JPH0715737B2 - Magnetic recording medium - Google Patents

Description

TECHNICAL FIELD The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having excellent surface smoothness, magnetic powder dispersibility, and durability. Is.

[Conventional technology]

Conventionally, polyester resins, polyurethane resins, and the like have been widely used as binders for magnetic recording media, but these resins in which ferromagnetic fine powders are dispersed are placed on a non-magnetic support to be manufactured. The obtained magnetic recording medium does not have sufficient magnetic powder dispersibility, surface smoothness and durability. Among these resins, polyurethane resin in particular has a good aspect of high durability, but on the other hand, it has a weak affinity with magnetic particles and is inferior in magnetic powder dispersibility and surface smoothness. ing.

Therefore, for the purpose of improving the disadvantage that the magnetic powder dispersibility of the polyurethane resin is poor, polar groups such as SM ₃ M group and COOM group (where M is H, Li, Na or K) are used alone. Polyurethane resin having (JP-A-55-38693, JP-A-5-38693)
7-92423, JP-A-59-30235, each publication), a tertiary OH group and SO ₃
Attempts have been made to use a polyurethane resin having both M group or SO ₃ M group and COOM group (JP-A-59-108023 and JP-A-57-92422) as a binder.

[Problems to be solved by the invention]

However, the polyurethane resins that have been developed so far have improved affinity and dispersibility for magnetic powders, but their effects are not always sufficient, and conversely, they were introduced into polyurethane resins. It can be seen that the polar group deteriorates the mechanical properties of the resin, and therefore, when a magnetic recording medium is manufactured using the resin, the magnetic recording medium cannot exhibit sufficient durability. For this reason, the advent of a binder that is sufficiently satisfactory in all of the magnetic powder dispersibility, durability, and plane smoothness is desired.

[Means for solving problems]

The present inventors have found that no polyurethane resin excellent in the above problems, that is, the dispersibility of magnetic particles has been obtained, and that the polyurethane resins obtained so far are not necessarily durable. In view of the fact that satisfactory effects have not been obtained, as a result of repeated intensive studies aimed at improving the dispersibility of magnetic particles while maintaining the mechanical properties of the polyurethane resin, it has a specific polar group. The present invention has been completed by finding that the above object can be achieved by adopting a polyurethane resin.

That is, the present invention provides a magnetic recording medium in which a magnetic layer in which ferromagnetic fine powder is dispersed in a binder is provided on a non-magnetic support, and as a binder, SO ₃ M group, COOM group (COOM group in the molecular chain M is H,
The present invention provides a magnetic recording medium characterized by using a polyurethane resin having at least one or more of three polar groups of Li, Na or K) and an OH group and a combination thereof.

The polyurethane resin used in the present invention is a resin having both the above-mentioned three polar groups in the molecule, both ends of the molecule are isocyanate groups, both ends are hydroxyl groups, and one end is an isocyanate. Any of the groups having a hydroxyl group at the other end may be used. The number average molecular weight of the polyurethane resin is 1,000 to 100,000, preferably 5,
Those of 000 to 50,000 are desirable. When the number average molecular weight of the polyurethane resin is less than 1,000, the mechanical properties, abrasion resistance and wet heat resistance of the resin are insufficient, and when the number average molecular weight exceeds 100,000, these performances are However, when the polyurethane resin is dissolved in a solvent to form a coating, it often becomes difficult to form the coating. If the number average molecular weight is 5,000 to 50,000,
In particular, it is advantageous in exhibiting the mechanical properties, abrasion resistance and wet heat resistance required for the polyurethane of the present invention.

Examples of the method for producing the polyurethane resin having three kinds of SO ₃ M group, COOM group and OH group in the molecule used in the present invention include the following methods.

(1) (a) Organic polyisocyanate (b) High molecular weight polyol having at least one or more of the above-mentioned three polar groups, and (c) At least one or more of the above-mentioned three polar groups. It is produced by reacting a chain extender having However, in this case, the polyurethane resin produced should include all of the above three polar groups.

