JPH08335311A - Binder for magnetic recording body and magnetic recording body - Google Patents

Abstract

PURPOSE: To improve the dispersibility of polyvinyl acetal resin to a styrene- acrylic acid copolymer by grafting the polyvinyl acetal resin with a copolymer of a styrene monomer with (meth)acrylic ester. CONSTITUTION: This binder is based on a graft polymer consisting of 0.5-30wt.% polyvinyl acetal resin having a polymn. degree of 200-4,000 and 70-99.5wt.% copolymer of a styrene monomer with (meth)acrylic ester. This binder has superior magnetic powder dispersing ability and forms a heat resistant magnetic coating film excellent in wear resistance. The reason for the properties of the binder is that the dispersibility of the polyvinyl acetal resin to a styreneacrylic acid copolymer is extremely improved because the styrene monomer is copolymerized with the (meth)acrylic ester by soln. polymn. in the presence of the polyvinyl acetal resin.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a binder for a magnetic recording material and a magnetic recording material, and more particularly to a magnetic coating film having excellent dispersibility of magnetic powder and excellent abrasion resistance and heat resistance. And a binder for a magnetic recording body capable of forming a magnetic recording medium.

[0002]

2. Description of the Related Art A magnetic recording material such as a magnetic tape or a magnetic disk is generally prepared by applying a magnetic paint obtained by mixing magnetic powder and a binder onto a substrate such as polyester.
It is manufactured by drying to form a magnetic coating film.

The above magnetic recording medium is required to have excellent electromagnetic characteristics and durability such as resistance to contact with a magnetic head. Therefore, the binder for a magnetic recording material can form a magnetic coating film having excellent dispersibility of magnetic powder in the case of forming a magnetic coating, and having excellent abrasion resistance and heat resistance. Various characteristics such as being present are required.

Conventionally, a vinyl chloride-vinyl acetate-vinyl alcohol polymer has been widely used as a binder satisfying the above requirements (Japanese Patent Publication No. 55-34493).

On the other hand, with the recent increase in the density of magnetic recording, it has become necessary to use magnetic powder having a smaller particle size. As a result, when the above vinyl chloride-vinyl acetate-vinyl alcohol polymer is used as a binder,
There is a problem that the dispersibility of fine magnetic powder is not sufficient.

Therefore, in place of the above-mentioned polymer, a polyvinyl acetal resin has been proposed as a binder resin having excellent affinity with magnetic powder and exhibiting good dispersibility for fine magnetic powder. (JP-A-61-2722)
4 and JP-A-1-236290).

It is also proposed to use a copolymer composed of an aliphatic lower unsaturated acid, acrylonitrile, and an aliphatic lower unsaturated acid ester as a binder for a magnetic recording material having excellent abrasion resistance. Has been done. (Japanese Patent Publication Sho 48-5483
Issue)

[0008]

DISCLOSURE OF THE INVENTION An object of the present invention is to provide a magnetic recording material which can form a magnetic coating film having more excellent magnetic powder dispersibility and excellent abrasion resistance and heat resistance. It is to provide a binder and a magnetic recording material.

[0009]

The binder for a magnetic recording medium according to the invention of claim 1 has (a) a degree of polymerization of 200 to 400.
0.5 to 30% by weight of polyvinyl acetal resin which is 0
And (b) a copolymer of styrene monomer and (meth) acrylic acid ester in an amount of 70 to 99.5% by weight as a main component.

A binder for a magnetic recording medium according to a second aspect of the present invention is obtained by copolymerizing a styrene monomer and a (meth) acrylic acid ester by a solution polymerization method in the presence of a polyvinyl acetal resin. It is characterized in that the main component is a copolymer.

A magnetic recording medium according to a third aspect of the invention is characterized by containing magnetic powder and the binder for the magnetic recording medium according to the first or second aspect. Hereinafter, the details of the present invention will be described.

