CA2036261A1 - Magnetic recording medium - Google Patents

Abstract

Abstract A magnetic recording medium comprises a non-magnetic substrate and a magnetic layer, provided on the substrate, comprising ferromagnetic powder and a binder comprising polyurethane resin which is obtained by reacting polyisocyanate with polymethylvalerolactonediol prepared by polymerizing .beta.-methyl-.delta.-valerolactone in the presence of a dihydroxy compound having a metal sulfonate group as a polymerization initiator. It is is improved in view of dispersibility of magnetic power in the layer.

Description

2~3~26~

Magnetic recording medium [Field of Industrial Application]
The present invention relates to a magneticrecording medium. In particular, the present invention is concerned with a magnetic recording medium wherein use is made of a binder having an excellent dispersibility.
[Prior Art]

A magnetic recording medium is prepared by coating the surface of a non-magnetic substrate, such as polyester film, with a magnetic coating material comprising a magnetic powder, a binder-, an organic solvent and, if necessary, other additives, and drying it.
In order to attain such electrical character-istics as high xeproduction output and high ~ ratio, the magnetic recording medium should have such various properties that the magnetic powder is homogeneously dispersed in the above-described coating material, the surface of the magnetic coating film is smooth and the resultant magnetic recording medium has excellent durability. For this reason, various binders have been studied for the purpose of satisfying the above-required properties.

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A magnetic recording medium wherein a polyurethane resin having, introduced thereinto, a component containing a metal sulfonate group is used as a binder has been proposed for the purpose of increasing the ratio of the residual magnetism, Br, to the saturation magnetization, Bm, i.e., Br/Bm (squareness ratio), in the direction of orientation o a magnetic tape, and lowering the void fraction (see Japanese Patent Publication No. 41565/1983).

The magnetic recording medium proposed in the Japanese Patent Publication No. 41565!1983, however, is unsatisfactory in the durability because it has a problem with the hydrolysis resistance. Further, it has a drawback that the thixotropy of the magnetic coating material is enhanced.
In order to solve this problem, a proposal has been made on the use of a polyurethane resin prepared by the ring-opening polymerization of s-capro]actone (see Japanese Patent Laid-Open No. 115920/1989).
However, also the magnetic recording medium prepared by this method is unsatisfactory in the performance.

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An object of the present invention is to provide a polyurethane resin for a magnetic recording medium having such various properties that it is a coating material for a magnetic recording medium wherein a novel polyurethane resin is used as a binder, a magnetic powder is homogeneously dispersed in the coating material, the surface of the magnetic coating film is smooth, the resultant magnetic recording medium has an excellent durability and the coating material has a low thixotropy.

( Summary oi the:invention ) The present inventors have made intensive studies with a view to solving the above-described problem and, as a result, have found that a polyurethane resin prepared by reacting a polyisocyanate with a starting polyol comprlsing polymethylvalerolactonediol prepared by subjecting ~-methyl-~-valerolactone to ring-opening polymerization in the presence of a polymerization initiator comprising a dihydroxy compound having a metal sulfonate group in its molecule instead of a polyurethane resin prepared by the ring-opening polymerization of ~-caprolactone can serve as a binder having an excellent dispersibility, which has led to the completion of the present invention.

21~3~2~

The magnetic recording medium of the invention comprises a non-magnetic substrate and a magnetic layer, provided on the substrate, comprising ferromagnetic powder and a binder comprising polyurethane resin which is obtained by reacting polyisocyanate with polymethylvalerolactonediol prepared by polymerizing ~-methyl- ~-valerolactone in the presence of a dihydroxy compound having a metal sulfonate group as a polymerization initiator. It is improved in view of dispersibility of magnetic power in the layer.

