JPH03185041A - Binder for magnetic recording material - Google Patents

Abstract

PURPOSE:To provide the subject binding agent having excellent magnetic powder dispersibility, giving a high rectangular ratio, proper hardness and flexibility to magnetic layers and having durability such as sliding resistance characteristics to magnetic heads by comprising a copolymer of vinyl chloride, a hydroxyl group-containing vinyl compound and a N-substituted maleimide in a specific ratio. CONSTITUTION:The objective binding agent comprises a copolymer of (A) 59-98wt.% of vinyl chloride, (B) 1-30wt.% of a hydroxyl-containing vinyl compound [e.g. 2-hydroxylethyl (meth)acrylate], (C) 1-40wt.% of a N-substituted maleimide (e.g. N-methylmaleimide) and preferably (E) 0.05-5wt.% of a polymerizable monomer containing a carboxyl group, sulfonic acid group, sulfonate salt group, phosphoric acid group or quaternary ammonium group.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a binder for magnetic recording media.

(Prior Art) Conventionally, magnetic recording bodies used for magnetic recording tapes, magnetic disks, etc. are manufactured by applying a magnetic paint containing magnetic powder and a binder onto a substrate such as a polyester film, orienting it, and drying it. Therefore, durability such as excellent electrical properties and resistance to sliding contact with the magnetic head is required. Therefore, binders for magnetic recording media are required to have good dispersibility of magnetic powder when used as a magnetic paint, abrasion resistance when forming a magnetic layer, heat resistance, etc. Generally, vinyl chloride - vinyl acetate - vinyl Alcohol-based polymer A mixture of this polymer and an incyanate compound was used. .

However, in recent years, magnetic powder has been made into finer particles as the density of magnetic recording media has increased, and the above-mentioned binder has been lacking in dispersibility.

(Problems to be Solved by the Invention) In view of the above-mentioned drawbacks, an object of the present invention is to be able to uniformly disperse finely divided magnetic powder, and to improve durability such as electrical properties and resistance to sliding contact with a magnetic head. An object of the present invention is to provide a binder for a magnetic recording body that can form an excellent magnetic layer.

(Means for Solving the Problems) The hydroxyl group-containing vinyl (b) used in the present invention may have a reactive double bond and a hydroxyl group, for example, 2
-Hydroxyethyl (meth)acrylate, 2-hydroquinepropyl (meth)acrylate, 3-chloro-2
-Hydroxypropyl (meth)acrylate, polyethylene glycol mono(meth)acrylate represented by the following formula, CH2=CR-COO+CHzCHzO-)+IHde (in the formula, n is an integer from 2 to 11, and R represents hydrogen or a methyl group) ), polypropylene glycol mono(meth)acrylate represented by the following formula, CH2= CRCOO+ C
)12 CHO+oHH3 (In the formula, n is an integer of 2 to 6, R represents hydrogen or a methyl group.) \ Reaction product of (meth)acrylic acid and polyhydric alcohol such as 2-hydroxyethyl-2-acryloyloxyphthalate, Examples include (meth)acrylic acid amides such as N-methylol (meth)acrylamide, and 2-hydroxyethyl (meth)acrylate and 2-hydroxypropyl (meth)acrylate are preferably used.

The N-substituted maleimide used in the present invention is a compound represented by the following formula.

In the formula, R is preferably an aliphatic group, an aliphatic group, or an aromatic group having 1 to 20 carbon atoms, such as N-methylmaleimide, N
-n-propylmaleimide, N-isopropylmaleimide, N-n-butylmaleimide, N-tert-butylmaleimide, Nn-hexylmaleimide, N-cyclohexylmaleimide, N-phenylmaleimide, N-2-
Chlorophenylmaleimide, N-2-methylphenylmaleimide, N-2-ethylphenylmaleimide, N-2
, 6-cyclohexylmaleimide, N-2,6-dimethylphenylmaleimide, N-benzylmale amide, N
-(2-chlorobenzyl)maleimide, N-(2-methylbenzyl)maleimide, N-naphthylmaleimide, etc., and N-7 chlorohexylmaleimide and N-phenylmaleimide are preferably used.