The polyvinyl acetal resin used in the inventions of claims 1 and 2 is obtained by acetalizing polyvinyl alcohol with an aldehyde. The polyvinyl alcohol may be a completely saponified product or a partially saponified product. The saponification degree is preferably 8
Polyvinyl alcohol of 8-100 mol% is used. Specific examples of the aldehyde include acetaldehyde, formaldehyde, propionaldehyde, butyraldehyde, amylaldehyde, hexylaldehyde, heptylaldehyde, 2-ethylhexylaldehyde, cyclohexylaldehyde, furfural,
Gliogizar, glutaraldehyde and the like can be mentioned, but the invention is not limited thereto. Moreover, you may use 2 or more types of aldehydes among the said aldehydes.
Note that formaldehyde includes paraformaldehyde and acetaldehyde includes paraacetaldehyde.

The degree of polymerization of the polyvinyl acetal resin used in the invention of claim 1 is required to be 200 to 4000, preferably 300 to 2000, and is particularly limited in the invention of claim 2. However, the degree of polymerization is preferably 200 to 4000, more preferably 300 to 2000. Degree of polymerization is 200
If it is less than the above range, it may be difficult to obtain a certain thickness when the magnetic coating material in which the magnetic powder and the binder are mixed is applied to the substrate.
On the other hand, when the degree of polymerization is more than 4000, the viscosity becomes high, coating becomes difficult, and a film may not be formed.

The degree of acetalization of the polyvinyl acetal resin used in the inventions of claims 1 and 2 is preferably 60% by weight or more. When the degree of acetalization is less than 60% by weight, the compatibility of the styrene-based monomer and (meth) acrylic acid ester copolymer with the polyvinyl acetal resin is poor, and the dispersibility of the magnetic powder and the magnetic properties of the magnetic coating after drying are low. It may be difficult to satisfy both the film strength.

Specific examples of the styrene-based monomer used in the present invention include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, α-methylstyrene, p-ethylstyrene and 2,4- Dimethyl styrene,
p-n-butylstyrene, p-ter-butylstyrene, p-n-hexylstyrene, p-n-octylstyrene, p-n-nonylstyrene, p-n-decylstyrene, p-n-dodecylstyrene, p -Methoxystyrene, p-phenylstyrene, p-chlorostyrene, 3,
4-dichlorostyrene etc. are mentioned. Of these, styrene is the most preferable.

Specific examples of the above-mentioned (meth) acrylic acid ester include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate,
N-Butyl (meth) acrylate, isobutyl (meth) acrylate, n-octyl (meth) acrylate, (meth)
Dodecyl acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate, 2-hydroxyethyl methacrylate, methyl α-chloroacrylate and the like can be mentioned. In addition, acid groups such as acrylic acid, methacrylic acid, maleic acid, itaconic acid, styrenesulfonic acid, sodium styrenesulfonate, 2-acrylamido-2-methylpropanesulfonic acid, 2-methacrylamido-2-methylpropanesulfonic acid, etc. Examples thereof include (meth) acrylic acid esters and (meth) acrylic acid esters containing a basic group such as dimethylaminoethyl, diethylaminomethacrylate and dimethylaminopropylmethacrylamide. Among these, methyl methacrylate, n-butyl (meth) acrylate, and 2-ethylhexyl acrylate are preferred.

In the first and second aspects of the invention, the content of the styrene-based monomer component is 40 to 95% by weight, more preferably 6%.
Copolymerization is preferably performed so that the content of the (meth) acrylic acid ester component is 0 to 90% by weight, and the content of the (meth) acrylic acid ester component is 5 to 60% by weight, more preferably 10 to 40% by weight.

When the content of the styrene-based monomer component is less than 40% by weight, the copolymer becomes hard and it is difficult to pulverize it, and it may not be possible to make fine powder. If the content of the (meth) acrylic acid ester component is less than 5% by weight, the wear resistance and durability of the magnetic recording material may be reduced.