Specifically, according to the present invention, there is provided a magnetic recording medium comprising a non-magnetic substrate and a magnetic layer provided thereon through coating with a ferromagnetic powder, characterized in that said magnetic layer comprises a polyurethane resin prepared by reacti.ng a polyisocyanate with polymethylvalero-lactonedi.ol prepared by polymerizing ~-methyl-~-valerolactone in the presence of a dihydroxy compound having a metal sulfonate group as a polymerization initiator.
The polyurethane resin used in the present invention has a low viscosity and therefore an 2'~3~

improved handleability. This is because the viscosity of the polyester of ~-methyl-o-valerolactone is much lower than that of the polyester of E-caprolactone.
In the polymerization of ~-methyl-~-valerolactone in the present invention, a dihydroxy compound having a metal sulfonate group is used as a polymerization initiator. The proportion of ~-methyl-~-valerolactone to the polymerization initiator in terms of the molar ratio is preferably 3-methyl-~-valerolactone : polymerization initiator = (50 : 1) to (2 : 1), preferably (3 : 1) to (20 : 1). When this value exceeds 2 : 1, there is a possibility of bringing ahout an unfavorable phenomenon such as a lowering in the solubility of the synthesized polymethylvalerolactonediol in the solvent.
Examples of the dihydroxy compound having a metal sulfonate group used as a polymerization initiator include dihydroxy compounds represented by the following general formula and comprising a benzene ring as a skeleton and one or two metal sulfonate groups.
(MO~S ~ R ~ X ' CH z )e - OH] z (110~S~ R ~X ~CHzCH20~H] z (MO~S~ R~X ~CH2CHO~H] 2 wherein a is 1 or 2, Q is an integer of 1 to 8, m and n are each an integer of 1 to 5, R is CH, S ~ or O ,.. .
X is -O- or -COO-, and M is Li, Na or K.
Preferred examples of the dihydroxy compound include those represented by the following formulae (I) to (V):

HOCH2CHzOOC ~ COOCHzCHzOH

COOCHzCHzOH OCH7.CHzOH

S 0 3 M ~ SO~M

COOCHzCHzOH OCHzCHzOH

(II) (m) ~ir~3$2~

OCHzCHzOH

MO 3S ~ SO ~M
OCHzCH20H
( ~ ) HOCHzCHzO ~ C ~ OCHzCH20H

S O I ~ S O
( V ) In the above-described formulae (I) to (V), M is Na or K.
Among the compounds represented by the above-described formulae (I) to (V), the compounds represented by the formulae (I) and (II) are particularly preferred. It is also possible to use an ethylene glycol solution thereof.
Polymethylvalerolactonediol is prepared according to a conventional process, i.e., by subjecting ~-methyl-~-valerolactone to ring-opening polymerization through the use of the above-described dihydroxy compound as a polymerization initiator, and contains a metal sulfonate group.
In the reaction, a catalyst may be used according 3 ~3 2 ~

to need. The reaction temperature is 50 to 300C, preferably 130 to 230C.
Examples of the catalyst include al~ali metals and organometallic compounds. Examples of the alkali metal include lithium, sodium and potassium, while examples of the organometallic compound include organotitanium compounds such as tetrabutyl titanate and tetrapropyl titanate, organotin compounds, such as dibutyltin oxide, and stannous chloride.
Among them, the organotitanium compounds are most desirable because they have a high catalytic activity and enables a less colored product to be produced in a short period of time.
The content of the metal sulfonate group in the polyurethane resin used in the present invention depends upon the content of the metal sulfonate group in the above-described polymethylvalerolactonediol and, in the case of its use in combination with other polyester polyol not containing a metal sulfonate group, the proportion of mixing of polymethylvalero-lactonediol with other polyester polyol and the concentration of the metal sulfonate group in polymethylvalerolactonediol. The content of the metal sulfonate group in the resultant polyurethane resin is 0.1 to 300 ~eq/g, preferably 1 to 200 ~eq/g, more pre-ferably about 9 to about 120 ~ueq/g.

g When the content of the metal sulfonate group in the polyurethane is less than 0.1 ~eq/g, no effect of improving the dispersibility of the magnetic powder can be expected. On the other hand, when the content exceeds 300 ~eq/g, the thixotropy of the resultant magnetic coating material increases, which raises a possibility of giving an adverse effect on the dispersion of the magnetic powder. Fuxther, there is a possibility that an increase in the hydrophilicity lowers the hydrolysis resistance of the polymer, which raises a possibility of giving an adverse effect on the durability, environmental resistance, etc. of the magnetic recording medium per se.
In the present invention, it is n-ecessary to incorporate the above-described polymethylvalero-lactonediol as a polyhydroxy compound having two or more active hydrogen atoms in its molecule. It may be used in combination with other polyhydroxy compound.
Examples of the other polyhydroxy compound include known polyhydroxy compounds usually having a molecular weight of 50 to 10,000 and commonly used for the production of polyurethane, for example, low-molecular glycols, polyethers, polyesters, polythioethers, polybutadiene glycols, silicon-containing polyols and phosphorus-containing polyols.

