer is 1/5 of the two types of abrasives having different average particle diameters, which is smaller in particle diameter. Electromagnetic conversion characteristics and running durability are not sufficient for the comparative columns 8 to 9 using larger ones.

Even when two types of abrasives are used, in the combination as in Comparative Example 10, the YS / N and still life are considerably improved, but they are still not sufficient, and there is much head wear.

Claims (2)

(57) [Claims] In a magnetic recording medium having at least two magnetic layers containing a ferromagnetic powder and a binder on the surface of a nonmagnetic support, the second magnetic layer (upper layer) has an average particle diameter of R ₁ μm. R ₂
μm two types of abrasives and carbon black having an average particle size of r ₂ μm, a dry thickness of the second magnetic layer being d μm, and a first magnetic layer having an average particle size of r ₁ μm. A magnetic recording medium characterized in that the following relational expression holds when black is included. Formula 1.R ₁ >d> R ₂ Formula 2.1 / 2 × R ₂ ≧ r ₂ > 1/5 × R ₂ ≧ r ₁ > 1/20 × R ₂ Wherein wherein said second magnetic layer is the average particle diameter R ₁ [mu] m abrasives A of nearly spherical granular 2/10 or less, the average particle diameter R ₂ [mu] m
The magnetic recording medium according to claim 1, wherein the magnetic recording medium contains at least 8/10 of a granular abrasive B having many corners.