To obtain the graft polymer of the present invention, a polyvinyl acetal resin having a functional group, a functional group capable of reacting with the functional group, and a (meth) acrylic acid ester and a styrene-based resin are contained. It may be reacted with a copolymer of monomers. As the reaction method, a method of copolymerizing the styrene-based monomer and the (meth) acrylic acid ester by a solution polymerization method in the presence of the polyvinyl acetal resin, and the polyvinyl acetal resin and a styrene-based monomer separately polymerized Examples thereof include a method of dissolving a monomer and a (meth) acrylic acid ester copolymer in a solution, and heating and mixing.

The above-mentioned functional group contained in the polyvinyl acetal resin is mainly a hydroxyl group, but in addition, sulfonic acid, sodium sulfonate, primary amine, secondary amine, tertiary amine, carboxylic acid, phosphoric acid, etc. Is mentioned. Therefore, the functional group contained in the copolymer of the styrene-based monomer and the (meth) acrylic acid ester is particularly limited as long as it is a functional group capable of reacting with the functional group contained in the polyvinyl acetal resin. Not a thing. Preferably, a carboxylic acid group is used as the functional group of the copolymer of the styrene-based monomer and the (meth) acrylic acid ester with respect to the hydroxyl group in the polyvinyl acetal resin.

The amount of the polyvinyl acetal resin contained in the binder of the invention of claim 1 is required to be 0.5 to 30% by weight. When the content is less than 0.5% by weight, the effect due to the addition of the polyvinyl acetal resin is not exhibited, and the strength of the magnetic film after drying the magnetic coating may be weakened. On the contrary, if the content is more than 30% by weight, the binder becomes hard and may not be crushed.

In the invention of claim 2, in order to obtain a binder by copolymerizing the styrene monomer and the (meth) acrylic acid ester solution in the presence of the polyvinyl acetal resin, a binder is generally used. Since the polyvinyl acetal resin does not dissolve in any of the styrene-based monomer and the (meth) acrylic acid ester, the polyvinyl acetal resin is put in an appropriate solvent and dissolved while heating if necessary. While adding the mixture of the monomer and the (meth) acrylic acid ester little by little, heating is carried out if necessary to carry out polymerization in the boiling point region of the solvent, and the reaction system is further heated after the completion of the copolymerization reaction to carry out the above solvent. Remove. According to this method, for example, compared to a hot melt blend of the resin obtained by copolymerizing the styrene-based monomer and the (meth) acrylic acid ester and the polyvinyl acetal resin, the resins are extremely uniform. The dispersed binder of the invention of claim 2 is obtained.

Specific examples of the above solvent include toluene, xylene, methyl isobutyl ketone, methyl ethyl ketone, ethyl acetate, tetrahydrofuran, cyclohexane, dimethylformamide and the like. Among these, toluene, xylene, methyl isobutyl ketone, and methyl ethyl ketone are preferably used, and toluene is particularly preferably used.

The amount of the polyvinyl acetal resin contained in the binder of the invention of claim 2 is preferably 1 to 50% by weight. When the content is less than 1% by weight, the effect of adding the polyvinyl acetal resin does not appear and the strength of the magnetic film after drying the magnetic coating may be weakened. On the contrary, if the content is more than 50% by weight, the binder becomes hard and may not be crushed.

Further, the binder for a magnetic recording medium according to the inventions of claims 1 and 2 may contain a polyfunctional polyisocyanate in order to further enhance the strength (wear resistance) of the magnetic film. Examples of such polyfunctional polyisocyanates include tolylene diisocyanate, diphenyl isocyanate, dianisidine diisocyanate, tridene diisocyanate, hexamethylene diisocyanate, reaction adducts of trimethylolpropane and tolylene diisocyanate, and the like. It is not limited to.