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Examples of the low-molecular glycols include ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,5-hexanediol, neopentyl glycol, diethylene glycol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, 1,4-cyclohexanedimethanol, ethylene oxide adduct and propylene oxide adduct of bisphenol A, ethylene oxide adduct and propylene oxide adduct of hydrogenated bisphenol A, polyethylene glycol, polypropylene glycol and polytetramethylene glycol. They may be used in combination with tri- and tetraols such as trimethylolethane, trimethylolpropane, glycerin and pentaerythritol.
Examples of the polyethers include polymers and copolymers of ethylene oxide, propylene oxide and tetrahydrofuran. Further, it is also possible to use polyethers prepared by the condensation of the above-described low-molecular glycols, or mixed ethers, and further products of addition polymerization of the above-described polyethers and the above-described low-molecular glycols with ethylene oxide and propylene oxide.
A thioglycol alone or a product of condensation thereof with other glycol is particularly suitable as the polythioethers.

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Examples of the polyesters include those prepared by the dehydration condensation of low-molecular glycols with a dibasic acid, and lactone polyols prepared by the ring-opening polymerization of a lactone, such as ~-caprolactone, in the presence of the above-described low-molecular glycol or the like.
It is also possible to use a trifunctional or higher polyol in the form of a mixture as part of the above-described polyhydroxy compound.
The use of a (monofunctional or higher) hydroxy compound having metal sulfonate group, such as 2-sodium sulfo-1,4-butanediol or 2-potassium sulfo-1,4-butanediol, in part of the glycol moiety enables the metal sulfonate group to be introduced into a hard segment portion of the polyurethane resin.
Examples of the polyisocyanate compound used for preparing a polyurethane resin through the reaction with a polyhydroxy compound in the present invention include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, p-phenylene diisocyanate, diphenyl-methane diisocyanate, m-phenylene diisocyanate, hexamethylene diisocyanate, tetramethylene diisocyanate, 3,3-dimethoxy-4,4'-biphenylene diisocyanate, 2,4-naphthalene diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, 4,4'-~d ~ 2 ~ ~

diphenylene diisocyanate, 4,4'-diisocyanatodiphenyl ether, 1,5-naphthalene diisocyanate, p-xylylene diisocyanate, m-xylylene diisocyanate, 1,3-diisocyanatomethylcyclohexane, 1,4-diisocyanato-methylcyclohexane, 4,4'-diisocyanatodicyclohexane, 4,4,-diisocyanatodicyclohexylmethane and isophorone diisocyanate. If necessary, minor amounts of 2,4,4'-triisocyanatodiphenyl, benzene triisocyanate, etc. may also be used.
Among the above-described organic diisocyanates, urethane resins wherein use is made of an organic diisocyanate containing a diisocyanate having a cyclohexyl group provide particularly good results.
Especially, urethane resins wherein us-e is made of a diisocyanate containing isophorone diisocyanate are excellent particularly in the dispersibility of the magnetic powder as well as in the surface smoothness of the magnetic layer.
Examples of the chain extender used for the production of the polyurethane resin include glycols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, 3-methyl-1,5-pentanediol and 1,5-pentanediol, low-molecular polyols such as trimethylolpropane and glycerin, and diamines such as ethylenediamine, ~ 3 t 3 propylenediamine, hydrazine, piperazine and isophoronediamine.
It is also possible to use the above-described polyhydroxy compound as the chain extender.
Examples of the solvent used for the production of the polyurethane resin in the present invention include compounds usually inert to an isocyanate group, such as ketones such as methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, esters such as ethyl acetate and butyl acetate, aromatic hydrocarbon solvents such as toluene and benzene, and tetrahydrofuran.
The polyurethane resin is prepared according to the conventional process. Examples of the catalyst used in this case include usual urethanization catalysts, i.e., those based on tin, iron and tertiary amine. Examples of the tin-based catalyst include dibutyltin dilaurate, dioctyltin dilaurate, dibutyltin dioctoate and stannous octoate. Examples of the iron-based catalyst include acetylacetonatoiron and ferric chloride. Examples of the tertiary amine-based catalyst include triethylamine and trlethylenediamine.
In the present invention, the process for preparing the polyurethane resin is properly selected from conventional processes taking into consideration ~J ~ P3~ ~

