GB2057471A - Magnetic Recording Media - Google Patents

Abstract

A magnetic recording medium has a magnetic layer, comprising a binder and magnetic powder, the binder comprising the compound and/or compound II as specified below and having been hardened under irradiation: Compound I A binary compound that can be produced by reacting at least one of monomers, oligomers, and polymers with an isocyanate compound, the monomers, oligomers and polymers containing in their molecule both a plurality of active hydrogen atoms that can undergo a reaction with isocyanate groups and at least one unsaturated bond while the isocyanate compound contains in its molecule a plurality of isocyanate groups. Compound II A ternary compound that can be produced by reacting three components, one of which is at least one of the compounds having in their molecule a plurality of active hydrogen atoms that can undergo a reaction with isocyanate groups, another of which is at least one of monomers, oligomers and polymers that contain in their molecule at least one unsaturated bond, and the remainder of which is an isocyanate compound that contains in its molecule a plurality of isocyanate groups.

Description

SPECIFICATION Magnetic Recording Media This invention relates to magnetic recording media.

In a conventional magnetic recording tape, a compound that is formed by reacting a polymer having a plurality of hydroxyl groups in its molecule with a di- or triisocyanate to form a urethane crosslinkage has been used for the binder of the magnetic layer on the tape. However, this type of binder has the demerit that its pot life is rather short, due to a high proportion of isocyanate groups used in the reaction. If by contrast the proportion of isocyanate groups is reduced to prolong the pot life, the binder compound obtained will be only poorly hardened, which means that the coated layer has an insufficient strength. Thus, it has been impossible or at least extremely difficult to provide a binder satisfactory both in hardening performance and pot life. Moreover, it has also been impossible to step-wise harden the binder through formation of urethane cross-linkages.

According to the present invention there is provided a magnetic recording medium carrying a magnetic layer thereon, said magnetic layer comprising a binder and magnetic powder, wherein said binder comprises compound I and/or compound II as specified below and said binder has been hardened under irradiation: Compound l: A binary compound produced by reacting at least one of monomers, oligomers, and polymers with an isocyanate compound, said monomers, oligomers and polymers containing in their molecule both a plurality of active hydrogen atoms that can undergo a reaction with isocyanate groups and at least one unsaturated bond, while said isocyanate compound contains in its molecule a plurality of isocyanate groups.

Compound II: A ternary compound produced by reacting three components, one of which is at least one of the compounds having in their molecule a plurality of active hydrogen atoms that can undergo a reaction with isocyanate groups, another of which is at least one of monomers, oligomers and polymers that contain in their molecule at least one unsaturated bond, and the remainder of which is an isocyanate compound that contains in its molecule a plurality of isocyanate groups.

The invention will now be described by way of example.

In embodiments of the invention, the isocyanate compound accounts for a comparatively small proportion in the binder formulation, which is a favourable factor in extending the pot life of magnetic paint. In spite of such a small proportion of the isocyanate compound, unsaturated bonds, particularly double bonds sensitive to radiation, are made available in the binder, so that when active chemical species, particularly radicals, are formed by ionisation under irradiation, they add to these unsaturated bonds to start a radical reaction forming a linear or three-dimensional network structure, while active hydrogen atoms react with isocyanate groups for simultaneous formation of a cross-linked structure of two or three dimensions. As a result, the binder can be hardened sufficiently to form a magnetic layer of high hardness.A proper selection of the quantity of isocyanate compound used allows extension and adjustment of the pot life of magnetic paint within a desired range, while avoiding any poor hardening of the binder; so assuring a desirable hardness of a magnetic layer comprising the binder. Thus, both the pot life of the magnetic paint and the hardness of the magnetic layer are improved, making a long pot life compatible with a sufficiently high hardness. Moreover, the coated magnetic layer can be hardened step-wise by adjusting the radiation doses, and therefore the target hardness of a magnetic layer can be readily and positively attained.

