GB2146269A - Magnetic recording disc and process for its production - Google Patents

Abstract

A magnetic recording disc which comprises a supporting member, a magnetic layer formed on one side of said supporting member, and a backing layer formed on the other side of said supporting member, wherein said backing layer is composed of a binder containing radiation-sensitive curable resin, and a filler material dispersed in the binder.

Description

SPECIFICATION Magnetic recording disc and process for its production This invention relates to a magnetic recording disc and a process for its production. More particularly, the present invention is concerned with a magnetic recording disc whereby problems such as curling or dropout of the disc have been solved.

In these days, magnetic discs have been widely used in the fields of computers and magnetic cameras. Accordingly, the amount of information to be recorded on such magnetic discs has increased year after year. As a result, there is an increasing demand for magnetic recording discs having a high recoridng density.

The higher the recording density becomes, the more pronounced the problem of dropouts becomes. Namely, in the current recoridng system wherein a magnetic head is utilized, the spacing loss between the disc and the recoridng head may be represented by 54.6 d/A [ dB ] where d represents a distance between the disc and the magnetic head, and A represents the recording wavelength. It should be understood from this formula that in a short wavelength recording with a high recording density, the rate of reduction in the output due to spacing is significantly greater than that of a longer wavelength. Accordingly, even a small foreign matter existing on the surface of the disc is inevitably detected as a dropout.

As possible causes for the dropouts, there may be mentioned the falling-off of magnetic powder from the surface of the magnetic disc coated with such powder material, which results from deterioration of the coated film due to repeated applications of stress, or the scraping-off of the base material during the disc running, and consequential electrostatic adhesion of the powder, dusts or the like on the surface of the supporting member, and their transfer from the supporting member onto the coated magnetic layer surface.In order to prevent these undesirable phenomena, there have been proposed various methods, for example, against the former cause, some proposals have been made to improve the toughness of the coated layer, and 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 supporting member opposite to the magnetic surface of the magnetic disc (i.e., the rear surface), thereby reducing the electrostatic phenomenon on the supporting member, or a coating composition of e.g.

silicon oxide kneaded with an organic binder is coated on the surface of the supporting member in an attempt to render the supporting member to be more tenacious and thereby to reduce the abrasion of the base material.

While these treatments are effective to suppress the tendency for an increase in the dropouts by the repeated running of the discs to a remarkable extent, the level of the dropouts cannot still be said to be adequately low under the existing circumstances, and it is necessary to further reduce the droupouts. In an attempt to reduce the dropouts, detailed studies of the causes of the dropouts have been made, and the following facts have been made clear.

With respect to the formation of a backing layer, if the backing layer is formed prior to the formation of the magnetic layer, it is likely that the irregularities of the backing layer surface will be transferred onto the magnetic layer during the surface smoothing opeation by calender treatment, and it will thereby be difficult to obtain an adequate smooth magnetic layer surface.

Thus, it is usual that a magnetic layer is fist formed on a supporting member, and then back coating treatment is applied onto the rear side of the supporting member. Since the backing layer is required to be sufficiently tenacious so as to prevent the dropouts even when the frequency of the disc running is increased, it is usual to employ a thermosetting resin as a binder for the backing layer. In that case, after the application of the backing layer, the tape, from which discs are to be punched out, will be wound up on a take-up reel and then subjected to thermosetting treatment. However, at the time when the coating has been just finished, no curing reaction has yet been started in the backing layer, and the coated layer is still weak. Yet, the backing layer is in close contact with the magnetic layer in the rolled-up condition.

Accordingly, the carbon black, graphite or other inorganic filler incorporated in the backing layer is likely to migrate from the coated surface to the surface of the magnetic layer which is in contact with the coated surface of the backing layer. It has been found that such migrated substance causes the dropouts or the clogging of the magnetic head. It is considered that similar phenomenon may take place even when thermoplastic resin is used.

