JP2581582B2 - Magnetic recording media - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a magnetic recording medium comprising a nonmagnetic support and a magnetic layer, 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. The magnetic recording medium is basically a magnetic recording medium in which a magnetic layer in which ferromagnetic powder is dispersed in a binder is laminated 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 functions of the abrasive and the lubricant usually play an important role.

However, in order to obtain excellent running durability by the abrasive, it is necessary to increase the amount of the abrasive to some extent, and therefore, the content of the ferromagnetic powder decreases. When an abrasive having a large particle diameter is used to obtain excellent running durability, the abrasive tends to excessively protrude from the surface of the magnetic layer. Therefore, the improvement of the running durability by the abrasive often causes a problem in that the above-described electromagnetic conversion characteristics are deteriorated.

When the running durability is improved by a lubricant, it is necessary to increase the amount of the added lubricant. Therefore, the binder is easily plasticized, and the durability of the magnetic layer tends to decrease.

As another method for improving the running durability, a method of increasing the hardness of a magnetic layer using a hard binder has been used. However, as an adverse effect of increasing the hardness of the magnetic layer, the brittleness of the magnetic layer becomes conspicuous, and the magnetic layer becomes more susceptible to damage. There is a problem that improvement in running durability cannot be obtained.

Therefore, a magnetic recording medium having sufficiently excellent running durability, such as low dropout and good still characteristics, and further excellent electromagnetic conversion characteristics has not yet been obtained.

[Object of the Invention] It is an object of the present invention to provide a magnetic recording medium which has less dropout and good still characteristics, that is, has excellent running durability.

[Summary of the Invention] The present invention relates to a magnetic recording medium comprising a nonmagnetic support and a first magnetic layer and a second magnetic layer provided in this order, wherein the first magnetic layer is a vinyl chloride copolymer as a binder. And a vinyl chloride copolymer having a number average molecular weight of less than 25,000, a polyurethane resin having a number average molecular weight of less than 25,000, and the second magnetic layer having an organic group having active hydrogen as a binder at a ratio of three or more per molecule. A magnetic recording medium comprising isocyanate and having a layer thickness in the range of 0.1 to 1.5 μm.

The second magnetic layer of the magnetic recording medium of the present invention, the first magnetic layer coating layer formed by coating the first magnetic layer coating solution on the surface of the non-magnetic support, while the wet state, continuous Preferably, it is a layer formed by applying a coating solution for the second magnetic layer on the coating layer.

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

1) The magnetic recording medium as described above, wherein the second magnetic layer contains a polyurethane resin as a binder in addition to the vinyl chloride copolymer, and the polyurethane resin has a number average molecular weight of 25,000 or more.

2) The magnetic recording medium as described above, wherein the polyurethane resin contained in the first magnetic layer has a glass transition temperature of less than 0 ° C.

3) The second magnetic layer contains a polyurethane resin in addition to the vinyl chloride copolymer as a binder, and the glass transition temperature of the polyurethane resin contained in the second magnetic layer is 0 ° C. or higher. The magnetic recording medium described above.

4) organic group having active hydrogen, -OH, -NH ₂ and -N
The above magnetic recording medium, which is at least one selected from the group consisting of HR (R is an alkyl group).

5) When the second magnetic layer contains a polyurethane resin,
The above-mentioned magnetic recording medium, wherein the polyurethane resin has three or more organic groups having active hydrogen per molecule.

[Effect of the Invention] As described above, the present invention is a magnetic recording medium having at least two magnetic layers, wherein the second magnetic layer contains a polyisocyanate and a vinyl chloride-based copolymer, and This vinyl chloride-based copolymer has three or more organic groups having active hydrogen per molecule in order to increase the crosslink density of the second magnetic layer.

As described above, since the vinyl chloride copolymer having an organic group having three or more active hydrogens per molecule has high reactivity with polyisocyanate, there is almost no vinyl chloride copolymer that does not participate in crosslinking. Since the reaction (crosslinking) with the polyisocyanate proceeds to an extent, the second magnetic layer becomes an extremely tough magnetic layer. This makes it possible to obtain a magnetic recording medium with less dropout and good still characteristics, that is, excellent running durability.

