JPH11250444A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a recording medium which has a magnetic layer having the good dispersion performance of a copolymer (binder resin) with fine ferromagnetic powder, thermal stability, compatibility with various resins, more particularly polyurethane resin, having, therefore, the good smoothness of a coating film substrate in spite of the viscosity of the magnetic coating material made suitable for high-speed coating work and having excellent wear resistance, good physical characteristic of a coated film and excellent durability and magnetic characteristics and is suitable for a magnetic tape, magnetic card, floppy disk, etc. SOLUTION: This magnetic recording medium is provided with the magnetic layer consisting of the magnetic coating material dispersed with the fine ferromagnetic powder in a binder on a nonmagnetic base. The binder of the magnetic recording medium consists essentially of a copolymer contg. (1) 30 to 50 wt.% alkyl (meth)acrylate unit, (2) 30 to 55 wt.% vinyl chloride unit, (3) 1 to 20 wt.% monomer unit having an epoxy group and ethylenically unsatd. double bonds and (4) 1 to 20 wt.% other radical polymeric monomer unit as component units.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic layer made of a magnetic paint in which ferromagnetic fine powder is dispersed in a binder on a nonmagnetic support, and is used as a magnetic tape, a magnetic card, a floppy disk or the like. Magnetic recording medium to be used.

[0002]

2. Description of the Related Art A magnetic recording medium such as a magnetic tape, a magnetic card, a floppy disk or the like is generally coated on a support such as a polyester film with a magnetic powder and a binder (synthetic resin). It is made by providing a film (magnetic layer).

In recent years, the recording density of magnetic recording media has been improved,
With the demand for improvement in the / N ratio and the C / N ratio, the magnetic powder has been made finer than ever, and has a very large magnetic moment, so that the particles are likely to aggregate with each other. As a result, in order to uniformly disperse the magnetic powder in the paint to form a smooth and highly filled magnetic layer and to improve the performance as a magnetic recording medium, the dispersion performance of the binder resin used as the binder must be improved. It is a critical factor.
Further, in order to further enhance the flexibility and friction resistance of the magnetic recording medium, it is an essential condition that the magnetic recording medium has excellent compatibility with the polyurethane resin used in combination.

On the other hand, in recent years, the material of ferromagnetic powder has shifted from iron oxide to iron oxide in which cobalt ions are adsorbed or doped, and furthermore, ferromagnetic metals such as iron, nickel, and cobalt, and iron oxides. Alloys have been used.

[0005] -OH against the ferromagnetic powder, as a technique to improve the dispersibility improving as described above, for example the binder _{resin, -COOH, -SO 3 M, -OSO} 3 M,
-PO ₃ M ₂ , -OPO ₃ M ₂ , -N <, -CN (where M
Is advantageous to introduce a hydrophilic functional group such as a hydrogen atom, NH ⁴⁺ or an alkali metal).
mer. Sci. 2666114 (1988).

[0006] However, a vinyl chloride resin is conventionally known as a binder resin used for the binder. However, when a functional group described in the above proposal is introduced into the vinyl chloride resin, the ferromagnetic powder is not obtained. Although excellent results can be obtained in dispersion, there is a drawback that sufficient durability as a magnetic recording medium cannot be obtained, and therefore, reliability is lacking.

[0007] Recently, in order to improve productivity, a binder resin suitable for high-speed coating with a reduced viscosity of a magnetic coating material has been required. When the monomer is copolymerized or the molecular weight is reduced, there is also a problem that the coating film becomes soft and durability deteriorates.

Further, as a binder resin used as a binder, an acrylic copolymer containing no vinyl chloride is known. However, this acrylic copolymer has a magnetic layer which is somewhat excellent in durability and reliability. Although it can be obtained, it has a drawback that it is difficult to manufacture a magnetic recording medium that sufficiently satisfies the required performance, because it can be used only for a limited range of polyurethane resin due to poor compatibility.

