JPH10320746A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve dispersibility in a coating layer and to improve travelling durability by using a polyurethane resin comprising polyols and polyisocyanate as a binder and introducing sulfoalkyl groups into OH groups in the molecule of the polyurethane resin by using a sultone compd. SOLUTION: As for the polyols, long-chain diols having >=500 to 5000 weight average mol.wt. produced by adding alkylene oxides to diols having a cyclic structure are incorporated by 0 to 50 wt.% into the polyurethane resin, and short chain diols having 200 to <500 weight average mol.wt. produced by adding alkylene oxides to diols having a cyclic structure are incorporated by 15 to 50 wt.% into the polyurethane resin. The polyurethane contains ether groups derived from polyetherpolyols by 2 to 6 mmol/g of the whole polyurethane resin. Further, the average polar group content in the polyurethane resin to which alkyl groups are introduced by using a sultone compd. is controlled to 1×10<-5> to 50×10<-5> eq/g.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION In a magnetic recording medium having a magnetic layer on which a ferromagnetic fine powder and a binder are dispersed on a nonmagnetic support, extremely excellent electromagnetic conversion characteristics, running durability and high temperature and humidity TECHNICAL FIELD The present invention relates to a magnetic recording medium having a storage property under a magnetic field, and more particularly to a magnetic recording medium which does not cause head contamination.

[0002]

2. Description of the Related Art Magnetic recording media are widely used as recording tapes, video tapes, floppy disks and the like. In a magnetic recording medium, a magnetic layer in which ferromagnetic powder is dispersed in a binder is laminated on a non-magnetic support. The magnetic recording medium is required to be at a high level in various characteristics such as electromagnetic conversion characteristics, running durability and running performance. That is, audio tapes for recording and reproducing music are required to have higher original sound reproduction capabilities. Also, video tapes are required to have excellent electromagnetic conversion characteristics such as excellent original image reproduction capability. At the same time as having such excellent electromagnetic conversion characteristics, the magnetic recording medium is required to have good running durability as described above. In order to improve durability and electromagnetic conversion characteristics, the binder, which is one of the main components of the magnetic layer, also plays an important role. Conventionally used vinyl chloride resin, cellulose resin, urethane resin, acrylic resin and the like have a problem in that the wear resistance of the magnetic layer is inferior and the running members of the magnetic tape are contaminated.

[0003] Dirt on the magnetic head causes deterioration of electromagnetic conversion characteristics. In particular, in a device for high-density recording, the rotational speed of a magnetic head is increasing, and in a digital video tape recorder, the rotational speed of a magnetic head is 9600 revolutions / minute, which is a 1800 analog video tape recorder for consumer use.
The rotation speed is much higher than the rotation speed / minute, 5000 rotations / minute for business use, the speed at which the magnetic recording medium slides with the magnetic head is increased, and the magnetic head is also small, such as a thin film head. There is a demand for improvement of magnetic head contamination due to components generated from a magnetic recording medium.

As a method of solving such a problem, a method of increasing the hardness of the magnetic layer using a hard binder has been used. For example, JP-B-58-41565 discloses a binder obtained by introducing a polar group into a polyurethane resin.
A polyurethane resin containing SO ₃ M is disclosed, and a polyester polyol having a metal sulfonate group is described as a compound for introducing a metal sulfonate group. In addition, Japanese Patent Application Laid-Open No. Hei 3-203811 discloses that
A polyurethane resin in which an oxysulfonic acid, that is, a sultone compound is reacted with a tertiary amino group-containing polyurethane resin to modify a tertiary amino group to a sulfobetaine group is described. However, a polar group-free diol used in combination with a polar group-containing diol causes a difference in urethanization rate, resulting in non-uniform introduction into the polyurethane skeleton, many molecules without a polar group being present, and dispersion. Both properties and durability were insufficient.

Further, Japanese Patent Application Laid-Open No. Hei 6-12654 discloses that
A polyurethane resin in which a polar group is introduced by reacting a sultone compound with a nitrogen atom of a urethane bond is described,
Japanese Patent Application Laid-Open No. 6-279746 discloses that a urethane bond has ω
- polyurethane resin obtained by introducing a SO ₃ M group with an alkane sultone is described. Since these are bonded to nitrogen atoms, the distribution of polar groups is relatively uniform.However, when the polar groups are adsorbed on magnetic or non-magnetic powder as compared with those present at the molecular terminals, the adsorption layer However, there was a problem that it was thin and inferior in dispersibility.

Japanese Unexamined Patent Publication (Kokai) No. 63-263629 describes that the amount of a component having a weight average molecular weight of less than 3000 in a binder resin is reduced to 3% by weight or less to prevent bleeding of a magnetic layer. JP-A-6-52539 discloses that
It is described that the bleeding of the magnetic layer is prevented by using 40 to 60% by weight of the polyurethane resin from which low molecular components have been removed. In the case of a diol that does not contain a polar group used in combination with a diol, a difference occurs in the urethanization rate, the introduction into the polyurethane skeleton becomes uneven, there are many molecules without a polar group, and both the dispersibility and durability are insufficient. there were.

[0007] The present applicant has filed Japanese Patent Application No. 8-14837.
No. 0 discloses that after synthesizing a polyurethane resin having a terminal NCO, a compound containing a polar group and active hydrogen in one molecule is reacted to introduce a polar group into the polyurethane resin. Since the compound has low solubility in the organic solvent used for synthesizing the polyurethane resin, there is a problem that the amount of the polar group introduced is limited.

[0008]

DISCLOSURE OF THE INVENTION The present invention provides a coating layer having high dispersibility, a magnetic layer having excellent surface smoothness and electromagnetic conversion characteristics, and having excellent running durability by preventing head dirt and clogging. It is an object of the present invention to provide a magnetic recording medium which is excellent in preservability under high temperature and high humidity and has a small amount of surface binder before and after running.