(2) (a) Organic polyisocyanate (b) It is produced by reacting the above-mentioned high molecular weight polyol having all three polar groups together, and (c) a chain extender, if necessary. In this case, the chain extender may not have the polar group or may have one or more polar groups.

(3) (a) Organic polyisocyanate (b) High molecular weight polyol (c) It is produced by reacting a chain extender having all of the above three polar groups.

Among these methods, the method (1) is particularly preferable from the viewpoints of easiness of production and mechanical strength of the produced polyurethane resin, and among them, as the high molecular weight polyol (b), SO ₃ M inside the molecular chain is preferably used. A high-molecular-weight polyol having a group and a COOM group, and having as a chain extender of (c) a diamine compound having at least one primary OH group inside the molecular chain or having at least one secondary OH group inside the molecular chain A combination of selected low molecular weight polyol compounds is particularly preferable from the viewpoint of mechanical strength.

In the present invention, the polyester polyol portion in the polyurethane molecule is referred to as a soft segment (soft portion), and the portion including the polyisocyanate and the chain extender is referred to as a hard segment (hard portion).

Examples of the organic polyisocyanate (a) in the method (1) include hexamethylene diisocyanate, lysine diisocyanate, isophorone diisocyanate, xylene diisocyanate, cyclohexane diisocyanate, toluidine diisocyanate, 2,4-tolylene diisocyanate, and 2,6-tolylene diisocyanate. , 4,4 ′
-Diphenylmethane diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate,
1,5-naphthylene diisocyanate, 4,4'-biphenyl diisocyanate, 3,3'-dimethylbiphenyl-4,4 '
-Diisocyanate, 3,3'-dimethoxybiphenyl-
Examples thereof include 4,4'-diisocyanate and the like, and hydrogenated products and mixtures of these aromatic isocyanates.
Among these, particularly preferably used in the present invention, from the viewpoint of strength of the polyurethane resin produced, 4,4'-diphenylmethane diisocyanate,
2,4- and 2,6-tolylene diisocyanate and mixtures thereof. These organic polyisocyanates are
It is also used in the methods (2) and (3).

The high molecular weight polyol having at least one of the above-mentioned three polar groups in the molecular chain of (b) in the method of (1) has both SO ₃ M group and COOM group in one molecule. Alternatively, it may be a mixture of a high molecular weight polyol having a SO ₃ M group and a high molecular weight polyol having a COOM group, and the mixture may have both a SO ₃ M group and a COOM group. In addition to these high molecular weight polyols, high molecular weight polyols usually used in the production of polyurethane can be used in combination.

As the high molecular weight polyol having a polar group in the molecular chain used in the method, a dibasic acid having a molecular weight of 50 to 500 and / or a low molecular weight glycol having a polar group in the molecule and a molecular weight of about 50 to 500 are used. A molecular weight of about 500 to 60, produced from a conventional dibasic acid and / or a low molecular weight glycol.
Mention may be made of 00 polyester polyols.

Among the above dibasic acids, examples of the compound having a SO ₃ M group include 5-sodium sulfoisophthalic acid, sodium sulfosuccinic acid, and alkyl esters thereof. Further, examples of the compound having a SO ₃ M group in the above low molecular weight glycol include straight chain diols such as 2-sodium sulfo-1,4-butanediol and 2-potassium sulfo-1,4-butanediol, and SO. Having ₃ M groups, the following formula A compound in which ethylene oxide and / or propylene oxide is added to the compound represented by A compound obtained by adding ethylene oxide and / or propylene oxide to a compound represented by the formula (4), a compound obtained by sulfonation, and an alkali metal chloride thereof (in this case, containing one or more SO ₃ M groups), 5-sodium sulfo. Screw-
Examples thereof include β-hydroxyethyl isofurate, 5-sodium sulfoisophthalic acid, and ethylene oxide or propylene oxide adduct of sodium sulfosuccinic acid. Examples of the compound having a COOM group include 2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid and 2,2
-Dimethylol valeric acid and the like.