The present invention has been made in order to improve the above circumstances, and a binder resin having excellent dispersibility in fine ferromagnetic powder, excellent heat stability, and excellent compatibility with a polyurethane resin used in combination. It is an object of the present invention to provide a magnetic recording medium which is suitable for high-speed coating by using a (copolymer), and in which a magnetic coating film obtained exhibits excellent performance in durability and abrasion resistance.

[0010]

Means for Solving the Problems and Embodiments of the Invention The present inventors have made intensive studies to achieve the above object, and as a result, dispersed a ferromagnetic fine powder in a binder on a non-magnetic support. In a magnetic recording medium provided with a magnetic layer made of a magnetic paint, the following alkyl (meth) acrylate unit:
When using a binder containing a copolymer containing a vinyl chloride unit, a monomer unit having an epoxy group and an ethylenically unsaturated double bond, and other radically polymerizable monomer units in a specific ratio as a binder resin. The binder has good dispersibility in fine ferromagnetic powder, and also has excellent thermal stability, and the binder resin has excellent compatibility with various resins such as polyurethane resin. It is excellent in durability and abrasion resistance, and it is possible to obtain a coating film having satisfactory characteristics even if the viscosity is lowered in order to be suitable for high-speed coating, and therefore has a magnetic layer excellent in various characteristics as described above, The present inventors have found that a magnetic recording medium suitable for high-speed coating that sufficiently satisfies the recent required characteristics can be obtained, and has accomplished the present invention.

That is, the present invention relates to a magnetic recording medium in which a magnetic layer made of a magnetic coating material in which ferromagnetic fine powder is dispersed in a binder is provided on a non-magnetic support, wherein the binder comprises 1) Alkyl (meth) acrylate unit 30 to 50% by weight (2) Vinyl chloride unit 30 to 55% by weight (3) Monomer unit having an epoxy group and an ethylenically unsaturated double bond 1 to 20% by weight (4 A) a magnetic recording medium characterized by comprising, as a main component, a copolymer containing 1 to 20% by weight of another radically polymerizable monomer unit as a constituent unit;

Hereinafter, the present invention will be described in more detail. The magnetic recording medium of the present invention is a magnetic recording medium comprising a non-magnetic support and a magnetic layer made of a magnetic paint in which a ferromagnetic fine powder is dispersed in a binder. It is a recording medium using a copolymer containing the above units (1) to (4) as constituent units as a binder resin of a binder.

In the copolymer used for the binder,
As the alkyl (meth) acrylate unit (1), an alkyl (meth) acrylate unit having an alkyl group having 1 to 18 carbon atoms is preferable, and as a monomer corresponding thereto, for example, methyl (meth) acrylate, Ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth)
Examples include acrylate, pentyl (meth) acrylate, hexyl (meth) acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate, and cyclohexyl (meth) acrylate. These monomers may be used alone or in combination of two or more. Note that (meth) acrylate is a general term for acrylate and methacrylate.

The content of the alkyl (meth) acrylate unit (1) is 30 to 50% by weight of the whole copolymer,
Preferably, it is 33 to 46% by weight. If the content of the unit (1) is less than 30% by weight, sufficient smoothness (gloss) and durability of the coating film cannot be obtained, and the viscosity of the magnetic coating material becomes high, and the content is more than 50% by weight. And the polyurethane resin have poor compatibility.

The content ratio of the vinyl chloride unit (2) is 30 to 55% by weight, preferably 34 to 50% by weight of the whole copolymer. When the content of the above (2) unit is 3
When the amount is less than 0% by weight, the compatibility with a synthetic resin such as a polyurethane resin is deteriorated. When the amount is more than 55% by weight, the solubility in a solvent is reduced, and the viscosity of the magnetic coating material is increased, and the dispersibility in the ferromagnetic fine powder is increased. Will decrease.

(3) The unit corresponding to the monomer unit having an epoxy group and an ethylenically unsaturated double bond is, for example, a unit containing glycidyl (meth) acrylate, allyl glycidyl ether or the like as a monomer. And
These monomers may be used alone or in combination of two or more.

