GB2125800A - Magnetic recording medium - Google Patents

Abstract

A magnetic recording medium having a magnetic recording layer comprising a binder containing magnetic particles, a fatty acid ester and a fatty acid is characterized in that said binder is a radiation sensitive resin. The choice of the ester and acid, which serve as a lubricant, is widened compared to that available in prior art systems in which the binder is a thermosetting resin.

Description

SPECIFICATION Magnetic recording medium The present invention relates to an improvement of a magnetic recording medium such as a video tape, a computer tape, an audio tape or a floppy disk.

In the magnetic recording medium of this type, it has been common to employ a thermosetting binder and to improve the durability, running property and environmental dependability of the magnetic recording medium in such a manner that a cross linking agent represented by a compound having a polyisocyanate group is chemically reacted with reactive functional groups represented by hydroxyl groups or amino groups in the binder to form a three-dimensional network structure in the binder and a lubricant is added thereto.

As the lubricant, it has been known to use, for instance, a silicone oil, a fatty acid, a fatty acid ester, molybdenum disulfide or graphite. Among them, it has been proposed to incorporate a fatty acid ester having a RCO group (where R is an alkyl group of CnH2n+1) having from 10 to 16 carbon atoms and a fatty acid having a melting point of from 44 to 700C to obtain a magnetic recording medium which exhibits, by virtue of a synergistic effect of the two components, superior wear-resistance and running property (Japanese Examined Patent Publication No. 39081/1976), and such a combination is widely used for thermosetting binders.

However, in the prior art represented by Japanese Examined Patent Publication No.39081/1976, it is essential to use a thermosetting binder, and consequently, it has the following drawbacks.

(a) A magnetic coating material containing the above-mentioned fatty acid ester and fatty acid as additives is coated on the substrate and dried, and then subjected to calender treatment. When the tape is wound up, the thermosetting reaction is not yet initiated and the magnetic coating layer is not yet strong enough, whereby the additives in the coating layer tend to cause blooming. Accordingly, there has been a restriction such that the types of the fatty acid ester and the fatty acid to be used as the additives are rather limited.

b) In the preparation of the conventional thermosetting magnetic recording medium, it was necessary that a substrate coated with a magnetic coating material was placed in an oven and subjected to thermosetting at a temperature of e.g. 700C for a period as long as from 2 to 20 hours. In order to avoid blocking or tightening of the wound tape during the thermosetting process, it was necessary to restrict the additives to certain types.

Accordingly, it is an object of the present invention to overcome the above-mentioned drawbacks and to provide a magnetic recording medium whereby the useful ranges of the fatty acid ester having an alkyl group and the fatty acid as the additives are widened.

According to the present invention, the above-mentioned object is attained by providing a magnetic recording medium having a magnetic recording layer composed of a binder containing a fatty acid ester having an alkyl group and a fatty acid, wherein the binder is a radiation sensitive resin.

Now, the present invention will be described in detail with reference to the preferred embodiments.

In the accompanying drawings, Figure 1 is a graph showing the results of the running property tests of the magnetic recording medium according to the present invention.

Figure 2 is a graph showing the results of the running property tests of the conventional thermosetting type magnetic recording medium.

In the present invention, a radiation sensitive resin is used as a binder instead of the conventional thermosetting resin, whereby the radiation sensitive resin contained in the magnetic coating layer can be hardened by irradiation prior to the winding-up operation subsequent to the calender treatment after the magnetic coating material containing the radiation sensitive resin is coated on a substrate and dried. Thus, the magnetic coating layer on the substrate is strengthened prior to the winding-up operation. Accordingly, the winding-up operation of the magnetic recording medium can be conducted free from blocking of the additives in the coating layer, whereby the useful ranges of the additives are widened as compared with the conventional thermosetting type binders.For instance, in the conventional case disclosed in Japanese Examined Patent Publication No.39081/1976, the useful additives are restricted to a fatty acid ester having a RCO group having from 10 to 16 carbon atoms and a fatty acid having a melting point of from 44 to 7000.

Whereas, in the present invention, it is possible to use a fatty acid ester having a RCO group having at least 9 carbon atoms and a fatty acid having a melting point of from 32 to 8100.

Further, the magnetic recording medium of the present invention is hardened by an application of a radiation, whereby the thermosetting process which used to be required for the conventional thermosetting type magnetic recording medium, is no longer necessary and the restriction to the useful types of the additives is eased.