The binder for a magnetic recording medium of the present invention comprises vinyl chloride (a), the above-mentioned hydroxyl group-containing vinyl (b), and N-substituted maleimide (c).
However, the content of vinyl chloride (a) is 59 to 98% by weight, as the solubility in organic solvents decreases, the copolymer becomes hard, and the flexibility decreases. When the content of hydroxyl group-containing vinyl (b) decreases, the dispersibility of the magnetic powder decreases, and when an incyanate compound is added, urethane bonds are not formed sufficiently, resulting in weak coating film strength and easy blocking. 1 to 3.
0ii1%, preferably 8 to 20% by weight.

In addition, N-11-maleimide reduces the solution viscosity of the copolymer, improves the dispersibility of magnetic powder, and improves heat resistance and
It improves wear resistance, etc., but if it is too small, this effect will not be achieved, and if it is too large, the copolymer will become brittle.
It is contained in the copolymer in an amount of 1 to 40% by weight, preferably 5 to 40% by weight.
Contains 30% imaginary amount.

When the average degree of polymerization of the above copolymer becomes small, it is difficult to use as a magnetic paint, and when a magnetic J- is formed, the surface strength of the magnetic layer becomes small; 150 to SOO is preferable because the IFL viscosity becomes high and coating becomes difficult.

Any method may be used to produce the above-mentioned copolymer, for example, a known suspension polymerization method, emulsion polymerization method, or solution polymerization method that uses a radical polymerization initiator and is commonly carried out in the polymerization of vinyl chloride. , bulk polymerization method, etc. The suspension polymerization method is preferably employed because the resulting resin contains relatively few impurities and the manufacturing cost is relatively low.

In the suspension polymerization method, IF! As the turbidity stabilizer, partially saponified polyvinyl alcohol, cellulose derivatives, etc. are used, and as the radical polymerization initiator, penzoyl peroxide, dilauroyl peroxide, tert
-Organic peroxides such as butyl peroxyneodecanoate are preferably used. The N-substituted maleimide may be added in its entirety into the polymerization vessel at once before the start of polymerization, or may be added in portions or continuously during polymerization.

In particular, N whose copolymerizability ratio with vinyl chloride is significantly different
When using an N-substituted maleimide such as -phenylmaleimide, it is preferable to add the entire amount in portions during polymerization or to add it continuously because heat resistance is further improved. In this case, the N-substituted maleimide is preferably dissolved in an organic solvent such as acetone or methanol, or dispersed in water. And the polymerization temperature is generally 30 to 9
The polymerization time is generally 2 to 20 hours at 0°C.

The second invention of the present invention is a copolymer in which a polymerizable monomer (d) containing a carboxyl group, a sulfonic acid group, a sulfonic acid group, a phosphoric acid group, or a quaternary ammonium group is added to the copolymer. Polymerized.

As the polymerizable monomer containing the above carboxyl group,
Examples include acrylic acid, methacrylic acid, succinic acid mono(meth)acryloyloxyethyl ester, phthalic acid mono(meth)acryloyloxyethyl ester, maleic acid n-butyl monoester, maleic acid, fumaric acid, etc. methacrylic acid, maleic acid
N-phthyl monoester and maleic acid are preferably used.

Examples of the sulfonic acid group or the polymerizable monomer containing a sulfonic acid group include 2-acrylamido-2-methylpropanesulfonylacunacinide, nail deium metaallylsulfonate, and the like.

Examples of the polymerizable monomer containing a phosphoric acid group include acindophosphoxyethyl (meth)acrylate, ambuthophosphoxypropyl (meth)acrylate, 3-
Examples include chloro-2-ambutophosphoxypropyl (meth)acrylate.

Further, C as the polymerizable monomer containing the quaternary ammonium base is, for example, 2-hydroxy-3-methacryloyloxypropyltrimethylammonium chloride, methacryloyl quinethyltrimethylammonium chloride, trimethyl-3-methacrylamidopropyl Examples include ammonium chloride.

If the content of the polymerizable monomer (d) is too small, the effect of dispersing the magnetic powder will be lost, and if it is too large, the solubility of the copolymer in organic solvents will decrease, the surface smoothness of the magnetic layer will decrease, and the moisture resistance will decrease. Sexuality decreases. In addition, when adding an inocyanate group, if the content of the polymerizable monomer (d) decreases, urethane bonds will not be formed sufficiently and the coating strength will become weak and blocking will occur easily. (d
) content is 0.05 to 5% by weight, preferably 0.1 to 3% by weight.