The above-mentioned polyfunctional polyisocyanate includes a vinyl alcohol moiety remaining in the polyvinyl acetal resin, a hydroxyl group contained in the copolymer of a styrene monomer and a (meth) acrylic acid ester in some cases, and a magnetic coating film. When formed, it is cross-linked to form a magnetic coating film having more excellent abrasion resistance and heat resistance.

However, when the amount of the polyfunctional polyisocyanate is too large, the cross-linking density becomes excessively high, so that the magnetic coating film becomes hard and brittle. On the other hand, when the blending amount of the polyfunctional polyisocyanate is too small, the effect of improving wear resistance and heat resistance is poor. Therefore, it is usually preferable to add 0.3 to 30 parts by weight of the polyfunctional polyisocyanate to 100 parts by weight of the binder.

In order to prepare a magnetic paint using the above-mentioned binder for a magnetic recording medium according to the present invention, magnetic powder, for example, cobalt-γ-iron oxide magnetic powder,
The binder may be added and kneaded together with a surfactant and other additives as necessary. In the preparation of this magnetic coating material, the above-mentioned binder was added to 100 parts by weight of magnetic powder.
Usually, it is used in the range of 10 to 100 parts by weight.

The magnetic coating material obtained as described above is applied to the surface of a substrate by a conventional method to orient the magnetic powder contained in the magnetic coating material, and the magnetic coating material is dried to form a magnetic coating film on the surface of the substrate. By doing so, a magnetic recording body is produced. The substrate is not limited, for example, polyester, triacetyl cellulose, polyamide, polyimide, polyamide imide, polystyrene,
Those formed of polyvinyl alcohol, nitrocellulose or the like are used, and particularly preferably, a substrate made of polyester is used.

[0030]

The binder for magnetic recording material and the magnetic recording material using the same according to claim 1, wherein the specific polyvinyl acetal resin is a copolymer of a styrene monomer and a (meth) acrylic acid ester. Since the graft resin is grafted, the graft resin is present between both resins which are originally incompatible with each other, so that the resin is in a partially but compatibilized state. As a result, the polyvinyl acetal resin is dispersed very well in the styrene-acrylic acid copolymer, and the dispersibility of the magnetic powder is improved and the glossiness is improved as compared with the case where it is not grafted. Further, since the polyvinyl acetal resin is uniformly dispersed, when the magnetic coating material is applied to the substrate, it is easy to apply and it is possible to apply it with a uniform thickness. Further, the magnetic coating film after drying the magnetic coating also has excellent wear resistance and heat resistance.

In the binder for a magnetic recording material and the magnetic recording material using the same according to claim 2, the binder comprises a styrene monomer and a (meth) acrylic acid ester in the presence of a polyvinyl acetal resin. It is copolymerized by the solution polymerization method, and the polyvinyl acetal resin is very well dispersed in the styrene-acrylic acid copolymer. Therefore,
The dispersibility of the magnetic powder becomes very good and the glossiness becomes good as compared with a mixture of both resins. Further, since the polyvinyl acetal resin is uniformly dispersed, when the magnetic coating material is applied to the substrate, it is easy to apply and it is possible to apply it with a uniform thickness. Further, the magnetic coating film after drying the magnetic coating also has excellent wear resistance and heat resistance.

[0032]

EXAMPLES The present invention will be clarified by giving non-limiting examples and comparative examples of the present invention.

Example 1 15 parts by weight of polyvinyl butyral resin having a degree of polymerization of 1000 and an acetalization degree of 63% by weight and 50 parts by weight of toluene were charged into a flask, and the inside of the flask was replaced with nitrogen gas until the boiling point of toluene was reached. Heated. 60% by weight of styrene, 10% by weight of methyl methacrylate, 15% by weight of n-butyl acrylate while stirring under reflux of toluene.
By weight, 85 parts by weight of a mixed solution of 10% by weight of acrylic acid, 5% by weight of β-hydroxyethyl acrylate and 2% by weight of benzoyl peroxide (polymerization initiator) was added dropwise over 2.5 hours to carry out solution polymerization. went. After completion of the dropping, aging was carried out for 2 hours while stirring under reflux of toluene. Then, while gradually raising the temperature in the flask to 180 ° C., the solvent was removed from the toluene under reduced pressure to obtain a binder for a magnetic recording body.