The present invention has been made with a view to eliminating the above-mentioned inconveniences encountered so far in the backing layer-forming step, and aims at reducing the dropouts due to the above-mentioned causes. Namely, according to the present invention, a backing layer is formed with a coating composition obtained by kneading a binder of a radiation sensitive resin (a resin which is curable when irradiated with radiation rays) with carbon black, graphite or other inorganic filler, and then radiation rays are irradiated thereto from the active energy ray source for curing treatment, or such curing treatment is conducted after the surface treatment, thereby to form a three dimensional crosslinking in the backing layer and thus render the coated layer strong before the tape is wound-up, whereby the dropouts can be minimized.

According to this method, it is after the completion of the crosslinking reaction of the coated film that the tape is wound-up, and accordingly even if the backing layer is in close contact with the magnetic layer, no migration from the backing layer to the magnetic layer will take place.

Curling of the magnetic disc is another important problem. In a one-side type magnetic disc, the curling is believed to be caused by a dynamic unbalance as between the coated layer and the supporting member. Namely, the coated layer has a Young's modulus greater than the Young's modulus of the supporting member, and therefore the disc tends to flex towards the coated layer side. The curling gives adverse effects to the head touch and the running property, and therefore should be reduced as far as possible. It is conceivable that by forming a backing layer containing carbon black, graphite or other inorganic filler on the side of the supporting member opposite to the magnetic layer so as to dynamically balance the magnetic layer, the supporting member and the backing layer, it is possible to minimize the curling of the disc which creates problems in the heat touch the running property.However, in the case of thermosetting binder, the thermosetting treatment is conducted in a rolled condition, whereby curling is likely to form in the direction of the roll winding, or shrinkage of the supporting member is likely to be led to increase the curling. Whereas, in the case of electron beam curable binder, the curling can be conducted on-line, whereby no problem in the case of thermosetting will be involved, and the curling can advantageously be minimized.

Thus, in the magnetic disc with a magnetic layer on one side of the supporting member, it is possible to simultaneously solve the problem of the curling of the disc and the problem of the dropouts by forming a backing layer with a filler incorporated in a binder, on the rear side of the supporting member, wherein a radiation sensitive curable resin is used as the binder.

Thus, the present invention provides a magnetic recording disc which comprises a supporting member, a magnetic layer formed on one side of said supporting member, and a backing layer formed on the other side of said supporting member, wherein said backing layer is composed of a binder containing a radiation-sensitive curable resin, and a filler material dispersed in the binder.

Further, the present invention provides a process for producing a magnetic recording disc, which comprises forming a magnetic layer on one side of a supporting member, then applying onto the other side of the supporting member, a coating material prepared by dispersing a filler material in a binder containing a radiation-sensitive curable resin, to form a backing layer, and thereafter irradiating active energy rays onto said backing layer to cure said radiation-sensitive curable resin.

The radiation curable or sensitive resin to be used in the present invention, is usually a resin containing at least two unsaturated double bonds in its molecular chain, which are capable of generating radicals for crosslinking when irradiated. Such a resin may also be obtained by subjecting a thermoplastic resin to radiation sensitive modification.

The radiation sensitive modification can be conducted, for instance, by introducing into the molecule a radiation cross-linkable or polymerizable group having a radical polymerizable unsaturated double bond such as an acrylic double bond attributable to e.g. acrylic acid, methacrylic acid or their ester compounds, an allyl-type double bond attributable to e.g.

diallylphthalate, or an unsaturated bond attributable to e.g. maleic acid or maleic acid derivatives. Any other radiation cross-linkable or polymerizable unsaturated double bond may likewise be employed.

Examples of the thermoplastic resins which can be modified into the radiation-sensitive resins will be given below.