Further, as a high molecular weight polyol having a polar group in the molecule, a dibasic acid and / or glycol having a polar group and having a molecular weight of about 50 to 500 is added with ethylene oxide and / or propylene oxide. The polyalkylene polyether polyols having a molecular weight of 500 to 8,000 obtained in this way can also be mentioned.
However, when a polyether polyol is used as the high molecular weight polyol, the resulting polyurethane resin has a low mechanical strength. Therefore, it is particularly preferable to use a polyester polyol in the present invention.

Examples of common dibasic acids used to make polyester polyols include succinic acid, maleic acid, adipic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid. Examples thereof include acids, dibasic acids such as hexahydroterephthalic acid and hexahydroisophthalic acid, acid esters thereof, acid anhydrides and mixtures thereof. In addition, examples of usual low molecular weight glycols include ethylene glycol, 1,
2-propylene glycol, 1,3-propylene glycol
1,4-butylene glycol, 2,2'-dimethyl-1,3-propanediol, dielen glycol, 1,5-pentamethylene glycol, 1,6-hexane glycol, cyclohexane-1,4-diol, cyclohexane- Examples include glycols such as 1,4-dimethanol, and mixtures thereof.

Examples of the usual high molecular weight polyol that can be used in combination with the high molecular weight polyol having a polar group inside the molecule include, for example, a low molecular weight polyol or a polyether polyol obtained by adding an alkylene oxide such as ethylene oxide or propylene oxide to an amine compound. And polytetramethylene ether glycol,
Further ethylene glycol, propylene glycol, 1,4
-Polyhydric alcohols such as butanediol, phthalic acid,
Polycondensation products with polybasic acids such as maleic acid, malonic acid, succinic acid, adipic acid, terephthalic acid, etc., having a hydroxyl group at the terminal polyester polyol, polycaprolactone polyol, polycarbonate polyol, acrylic polyol, castor oil, tall oil Etc. can be mentioned. Further, a liquid rubber having a hydroxyl group, an amino group, an imino group, a carboxyl group, an active hydrogen group such as a mercapto group or the like at the molecular end, or a mixture thereof can also be used.

The chain extender having at least one of the above-mentioned three polar groups in the molecular chain of (c) in the method (1) preferably has a molecular weight of about 50 to 500. Examples of the diamine compound having at least one primary OH group therein include N-hydroxyethylethylenediamine, N, N'-bishydroxyethylethylenediamine, 1,3-diamino-2-propanol and the like. . Examples of the low molecular weight polyol compound having at least one secondary OH group in the molecule include glycerin, ethylene oxide adduct of glycerin, 1,2,6
Mention may be made of hexanetriol, 4- [bis (2-hydroxyethyl)]-2-hydroxypentane and propylene oxide adducts of diethanolamine. Further, those having a tertiary OH group in the molecule include, for example, 1,2,3-hydroxy-2-methylpropane and 1,2,3-
Hydroxy-2-ethylpropane, 1,2,4-hydroxy-2-methylbutane, 1,2,5-hydroxy-2-methylpentane, 1,3,5-hydroxy-3-methylpentane,
1,3,6-Hydroxy-3-methylhexane, 1,2,3,6-hydroxy-2,3-dimethylhexane, 1,2,4,6-hydroxy-2,4-dimethylhexane, etc. alone or A mixture may be mentioned. Further, in the method, a chain extender usually used in the synthesis of polyurethane resin can be used in combination, for example, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-butane. Diol, 1,4-butanediol, neopental glycol, diethylene glycol, 1,5-pentamethylene glycol, 1,6-hexane glycol, cyclohexane-1,4-diol, cyclohexane-1,4-dimethanol, bis- Low molecular weight glycols such as β-hydroxyethylhydroquinone, ethylenediamine, 1,2-propylenediamine, 1,3-propylenediamine, 1,4-butanediamine, 1,6-hexanediamine, 4,4′-methylenebis-2 -Chloroaniline, 4,4'-methylenebis-2-
Ethylaniline, 4,4'-methylenebisdiphenylamine, isophoronediamine, 2,4-diaminotoluene, 2,6
And low molecular weight diamines such as diaminotoluene and mixtures thereof.

