MAGNETIC RECORDING MEDIUM

Abstract: A magnetic recording medium which contains a mod­ ified polymer obtained by modifying a polymer by subjecting the polymer in a binder to graft polymerization of a nitrogen-containing polymerizable unsaturated compound by the emission of radiation. Since the modified polymer is obtained by graft polymerization by the emission of radia­ tion, its range of application after production is wide, thereby providing a large industrial value. Since the modified polymer has nitrogen-containing side chains, a magnetic layer contributes to improvements in the dispersibility and packing properties of magnetic powder. Accordingly, a magnetic recording medium which has excellent magnetic characteristics and durability can be provided.

Description: SPECIFICATION MAGNETIC RECORDING MEDIUM Technical Field This invention relates to a magnetic recording medium and, more particularly, to a magnetic recording medium having a superior durability. Background Art In order to enhance the durability of a magnetic recording medium, there has been employed a method, for example, of providing an improved binding of magnetic powder to a binder using a binder having a high compatibility with the magnetic powder, thus improving dispersibility and packing properties to decrease the rubbing-off of the powder. In this method, it has been proposed that there be employed as a binder a polymer which is modified by the polymerization of its copolymeric ingresients highly compatible with the magnetic powder, thereby imprcving the bincing property of the binder to the magnetic powder. This method, however, requires that ail the copolymeric ingredients are capable of being polyinerized. Even if this requirement is met, a binder containing a desired amount of copolymeric ingredients cannot be produced or a block copolymer is produced in the absence of a proper relative polymer zation ratio, so that conditions of manufacture are sianificantiy limited. This method is accordingly less valuable and less practical in industrial applications. Disclesure of Invention The object of this invention is to provide a magnetlc recording medium having a superior durability by using binder containing a modified polymer. The mocifie polymer used in this invention can be cbtained by mocifyinc a polymer conventionally used as a binder for maanetic recording media by graft-poly merizing a nitrogen-containing polymerizable unsaturated compound onto the polymer by the irradiation of radiation. That is, such a modified polymer can be obtained by first irradiating the polymer to be graft-polymerized with a radiation such as gamma rays, electron beams or the like, to generate organic free radicals and then graft-polymerizing the nitrogen-containing polymerizable unsaturated compound to the free radical to modify the polymer. The modified polymer can be also obtained by using a mixture of a polymer usually used as a binder for magnetic recording media and a nitrogen-containing polymerizable unsaturated compound and irradiating the mixture with radiation to perform the graft-polymerization. Any polymer which can generate free radicals upon being irradiated with radiation and is not degraded by the radiation may be used as the polymer to be modified by the graft polymerization of a nitrogen-containing polymerizable unsaturated compound to the polymer. It is to be ncted that, since any organic material without exception can produce free radicals if a radiation having a high penetrating power is used, and since a polymer likely to be degraced by such radiation can procure enough free radicals to react with the flitrogen- containing poibrierizable monomer even if the radiation is applied at small dose under conditions which control degradation, the applicable range of the foregoing method is very broad. It is preferred that the polymer to which the aforesaid method can be applied has a molecular weight ranging from about 10,000 to about 100,000. Such a polymer may be enumerated by, for example, a vinyl chloride-vinyl acetate copolymer, a vinyl chloride-vinyl propionate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, a vinyl chloride-vinyl propionate-vinyl alcohol copolymer, a vinyl chloride-vinyl acetate-maleic acid copolymer, vinyl chloride-vinylidene chloride copolymer, a vinyl chloriae-acrylonitrile copolymer, an acrylic ester-acrylonitrile copolymer, an acrylic ester-vinylidnene chloride copolymer, a methacrylic ester-vinylidene chloride copolymer, a methacrylic ester-styrene copolymer, a thermoplastic resin, a phenoxy resin, polyvinyl fluoride, a vinylidene chloride-acrylonitrile copolymer, a butadieneacrylonitrile copolymer, an acrylonitrile-butadiene-acrylic acid copolymer, an acrylonitrile-butadiene-methacrylic acid copolymer, polyvinyl butyral, a cellulose derivative, a styrene-butadiene copolymer, a polyester resin, a phenol resin, an epoxy resin, a thermo-setting polyurethane resin, a urea resin, a melamine resin, an alkyd