Furthermore, it has been found that to obtain a video tape product which is capable of presenting a stable static picture image for a long period of time without exudation under a temperature condition as wide as from 0 to 60 C, it is possible to use a fatty acid ester having a RCO group having at least 9 carbon atoms, preferably from 10 to 18 carbon atoms and a fatty acid having a melting point of from 32 to 8100. Moreover, it has been found that when used independently, these lubricants do not provide a satisfactory result, and the desired synergistic effect is obtainable only when the two types of the lubricants are used in combination.

The radiation sensitive resin to be used in the present invention is a resin containing at least one unsaturated double bond in its molecular chain, which generates radicals when subjected to irradiation and is thereby capable of being hardened by cross linking or polymerization. It is known that there are two types of the polymers, i.e. the first type is the polymers which decompose when irradiated and the second type is the polymers which undergo intermolecular cross linking when irradiated. As the second type, there may be mentioned, for instance, a polyethylene, a polypropylene, a polystyrene, a polyacrylate, a polyacrylamide, a polyvinyl chloride, a polyester, a polyvinyl pyrrolidone rubber, a polyvinyl alcohol and a polyacrolein. These cross linkable polymers may be used for the magnetic layer.

Further, the radiation sensitive resin to be used in the present invention may also be prepared by modifying a thermoplastic resin to be radiation sensitive. This is preferred from the viewpoint of the hardening speed. The modification of a thermoplastic resin to a radiation sensitive resin may be conducted by introducing into the molecule of the resin a group which is cross-linkable or polymerizable by irradiation, such as an acrylic double bond derived from e.g. acrylic acid, methacrylic acid or their ester compounds having an unsaturated double bond having a radical polymerization property, an allyl-type double bond derived from diallyl phthalate or an unsaturated bond derived from maleic acid or a maleic acid derivative.

Any other unsaturated double bond which are cross-linkable or polymerizable by irradiation, may also be used. Specific examples will be given below.

(A) Vinyl chloride-type copolymers Avinyl 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-terminal OH side chain alkyl group copolymer and the like. As commercial products, there may be mentioned, for instance, those known by the trade names VROH, VYNC, VYEGX and VERR of UCC Company.

The above-mentioned copolymers may be modified to radiation sensitive resins by introducing acrylic double bonds, maleic acid-type double bonds or allyl-type double bonds in accordance with a method which will be described hereinafter.

(B) Saturated polyester resins Saturated polyester resins obtainable by esterification of a saturated basic acid such as phthalic acid, isophthalic acid, terephthalic acid, maleic acid, a maleic acid derivative, succinic acid, adipic acid or sebacic acid, with a polyhydric alcohol such as ethylene glycol, diethylene glycol, glycerin, trimethylolpropane, 1,2-propylene glycol, 1 ,3-butanediol, dipropylene glycol, 1 ,4-butanediol, 1 ,6-hexanediol, pentaerythritol, sorbitol, glycerin, neopentyl glycol or 1,4-cyclohexanedimethanol, or modified resins (Vylon 53S) obtained by modifying these polyester resins with SO3Na.

These resins are likewise modified in accordance with the method described hereinafter.

(C) Unsaturated polyester resins Polyester compounds containing radiation-curable unsaturated double bonds in their molecular chains.

For instance, there may be mentioned unsaturated polyester resins, prepolymers or oligomers containing radiation-curable unsaturated double bonds, which are obtained from the saturated polyester resins obtained by the esterification of the polybasic acid with the polyhydric alcohol as disclosed as thermosetting resins in the above item (B) by using maleic acid as a part of the polybasic acid.

As the polybasic acid component and the polyhydric alcohol component of the saturated polyester resins, there may be mentioned various compounds mentioned in the above item (A). As the radiation-curable unsaturated double bonds, there maybe mentioned maleic acid and fumaric acid.

The radiation-curable unsaturated polyester resins may be prepared by a process wherein maleic acid or fumaric acid is added to at least one polybasic acid component and at least one polyhydric alcohol component, and the mixture is subjected to a dehydration or alcohol-removal reaction in a conventional manner i.e. in a nitrogen atmosphere at a temperature of from 180 to 2000C in the presence of a catalyst and then heated to a temperature of from 240 to 2800C to conduct a condensation reaction under reduced pressure of from 0.5 to 1 mmHg, whereby a polyester resin is obtainable. The content of maleic acid or fumaric acid is usually from 1 to 40 molar %, preferably from 10 to 30 molar % in the acid component in view of the cross linking during the preparation and the radiation curability.