The lower first magnetic layer has a number average molecular weight as a binder of less than 25,000 (preferably having a glass transition temperature of 0 ° C.).
), The adhesion of the magnetic layer to the non-magnetic support is improved, and the dispersibility of the ferromagnetic powder is improved, so that the surface smoothness of the first magnetic layer is improved. Therefore, durability such as running durability is further improved. Then, in order to sufficiently reflect the high surface smoothness of the lower first magnetic layer also on the surface state of the upper second magnetic layer, the thickness of the second magnetic layer is as thin as 0.1 to 1.5 μm. As a layer, the surface of the second magnetic layer also ensures high smoothness.

[Detailed Description of the Invention] 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. Have.

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, Films or sheets made of vinyl resins such as polyvinylidene chloride, synthetic resins such as polycarbonate, polyamide, polyamideimide, and polyimide; non-magnetic metal foils such as aluminum and copper; metal foils such as stainless steel foil; paper and ceramic sheets 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 magnetic recording medium of the present invention, the first magnetic layer contains a vinyl chloride copolymer as a binder and a polyurethane resin having a number average molecular weight of less than 25,000, and the second magnetic layer contains a vinyl chloride copolymer. Contains polymers and polyisocyanates. And the vinyl chloride copolymer contained in the second magnetic layer has an organic group having active hydrogen at a rate of three or more per molecule, and the layer thickness of the second magnetic layer is 0.1 to
It is characterized in that it is a thin layer in the range of 1.5 μm.

The second magnetic layer of the present invention contains a polyisocyanate and a vinyl chloride-based copolymer, and the vinyl chloride-based copolymer has, per molecule, an organic group having active hydrogen to increase the crosslinking density of the second magnetic layer. It has three or more. As described above, since the vinyl chloride copolymer having an organic group having three or more active hydrogens per molecule has high reactivity with polyisocyanate, there is almost no vinyl chloride copolymer that does not participate in crosslinking. Since the reaction (crosslinking) with the polyisocyanate proceeds to an extent, the second magnetic layer becomes an extremely tough magnetic layer. As a result, it is possible to obtain a magnetic recording medium with less dropout and good still characteristics.

Examples of the organic group having an active hydrogen include -OH, -SH,
_{-NH 2, = NH, -CSNH 2} , -CSNHR, -C (= NH) OH, -
C (= NH) NH, = NOH, -NHNH 2, -CONHNH 2, -NRNH 2,
_{-NHNHR, = NNH 2, -N-} NHR, -NHCONH 2, -NHCONHR,
_{-NHCSNH 2, -NHCSNHR, -NRCSNH 2,} -NRCONH 2, -NHC
_{(= NR) NH 2, -NHC} (= NH) NHR, biguanidine and biuret (wherein, R represents an alkyl group), and the like. Preferably -OH, -NH ₂ and -NHR.
One of these functional groups may be contained in one molecule of the polyurethane resin, or two or more thereof may be contained.

In the first magnetic layer of the present invention, the polyurethane resin used is preferably of relatively low molecular weight, and the running durability is improved so that the dispersibility of the ferromagnetic powder and the smoothness of the magnetic layer surface are improved. In terms of meaning, it is preferable that the molecule is low-molecular and flexible.

Therefore, in order to make the first magnetic layer flexible as described above, it is necessary to use a polyurethane resin in addition to the vinyl chloride copolymer as a binder, and the number of polyurethane resins It is required that the average molecular weight is less than 25,000. More preferably, the glass transition temperature is 0.
It is below ° C. This improves the adhesion of the magnetic layer to the non-magnetic support, thereby improving durability such as running durability.

Also, in the second magnetic layer of the present invention, it is preferable to use a polyurethane resin in combination with the binder in order to further strengthen the magnetic layer. The polyurethane resin contained in the second magnetic layer preferably has a number average molecular weight of 25,000 or more, and more preferably has a glass transition temperature of 0 ° C. or more. In particular, the polyurethane resin contained in the second magnetic layer preferably has three or more organic groups having active hydrogen per molecule.