In embodiments of the invention, for the binder formulation as above described, 3 to 1 5 parts by weight of the isocyanate compound are preferably used per 100 parts by weight of the compound having double bonds and/or active hydrogen atoms. Less than 3 parts by weight of the isocyanate compound will cause poor hardening of the binder compound resulting in poor mechanical strength of the coated layer while use of more than 15 parts by weight of the isocyanate compound will produce a binder that has a shorter pot life.The ratio of double bonds to active hydrogen atoms is preferably from 1:1 to 10:1, because if double bonds are reduced to decrease this ratio below 1 the pot life is shortened, while if the ratio exceeds 10:1, there are too many double bonds, which cause too much hardening (or poor elasticity) of the coated layer; both cases causing undesirable results. The molecular weight of monomers, oligomers and polymers per unsaturated bond is preferably from 400 to 1000 while that per active hydrogen atom is preferably between 1000 and 4000, because molecular weight below 1 000 will cause too much hardening of the coated layer while molecular weight above 4000 provides only a low density of cross-linkages, resulting in poor mechanical strength of the coated layer.

In the binder formulation, active hydrogen atoms are provided for example in the form of a hydroxyl, carboxyl, amino or amide group, which means that a compound having two or more functional groups of the above kind in each molecule is used. These active hydrogen atoms may also belong to the urethane bond. Actual examples of compounds that contain in their molecules both one or more unsaturated bonds and a plurality of active hydrogen atoms are polyester copolymers of various degrees of polymerisation that can be formed by reacting trimethylolpropane monoacrylate, hexandediol and adipic acid. In the above reaction, trimethylolpropane monoacrylate may be replaced with a diacrylate of an aliphatic polyol such as pentaerythritol, or a diacrylate of an alicyclic diglycidyl ether.Moreover, hexanediol may be replaced for example with ethylene glycol; propylene glycol; 1,3propanediol; 1,4-butanediol; 1 ,5-pentanediol; neopentyl glycol; diethylene glycol; dipropylene glycol; 2,2,4-trimethyl-1 3-pentanediol; 1 ,4-cyclohexane dimethanol; an ethylene oxide addition product or propylene oxide addition product of bisphenol A or hydrogenated bisphenol A; polyethylene glycol; polypropylene glycol; or polytetramethylene glycol. Tri- or tetraol of trimethylolethane, trimethylolpropane, glycerin or pentaerythritol may be added in a small amount.Moreover, adipic acid may be replaced with an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid, orthophthalic acid or naphthalene-1 5-dicarboxylic acid; an aromatic hydroxycarboxylic acid such as p hydroxy-benzoic acid or p-(hydroxyethoxy)-benzoic acid; or an aliphatic dicarboxylic acid such as succinic acid, azelaic acid, sebacic acid or dodecanedicarboxylic acid. In this case, if an aromatic carboxylic acid and an aliphatic carboxylic acid are used in combination, their molar ratio is selected to be between 50/50 and 100/0, at least 30 mol% of the aromatic carboxylic acid being preferably accounted for by terephthalic acid. A tri- or tetra-carboxylic acid such as trimellitic acid, trimesic acid or pyromellitic acid may be added in a small amount.It is noted that the acid component such as adipic acid may be replaced with an aromatic or aliphatic diisocyanate such as tolylene diisocyanate, when polyurethane is formed. A preferable blend ratio of the above acrylate and polyol is between 80/20 and 10/90 when the acrylate is a monoacrylate or between 40/60 and 5/95 when the acrylate is a diacrylate.

Examples of the compound having a plurality of active hydrogen atoms in its molecule are vinyl chloride-vinyl acetate-vinyl alcohol copolymer, ethyl cellulose, nitrocellulose, polyvinyl butyral, butyl acrylate-acrylonitrile-acrylic acid copolymer, butyl acrylate-acrylonitrile-methacrylic acid copolymer, amino resin, melamine resin and poiybutadiene with active terminal groups.