[0009]

According to the present invention, there is provided a magnetic recording medium having a magnetic layer in which at least one ferromagnetic powder is dispersed on a nonmagnetic support, wherein the binder is a polyurethane resin comprising a polyol and a polyisocyanate. The polyurethane resin is a magnetic recording medium containing a polyurethane resin in which a sulfoalkyl group is introduced into a OH group in a molecule using a sultone compound. The polyurethane resin has a weight average molecular weight of 500 or more obtained by adding an alkylene oxide to a diol having a cyclic structure as the polyol.
5000 long-chain diols are added to the polyurethane resin from 0 to 5
The polyurethane resin contains 15 to 50% by weight of a short-chain diol having a weight average molecular weight of 200 to less than 500 in which a diol having a cyclic structure is added to an alkylene oxide and the ether group derived from the polyether polyol contains 0% by weight. The above magnetic recording medium is a polyurethane resin containing 2 to 6 mmol / g with respect to the above. The above magnetic recording medium, wherein the intramolecular OH group of the polyurethane resin comprising the polyol and the polyisocyanate is at a molecular terminal. The above magnetic recording medium, wherein the intramolecular OH group of the polyurethane resin comprising the polyol and the polyisocyanate is two or more per molecule. An average polar group content contained in a polyurethane resin into which a sulfoalkyl group has been introduced using the sultone compound is 1 × 1.
The magnetic recording medium as described above, wherein the magnetic recording medium has a density of 0 ⁻⁵ to 50 × 10 ⁻⁵ eq / g. On a non-magnetic support, a lower non-magnetic layer containing at least a non-magnetic powder and a binder, and an upper magnetic layer containing at least a ferromagnetic powder and a binder on which a plurality of at least two or more layers are provided. A magnetic recording medium comprising the polyurethane resin as a binder for at least one of the lower nonmagnetic layer and the upper magnetic layer.

[0010]

DETAILED DESCRIPTION OF THE INVENTION The present invention focuses on the type and amount of a polar group of a polyurethane resin used for a magnetic layer and a non-magnetic layer on a non-magnetic support, and a method of introducing the polar group.
This is a result of intensive studies on manufacturing a magnetic recording medium having excellent electromagnetic conversion characteristics such as durability and storability, and practical characteristics.

The present invention is directed to a magnetic recording medium having a magnetic layer containing at least a ferromagnetic powder and a binder on a nonmagnetic support, or a lower layer containing at least a nonmagnetic powder and a binder on a nonmagnetic support. In a magnetic recording medium having two or more layers including a non-magnetic layer and an upper magnetic layer including at least a ferromagnetic powder and a binder thereon, the magnetic layer or the lower non-magnetic layer or the upper magnetic layer As an agent, a sulfonic acid group is introduced by using a sultone compound at a terminal OH group in a solution state after synthesizing a polyurethane resin. As a result, the polar group is more uniformly introduced into the binder molecules, the number of binder molecules having no polar group is reduced, the amount of adsorption is increased, and the dispersibility is improved. The resulting magnetic recording medium is
A magnetic recording medium exhibiting excellent electromagnetic conversion characteristics and having reduced migration of low molecular components is obtained. Then, -SO ₃ M (wherein, M is hydrogen, alkali metal, alkaline earth metal, at least one selected from ammonium salts) to the terminal has a polar group, the polar group content 1
It has been found that it is preferable to use a polyurethane resin in the range of × 10 ^-5 to 50 × 10 ^-5 eq / g.

That is, in the present invention, a sulfonic acid group is introduced into a hydroxyl group portion by synthesizing a polyurethane resin and then introducing a sulfonic acid group with a sultone compound or a lactone compound. In the present invention, an arbitrary amount of a sulfonic acid group or a carboxylic acid group can be introduced by selecting and reacting an arbitrary amount of a sultone compound or a lactone compound with respect to the urethane bond of the polyurethane.

The polyurethane resin of the present invention can be sulfonated with propane sultone or butane sultone by adding a dimethylformamide (DMF) solution of sodium hydride (NaH) to a solution in which the polyurethane resin is dissolved. An arbitrary amount of sulfonic acid can be introduced by arbitrarily selecting the amount of propane sultone or butane sultone and causing the reaction.

Conventionally, using sodium hydride,
Although a method of reacting alkane sultone is known,
When sodium hydride was added, the mixture was anionized while cooled to -5 to 0 ° C.
It was found that a polar group was introduced into a hydroxyl group preferentially over a nitrogen atom by a heating reaction at about ° C. Also,
These occur preferentially with butane sultone over with propane sultone.

Conventionally, polyurethane resins and vinyl chloride resins having good mechanical strength and high solubility in selected solvents have been used together with polyisocyanate curing agents as binders for magnetic recording media.

Further, in a magnetic recording medium having a ferromagnetic powder and a non-magnetic lower layer used for a magnetic layer to increase the recording density, as a means for further improving the dispersibility of the lower non-magnetic powder, the above-mentioned polyurethane resin may be used. -SO ₃ M, -O
SO ₃ M, —PO (OM) ₂ , —COOM (where M is a hydrogen atom, an alkali metal or ammonium salt), 4
It has been proposed to introduce a polar group such as a quaternary ammonium base or an amino group, but in order to introduce a polar group into a polyurethane resin, a diol containing a polar group or a reactive group for introducing a polar group is used. These polar group-containing diols and other polar group-free diols used in combination with the isocyanate compound have a difference in the rate of urethanization reaction with the isocyanate compound. Are non-uniform. A binder obtained by a conventionally known method averages 10 × 10 ⁻⁵ e.
A detailed examination of the q / g binder containing a polar group reveals that the binder molecule containing no polar group is contained in a high proportion, and the polyurethane free of such a polar group is contained. The molecules were hardly adsorbed on the ferromagnetic powder and the non-magnetic powder, and as a result, the dispersibility was reduced and the electromagnetic conversion characteristics were deteriorated as compared with those existing uniformly. Regarding the storage stability, polyurethane molecules that are not adsorbed by the magnetic powder and the non-magnetic powder in the coating film bleed over the surface of the magnetic layer with the passage of time, especially when stored under high temperature and high humidity. It may cause contamination of the system.

The polyurethane resin skeleton of the present invention includes:
Polyol having a weight average molecular weight of 500 to 5,000 in which a diol having a cyclic structure is added with an alkylene oxide is contained in a polyurethane resin in an amount of 0 to 50% by weight, and a diol having a cyclic structure or a diol having a cyclic structure to which an alkylene oxide is added. A polyurethane resin containing 15 to 50% by weight of at least one of short chain diols having a weight average molecular weight of 200 to less than 500 and containing 2 to 6 mmol / g of ether groups derived from the diol and the polyol based on the whole polyurethane resin. It has been found that a magnetic recording medium which is excellent not only in dispersibility and electromagnetic conversion characteristics but also in running durability and storability can be obtained.

The polyurethane resin used for the polyurethane resin skeleton of the present invention can be produced from a polyol such as polyester polyol, polycarbonate polyol, polyether polyol, polyester ether polyol and polyisocyanate. May be used in combination. Among these, a polyurethane resin derived from a short-chain diol having a cyclic structure and a long-chain diol containing an ether group is preferable because it has excellent not only electromagnetic conversion characteristics but also running durability and storage stability.

