JPH06342516A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium formed with a magnetic coating film uniformly ferromagnetic fine powder and excellent in slidability, traveling property, durability and electromagnetic conversion characteristic. CONSTITUTION:In the first stage, the ferromagnetic fine powder and the copolymer soln. containing the copolymer of vinyl chloride/the unsatd. monomer having -SO3X or -OSO3X/the unsatd. monomer having hydroxy group in the concn. of >=20wt.% are kneaded, and in the second stage, the magnetic layer is formed by using the magnetic coating material prepared by adding and dispersing rubbers resin soln. and an additive in the kneaded liq. which is obtained by kneading the ferromagnetic fine powder and the copolymer soln.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called coating type magnetic recording medium in which a magnetic coating mainly composed of ferromagnetic fine powder and a binder is formed on a non-magnetic support, and more particularly to electromagnetic conversion. The present invention relates to improvement of characteristics, running property and durability.

[0002]

2. Description of the Related Art In magnetic recording materials, particularly audio cassette tapes, tapes having low distortion and excellent electromagnetic conversion characteristics are required because they are used for recording music in recent years. Further, due to the popularization of car stereos, radio-cassettes, etc., it is required to have excellent running performance and durability.

On the other hand, in a video cassette tape, recording at a very high density has been performed by shortening the recording wavelength or narrowing the track width. Therefore, a tape having a high output and a high S / N ratio and excellent electromagnetic conversion characteristics is required. Further, while the total thickness is as thin as 10 μm or less, the popularization of portable VTRs has led to demand for VTR tapes having a running durability that is not as good as that of conventional VTR tapes.

For these audio cassette tapes and video cassette tapes, a ferromagnetic paint prepared by mixing and dispersing ferromagnetic fine powder, a resin binder, an organic solvent, various additives, etc. on a non-magnetic support is applied. As a result, the so-called coating type magnetic recording medium having a magnetic layer is predominant. Therefore, in such a coating type magnetic recording medium, in order to meet the above requirements, the composition of the magnetic paint, the preparation method, and the like have been optimized for the purpose of improving electromagnetic conversion characteristics, running properties, and durability. .

That is, in the above-mentioned coating type magnetic recording medium, the electromagnetic conversion characteristics, runnability and durability are greatly affected by the sliding resistance of the magnetic coating film, the degree of dispersion of the magnetic powder in the magnetic coating film and the like. .

Up to now, it has been considered that the sliding resistance of such a magnetic coating film, the degree of dispersion of the magnetic powder in the magnetic coating film, etc. are mainly dependent on the properties of the binder constituting the magnetic coating film together with the magnetic powder, With respect to various polymers that can be used as a binder, the sliding resistance and the dispersibility when magnetic powders are dispersed have been investigated extensively, and their use has been attempted. However, there is a limit to the improvement of properties only by changing the type of binder used, and it is the actual situation that a sufficiently satisfactory magnetic coating film cannot be formed.

Therefore, Japanese Patent Publication No. 4-81253 proposes a magnetic recording medium which can meet the above requirements by kneading a magnetic coating composition in two stages. That is, in this publication, first, in a first step, ferromagnetic fine powder and a vinyl chloride / vinyl acetate / vinyl alcohol copolymer solution containing vinyl alcohol are kneaded, and then in a second step, When a dispersion of a ferromagnetic fine powder kneaded product obtained by kneading this copolymer solution and ferromagnetic fine powder with a rubber-based resin solution and additives is used as a magnetic paint, the running property and durability are improved. It has been shown that a magnetic recording medium having excellent electromagnetic conversion characteristics can be obtained.

However, when the copolymer solution of vinyl chloride / vinyl acetate / vinyl alcohol copolymer containing vinyl alcohol and the ferromagnetic fine powder are kneaded, the copolymer is thermally decomposed to cause dehydrochlorination reaction. . In the above-mentioned magnetic recording medium, the magnetic coating film formed by this influence still cannot obtain the expected running property and durability.

Therefore, the present invention has been proposed in view of such conventional circumstances, and a magnetic coating film having a sliding resistance and a highly dispersed magnetic powder is formed, and the running property and durability are improved. It is an object of the present invention to provide a magnetic recording medium having excellent properties and electromagnetic conversion characteristics.