(I) Vinyl chloride type copolymers: There may be mentioned a vinyl chloride-vinyl acetate-vinyl alcohol copolymer, a vinyl chloride-vinyl alcohol copolymer, a vinyl chloride-vinyl alcohol-vinyl propionate copolymer, a vinyl chloride-vinyl acetate-maleic acid copolymer, a vinyl chloride-vinyl acetate-OH-terminated side chain alkyl group copolymer, such as VROH, VYNC or VYEGX, manufactured by UCC (Union Carbide Corporation, U.S.A.), and a maleic acid modified VERR also manufactured by UCC.

These copolymers can be modified to radiation-sensitive resins by introducing into them acrylic double bonds, maleic double bonds or allyl-type double bonds by the process described later.

(II) Unsaturated polyester resins: There may be mentioned saturated polyester resins obtained by the esterification of saturated polybasic acids such as phthalic acid, isophthalic acid, terephthalic acid, maleic acid, maleic acid derivatives, succinic acid, adipic acid or sebacic acid, with polyhydric alcohols such as ethylene glycol, diethylene glycol, glycerol, trimethylol propane, 1 ,2-propylene glycol, 1,3-butanediol, dipropylene glycol, 1,4-butanediol, 1,6-hexanediol, pentaerythritol, sorbitol, glycerin, neopentyl glycol or 1,4-cyclohexane dimethanol, and resins obtained by the modification of these polyester resins with e.g. SO,Na (e.g. Vylon 53S).

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

(III) Unsaturated polyester resins: There may be mentioned polyester compounds containing radiation-curable unsaturated double bonds in the molecular chains, such as unsaturated polyester resins, prepolymers, and oligomers containing radiation curable unsaturated double bonds, which correspond to the saturated polyester resins prepared by the esterification of polybasic acids with polyhydric alcohols as referred to as the thermoplastic resins in the above (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 above (1), and examples of the radiation-curable unsaturated double bonds include maleic acid, fumaric acid, and so forth.

The radiation-curable unsaturated polyester resin can be produced by adding maleic acid, fumaric acid, or the like to at least one polybasic acid component and at least one polyhydric alcohol component, and then subjecting the mixture to a dehydration or dealcoholization reaction in the conventional manner, i.e. at a temperature of from 180 to 200 C in a nitrogen atmosphere in the presence of a catalyst, thereafter raising the temperature to a range of from 240 to 280 C whereupon a condensation reaction is effected under a reduced pressure of from 0.5 to 1 mmHg, to obtain a polyester resin.The content of maleic acid, fumaric acid, or the like may be in a range of from1 to 40 mol %, or preferably from 10 to 30 mol %, in the acid component in view of the degree of cross-linking at the time of its production, the radiationcurability, and so on.

(IV) Polyvinyl alcohol type resins: There may be mentioned polyvinyl alcohol, butyral resins, acetal resins, formal resins, and copolymers of these components.

The hydroxyl groups contained in these resins can be modified to be radiation-sensitive by the process described hereinafter.

(V) Epoxy type resins and phenoxy resins: There may be mentioned epoxy resins obtained by the reaction of bisphenol A with epichlorohydrin, methylepichlorohydrin or the like, such as EPIKOTE 152, 154, 828, 1001, 1004 and 1007 (manufactured by Shell Chemical Company); DEN431, DER732, DER511 and DER 331 (manufactured by Dow Chemical Company); EPICLON-400 and EPICLON-800 (manufactured by Dai-Nippin Ink K.K.); phenoxy resins such as PKHA, PKHC and PKHH which are the highly polymerized resins among the above-mentioned epoxy resins, and are manufactured by Union Carbide Corporation; and copolymers of brominated bisphenol A with epichlorohydrin, such EPICLON 145, 152, 153 ahd 1120(manufactured by Dai-Nippon Ink 8 Chemicals Co.) and others.

The radiation-sensitive modification is effected by utilization of the epoxy groups contained in these resins.

(Vi) Cellulose derivatives: Cellulose derivatives of various molecular weights are also effective as thermoplastic components. Particularly effective and preferable among these cellulose derivatives are nitrocellulose, cellulose aceto-butylate, ethyl-cellulose, butyl-cellulose, acetyl-cellulose, and so forth.