A multilayer magnetic recording medium comprising two magnetic layers as in the present invention, wherein a vinyl chloride-vinyl acetate copolymer having active hydrogen, a polyester having two hydroxyl groups and a polyisocyanate are used for a lower layer, JP-A-57-138044 and JP-A-57-138045 disclose that a magnetic recording medium with less sensitivity unevenness and output fluctuation can be provided by sufficiently curing the lower layer.
However, the above-mentioned invention is the same as that of the present invention in that a resin having active hydrogen is used, but also has a different configuration, and is used not in the lower layer but in the upper layer, and the purpose is to improve the running durability. This is qualitatively different from the above prior art, in that the durability is improved.

Vinyl chloride copolymers usable in the present invention include, for example, vinyl chloride / vinyl acetate copolymer, vinyl chloride / vinyl acetate / vinyl alcohol copolymer, vinyl chloride / vinyl acetate / acrylic acid copolymer, vinyl chloride vinylidene chloride copolymers, vinyl-acrylonitrile copolymer chloride, and a hydroxyl group, a carboxyl group, a phosphoric acid group, a phosphoric acid ester group, the vinyl chloride polar group is introduced, such as -SO ₃ Na or -SO ₂ Na System copolymers.

Furthermore, polyurethane-based resins usable in the present invention include, for example, polyester polyurethane resins, polyether-based polyurethane resin, hydroxyl group, carboxyl group, phosphoric acid group, a phosphoric acid ester group, such as -SO ₃ Na or -SO ₂ Na Polyurethane-based resins into which polar groups have been introduced, and polycarbonate polyurethane resins.

In addition to the vinyl chloride-based copolymer and the polyurethane-based resin, the binder resin used for forming each magnetic layer is not particularly limited. For example, ethylene-vinyl acetate copolymer, cellulose derivatives such as nitrocellulose resin, acrylic resin, polyvinyl acetal resin, polyvinyl butyral resin, epoxy resin, and phenoxy resin.

These can be used alone or in combination.

However, the specific vinyl chloride copolymer contained in the first magnetic layer is preferably contained in the binder in an amount of at least 20% by weight, and the specific vinyl chloride copolymer contained in the second magnetic layer. Preferably, the copolymer is contained in the binder in an amount of 20% by weight or more. When the value is less than the above value, the effect of the present invention is not exhibited.

In the present invention, a polyisocyanate compound is used for the magnetic layer. The second magnetic layer needs to contain the compound, and it is preferable to use the first magnetic layer. 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 3 mol of tolylene diisocyanate and 1 mol of trimethylolpropane (eg, Desmodur L-75).
(Manufactured by Bayer AG)), a reaction product of 3 moles of a diisocyanate such as xylylene diisocyanate or hexamethylene diisocyanate with 1 mole of trimethylolpropane, a buret addition compound of 3 moles of hexamethylene diisocyanate, and an isocyanurate compound of 5 moles of tolylene diisocyanate And isocyanurate adducts of 3 moles of tolylene diisocyanate and 2 moles of hexamethylene diisocyanate, and polymers of isophorone diisocyanate and diphenylmethane diisocyanate.

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

As in the present invention, when a vinyl chloride-based copolymer having an organic group having active hydrogen (preferably in combination with a polyurethane-based resin) uses a polyisocyanate compound as a curing agent, the vinyl chloride-based copolymer (preferably In addition, the mixing weight ratio between the polyurethane resin) and the polyisocyanate compound is usually 1: 0.8 to 1: 2 (preferably 1: 1 to 1: 1.5)
Is set within the range. In this way, even when the ferromagnetic metal fine powder having a low hardness is used, the softening of the binder accompanying the use of the polyurethane resin can be effectively prevented.

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 type of ferromagnetic metal or alloy (e.g., 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.

The method for producing these ferromagnetic powders is already 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,
Granular, dice-like, rice-granular, plate-like and the like are used. In particular, it is preferable to use acicular ferromagnetic powder.

