GB2130121A

GB2130121A - Magnetic recording medium and method for producing the same

Info

Publication number: GB2130121A
Application number: GB08324679A
Authority: GB
Inventors: Yuichi Kubota; Masaharu Nishimatsu; Toshiaki Ide; Kiyoshi Noguchi; Koji Kobayashi
Original assignee: TDK Corp
Current assignee: TDK Corp
Priority date: 1982-10-06
Filing date: 1983-09-14
Publication date: 1984-05-31
Also published as: GB8324679D0; DE3334305A1; GB2130121B; JPS5965929A

Abstract

A magnetic recording medium comprises in combination a supporting member, a ferromagnetic thin film formed on one surface side of said supporting member, and a back layer provided on the other surface side of the supporting member The back layer is composed of filler material dispersed in a binder containing a radiation-sensitive curable resin, and then cured. <IMAGE>

Description

SPECIFICATION Magnetic recording medium and method for producing the same This invention relates to a magnetic recording medium and a method for producing the same.

More particularly, the invention is concerned with the magnetic recording medium with reduced friction coefficient and suppressed curling on its vapor-deposited surface by applying a back-coat on a thin film type magnetic recording medium obtained from formation of a ferromagnetic thin film on a substrate by means of electro-plating, chemical plating, vacuum-evaporating, sputtering, ion-plating, and so forth, and a method for producig the same.

In these days, magnetic tapes have become widely used in various fields of technology such as audio and video recording/reproducing, and computer soft ware as well. With such varieties of use of the magnetic recording tapes, informations to be recorded in such recording medium has increased year by year with the consequence that the recording medium has become required to have higher and higher recording density.

In the current recording medium utilizing a magnetic head, the spacing loss between the tape and the recording head is expressed as d 54.6 -- [ dB ] (where: d denotes a distance between the tape and the magnetic head, and A represents the recording wavelength). As can be seen from this formula, in a short wavelength recording with a high recording density, the rate of reduction in the record output due to spacing is significantly greater than that of a longer wavelength. As the result of this, even a small foreign substance existing on the surface of the tape is inevitably detected as dropout.

As the possible causes for the dropouts, there may be contemplated the falling-off of magnetic powder from the surface of the magnetic tape coated with such powder material, which results from deterioration of the coated film due to repeated applications of stress, the scraping-off of the base during the tape running, electrostatic adhesion of dust, etc. on the surface of the base, followed by its transfer onto the coated surface, and so forth.In order to prevent these undesirable phenomena, there have been worked out various methods such that, for example, against the latter cause, a coating composition of carbon black, graphite, or the like kneaded with an organic binder, or an antistatic agent is coated on the surface of the substrate opposite to the magnetic surface of the magnetic tape (i.e., the back surface), thereby reducing the electrification phenomenon on the base member, or a coating composition of silicon oxide, etc. kneaded with an organic binder is coated on the surface of the substrate in an attempt to render the base member to be more tenacious and to thereby reduce the scraping-off of the base.While these treatments make it possible to suppress the tendency of increase in the dropouts against the repeated running of the tape to a remarkable extent, the level of the dropouts cannot still be said to be perfectly satisfactory under the existing circumstances, hence further effort in reducing the level is required.

As the result of having made detailed studies of the causes for the dropouts to generate so as to reduce them further, the following facts have been made clear.

Since the back surface (or layer) is required to be sufficiently tenacious so as not to cause the dropouts to increase, even if the frequency of the tape running is increased, a thermosetting resin is usually used as a binding agent. In that case, the tape is wound up on a take-up reel after the back layer has been coated, followed by the thermosetting treatment. However, at the time instant when the coating has just been finished, no curing reaction has yet to take place within the back layer, hence the coated film is still deficient in its strength, and the back layer is in a tight contact with the magnetic surface. On account of this, carbon black, graphite, or other inorganic filler incorporated in the back layer tends to readily migrate from the coated surface to the surface of the magnetic layer on the opposite side, to which the coated surface of the back layer is contacted.Such migrated substance has been found to cause the dropouts and the clogging of the magnetic head. On the other hand, even when the back layer is coated before the magnetic layer, this back layer readily migrates to the base surface, and the migration of such filler material has also been found to be the cause of the dropouts and the clogging of the magnetic tape head in the same manner as mentioned above. It is considered that similar phenomenon may take place even with the thermosetting resin. By providing the back layer, it is possible to restrain this increase in the dropouts due to the repeated running of the tape. It is by this migration of the filler material that the dropouts are not so low at the rate of generation thereof in a stage where the number of times of the running is relatively small.