The content of the unit (3) is 1 to 20% by weight, preferably 3 to 16% by weight of the whole copolymer.
If the content of the above unit (3) is less than 1% by weight, the thermal stability of the coating film is deteriorated, causing dehydrochlorination and corroding the apparatus. If the content is more than 20% by weight, the polymerization reaction is hindered. A product having a high degree of polymerization cannot be obtained.

Examples of the monomer corresponding to the unit (4) other radical polymerizable monomer units include vinyl acetate and
Monomers having an unsaturated double bond other than the monomers corresponding to the above (2) and (3) units, such as vinyl propionate, vinylidene chloride, ethylene, propylene, butylene, isobutylene, butadiene, and benzyl (meth) (Meth) acrylates other than the alkyl (meth) acrylate corresponding to the above (1) unit, such as acrylate, phenyl (meth) acrylate, and acetoacetoxyethyl (meth) acrylate; further, methyl vinyl ether, ethyl vinyl ether, isobutyl vinyl ether, n -Butyl vinyl ether, 2-ethylhexyl vinyl ether, n
-Octyl vinyl ether, lauryl vinyl ether,
Examples include alkyl vinyl ethers such as cetyl vinyl ether and stearyl vinyl ether. Among these, preferable monomers for introducing the unit (4) are vinyl acetate and vinyl propionate.

These monomers may be used alone or in combination of two or more. The content of the unit (4) is 1 to 20% by weight, preferably 3 to 18% by weight of the whole copolymer. weight%
And (4) When the content of the unit is less than 1% by weight,
The solubility in the solvent decreases, the viscosity of the magnetic paint increases, the dispersibility in the ferromagnetic fine powder decreases, and the compatibility with the polyurethane resin deteriorates. If it exceeds 20% by weight, the durability of the coating film is sufficient. The property cannot be obtained.

The copolymer composed of a combination of the above units (1) to (4) may be formed randomly or in blocks, but has a number average molecular weight of 5,
000 to 100,000, especially 10,000 to 50,000
It is preferably in the range of 00. Number average molecular weight is 5,
If it is lower than 000, the physical strength such as the brittleness of the magnetic coating film is reduced, and the durability of the magnetic tape or the like may be lowered. On the other hand, if it exceeds 100,000, the viscosity of the paint at a predetermined concentration becomes high. Workability may be reduced and handling may become difficult. For adjusting the molecular weight, a molecular weight modifier such as n-dodecyl mercaptan, thioglycol or trichloroethylene can be used.

The copolymer according to the present invention as described above,
Generally, it can be produced by an ordinary method employing a solution polymerization method, a precipitation polymerization method, a suspension polymerization method, an emulsion polymerization method, or the like.

Here, specific examples of the polymerization initiator used in the above copolymerization include persulfates such as ammonium persulfate, potassium persulfate and sodium persulfate, hydrogen peroxide, diisopropyl peroxydicarbonate, t-
Organic peroxide-based polymerization initiators such as butyl hydroperoxide, benzoyl peroxide, cumene hydroperoxide and dibutyl peroxide, and nitrogen-containing polymerization initiators such as azobisisobutyronitrile and azobisisovaleronitrile are exemplified. However, particularly preferred are persulfates such as ammonium persulfate, potassium persulfate, and sodium persulfate. These polymerization initiators may be used alone, but polymerization can also be carried out by using them together with a reducing agent such as sodium acid sulfite, Rongalit, L-ascorbic acid, saccharides and amines. The amount of the polymerization initiator used is preferably 0.01 to 4 parts by weight based on 100 parts by weight of the total amount of the monomers used.

The suspension stabilizer used in the suspension polymerization includes, for example, polyvinyl alcohol polymers such as polyvinyl alcohol and partially saponified polyvinyl acetate;
Methyl cellulose, hydroxypropyl cellulose, cellulose derivatives such as carboxymethyl cellulose, polyvinylpyrrolidone, polyacrylamide, maleic acid
Examples include synthetic polymer compounds such as styrene copolymer and maleic acid-methyl vinyl ether copolymer, and natural polymer compounds such as starch and gelatin. The use amount of the suspension stabilizer is preferably 0.01 to 1 part by weight based on 100 parts by weight of the total amount of the monomers to be copolymerized.