These cellulose derivatives are modified to radiation-sensitive resins by activating the hydroxyl groups in the resins by the process mentioned hereinafter.

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

Further, by blending a thermoplastic elastomer or prepolymer with the above-described radiation-sensitive, modified thermoplastic resin, it is possible to make the coating film much more tenacious. Furthermore, when such an elastomer or prepolymer is likewise modified to be radiation-sensitive, a better result can be obtained, as will be described hereinbelow.

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

(I) Polyurethane elastomers, prepolymers and telomers: The use of polyurethane elastomers is particularly effective in that their abrasion resistane and adhesion to EPT 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'-diphenylmeth- ane diisocyanate, 3, 3'-dimethylbiphenylene diisocyanate, 4, 4'-biphenylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane diisocyanate, Desmodule L, Desmodule 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, maleic 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 caprolactum, 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 allyl-type 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 radiation-sensitive.

(II) Elastomers of acrylonitrile-butandiene copolymers: Prepolymers of acrylonitrile-butandiene copolymers containing therein terminal hydroxyl groups, such as "poly BD Liquid Resin" produced by Sinclair Petrochemicals Corp. and available in general market, or elastomers such as "Hycar 1 432J" manufactured 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-1 5", manufactured by Sinclair Petrochemicals Corp., are particularly suitable in respect of their compatibility with thermoplastic resins. In the "R-1 5" 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 unsaturated double bond to the terminal end of the molecule, whereby the prepolymer becomes much more advantageous as the binder.

Further, cyclized products of polybutadiene such as "CBR-M901" manufactured 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 crosslinking and polymerization by radiation because of the,,radicals of the unsaturated bonds inherent to polybutadiene. Thus, they have excellent properties as the binder.

Further, suitable among other thermoplastic elastomers and their prepolymers are: styrenebutadiene rubbers, chlorinated rubbers, acrylic rubbers, isoprene rubbers and their cyclized products (e.g. "CIR701", manufactured by Japan Synthetic Rubber K.K.), and elastomers such as epoxy-modified rubbers, or internally plasticized saturated linear polyesters (e.g. "Vylon #300", a product of Toyo Spinning K.K.). These may also be used effectively by subjecting them to the modifying treatment for the radiation-sensitization described hereinbelow.

Examples for the synthesis of the radiation-sensitive binders, will be given hereinafter.

Among high-polymer substances, there have been known those which undergo decomposition when irradiated with radiation rays and those which undergo cross-linking among the molecules by the irradiation with radiation rays. Those which undergo cross-linking among the molecules include polyethylene, polypropylene, polystyrene, polyacrylic acid ester, polyacrylamide, polyvinyl chloride, polyester, polyvinyl pyrrolidone rubbers, polyvinyl alcohol and polyacrolein. Such cross-linking type polymers can be used as radiation curable coating resins for the backing layer, as they are, i.e. without subjecting them to any particular modifying treatment as mentioned above, since they undergo a cross-linking reaction without such treatment.

Furthermore, according to this method, even a non-solvent type resin can effectively used for the back coating, since it can be cured in a short period of time without requiring any solvent.

As the active energy rays to be used for cross-linking of the backing layer of the present invention, there may be employed electron beams generated from a radiation accelerator as the source of radiation, y-rays generated from Co60 as the source of radiation, ssrays generated from Sr90 as the source of radiation, or X-rays generated from an X-ray generator as the source of radiation.

From the viewpoints of the easy control of the absorption dose, the introduction to the production line, or the shielding of ionized radiation, it is particularly advantageous to use the radiation rays from the radiation accelerator as the source of radiation.

With respect to the characteristic of the radiation rays to be used for the curing of the backing layer, it is preferred, from the aspect of the penetrating power, to irradiate by means of the radiation accelerator having an acceleration voltage of from 100 to 750 KV, or preferably from 150 to 300 KV, at an absorption dose in a range of from 0.5 to 20 Mrad.