1~

the degree of polymerization of the intended polyurethane resin, the kind of starting material used, etc. Examples of the process for preparing the polyurethane resin include one which comprises reacting a polyhydroxy compound having an active hydrogen atom with a stoichiometrically excess polyisocyanate compound, if necessary, in the presence of a solvent usually inert to the isocyanate group and further, if necessary, a usual urethanization catalyst, at a temperature of 10 to 150C, preferably 20 to 130C to prepare a prepolymer having an isocyanate group at its terminal, and reacting the prepolymer with a chain extender such as a diol, diamine or triol to prepare a polyurethane having a hydroxyl group at its terminal, and one wherein a polyhydroxy compound, a chain extender and a polyisocyanate compound are simultaneously reacted with each other in such a feed ratio that the hydroxyl group is stoichiometrically excess. Further, it is also possible to use a method wherein a polyisocyanate compound is reacted with a polyhydroxy compound without use of a chain extender to prepare a hydroxy-terminated polyurethane resin, a method which comprises reacting a polyhydroxy compound with a polyisocyanate compound to prepare a hydroxy-terminated compound and further reacting the hydroxy-terminated compound with the polyisocyanate compound to prepare a hydroxy-terminated polyurethane resin, and other methods.
The number-average molecular weight of the polyurethane resin of the present invention thus prepared is 5,000 to 200,000, preferably 7000 to 70,000. The weight-average molecular weight thereof is about 5,000 to 500,000, preferably 8,000 to 150,000.
No additivity exists between the number-average molecular weight and the weight-average molecular weight because polymers synthesized so as to have the same number-average molecular weight do not always have the same weight-average molecular weight. The ratio of the weight-average molecular weight to the number-average molecular weight is generally about 1 to 5.
The magnet:ic layer of the magnetic recording medium of the present invention is formed by kneading a binder, a magnetic powder and optional additives with a suitable solvent to prepare a magnetic coating material, coating the surface of a non-magnetic substrate with the magnetic coating material, and drying the coated substrate.

Examples of the binder include the polyurethane resin prepared in the present invention and further mixtures of the polyurethane with compounds commonly employed as a binder component, such as vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylic ester-vinylidene chloride copolymer, acrylic ester-styrene copolymer, methacrylic acid-acrylonitrile copolymer, methacrylic ester-vinylidene chloride copolymer, methacrylic ester-styrene copolymer, urethane resin, polyvinyl fluoride, vinylidene chloride-acrylonitrile copolymer, butadiene-acrylonitrile copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate butyrate, cellulose propionate, nitrocellulose, etc.), styrene-butadiene copolymer, polyester resin, chlorovinyl ether-acrylic ester copolymer, amino resin, phenolic resin, epoxy resin, phenoxy resin, alkyd resin, silicone resin, etc.
Among the above-described resins, binders having a metal sulfonate group, a metal carboxylate group, an amino group, etc. may also be used in combination with the polyurethane.
Further, it is also possible to add a polyisocyanate compound in the preparation of a ~ ~3 S~
1~, 65702-381 magnetic layer of the magnetic recording medium of the present invention. Examples of the polyisocyanate compound include Desmodur*L and Desmodu~ N
manufactured by Bayer, Coronate L and Coronate*HL
manufactured by Nippon Polyurethane Industry Co., Ltd., and Takenate*D102 manufactured by Takeda Chemical Industries, Ltd. These polyisocyanate compounds are used in a proportion of 1 to 50% by weight based on the total amount of the binder.
Examples of the magnetic powder used in the magnetic recording medium of the present invention include known magnetic powders commonly used in the art, for example, y-iron oxide, cobalt-coated y-iron oxide, cobalt-doped Y-iron oxide, CrO2, barium ferrite, magnetic alloy powder and magnetic metal powder.
There is no particular llmitation on the form of the ferromagnetic powder, and acicular, granular, dice-like, rice-like granular and flaky powders may usually be employed. The specific surface area of the ferromagnetic powder is preferably 45 m2/g or more from the viewpoint of electromagnetic transduction.
The total content of the binder in the magnetic layer of the magnetic recording medium according to the present invention is generally 5 to 100 parts by *Trade-mark ~3~
1~