In producing the polyurethane resin which is the binder in the magnetic recording medium of the present invention, a conventionally known method can be adopted. For example, after sufficiently mixing the reactants, the reaction mixture is poured into a flat plate or a vat and heated. And then crushing it after cooling, or a single or mixed solvent organic such as dimethylformamide, toluene, xylene, benzene, dioxane, tetrahydrofuran, cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate. A production method such as a solution reaction method of reacting in a solvent can be adopted. At this time, a reaction catalyst may be added to reduce the reaction temperature or the reaction time. Specific examples of the reaction catalyst include, for example, triethylenediamine, tetramethylethylenediamine, amine compounds such as tetramethylhexanediamine and salts thereof, and dibutyltin dilaurate, tin octylate, lead octylate, organic metals such as manganese octylate. Examples thereof include compounds and mixtures thereof. Further, for the purpose of increasing the stability of the polyurethane resin, antioxidants, ultraviolet absorbers, hydrolysis inhibitors and the like can be blended alone or in combination.

Further, in the present invention, in order to improve the durability of the polyurethane resin, the polyurethane resin is cured by using an appropriate curing agent before use.

The curing agent that can be used in the present invention,
Examples thereof include a low molecular weight polyisocyanate having two or more isocyanate groups, and a compound having a molecular end having an isocyanate group by reacting a low molecular weight polyol with a polyisocyanate compound. The molecular weight thereof is preferably about 150 to 7,000. Among these, a low molecular weight polyisocyanate having two or more isocyanate groups is particularly preferable.

The low molecular weight polyisocyanate having two or more isocyanate groups is (a) in the above method (1).
In addition to those described above, mention may be made of reaction products of 3 mol of diisocyanates such as tolylene diisocyanate and hexamethylene diisocyanate with 1 mol of trimethylolpropane, and polyfunctional isocyanate compounds such as modified diphenylmethane diisocyanate and polyphenylmethane polyisocyanate. it can. These compounds are available from Nippon Polyurethane Co., Ltd. as "Coronate L", "Coronate HL", "Coronate 2030", "Millionate M".
"Death module L", "Death module N", "Death module IL", "Death module HL", "Death module R" from Sumitomo Bayer Urethane Co., Ltd. under product names such as "R" and "Millionate MTL" , "Death Module RF"
"Takenate D-10" from Takeda Pharmaceutical Co., Ltd.
2 "," Takenate D-110N "," Takenate D-202 "
Etc. are each marketed under the trade name.

In the present invention, the mechanical strength of the magnetic layer is obtained by adding 5 to 60 parts by weight of a low molecular weight polyisocyanate having two or more isocyanate groups to 100 parts by weight of a polar group-containing polyurethane resin and curing the same. Abrasion resistance, heat resistance, moist heat resistance, solvent resistance, and adhesion to a substrate can be greatly improved.

The polyurethane resin having three kinds of polar groups used in the present invention may be used in combination with a generally known polyurethane resin and other resins. For example, as the resin used in combination, nitrocellulose, cellulose acetate,
Cellulose acetate such as cellulose acetate butyrate; vinyl chloride / vinyl acetate copolymer, vinyl chloride / vinyl acetate / vinyl alcohol copolymer, vinyl chloride / vinyl acetate / maleic acid copolymer, etc .; vinylidene chloride resin; / Vinylidene chloride resin such as vinyl chloride copolymer and vinylidene chloride / acrylonitrile copolymer; polyester resin such as alkyd resin and linear polyester;
Acrylic resin such as (meth) acrylic acid / acrylonitrile copolymer, methyl (meth) acrylate / acrylonitrile copolymer; acetal resin such as polyvinyl acetal, polyvinyl butyral; phenoxy resin, epoxy resin, polyamide resin, butadiene / acrylonitrile copolymer Examples thereof include polymers and styrene / butadiene copolymers. These resins are used alone or in combination.