For the curing of the backing layer according to the present invention, a low dose type radiation accelerator (an electron-curtain system) manufactured by Energy Science Co., U.S.A. is extremely advantageous from the viewpoints of its easy introduction into the tape coating process line and the shielding of the secondary X-rays within the accelerator.

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, for the cross-linking by radiation, it is important to irradiate the backing layer with the radiation rays in an inert gas stream such as nitrogen (N2) gas, helium (He) gas, and so forth. It is extremely disadvantageous to conduct the radiation in the air because 03, etc. generated by the radiation for the cross-linking of the binder components serve to prevent the radicals generated in the polymer from acting advantageously for the cross-linking reaction.

It is therefore important to maintain the atmosphere at a portion where the active energy rays are irradiated to be an inert gas atmosphere such as N2, He or CO2, with the oxygen concentration being as low as 1 % at the maximum.

Fillers to be incorporated in the backing layer together with the above-mentioned binder include: (1) graphite and carbon black having electric conductivity; and (2) inorganic fillers such as SiO2, TiO2, Al203, Cur2 03, SiC, CaCO3, zinc oxide, geothite, a-Fe203, talc, kaolin, CaSO4, boron nitride, Teflon (R.T.M.) powder, graphite fluoride and molybdenum disulfide. The amount of such fillers to be incorporated should appropriately be from 20 to 100 parts by weight relative to 100 parts by weight of the binder in the case of the electrically conductive fillers (1), and from 10 to 300 parts by weight in the case of the inorganic fillers (2). When the amount of the fillers is too large, there will be disadvantages such that the coating film becomes brittle and the number of dropouts increases.

It is of course possible to employ conventional dispersants or lubricants for the formation of the backing layer.

As the supporting member, there may be employed a polyethylene terephthalate film, a polyimide film or polyamide film.

Likewise, as the magnetic layer, conventional coating conventional coating-type magnetic layers may be employed, wherein oxide magnetic powders such as a-Fe203, Fe304 and Cocoated or doped a-Fe203, or metal or alloy magnetic powders such as Fe, Ni, Co or their alloys, may optionally be used.

If the magnetic layer is made of an electron ray curable magnetic coating material, it is extremely effective for the prevention of curling to form the backing layer from the same material as the magnetic layer. It is thereby possible to conduct the entire operation on-line, thus eliminating a rolled state, whereby the curling in the longitudinal direction is minimized.

The thickness of the backing layer is usually selected within a range of from 1 to 10 ym, preferably from 2 to 4 ym.

By providing such a backing layer, it is possible to improve the reduction of the dropouts, the antistatic property, the running characteristics and reduction of flexing towards the magnetic layer side of the disc.

Further, the magnetic discs to be provided with such a backing layer include discs for computers and discs for magnetic cameras. Dropouts are critical to the former, and the flexing of the discs is critical to the latter. Accordingly, it is extremely effective to apply the backing layer of the present invention to such discs.

Now, the present invention will be described in further detail with reference to Examples.

Firstly, Examples for the syntheses of the radiation-sensitive binders will be given.

(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 heating. After raising the temperature to 80 C, 61.4 parts by weight of 2hydroxyethyl 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 80 C in a nitrogen (N2) stream until the conversion of NCO reached 90%. After completion of the reaction, the reaction system was cooled and diluted by addition of 1250 parts by weight of methyl ethyl 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 80 C in a fournecked flask of one-liter capacity in a nitrogen (N2) steam. 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 cooling to control the temperature inside the reaction vessel to be in a range of from 80 to 85 C. After completion of the dropwise addition, the mixture was stirred for three hours at 80 C to complete the reaction.

After completion of the reaction, the reaction product was taken out of the reaction vessel and cooled to obtain 2-hydroxyethyl methacrylate (2HEMA) adduct of tolylene diisocyanate (TDI) as a white paste.