The above resin component, curing agent and ferromagnetic powder are kneaded and dispersed together with a solvent (eg, methyl ethyl ketone, dioxane, cyclohexanone, ethyl acetate) usually used in the preparation of a magnetic paint 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, an abrasive (eg,
α-Al ₂ O ₃ , Cr ₂ O ₃ ), antistatic agent (eg, carbon black), lubricant (eg, fatty acid, fatty acid ester, silicone oil), dispersant, and other commonly used additives or fillers Of course, it may contain (agent).

The coating is performed by applying the magnetic paint prepared from the above materials on a non-magnetic support by the following method. First, a magnetic layer-forming component such as a resin component for the first magnetic layer, a ferromagnetic powder, and a curing agent that is optionally blended are kneaded and dispersed together with a solvent to prepare a coating solution for the first magnetic layer. Then, a coating solution for the second magnetic layer is prepared for the second magnetic layer in the same manner as for the first magnetic layer.

In the method for producing a magnetic recording medium of the present invention, for example, the coating solution for the first magnetic layer is preferably applied to the surface of the non-magnetic support under running, preferably the thickness of the first magnetic layer after drying is in the range of 0.1 to 5 μm. Is applied so that the coating layer is in a wet state, and the coating liquid for the second magnetic layer is continuously dried on the coating layer in a thickness of 0.1 to 1.5 μm after drying the second magnetic layer. It is preferable to apply so as to be inside. 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).

Examples of the coating machine for applying the magnetic paint include 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, and spray. Coat, spin coat, etc. can be used. In the present invention, an extrusion coat for simultaneous multilayer coating having two slots as described in Japanese Patent Application No. 62-124631 is desirable.

The second magnetic layer obtained by using the above-mentioned manufacturing method, even if the magnetic layer has a very thin layer thickness of 1.5 μm or less, has a uniform layer thickness, and the surface thereof is applied in an extremely smooth state. Can be. 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 a 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 nonmagnetic 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 process, for example, a super calendar roll or the like is used. By performing the surface smoothing treatment, the voids generated by the removal of the solvent during drying disappear, and the filling rate of the ferromagnetic powder in the magnetic layer is improved.
A magnetic recording medium with high electromagnetic conversion characteristics can be obtained.

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 respect to the upper and lower two-layer system, it 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 ₃ 100 parts [Hc: 650 Oe, S BET specific surface area: 35 m ² / g] Vinyl chloride / vinyl acetate / maleic acid copolymer 12 Parts (composition ratio: 86: 13: 1, degree of polymerization 350) polyester polyurethane resin 6 parts (Mn = 23000, Tg = -5 ° C) carbon black 8 parts butyl stearate 1 part stearic acid 2 parts butyl acetate 200 parts second Coating solution for magnetic layer 100 parts Co-γ-Fe ₂ O ₃ [Hc: 700 Oe, S BET specific surface area: 40 m ² / g] Vinyl chloride / vinyl acetate / maleic acid / vinyl alcohol copolymer 10 parts (composition ratio : 85: 10: 1: 4, -OH: 18 units / molecule, degree of polymerization 400) Polyester polyurethane resin 7 parts (Mn = 28000, Tg = -3 ° C, -OH: 3 units / molecule) Polyisocyanate 5 parts ( Coronate L, manufactured by Nippon Polyurethane Co., Ltd.) Carbon black 3 parts α-Al ₂ O ₃ 5 parts (average particle diameter: 0.6 μm) Butyl stearate 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. 6 parts of polyisocyanate (coronate L above) and butyl acetate 40 were added to the resulting dispersion.
And the mixture was filtered using a filter having an average pore diameter of 1 μm to prepare coating liquids for forming the first magnetic layer and for forming the second magnetic layer, respectively.

The application of the coating liquids for forming the first and second magnetic layers was performed as follows using a simultaneous multi-layer extrusion coat having a first magnetic layer coating slot and a second magnetic layer coating slot.

The obtained first magnetic layer coating solution has a thickness of 3.5 μm after drying.
m, while running a polyethylene terephthalate support having a thickness of 14 μm at a speed of 60 m / min, using a extrusion coat having a slot for coating the first magnetic layer on the surface of the support, and immediately thereafter ( Apply the second magnetic layer coating solution to the first magnetic layer while the first magnetic layer is wet using an extrusion coat having a second magnetic layer coating slot so that the thickness after drying is 0.5 μm. While the layer was in a wet state, it was oriented by the above magnet, dried, subjected to a supercalender treatment, and slit to a 1/2 inch width to produce a video tape.