In the binder for magnetic recording material obtained as described above, it is determined whether or not the polyvinyl acetal resin and the copolymer of the styrene monomer and the (meth) acrylic acid ester are grafted. It was confirmed by the decrease of.
Specifically, the grafting was confirmed by the reduction of the carboxylic acid group contained in the styrene-acrylic acid copolymer as a functional group. The method was carried out by comparing the acid value in the state where both resins were mixed without grafting and the acid value in the state where they were grafted by heating and mixing. The acid value was measured by dissolving a certain sample in methyl ethyl ketone and titrating with a potassium hydroxide / isopropyl alcohol solution using a phenolphthalein solution as an indicator. At that time, the acid value is represented by the consumption amount (mg) of potassium hydroxide required for neutralizing 1 g of the sample.

The binder for magnetic recording material obtained as described above had an acid value of 48 KOHmg / g, and a styrene-acrylic acid copolymer was separately polymerized, and polyvinyl butyral resin was mixed at the same addition ratio. In case of acid value is 65KOH
It was mg / g. From this, it is understood that the carboxylic acid group which is a functional group reacts with the hydroxyl group in the polyvinyl butyral resin and decreases.

The binder for magnetic recording material obtained as described above was mixed with a mixed solvent [methyl ethyl ketone / toluene = 50.
/ 50 (weight ratio)] to obtain a binder solution for a magnetic recording material having a concentration of 10% by weight. In this binder solution, magnetic powder (particle size 0.8 μm) made of cobalt-γ-iron oxide in an amount 2.5 times as much as the binder solution by weight, and 20% by weight with respect to the binder for magnetic recording material. % Of tolylene diisocyanate was added, and the mixture was kneaded for 4 hours with a paint conditioner.

The magnetic paint prepared as described above was added to
It was applied on a polyethylene terephthalate (PET) film having a thickness of 14 μm so that the thickness after drying would be 5 μm. Next, after subjecting the coating film to magnetic field orientation treatment, the coating film was dried, and a PET film having a magnetic coating film formed on one surface was wound up. Further, PE having the above magnetic coating film formed thereon.
The T film was calendered and cut into a 1/2 inch width to prepare a magnetic recording material. In addition, the measurement of the degree of polymerization in the description of Examples and Comparative Examples in this specification is performed according to JISK.
Done according to -6728.

Example 2 50 parts by weight of toluene was put into a flask, the inside of the flask was replaced with nitrogen gas, and then heated to the boiling point of toluene. While stirring under reflux of toluene, 72% by weight of styrene, 10% by weight of methyl methacrylate, 10% by weight of 2-ethylhexyl acrylate, 5% of methacrylic acid were stirred.
By weight, a mixed solution of 3% by weight of β-hydroxyethyl methacrylate and 2% by weight of benzoyl peroxide (polymerization initiator) was added dropwise over 2.5 hours to carry out solution polymerization. Then, toluene was removed under reduced pressure while gradually increasing the temperature in the flask to 180 ° C. to obtain a copolymer of a styrene-based monomer and a (meth) acrylic acid ester. 97 parts by weight of this copolymer, 3500 parts by weight of polyvinyl butyral resin having a degree of polymerization of 3500 and an acetalization degree of 72% by weight, and 100 parts by weight of toluene were charged into a flask, and the inside of the flask was replaced with nitrogen gas. Heat to boiling. Stirring was carried out for 2 hours in the state where toluene was refluxed, and then the temperature in the flask was gradually increased to 180 ° C. and the toluene was removed under reduced pressure to obtain a binder for magnetic recording media.