(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 191.2 parts by weight of toluene and 71.4 parts by weight of cyclohexanone, and dissolved under heating. After raising the temperature to 80 C, 7.4 parts by weight of the 2-hydroxyethyl methacrylate adduct of tolylene diisocyanate was added to the solution, followed by further addition of 0.015 part by weight of tin octylate and 0.01 5 part by weight of hydroquinone. Then, the reaction was carried out at 80 C in a nitrogen (N2) steam until the conversion of NCO reached at least 90%.After completion of the reaction, the reaction product was cooled and diluted with methyl ethyl ketone.

(c) Synthesis of an acryl-modified product of a saturated polyester resin (radiation-sensitive modified resin): 100 Parts by weight of "Vylon RV-200" manufactured by Toyo Spinning Co., was dissolved under heating in 11 6 parts by weight of toluene and 116 parts by weight of methyl ethyl ketone. After raising the temperature to 80 C, 3.55 parts by weight of the 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. Then, the reaction was carried out at 80 C in a nitrogen (N2) stream until the conversion of NCO reached at least 90%.

(d) Synthesis of an acryl-modified product of an epoxy resin (radiation-sensitive modified resin): 400 Parts by weight of "Epikote 1007" manufactured by Shell Chemical Co., was dissolved under heating in 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 solution, and the temperature was raised to 809C. Then, 69 parts by weight of acrylic acid was added dropwise, and the reaction was carried out at 80"C until the acid value became 5 or lower.

(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", manufactured by 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 charged into a reaction vessel, and dissolved under heating at 80 C.

Then, 43.5 parts by weight of TDI was added dropwise into the reaction vessel, while cooling to control the temperature inside of the reaction vessel to be in a range of from 80 to 90 C. After completion of the dropwise addition, the reaction was conducted at 80 C until the conversion of NCO reached at least 95%.

(f) Synthesis of an acryl-modified product of a polyether type terminal urethane-modified elastomer (radiation-sensitive elastomer): 250 Parts by weight of a polyether, "PTG-500" manufactured by 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 charged into a reaction vessel, and dissolved under heating at 80 C. Then 43.5 parts by weight of TDI was added dropwise into the reaction vessel, while cooling to control the temperature inside of the reaction vessel to be in a range of from 80 to 90 C. After completion of the dropwise addition, the reaction was conducted at 80 C until the conversion of NCO reached at least 95%.

(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-1 5" manufactured by 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 charged into a reaction vessel, and dissolved under heating at 80 C. Then, 43.5 parts by weight of TDI was added dropwise, while cooling to control the temperature inside of the reaction vessel to be in a range of from 80 to 90 C. After completion of the dropwise addition, the reaction was conducted at 80 C until the conversion of NCO reached at least 95%.

EXAMPLE 1: Parts by weight Carbon black "ASAHI HS500" 50 (particle size of 80 mym) produced by Asahi Carbon Co.

Copolymer of acryl-modified vinyl 30 chloride, vinyl acetate, and vinyl alcohol (experimental product) Acryl-modified polyurethane elastomer 20 (experimental product) Mixed solvent (MIBK/toluene= 1/1) 300 The mixture of the above ingredients was dispersed in a ball mill for five hours, and the 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 backing layer upon its drying would be 3 lim. Thereafter, this backing layer was irradiated with electron beam in a nitrogen (N2) gas using an electron curtain type electron beam accelerator with an accelerating voltage of 150 KeV, an electrode current of 10 mA, and an absorption dose of 10 Mrad, thereby curing the layer.After the curing, the coated film was wound up and then stamped out into a 5" floppy disc. This sample is designated as Sample No. 1.

The magnetic layer was formed by dispersing a mixture having the following composition in a ball mill for 5 hours, adding 10 parts by weight of isocyanate (Colonate L, manufactured by Nippon Polyurethane) and coating the mixture in a thickness of 3 ym as dried.