On the one hand, examples of the compound that contains one or more unsaturated bonds are acrylic acid and acrylates such as ethyl acrylate, propyl acrylate, butyl acrylate, amyl acrylate, ethylhexyl acrylate, octyl acrylate, t-octyl acrylate, 2-methoxyethyl acrylate, 2-butoxyethyl acrylate, 2phenoxyethyl acrylate, chloroethyl acrylate, hydroxyethyl acrylate, cyanoethyl acrylate, hydroxypropyl acrylate, dimethylaminoethyl acrylate, 2,2-dimethylhydroxypropyl acrylate, 2-hydroxybutyl monoacrylate, 1,4-hydroxybutyl monoacrylate, 5-hydroxypentyl acrylate, diethylene glycol monoacrylate, trimethylolpropane monoacrylate, pentaerythritol monoacrylate, glycidyl acrylate, 2-hydroxy-3-chloropropyl acrylate, benzyl acrylate, methoxybenzyl acrylate, and phenyl acrylate.Use may also be made of methacrylic acid and methacrylates such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, amyl methacrylate, hexyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, cyanoacetoxyethyl methacrylate, benzyl methacrylate, octyl methacrylate, sulphopropyl methacrylate, N-ethyl-N-phenylaminoethyl methacrylate, ethylene glycol monomethacrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl methacrylate, 2-hydroxypropyl methacrylate, 2-hydroxybutyl monomethacrylate, 4-hydroxybutyl methacrylate, 5-hydroxy-pentyl methacrylate, 2,2-dimethyl-3 hydroxypropyl methacrylate, diethyleneglycol monomethacrylate, trimethylolpropane monomethacrylate, pentaerythritol monomethacrylate, glycidye methacrylate, 2-methoxyethyl methacrylate, 2-(3-phenylpropylhydroxy) ethyl methacrylate, dimethyla minophenoxyethyl methacrylate, phenyl methacrylate, cresyl methacrylate or naphthyl methacrylate. Instead of acrylamide and its derivatives N-substituted acrylamides for example may be used. Examples of N substituted acrylamides are N-methacrylamide, N-ethylacrylamide, N-propylacrylamide, N isopropylacrylamide, N butylacrylamide, N-t-butylacrylamide, N heptylacrylamide, N-t octylacrylamide, N-cyclohexylacrylamide, N-benzylacrylamide, N-(hydroxymethyl)-acrylamide, N (methoxyethyl)-acrylamide, N-(N,N-dimethyla minoethyl)-acrylamide, N-(beta-hydroxyethyl) acrylamide, N-phenylacrylamide, N (hydroxyphenyl) acrylamide, N-tolylacrylamides, N naphthylacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N,N-dibutylacrylamide, N,Ndiisobutylacrylamide, diacetone acrylamide, N-methyl-N-benzylacrylamide, N-benzyl-N-#hydroxyethyl)- acrylamide, N-(beta-cyanoethyl)-acrylamide, N-acryloylmorpholine, N-methyl-N-acryloylpiperazine, N acryloylpiperizine, N-acryloyiglycine, N-( 1,1 -dimethyl-3-hydroxybuty)acrylamide, N-(beta morpholinoethyl) acrylamide, N-acryloylhexamethyleneimine, N-(beta-hydroxyethyl)-N methylacrylamide, and N-2-acetamideethyl-N-acetylacrylamide.In addition, methacrylamide and its derivatives, for example, N-substituted methacrylamides such as N methylmethacrylamide, N-t methacrylamide, N-t-octylm ethacrylamide N-benzylmethacrylamide, N-cyclohexylmethacrylamide, N phenyl methacrylamide, N,N-dimethylmethacrylamide, N,N-diethyl-methacrylamide, N,N dipropylmethacrylamide, N-(beta-hydrozyethyl)-N-methylmethacrylamide, N-methyl-N phenylmethylacrylamide, and N-ethyl-N-phenylmethacrylamide, may also be used. In addition to