[0010]

In order to achieve the above-mentioned object, the present inventors have examined the composition of the binder monomer, the dispersion, the kneading method, the addition order of the composition, etc. for the magnetic coating material. As a result, a vinyl chloride /-SO ₃ X group or --OSO was used as a copolymer for kneading the magnetic coating composition in two predetermined steps and kneading with the ferromagnetic fine powder in the first step.
₃ By using a copolymer of an unsaturated monomer having an X group / an unsaturated monomer having a hydroxyl group, a magnetic coating film can be formed without inducing a reaction that is disadvantageous for ensuring running property and durability. It has been found that a magnetic recording medium which is formed and has excellent running property, durability and electromagnetic conversion characteristics can be obtained.

The magnetic recording medium of the present invention has been completed on the basis of such findings, and in the first step, ferromagnetic fine powder and vinyl chloride /-SO ₃ X group or --OSO are used.
₃ X group (where X is H, Li, Na, K, Rb, Cs,
After kneading with a copolymer solution containing a copolymer of an unsaturated monomer having any of Fr) / an unsaturated monomer having a hydroxyl group at a concentration of 20% by weight or more, the second In the step, a magnetic paint prepared by adding and dispersing a rubber resin solution and an additive to a kneaded product obtained by kneading the ferromagnetic fine powder and a copolymer solution is applied on a non-magnetic support. It is characterized by being done.

Further, the rubber resin is polyester polyurethane.

In the magnetic recording medium to which the present invention is applied, the magnetic layer is formed by applying a magnetic coating material prepared by mixing and dispersing ferromagnetic fine powder, a binder, various additives and the like on a non-magnetic support. It is a so-called coating type magnetic recording medium formed.

In the present invention, in order to make the above-mentioned coating type magnetic recording medium excellent in running property, durability and electromagnetic conversion characteristics, as a magnetic paint for forming a magnetic layer, a ferromagnetic fine powder, Vinyl chloride / -SO _{3 as a} binder
X group or an unsaturated monomer of the copolymer and the rubber-based resin having an unsaturated monomer / hydroxyl having -OSO ₃ X groups, prepared by various additives and kneaded in two stages, is distributed It is assumed that the used magnetic paint is used.

That is, in order to prepare the above magnetic paint,
First, in the first step, ferromagnetic fine powder and vinyl chloride / -SO
_The mixture is kneaded with a copolymer solution containing a copolymer of unsaturated monomer having ₃ X group or —OSO ₃ X group / unsaturated monomer having hydroxyl group at a concentration of 20% by weight or more. At this stage, the ferromagnetic fine powder is in a highly dispersed state by using only the above-mentioned copolymer solution as the dispersion medium. At this time, if a rubber resin or an additive is added, such a highly dispersed state cannot be obtained. When kneaded with the ferromagnetic fine powder, the copolymer having the above-mentioned monomer composition is stable and does not cause a reaction which is disadvantageous in securing the running property and durability of the magnetic recording medium.

Then, in the second step, a rubber-based resin solution and an additive in which a rubber-based resin is dissolved in a solvent is added to the ferromagnetic fine-powder kneaded material in which the ferromagnetic fine powder is highly dispersed,
A magnetic paint is prepared by dispersing.

When the magnetic coating material thus prepared is applied onto a non-magnetic support, ferromagnetic fine powder is uniformly dispersed and a magnetic layer excellent in sliding resistance is formed, so that the running property is improved. Therefore, a magnetic recording medium excellent in durability and electromagnetic conversion characteristics can be obtained.

First, the vinyl chloride used in the first step
Nil / -SO₃X group or -OSO ₃Tired of having X groups
Suitable for copolymers of hydrated monomers / unsaturated monomers having hydroxyl groups
, -SO₃X group or -OSO₃Tired of having X groups
Sum monomer component is a strong acid containing sulfur against unsaturated monomer.
Of strong acids containing alkali metal salts or sulfur
It can be obtained by adding sodium salt.