In this case, the content of vinyl chloride (a) is 59 to 97% by weight.

The structure of the binder for a magnetic recording medium of the present invention is as described above, but it is preferable to use an io
A monomer copolymerizable with vinyl chloride may be copolymerized within 2% of Mi.

Examples of the monomers include α-olefins such as ethylene/propylene, vinyl esters such as vinyl acetate, vinyl ethers such as butyl vinyl ether, cetyl vinyl ether, and phenyl vinyl ether, and acrylic esters such as methyl acrylate and ethyl acrylate. methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, and phenyl methacrylate; aromatic vinyls such as styrene and α-styrene; vinyl cyanides such as acrylonitrile and methacrylate; halogenated vinylidene chloride and vinyl fluoride; vinyls,
Examples include unsaturated dicarboxylic acid esters such as dimethyl maleate and dimethyl fumarate, and unsaturated dicarboxylic acid anhydrides such as maleic anhydride.

To create a magnetic 6!1 material from the binder for magnetic recording material of the present invention, the binder is dissolved in an organic solvent and cobalt-
Magnetic powder such as γ-iron oxide may be dispersed.

The amount of binder added is generally 10-100 parts by weight per 100 parts by weight of magnetic powder, and the content of binder in the magnetic paint is generally 5-30% by weight. For example, C is toluene.

Examples include methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone.

Further, the magnetic layer is formed by applying and drying a magnetic coating, but an incyanate compound may be added to the magnetic paint in order to improve the abrasion resistance and heat resistance of the magnetic layer.

Examples of the incyanate compound include tolylene diisocyanate, diphenylmethane diisocyanate, dianisidine diisocyanate, tridene diisocyanate, hexamethylene diisocyanate, metaxylylene diisocyanate *-), ? Reaction product of 1 mol of limethylolpan and 13 mol of tridene diisocyanate (sold under the name Coronate L by Nippon Polyurethane)
etc. can be mentioned.

If the amount of the above incyanate compound added increases, the crosslinking density will increase and the magnetic layer will become brittle.
It is preferable to add 0.3 to 30 parts by weight per part by weight.

(Example) Next, the present invention will be explained in detail. The physical properties were measured as follows.

Degree of polymerization: Compliant with JIS K6721.

Composition: Chlorine-containing waste was measured using an oxygen flask combustion method, and an elemental analysis bag wi (Hatsuhyo Seisakusho, CHN Coater MT) was used.
-5) The component composition was determined from l.

Squareness ratio...Yokogawa Electric, DC magnetization characteristic recording device ffT/
Measured with 3256-30.

Glossiness Nippon Denshoku Kogyo, digital bending gloss needle VG-ID
It was measured with

Heat adhesion: Polyester films with magnetic layers formed thereon are stacked so that the magnetic layers are in contact with each other, and
After applying a cli load and heating at 120°C for 15 minutes,
One film was lifted and both films were peeled off, and the one in which both films were peeled off was designated as 01.The one in which both films were adhered was designated as X.

Gel fraction...The measurement test was immersed in a mixed solvent of toluene/-methyl isobutyl ketone (weight ratio 1/1) and U at 50°C for one day and night, and the undissolved content was expressed as the weight percentage of test t1.

Example! 15I! 8 kg of ion-exchanged water, 12 hydroxypropylethyl cellulose, and 8.8 y of dilauroyl i4-oxide were placed in a jacketed pressure-resistant container equipped with a stirrer, and after sealing the container and removing any remaining air, 3 kg of vinyl chloride was press-fitted. , and then heated to 70°C using a jacket. Immediately after the temperature inside the vessel reached 70°C, the I@ solution containing 2-hydroxypropyl acrylate 1O2 and N-cyclohexylmaleimide 7, OP dissolved in acetone IOP was added in an amount of 40 times at 5-minute intervals. The mixture was stirred to polymerize the red carcass. After the final addition, the mixture was cooled for 5 minutes, and after cooling, the remaining vinyl chloride was discharged outside the vessel. Next, the polymerization slurry was taken out, washed with ion-exchanged water, and dried to obtain a copolymer (A-1). . The composition and degree of polymerization of the obtained copolymer (A-1) were measured, and the results are shown in Table 1.