The present invention has been made with a view to eliminating the abovementioned inconveniences encountered so far in the back layer forming step of the magnetic recording tape, and aims at decreasing the dropouts due to the afore-described causes. Such decrease in generation of the dropouts can be effected by irradiating radiation rays from an active energy ray source onto the back layer after it has been formed with a coating composition of carbon black or graphite or other inorganic filler, which is kneaded with a radiation-sensitive resin (a resin which is curable upon irradiation of it with radiation rays) as a binder, thereby subjecting the back layer to the curing treatment, or by subjecting the back layer to the curing treatment, as it is, after the surface treatment thereof having been effected to produce three-dimensional cross-linking within the back layer to render the coated film to be tough, followed by winding the tape on a take-up reel. According to this method, since it is after the cross-linking reaction of the coated film having been completed that the tape is wound up, irrespective of whether the back layer is coated first, or the magnetic layer, there is no possibility of migration of the filler material from the back layer toward the magnetic layer, even when the back layer comes into close contact with the magnetic layer as the result of the tape winding on the take-up reel.

According to the present invention, in one aspect of it, there is provided a magnetic recording medium which comprises in combination: a supporting member; a ferromagnetic thin film formed on one surface side of said supporting member; and a back layer provided on the other surface side of said supporting member, said back layer being composed of filler material dispersed in a binder containing a radiation-sensitive curable resin, and cured.

According to the present invention, in another aspect of it, there is provided a method for producing a magnetic recording medium comprising a supporting member, a ferromagnetic thin film formed on one surface side of said supporting member, and a back layer provided on the other surface side of said supporting member, said method comprising: dispersing a filler material in a binder containing a radiation-sensitive curable resin; forming the back layer with the material; and thereafter irradiating active energy rays onto said back layer to cure said radiation-sensitive resin in said binder.

The foregoing object, other objects as well as specific ingredients for the supporting member (or substrate), the ferromagnetic thin film, and the back layer for the magnetic recording medium, and the performance thereof according to the present invention will become more apparent and understandable from the following detailed description thereof, when read in conjunction with the accompanying drawing.

In the accompanying drawing, a single figure is a graphical representation showing a relationship between the dropouts and the number of times of the tape running measured on various test specimens.

In the following, the present invention will be described in specific details as to its concrete construction.

The radiation-sensitive resin to be used in the present invention is such one that contains in the chain of its molecules two or more unsaturated double bonds which generate radicals by the radiation rays to form a cross-linked structure. This radiation-sensitive resin can also be obtained by modifying a thermoplastic resin to become radiation-sensitive.

A concrete example of the modification for the radiation-sensitivity is to introduce into the molecules those groups which are cross-linkable or polymerizable by irradiation of the radiation rays, such as acrylic double bond showing the unsaturated double bond having the radical polymerizability like acrylic acid, methacrylic acid, or ester compounds of these acids; allyl-type double bond like diallyl phthalate; unsaturated bonds of, for example, maleic acid, maleic acid derivatives, etc.; and others.

Besides these, any other unsaturated bonds which can be cross-linked and polymerized by irradiation of the radiation rays may also be used for the purpose.

Examples of the thermoplastic resins which can be modified into the radiation-sensitive resins are enumerated in the following.

(I) Vinyl chloride type copolymers: There may be exemplified the following synthetic resin polymers as belonging to this group: copolymers of vinyl chloride, vinyl acetate, and vinyl alcohol; copolymers of vinyl chloride and vinyl alcohol; copolymers of vinyl chloride, vinyl alcohol, and vinyl propionate; copolymers of vinyl chloride, vinyl acetate, and maleic acid; copolymers of vinyl chloride, vinyl acetate, and OH-terminated, alkyl group side chain such as, for example, VROH, VYNC VYEGX, etc. manufactured and sold by UCC (Union Carbide Corporation, U.S.A.), and VERR, etc. also manufactured and sold by UCC.

The modification of these thermoplastic resins to be radiation-sensitive is effected by introducing into the above-enumerated copolymers the acrylic double bonds, maleic acid type double bonds, allylic type double bonds, and so forth through the process steps to be described later.

(oil) Unsaturated polyester resins: There may be exemplified the following synthetic resin polymers as belonging to this group: saturated polyester resins to be obtained by the ester bonding between saturated polybasic acids such as phthalic acid, isophthalic acid, terephthalic acid, succinic acid, adipic acid, sabacic acid, and so forth and polyhydric alcohols such as ethylene glycol, diethylene glycol, glycerin, trimethylol propane, 1 ,2- propylene glycol, 1 ,3-butanediol, dipropylene glycol, 1 ,4-dutanediol, 1 ,6-hexanediol, pentaerythritol, sorbitol, neopentyl glycol, 1 4-cyclohexane dimethanol, and so forth; and resins obtained by modification of these polyester resins with SO3Na, etc. (e.g. Vylon 53S).