Examples of the alkali metal salt of a strong acid or the ammonium salt of a strong acid added to the above unsaturated monomer include sulfites such as sodium sulfite, sodium thiosulfate, sodium bisulfite, ammonium sulfite and potassium sulfite, and sulfuric acid. Examples thereof include hydrogensulfates such as sodium hydrogen, potassium hydrogensulfate and ammonium hydrogensulfate, and aminosulfonates such as sodium taurine, sodium sulfamate and potassium sulfanilate.

On the other hand, examples of the unsaturated monomer component having a hydroxyl group which constitutes the above-mentioned copolymer include vinyl acetate, vinyl propionate and other carboxylic acid vinyl esters, methyl vinyl ether, isobutyl vinyl ether, cetyl vinyl ether and other vinyl ethers, and acrylics. Unsaturated carboxylic acids such as acids, methacrylic acid, maleic acid, itaconic acid, unsaturated carboxylic acid anhydrides such as maleic anhydride, diethyl maleate, butylbenzyl maleate, maleic acid-di-2-hydroxyethyl, itaconic acid Dimethyl, methyl (meth) acrylate, ethyl (meth) acrylate,
Lauryl (meth) acrylate, (meth) acrylic acid-2
-Unsaturated carboxylic acids such as hydroxypropyl, allyl alcohol, unsaturated alcohols such as 3-buten-1-ol, and the like.

When selecting the constituent monomers of the above-mentioned copolymer from the above-exemplified monomers, the solubility of the resin is adjusted while adjusting the compatibility with the rubber-based resin used in the second step and the softening point. In addition to the purpose of improving the coating property, it is preferable to appropriately select it depending on the characteristics of the coating film and the necessity of improving the coating process.

The above copolymer is obtained by copolymerizing vinyl chloride with an unsaturated monomer having such a —SO ₃ X group or —OSO ₃ X group and an unsaturated monomer having a hydroxyl group. .

Here, the vinyl chloride constituting the above copolymer
Component, -SO₃X or -OSO ₃Tired of having X groups
Ratio of sum monomer component and unsaturated monomer component having hydroxyl group
Is a vinyl chloride component / -SO₃X or -OSO₃X group
Unsaturated Monomer Component Having / Unsaturated Monomer Having Hydroxyl Group
Body is (70-90) / (5-25) / (5-25)
Preferably.

The degree of polymerization of the above copolymer is about 200-
It is preferably 600, and more preferably 300 to 400. When the degree of polymerization of the copolymer is less than 200,
When a magnetic coating film having sufficient durability is not formed and the amount exceeds 500, the solubility in the solvent is insufficient when the copolymer solution is prepared by dissolving the copolymer in the solvent.

It is theoretically impossible for the above copolymer to have a molecular weight distribution of 1.0 or less.
When it is 0 or more, the durability of the magnetic coating film is insufficient. Therefore, it is preferable to select a synthesis method or the like so that the molecular weight distribution does not become 3.0 or more.

The copolymer obtained by copolymerizing the three types of monomer components as described above is dissolved in a solvent to form a copolymer solution in which the ferromagnetic fine powder is kneaded. As a solvent for dissolving the copolymer, a solvent having a boiling point of 50 ° C. or higher and 200 ° C. or lower is used alone or in combination. When a mixture of a plurality of solvents is used, even if it does not dissolve the above-mentioned copolymer alone, it can be used as long as it is within the range in which the above-mentioned copolymer dissolves when used as a mixed solvent. Is.

The copolymer concentration in this copolymer solution is 2
Must be 0% by weight or more, 30% by weight or more, 50
It is preferable to set the content to be not more than weight%. By making the copolymer solution relatively concentrated in this way, the ferromagnetic fine powder is highly dispersed in the copolymer solution, and it is possible to form a magnetic coating film with excellent electromagnetic conversion characteristics. become. However, in the copolymer solution, when the copolymer concentration is about 50% by weight or more, the viscosity becomes too high to form a gel and the handling becomes difficult.