The binder for magnetic recording material obtained as described above has an acid value of 28 KOHmg / g, and is obtained by separately polymerizing a styrene-acrylic copolymer and mixing polyvinyl butyral resin at the same addition ratio. Has an acid value of 36 KOHm
It was g / g. Further, a magnetic recording material was produced in the same manner as in Example 1 using the above-mentioned binder for magnetic recording material.

Example 3 As shown in Table 1 below, a binder for a magnetic recording body was obtained in the same manner as in Example 1 except that the monomer composition and the like were changed, and further a magnetic recording body was prepared.

The acid value of the binder for magnetic recording bodies obtained as described above was 31 KOHmg / g, and a styrene-acrylic acid copolymer was separately polymerized, and a polyvinyl acetoacetal resin was mixed at the same addition ratio. Acid value is 45
It was KOH mg / g.

Example 4 As shown in Table 1 below, a binder for a magnetic recording medium of Example 4 was prepared in the same manner as in Example 1 except that the monomer composition and the like were changed, and Example 1 A magnetic recording material was obtained in the same manner as in.

Example 5 As shown in Table 1 below, a binder for a magnetic recording medium of Example 5 was prepared in the same manner as in Example 1 except that the monomer composition and the like were changed, and Example 1 A magnetic recording material was obtained in the same manner as in.

Example 6 As shown in Table 1 below, a binder for a magnetic recording medium of Example 6 was prepared in the same manner as in Example 1 except that the monomer composition and the like were changed, and Example 1 A magnetic recording material was obtained in the same manner as in.

Comparative Example 1 A binder for a magnetic recording medium was obtained in the same manner as in Example 3 except that the polyvinyl acetoacetal resin was not used, and a magnetic recording medium was prepared.

Comparative Example 2 A binder for a magnetic recording medium was obtained in the same manner as in Example 1 except that 20% by weight of methyl methacrylate was used instead of 10% by weight of acrylic acid. A recording body was prepared.

Comparative Example 3 A binder for a magnetic recording medium was obtained in the same manner as in Example 6 except that the polyvinyl butyral resin was not used, and a magnetic recording medium was prepared.

Reference Example Solution polymerization was carried out in the same manner as in Example 4 except that the polyvinyl butyral resin was not used to obtain a styrene-acrylic copolymer.

90 parts by weight of this copolymer, 10 parts by weight of polyvinyl butyral resin and 100 parts by weight of toluene were put into a flask, the inside of the flask was replaced with nitrogen gas, and then heated to the boiling point of toluene. Stirring was carried out for 2 hours in the state where toluene was refluxed, and then the temperature in the flask was gradually raised to 180 ° C. and the toluene was removed under reduced pressure to obtain a binder for magnetic recording media. This magnetic recording material binder has the same composition and formulation as the magnetic recording material binder in Example 4, but the mixing method of the styrene-acrylic copolymer and the polyvinyl butyral resin is different. Using the binder for magnetic recording medium obtained as described above, a magnetic recording medium was prepared in the same manner as in Example 1.

Evaluation The monomer composition and functional group type of the styrene-acrylic copolymers of Examples 1 to 6, Comparative Examples 1 to 3 and Reference Example, the composition of the polyvinyl acetal resin, the degree of polymerization, the degree of acetalization, Table 1 shows the types of functional groups and the addition ratios.

[0051]

[Table 1]

Abbreviations of monomers in Table 1 have the following meanings. ST: Styrene, MMA: Methyl Methacrylate, B
A: butyl acrylate, 2EHA: 2-ethylhexyl acrylate, AAc: acrylic acid, MAAc: methacrylic acid, β-HEMA: β-hydroxyethyl methacrylate.

Examples 1 to 6 produced as described above,
The glossiness, abrasion resistance and heat resistance of the nuclear magnetic recording materials of Comparative Examples 1 to 3 and Reference Example were measured and evaluated in the following manner. Table 2 shows the results.