These resins are modified to be radiation-sensitive by the process steps to be mentioned hereinafter.

(III) Unsaturated polyester resins: There may be exemplified, as belonging to this group, those polyester compounds containing radiation-curable unsaturated double bonds in the chain of its molecules such as, for example, unsaturated polyester resins, prepolymers, and oligomers which are the saturated polyester resins composed of the ester bonds of polybasic acids and polyhydric alcohols as referred to the thermoplastic resins in the abovementioned group (II) with the exception that a part of the polybasic acid has been replaced by maleic acid.

Examples of the polybasic acids and polyhydric alcohols for the saturated polyester resins include those compounds as enumerated in the abovementioned group (I), and examples of the radiationcurable unsaturated double bonds include maleic acid, fumaric acid, and so forth.

The radiation-curable unsaturated polyester resin can be produced in the following manner: that is to say, maleic acid, fumaric acid, or the like is added to one or more kinds of polybasic acid components and one or more kinds of polyhydric alcohol components, and then the batch is subjected to dehydration or dealcoholization reaction by the conventional manner, i.e., at a temperature range from 180 to 200 degress Centigrade, in the nitrogen atmosphere and in the presence of a catalyst, thereafter the temperature is elevated to a range of from 240 to 280 degrees Centigrade, at which temperature level the condensation reaction is effected under a reduced pressure of from 0.5 to 1 mm Hg, thereby obtaining a polyester resin.The content of maleic acid, fumaric acid, or the like may be in a range of from 1 to 40 mol%, or preferably from 10 to 30 mol%, in consideration of the degree of cross-linking at the time of its production, the radiation-curability, and so on.

(IV) Polyvinyl alcohol type resins: There may be enumerated the following synthetic resin polymers as belonging to this group: polyvinyl alcohol, butyral resins, acetal resins, formal resins, and copolymers of these components.

The hydroxy groups contained in these resins are modified to be radiation-sensitive by the process steps to be described hereinafter.

(V) Epoxy type resins and phenoxy resins: There may be exemplified the following synthetic resins as belonging to this group: epoxy resins obtained by the reaction of bisphenol A and epichlorohydrin, methylepichlorohydrin, or the like, such as EPIKOTE R.T.M. 152, 154, 828, 1001, 1004, 1007 (manufactured and sold by Shell Chemical Company);DEN431, DER732, DER511, DER331 (manufactured and sold by Dow Chemical Company); EPICLON-400 and EPICLON-800 (manufactured and sold by Dai-Nippon Ink K.K.); phenoxy resins such as PKHA, PKHC, PKHH which are the highly polymerized resins among the abovementioned epoxy resins, and are manufactured and marketed by Union Carbide Corporation; and copolymers of brominated bisphenol A and epichlorohydrin such as EPICLON 145, 1 52, 1 53, and 1120 (manufactured and sold by Dai-Nippon Ink s Chemicals Co.) and others.

The modification of these resins to be radiation-sensitive is effected in utilization of the epoxy group contained in these resins.

(lav) Cellulose derivatives: Cellulose derivatives of various molecular weights are also effective as thermoplastic components.

Particularly effective and preferably among these cellulose derivatives are nitrocellulose, cellulose aceto-butylate, ethyl-cellulose, butyl-cellulose, acetyl-cellulose, and so forth.

Those cellulose derivatives are modified to be radiation-sensitive in utilization of the hydroxyl group in the resin through the process steps to be mentioned hereinafter.

Besides the above, the resins which may also be used effectively for the modification to be radiation-sensitive are polyfunctional polyester resins, polyether ester resins, polyvinyl pyrrolidone resins and derivatives thereof (e.g., PVP olefin copolymers), polyamide resins, polyimide resins, phenol resins, spiro-acetal resins, acrylic resins containing therein one or more kinds of acrylic or methacrylic acid ester having a hydroxyl group as the polymerization component, and others.

Further, by blending a thermoplastic elastomer or prepolymer with the above-described radiationsensitive, modified thermoplastic resins, it is possible to make the coating film much more tenacious.

Furthermore, when these elastomers or prepolymers are modified to be radiation-sensitive in the same manner as mentioned above, a better result can be obtained, as will be described hereinbelow.

In the following, examples of the elastomers and prepolymers which may be combined with the above-described radiation-sensitive resins are enumerated.

(I) Polyurethane elastomers, prepolymers and telomers: The use of polyurethane elastomers is particularly effective in respect of that their abrasion resistance and adhesivity to PET films are satisfactory.