resin, a urea-formaldehyde resin or the like, or a mixture thereof. The radiation to be irradiated for graft-polymerizing a nitrogen-containing polymerizable unsaturated compound onto the foregoing polymer may be, for example, gammarays, an electron beam or the like, and the dose of the radiation is preferably in a range from about 0.1 to about 10 Mrad. The nitrogen-containing polymerizable unsaturated compound to be graft-polymerized onto the foregoing polymer may include, for example, a nitrooen-containing heterocyclic vinyl compound such as 2-vinylpyridine, 4-vinylpyridine or the like; a dia lkylaminoalkyl acrylate or methacrylate such as dimethylarninopropyl acrylate, dimethylaminopropyl methacrylate, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylamiàe a dialkylamino alkyl acrylarnide or methacrylamiae such as dimethyl arninoethyl acrylamide, dimethylaminoethyl methacryl-amiae, dirnethylamînopropyl acrylamide, dimethylaminopropyl methacrylamiae or the like. The amount of the polymerizable unsaturated compound is preferred to be about 0.1 to about 10 parts by weight per 100 parts by weight of the polymer to be modified. Where the amount of the polymerizable unsaturated compound is toc small, the contribution to the affinity for a magnetic powder by the nitrogen-containing hydro philic group thus introduced is so insufficient that the dispersibility and packing properties can not be provided to the desired extent. Where the amount is toc large, solubility of the modified polymer in a solvent is rendered poor, or it becomes so insoluble that compatibility with other binders is rendered poor. In the case where a magnetic paint is prepared using the modified polymer, it presents the disadvantage that the viscosity of the paint becomes extremely poor to a practically inapplicable extent. The method of modifying a conventional polymer by reacting with the above-described polymerizable unsaturated compound by the irradiation of radiation is preferably performed by a graft polymerization and the graft-polymerization may be conducted in a conventional manne The binder containing the modified polymer to be used in this invention can exhibit a sufficient effect in aamixture with the aforementioned conventional binder. When the binder comprising such a modifiea polymer is used in aamixture with the conventional binder, the desired effect is cbtained where the modified polymer is ccntained at least about 15 by weight based on the total binder amount. The modif led polymer to be used in this invention irtay be used in a system in which a polyfunctional isocyanate compound is combined or in a thermosetting type system in which the modifies polymer is combined with the conventional binder as above described and 2 polyfunctional isocyanate compound. The magnetic powder may be any one which is conven tionally employed for magnetic recording media. Examples of such a magnetic powder may include, an oxide magnetic powder such as gamma -Fe2O3, Fe304, an iron oxide in an intermediate oxide state between gamma-Fe2o3 and Fe3O4, Co-containins gamma-Fe2O3, Co-containing Fe3O4, a Co- containing iron oxide in an intermediate oxide state between gamma-Fe2O3 and Fie304, the aforesaid iron oxide each further containing one or more metal elements (in particular, transition metal elements), the aforesaid iron oxides each having coating composed mainly of cobalt oxide or cobalt hydroxide, CrO2, CrO2 the surface of which is subjected to a reduction trament to form layer, and so forth, and a ferromagnetic fine a Cr2 3 powder such as a metal such as Fe, Co, Ni, and so forth and an alloy such as an Fe-Co alloy, an Fe-Ni alloy, an Fe-Co-Ni alloy, a Co-Ni-P alloy, a Co-Ni-Fe-B alloy, an Fe-Ni-Zn alloy, an Fe-Mn-Zn alloy, an Fe-Co-Ni-P alloy and so forth. The magnetic paint to be used in this invention containing the modified polymer and a magnetic powder may further contain, if necessary, an abrasive such as aluminum oxide, chromium oxide, silicon oxide or the like; an antistatic agent such as carbon black, or the like; and a lubricant such as molybdenum disulfide, graphite, silicone cil, olive cil or the like. A non-magnetic substrate on which a magnetic layer is formed by coati with the magnetic paint containing the above-described components may be a film or sheet comprising a non-magnetic material such as, for example, a polyester such as polyethylene terephthalate or the like; a polyolefin such as polypropylene or the like; a cellulose derivative such as cellulose triacetate, cellulose diacetate or the like; a polycarbonate; poly vinyl chloride; a polyimide; a metal such as aluminum copper or the like; paper or the like. A solvent capable of being used for the preparation of the magnetic paint may include, for example, a ketone such as acetone, methyl ethyl keton, methyl isobutyl ketone, cyclohexanone or the like; an alcohol such as methanol or the like; an ester such as methyl acetate, ethyl acetate, butyl acetate, ethyl