On the other hand, the strong magnetic material to be kneaded with the above copolymer solution.
As a fine powder, it is usually used in this type of medium.
Any of the above can be used. For example, γ-
Fe ₂O₃, Fe₃O_Four， Co modified iron oxide, iron as the main component
Other than synthetic fine powder, modified barium ferrite, modified
Examples thereof include strontium ferrite.

For kneading the ferromagnetic fine powder and the copolymer solution, a conventionally known kneading device such as an open kneader, a pressure kneader, a helical rotor, a continuous kneader, a three roll mill, a taper roll and an internal mixer is used. it can. A ferromagnetic fine powder kneaded material having a viscosity of about 1000 poise or higher is prepared by these kneading devices.

In the second stage, the ferromagnetic fine powder kneaded product obtained by kneading the ferromagnetic fine powder and the copolymer solution in this way is mixed with a rubber-based resin solution prepared by dissolving a rubber-based resin in a solvent. Add additives and disperse.

Examples of the rubber-based resin include polyurethane rubber (polyurethane resin), styrene-butadiene rubber, butadiene rubber, isobutylene rubber, chloroprene rubber,
Examples thereof include isobutylene / isoprene rubber, acrylonitrile butadiene rubber, chlorinated butyl rubber, acrylic rubber, and epichlorohydrin rubber, and polyurethane rubber (polyurethane resin) is particularly preferable.

The solvent in which the rubber-based resin is dissolved has a boiling point of 50 ° C. as in the case of dissolving the copolymer.
A solvent having a temperature of 200 ° C. or higher is used alone or as a mixture. On the other hand, the additives to be added to the ferromagnetic fine powder dispersion together with the rubber-based resin are usually added to the magnetic layer in this type of medium such as a dispersant, a lubricant, a stabilizer, an abrasive and an antistatic agent. It is what is done.

As the dispersing machine for dispersing the rubber-based resin solution, various additives and the ferromagnetic fine powder kneaded material, for example, a ball mill, an attritor, a sand glider, a vibration mill, or the like can be used.
A conventionally known one can be used, and a ball mill and a sand glider are particularly suitable.

The magnetic coating material is prepared as described above, and it is desirable that the prepared magnetic coating material has a viscosity of about 1 to 500 poise, preferably about 10 poise to 200 poise. Therefore, in the above magnetic coating material preparation process, it is preferable to consider the conditions to be set so that the viscosity falls within the above range.

In the finally prepared magnetic paint, the total amount of the copolymer and the rubber resin is the ferromagnetic fine powder 10.
It is preferably 10 to 100 parts by weight with respect to 0 parts by weight, and more preferably 15 to 40 parts by weight.

[0036]

In preparing the magnetic coating material, first, in the first step, the ferromagnetic fine powder and vinyl chloride /-SO ₃ X group or-
When a copolymer solution containing a copolymer of an unsaturated monomer having an OSO ₃ X group / an unsaturated monomer having a hydroxyl group at a concentration of 20% by weight or more is kneaded, ferromagnetic fine powder is obtained. Highly dispersed in the monomer-based copolymer solution. At this time, if a rubber resin or an additive is added, the ferromagnetic fine powder cannot be made into such a highly dispersed state. Upon kneading with the ferromagnetic fine powder, the copolymer having the above-mentioned monomer composition is stable, and the running property of the magnetic recording medium
It does not cause a reaction that is disadvantageous in ensuring durability.

Then, in the second step, a rubber-based resin obtained by dissolving a rubber-based resin in a solvent and an additive are added to and dispersed in the ferromagnetic fine-powder kneaded product in which the ferromagnetic fine powder is highly dispersed. By doing so, a magnetic paint is prepared.

When the magnetic coating material thus prepared is applied onto a non-magnetic support, a ferromagnetic fine powder is uniformly dispersed and a magnetic layer excellent in sliding resistance is formed, so that the running property is improved. A magnetic recording medium having excellent durability and electromagnetic conversion characteristics can be obtained.

[0039]

EXAMPLES Hereinafter, specific examples of the present invention will be described, but it goes without saying that the present invention is not limited to these examples.