Example 2 In Example 1, the vinyl chloride / vinyl acetate / maleic acid / vinyl alcohol copolymer in the coating solution for the second magnetic layer was used (composition ratio: 85: 10: 1: 4, -OH: 18). A video tape was manufactured in the same manner as in Example 1 except that the composition was changed from the composition of (number of molecules / molecule) to that of (composition ratio: 88: 10: 1: 1, -OH: 4 pieces / molecule).

[Example 3] In Example 1, the vinyl chloride / vinyl acetate / maleic acid / vinyl alcohol copolymer (composition ratio: 85: 10: 1: 4, -OH: 18 units) in the coating solution for the second magnetic layer was used. / Molecule) instead of amino acid-containing vinyl chloride / vinyl acetate copolymer (-NH ₂ : 3
A videotape was produced in the same manner as in Example 1 except that the number of particles / molecule was used.

[Comparative Example 1] In Example 1, a vinyl chloride / vinyl acetate / maleic acid / vinyl alcohol copolymer (composition ratio: 85: 10: 1: 4, -OH: 18 units) in the coating solution for the second magnetic layer was used. / Molecule) instead of vinyl chloride / vinyl acetate / maleic acid copolymer (composition ratio: 8
7: 12: 1, no organic group having active hydrogen) was used in the same manner as in Example 1 except that a video tape was produced.

[Comparative Example 2] In Example 1, the polyester polyurethane resin in the coating solution for the second magnetic layer was changed from (Mn = 23000, Tg = -5 ° C) to (Mn = 28000, Tg = -5 ° C). A video tape was manufactured in the same manner as in Example 1 except for the above.

Table 1 shows the physical properties of each of the video tapes obtained in Examples 1 to 3 and Comparative Examples 1 and 2 measured by the following measurement methods.

Measurement method (1) Still durability A constant video signal was recorded on a video tape obtained using a VHS video tape recorder (NV-8200 [Matsushita Electric Industrial Co., Ltd.]), and the reproduced still image was sharp. The time to loss was measured. (Measurement was performed at 5 ° C. and 80% RH.) (2) Number of clogging A fixed video signal was recorded on the obtained video tape using the above-described VHS video tape recorder, and was recorded for 10 minutes at 120 minutes.
The vehicle was run repeatedly and the number of times the output decreased by 3 dB or more during the run was examined. (Measurement was performed at 5 ° C. and 30% RH.) (3) Adhesive force between nonmagnetic support and magnetic layer 180 ° peeling using a video tape of 1/2 inch width and 5 cm length The required force was measured, and this was defined as the adhesive force.

As is apparent from Table 1, the first magnetic layer contains a vinyl chloride copolymer and a polyurethane resin having a number average molecular weight of less than 25,000, and the second magnetic layer contains an organic group having active hydrogen in one molecule. The magnetic recording medium of the present invention using a vinyl chloride / vinyl acetate copolymer having three or more rubbers has good clogging frequency, still durability and good adhesion of the magnetic layer, and excellent durability such as running durability. You can see that it is.

On the other hand, a conventional magnetic recording medium using a vinyl chloride / vinyl acetate copolymer having no active hydrogen is inferior in the above performance (Comparative Example 1),
Even when a vinyl acetate copolymer is used, when the number average molecular weight of the polyurethane resin contained in the first magnetic layer is 25,000 or more, still durability and adhesive strength are inferior (Comparative Example 2).

Comparative Example 3 (1) A first magnetic layer coating liquid and a second magnetic layer coating liquid having the following compositions were prepared.