weight, preferably 10 to 40 arts by weight based on 100 parts by weight of the ferromagnetic powder.
It is preferred that the magnetic layer of the magnetic recording medium according to the present invention additionally comprise an inorganic particle having a Mohs hardness of 5 or more.
There is no particular limitation on the inorganic particle used as far as it has a Mohs hardness of 5 or more. Examples of the inorganic particle having a Mohs hardness of 5 or more include A1203 (Mohs hardness: 9), Tio (Mohs hardness: 6), TiO2 (Mohs hardness: 6.5), SiO2 (Mohs hardness: 7), SnO2 (Mohs hardness: 6.5), Cr203 (Mohs hardness: 9) and ~-Fe203 (Mohs hardness: 5.5). They may be used alone or in the form of a mixture thereof.
An inorganic particle having a Mohs hardness of 8 or more is particularly preferred. The use of a relatively soft inorganic particle having a Mohs hardness lower than 5 brings about drawbacks that the inorganic particle tends to fall off from the magnetic layer, the clogging of the head tends to occur due to a sub~tantial absence of any polishing action of the head and the travelling durability becomes poor.
The content of the inorganic particle is usually 0.1 to 30 parts by weight, preferably 1 to 15 parts by ~ s.t$~

weight based on 100 parts by weight of the ferxomagnetic powder.
It is desirable to incorporate carbon black (particularly one having a mean particle diameter of 10 to 300 nm), etc., besides the above-described inorganic particle.
In the preparation of the magnetic coating material, the above-described materials may be used in combination with known additives such as dispersants, antistatic agents and lubricants.
Examples of the dispersant include known one such as lecithin.
The amount of the dispersant, when used, is usually 0.1 to 10 parts by weight based on 100 parts by weight of the ferromagnetic powder used.
Examples of the antistatic agent include electrically conductive impalpable powders such as carbon black and carbon black graft polymer; natural surfactants such as saponin; cationic surfactants such as higher alkylamines, quaternary ammonium salts, salts of pyridine and other heterocyclic compounds, phosphoniums and sulfoniums; anionic surfactants containing a polar group, such as salts of phosphonic acid, salts of phosphoric acid, salts of phosphoric ester, salts of sulfonic acid, salts of sulfuric acid and salts of sulfuric ester; and amphoteric surfactants such as amino acids, aminosulfonic acids, and sulfuric or phosphoric esters of amino alcohols When the above-described electrically conductive impalpable powder is used as the antistatic agent, for example, the amount thereof is 0.1 to 50 parts by weight based on 100 parts by weight of the ferromagnetic powder, while when the surfactant is used, the amount thereof is 0.10 to 10 parts by weight.
Examples of the lubricant include the above-described fatty acids, higher alcohols, esters of monobasic fatty acids having 12 to 20 carbon atoms with monohydric or polyhydric alcohols having 3 to 20 carbon atoms, such as butyl stearate and sorbitan oleate, known lubricants and lubricants for plastics, such as mineral oils, vegetable and animal oils, oligomers of olefins, fatty acid amides, silicone oil and modified silicone oil, and further impalpable powder of graphite, impalpable powder of molybdenum disulfide and impalpable powder of tetrafluoroethylene polymer.
The function and effect of the above-described additives, such as dispersants, antistatic agents and lubricants, are not strictly limited to the above-~ r 2~

described function and effect only. For example,there is a possibility that the dispersant might function as a lubricant or an antistatic agent.
Therefore, it is a matter of course that the function and effect of the compounds recited above according to the above-described classification are not limited to the matters described in the above classified items.
When a substance having a plurality of functions and effects is used, the amount of addition thereof is preferably determined by taking into consideration the function and effect thereof.
The process for preparing the magnetic recording medium of the present invention will now be described.
At the outset, a ferromagnetic powder, a binder containing a polyurethane resin which is one of the features of the present invention, and other optional fillers and additives are kneaded together with a solvent to prepare a magnetic coating material.
Examples of the solvent used in the kneading include cyclohexanone, methyl ethyl ketone, tetrahydrofuran, dioxane and dimethylformamide. They may he used alone or in the form of a mixture thereof. Alternatively, use may be made of mixed solvents comprising a mixture of the above-described solvents with methyl isobutyl ketone, toluene or the like.