Example 1 The ferromagnetic fine powder, the copolymer solution, the rubber resin solution and the additives used in the preparation of the magnetic paint in this example are shown below. Ferromagnetic fine powder: γ-Fe ₂ O ₃ powder (coercive force Hc400
Oe, acicular ratio 10/1, average particle length 0.3 μm, specific surface area 22 m ² / g) Copolymer solution: vinyl chloride / -SO ₃ X or -OSO
₃ % by weight of copolymer of unsaturated monomer having X group / unsaturated monomer having hydroxyl group in cyclohexanone solution
(In this example, as the copolymer, an unsaturated monomer having vinyl chloride /-SO ₃ Na group / unsaturated monomer having a hydroxyl group (( Polymerization degree 320) was used) Rubber resin solution: Polyester polyurethane resin (manufactured by Nippon Polyurethane Industry Co., Ltd., trade name N-2304) dissolved in methyl ethyl ketone at a concentration of 30% by weight Additives: oleic acid, silicone oil ( Degree of polymerization about 6
0) In this example, the above magnetic coating composition was kneaded and dispersed in two steps to prepare a magnetic coating material to prepare a magnetic recording medium.

That is, first, the ferromagnetic fine powder 100
18 parts by weight of the copolymer solution (solid content) are mixed,
The mixture was kneaded for 2 hours in an open kneader to prepare a kneaded product of ferromagnetic fine powder. (First stage)

Next, the ferromagnetic fine powder kneaded material thus prepared was charged into a ball mill, and 10 parts by weight of the rubber resin solution, 2 parts by weight of oleic acid, and 0.5 parts by weight of silicone oil were added. At the same time, the total amount of the liquid was adjusted to 350 parts by weight by adding butyl acetate. In this way, various magnetic coating compositions were put into the ball mill and dispersed for 48 hours to prepare a magnetic coating, and the average particle size was 3 μm.
m filter. . (Second stage)

The magnetic coating material thus prepared was coated on a polyethylene terephthalate (PET) film having a thickness of 7 μm by a reverse roll method so that the dry thickness was 5 μm. Magnetic field orientation treatment was performed with an electromagnet, and the coating film was dried. After drying, the coating film was subjected to a super calendar roll treatment to form a magnetic layer having a smooth surface, and the magnetic layer was cut to a width of 3.8 mm to prepare a magnetic tape.

Example 2 As a copolymer solution, vinyl chloride /-SO ₃ X or-
A copolymer of an unsaturated monomer having an OSO ₃ X group / an unsaturated monomer having a hydroxyl group is added to a cyclohexanone solution in an amount of 20.
A magnetic tape was produced in the same manner as in Example 1 except that the one dissolved at a concentration of wt% was used.

Comparative Example 1 As a copolymer solution, vinyl chloride /-SO ₃ X or-
A copolymer of an unsaturated monomer having an OSO ₃ X group / an unsaturated monomer having a hydroxyl group is contained in a cyclohexanone solution.
A magnetic tape was produced in the same manner as in Example 1 except that the one dissolved at a concentration of wt% was used.

Comparative Example 2 As a copolymer solution, vinyl chloride /-SO ₃ X or--
A copolymer of an unsaturated monomer having an OSO ₃ X group / an unsaturated monomer having a hydroxyl group is added to a cyclohexanone solution in an amount of 10
A magnetic tape was produced in the same manner as in Example 1 except that the one dissolved at a concentration of wt% was used.

Comparative Example 3 A magnetic tape was produced in the same manner as in Example 1 except that a vinyl chloride / vinyl acetate / vinyl alcohol copolymer was used as the copolymer dissolved in the copolymer solution.

Comparative Example 4 A magnetic tape was prepared in the same manner as in Example 1 except that a vinyl chloride / unsaturated monomer copolymer having a hydroxyl group was used as the copolymer dissolved in the copolymer solution. .

Comparative Example 5 A magnetic tape was prepared in the same manner as in Example 1 except that a vinyl chloride / vinyl acetate / vinyl alcohol copolymer containing vinyl alcohol was used as the copolymer dissolved in the copolymer solution. It was made.