(D) Polyvinyl alcohol-type resins A polyvinyl alcohol, a butyral resin, an acetal resin, a formal resin and a copolymer of these components.

The hydroxyl groups contained in these resins may also be modified to be radiation sensitive in accordance with the method described hereinafter.

(E) Epoxy resins andphenoxy resins Epoxy resins obtained by the reaction of bisphenol Awith epichiorohydrin or methylepichlorohydrin. For instance, there may be mentioned Epikote 152, 154,828, 1001, 1004 and 1007, manufactured by Shell Chemical Company, DEN431, DER732, DER5l 1 and DER331, manufactured by Dow Chemical Company, or Epiclon 400 and Epiclon 800, manufactured by Dainippon Ink Co. Further, as resin derivatives of the above-mentioned epoxy resins having a high degree of polymerization, there may be mentioned a phenoxy resin (PKHA, PKHC and PKHH) manufactured by UCC Company, a copolymer of brominated bisphenol-A with epichlorohydrin, or Epiclon 145, 152, 153 and 1120, manufactured by Dainippon Ink Co.

The epoxy groups contained in these resins may also be modified to be radiation sensitive.

(F) Cellulose derivatives Cellulose derivatives having various molecular weights are also effective as thermoplastic components.

Especially effective among them are nitrocellulose, cellulose acetobutyrate, ethylcellulose, butylcelliulose and acetylcellulose.

The hydroxyl groups contained in these resins may also be modified to be radiation sensitive in accordance with the method described hereinafter.

As other resins which may be modified to be radiation sensitive, there may be effectively used a polyether ester resin, a polyvinyl pyrrolidone resin and its derivative (PVP olefin copolymer), a polyamide resin, a polyimide resin, a phenol resin, a spiroacetal resin, and acrylic resins containing at least one kind of an acryl ester and a methacryl ester containing a hydroxyl group, as a polymer component.

As other useful binder components, there may be mentioned monomers such as acrylic acid, methacrylic acid, an acryl amide and methacryl amide. Binders having double bonds may be prepared by modifying various polyesters, polyols, or polyurethanes with a compound having an acrylic double bond. A polyhydric alcohol and a polyvalent carboxylic acid may be incorporated, as the case requires, to obtain resins having various molecular weights. The above-mentioned specific examples represent only a part of the useful radiation sensitive resins.

Further, it is possible to form a stronger coating layer by incorporating a thermoplastic elastomer or prepolymer into the above-mentioned radiation-sensitively modified thermoplastic resins. Moreover, if this elastomer or prepolymer is likewise modified to be radiation sensitive, the effectiveness will be greater.

Specific examples of the elastomer or prepolymer which may be combined with the above-mentioned radiation-sensitively modified thermoplastic resins, will be given below.

(A) Polyurethane elastomers, prepolymers and telomers Polyurethane elastomers are particularly effective from the view-points of wear resistance and adhesion to a polyester terephthalate film. As specific examples of the useful urethane compounds, there may be mentioned polyurethane elastomers and telomers obtained by condensation polymerization of various polyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisbcyanate, 1 ,3-xylene diisocyanate, 1 A-xylene diisocyanate, 1 ,5-naphthalene diisocyanate, m-phenylene diisoycanate, p-phenylene diisocyan ate, 3,3'-dimethyl-4,4'-diphenyl methadiisocyanate, 3,3'-dimethylbiphenylene diisocyanate, 4,4'biphenylene diisocyanate, hexamethylene diisocyanate, isophoro diisocyanate, dicyclohexylmethane diisocyanate, Desmodule L or Desmodule N, with various polyesters such as linear saturated polyester (a condensation polymerization product of a polyhydric alcohol such as ethylene glycol, diethylene glycol, glycerine, trimethylol propane, 1,4-butanediol, 1,6-hexanediol, pentaerythritol, sorbitol, neopentyl glycol or 1,4-cyclohexanedimethanol, with a saturated polybasic acid such as phthalic acid, isophthalic acid, terephthalic acid, maleic acid, succinic acid, adipic acid or sebacic acid), a linear saturated polyether (polyethylene glycol, polypropylene glycol or polytetraethylene glycol), caprolactam, a hydroxyl-containing acrylic acid ester or a hydroxyl-containing methacrylic acid ester.