The polyester polyol can be synthesized by, for example, polycondensation of a dihydric alcohol and a dibasic acid, and ring-opening polymerization of lactones, for example, caprolactone. Representative dihydric alcohols include ethylene glycol, propylene glycol, butanediol, 1,6
And glycols such as hexanediol and cyclohexanedimethanol. Typical dibasic acids include adipic acid, pimelic acid, azelaic acid, sebacic acid, phthalic acid, terephthalic acid, and the like.

Examples of the polycarbonate polyol include, for example, a weight average molecular weight of 300 to 20,000 and a hydroxyl value of 200 to 300 synthesized by condensation or transesterification of the polyol with phosgene, chloroformate, dialkyl carbonate or diallyl carbonate.
And a polycarbonate polyester polyol having a molecular weight of 400 to 30,000 and a hydroxyl value of 5 to 300 obtained by condensation of the polycarbonate polyol with a divalent carboxylic acid.

The short-chain diol having a cyclic structure preferably has a weight average molecular weight of 50 to 500.
If it is less than 50, the coating film becomes brittle and the durability is reduced.
If it is 500 or more, the glass transition temperature Tg of the coating film decreases, and the coating film becomes soft, so that the durability decreases.

Specifically, bisphenol A, hydrogenated bisphenol A, bisphenol S, bisphenol P
And ethylene oxide, propylene oxide adduct thereof, cyclohexanedimethanol, cyclohexanediol, hydroquinone, bis (2-hydroxyethyl) tetrabromobisphenol A, bis (2-hydroxyethyl) tetrabromobisphenol S, bis (2
-Hydroxyethyl) tetramethylbisphenol S,
Bis (2-hydroxyethyl) diphenylbisphenol S, bis (2-hydroxyethyl) diphenylbiphenol, bis (2-hydroxyethyl) thiodiphenol, bis (2-hydroxyethyl) bisphenol F,
Biphenol, bisphenol fluorene, and bisphenol fluor orange hydroxyethyl ether are preferred.

Among them, bisphenol A, hydrogenated bisphenol A, bisphenol S, bisphenol P, and diols having an aromatic or alicyclic group such as ethylene oxide, propylene oxide adduct thereof, cyclohexane dimethanol and cyclohexane diol are preferred. Further, hydrogenated bisphenol A and their ethylene oxide and propylene oxide adducts are preferred.

The content of the short-chain diol having a cyclic structure in the polyurethane is preferably 20 to 35% by weight. If it is less than 20% by weight, the mechanical strength is reduced and the durability is reduced. 3
If the content is 5% by weight or more, the solubility in a solvent decreases, and the dispersibility decreases. Further, the coating film tends to be brittle, and the durability is also reduced.

As the long-chain diol containing an ether group, a long-chain diol having a weight average molecular weight of 500 or more and 5,000 is preferable. If it is less than 500, the characteristics of a long-chain diol cannot be seen. , And becomes soft, so that the durability decreases. Specifically, the following can be used.

Bisphenol A, hydrogenated bisphenol A, bisphenol S, bisphenol P These polyethylene oxide and polypropylene oxide adducts. Polypropylene glycol, polyethylene glycol, polytetramethylene glycol. Furthermore, bisphenol A, hydrogenated bisphenol A and their ethylene oxide and propylene oxide adducts are preferred. The content of the long-chain diol containing an ether group in the polyurethane is preferably from 25% by weight to 45% by weight. More preferably, the range is 30% by weight to 40% by weight. If the amount is less than 25% by weight, the solubility in a solvent is reduced, so that the dispersibility is reduced. If the amount is more than 45% by weight, the coating film strength is reduced, and the durability is reduced. The ether group content in the polyurethane is 2 mmol / g
~ 4 mmol / g is preferred. If it is less than 2 mmol / g, the dispersibility decreases due to a decrease in the adsorptivity to the magnetic substance,
If it is 5 mmol / g or more, the dispersibility decreases due to the decrease in solubility in a solvent.

The polyisocyanate used to form a polyurethane resin by reacting with a polyol includes hexamethylene diisocyanate, tolidine diisocyanate, isophorone diisocyanate, 1,3-xylylene diisocyanate, 1,4-xylylene diisocyanate, and cyclohexane diisocyanate. , Toluidine diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4'-
Diphenylmethane diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate,
Examples thereof include 1,5-naphthalene diisocyanate, 4,4-diphenylmethane diisocyanate, and 3,3-dimethylphenylene diisocyanate. Examples of the chain extender include the above-mentioned polyhydric alcohols, aliphatic polyamines, alicyclic polyamines, and aromatic polyamines.

Further, the OH group in the polyurethane resin is
It is preferable to have a branched structure OH group from the viewpoint of curability and durability, and it is preferably 2 to 40 per molecule, more preferably 3 to 20 per molecule. If the number of OH groups is less than 2 per molecule, OH
Since the number of groups decreases and the reactivity with the isocyanate curing agent decreases, the strength of the coating film decreases, and the durability decreases. If the number is 40 or more per molecule, the solubility in a solvent is reduced, so that the dispersibility is reduced. As the compound used for adjusting the OH group content, a compound having three or more OH groups can be used.

The polyurethane resin preferably has a molecular weight of 5,000 to 100,000,
Those having 8,000 to 50,000 are more preferable. 5000
If it is less than 10, the resulting magnetic coating film becomes brittle and the physical strength decreases, which also affects the durability of a magnetic tape or the like. 10
If it exceeds 0000, the solubility in a solvent is reduced and the dispersibility is reduced. Further, the viscosity of the paint at a predetermined concentration becomes high, so that workability is remarkably deteriorated and handling becomes difficult.

Glass transition temperature Tg of polyurethane resin
Is preferably 50 ° C. to 200 ° C., more preferably 70 ° C.
C. to 170C. More preferably 90 ° C to 130
° C. If the temperature is lower than 50 ° C., the strength of the coating film at a high temperature is reduced, so that the durability and the storage stability are reduced. When the temperature exceeds 200 ° C., calender moldability decreases, and electromagnetic conversion characteristics decrease.

In the magnetic recording medium of the present invention,
Other binder resins can be used in combination. Although it can be used in combination with many binder resins, the polyurethane resin of the present invention is preferably used in combination with a vinyl chloride resin having good mechanical strength and high solubility in a selected solvent. For magnetic recording media with a ferromagnetic powder and a non-magnetic lower layer used for the magnetic layer to increase the recording density, a vinyl chloride resin containing a polar group is used to further improve the dispersibility of the lower non-magnetic powder together with the upper magnetic layer. Is preferably used in combination.