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There is no particular limitation on the method of kneading, and the order of addition of individual components may be properly determined.
The preparation of the magnetic coating material may be conducted by means of usual kneaders, for example, twin-rol~ mill, triple-roll mill, ball mill, pebble mill, trommel, sand grinder, Szegvari,attritor, high-speed impeller disperser, high-speed stone mill, high-speed impact mill, disper, kneader, high-speed mixer, homogenizer, ultrasonic disperser, etc.
A non-magnetic substrate is coated with the magnetic coating material thus prepared. The coating may be conducted directly on the non-magnetic substrate. ~lternatively, the coating may be conducted on the non-magnetic substrate through an adhesive layer.
Examples of the method of coating the non-magnetic substrate with the magnetic coating material include air doctor coating, blade coating, rod coating, extrusion coating, air knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, gravure coating, kiss coating, cast coating, spray coating and spin coati.ng.
Further, it is also possible to use methods other than those described above.

~f,jf3~,?J~ff Examples of the non-magnetic substrate include polyesters (for example, polyethylene terephthalate and polyethylene naphthalate), polyamides, polyolefins, cellulose derivatives, non-magnetic metals and paper. They may be in the form of film, tape, sheet, card, disk, etc.
The magnetic layer is provided by coating so that the thickness after drying is generally about 0.1 to 10 ~m, usually 0.1 to 7.0 ~m.
The magnetic layer applied on the non-magnetic substrate, when the magnetic recording medium is used in a tape form, is usually subjected to a treatment for orienting the ferromagnetic powder in the magnetic layer, i.e., magnetic orientation tre-atment, and then dried~ If necessary, a surface smoothing treatment is applied. The magnetic recording medium subjected to the surface smoothing treatment is then cut into a desired shape.
Since the polyurethane resin prepared by a method characteristic of the present invention is incorporated in a binder for a magnetic recording medium, the magnetic recording medium of the present invention thus prepared has a much higher capability of dispersing the magnetic powder, a better surface smoothness of the magnetic coating and better moist ~,~

2 ~

heat resistance and durability than those of he conventional magnetic recording medium wherein use is made of a conventional thermoplastic polyurethane resin.
As compared with the conventional polyurethane resin prepared through the use of a polycondensation type polyester glycol containing a metal sulfonate group, the binder containing the polyurethane resin of the present invention, by virtue of the ease of introducing a metal sulfonate group, selective introduction of one or two metal sulfonate groups per molecule of polymethylvalerolactonediol and homogeneous distribution of polar groups in the resultant polyurethane resin, has advantages such as less increase in the thixotropy of the magnetic coating material caused by the interaction between the metal sulfonates themselves not participating in the adsorption on the magnetic powder, possible minimization of the amount of introduction of the metal sulfonate group by virtue of its high effectiveness, and possible suppression of a lowering in the moist heat deterioration due to an increase in the hydrophilicity by virtue of the superiority in the hydrolysis resistance to the polyurethane resin prepared by making use of a conventional usual ~ ~ ~ 'f 3 ~
2;) aromatic or aliphatic polycondensed polyester glycol.
[Examples]
The present invention will now be described in more detail by way of the following Examples, though it is not limited to these Examples only.
Preparation of polvmethvlvalerolactonediol:
Preparation Examples A to D
The inside of a separable flask equipped with a thermometer, an agitator, a nitrogen gas inlet tube and a dropping funnel was sufficiently purged with a nitrogen gas and then charged with 91.8 parts by weight of sodium di-2-hydroxyethyl 5-sulfoisophthalate and 0.65 part by weight of metallic sodium. The mixture was heated while agitating for effecting dissolution. 1000 parts by weight of ~-methyl-~-valerolactone was dropwise added thereto, and the mixture was maintained at a reaction temperature of 40C for about 1 hr to prepare polymethylvalero-lactonediol (A).
The polymethylvalerolactonediol (A) thus prepared had a hydroxyl value of 113 and a content of a sodium sulfonate group of 260 ~eq/g.
Polymethylvalerolactonediols B to D were prepared in the same manner as that described above, except that the amounts of the sodium di-2-hydroxyethyl 5-2~

sulfoi.sophthalate and ethylene glycol were varied asspecified in Table 1.
Preparation Example E
Polycaprolactonediol (E) was prepared in the same manner as that described in Japanese Patent Laid-Open No. 115920/1989, except that ~-caprolactone was used instead of ~-methyl-~-valerolactone.
Preparation Example F
An isophthalate type condensation polyester (F) was prepared through the use of starting materials listed in the column of Preparation Example F in Table 1.