The emulsifier used in the emulsion polymerization method includes, for example, anionic emulsifiers such as alkyl or alkyl allyl sulfate, alkyl or alkyl allyl sulfonate, alkyl allyl sulfosuccinate, polyoxyethylene alkyl phenyl ether, polyoxyethylene Nonionic emulsifiers such as alkyl ethers and polyoxyethylene carboxylic acid esters are exemplified. The amount of the emulsifier used is preferably 0.5 to 5 parts by weight based on 100 parts by weight of the total amount of the monomers to be copolymerized.

In the present invention, a polymerization reaction can be carried out under ordinary conditions by using the above method.
Monomers, polymerization initiators, suspension stabilizers, emulsifiers and the like can be added to the polymerization system at the same time at the start of polymerization,
It can be added in portions during the polymerization.

The above-mentioned monomers (1) to (4) may be polymerized at one time, but after partially copolymerizing some of the monomers in advance, the remaining monomers are copolymerized. Alternatively, specifically, after previously copolymerizing a part of the monomer mainly composed of vinyl chloride, or copolymerizing the remaining monomer mainly composed of acryl in this system, Or conversely, after copolymerizing a monomer or the like mainly containing acrylic, the remaining monomer mainly containing vinyl chloride can be copolymerized in this system.

In the case of emulsion polymerization, an aqueous solution of an inorganic salt such as sodium chloride, calcium chloride, sodium sulfate, calcium sulfate or the like, or a water-soluble organic solvent is added according to a known salting-out method after the polymerization. After the particles are agglomerated, for example, by filtration, washed with water, and dried.

In the present invention, the binder resins (1) to (4) described above as binders in the production of magnetic paints are used.
A copolymer comprising the monomer of (4) is used. The resin used for the binder may be composed of the above copolymer alone, but is preferably a resin obtained by using another resin in combination with the above copolymer.

In this case, specific examples of the other resin include polyurethane resin, nitrocellulose, polyester resin, epoxy resin, polyamide resin, phenol resin,
Various resins such as polymers such as alkyd resins and polyvinyl butyral resins and copolymers thereof are exemplified, and polyurethane resins are particularly preferable.

The content of the copolymer comprising the monomers (1) to (4) in the binder is 100
The amount is preferably 30 to 70 parts by weight, more preferably 40 to 70 parts by weight.
~ 60 parts by weight are preferred. If the amount is less than 30 parts by weight, the magnetic performance is not good due to insufficient dispersion of the ferromagnetic fine powder, and if the amount of the polyurethane resin or the like is relatively large, the coating film becomes soft and the friction with the magnetic head may increase. If it exceeds 70 parts by weight, the coating film may be hard and the durability may be poor.

In the magnetic recording medium of the present invention, a ferromagnetic fine powder is dispersed on a non-magnetic support in a binder containing a copolymer comprising the above units (1) to (4) as a binder resin. It has a magnetic layer formed of a magnetic paint.

The non-magnetic support used in the magnetic recording medium of the present invention includes, for example, synthetic resins such as polyester, polyolefin, cellulose acetate and polycarbonate, non-magnetic metals and ceramics. The form is not particularly limited, and various forms such as a film, a tape, a sheet, and a plate-like body can be used.

As the ferromagnetic fine powder, specifically, γ
-Fe ₂ O ₃ , Fe ₃ O ₄ and those obtained by adsorbing or doping cobalt ions, CrO ₂ , and further Fe, Co,
Examples thereof include needle-shaped fine particles of a metal or an alloy containing Fe-Co, Ni, and the like, and various other conventionally known magnetic powders.

The mixing ratio of the ferromagnetic fine powder to the binder is preferably 8 to 50 parts by weight, more preferably 13 to 20 parts by weight, per 100 parts by weight of the ferromagnetic fine powder.