Parts by weight Cobalt-adsorbed acicular y-Fe203 120 (length: 0.4 #m, diameter: 0.05 ym) Vinyl chloride-vinyl acetate- 30 vinyl alcohol copolymer (VAGH manufactured by U.C.C. Co.) Polyurethane elastomer (N5033 20 manufactured by Nippon Polyurethane Co.) Carbon black (Antistatic, Mitsubishi 5 Carbon Black MA-600) a-AI203 powder (0.5 ym) 2 Solvent (methyl ethyl ketone/toluene: 50/50200 EXAMPLE 2: Parts by weight SiO2 (particle size of 2 ism) 50 Copolymer of acryl-modified vinyl 30 chloride, vinyl acetate, and vinyl alcohol (experimental product) Acryl-modified polyurethane elastomer 20 (experimental product) Mixed solvent 300 The mixture of the above ingredients was processed in the same manner as in Example 1 above, to obtain Sample No. 2.

EXAMPLE 3: Parts by weight Carbon black "ASAHI HS500" 50 a product of Asahi Carbon Co.

Acryl-modified polyester resin 60 (experimental product) Mixed solvent 300 The mixture of the above ingredients was processed in the same manner as in Example 1 above, to obtain Sample No. 3.

EXAMPLE 4: Parts by weight Carbon black "ASAHI HS500" 50 a product of Asahi Carbon Co.

Acryl-modified polyurethane elastomer 60 (experimental product) Copolymer of vinyl chlorode, vinyl 70 acetate, and vinyl alcohol ("VAGH" produced by UCC) Mixed solvent 300 The mixture of the above ingredients was processed in the same manner as in Example 1 above, to obtain Sample No. 4.

EXAMPLE 5: Parts by weight Cobalt-adsorbed acicular y-Fe2O3 120 (length: 0.4 ym, diameter: 0.05 ym, Hc: 600 Oe) Carbon black (Antistatic, Mitsubishi 5 Carbon black MA-600) a-A1203 powder (0.5 ym spherical) 2 Dispersant (Soybean oil purified lecithin) 3 Solvent (methyl ethyl ketone/ 100 toluene: 50/50) The above components were mixed in a ball mill for 3 hours to sufficiently wet the acicular magnetic iron oxide with the dispersant.

Separately, a mixture having the following composition was thorougly mixed and dissolved.

Copolymer of partially saponified 15 parts by weight vinyl chloride-vinyl acetate copolymer with maleic acid (X % by weight) Acrylic double bond-containing 15 parts by weight polyester urethane elastomer (b) (as solid) Solvent (methyl ethyl ketone/ 200 parts by weight toluene: 50/50) Lubricant (higher fatty acid- 3 parts by weight modified silicone oil) The solution thereby obtained was introduced into the ball mill containing the magnetic powder composition treated as mentioned above, and the mixture was thoroughly mixed and dispersed for 42 hours.

The magnetic coating material thus prepared, was coated on one side of a polyester film in a thickness of 3 ym as dried, and cured by electron beams. Further, the same coating material was also applied to the rear side in a thickness of 3 lim as dried, and cured by electron beams.

The coated film thus obtained was punched out to obtain a 5" floppy disc. This sample is designated as Sample No. 5.

COMPARATIVE EXAMPLE 1: Parts by weight Carbon black "ASAHI He500" 50 Copolymer of vinyl chloride, vinyl 30 acetate, and vinyl alcohol ("VAGH" produced by UCC) Polyurethane elastomer ("5033" 20 produced by Nippon Polyurethane Co.) Mixed solvent 300 The mixture of the above ingredients was prepared into a coating composition in the same manner as in Example 1, after which 10 parts by weight of isocyanate ("COLONATE L" produced by Nippon Polyurethane Co.) was added, and the mixture was coated in a thickness of 3,um as dried. Thereafter, the coated film was allowed to stand for 24 hours at a temperature of 60 C for thermosetting treatment. The treated film was then punched out into a dics of 5" f.

This sample is designated as Sample No. 6.

COMPARATIVE EXAMPLE 2: The same magnetic medium 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. 7. This was a 5" + floppy disc and made of a polyester film, on which a magnetic layer was coated.