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v c v c e o v c ~ v v--O :~: v _ V O O ~ O O r O V v V
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Preparation of polyurethane resin:
Preparation Example 1 A four-necked flask equipped with an agitator, a thermometer, a condenser and a nitrogen gas inlet tube was charged with 323 parts by weight of methyl ethyl ketone and 100 parts by weight of polymethylvalero-lactonediol (A) (OHV: 113) prepared in the Preparation ~xample A, and the mixture was heated and agitated to prepare a homogeneous solution. 89.3 parts by weight of 4,4'-diphenylmethane diisocyanate was poured into the homogene~us solution, and they were mixed at a solution temperature of 80C while agitating and allowed to react it that temperature for 2 to 3 hr to synthesize an isocyanate-terminated prepolymer.
Then, 11.9 parts by weight of 1,4-butanediol as a chain extender and 13.8 parts by weight of neopentyl glycol were added to the prepolymer, and the chain extension reaction was conducted at 70 to 85C for 10 to 15 hr to prepare a polyurethane resin.
Dibutyltin dilaurate was added as a urethanization catalyst in an amount of 0.11 part by weight based on the solid content to promote the reaction.
The above-described reactlon was regarded as completed when the concentration of the residual ~ ~,J~
2'~

isocyanate group in the reaction system is 0.01% by weight or less based on the solid content of the system.
Preparation Examples 2 to 7 Polyurethanes were prepared in the same manner as that of Preparation Example 1, except that individual starting materials were used in amounts specified in Table 2 and diols ~ to E and polyester F prepared in the above-described Preparation Examples B to F were used.

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~ 1 65702-381 Examples 1 to 4 and Comparatlve Examples 1 to 3 Preparation of magnetic coatinq material:
A mixture having the following mixing proportions was kneaded by means of a sand mill at 1200 rpm for 3 hr to prepare a magnetic coating material.
. metallic powder 100 parts by weight (specific surface area: 50 m2/g coerclve force (Hc): 1450 Oe saturation/magnetization (as): 122 emu/g) . polyurethane resin prepared in Preparation Examples 1 to 7 10 parts by weight . vinyl chloride/vinyl acetate/
vinyl alcohol copolymer 10- parts by weight . carbon black2 parts by weight . alumina5 parts by weight . tridecyl stearate1 part by weight . palmitic acid1 part by weight . methyl ethyl ketone200 parts by weight . cyclohexanone155 parts by weight Preparation of maqnetic tape-S parts by weight of a polyisocyanate compound (Coronate*L; a product of Nippon Polyurethane Industry Co., Ltd.) was added as a curing agent to 100 parts by weight of the magnetic coating material prepared by *Trade-mark r l~

the above-described method. They were mixed and agitated, and the mixture was applied to a 10 ~m-thick polyethylene terephthalate film by means of an applicator of 40 ~m, subjected to orientation treatment, dried, calendered for planishing, and slit to a width of 8 mm, thereby preparing a magnetic tape for video.
Evaluation of ma~netic tape:
The magnetic tape prepared above was subjected to the following evaluation.
(1) Reflectance (at 60C) (~) Residual magnetic flux density (Br) (3) Squareness ratio (Sq) (4) S/N ratio ~

(5) Durability: The magnetic tape was stored at 40C and 80 %RH for 30 days, set in a tape-driving unit and subjected to measurement of the number of runs necessary for lowering the output by 10% from the initial output. This value was used as a measure of the durability.
The results of evaluation are given in Table 3.

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P~ h m u~

Claims (15)

1. A magnetic recording medium comprising a non-magnetic substrate and a magnetic layer provided on the substrate, the said magnetic layer comprising ferromagnetic powder and a binder comprising a polyurethane resin which is obtained by reacting a polyisocyanate with polymethylvalerolactonediol prepared by polymerizing .beta.-methyl-.delta.-valerolactone in the presence of a di-hydroxy compound having a metal sulfonate group as a polymerization initiator.