These elastomers may be combined, as they are, with various radiation-sensitiveiy modified thermoplastic resins. However, it will be very effective to modify the urethane elastomers to be radiation sensitive by reacting them with a monomer having an acrylic double bond or a allyl-type double bond which is reactive with the terminal isocyanate group or hydroxyl group of the urethane elastomers.

(B) Acrylnitrile-butadiene copolymer elastomers An acrylnitrile-butadiene copolymer prepolymer having a terminal hydroxyl group, manufactured by Sinclair Petrochemicals Inc. and commercially available as Poly BD Liquid Resin, or an elastomer such as Hycar 1432J manufactured by Nihon Zeon Co., is particularly suitable as an elastomer component of the type which undergoes cross linking and polymerization when the double bond in the butadiene generates a radical by the application of radiation.

(C) Polybutadiene elastomers A prepolymer having low molecular weight hydroxyl groups, such as Poly BD Liquid Resin R-15 manufactured by Sinclair Petrochemicals Inc. is particularly suitable in view of the compatibility with a thermoplastic resin. In R-15 prepolymer, the terminal of the molecule is a hydroxyl group. Accordingly, it is possible to increase the radiation sensitivity by adding an acrylic unsaturated double bond to the terminal of the molecule. Thus, it will be more advantageous as a binder.

Further, a cyclization product of polybutadiene such as CBR-M901 manufactured by Nippon Gosei Gum K.K. also provides a superior property when combined with a thermoplastic resin. The cyclized polybutadiene has a particularly good cross-linking polymerization efficiency by irradiation by virtue of the radical of the unsaturated bond intrinisic to the polybutadiene, and thus has a superior property as the binder.

Among other suitable thermoplastic elastomers and their prepolymers, elastomers such as a styrenebutadiene rubber, a chlorinated rubber, an acryl rubber, an isoprene rubber and its cylication product (CIR701 manufactured by Nippon Gosei Gum K.K.), an epoxy-modified rubber and a plasticized saturated linear polyester (Vylon #300 manufactured by Toyobo) may be effectively used by subjecting them to modification treatment for radiation sensitizing.

When a solvent is used in the present invention, it may be a ketone such as acetone, methylisobutyl ketone or cyclohexanone; an alcohol such as methanol, ethanol, isopropanol or butanol, which was not suitable for use in the isocyanate type thermosetting binders; a solvent having an ether bond such as tetrahydrofuran or dioxane; a solvent such as dimethylformamide, vinylpyrrolidone or nitropropane; or an aromatic hydrocarbon diluent or solvent such as toluene or xylene.

As the substrate on which the coating is applied, a polyethyleneterephthalate film which is commonly used as a substrate for a magnetic recording medium, may be used. Further, when heat-resistance is required, a polyimide film or polyamideimide film is employed as a thin base, which is usually subjected to monoaxial or biaxial stretching treatment before use.

The fine magnetic powder to used in the present invention is composed mainly of-'-Fe2O3, Co deoped #-Fe2O3, Co-doped -Fe203-Fe304 solid solution, y-Fe203 coated with a Cr02 or Co compound. Fe3O4 coated with a Co compound (inclusive of intermediately oxidized "-Fe2O3; here, the Co compound is meant for a compound wherein the magnetic anisotropy of cobalt is utilized for the improvement of the coercive force, such as cobalt oxide, cobalt hydroxide, cobalt ferrite or a cobalt ion absorbed substance.), or a ferromagnetic metal element such as Co, Fe-Co, Fe-Co-Ni or Co-Ni.As the method for their preparation, there may be mentioned a wet reduction method by means of a reducing agent such as NaBH4, a dry reduction method in which the surface of iron oxide is treated with a Si compound, followed by reduction with e.g. H2 gas, or a vacuum vapour-deposition method in which vapour deposition is conducted in a low pressure argon gas stream. Further, fine powder of single crystal barium ferrite may also be used.

The above-mentioned fine magnetic particles may be in a needle-like form or a generally spherical form.

They are optionally selected depending upon the particular purpose of the magnetic recording medium.