The obtained copolymer (A-1) was mixed with toluene-methyl in butyl ketone (weight ratio 1:1). i Combined solution solution, 1
A 5% by weight copolymer solution was obtained, and 4 times (M amount) of cobalt-γ-iron oxide and 32 times (by weight) of a stainless steel pole with a diameter of 1/8 inch were supplied to the obtained solution. The mixture was mixed in a paint conditioner for 4 hours to obtain a magnetic paint.

The resulting paint was applied to a 25 μm thick polyester film, oriented and dried to form a 6 μm thick magnetic layer.The squareness ratio and glossiness were measured.The results are shown in Table 1.

Examples 2 to 5, Comparative Example 1.2 A copolymer (A-2
-5, A-7, 8) were obtained, and the composition and degree of polymerization of the copolymer were measured and the results are shown in Table 1. Also, examples! A magnetic coating material was obtained in the same manner as in Example 1, a magnetic layer was formed, and the squareness ratio and glossiness were measured. The results are shown in Table 1.

Example 6 6.8 kg of methanol and α-cumyl peroxyneodecanoate 53F were charged into a jacketed pressure-resistant container equipped with a 25/2 stirrer, and after sealing and removing the remaining air, vinyl chloride 3 .5 ~ was press-fitted, and then heated to 43° C. using a jacket. Immediately after the internal temperature reached 43°C, a mixed solution of 6.75P of 2-hydroxypropyl acrylate, 42P of N-chlorohexylmaleimide, 25f of methanol, and 32.5f of vinyl chloride was added 40 times at 5-minute intervals. The mixture was added in portions and polymerized with stirring. After cooling for 5 minutes after the final addition, and when the temperature reached 25°C or lower, excess ion-exchanged water was supplied to remove residual vinyl chloride, and the polymerization slurry was taken out, washed, and dried to form a copolymer (A-6). ) was obtained.

The composition and degree of polymerization of the obtained copolymer were measured, and the results are shown in Table 1.

Further, a magnetic layer was formed in the same manner as in Example 1, and the squareness ratio and glossiness were measured. The results are shown in Table 1.

Comparative Example 3 Vinyl chloride-vinyl acetate-vinyl alcohol copolymer (
Example using weight ratio 91/3/6, degree of polymerization 440) (A-9)! A magnetic layer was formed in the same manner as in Example 1, and the squareness ratio and glossiness were measured. The results are shown in Table 1.

(Margin below) Table Examples 7 to 12, Comparative Example 4 Copolymers A-1 to obtained in Examples 1 to 6 and Comparative Example 3
6 and 9 were each dissolved in a mixed solution of toluene-methyl isobutyl ketone (weight ratio 1:1), and 15 hk
% copolymer solution was obtained, and in the resulting bath liquid, a stainless steel pole with a diameter of 178 inona, which was 32 times (by weight) as much as cobalt-γ-iron oxide and 32 times (by weight) as much as the copolymer, was painted. 7. Mixed with a trowel for 4 hours. -to compound (Nippon Boliureta/Korofu-1,
A magnetic coating material was obtained by adding 065y to 15yl of copolymer and mixing Lj7j form (70 wt%). Using the obtained paint, the shape ('EL, squareness ratio, glossiness, and heating density InoFl) of one magnetic circle was measured in the same manner as in Example 1 for 7 particles, and the results are shown in Table 2. .

L synergistic combination solution) Koinonai-1 Compound (■ Polyuretano, Koroi-1-L, solid content 70 stores [raw)
Add copolymer 15y (to this, add 0652 &) to obtain a paint, and place the resulting paint on a glass plate.
, dry at 100°C for 2 hours and at 50°C for 24 hours to prepare sample #1 for gel fraction measurement. The gel fraction of the samples obtained was determined and shown in Table 2.

(Margin below) Table Example 13 One batch of added rice was mixed with 10y of 2-hydroxypropyl acrylate and LOy%N of n-butyl maleate monoester.
- Ncrohequinlumaleimide 7.02 and acetone 10
A copolymer (B-1) was obtained in the same manner as in Example 1 except that the mixture was changed to a mixed solution of y. The composition and degree of polymerization of the obtained copolymer were measured, and the results are shown in Table 3.

Further, a magnetic paint was obtained in the same manner as in Example 1, a magnetic layer was formed, and the squareness ratio and glossiness were measured, and the results are shown in Table 3.