In addition, other binder resins may be used together with the polyurethane resin and the binder resin of the present invention in an amount equal to or less than the total amount of these. As the binder resin that can be used in combination, a thermoplastic resin, a thermosetting resin, a reactive resin, and a mixture thereof that have been conventionally used as a binder resin for a magnetic recording medium can be used.

The polar group contained in the binder resin of the present invention includes -SO ₃ M (M is a hydrogen atom, an alkali metal or an ammonium salt for the purpose of improving the dispersibility of the ferromagnetic fine powder and the nonmagnetic powder. Is preferable because of excellent dispersibility. The preferred polar group content is 1 × 10 ⁻⁵ to 50 × 1.
0 ⁻⁵ eq / g binder resin, more preferably 3 × 10 ⁻⁵ to
30 × 10 ⁻⁵ eq / g binder resin. 1 × 10 ^-5 e
If the amount is less than q / g binder resin, no effect is obtained, and 50 × 10
^{If the amount} exceeds ^-5 eq / g binder resin, the viscosity of the paint becomes high, the workability becomes extremely poor, and the handling becomes difficult.

In addition to the polar group of the present invention, a polar group may be introduced by using another vinyl chloride resin containing a polar group and a polyurethane resin containing a polar group in an amount equal to or less than the total amount of these. As the other polar group that can be used in combination, a polar group conventionally used as a binder resin for a magnetic recording medium and a mixture thereof can be used.

The polar group-containing vinyl chloride resin which can be used in the present invention can be prepared by a method of reacting a polar group-containing compound with a vinyl chloride resin, or by using a copolymerizable polar group-containing compound with a vinyl chloride monomer or other copolymer. It is obtained by copolymerization with possible compounds. For these production methods, for example, a method described in “Experimental method for polymer synthesis” (Takayuki Otsu, published by Kagaku Dojinsha 1972) can be used.

The polar group-containing vinyl chloride resin can also be synthesized by adding the polar group to a vinyl chloride resin containing no polar group by a reaction. For example, when -SO ₃ M is introduced into a vinyl chloride resin, a vinyl chloride monomer and a copolymerizable compound having a glycidyl group and, if necessary, another compound copolymerizable with these are copolymerized. Simultaneously with the copolymerization or after obtaining the copolymer, -SO
₃ is obtained by reacting a compound having a M.
Examples of the copolymerizable compound for introducing a glycidyl group include glycidyl acrylate, glycidyl methacrylate, and glycidyl vinyl ether. These may be used alone or in combination of two or more.

The copolymerizable polar group-containing compound can be copolymerized with a vinyl chloride monomer and other copolymerizable compounds. As the copolymerizable polar group-containing compound, a copolymerizable compound containing the polar group can be used. The copolymerizable compound for introducing a -SO ₃ M 2-acrylamido-2-methylpropanesulfonic acid, vinylsulfonic acid, acrylic acid, methacrylic acid, unsaturated hydrocarbons such as p- styrenesulfonate Sulfonic acid and salts thereof, and acrylic acid or methacrylic acid sulfoalkyl esters such as methacrylic acid sulfoethyl ester and methacrylic acid sulfopropyl ester, and salts thereof can be exemplified. These may be used alone or in combination of two or more. Furthermore, polar groups to be introduced may be two or more, for example, -COO in addition to the -SO ₃ M
When M needs to be introduced, specific examples of the copolymerizable compound for introducing -COOM include acrylic acid, methacrylic acid, and maleic acid, and salts thereof.

The copolymerizable compound for introducing an amino group and a quaternary ammonium base includes 1
Monomers such as acrylates, methacrylates, or allyl ethers having a primary, secondary, or tertiary aliphatic amino group, alicyclic amino group, alkanolamino group, or quaternary ammonium base can be used.

Further, as the vinyl chloride unit, 57 to 98
% By weight is preferred. If the amount is less than 57% by weight, the strength of the obtained resin coating film is reduced, and if it exceeds 98% by weight, the solubility of the resin in organic solvents such as ketones and esters is hindered.

The content of the vinyl alcohol unit is preferably 2 to 16% by weight. If the content is less than 2% by weight, the solubility in organic solvents such as ketones and esters when preparing the coating, non-magnetic powder,
The effect on the dispersibility of the magnetic powder, the reactivity with the isocyanate compound used in combination with the present resin, and the compatibility with other resins cannot be obtained. If it exceeds 16% by weight, the viscosity of the paint becomes too high, and When added, the pot life becomes short, which is disadvantageous in practical use.

The other vinyl unit is preferably from 0 to 26% by weight. If it exceeds this range, the mechanical properties and dispersibility of the entire resin will be reduced. Other copolymerizable monomers include vinyl acetate, vinyl propionate, (meth) acrylic acid ester, (meth) acrylic acid, (anhydrous) maleic acid, acrylonitrile, vinylidene chloride, hydroxyethyl (meth) acrylate, glycidyl (meth) Acrylate and the like can be mentioned. Here, (meth) acrylic acid means at least one of acrylic acid and methacrylic acid. Further, the obtained vinyl chloride copolymer can be saponified to convert vinyl acetate, vinyl propionate, and the like into a vinyl alcohol component.

The vinyl chloride resin has an average polymerization degree of 200 to 200.
800 is preferred. Further, 250 to 700 is more preferable. If it is less than 200, the resulting magnetic coating film becomes brittle, and the obtained magnetic coating film becomes brittle, resulting in a decrease in physical strength, which also affects the durability of a magnetic tape or the like. If it exceeds 800, the viscosity of the paint at a predetermined concentration becomes high, the workability becomes extremely poor, and the handling becomes difficult.

Further, a curing agent such as a polyisocyanate compound can be used together with the binder of the present invention.
Examples of the polyisocyanate compound include a reactive product of 3 mol of tolylene diisocyanate and 1 mol of trimethylolpropane (eg, Desmodur L-75 (manufactured by Bayer)), diisocyanate such as xylylene diisocyanate or hexamethylene diisocyanate. Reaction product of 1 mol of trimethylolpropane, burette addition compound with 3 mol of hexamethylene diisocyanate, isocyanurate compound of 5 mol of tolylene diisocyanate, isocyanurate addition of 3 mol of tolylene diisocyanate and 2 mol of hexamethylene diisocyanate Mention may be made of compounds, polymers of isophorone diisocyanate and diphenylmethane diisocyanate. The polyisocyanate compound contained in the magnetic layer is preferably contained in the binder in the range of 10 to 50% by weight, more preferably in the range of 20 to 40% by weight. Further, polymerization initiators, suspension stabilizers, emulsifiers and molecular weight regulators are described, for example, in JP-A-60-23830.
No. 6 can be used.