the above compounds having an unsaturated bond or bonds, polyacrylates and polymethacrylates (here the prefix 'poly' refers to a dimer or higher polymer) of polyalcohols may be used. Here, polyalcohols refer, for example, to polyethylene glycols; polypropylene oxide, polybutylene oxide, polycyclohexene oxide, polyethylene oxide; polypropylene oxide, polystyrene oxide; polyoxetane; polytetrahydrofuran; cyclohexanediols; xylylenediol; di-(beta-hydroxyethoxy) benzene; glycerin; diglycerin; neopentyl glycol; trimethylolpropane, triethylolpropane; pentaerythritol; dipentaerythritol; sorbitan; sorbitol; butanediols; butanetriols; 2-butene-1 ,4-diol; 2-n-butyl-2-ethyl-propane-1 ,3-diol; 2-butyn-1 ,4-diol; 3-chloro-1 ,2- propanediol; 1 ,4-cyclohexanedimethanol; 3-cyclohexene-1 , 1 -dimethanol; decahydronaphthalenediols; 2,3-dibromo-2-butene-1 ,4-diol; 2,2-diethyl- 1 3-propanediol; 1,5-dihydroxy-1,2,3,4- tetrahydronaphthalene; 2 ,5-dimethyl-2,5-hexanediol; 2,2-dimethyl- 1 ,3-propanediol; 2,2-diphenyl-1 ,3propanediol; dodecanediols; mesoerythritol, 2-ethyl-1 ,3-hexanediol; 2-ethyl-2-(hydroxymethyl)-1 ,3propanediol; 2-ethyl-2-methyl-1 3-propanediol; heptanediol; hexanediol; 3-hexene-2,5-diol; hydroxybenzyl alcohol; hydroxyethylresorcinols; 2-methyl- 1 ,4-butanediol; 2-methyl-2,4-pentanediol, nonanediols; octanediols; pentanediols; 1 -phenyl-1 ,2-ethanediol; propanediols; 2,2,4,4-tetramethyl1 ,3-cyclobutanediol; 2,3,5,6-tetramethyl-p-xylene-alpha-alpha'diol; 1 ,1 ,4,4-tetraphenyl-1 4- butanediol; 1,1 ,4,4-tetraphenyl-2-butyn-1 ,4-diol; 1 2,6-trihydroxyhexane; 1,1 '-bis-2-naphthol; dihydroxynaphthalene; 1,1 '-methylene-bis-2-naphthol; 1 ,2,4-benzenetriol; bisphenols; 2,2-bis(4hydroxyphenyl)butane; 1 1 -bis(4-hydroxyphenyl)-cyclohexane; bis(hydroxyphenyl)methanes; catechol; 4-chlororesorcinol; 3,4-dihydroxyhydocinnamic acid; hydroquinone; hydroxybenzyl alcohol; methylhydroquinone; methyl-2,4,6-trihydroxybenzoate; phloroglucinol; pyrogallol; resorcinol; glucose; alpha-( 1 -aminoethyl)-p-hydroxybenzyl alcohol; 2-amino-2-ethyl- 1 ,3-propanediol; 2-amino-2-methyl1,3-propanediol; 3-amino-1 2-propanediol; N-(3-aminopropyl)-diethanolamine;N,N'-bis-(2hydroxyethyl)-piperazine; 2,2-bis(hydroxymethyl-2,2 ',2"-nitrilotriethanol; 2,2bis(hydroxymethyl)propionic acid; 1 ,3-bis(hydroxymethyl)-urea; 1 ,2-bis(4-pyridyl)- 1 ,2-ethanediol; N-nbutyl-diethanolamine; diethanolamine; N-ethyldiethanolamine; 3-mercapto- 1 ,2-propanediol; 3piperidino- 1 ,2-propanediol; 2-(2-pyridyl)- 1 ,3-propanediol; triethanolamine; alpha-( 1 -aminoethyl)-phydroxybenzyl alcohol; 3-amino-4-hydroxyphenyl; and sulphones.Among the above acrylic esters or acrylates and methacrylic esters or methacrylates, those preferable in view of their availability are ethylene dimethacrylate, ethyleneglycol diacrylate, diethyleneglycol dimethacrylate, polyethyleneglycol diacrylates, pentaerythritol triacrylate, pentaerythritol dimethacrylate, dipentaerythritol pentaacrylate, glycerin triacrylate, diglycerin dimethacrylate; 1,3-propanediol diacrylate, 1,2,4-butanetriol trimethacrylate, 1,4-cyclohexanediol diacrylate, 1 5-pentanediol diacrylate, neopentyl glycol-diacrylate, and the triacrylic ester of ethylene-oxide-added trimethylolpropane.