Examples 14 to 17 A copolymer (B -2~5)
The composition and polymerization history of the obtained copolymer were measured and are summarized in Table 3. In addition, magnetic paint was applied to the surface of the sample, and the squareness ratio and gloss level were determined to be 17, and the results are shown in Table 3. ,
Example 18-1! ! 1 minute/1 minute addition M+2-hydroquinepropyl acrylate 6,75 y male f acid 0.39. A copolymer (B-6) was obtained in the same manner as in Example 6, except that a mixed solution of N-chlorohexylmaleimide (4), methanol (25 mm), and vinyl adduct (32.5 yσ) was used. 1
The composition and degree of polymerization of the copolymer obtained were measured and the results are shown in Table 3.

Also, in the same way as in Practical Example 1, magnetic paint was obtained. Form 1 ml of magnetic J, Measure squareness ratio and glossiness established, The results are shown in Table 3.

Examples 9 to 2 in Table 1 Using the copolymers B- to 6 obtained in Examples 1 to 3, a magnetic layer was formed in the same manner as in Example 7, and the squareness ratio, glossiness, and heat adhesion were determined. The gel fraction was also measured, and the results are shown in Table 4.

Table 4 Example 25 One dose of /li+, 10y of 2-hydroquine propyl acrylate, LOy of 2-acrylamide-2methylpropanesulfonyl acrylate.

N-ncrohequinlumaleimide7. A copolymer (c-1) was obtained in the same manner as in Example 1 except that the mixture was changed to a mixture of Oy and (J) acetone (10y).

The composition and degree of polymerization of the obtained copolymer were measured, and the results are shown in Table 5.

Further, a magnetic coating material was obtained in the same manner as in Example 1, a magnetic layer was formed, and the squareness ratio and glossiness were measured. The results are shown in Table 5.

Examples 26 to 29 Example 2 except that the lines and amounts of N-substituted maleimide, hydroxyl group-containing vinyl, and phosphonic acid group-containing polymerizable monosagi were changed.
Copolymers (c-2 to c-5) were obtained in the same manner as in Example 5, and the composition and degree of polymerization of the copolymers were measured and shown in Table 5. Further, a magnetic paint was obtained, a magnetic layer was formed, and the squareness ratio and glossiness were measured, and the results are shown in Table 5.

Example 30 The amount added per batch was 6.759 y of 2-hydroxypropyl acrylate, and 0.3 y of 2-acrylamido-2-methylpropanesulfonic anrad.

N-chlorohexyl maleimide 4, methanol 25
A copolymer (c-6) was obtained in the same manner as in Example 6, except that a mixed solution of 325y and hinyl chloride was used. The composition and degree of polymerization of the obtained t-copolymer were measured, and the results are shown in Table 5.

Further, a magnetic paint was obtained in the same manner as in Example 1, a magnetic layer was formed, and the squareness and gloss were measured. The results are shown in Table 5.

Examples 31 to 36 Magnetic layers were formed in the same manner as in Example 7 using the copolymers C-1 to 6 obtained in Examples 25 to 30, and the squareness ratio, glossiness, and heat adhesion were measured. At the same time, the gel fraction was measured and the results are shown in Table 6.

Table 6 Example 37 The amount added per batch was 2-hydroquinepropyl acrylate 102, antphosphoxypropyl methacrylate L
A copolymer (13-1) was obtained in the same manner as in Example 1 except that the mixture was changed to a mixture of 7.0y of oy% N-chlorohequinlumaleimide and 10y of acetone. The composition and degree of polymerization of the obtained copolymer were measured and the results are shown in Table 7.

In addition, a magnetic paint was obtained in the same manner as in Example II 1, a magnetic layer was formed, and the squareness ratio and glossiness were measured, and the results are shown in Table 7.

Practical Examples 38 to 41 Example 37 except that N-m% maleimide, hydroxyl group-containing hynyl, and phosphoric acid group-containing chassis monomers 8M and the mouse were changed.
The copolymers (D-2 to 5) were obtained by filtration through Mukomune 4.
The composition and degree of polymerization of the copolymer were measured and shown in Table 7. In addition, a magnetic paint was obtained, a magnetic I- was formed, and the squareness ratio and glossiness were measured, and the results are shown in Table 1.