When the magnetic layer is composed of an upper magnetic layer and a non-magnetic lower layer, the non-magnetic powder used for the non-magnetic lower layer can be either an inorganic substance or an organic substance. Carbon black and the like can also be used. Examples of the nonmagnetic powder include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, and the like. Specifically, TiO ₂ , TiO _X , cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO ₂ , SiO ₂ ,
Cr ₂ O ₃ , α-alumina having an α conversion of 90% or more, β-alumina, γ-alumina, α-iron oxide, goethite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide, Copper oxide, MgC
O ₃ , CaCO ₃ , BaCO ₃ , SrCO ₃ , BaSO ₄ ,
Silicon carbide, titanium carbide or the like is used alone or in combination of two or more.

Of these, α-iron oxide and titanium oxide are preferred, and α-iron oxide is particularly preferred.

The pH of the non-magnetic powder used for the lower layer
Is measured by the JIS-A method. The non-magnetic powder is converted into an alkaline suspension, heated (for example, at 60 to 200 ° C.), surface-treated with an inorganic substance, and both are used in combination to form
H can be in a preferable range. pH = 8-11
If the pH is less than 8, the μ value increases under high temperature and high humidity, the running property is deteriorated, and the change in the amount of the surface binder increases. If the pH exceeds 11, the amount of released fatty acids decreases, so that the friction coefficient increases, the running property deteriorates, and the change in the amount of surface binder increases.

On the surface of these nonmagnetic powders, Al ₂ O ₃ ,
SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , Z
It is preferable to perform surface treatment with nO. Particularly preferred for dispersibility are _{Al 2 O 3, SiO 2,} TiO 2, ZrO 2, further preferred are _{Al 2 O 3, SiO 2,} ZrO 2. These may be used in combination or may be used alone. Further, a co-precipitated surface treatment layer may be used according to the purpose, or a method of first treating with alumina and then treating the surface layer with silica, or vice versa, may be employed. Although the surface treatment layer may be a porous layer depending on the purpose, it is generally preferable that the surface treatment layer be homogeneous and dense.

By mixing carbon black in the lower layer, Rs, which is a known effect, can be reduced, and a desired micro Vickers hardness can be obtained. For this purpose, furnace black for rubber, thermal black for rubber, carbon black for color, acetylene black and the like can be used. The specific surface area of the carbon black is 100 to 500 m ² / g, preferably 150 to 400 m ²
/ G, DBP oil absorption is 20 to 400 ml / 100 g, preferably 30 to 200 ml / 100 g. The average particle size of the carbon black is 5 nm to 80 nm, preferably 10 nm to 80 nm.
To 50 nm, more preferably 10 to 40 nm.
Carbon black has a pH of 2 to 10 and a water content of 0.1 to
Preferably, the tap density is 10% and the tap density is 0.1 to 1 g / ml.
Specific examples of carbon black used in the present invention include BLACKPEARLS 200 manufactured by Cabot Corporation.
0, 1300, 1000, 900, 800, 880, 7
00, VULCAN XC-72, manufactured by Mitsubishi Chemical Corporation, # 3
050B, 3150B, 3250B, # 3750B, #
3950B, # 950, # 650B, # 970B, # 8
50B, MA-600, manufactured by Columbia Carbon, CO
NDUCTEX SC, RAVEN 8800,8000,7000,5
750, 5250, 3500, 2100, 2000, 1800, 1500, 1255, 1250, Ketjen Black EC manufactured by Akzo, and the like.

In the lower layer of the present invention, a magnetic powder can also be used. As the magnetic powder, γ-Fe ₂ O ₃ , Co
Modified γ-Fe ₂ O ₃ , alloy mainly composed of α-Fe, Cr
O ₂ or the like is used. Particularly, Co-modified γ-Fe ₂ O ₃ is preferable. The ferromagnetic powder used in the lower layer of the present invention preferably has the same composition and performance as the ferromagnetic powder used in the upper magnetic layer. However, it is known that the performance is changed between the upper and lower layers according to the purpose. For example, in order to improve long-wavelength recording characteristics, it is desirable that Hc of the lower magnetic layer is set lower than that of the upper magnetic layer, and that Br of the lower magnetic layer is higher than that of the upper magnetic layer. It is valid. In addition to the above, an advantage obtained by adopting a known multilayer structure can be provided.

A binder for the lower magnetic layer or the lower non-magnetic layer,
Lubricants, dispersants, additives, solvents, dispersion methods and the like can be applied to those of the magnetic layer. In particular, with respect to the amount and type of the binder, the amount of the additive and the type of the dispersant, and the like, the known technology for the magnetic layer can be applied. A magnetic coating prepared from the above materials is applied on a non-magnetic support to form a magnetic layer.

Examples of the nonmagnetic support that can be used in the present invention include biaxially stretched polyethylene naphthalate,
Known materials such as polyethylene terephthalate, polyamide, polyimide, polyamide imide, aromatic polyamide, and polybenzoxdazole can be used. Preferred are polyethylene naphthalate and aromatic polyamide. These non-magnetic supports may be previously subjected to corona discharge, plasma treatment, easy adhesion treatment, heat treatment, and the like. The nonmagnetic support that can be used in the present invention has a surface having excellent smoothness such that the center line average surface roughness is in the range of 0.1 to 20 nm, preferably 1 to 10 nm at a cutoff value of 0.25 mm. Is preferred. It is preferable that these nonmagnetic supports have not only a small center line average surface roughness but also no coarse protrusions of 1 μ or more.

In the method for producing a magnetic recording medium of the present invention, for example, a coating solution for a magnetic layer is preferably applied to the surface of a running nonmagnetic support, preferably in a thickness of 0.05 to 5 μm after drying the magnetic layer. , More preferably 0.07 to 1 μm. Here, a plurality of magnetic paints may be sequentially or simultaneously applied in a multilayer manner. Examples of the coating machine for applying the magnetic paint include air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure coat, kiss coat, cast coat, and spray. Coat, spin coat, etc. can be used. These are described in, for example, “Latest Coating Technology” published by Sogo Gijutsu Center (May 1983
March 31).

When the present invention is applied to a magnetic recording medium having two or more layers, the following can be proposed as an example of a coating apparatus and method. (1) Gravure generally applied in the application of magnetic paint,
First, the lower layer is applied by a coating device such as a roll, a blade, an extrusion or the like, and while the lower layer is in an undried state, it is disclosed in Japanese Patent Publication No. 46186/1994, Japanese Patent Application Laid-Open No. 60-238179.
The upper layer is applied by a support pressurization type extrusion coating device as disclosed in Japanese Patent Application Laid-Open No. 2-265672 and JP-A-2-265672. (2) JP-A-63-88080, JP-A-2-179
No. 71 and Japanese Patent Application Laid-Open No. 2-265672 disclose the upper and lower layers almost simultaneously by one coating head having two coating liquid passage slits. (3) The upper and lower layers are coated almost simultaneously by an extrusion coating device with a backup roll as disclosed in JP-A-2-174965.