Acrylamides and methacrylamides that may be used for the aforementioned unsaturated compound are methylenebisacrylamide and methylenebismethacrylamide, as well as polyacrylamides and polymethacrylamides derived from ethylenediamine, diaminopropane, diaminobutane, pentamethylenediamine, hexamethylene bis(2-aminopropyl)-amine, diethylenetriaminediamine, heptamethylenediamine, octamethylenediamine and polyamines with their polyamine chain interrupted by a hetero atom, and polyacrylamides and polymethacrylamides of cyclic polyamines, such as phenylenediamine, xylylenediamine, beta-(4-aminophenyl)ethylamine, diaminobenzoic acid, diaminotoluene, diaminoanthroquinone and diaminofluorene.

Compounds containing two or more different unsaturated bonds that can undergo an additional polymerization reaction in a molecule, such as N-beta-hydroxyethyl-beta-(methacrylamide) ethylacrylate, N-N-bis(beta-methacryloxyethyl)-acrylamide and allyl methacrylate may preferably be used for the unsaturated compound.

Isocyanate compounds containing two or more isocyanate groups, and preferably three or more isocyanate groups, that are used are 2,4-tolylene diisocyanate; 2,6-tolylene diisocyanate; p-phenylene diisocyanate; polymethylenepolyphenyl diisocyanate; diphenylmethane diisocyanate; m-phenylene diisocyanate; hexamethylene diisocyanate; butylene 1 ,4-diisocyanate; octamethylene diisocyanate; 3,3'-dimethoxy-4,4'-biphenylene diisocyanate; 1,1 8-octadecamethylene diisocyanate; polymethylene diisocyanate; naphthalene 2,4-diisocyanate; 3,3'-dimethyl-4,4'-biphenylene diisocyanate; 1methoxyphenylene 2,4-diisocyanate; diphenylene 4,4'-diisocyanate, diphenyl ether 4,4'-diisocyanate; naphthalene 1 ,5-diisocyanate; dicyclohexylmethane diisocyanate; p-xylene diisocyanate; m-xylene diisocyanate; diphenylene diisocyanate hydride; diphenylmethane diisocyanate hydride; benzene triisocyanate; toluene 2,4,6-triisocyanate; 3-methyl diphenylmethane 4,6,4'-triisocyanate; diphenyl 2,4,4'-triisocyanate; diphenyl ether 2,4,4'-triisocyanate; the triisocyanate obtained by reacting 3 moles of tolylene diisocyanate and 1 mole of trimethylolpropane; Desmodur L (75% ethyl acetate solution supplied by Farbenfabriken Bayer A.G.); and Desmodur N (triisocyanate from Farbenfabriken Bayer A.G.

obtained by reacting 3 moles of hexamethylene diisocyanate and 1 mole of water).

The magnetic layer can be formed on a base by coating using a conventional method. To irradiate the coated magnetic layer, ionizing radiation such as an electron beam, a neutron beam or gammarays, are used preferably at a dose of 1 to 10 Mrad and more preferably at a does of 2 to 7 Mrad, with a radiation energy preferably of 100 KeV or more. The above radiation dose is enough to start a desirable radical reaction.