Examples of such effective urethane compounds are: polyurethane elastomers, prepolymers, and telomers which are composed of polycondensates of various polyhydric isocyanates, as the isocyanate components, such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 1 ,3-xylene diisocyanate, 1 4- xylene diisocyanate, 1 ,5-naphthalene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, 3,3'-dimethyl-4,4'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 3,3'-dimethylbiphenylene diisocyanate, 4,4'-biphenylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane diisocyanate, Desmodur L, Desmodur N, and so on; and various polyesters such as linear saturated polyesters (e.g. those obtained by polycondensation of polyhydric alcohols such as ethylene glycol, diethylene glycol, glycerin, trimethylolpropane, 1,4- butanediol, 1 6-hexanediol, pentaerythritol, sorbitol, neopentyl glycol, 1 ,4-cyclohexane dimethanol, and so forth with saturated polybasic acids such as phthalic acid, isophthalic acid, terephthalic acid, succinic acid, adipic acid, sebasic acid, and so forth), linear saturated polyethers (e.g. polyethylene glycol, polypropylene glycol, polytetraethylene glycol, and so forth) as well as caprolactam, hydroxyl-containing acrylic acid esters, hydroxyl-containing methacrylic acid esters, and so forth.

While these elastomers may be combined, as they are, with various radiation-sensitive thermoplastic resins, it will be highly effective to further react these elastomers with a monomer having an acrylic double bond, an allylic double bond, or the like which reacts with the terminal isocyanate group or hydroxyl group in the urethane elastomer, to thereby modify the resins to be radiationsensitive.

(II) Elastomers of copolymers of acrylonitrile and butadiene: Prepolymers of acrylonitrile-butadiene copolymer containing therein a terminal hydroxyl group, such as "poly BD Liquid Resin" produced by Sinclair Petrochemicals Corp. and available in general market, or elastomers such as "Hycar R.T.M. 1 432J" manufactured and sold by Japanese Zeon K.K., and so forth are particularly suitable as the elastomeric components, with which the double bonds in butadiene produce radicals by the radiation rays to cause cross-linking and polymerization.

(III) Polybutadiene elastomers: Low molecular weight prepolymers having the terminal hydroxyl groups, such as "Poly BD liquid Resin R-15", etc. manufactured and sold by Sinclair Petrochemicals Corp. are particularly suited in respect of their compatibility with thermoplastic resins. In the "R-15" prepolymer, since the terminal end of the molecule is occupied by the hydroxyl group, it is possible to enhance the radiation sensitivity by adding an acrylic type unsaturated double bond to the terminal end of the molecule, whereby the prepolymers become much more advantageous as the binder.

Further, cyclized products of polybutadiene such as, for example, "CBR-M9O1" manufactured and sold by Japan Synthetic Rubber Co. also exhibit excellent performance by their combination with the thermoplastic resins. In particular, cyclized polybutadienes are highly efficient in their cross-linking and polymerization underthe radiation rays due to the radicals of the unsaturated bonds inherent in polybutadiene, so that they have excellent properties as the binder.

Besides the above, suitable among other thermoplastic elastomers and their prepolymers are: styrene-butadiene rubbers, chlorinated rubbers, acrylic rubbers, isoprene rubbers and their cyclized products (e.g. "C1R701 ", a product of Japan Synthetic Rubber K.K.), and further those elastomers such as epoxy-modified rubbers, internally plasticized saturated linear polyesters (e.g. "Vylon &num;300", a product of Toyo Spinning K.K.), and so forth may also be used effectively by subjecting them to the modifying treatment for the radiation-sensitization to be described hereinbelow.

In the following, several examples of synthesizing the radiation-sensitive binders will be explained.

Production of tolylene diisocyanate adduct a) Synthesis of an acryl-modified product of a vinyl chloride/vinyl acetate copolymer type resin (radiation-sensitive modified resin): 750 Parts by weight of Vinylite VAGH, 1250 parts by weight of toluene, and 500 parts by weight of cyclohexanone were charged into a four-necked flask of a 5-liter capacity and dissolved under heat.

After raising the temperature to 800C, 61.4 parts by weight of 2-hydroxyethyl methacrylate adduct of tolylene diisocyanate was added. Further,0.012 part by weight of tin octylate and 0.012 part by weight of hydroquinone were added, and the reaction was carried out at a temperature of 800C in a nitrogen (N2) current until the rate of reaction of NCO reached 90%. After completion of the reaction, the reaction system was cooled and diluted by addition of 1250 parts by weight of methylethyl ketone.