A backcoat layer (backing layer) is formed on the surface of the non-magnetic support used in the present invention on which the magnetic paint is not applied.
May be provided. The back coat layer is provided by applying a back coat layer forming paint in which a particulate component such as an abrasive and an antistatic agent and a binder are dispersed in an organic solvent on a surface of the non-magnetic support on which the magnetic paint is not applied. Layer. Various inorganic pigments and carbon black can be used as the particulate component, and resins such as nitrocellulose, phenoxy resin, vinyl chloride resin, and polyurethane can be used alone or as a mixture of these as a binder. . Note that an adhesive layer may be provided on the surface of the non-magnetic support on which the magnetic paint and the back coat layer forming paint are applied.

The applied layer of the magnetic paint is dried after subjecting the ferromagnetic powder contained in the layer of the magnetic paint to a magnetic field orientation treatment. After being dried in this manner, the coating layer is subjected to a surface smoothing treatment. For the surface smoothing treatment, for example, a super calender roll or the like is used. By performing the surface smoothing treatment, voids generated by the removal of the solvent during drying disappear, and the filling rate of the ferromagnetic powder in the magnetic layer is improved, so that a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. it can. As the calendering roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, or polyamideimide is used. Moreover, it can also process with a metal roll.

The magnetic recording medium of the present invention has a center line average roughness of 0.1 to 0.1 at a cutoff value of 0.25 mm.
It is preferable that the surface has an extremely excellent smoothness of 4 nm, preferably 1 to 3 nm. As a method for this, for example, a magnetic layer formed by selecting a specific ferromagnetic powder and a binder as described above is subjected to the above calendering treatment. As the calendering conditions, the temperature of the calender roll is in the range of 60 to 100 ° C, preferably in the range of 70 to 100 ° C, particularly preferably 80 to 10 ° C.
The temperature is in the range of 0 ° C. and the pressure is in the range of 100 to 500 kg / cm, preferably in the range of 200 to 450 kg / cm, particularly preferably in the range of 300 to 400 kg / cm. Is preferred. The obtained magnetic recording medium can be used after being cut into a desired size using a cutting machine or the like.

[0057]

The present invention will be described below in more detail with reference to Examples of the present invention. "Parts" in the following description are "parts by weight"
And% indicates% by weight. Production Example 1 (Synthesis of Polyurethane as Skeleton) For the production of a polyurethane resin as a skeleton, a conventionally known method can be used. For example, it can be manufactured by the method disclosed in Japanese Patent Publication No. 58-41565. Specifically, a reaction vessel equipped with a reflux condenser and a stirrer and previously purged with nitrogen was charged with hydrogenated bisphenol A (Rikabinol HB manufactured by Nippon Rika) 330
Part, bisphenol A polypropylene oxide adduct (average molecular weight 1000) 570 parts, cyclohexanone 3
000 parts at 60 ° C. under a nitrogen stream. Next, as a catalyst, di-n-dibutyltin dilaurate was added in an amount of 60 ppm based on the total amount of the raw materials, and the mixture was further dissolved for 15 minutes. Then, diphenylmethane diisocyanate 5
After adding 00 parts and trimethylolpropane 13 parts, the mixture was heated and reacted at 90 ° C. for 4 hours to obtain a polyurethane resin A. The average OH content of the obtained polyurethane resin A was 4 per molecule.

(Introduction of Polar Group) For the introduction of a polar group after the synthesis of the polyurethane resin, a conventionally known method can be used. For example, POLYMER, 30 , 1204 (198
It can be synthesized by the method disclosed in 9). In particular,
The polyurethane resin A obtained in Production Example 1 was diluted with dimethylacetamide in a reaction vessel equipped with a thermometer, a stirrer, and a partial reflux condenser under a nitrogen stream to make a 5% solution, and 13 parts of sodium hydride was added. While vigorously stirring at -5 to 0 ° C, 3 parts of butane sultone was dropped into the flask. After completion of the dropwise addition, cooling was stopped, and the mixture was heated to 50 ° C. in a warm water bath. After the heating and stirring were stopped, the contents of the flask were poured into water to stop the reaction, and the polyurethane resin A1 was solidified and dried. Table 1 shows the properties of the obtained polyurethane resin A1.

Production Examples 2 to 4 Polyurethane resins A2 to A4 were obtained in the same manner as in Production Example 1, except that the amount of sodium hydride was changed to the amount shown in Table 1. Table 1 shows the properties of the obtained polyurethane resins A2 to A4.

Production Example 5 A polyurethane resin B was obtained in the same manner as in Production Example 1, except that 470 parts of hydrogenated bisphenol A was used without using the polypropylene oxide adduct of bisphenol A.

Production Example 6 830 parts of dimethyl terephthalate, 420 parts of neopentyl glycol, 0.64 part of zinc acetate and 0.03 part of sodium acetate were added to a reaction vessel equipped with a thermometer, a stirrer and a partial reflux condenser. A transesterification reaction was performed at a temperature of from 220C to 220C. Then hydroxycaproic acid 7
After adding 40 parts and reacting at 210 ° C. to 250 ° C. for 2 hours, the pressure of the reaction system was reduced to 20 mmHg in 30 minutes, and a polycondensation reaction was further performed at 5 to 20 mmHg and 250 ° C. for 30 minutes. The molecular weight of the obtained polyester polyol is 2
000. Then, 100 parts of neopentyl glycol, 500 parts of diphenylmethane diisocyanate, and 14 parts of trimethylolpropane were added to the polyester polyol as a chain extender, and the mixture was added at 90 ° C.
The mixture was heated for a period of time to obtain a polyurethane resin C.

Preparation Example 7 In Preparation Example 6, a transesterification reaction was carried out with 1300 parts of dimethyl terephthalate and 570 parts of neopentyl glycol.
Next, 220 parts of cyclohexanedimethanol and 210 parts of hydroxypivalic acid were added and subjected to a polycondensation reaction to obtain a polyurethane resin D by the same production method except that a polyester polyol having a molecular weight of 2,000 was used. Table 1 shows the properties of the obtained polyurethane resins B to D.