In the radiation-curable magnetic recording medium of the present invention, the binder may contain various antistatic agents or abrasive agents which are commonly used in the usual magnetic recording media, depending upon the particular purpose.

As the activating energy rays to be used for curing the magnetic coating layer containing the above-mentioned radiation sensitive resin, electron rays generated by an electron ray accelerator as their source is particularly effective for the reasons as mentioned below. However, other rays such as y-ray generated from CoS0, 13-rays generated from Sir30 and X-rays generated by an X-ray apparatus, may also be employed.

Namely, the electron ray accelerator is advantageously be used as the ray source in that the control of the amount of the absorbed dose, the self-shielding of the ionized radiation for the introduction to the production line, and the connection between the various equipments in the process line and the sequence control, can readily be made. As the electron accelerator, there have been practically used various accelerators which differ from one another primarily in the system to obtain a high voltage, such as a Cockcroft type, a van de Graaff type, a resonance transformer type, an iron-core insulated transformer type or a linear selector type.However, in the magnetic recording medium for general purposes, the magnetic layer is usually as thin as 10 pwm or less, and accordingly it is unnecessary to use a high acceleration voltage of at least 1000 KV which is normally used in the above-mentioned accelerator. Namely, an electron ray accelerator having a low acceleration voltage of not more than 300 KV is sufficient. In the low acceleration voltage accelerator, not only the cost for the system itself is small but also the cost for the equipments for shielding the ionized radiation is advantageously low. The advantage of the low cost for the shielding equipments will be illustrated in Table 1.

TABLE 1 Thickness of the Acceleration shielding material voltage {KV) Shielding material (cm) 150 lead 0.5 200 lead 2 300 lead 3 500 concrete 85 750 concrete 110 1000 concrete 125 2000 concrete 175 3000 concrete 190 (Report of the Study Group for radiation utilization, page 8, Japan Atomic Energy Council, August 1979) As shown in Table 1, in the case of an electron accelerator of 300 KV or less, the leaking X-rays can adequately be shielded by covering the entire acceleration tube including the electron ray irradiation portion with a lead plate having a maximum thickness of 3 cm, as the shielding material. Therefore, it is unnecessary to separately provide an expensive electron ray irradiation chamber, and the system itself can be incorporated as a system in the production line for the magnetic recording medium.For instance, it is possible to conduct drying and curing of the magnetic tape or the magnetic sheet by the electron rays on an on-line system.

As a specific system, a low voltage type electron accelerator (Electro Curtain System) manufactured by US Energy Science Co. (ESJ Co.), an electron accelerator of RPC Co. (Broad Beam System) or a Self-Shielding type scanning low voltage type electron accelerator of Polymer Physics Co., West Germany, may suitably be used. When a binder coating layer containing the above-mentioned radiation sensitive resin is cured by means of a low voltage accelerator of from 150 to 300 KV, if the amount of the absorbed dose exceeds 5 Mrad, there will be disadvantages in the running durability under a high temperature and high humidity condition, for instance, the magnetic layer is likely to fall off at the head in the case of an audio or memory tape, or the amount of deposition onto the rotary cylinder is likely to increase in the case of a video tape.

Whereas, when the amount of the absorbed dose is from 0.5 to 5 Mrad, the polymerization and cross-linking density by the electron rays is proper, whereby the magnetic coating layer will be well balanced with proper flexibility and stiffness, and the wear-resistance between the magnetic layer and the head will be improved.

Thus, a superior magnetic recording medium which is free from deposition on the head or the deposition on the cylinder will be obtained.

When the cross-linking is conducted by irradiation, it is important that the radiation is applied to the recording medium in an inert gas stream such as N2 gas or He gas. As in the case of the magnetic coating layer of the magnetic recording medium, a coating layer having a very high density of the magnetic pigment is highly porous. Accordingly, if the cross-linking of the binder component is carried out by irradiation in air, O3 generated by the irradiation will adversely affect the function of the radical formed in the polymer, whereby the radical is prevented from effectively serving for the cross-linking reaction. Since the magnetic coating layer is porous, this adverse effect takes place to impair the cross-linking of the binder not only at the surface of the magnetic layer but also at the interior of the layer.Therefore, it is important that the atmosphere in which the active energy rays are irradiated is kept to be an inert gas atmosphere such as N2, He or CO2 having an oxygen concentration of not more than 1%, desirably not more than 3000 ppm, at the maximum.