*production of 2-hydroxyethyl methacrylate (2HEMA) adduct of tolylene diisocyanate (TDI): 348 Parts by weight of tolylene diisocyanate was heated to a temperature of 800C ina fournecked flask of one-liter capacity with use of nitrogen (N2) current. Thereafter, 260 parts by weight of 2hexamethylene methacrylate, 0.07 part by weight of tin octylate, and 0.05 part by weight of hydroquinone were added dropwise into a reaction vessel, while controlling the temperature inside the reaction vessel to be maintained in a range of from 80 to 850C. After completion of the dropping, the reactants were stirred for three hours at 800C to complete the reaction. After termination of the reaction, the reaction product was taken out of the reaction vessel and cooled. As the result, 2- hydroxyethyl methacrylate (2HEMA) adduct of tolylene diisocyanate (TDI) in white paste was obtained.

b) Synthesis of an acryl-modified product of a butyral resin (radiation-sensitive modified resin) 100 Parts by weight of a butyral resin, "BM-S", produced by Sekisui Chemical Co. was charged into a four-necked flask of a 5-liter capacity together with 1 91.2 parts by weight of toluene and 71.4 parts by weight of cyclohexanone, followed by dissolving the reactants under heat. After raising the temperature to 800 C, 7.4 parts by weight of 2-hydroxyethyl methacrylate adduct of tolylene diisocyanate was added to the reactants, followed by further addition of 0.01 5 part by weight of tin octylate and 0.015 part by weight of hydroquinone, after which the reaction was effected at 800C in a nitrogen (N2) stream until the rate of reaction of NCO reached 90% or higher.After termination of the reaction, the reaction product was cooled and diluted with methyl ethyl ketone.

c) Synthesis of acryl-modified product of a saturated polyester resin (radiation-sensitive modified resin) 100 Parts by weight of "Vylon RV-200", a product of Toyo Spinning Co., was dissolved under heat in 11 6 parts by weight of toluene and 11 6 parts by weight of methyl ethyl ketone. After elevating the temperature to 800C, 3.55 parts by weight of 2HEMA adduct of TDI was added followed by further addition of 0.007 part by weight of tin octylate and 0.007 part by weight of hydroquinone, and the reaction was carried out at 800C in a nitrogen (N2) stream until the rate of reaction of NCO reached 90% or higher.

d) Synthesis of acryl-modified product of an epoxy resin (radiation-sensitive modified resin) 400 Parts by weight of "Epikote 1007" a product of Shell Chemical Co., was dissolved under heat into 50 parts by weight of toluene and 50 parts by weight of methyl ethyl ketone. Thereafter, 0.006 part by weight of N,N-dimethylbenzylamine and 0.003 part by weight of hydroquinone were added to the reactant, and the temperature was elevated to 800 C. Then, 69 parts by weight of acrylic acid was added dropwise and the reaction was carried out at 800C until the acid value became 5 or below.

e) Synthesis of an acryl-modified product of a urethane elastomer (radiation-sensitive elastomer) 250 Parts by weight of an isocyanate-terminated diphenylmethane diisocyanate (MDI) type urethane prepolymer, "Nipporan 4040", a product of Nippon Polyurethane Industry Co., 32.5 parts by weight of 2HEMA, 0.07 part by weight of hydroquinone, and 0.009 part by weight of tin octylate were placed in a reaction vessel, and dissolved under heat of 800 C. After the dissolution, 43.5 parts by weight of TDI was added dropwise into the reaction vessel, while controlling the inside of the reaction vessel to be maintained at a temperature range of from 80 to 900C. After completion of the dropping, the reaction was conducted at 800C until the rate of reaction of NCO became 95% or higher.

f) Synthesis of an acryl-modified product of a polyether type terminated urethane-modified elastomer (radiation-sensitive elastomer) 250 Parts by weight of a polyether, "PTG-500", a product of Nippon Polyurethane Industry, 32.5 parts by weight of 2HEMA, 0.007 part by weight of hydroquinone, and 0.009 part by weight of tin octylate were placed in a reaction vessel, and dissolved under heat of 80 C. After the dissolution, 43.5 parts by weight of TDI was added dropwise into the reaction vessel, while controlling the inside of the reaction vessel to be maintained at a temperature in a range of from 80 to 900C. After completion of the dropping, the reaction was conducted at 800C until the rate of reaction of NCO became 95% or higher.

g) Synthesis of an acryl-modified product of a polybutadiene elastomer (radiation-sensitive elastomer) 250 parts by weight of a low molecular weight hydroxyl-terminated polybutadiene, "Poly-BD Liquid Resin R-15", a product of Sinclair Petrochemicals, Inc., 32.5 parts by weight of 2HEMA, 0.007 part by weight of hydroquinone, 0.009 part by weight of tin octylate were placed in a reaction vessel, and dissolved under heat of 800C. After the dissolution, 43.5 parts by weight of TDI was added dropwise, while controlling the inside of the reaction vessel to be maintained at a temperature range of from 80 to 900C. After completion of the dropping, the reaction was conducted at 800C until the rate of reaction of NCO became 90% or higher.