Comparative Example 6 The procedure of Example 1 was repeated except that the kneaded material obtained by mixing the rubber-based resin solution together with the ferromagnetic fine powder and the copolymer solution and kneading the mixture with an open kneader was used as the magnetic paint. A magnetic tape was produced.

Comparative Example 7 Ferromagnetic fine powder, rubber solution together with copolymer solution,
A kneaded product was prepared by mixing oleic acid and silicone oil and kneading with an open kneader. The kneaded product was put into a ball mill, and butyl acetate was further added, and then dispersed. A magnetic tape was produced in the same manner as in Example 1 except that the thus obtained dispersion liquid was used as a magnetic paint.

Comparative Example 8 Ferromagnetic fine powder, copolymer solution, rubber resin solution, oleic acid, silicone oil and butyl acetate were put into a ball mill and dispersed. A magnetic tape was produced in the same manner as in Example 1 except that the thus obtained dispersion liquid was used as a magnetic paint.

Comparative Example 9 Ferromagnetic fine powder, copolymer solution and butyl acetate were weighed out according to the following composition and put into a ball mill and dispersed for 24 hours. Ferromagnetic fine powder (γ-Fe ₂ O ₃ powder): 100 parts by weight Vinyl chloride / unsaturated monomer having —SO ₃ X or —OSO ₃ X group / copolymerization of unsaturated monomer having hydroxyl group Copolymer solution in which the combined product was dissolved in butyl acetate at a concentration of 30% by weight: 60 parts by weight Butyl acetate: 177.5 parts by weight After dispersion, further add a rubber-based resin solution and various additives according to the following composition. It was put into a ball mill and dispersed for 24 hours.

Rubber polyurethane resin solution in which polyester polyurethane resin was dissolved in ethyl acetate at a concentration of 30% by weight: 10 parts by weight Oleic acid: 2 parts by weight Silicone oil (degree of polymerization: about 60): 0.5 parts by weight A magnetic tape was produced in the same manner as in Example 1 except that the obtained dispersion was used as a magnetic paint.

Table 1 summarizes the details of the addition stages of various resin coating compositions. In Table 1, a is a case of addition at the time of kneading with an open kneader, b is a case of addition at the start of ball mill dispersion, and c is a case of addition after 24 hours of ball mill dispersion.

[0056]

[Table 1]

With respect to the magnetic tape manufactured as described above, the maximum output level of the frequency 315 Hz signal and the maximum output level of the frequency 10 kHz signal were measured, and a temperature of 25 ° C. was measured using 100 commercially available cassette decks.
Relative humidity 50% environment or temperature 40 ℃, relative humidity 8
A magnetic tape was run in a 0% environment while playing back a signal with a frequency of 10 kHz, and the runnability, reduction in playback output level, head contamination, and squeaking of the tape were evaluated.

The results are shown in Table 2. In Table 2, the maximum reproduction output is a relative value when the maximum reproduction output of the reference tape (reference tape manufactured by Sony Corporation) is 0 dB. A, B, C, and D represent the following cases in the runnability, the level drop, the head dirt, and the squeal of the tape. Runability A: When there is no winding disorder on the magnetic tape B: When winding disorder occurs on 1 to 5 cassette decks C: When winding disorder occurs on 5 to 10 cassette decks D: 11 or more units When winding disorder occurs in the cassette deck Level drop A: There is a level drop of less than 3 dB B: There is a level drop of less than 6 dB C: There is a level drop of less than 9 dB D: There is a level drop of 9 dB or more Case Head dirt A: There is no dirt at all, but it is difficult to confirm even if it exists. B: If there is some dirt but there is no problem. C: If there is much dirt. Tape squeaking. Tape squeaking. A: Tape When there is no squeal B: When squeaking occurs temporarily on the magnetic tape of 1 to 2 volumes C: When squeaking occurs temporarily on the magnetic tape of 3 to 5 windings D: Magnet of 5 or more windings When the air tape temporarily squeals and the magnetic tape of 1 to 2 or more turns constantly.