The ferromagnetic fine powder and the binder can be mixed and dispersed by a usual method. However, in order to uniformly disperse both components, if necessary, a lubricant, which is conventionally generally used, may be used. Abrasive, antistatic, dispersing aid, rust inhibitor,
Furthermore, for example, in addition to a coating medium such as methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, and toluene,
Various organic solvents can be used as long as the effects of the present invention are not impaired.

When the copolymer is prepared by an emulsion polymerization method, the magnetic powder is added to the emulsion dispersion after polymerization together with a dispersing agent without employing a known salting-out method after the polymerization, as described above. It is also possible to add, mix and disperse to prepare an aqueous emulsion magnetic paint.

As a coating means for forming the magnetic layer on the support, a known method can be adopted, and by appropriately performing a smoothing treatment such as a calendering treatment, the high performance of the present invention can be attained. A magnetic recording medium can be obtained.

[0038]

According to the magnetic recording medium of the present invention, the dispersibility of the above-mentioned copolymer (binder resin) in ferromagnetic fine powder,
It has good thermal stability and good compatibility with various resins, especially polyurethane resin.Thus, even if the viscosity of the magnetic paint is suitable for high-speed coating work, the coating surface has good smoothness and good abrasion resistance. It is excellent and has a magnetic layer with good physical properties of coating film and excellent durability and magnetic properties, and is effective as a magnetic tape, a magnetic card, a floppy disk and the like.

[0039]

EXAMPLES Hereinafter, the present invention will be described specifically with reference to Synthesis Examples, Examples, and Comparative Examples, but the present invention is not limited to the following Examples. In the examples below, parts and% are parts by weight and% by weight, respectively.

The analysis of the monomer units in each synthesis example was performed as follows. a. (Meth) acrylate unit Unreacted (meth) in slurry or emulsion after polymerization
Analyze the amount of acrylate by gas chromatography,
The value obtained by excluding this from the charged amount was defined as the polymerization amount, and was defined as the value of the unit in the copolymer. b. Vinyl chloride unit The dry copolymer was burned in the presence of a platinum catalyst, and the hydrochloric acid content absorbed in deionized water was determined by neutralization titration with silver nitrate. c. Unit containing epoxy group and ethylenically unsaturated double bond The dried copolymer was dissolved in acetone, a certain amount of hydrochloric acid was added, and the consumed hydrochloric acid content was determined by neutralization titration. d. Vinyl acrylate, vinyl propionate unit (radical polymerizable unit) The dried copolymer is dissolved in acetone, and a NaOH-methanol solution is added to react at 70 to 80 ° C. to consume N.
The aOH content was determined by neutralization titration.

[Synthesis Example 1] (Synthesis of Polymer 1) After the inside of a polymerization vessel equipped with a stirrer, a thermometer and a nitrogen gas inlet was replaced with nitrogen, 540 parts of deionized water, 150 parts of vinyl chloride, and allyl glycidyl were added. 36 parts of ether, 30 parts of acetic acid vinyl, 8 parts of potassium persulfate, 2.7 parts of sodium lauryl sulfate, and 1 part of sodium carbonate were charged, and 56 parts were stirred.
The reaction was started by heating to ° C. Internal pressure is 1kg in gauge pressure
/ Cm ² , the unreacted gas was exhausted and cooled to obtain an emulsion.

Next, another polymerization vessel equipped with a stirrer, a thermometer and a nitrogen gas inlet was purged with nitrogen, and 760 parts of the emulsion obtained above and methyl methacrylate 14 were added thereto.
6 parts, Thiocalcol # 20 (trade name, manufactured by Kao Corporation)
7 parts and 2.7 parts of Neugen ET-83 (trade name, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) were charged, and the mixture was heated to 70 ° C. with stirring to start the reaction. After stirring for 8 hours, the mixture was cooled to complete the polymerization reaction.

After completion of the polymerization, 50 parts of calcium chloride and 500 parts of hot deionized water were added to precipitate a copolymer. The precipitated copolymer was washed and filtered five times with 1,000 parts / time of deionized water, and then dried to obtain 285 parts of a copolymer (polymer 1). This polymer has 44.2% of methyl methacrylate units, 45.3% of vinyl chloride units, 5.3% of allyl glycidyl ether units,
The vinyl acetate unit was 5.2%. The number average molecular weight of the copolymer was 32,000.