Comparative Production Example 1 In Production Example 1, 300 parts of hydrogenated bisphenol A, 150 parts of an adduct of bisphenol A with polypropylene oxide, and 20 parts of 5-sodium sulfoisophthalic acid were added.
Polyurethane resin e was obtained by the same production method except that a part was added.

Comparative Production Example 2 In Production Example 5, hydrogenated bisphenol A was used in an amount of 300 parts, and sulfoisophthalic acid ethylene oxide adduct was added in an amount of 20 parts.
A polyurethane resin f was obtained in the same manner as above except that part of the resin was added.

Comparative Production Example 3 In Production Example 6, 830 parts of dimethyl terephthalate, 45 parts of dimethyl 5-sodium sulfoisophthalate, and 420 parts of neopentyl glycol were added and transesterification was carried out.
Next, 740 parts of hydroxycaproic acid was added and a polycondensation reaction was carried out to obtain a polyurethane resin g in the same manner except that a polyester polyol having a molecular weight of 2,000 was used.

Comparative Production Example 4 In Production Example 7, 1300 parts of dimethyl terephthalate, 50 parts of dimethyl 5-sodium sulfoisophthalate and 570 parts of neopentyl glycol were added and transesterification was carried out, followed by 220 parts of cyclohexanedimethanol and 210 parts of hydroxypivalic acid. In addition, a polyurethane resin h was obtained in the same manner except that a polyester polyol having a molecular weight of 2,000 obtained by performing a polycondensation reaction was added.

[0067]

[Table 1]

However, in Table 1, HBPA: hydrogenated bisphenol A BPA-12PO: polypropylene oxide adduct of bisphenol A (average PO addition number = 12 / one molecule of BPA) MDI: 4,4'-diphenylmethane diisocyanate PA: phthalic acid CL: hydroxycaproic acid CM: cyclohexane dimethanol HPV: hydroxypivalic acid NPG: neopentyl glycol Weight average molecular weight: gel permeation chromatography HLC-8020 manufactured by Tosoh (column configuration: G200
0HXL × 30cm × 1 + G4000HXL × 30cm
× 1 + G5000HXL × 30 cm × 1, eluent: tetrahydrofuran, flow rate: 1 ml / min, temperature: 40 ° C., detector: RI), and the weight average molecular weight was determined from the molecular weight distribution curve in terms of standard polystyrene. OH content of the polyurethane resin: It is represented by the number of OH groups per molecule from the OH value obtained by the test method of JIS K0070 and the number average molecular weight in terms of polystyrene obtained by using GPC. However, in Comparative Examples, sulfoisophthalic acid was used for introducing polar groups, but the composition of the polyurethane resin was not described. An example of a magnetic recording medium using the compound produced in the above Production Example will be described.

Example 1 (Preparation of Magnetic Coating Liquid) Ferromagnetic alloy powder 100 parts Composition: Fe / Zn / Ni = 92/4/4 (atomic ratio), Hc: 2000 Oe Crystallite size: 15 nm, BET specific surface area: 59 m ² / g Long axis diameter: 0.12 μm, needle ratio: 7, σs: 140 emu / g Polyurethane resin (Production example A1) 10 parts Vinyl chloride resin 8 parts (MR110: manufactured by Zeon Corporation, polar group-containing vinyl chloride) Α-Al ₂ O ₃ (particle size 0.3 μm) 2 parts carbon black (particle size 40 nm) 2 parts cyclohexanone 110 parts methyl ethyl ketone 100 parts toluene 100 parts butyl stearate 2 parts stearic acid 1 part The above magnetic coating composition About, each component was kneaded with an open kneader for 60 minutes, and then dispersed in a sand mill for 120 minutes. The resulting dispersion is mixed with a trifunctional low molecular weight polyisocyanate compound (Coronate 304 manufactured by Nippon Polyurethane).
After adding 6 parts of 1) and stirring and mixing for another 20 minutes, 1
The solution was filtered using a filter having an average pore diameter of μm to prepare a magnetic coating liquid. The obtained magnetic coating material is applied on a polyethylene naphthalate support having a thickness of 10 μm so that the thickness after drying becomes 3.0 μm, and the magnetic layer is subjected to magnetic field orientation with a 3000 Gauss magnet while the magnetic layer is not dried. After drying, a 7-stage calender consisting only of metal rolls was used at a speed of 100 m / min, a linear pressure of 300 kg / cm, and a temperature of 90.
After performing a surface smoothing treatment at 70 ° C., a heat curing treatment was performed at 70 ° C. for 24 hours, and the resultant was cut into a 6.35 mm width to produce a magnetic tape.

Example 2 A magnetic tape of Example 2 was produced in the same manner as in Example 1 except that the polyurethane resin used in the magnetic coating composition was changed as shown in Table 2.

Example 3 (Preparation of Upper Layer Magnetic Coating Liquid) Ferromagnetic Alloy Powder 100 parts Composition: Fe / Zn / Ni = 92/4/4 (atomic ratio), Hc: 2000 Oe, Crystallite Size: 15 nm, BET Ratio Surface area: 59 m ² / g, major axis diameter: 0.12 μm, needle ratio: 7, s: 140 emu / g Polyurethane resin (Production example A1) 10 parts Vinyl chloride resin 8 parts (MR110: manufactured by Zeon Corporation, polar group Containing vinyl chloride resin) α-Al ₂ O ₃ (particle size 0.3 μm) 2 parts Carbon black (particle size 40 nm) 2 parts Cyclohexanone 110 parts Methyl ethyl ketone 100 parts Toluene 100 parts Butyl stearate 2 parts Stearic acid 1 part (lower layer) Preparation of non-magnetic coating liquid for non-magnetic) 85 parts of non-magnetic inorganic powder α-iron oxide, major axis diameter: 0.12 μm, needle ratio: 7, BET ratio Area 55m ² /G,pH6.5 polyurethane resin (Production Example A1) 10 parts of vinyl chloride resin 8 parts (MR 110: Nippon Zeon, polar group-containing vinyl chloride resin) Cyclohexanone 140 parts Methyl ethyl ketone 170 parts Butyl stearate 2 parts Stearic acid 1 part For each of the above-mentioned magnetic coating composition for upper layer and non-magnetic coating composition for lower layer, each component was treated with an open kneader by 6 parts.
After kneading for 0 minutes, the mixture was dispersed in a sand mill for 120 minutes.
To the obtained dispersion, 6 parts of a trifunctional low molecular weight polyisocyanate compound (Coronate 3041 manufactured by Nippon Polyurethane) was added, and the mixture was further stirred and mixed for 20 minutes. Then, the mixture was filtered using a filter having an average pore diameter of 1 μm to obtain a magnetic coating material. A non-magnetic paint was prepared. The obtained non-magnetic paint is applied on a polyethylene naphthalate support having a thickness of 10 μm so that the thickness after drying becomes 2.5 μm. Immediately thereafter, the thickness after drying the magnetic paint becomes 0.5 μm. At the same time. In a state where both layers are not dried, a magnetic field orientation is performed using a 3000 Gauss magnet, and after drying, a speed of 100 is performed using a seven-stage calender composed of only metal rolls.
After performing a surface smoothing treatment at m / min, a linear pressure of 300 kg / cm and a temperature of 90 ° C., a heat curing treatment was performed at 70 ° C. for 24 hours, and the resultant was cut into a 6.35 mm width to prepare a magnetic tape.