Besides the above-enumerated modified products, there have been known those high-polymer substances which degrade by irradiation of radiation rays, and which bring about cross-linking among molecules by the irradiation of radiation rays. Examples of those that bring about the cross-linking among the molecules are: polyethylene, polypropylene, polystyrene, polyacrylic acid ester, polyacrylamide, polyvinyl chloride, polyester, polyvinyl pyrrolidone rubbers, polyvinyl alcohol, polyacrolein, etc. Since such cross-linking type polymers can bring about the cross-linking reaction without subjecting the same to the particular modifying treatment as mentioned in the foregoing, they are able to be used, as they are, as the back coating resins for the cross-linking by radiation.

Furthermore, according to this method, even a non-solvent type resin which does not need to use any solvent can be cured in a short period of time, so that it is also possible to use such resin for the purpose of the back coating.

Moreover, as the active energy rays to be used for cross-linking of the back coat according to the present invention, there may be employed electron beam generated from a radiation accelerator as the source of radiation, y-rays generated from Co60 as the source of radiation, ss-rays generated from Sr90 as the source of radiation, X-rays generated from an X-ray generator as the source of radiation, ultraviolet rays, and so forth.

In particular, from the standpoints of control of the absorption dose, introduction into the production line, interception of ionized radiation, and so forth, use of the radiation rays from the radiation accelerator as the source of irradiation is advantageous.

The characteristics of the radiation rays to be used at the time of curing the back layer should preferably be such that, from the aspect of the penetrating power, the irradiation may be done by use of the radiation accelerator having an acceleration voltage of from 100 to 750 KV, or preferably from 1 50 to 300 KV, and the absorption dose in a range of from 0.5 to 20 mega-rad.

For the curing of the back layer according to the present invention, a low dose type radiation accelerator (an electro-curtain system) produced by Energy Science Co., U.S.A. is extremely advantageous from the points of its introduction into the tape coating process line, interception of the secondary X-rays within the accelerator, and others.

Needless to say, it is possible to use a Van de Graaff type accelerator which has so far been used widely as the radiation accelerating apparatus.

Further, at the cross-linking by radiation, it is important to irradiate the back layer with the radiation rays in an inert gas stream such as nitrogen (N2) gas, helium (He) gas, and so forth. Irradiation of the radiation rays in the air is extremely disadvantageous because 03, etc. generated by irradiation of the radiation rays at the time of cross-linking of the binder components inhibit the radicals generated in the polymer to advatageously act on the cross-linking reaction.

It is therefore important that the atmosphere at a portion where the active energy rays are irradiated be maintained in an inert gas atmosphere such as N2, He, CO2, and so on with the oxygen concentration being as particularly low as 5% at its maximum.

Examples of the filler to be used together with the abovementioned binder include: 1) graphite and carbon black having electric conductivity; and 2) inorganic fillers such as SiO2,TiO2,Al203, Cr203, SiC, CaCO3, zinc oxide, geothite, aFe2O3, talc, kaolin, Cay04, boron nitride, Teflon R.T.M. powder, graphite fluoride, molybdenum disulfide, and so on. The quantity of such fillers to be used should appropriately be from 20 to 100 parts by weight with respect 1 00 parts by weight of the binder in the case of using the abovementioned electrically conductive fillers 1), and from 10 to 300 parts by weight in the case of using the inorganic fillers 2).When the quantity of the filler is too large, there will be resulted disadvantages such that the coating film becomes brittle and the amount of dropouts increases accordingly.

The magnetic tapes, on which the above-described back layer is to be provided, include audio tapes, video tapes, tapes for computers, endless tapes, and so forth. Of these, use of such back layer on the video tapes and the tapes for computer use, where the dropout constitutes one of the most important characteristics thereof, produces considerable effect.

Incidentally, the ferromagnetic thin film magnetic recording medium can be obtained by any of the various methods such as electric plating, chemical plating, vacuum evaporation, sputtering, ion-plating, and others. Taking the vacuum evaporation method as an example, an ingot of cobalt/nickel alloy (atomic ratio of 8/2) is prepared, and then the ferromagnetic thin film in a long length is formed on a polyethylene terephthalate base by the vacuum evaporation method. The evaporative deposition is effected by the electron beam heating. In this case, the vacuum deposition method is the so-called "slant evaporation method", in which the center incident angle of the electron beam is maintained at 70 degrees. For cooling the base film, a cylindrical can is used, wherein the cooling temperature is kept at 50C.The vacuum vessel is evacuated to a pressure value of 3 x 10-3 Pa, into which oxygen gas is introduced until its pressure becomes 6.3 x 10-2 Pa, thereby carrying out the evaporative deposition.