butyrate or the like; a glycol ether such as ethylene glycol dimethyl ether, ethylene glydol monoethyl ether, dioxane or the like; an aromatic hydrocarbon such as benzene, toluene, xylene or the like; an aliphatic hydrocarbon such as hexane, heptane or the like, and a mixture thereof In the magnetic recording medium in accordance with this invention, the magnetic powder used therein shows good adhesion to the nonmagnetic substrate and also the magnetic powder has improved dispersibility, packing properties and surface properties, so that the magnetic recording medium has excellent magnetic characteristics and durability. Best Mode of Carrying Out the Invention This invention will be described further by way of examples. Example 1 A modified polyurethane was prepared by mixing 100 parts by weight of a polyurethane (tradename "N-2304". manufacturea by Nippon Polyurethan Kogyo K.K.) with 5 parts by weight of 2-vinylpyridine, forming the mixture in a sheet fortin having a thickness of about 100#m, and irradiating it with an electron beam of 5 Mrad. With the mocifiec polyurethane, magnetic paint having the followinc composition was prepared Modifies polyurethane 100 parts by weight #-Fe2O3 400 Squalane 9 Cr203 4 Methyl ehtyl ketone 500 Methyl isobutyl ketone 500 The paint composition containing the above components was milled to give a magnetic paint, and the magnetic paint was coated on a polyester base film having a thickness of 16Um to a thickness of 4#rn. The film was dried and then cut into magnetic tapes. Example 2 h paint having the following composition was coated on a polyester base film having a thickness of 16pm to a thickness of 4pm and then magnetic tapes were prepared in the same manner as in Example 1. #-Fe 203 400 parts by weight Modified polyurethane 30 Vinyl chloride-vinyl acetate vinyl alcohol copolymer (tradename "VAGH", manufactured by Union Carbide Corp,) 70 Squalane 9 " Cr203 4 Methyl ethyl ketone 500 Methyl isobutyl ketone 500 Comparative Example 1 By following the same procedure as in Example 1 except that the unmodified polyurethane (tradename "N 2304") was used in place of the modified polyurethane, magnetic tapes were prepared. Comparative Example 2 Magnetic tapes were prepared in the same procedure as in Example 2 with the exception that the unmocified polyurethane (tradename "N-2304") was used in place of the modifed polyurethane. The magnetic tapes prepared according to the afore mentioned Examples and Comparative Examples were measurea for Bm, Rs, gloss, rubbing-off rating, and still. The results are shown in the following table. In this measure ment, gloss was determined with a commercially available gloss meter by measuring an amount of reflected light arising from incident light at an incident angle of 750 The larger the value thereof, that is, the doser to 100%, the better the surface quality. The rubbing-off rating was determined by an amount of powder attached to a magnetic head, tape guide and so on after running the tape. The still was defined as the time required for the reduction of output signals from an initial level to a predetermined level by repeated scanning of the same portion of a magnetic tape with the magnetic head of a household VTR. Table Bm Rs(%) Gloss Rabbing- Still (%) Off Rat- (min.) ings Example 1 1,420 82 93 good 920 Example 2 1,440 83 104 good 840 Comparative 1,360 75 69 bad 800 example 1 Comparative 1,320 77 77 bad 820 example 2 1,320 77 77 From these results, it has been confirmed that the magnetic tapes of this invention are excellent in not only electromagnetic conversion characteristics but also surface properies ana durability.

Claims: CLAIMS 1. A magnetic recording medium having a magnetic layer composed mainly of magnetic powder and a binder formed on a nonmagnetic substrate, characterized in that said binder comprises a polymer modified by graft polymerizing a nitrogen-containing polymerizable unsaturated compound onto a polymer by irradiating with radiation. 2. A magnetic recording medium according to claim 1, characterized in that nitrogen-containing polymerizable unsaturated compound is contained in an amount ranging from 0.1 to 10 parts by weight with respect to 100 parts by weight of the polymer to be modified. 3. A magentic recording medium according to claim 1, characterized in that the polymer to be modidifed has a molecular weight in a range from 10,000 to 100,000, 4. A magnetic recording medium according to claim 1, charcterized in that the modified polymer is contained in an amount of not less than 1% by weight based on a total binder amount. 5. A magnetic recording medium according to daim 1, characterized in that the radiation is an electron beam. 6. A magnetic recording medium according to claim 1, characterzied in that the radiation is irradiated at a dose ranging from 0.1 to 10 Mrad.

Inventors: NAKAJIMA, KAORU
Applicants: SONY CORPORATION
Application year: 1983
Main classification: G11B5
IPC codes: C09D175, C09D5, G11B5