[Synthesis Example 2] (Synthesis of Polymer 2) After the inside of a polymerization vessel equipped with a stirrer, a thermometer and a nitrogen gas inlet was replaced with nitrogen, 1,000 parts of deionized water, 132 parts of methyl methacrylate, 167 parts of vinyl chloride, 34 parts of allyl glycidyl ether, 29 parts of vinyl acid acid, 10 parts of potassium persulfate, 4 parts of sodium lauryl sulfate and 2 parts of sodium carbonate were charged and heated to 70 ° C. with stirring to start the reaction. . After stirring for 8 hours, the mixture was cooled to complete the polymerization reaction.

After completion of the polymerization, salting out, washing-filtration, and drying were carried out in the same manner as in Synthesis Example 1 to obtain 310 parts of a copolymer (polymer 2). This polymer has 39.6% of methyl methacrylate units and 50.4% of vinyl chloride units.
%, Allyl glycidyl ether unit 4.8%, and vinyl acetate unit 5.2%. The number average molecular weight of the copolymer was 28,000.

[Synthesis Example 3] (Synthesis of Polymer 3) A polymerization vessel equipped with a stirrer, a thermometer and a nitrogen gas inlet was purged with nitrogen, and then 600 parts of deionized water, 130 parts of methyl methacrylate, and butyl methacrylate were added. 25 parts, 0.8 parts of thiocalcol # 20 (manufactured by Kao Corporation, trade name), 4 parts of potassium persulfate, 2 parts of sodium lauryl sulfate, and 1 part of sodium carbonate were charged and heated to 70 ° C. while stirring to carry out a reaction. Started. After stirring for 8 hours, the mixture was cooled to obtain an emulsion.

Next, another polymerization vessel equipped with a stirrer, a thermometer and a nitrogen gas inlet was replaced with nitrogen, and 760 parts of the emulsion obtained above, 165 parts of vinyl chloride, 21 parts of allyl glycidyl ether, Vinyl propionate 21
Department, Neugen ET-83 (Daiichi Kogyo Seiyaku Co., Ltd., trade name)
2 parts were charged and heated to 60 ° C. with stirring to start the reaction. After stirring for 5 hours, the mixture was cooled to complete the polymerization reaction.

After completion of the polymerization, salting out, washing-filtration and drying were carried out in the same manner as in Synthesis Example 1 to obtain 275 parts of a copolymer (polymer 3). This polymer has a methyl methacrylate unit of 38.1%, a butyl methacrylate unit of 7.4%, a vinyl chloride unit of 48.0%, an allyl glycidyl ether unit of 3.0%, and a vinyl propionate unit of 3.10%.
5%. The number average molecular weight of the copolymer is 29,
000.

Tables 1 and 2 show the types and amounts of the monomers used in Synthesis Examples 1 to 3 and the compositions of the obtained copolymers (polymers 1 to 3).

[Synthesis Examples 4, 5, 7, 8] (Polymer 4,
Synthesis of 5, 7, 8) In the same manner as in Synthesis Example 1, monomers of the types and amounts shown in Table 1 were copolymerized, salted out by the same method, and each composition as shown in Table 2 was obtained. Copolymer (Polymer 4, 5, 7, 8)
I got

[Synthesis Examples 6, 9, 10] (Polymer 6,
Synthesis of 9, 10) In the same manner as in Synthesis Example 2, monomers of the types and amounts shown in Table 1 were copolymerized, salted out by the same method, and copolymers having the respective compositions shown in Table 2 (Polymers 6, 9, 10) were obtained.