As a dispersing machine, a ball mill is usually employed. However, various other apparatus such as a sand grind mill, a roll mill, a high speed impleller dispersing machine, a homogenizer and a ultrasonic dispersing machine may also be employed.

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

Example 1 Magnetic powder Co-doped needle-like #Fe2O3 100 parts by weight (long axis: 0.4 m, short axis: 0.05 Clam, Hc: 6000 Oe) Antistatic agent Carbon black (Mitsubishi Carbon Black MA-600) 5 parts by weight 01-A1203 (particles of 0.5 Fm) 2 parts by weight Dispersing agent Soybean oil purified lecitin 3 parts by weight Solvent Methylethyl ketone/toluene (50/50) 100 parts by weight The above composition was mixed in a ball mill for 3 hours to thoroughly wet the needle-like magnetic iron oxide "-Fe2O3 with the dispersing agent Then, the following mixture is thoroughly mixed and dissolved.

Vinyl chloride-vinyl alcohol copolymer (polymerization degree: about 300) 15 parts by weight Acrylic double bond-introduced poly ether urethane elastomer 15 parts by weight (calculated as solid content) Solvent Methylethyl ketone/toluene (50/50) 200 parts by weight Fatty acid ester X-type 2 parts by weight Fatty acid Y-type 1 part by weight The dissolved mixture is introduced into the ball mill in which the magnetic material was previously treated, and the mixing and dispersing were again conducted for 42 hours.

The magnetic coating material thus obtained was coated on a polyester film having a thickness of 15 am and oriented by a permanent magnet (1600 gauss), and the solvent was dried by means of an infrared lamp or heated air and surface smoothing treatment was applied. Then, by means of an Electro Curtain Type electron ray accelerator manufactured by ESI Co., electron ray irradiation was carried out in a N2 gas atmosphere having a residual 2 concentration of about 500 ppm at a acceleration voltage of 150 KV, at an electrode current of 10 mA for a total irradiation of from 0.5 to 8 Mrad, whereby the polymerization, drying and curing reaction of the magnetic coating layer were carried out. The magnetic recording medium thus obtained was cut in a width of 1/2 inch to obtain a video tape.

The above-mentioned fatty acid ester was designated as X-type and the fatty acid was designated as Y-type. Various kinds of X-type and Y-type were selected for experiment. From the experiments, it was found that in a wide range of combinations comprising the fatty acid ester having a RCO group having from 10 to 18 carbon atoms and the fatty acid having a melting point of from 32 to 81 C, video tapes which are free from exudation and capable of presenting a stable static picture image for a long period of time within a temperature range of from 0 to 600C were obtained. This will be described in more detail.

Firstly, the fatty acid ester and the fatty acid were independently added, and an experiment for the reproduction of a static picture image was conducted in an environment of -10"C and a running test for the exudation phenomenon was conducted under a high temperature and high humidity condition at a temperature of 60 C under a relative humidity of 60%. In the case where the fatty acid ester was added alone, the tape stopped running shortly after being subjected to the running test under the high temperature and high humidity condition at 600C under relative humidity of 60%. On the other hand, in the case where the fatty acid was added alone, the static picture image-reproducing time in the environment of -10 C was as short as 15 minutes or less.

Then, with respect to various combinations of the following fatty acid esters X-type and the fatty acids Y-type, running tests were conducted.

Fatty acid esters X-type (a) caprylic acid ester C7H15COOC4Hg (b) capric acid ester C9H19COOC4H9 (c) myristic acid ester C13H27COOC4H9 (d) stearic acid ester C18H37COOC4H9 Fatty acids Y-type (1) caprylicacid melting point 170C (2) capric acid melting point 320C (3) myristic acid melting point 540C (4) stearic acid melting point 690C (5) behenicacid melting point81"C (6) cerotic acid melting point 880C The results of the running tests are shown in Figure 1. The tests were conducted in such a manner that the video tape was stored for 40 hours under a high temperature and high humidity condition at a temperature of 600C under relative humidity of 60%, and then this video tape was taken out and permitted to run at a normal temperature by means of EIAJ Standard Open Reel VTR (NV-3120 manufactured by Matsushita mixing was conducted for 20 minutes. The magnetic coating material thus obtained was coated on a polyester film having a thickness of 15 Clam, and it was oriented by a permanent magnet (1600 gauss) and then dried by an infrared lamp or heated air to remove the solvent. Then, surface smoothing treatment was applied.Further, the coated film was held in an oven at a temperature of 800C for 48 hours to promote the cross-linking reaction of the isocyanate compound. The tape thereby obtained was cut in a width of 1/2 inch to obtain a video tape.