Examples 4 to 9 The same procedure as in Example 3 was repeated except that the polyurethane resin used in the lower non-magnetic coating composition and the upper magnetic coating composition was changed as shown in Table 2. A magnetic tape was produced.

Comparative Example 1 A magnetic tape of Comparative Example 1 was produced in the same manner as in Example 1 except that the polyurethane resin used in the magnetic coating composition was changed to the one shown in Table 2.

Comparative Examples 2 to 5 The polyurethane resins used in the lower non-magnetic coating composition and the upper magnetic coating composition were changed to those shown in Table 2, and Comparative Examples 2 to 5 were prepared in the same manner as in Example 3. A magnetic tape was produced. Next, the characteristics of the video tapes of the examples and the comparative examples were measured by the following measurement methods, and the measurement results are shown in Table 3.

[0075]

[Table 2]

[0076]

[Table 3]

[Measurement Method] Surface Roughness Ra: The center line average roughness obtained by an optical interference method using a digital optical profile meter (manufactured by WYKO) was defined as Ra. Electromagnetic conversion characteristics: Recording and reproduction were performed on a sample tape using a drum tester at a recording wavelength of 0.5 μm and a head speed of 10 m / sec. The relative reproduction output with respect to the tape of Example 1 was evaluated. Repetitive running property (output decrease, head dirt, change in surface binder amount): A tape of 60 minutes length was used at 40 ° C., 80% R using a digital tape recorder (NV-BJ1 manufactured by Matsushita Electric).
In the H environment, run continuously and repeatedly 100 times, observe the dirt on the video head, and continuously reproduce the video output,
Output reduction was measured with the first output set to 0 dB. The video head dirt was evaluated by the following evaluation scores. Evaluation score 5: No stain was observed. 4: The head has a little dirt on the shoulder portion on the tape exit side. 3: A stain is recognized on the shoulder portion of the head on the tape exit side. 2: Stain is also observed in the head gap. 1: Recording / reproduction is practically impossible on the entire sliding portion of the head. The amount of surface binder was changed by extracting the tape before and after running with n-hexane at room temperature for 30 minutes in order to remove lubricants and the like existing on the surface of the magnetic layer.
5400MC) using a Mg anode at 400 W (15 kV) for 10 minutes. For N, 2P3 / 2 of Fe
The peak intensity ratio of 1S of N with respect to the peak of No. and the peak intensity ratio of 1S of Cl with respect to the peak of 2P3 / 2 of Fe were obtained with respect to Cl. The storability was determined by storing the magnetic tape at 60 ° C. and 90% RH for one week, and evaluating the change in friction coefficient (μ value) before and after the storage by the following evaluation score. Evaluation score 5: No change in μ value was observed. 4: Those whose μ value increase is less than 0.05. 3: The increase in μ value is 0.05 to 0.1. 2: Those whose μ value increase is larger than 0.1. 1: Sticking occurs.

[0078]

According to the present invention, a polyurethane resin having a polar group is more uniformly used from a low molecular weight to a high molecular weight region by introducing a polar group by reacting a sultone compound with a terminal OH group using a diol having a polar group. Since the group can be introduced, the dispersibility is improved, and the binder component has a high adsorptivity to the magnetic powder and the non-magnetic powder. Further, the present invention provides a magnetic recording medium which does not cause clogging or the like and has excellent running durability. In particular, it is possible to obtain a magnetic recording medium which is excellent in storage stability under high temperature and high humidity and has a small amount of surface binder before and after running.

Further, by increasing the content of the short-chain diol component as the skeleton of the polyurethane resin, the polyol content is smaller than that of the known polyurethane, and the cyclic structure is contained in the main chain. A characteristic of high Tg is obtained, and in particular, a material having excellent repetition running property under a high temperature environment is obtained. In particular, 40 ° C 80
Since low molecular components hardly precipitated on the surface of the coating film in a high temperature environment of% RH, head contamination and the like during repetitive running could be remarkably improved.

Claims (6)

[Claims] 1. A magnetic recording medium having a magnetic layer in which at least one ferromagnetic powder is dispersed on a non-magnetic support, wherein the binder is a polyurethane resin comprising a polyol and a polyisocyanate, A magnetic recording medium comprising a polyurethane resin having a sulfoalkyl group introduced into a OH group thereof using a sultone compound. 2. The polyurethane resin according to claim 1, wherein the polyurethane resin contains 0 to 50% by weight of a long-chain diol obtained by adding an alkylene oxide to the diol having a cyclic structure as the polyol and having a weight average molecular weight of 500 to 5,000 in the polyurethane resin. The polyurethane resin contains 15 to 50% by weight of a short-chain diol having a weight average molecular weight of 200 to less than 500 to which an alkylene oxide has been added, and 2 to 6 mmol / g of an ether group derived from a polyether polyol is contained in the entire polyurethane resin. 2. The magnetic recording medium according to claim 1, wherein the magnetic recording medium is a polyurethane resin. 3. The magnetic recording medium according to claim 1, wherein an intramolecular OH group of the polyurethane resin comprising the polyol and the polyisocyanate is at a molecular terminal. 4. The magnetic recording medium according to claim 1, wherein the polyurethane resin comprising the polyol and the polyisocyanate has two or more OH groups per molecule. 5. An average polar group content contained in a polyurethane resin having a sulfoalkyl group introduced by using the sultone compound is 1 × 10 ⁻⁵ to 50 × 10 ⁻⁵ eq / g. The magnetic recording medium according to claim 1. 6. At least two or more layers in which a lower nonmagnetic layer containing at least a nonmagnetic powder and a binder is provided on a nonmagnetic support and an upper magnetic layer containing at least a ferromagnetic powder and a binder is provided thereon. In a magnetic recording medium having a plurality of layers,
A magnetic recording medium comprising the polyurethane resin according to any one of claims 1 to 5 as a binder of at least one of the lower nonmagnetic layer and the upper magnetic layer.