[0052]

[Table 1]

[0053]

[Table 2]

Examples 1 to 6 and Comparative Examples 1 to 4 The compatibility (thermal stability) with the polyurethane resin of the copolymers (polymers 1 to 10) obtained in the above synthesis examples was examined. Next, using these copolymers, a magnetic tape was prepared by the following method,
The coating properties and magnetic properties were examined. Table 3 shows the results. In addition, the measurement of each characteristic was performed as follows. Evaluation method: a. Compatibility with Polyurethane Resin 100 parts of a 20% solution of each copolymer using a mixed solvent of methyl ethyl ketone / methyl isobutyl ketone / toluene = 1/1/1 (parts) and 50 parts of various polyurethane resins are mixed. After stirring for 1 hour, apply on glass plate
The coating was dried, and the compatibility was visually evaluated on a four-point scale based on the presence or absence of a bump on the coating film and the degree of cloudiness. ◎: Excellent ,: Good, Δ: Acceptable, ×: Not Acceptable b. Heat Stability 1 part of the copolymer was precisely weighed in a 15 cc test tube, and the opening was plugged with absorbent cotton sandwiching Congo Red test paper.
The oil was placed in an oil bath at ℃ and the time required for the Congo Red test paper to change its color due to the generated hydrochloric acid was measured. c. Coating viscosity The viscosity of the kneaded and prepared magnetic coating liquid at a temperature of 25 ° C was measured with an E-type viscometer. d. Gloss The 60-degree reflectance of the magnetic paint-coated surface of the magnetic tape before calendering was compared with that of a standard glass plate using a gloss meter (manufactured by Murakami Color Research Institute). e. Squareness ratio Measured using a vibrating sample magnetometer (manufactured by Toyo Kogyo Co., Ltd.). f. Durability After leaving the produced magnetic tape in a constant temperature and humidity room at 65 ° C. and a relative humidity of 90% for 168 hours, a load of 100 g is applied thereto, and the magnetic tape is rotated 1,000 times at 150 rpm by being brought into contact with a rotating drum on which abrasive paper is stuck. Then, the degree to which the magnetic paint adhered to the abrasive paper was visually evaluated in four steps. ◎: No stain on the polishing paper ○: Slightly stained △: Slightly stained ×: Many stains g. Runnability The force generated between the coating film and the rotating drum was measured by a U gauge in an atmosphere of 65 ° C. and a relative humidity of 80% in the same manner as in the evaluation of the durability, and evaluated in four steps in ascending order of running resistance. ：: extremely low, ○: low, Δ: medium, ×: high Magnetic tape preparation method: (1) Preparation of magnetic paint The following components were mixed for 90 minutes with a laboratory mixer, and further 3 hours with a sand mill containing glass beads. After kneading, the mixture was filtered to obtain a coating liquid. Binder resin (Polymer 1 to 10) 12 parts Magnetic powder (metal powder having a specific surface area of 50 m ² / g) 100 parts Polyurethane resin (trade name: U-8300: manufactured by Toyobo Co., Ltd.) 8 parts Methyl ethyl ketone 90 parts Cyclohexanone 90 parts Toluene 90 parts (2) Preparation of Magnetic Tape The above-mentioned coating liquid was applied to a thickness of 6 μm on a polyester film having a thickness of 15 μm, subjected to a magnetic field orientation treatment, dried, and then surface-treated with a super calender to prepare a magnetic tape.

[0055]

[Table 3]

Claims (3)

[Claims] 1. A magnetic recording medium comprising a nonmagnetic support and a magnetic layer made of a magnetic paint in which ferromagnetic fine powder is dispersed in a binder, wherein the binder comprises (1) an alkyl ( (Meth) acrylate unit 30 to 50% by weight (2) vinyl chloride unit 30 to 55% by weight (3) Monomer unit having an epoxy group and an ethylenically unsaturated double bond 1 to 20% by weight (4) Other radicals A magnetic recording medium comprising, as a main component, a copolymer containing 1 to 20% by weight of a polymerizable monomer unit as a constituent unit. 2. A copolymer having a number average molecular weight of 5,000.
2. The magnetic recording medium according to claim 1, wherein the number is from 0 to 100,000. 3. The magnetic recording medium according to claim 1, wherein the binder contains the copolymer and a polyurethane resin.