The thickness of the coating film is maintained at approximately 800 A by adjusting the driving speed of the base film and the power to be imparted to an electron gun.

The ferromagnetic thin film to be obtained in this manner has a coercive force of approximately 1,000 Oe and Br of about 8,000 G, which is favorably suited as the magnetic recording medium for video-recording.

With a view to enabling those persons skilled in the art to more readily put the present invention into practice, several preferred examples are presented hereinbelow. It should, however, be noted that these examples are merely illustrative and not so restrictive, and that any changes and modifications in the ingredients used and their rate of mixing as well as the manner of the treatment may be made within the spirit and scope of the invention as recited in the appended claims.

EXAMPLE 1 Ingredients Parts by Weight Carbon black "ASAHI HS500" (particle diameter of 81 my) produced by Asahi Carbon Co. 50 Copolymer of acryl-modified vinyl chloride, vinyl acetate, and vinyl alcohol (experimental product) 30 Acryl-modified polyurethane elastomer (experimental product) 20 Mixed solvent (MlBK/toluene=1/1) 300 The mixture of the above ingredients was dispersed in a ball mill for five hours, and the thus dispersed mixture was coated on the rear surface of a polyester film, on which a magnetic surface had already been formed, in such a manner that the thickness of the back layer upon its drying may be 3 microns.Thereafter, this back layer was irradiated with electron beam in a nitrogen (N3) gas using an electron curtain type electron beam accelerator with an accelerating voltage of 1 50 KV, an electrode current of 10 mA, and an absorption dose of 10 mega-rad, thereby curing the layer. After the curing, the coating film was wound up on a reel, cut into a video-tape width of 1/2 inch, followed by measurement of the dropouts with use of a VHS deck equipment. This sample is designated as Sample No. 1.

EXAMPLE 2 Ingredients Parts by weight SiO2 (particle diameter of 2 microns) 50 Copolymer of acryl-modified vinyl chloride, vinyl acetate, and vinyl alcohol (experimental product) 30 Acryl-modified polyurethane elastomer (experimental product) 20 Mixed solvent 300 The mixture of the above ingredients was prepared in the same manner as in Example 1 above, thereby obtaining Sample No. 2.

EXAMPLE 3 Ingredients Parts by Weight Carbon black "ASAHI HS500" a product of Asahi Carbon Co. 50 Acryl-modified polyester resin (experimental product) 60 Mixed solvent 300 The mixture of the abovementioned ingredients was prepared in the same manner as in Example above, thereby obtaining Sample No. 3.

EXAMPLE 4 Ingredients Parts by Weight Carbon black "ASAHI He500" a product of Asahi Carbon Co. 50 Acryl-modified polyurethane elastomer (experimental product) 30 Copolymer of vinyl chloride, vinyl acetate, and vinyl alcohol ("VAGH" produced by UCC) 70 Mixed solvent 300 The mixture of the abovementioned ingredients was prepared in the same manner as in Example 1 above, thereby obtaining Sample No. 4.

COMPARATIVE EXAMPLE 1 Ingredients Parts by Weight Carbon black "ASAHI HS500" 50 Copolymer of vinyl chloride, vinyl acetate, and vinyl alcohol ("VAGH" produced by UCC) 30 Polyurethane elastomer ("N5033" produced by Nippon Polyurethane Co.) 20 Mixed solvent 300 The abovementioned mixture of the ingredients was prepared into paint composition in the same manner as in Example 1 above, after which 10 parts by weight of isocyanate ("COLONATE L" produced by Nippon Polyurethane Co.) was added, and the mixture was coated on a base material in such a manner that thickness of the coated film upon its drying may be 3 microns. Thereafter, the coated film was allowed to stand for 24 hours at a temperature of 600C to subject it to the thermosetting treatment. The thus treated film was then wound up on a reel and cut into a width of 1/2 inch.This sample is designated as Sample No. 5.

COMPARATIVE EXAMPLE 2 The same magnetic tape as those used in the foregoing Examples and Comparative Example 1, with the exception that no back coating was applied thereto, was designated as Sample No. 6. This is a video tape for VHS and made of a polyester film, on which a magnetic coating film has been applied.

The results of dropouts of the above samples are shown in the following Table 1.