With respect to Samples No. 1 to No. 7, dropouts and curling were evaluated.

Dropout test A single signal of 125 KHz was recorded and reproduced. A number of defects where the reproduced signal was at a level of less than 40% of the average reproduction level for at least 8,u seconds, were counted with respect to the entire track.

Table 1 Samples No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 A number of sheets 8 6 9 10 6 25 13 containing at least one dropout, out of 100 disc sheets It is apparent from Table 1 that Samples No. 1 to No. 5 which were subjected to the radiation curing treatment show muchless dropouts as compared with the usual thermosetting type Sample No. 5.

Curling test A punched 8" f disc was placed on a horizontal plane, and a distance d of the periphery of the disc from the horizontal plane, was measured by a microscope. The results thereby obtained are shown in Table 2.

Table 2 Samples No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 Curling (mm) 0.3 0.5 0.4 0.5 0.2 1.1 2.3 From the above results, it is evident that curling is less in the Samples having backing layers as compared with the Samples having no backing layers. Further, among the Samples having backing layers, Samples No. 1 to No. 5 subjected to the radiation curing treatment have muchless curling as compared with Sample No. 6 made of a conventional thermosetting type resin.

Claims (10)

1. A magnetic recording disc which comprises a supporting member, a magnetic layer formed on one side of said supporting member, and a backing layer formed on the other side of said supporting member, wherein said backing layer is composed of a binder containing radiation-sensitive curable resin, and a filler material dispersed in the binder.

2. The magnetic disc according to Claim 1, wherein said filler material is a powder of electrically conductive material such as carbon black or graphite.

3. The magnetic recording disc according to Claim 1, wherein said filler material is a powder of highly tenacious material such as SiO2, TiO2, Awl203, Cr203, SiC. CeO2, CaCO3, zinc oxide, geothite, a-Fe203, talc, kaolin, CaSO4, boron nitride, Teflon powder, graphite fluoride molybdenum disulfide or zirconia.

4. The magnetic recording disc according to Claim 1 wherein said filler material is mixture of a powder of electrically conductive material of carbon black or graphite, and a powder of highly tenacious material of SiO2, TiO2, Awl203, Cr203, SiC, CeO2, CaCO3, zinc oxide, geothite, a Fe203, talc, kaolin, CaSO4, boron nitride, Teflon powder, graphite fluoride, molybdenum disulfide or zirconia.

5. A process for producing a magnetic recording disc, which comprises forming a magnetic layer on one side of a supporting member, then applying onto the other side of the supporting member, a coating material prepared by dispersing a filler material in a binder containing a radiation-sensitive curable resin, to form a backing layer, and thereafter irradiating active energy rays onto said backing layer to cure said radiation-sensitive curable resin.

6. The process for producing the magnetic recording disc according to Claim 5, wherein said filler material is a powder of electrically conductive material such as carbon black or graphite.

7. The process for producing the magnetic recording disc according to Claim 5, wherein said filler material is a powder of highly tenacious material such as SiO2, TiO2, Al203, Cr2O3, SiC, CeO2, CaCO3, zinc oxide, geothite, a-Fe2 03, talc, kaolin, CaSO4, boron nitride, Teflon powder, graphite fluoride, molybdenum disulfide or zirconia.

8. The process for producing the magnetic recording disc according to Claim 5, wherein said filler material is a mixture of a powder of electrically conductive material of SiO2, TiO2, Al203, Cr2O3, SiC, CeO2, CaCO3, zinc oxide, geothite, a-Fe2O3, talc, kaolin, CaSO4, boron nitride, Teflon powder, graphite fluoride, molybdenum disulfide or zirconia.

9. The process for producing the magnetic recording disc, according to Claim 5, wherein said irradiation of the active energy rays is effected in an inert gas atmosphere by means 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 Mrad.

10. A magnetic recording disc according to clam 1, substantially as described.