First magnetic layer coating solution Co-γ-Fe ₂ O ₃ [Hc: 650 Oe, S BET specific surface area: 35 m ² / g] 100
Part: Vinyl chloride / vinyl acetate / maleic anhydride copolymer [manufactured by Nippon Zeon Co., Ltd., 400 × 110A] 3 parts Polyurethane resin [manufactured by Nippon Polyurethane Co., Ltd., trade name: Nipporan N2301] 24 parts Polyisocyanate compound [Japan Polyurethane Co., Ltd.
Manufactured and trade name: Coronate L] 3 parts Carbon black [Average particle diameter: 40 mμ] 2 parts Cr ₂ O ₃ 5 parts Methyl ethyl ketone 300 parts Second magnetic layer coating solution Co-γ-Fe ₂ O ₃ [Hc: 700 Oe, S BET specific surface area: 40m ² / g] 100
Part: vinyl chloride / vinyl acetate / maleic anhydride copolymer [manufactured by Nippon Zeon Co., Ltd., 400 × 110A] 12 parts Polyurethane resin [manufactured by Nippon Polyurethane Co., Ltd., trade name: Nipporan N2301] 6 parts Polyisocyanate compound [Japan Polyurethane Co., Ltd.
Manufactured and trade name: Coronate L] 12 parts Carbon black [Average particle diameter: 40 mμ] 2 parts Cr ₂ O ₃ 5 parts Methyl ethyl ketone 300 parts [The number average molecular weight of Nipporan N2301 is 230.
The glass transition temperature (Tg) is 33 ° C., and the number of active hydrogens (hydrogen of hydroxyl group) per molecule is 2. (2) A first magnetic layer coating solution and a second magnetic layer coating solution are sequentially coated on a 15 μm-thick polyethylene terephthalate support so that each of the two dried layers has a thickness of 2 μm. After performing a magnetic field orientation treatment using a cobalt magnet in a state where the liquid is not powdered, the solvent is removed by heating to 100 ° C. for 1 minute, and the first magnetic layer and the second magnetic layer are laminated on the support I got a body. This laminate was subjected to a super calender treatment and slit to a 1/2 inch width to produce a video tape.

(3) The electromagnetic conversion characteristics (video S / N) and running durability (S / N reduction, clogging, still durability) of the video tape obtained above were evaluated by the following methods and the above-mentioned methods. The following results were obtained.

1) Video S / N A fixed video signal was recorded on a video tape obtained using a VHS video tape recorder (NV-8200 [Matsushita Electric Industrial Co., Ltd.]), and the S / N was measured. At the same time, the same measurement was performed on the video tape of Example 1. The measured value of the video tape of Example 1 was 0 dB for the S / N of the video tape of Comparative Example 4 manufactured above.
And expressed as relative values.

2) Reduction of S / N A constant video signal is recorded on a long video tape (120 minutes length) obtained using the above VHS video tape recorder,
The initial S / N value and the S / N value after running 10 times were measured, and the difference was expressed in dB.

 The measurement results are shown below.

That is, although the running durability of the video tape of Comparative Example 3 is improved as compared with the previous one, it does not reach much to the video tape of the present invention, and the electromagnetic conversion characteristics are in accordance with the present invention. You can see that it does not reach the level of videotape.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-131021 (JP, A) JP-A-1-205724 (JP, A) JP-A-1-263925 (JP, A) JP-A-60-1985 47228 (JP, A) JP 7-27631 (JP, B2)

Claims (2)

(57) [Claims] 1. A magnetic recording medium comprising a nonmagnetic support and a first magnetic layer and a second magnetic layer provided in this order on a surface of the nonmagnetic support, wherein the first magnetic layer comprises a vinyl chloride copolymer as a binder and a number average molecular weight. Contains less than 25,000 polyurethane-based resin, and the second magnetic layer contains a vinyl chloride-based copolymer having at least three organic groups having active hydrogen as a binder per molecule, and a polyisocyanate. And a layer thickness in the range of 0.1 to 1.5 μm. 2. The method according to claim 1, wherein the second magnetic layer is formed by applying a coating solution for the first magnetic layer on the surface of the nonmagnetic support, and the first magnetic layer coating layer is in a wet state. 2. The magnetic recording medium according to claim 1, wherein the magnetic recording medium is a layer formed by applying a coating liquid for a second magnetic layer onto the magnetic recording medium.