TABLE 1

W Sample No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 t No. ofdropouts in an average length of the total tenvolumes of the 10 1 8 12 15 25 13 sample tapes (l /min.) 15 lisec or longer 18 dB or more 200C, 60% of relative humidity, and VHS deck As can be seen from Table 1, Sampels No. 1 through No. 4 which have been subjected to the radiation curing treatment indicate much less dropouts as compared with the ordinarythermosetting type sample of No. 5.Further, the relationship between the number of tape runs and the dropouts is as shown in Figure 1.

From Figure 1, it can be seen that the tendency of increase in the dropouts with respect to the number of runs of the samples with the back coating having been applied thereon is much less than that of the non-treated samples. Furthermore, those samples which have been subjected to the radiation curing treatment maintain their initial low number of dropouts, as they are, even when they are compared with the conventional heat-cured samples. This low value of the dropout is still retained even after 100 times of the tape run.

It goes without saying that those dispersants and lubricants which have been used conventionally can also be used to the same extent even in such back layer forming method. In the following Table 2, the conditions (or outer appearance) of the wound tapes are shown after they have made FF-running on a VHS deck.

TABLE 2

Sample No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 Wound Condition of Tapes Excellent Pass Excellent Excellent Excellent Poor Evaluation in four ranks: Excellent; Good; Pass; Poor.

From these results, it is seen that those tapes with the back layer containing therein carbon black exhibit highly excellent wound conditions with suppressed curling on the deposited surface, and that those tapes with the back layer added with SiO2 also give better results than those having no back coating.

(N.B.) Method of measurement of the number of dropouts: The number of dropouts was counted on the tape which had recorded and reproduced a single signal of 4 MHz, wherein the signal as reproduced showed 15 micro-second or longer in the time period, during which its level lowered by 18 dB or more the average reproduction level.

Claims

1. A magnetic recording medium which comprises in combination: a supporting member; a ferromagnetic thin film formed on one surface side of said supporting member; and a back layer provided on the other surface side of said supporting member, wherein said back layer is composed of filler material dispersed in a binder containing a radiation-sensitive curable resin, and then cured.

2. The magnetic recording medium according to Claim 1, wherein said filler material is electrically conductive powder of a material selected from the group consisting of carbon black, graphite, and the like.

3. The magnetic recording medium according to Claim 1 , wherein said filler material is powder of highly tenacious materials selected from the group consisting of SiO3,TiO2, Al2 03, Cr2O3, SiC, CeO2, CaCO3, zinc oxide, geothite, aFe2O3, talc, kaolin, CaSO4, boron nitride, Teflon powder, graphite fluoride, molybdenum disulfide, and zirconia.

4. The magnetic recording medium according to Claim 1, wherein said filler material is a mixture of said electrically conductive substance of carbon black, graphite, and the like, and powder of highly tenacious material of SiO2, Tit3, Awl203, Cr2O3, SiC, CeO2, CaCO3,zin oxide, geothite, aFe2O3, talc, kaolin, CaSO4, boron nitride, Teflon powder, graphite fluoride, molybdenum disulfide, zirconia, and so forth.

5. A method for producing a magnetic recording medium comprising a supporting member, a ferromagnetic thin film formed on one surface side of said supporting member, and a back layer provided on the other surface side of said supporting member, said method comprising: dispersing a filler material in a binder containing therein a radiation-sensitive curable resin; forming the back layer with said bound material; and thereafter irradiating active energy rays onto said back layer to cure said radiation-sensitive resin.

6. The method for producing the magnetic recording medium according to Claim 5, wherein said filler material is electrically conductive powder of a material selected from the group consisting of carbon black, graphite, and the like.

7. The method for producing the magnetic recording medium according to Claim 5, wherein said filler material is powder of highly tough materials selected from the group consisting of SiO,, TiO Al2O3, Cr2O3, SiC, CeO2 CaCO3, zinc oxide, geothite, c#Fe2O3, talc, kaolin, Cay04, boron nitride, Teflon powder, graphite fluoride, molybdenum disulfide, zirconia, and the like.

8. The method for producing the magnetic recording medium according to Claim 5, wherein said filler material is a mixture of said electrically conductive substance of carbon black, graphite, and the like, and powder of highly tenacious material of SiO2,TiO2,Al2 As203, Cr2O3, SiC, CeO2, CaCO3, zinc oxide, geothite, #Fe2O3, talc, kaolin, CaSO4, boron nitride, Teflon powder, graphite fluoride, molybdenum disulfide, zirconia, and so forth.

9. The method for producing the magnetic recording medium, according to Claim 5, wherein said irradiation of the back layer is effected in an inert gas atmosphere by use of an electron beam accelerator having an acceleration voltage of from 100 to 750 KV and an absorption dose of from 0.5 to 20 mega-rad.

10. A magnetic recording medium, substantially as described.