The magnetic layer may comprise gamma-Fe203, mixed crystal of gamma-Fe203 and Fe3O4, cobalt-doped gamma-Fe203 or Fe3O4, CrO2, barium ferrite, ferrimagnetic alloys, such as Fe-Co, Co-Ni, Fe-Co-Ni, Fe-Co-B, Fe-Co-Cr-B, Mn-Bi, Mn-Al and Fe-Co-V, and iron nitride. The magnetic layer may also contain a reinforcing agent or agents such as aluminium oxide, chromium oxide and silicon oxide, as well as lubricant such as squalane, antistatic agent such as carbon black, and dispersing agent such as lecethin. The constituent materials of the magnetic layer are dissolved in an organic solvent to provide a magnetic paint, with which a base is coated to form a magnetic layer thereon.The organic solvent of the magnetic paint can be selected from ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; alcohols such as methanol, ethanol, propanol and butanol; esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate and glycol acetate; monoethyl ether; glycol ethers such as ethyleneglycol dimethyl ether, ethyleneglycol monoethyl ether and dioxane; aromatic hydrocarbons such as benzene, toluene, and zylene; aliphatic hydrocarbons such as hexane and heptane; and nitropropane.A non-magnetizable base is used to coat the magnetic paint thereon, and may be composed of a polyester such as polyethylene terephthalate, a polyolefin such as polypropylene, a cellulose derivative such as cellulose triacetate or cellulose diacetate, polycarbonate, polyvinyl chloride, polyimide, polyamide, polyhydrazide, a metal layer such as aluminium or copper foil, or paper.

Specific examples will now be described: Example 1 First, a magnetic paint was prepared with the following formulation: parts by weight gamma-ferric oxide (gamma-Fe203) 400 Polyacrylate containing active hydrogen atoms (a polymer of molecular weight 20000 obtained by copolymerizing pentaerythritol diacrylate, hexanediol, adipic acid and trimethylolpropane in a blend ratio of 16:30:50:4) 100 Trifunctional isocyanate compound (Desmodur L from Farbenfabriken Bayer A.G.) 10 Lecithin (dispersing agent) 1 Cr203 (reinforcing agent) 3 Olive oil (lubricant) 0.5 Methyl ethyl ketone (solvent) 600 Methyl isobutyl ketone (solvent) 600 These constituent materials were mixed in a ball mill to provide a magnetic paint, with which a base or a piece of polyethylene terephthalate film was coated.The coated film was dried, calender treated, and then subjected to thermo-hardening at 600C for 24 hours. Thereafter, the magnetic layer was irradiated with an electron beam at a dose of 5 Mrad and an acceleration voltage of 300 kV.

Characteristic performances of a magnetic tape thus obtained were measured, which resulted in a coating strength of 1 2.5 x 104 kg/cm2 and a dynamic coefficient of friction of 0.210.

Example 2 First, a magnetic paint was prepared with the following formulation: parts by weight gamma-Fe203 Bifunctional thermoplastic polyurethane resin (Paraplen 88 from Nippon Polyurethane Company) 40 Multifunctional acrylate (Aronix from Toa Gosei Kagaku Company) 60 Trifunctional isocyanate compound (Desmodur L) 10 Lecithin (dispersing agent) 1 Cr203 (reinforcing agent) 3 Olive oil (lubricant) 0.5 These constituent materials were mixed in a ball mill to provide a magnetic paint, with which a base or a piece of polyethylene terephthalate film was coated. The coated film was dried, calender treated, and then subjected to thermo-hardening at 600C for 24 hours. Thereafter, a magnetic layer was irradiated with an electron beam at a dose of 5 Mrad and an acceleration voltage of 300 kV.The coated film thus obtained was slit to give 1.27 cm wide magnetic tapes.