2. The medium as claimed in Claim 1, in which the poly-urethane contains 0.1 to 300 µeq/g of the metal sulfonate group.

3. The medium as claimed in Claim 1, in which a molar ratio of the lactone to the polymerization initiator ranges between 50:1 and 2:1.

4. The medium as claimed in Claim 1, in which the di-hydroxy compound having a metal sulfonate group is represented by one of the following formulae:
, and (in which a is 1 or 2, ? is an integer of 1 to 8, m and n each are an integer of 1 to 5, X is -O- or -COO-, M is Li, Na or K
and R is or .

5. The medium as claimed in Claim 1, in which the poly-urethane has a number-average molecular weight of 5,000 to 200,000 or a weight-average molecular weight of 5,000 to 500,000.

6. The medium as claimed in Claim 1, in which the magnetic layer comprises 100 parts by weight of the ferromagnetic powder and 5 to 100 parts by weight of the binder.

7. A magnetic recording medium comprising a non-magnetic substrate and a magnetic layer provided on the substrate, wherein the said magnetic layer comprises ferromagnetic powder and a binder comprising a polyurethane resin which has a content of a metal sulfonate group of 0.1 to 300 µeq/g and a number-average molecular weight of 5,000 to 200,000 and a weight-average molecular weight of 5,000 to 500,000 and is obtained by reacting a poly-isocyanate with a polyhydroxy compound that includes polymethyl-valerolactonediol prepared by polymerizing .beta.-methyl-.delta.-valero-lactone in the presence of a dihydroxy compound having a metal sulfonate group as a polymerization initiator in a molar ratio of the valerolactone to the dihydroxy compound within the range from 50: to 2:1.

8. The medium as claimed in Claim 7, in which the di-hydroxy compound having a metal sulfonate group is represented by one of the following formulae:
, and (in which a is 1 or 2, ? is an integer of 1 to 8, m and n each are an integer of 1 to 5, X is -O- or -COO-, M is Li, Na or K and R is or .

9. The medium as claimed in claim 8, wherein the poly-hydroxy compound employed for obtaining the polyurethane resin also includes at least one other polyhydroxy compound having a molecular weight of 50 to 10,000 and selected from the group consisting of low-molecular weight glycols, polyethers, polyesters, polythioethers, polybutadiene glycols, silicon-containing polyols and phosphorus-containing polyols.

10. The medium as claimed in claim 8, wherein the poly-hydroxy compound employed for obtaining the polyurethane resin also contains a chain extender selected from the group consisting of a low molecular weight glycol, trimethylolpropane, glycerin and a diamine.

11. The medium as claimed in any one of claims 1 to 10, wherein the polyurethane resin has a content of a metal sulfon-ate group of 1 to 200 µeq/q.

12. The medium as defined in claim 1, 2, 3, 5, 6, 7, 9 or 10, wherein the dihydroxy compound having a metal sulfonate group is represented by the formula:

(I) (II) (III) (IV) or (wherein M is Li, Na or K).

13. The medium as defined in any one of claims 1 to 10, wherein the polyisocyanate employed for obtaining the polyurethane resin is a diisocyanate having a cyclohexyl group.

14. The medium as defined in claim 13, wherein the di-isocyanate having a cyclohexyl group is isophorone diisocyanate.

15. The medium as defined in any one of claims 1 to 10, wherein the binder further comprises, in addition to the said polyurethane resin, at least one other binder resin selected from the group consisting of vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylo-nitrile copolymer, acrylic ester-vinylidene chloride copolymer, acrylic ester-styrene copolymer, methacrylic acid-acrylonitrile copolymer, methacrylic ester-styrene copolymer, polyvinyl fluoride, urethane resin other than the said polyurethane resin, vinylidene chloride-acrylonitrile copolymer, butadiene-acrylonitrile copoly-mer, polyamide, polyvinyl butyral, cellulose derivative, styrene-butadiene copolymer, polyester, chlorovinyl ether-acrylic ester copolymer, amino resin, phenolic resin, epoxy resin, phenoxy resin, alkyd resin and silicone resin.