The magnetic recording medium of the present invention uses the polyurethane resin produced as described above as a binder, and a magnetic coating obtained by dispersing the binder, ferromagnetic fine powder, additives and the like together with an organic solvent on a non-magnetic support. It is manufactured by coating and drying. At this time, the ratio of the binder to the ferromagnetic fine powder is 10
0 to 100 parts by weight of binder, preferably 20
~ 50 parts by weight. That is, when the amount of the binder is less than 10 parts by weight with respect to 100 parts by weight of the ferromagnetic fine powder, the binding force between the binder and the magnetic powder is reduced, and mechanical strength and abrasion resistance are unsatisfactory. If the binder is used in an amount of more than 100 parts by weight, the density of the magnetic powder in the magnetic layer will decrease, and sufficient performance (reproduction output etc.) as a magnetic recording medium will not be obtained. When the binder is particularly mixed in an amount of 20 to 50 parts by weight with respect to 100 parts by weight of the magnetic powder, the mechanical strength which is the object of the present invention,
It is preferable not only from the viewpoint of enhancing wear resistance but also from the viewpoint of reproduction output of the magnetic recording medium.

Ferromagnetic fine powders that can be used in the present invention include γ-Fe ₂ O ₃ , mixed crystals of γ-Fe ₂ O ₃ and Fe ₃ O ₄ , Co-modified iron oxide, CrO ₂ , iron, etc. Other alloy fine powders containing iron as a main component can be preferably used.
The shapes of these ferromagnetic fine powders are needle-like, plate-like,
Any shape such as spherical shape can be used. Moreover, the specific surface area of these ferromagnetic fine powders is 7 by the BET method.
The range is 5 m ² / g or less, but 40 m ² / g or less is preferable. If the specific surface area of the magnetic powder is larger than 75 m ² / g,
Not only a high reproduction output cannot be obtained, but the strength of the magnetic layer is inferior, which is disadvantageous in terms of durability. Specific surface area of magnetic powder is 40
Those of m ² / g or less are very preferable in the sense that a high reproduction output can be obtained while maintaining the durability of the magnetic recording medium.

Aluminum oxide, chromium oxide, silicon oxide is added as a strengthening agent to the magnetic layer consisting of a binder and ferromagnetic fine powder, a plasticizer such as dibutyl phthalate and triphenyl phosphate, zinc stearate, silicone oil. It is also possible to add a lubricant such as the above, a dispersant such as soybean oil lecithin, and various antistatic agents such as carbon black.

These materials constituting the magnetic layer are dissolved in an organic solvent to prepare a magnetic coating material, and the magnetic coating material is applied onto a support to produce a magnetic recording medium. As a solvent for adjusting the magnetic paint, acetone, a ketone-based solvent such as methyl ethyl ketone, an alcohol-based solvent such as methanol and ethanol, an ester-based solvent such as ethyl acetate and butyl acetate, an ether-based solvent such as dioxane and tetrahydrofuran, benzene and toluene, etc. Examples thereof include aromatic hydrocarbon solvents and aliphatic hydrocarbon solvents such as hexane.

The support for applying the magnetic coating material may be a non-magnetic one, for example, polyester such as polyethylene terephthalate, polyolefin such as polypropylene, cellulose derivative such as cellulose triacetate, polycarbonate, polyvinyl chloride, polyimide, polyamide,
Alternatively, a metal such as aluminum may be used.

[Action]

Although the mechanism of action of the magnetic group-containing polyurethane resin in the present invention is not always clear, either one of the SO ₃ M group or the COOM group which the polyurethane resin has inside the molecular chain is a salt with the metal in the magnetic powder. , And the other polar group forms a hydrogen bond with the adsorbed water containing the magnetic powder.
While various kinds of effects work and a high dispersion effect is obtained,
Further, the OH group in the molecular chain of the polyurethane resin reacts with a low molecular weight polyisocyanate having two or more isocyanate groups to structurally strengthen the urethane resin, thereby obtaining high durability of the magnetic recording medium. It is considered to be a thing.

〔Example〕

Hereinafter, the present invention will be specifically described with reference to examples.
The invention is not limited to these examples only. In the examples, "part" means "part by weight".

(Synthesis Example of Polyester Polyol) With the formulation shown in Table 1, three types of polyester polyols (A) and (B) having a SO ₃ Na group and / or a COOH group and a molecular weight of about 2,000 are prepared according to a known method. (C) was synthesized.