The magnetic layer hardened under irradiation with an electron beam gave favourable estimates of 12.1 x104 kg/cm2 and 0.214 for the coating strength and dynamic coefficient of friction (measured by Euler's method against an aluminium drum), respectively. By contrast, when the magnetic paint of the above formulation was used to form a magnetic layer but no electron beam irradiation was used, measurements of both the coating strength and the dynamic coefficient of friction gave poor results; 0.315x104mg/cm2 and 0.761, respectively. When the isocyanate compound was not added in the above formulation of magnetic paint, the coated magnetic layer performed poorly, the coating strength and the dynamic coefficient of friction being estimated to be 0.29 xl 104 and 0.82, respectively, even when irradiated with an electron beam.

The magnetic paint obtained in this example had a pot life of about 12 hours at 250C. When the quantity of isocyanate compound was increased to 30 parts by weight in the above formulation and there was no irradiation with an electron beam, the resultant magnetic tape showed a comparatively high coating strength of 1 1.8 x 104 kg/cm2, but the magnetic paint performed satisfactorily only for about 5 hours at 250C, a substantial cut in the pot life.

Example 3 A magnetic tape was prepared by using ethylene dimethacrylate instead of multifunctional acrylate (Aronix) in the paint formulation of Example 2 and following the procedure as in Example 2.

Characteristic performances of the tape were examined. Favourable results were obtained with the coating strength and dynamic coefficient of friction estimated to 11 1.5x104 kg/cm2 and 0.240 when the magnetic layer was irradiated with an electron beam. By contrast, when not irradiated with an electron beam, the magnetic layer gave poor results, the corresponding estimates being 0.31 Ox 104 kg/cm2 and 0.765.

Claims (11)

Claims

1. A magnetic recording medium carrying a magnetic layer thereon, said magnetic layer comprising a binder and magnetic powder, wherein said binder comprises compound I and/or compound II as specified below and said binder has been hardened under irradiation: Compound l: A binary compound produced by reacting at least one of monomers, oligomers, and polymers with an isocyanate compound, said monomers, oligomers and polymers containing in their molecule both a plurality of active hydrogen atoms that can undergo a reaction with isocyanate groups and at least one unsaturated bond, while said isocyanate compound contains in its molecule a plurality of isocyanate groups.

Compound ll: A ternary compound produced by reacting three components, one of which is at least one of the compounds having in their molecule a plurality of active hydrogen atoms that can undergo a reaction with isocyanate groups, another of which is at least one of monomers, oligomers and polymers that contain in their molecule at least one unsaturated bond, and the remainder of which is an isocyanate compound that contains in its molecule a plurality of isocyanate groups.

2. A magnetic recording medium according to claim 1, wherein the binder formulation contains 3 to 15 parts by weight of said isocyanate compound per 100 parts by weight of said monomers, oligomers and polymers.

3. A magnetic recording medium according to claim 1, wherein the binder formulation contains unsaturated bonds of said monomers, oligomers and polymers and active hydrogen atoms in a ratio of 1:1 to 10:1.

4. A magnetic recording medium according to claim 1 wherein said monomers, oligomers and polymers used in the binder formulation have molecular weights per unsaturated bond between 400 and 1000.

5. A magnetic recording medium according to claim 1 wherein said monomers, oligomers and polymers used in the binder formulation have molecular weights per active hydrogen atom between 1000 and 4000.

6. A magnetic recording medium according to claim 1 wherein the binder formulation uses active hydrogen atoms that belong to hydrozyl, carboxyl, amino or amide groups, or a urethane bond.

7. A magnetic recording medium according to claim 1 wherein an electron beam is used for said irradiation.

8. A magnetic recording medium according to claim 1 or claim 7 wherein a radiation dose of 1 to 10 Mrad is used.

9. A magnetic recording medium according to claim 8 wherein a radiation dose of 2 to 7 Mrad is used.

10. A magnetic recording medium according to claim 1 or any one of claims 7 to 9 wherein a radiation energy of 100 keV or more is used for said irradiation.

11. A magnetic recording medium substantially as hereinbefore described with reference to any one of Examples 1 to 3.