Example 42 The amount of 2-hydroxypropyl acrylate added per batch is 6,75? , Antsdophosphoquinebrovir methacrylate 0.39, Maleimide 4f for N-cyclohequine,
A copolymer (D-6) was obtained in the same manner as in Example 6 except that a mixed solution of 25 methanol and 32.5 M' of vinyl chloride was used. The composition and degree of polymerization of the obtained copolymer were measured and the results are shown in Table 7.

Further, a magnetic coating material was obtained in the same manner as in Example 1, a magnetic layer was formed, and the squareness ratio and glossiness were measured. The results are shown in Table 7.

(Margin below) Table Examples 43 to 48 Using the copolymers D-1 to 6 obtained in Examples 37 to 42, a magnetic layer was formed in the same manner as in Example 7, and the squareness ratio, gloss In addition to measuring the heat adhesion and heat adhesion, the gel fraction was also measured, and the results are shown in Table 8.

Table 8 Example 49 Addition amount per batch: 2-hydroquinepropyl acrylate 1Oy1 methacryloyloxyethyltrimethylammonium chloride LOp, N-cyclohexylmaleimide 7, Of! And 10g of acetone! The copolymer (E-1
) get two.

The composition and degree of polymerization of the obtained copolymer were measured and the results are shown in Table 9.

In addition, a magnetic paint was obtained in the same manner as in Example 1, a magnetic I- was formed, the squareness ratio and glossiness were measured, and the results were reported in the ninth example.
Shown in the table.

Examples 50 to 53 Copolymers (E- 2-5)
The composition and degree of polymerization of the military polymer were measured and are shown in Table 9. In addition, a magnetic layer was obtained, a magnetic layer was formed, and the squareness ratio and glossiness were measured, and the results are shown in Table 9.

Example 54 The amounts added per batch were 6.75 y of 2-hydroxypropyl acrylate and 0.3 y of methacryloyl quinethyltrimethylammonium chloride.

N-chlorohexyl maleimide 4, methanol 25
A copolymer (E-6) was obtained in the same manner as in Example 6, except that a mixed solution of 32.5y of vinyl chloride and 32.5y of vinyl chloride was used.

The composition and degree of polymerization of the obtained copolymer were measured and the results are shown in Table 9.

Further, a magnetic coating material was obtained in the same manner as in Example 1, a magnetic layer was formed, and the squareness ratio and glossiness were measured. The results are shown in Table 9.

Table 9 Examples 55 to 60 Using the copolymers E-1 to 6 obtained in Examples 49 to 54, a magnetic layer was formed in the same manner as in Example 7, and the squareness ratio, glossiness and heating were determined. The adhesion was measured as well as the gel fraction, and the results are shown in Table 10.

Table 10 (Effects of the Invention) The binder for a magnetic recording medium of the present invention is as described above, and is a copolymer of specific amount O vinyl chloride, hydroxyl group-containing vinyl, and N-@converted maleimide. The magnetic powder has excellent dispersibility, and the formed magnetic layer has a high squareness ratio and has appropriate hardness and flexibility, so it has excellent sliding contact resistance and durability with the magnetic head. ing. Furthermore, if an isoneate compound is added, the hydroxyl group and the isoneate group undergo a crosslinking reaction, resulting in better wear resistance and heat resistance.

In the second invention of the present invention, since a turtleizable monopolymer containing a carboxyl group, a sulfonic acid group, a sulfonic acid group, a phosphoric acid group, or a quaternary ammonium base is further copolymerized, the magnetic I- The squareness ratio is higher, and the durability, heat resistance, etc. are also better.

Claims (1)

[Claims] 1. (a) vinyl chloride, (b) hydroxyl group-containing vinyl, and (c
) A copolymer with N-substituted maleimide, in which the component (a) is 59 to 98% by weight, the component (b) is 1 to 30% by weight, and the component (c) is 1 to 40% by weight. Characteristic binder for magnetic recording media. 2. Polymerizable material containing (a) vinyl chloride, (b) hydroxyl group-containing vinyl, (c) N-substituted maleimide, and (d) carboxyl group, sulfonic acid group, sulfonic acid group, phosphoric acid group, or quaternary ammonium group A copolymer with a monomer, containing 59 to 97% by weight of component (A)
, component (b) is 1 to 30% by weight, component (c) is 1 to 40% by weight.
A binder for a magnetic recording medium, characterized in that the content of component (d) is 0.05 to 5% by weight.