(Production Example of Polyurethane Resin) Production Example 1 300 parts of methyl ethyl ketone, 20 parts of polyester polyol (A), polybutylene adipate (molecular weight of 2,000114 in a reaction vessel equipped with a thermometer, a stirrer and a reflux condenser.
Parts, 1.8 parts of 1,3,5-hydroxy-3-methylpentane, 18 parts of butanediol, 46.5 parts of a mixture of 80/20 of 2,4- and 2,6-tolylene diisocyanate, 0.5 parts of dibutyltin dilaurate as a catalyst. In addition, the mixture was reacted at 80 ° C for 8 hours.

The resulting polyurethane resin contains SO ₃ N in one molecule of the polymer.
It had one a group, one COOH group, and one secondary OH group, and its molecular weight was 20,000.

Production Example 2 In the same container as in Production Example 1, 360 parts of methyl ethyl ketone,
Tetrahydrofuran 89 parts, polyester polyol (A) 40 parts, polyester polyol (B) 40 parts, polybutylene adipate (molecular weight 2,000) 108 parts, ethylene glycol 17 parts, propylene oxide 1 mol adduct of diethanolamine 3.6 parts, 2,4- Also, 91 parts of a 2,6-tolylene diisocyanate mixture (weight ratio 80/20) and 0.5 part of dibutyltin dilaurate as a catalyst were added, and the mixture was reacted at 80 ° C. for 12 hours.

The resulting polyurethane resin contains SO ₃ N in one molecule of the polymer.
It had two a groups, one COOH group, and one secondary OH group, and its molecular weight was 15,000.

Production Example 3 In a container similar to Production Example 1, 240 parts of methyl ethyl ketone,
Cyclohexanone 60 parts, polyester polyol (A)
20 parts, polyester polyol (C) 20 parts, polybutylene adipate (molecular weight 2,000) 42 parts, butanediol 5.7
Parts, 3.6 parts of a 1-mole propylene oxide adduct of diethanolamine, 29 parts of 4,4′-diphenylmethane diisocyanate, and 0.1 part of dibutyltin dilaurate as a catalyst were added, and the mixture was reacted at 80 ° C. for 10 hours.

The resulting polyurethane resin contains SO ₃ N in one molecule of the polymer.
It had one a group, two COOH groups, and two secondary OH groups, and its molecular weight was 12,000.

Production Example 4 In the same container as in Production Example 1, 69 parts of polyester polyol (A) and 14 parts of polytetramethylene ether glycol were added.
0 parts, 4,4'-diphenylmethane diisocyanate 52 parts,
Add 0.1 part of dibutyltin dilaurate as a catalyst,
After reacting at 0 ° C for 1 hour, 112 parts of methyl ethyl ketone was added. The NCO% of the obtained prepolymer was 2.33%. .

A container equipped with a thermometer, a stirrer, a reflux condenser and a dropping funnel was charged with 6.9 parts of N-hydroxyethylethylenediamine, 7.5 parts of diethanolamine and 292 parts of methyl ethyl ketone, and 360 parts of the prepolymer was stirred for 1 hour at room temperature. The mixture was added dropwise, and then heated at 60 ° C. for 2 hours.

The obtained polyurethane resin has 1 SO ₃ Na group in one molecule.
It had one COOH group, one COOH group and two primary OH groups, and its molecular weight was 8,000.

Comparative Production Example 1 "Estan 5715" (manufactured by BF Goodrich) was prepared as a polyurethane resin having no polar group inside the molecular chain.

Comparative Production Example 2 Polyester polyol (A) used in Production Example 1
A polyurethane resin was produced in the same manner as in Production Example 1 except that 20 parts of the polyester polyol (B) was used instead of.

Comparative Production Example 3 Polyester polyol (A) used in Production Example 2
And (B) instead of polyester polyol (C)
A polyurethane resin was produced in the same manner as in Production Example 1 except that 40 parts and 40 parts of polybutylene adipate (molecular weight 2,000) were used.

Comparative Production Example 4 Instead of the propylene oxide 1 mol adduct of diethanolamine used as the chain extender in Production Example 3,
A polyurethane resin was produced in the same manner as in Production Example 1 except that 2.3 parts of diethanolamine was used.