[0059]

[Table 2]

As can be seen from Table 2, there are 20 ferromagnetic fine powders, vinyl chloride / -SO ₃ X or -OSO ₃ X group-containing unsaturated monomer / hydroxyl group-containing unsaturated monomer copolymer. The magnetic tapes of Examples 1 and 2 using the magnetic coating material in which the copolymer solution, the rubber-based resin solution, and the additive which are dissolved in a concentration of not less than wt% are kneaded and dispersed in a predetermined procedure are copolymer used in the coalescence solution vinyl chloride / -SO ₃ X
Alternatively, the kneading and dispersing procedures of the magnetic tapes of Comparative Examples 3 to 5 or each magnetic coating composition which is not a copolymer of an unsaturated monomer having an —OSO ₃ X group / an unsaturated monomer having a hydroxyl group are predetermined. Compared to the magnetic tapes of Comparative Example 6 to Comparative Example 9 which are different from the above procedure, they are excellent in running property, durability and electromagnetic conversion characteristics.

For example, as the magnetic tape of Comparative Example 4, a copolymer of vinyl chloride / hydroxyl group-containing unsaturated monomer containing no polar group is used as the copolymer kneaded with the ferromagnetic powder in the first step. In that case, the dispersion of the ferromagnetic fine powder becomes insufficient, and good characteristics cannot be obtained. Vinyl chloride containing vinyl alcohol like the magnetic tape of Comparative Example 5 /
When a vinyl acetate / vinyl alcohol copolymer is used, the copolymer is thermally decomposed at the time of kneading, and due to this effect, the level is liable to be lowered and the head is easily stained, that is, the durability is low. .

Further, as in Comparative Examples 6 and 7, when the ferromagnetic fine powder and the copolymer solution are kneaded, the ferromagnetic fine powder is dispersed even when a rubber resin or an additive is mixed. It becomes insufficient and good characteristics cannot be obtained. From this, the procedure of kneading and dispersing the magnetic coating composition is divided into two predetermined steps, and a vinyl chloride /-SO ₃ X or --OSO ₃ X group is used as a copolymer to be kneaded with the ferromagnetic fine powder in the first step. It can be seen that the use of a copolymer of an unsaturated monomer having a hydroxyl group / an unsaturated monomer having a hydroxyl group is effective in obtaining a magnetic tape having excellent performance.

However, as in Comparative Example 1 and Comparative Example 2, the first
In the step, the concentration of the copolymer solution to be kneaded with the ferromagnetic fine powder is 2
When the content is less than 0% by weight, the dispersion of the ferromagnetic fine powder cannot be said to be sufficient and the characteristics are insufficient as compared with the first and second embodiments. From this, it is understood that the copolymer solution to be kneaded with the ferromagnetic fine powder in the first step needs to be sufficiently concentrated.

[0064]

As is apparent from the above description, in the magnetic recording medium of the present invention, the ferromagnetic fine powder and vinyl chloride /-SO ₃ X group or --OSO ₃ X group are added in the first step. After kneading with a copolymer solution containing a copolymer of unsaturated monomer having / unsaturated monomer having hydroxyl group at a concentration of 20% by weight or more, in the second step, the ferromagnetic fine powder is obtained. Since a magnetic coating material prepared by adding and dispersing a rubber-based resin solution and an additive to a kneaded product obtained by kneading and a copolymer solution, a ferromagnetic fine powder with excellent sliding resistance is obtained. A magnetic coating film that is evenly dispersed is formed,
It is possible to obtain a magnetic recording medium having excellent durability and electromagnetic conversion characteristics.

Claims (2)

[Claims] 1. In the first step, ferromagnetic fine powder and vinyl chloride /-SO ₃ X group or --OSO ₃ X group (where X is H, Li, Na, K, Rb, Cs or Fr). In the second step, after kneading with a copolymer solution in which the copolymer of unsaturated monomer having a) / unsaturated monomer having a hydroxyl group is contained at a concentration of 20% by weight or more, A magnetic paint prepared by adding a rubber-based resin solution and an additive to a kneaded material obtained by kneading the ferromagnetic fine powder and a copolymer solution, and applying the magnetic paint on a non-magnetic support. A magnetic recording medium characterized by the above. 2. The magnetic recording medium according to claim 1, wherein the rubber-based resin is polyester polyurethane.