(1) Gloss: A gloss meter (Gloss meter, manufactured by Murakami Color Research Co., Ltd.) was used, and both the incident angle and the reflection angle were set to 60 °.

(2) Abrasion resistance: The magnetic recording material was made to run at a running speed of 50 cm / sec while being in sliding contact with the magnetic head, and the running loss test was carried out 300 times to measure the amount of wear loss. In Table 2, the wear reduction of Comparative Example 1 is 100
%, And the wear reductions of Examples 1 to 3 and Comparative Example 2 are shown as values relative to the wear reduction of Comparative Example 1.

(3) Heat resistance: evaluated by a heat adhesion test. That is, a plurality of magnetic recording bodies were prepared, heated at a temperature of 120 ° C. for 15 minutes, and then a plurality of magnetic recording bodies were stacked to examine the block property. The meanings of the evaluation symbols of evaluation 2 are as follows. ○ …… Good, △ …… Slightly bad, × …… Poor

[0057]

[Table 2]

As is clear from Table 2, in Comparative Example 1, it is considered that the dispersion of the magnetic powder was deteriorated and the glossiness was lowered because the polyvinyl acetoacetal resin was not added. The strength was weak and the heat resistance was not good. All of these results were inferior to those of Example 3.

In Comparative Example 2, the polyvinyl butyral resin was not grafted as compared with Example 1. Therefore,
Since the dispersibility of the polyvinyl butyral resin was not as good as that of Example 1, the glossiness was good, but the abrasion resistance was poor, and the result was inferior to the result of Example 1.

In Comparative Example 3, since the polyvinyl butyral resin was not added, the magnetic powder was poorly dispersed,
It is considered that the glossiness was decreased, the strength of the coating film was weak, and the heat resistance was not good. All these results
It was inferior to the result of Example 6.

In the reference example, the mixing method of the polyvinyl butyral resin is different from that of Example 4, and the dispersibility of the polyvinyl butyral resin is inferior to that of Example 4, although the glossiness is relatively good, The wear resistance was inferior to the result of Example 4.

[0062]

As described above, in the binder for a magnetic recording medium and the magnetic recording medium using the same according to the first aspect of the present invention,
Since the polyvinyl acetal resin is grafted with the copolymer of the styrene monomer and the (meth) acrylic acid ester, the dispersibility of the polyvinyl acetal resin in the styrene-acrylic acid copolymer is very good. Has been done. Further, in the binder for a magnetic recording medium and the magnetic recording medium using the same according to the second aspect of the invention, in the presence of the polyvinyl acetal resin, the styrene-based monomer and the (meth) acrylic acid ester are solution-weighted. Since the copolymerization is carried out by the legal method, the dispersibility of the polyvinyl acetal resin in the styrene-acrylic acid copolymer is very good. Therefore, according to the first and second aspects of the invention, the dispersibility of the magnetic powder becomes good, and the glossiness of the obtained magnetic recording medium becomes good. In addition, when a magnetic coating material is prepared and the magnetic coating material is applied, coating is easy, a film having a uniform thickness is obtained, and the obtained magnetic film is excellent in heat resistance and abrasion resistance. Become.

Claims (3)

[Claims] 1. (a) 0.5 to 30% by weight of a polyvinyl acetal resin having a degree of polymerization of 200 to 4000,
(B) A binder for a magnetic recording material, which comprises, as a main component, a graft polymer composed of 70 to 99.5% by weight of a copolymer of a styrene-based monomer and a (meth) acrylic acid ester. . 2. In the presence of a polyvinyl acetal resin,
A binder for a magnetic recording material, which comprises, as a main component, a copolymer obtained by copolymerizing a styrene-based monomer and a (meth) acrylic acid ester by a solution polymerization method. 3. A magnetic recording body comprising magnetic powder and the binder for a magnetic recording body according to claim 1.