Comparative Production Example 5 Polyester polyol (A) used in Production Example 4
A polyurethane resin was produced in the same manner as in Production Example 4 except that 69 parts of the polyester polyol (C) was used instead of.

Table 2 shows the numbers and molecular weights of polar groups contained in the molecular chains of the polyurethane resins produced in the above Production Examples and Comparative Production Examples.

Example 1 The magnetic powder dispersibility of the polyurethane resins produced in Production Examples 1 to 4 and Comparative Production Examples 1 to 5 was evaluated, and the durability of magnetic disks produced using the polyurethane resin was measured.

<Evaluation of Magnetic Powder Dispersibility> 125 parts γ-Fe ₂ O ₃ 100 parts Methyl ethyl ketone 278 parts Cyclohexanone 139 parts Polyurethane resin solution of Preparation Examples 1 to 4 or Comparative Preparation Examples 1 to 5 (nonvolatile content 40%) After kneading in a ball mill for 24 hours, apply the resulting magnetic coating on a 70-micron-thick polyethylene terephthalate substrate film, dry to a thickness of 15 microns after drying, and dry under an electron microscope (150x). The surface condition of the magnetic layer was observed by the method (1) to evaluate the dispersibility of the magnetic powder. The results are shown in Table 3.

<Durability evaluation of magnetic disk> 75 parts of polyurethane resin solution of Production Examples 1 to 4 or Comparative Production Examples 1 to 5 (nonvolatile content 40%) Vinylite VAGH (Vinyl chloride-vinyl acetate copolymer manufactured by Union Carbide Co., USA) 20 parts γ-Fe ₂ O ₃ 100 parts Carbon black 3 parts Lubricant 2 parts Methyl ethyl ketone 200 parts Cyclohexanone 100 parts After the above composition is kneaded in a ball mill for 24 hours, 10 parts of a curing agent (Coronate L manufactured by Nippon Polyurethane Co.) After further kneading for 1 hour, the obtained magnetic paint was applied onto a polyethylene terephthalate substrate film having a thickness of 70 μm so that the thickness after drying would be 2 μm, followed by drying and aging. Then, the film was cut into a predetermined magnetic disk size, mounted on a drive, and the time required for the reproduction output to decrease to 50% was measured to evaluate the durability. The results are shown in Table 3.

[Effects of the Invention] As specifically shown in the examples, the magnetic recording medium of the present invention uses a polyurethane resin having three kinds of polar groups in its molecular chain as a binder, and therefore its performance is improved. Is extremely high. This effect was obtained by introducing a SO ₃ M group, a COOM group and three kinds of polar groups together into the molecular chain of the polyurethane resin,
Compared with the conventional polyurethane resin having one or two of the above polar groups in combination, it exhibits extremely remarkable dispersibility and durability.

Therefore, the magnetic recording medium using the highly dispersible and highly durable polyurethane resin of the present invention can meet the desire for the emergence of a magnetic recording medium having excellent electromagnetic conversion characteristics and high durability. Conceivable.

Claims (5)

[Claims] 1. A magnetic recording medium comprising a magnetic layer having a ferromagnetic fine powder dispersed in a binder provided on a non-magnetic support, wherein a SO ₃ M group or COOM group (M Is H,
A magnetic recording medium characterized by using a polyurethane resin having at least one or more of three polar groups of Li, Na or K) and OH groups, and a combination thereof. 2. The number average molecular weight of the polyurethane resin is
The magnetic recording medium according to claim 1, which is 1,000 to 100,000. 3. The number average molecular weight of the polyurethane resin is
The magnetic recording medium according to claim 2, which is 5,000 to 50,000. 4. The magnetic recording medium according to claim 1, wherein the mixing ratio of the polyurethane resin and the ferromagnetic fine powder is 10 to 100 of the polyurethane resin to 100 of the ferromagnetic fine powder in a weight ratio. 5. The magnetic recording medium according to claim 4, wherein the mixing ratio of the polyurethane resin and the ferromagnetic fine powder is 20 to 50 of the polyurethane resin to 100 of the ferromagnetic fine powder in a weight ratio.