Figure 2 shows the results of the running tests of this Comparative Example. The test method and the manner of the presentation of the graph are similar to those presented in Example 1. It is evident from Figure 2 that in the case of the thermosetting type, the running friction became substantially great with the combinations of the fatty acid esters (b) to (d) with the capric acid (2) and behenic acid (5), which exhibited good running properties in the case of the radiation cured type, and no running was possible from the very beginning. Only the combinations with myristic acid (3) and stearic acid (4) as the fatty acid Y-type, showed practical usefulness. Namely, in the case of the thermosetting type, the applicable range of the additives is substantially narrower than that in the case of the radiation-cured type according to the present invention.

Example 2 Needle-like magnetic powder of Fe alloy (long axis: 0.3 Clam, short axis: 0.04 Clam, HC: 1100owe) 120 parts by weight Dispersing agent Oleic acid 2 parts by weight Solvent Methylethyl ketone/toluene (50/50) 100 parts by weight The above composition was mixed for 3 hours by a high power mixer to thoroughly wet the fine magnetic alloy powder.

Then, the following mixture was thoroughly mixed and dissolved.

Polyvinylbutyral resin 18 parts by weight (BL 3 manufactured by Sekisui (calculated as solid Chemical Co. Ltd.) content) Acetyl groups: 5 molar% Butyral groups: 40 molar % Formal groups: 20 molar% Hydroxyl groups: 35 molar% Polymerization degree: about 300 Acrylic double bond-introduced urethane 12 parts by weight elastomer (calculated as solid content) Solvent Methylethyl ketone/toluene (50/50) 200 parts by weight Butyl myristate 3 parts by weight Capric acid 1 part by weight The dissolved mixture was mixed with the previously-treated magnetic powder by a high speed mixer for 1 hour and 10 minutes, then the mixing and dispersing were conducted for 4 hours by means of a sand grind mill.

The magnetic coating material thus obtained was coated on a polyester film having a thickness of 15,am.

After subjecting it to a necessary treatments such as the magnetic field orientation, the drying to remove the solvent and the surface smoothing treatment, the coating layer was cured by irradiating electron rays in a N2 Electric Co.), whereby the running property relative to the running time was investigated with a Tension Analyzer IVA-500 Model manufactured by Nippon Jidoseigyo Co. being set between the head drum and the pinch roller. In Figure 1, the axis of abscissa represents the fatty acid esters X-type and the axis of ordinate represents the friction by VTR as a function of the fatty acids Y-type as a parameter.

It is evident from Figure 1 that when the caprylic acid ester (a) having an alkyl group of 8 carbon atoms was used as the fatty acid ester X-type, the running friction became extremely great and no running was possible from the very biginning. With respect to the fatty acids Y-type, caprylic acid (1) and cerotic acid (6) having melting points of 170C and 88 C, respectively, showed the same results. These samples which did not run were examined, whereby it was found that the respective coating layers had adhesiveness.

Whereas, when the caprylic acid ester (b) to the stearic acid ester (d) having an alkyl group of from 10 to 19 carbon atoms were used, the combinations with the fatty acids Y-type having a melting point of from 32 to 810C gave a small running friction at a level of from 100 to 170, whereby the running property was extremely good.

In order to compare the radiation-curable magnetic recording medium of the present invention with the conventional thermosetting type magnetic recording medium, samples of the thermosetting type magnetic recording medium were prepared in accordance with the following Comparative Example.

Comparative Example Magnetic powder Co-doped needle-like #-Fe2O3 (long axis: 0.4,um, short axis: 0.05 Fm, Hc: 600 Oe) 120 parts by weight Antistatic agent Carbon black (Mitsubishi Carbon Black MA-600) 5 parts by weight a-AI203 (particles of 0.5 Fm) 2 parts by weight Dispersing agent Soybean oil purified recitin 3 parts by weight Solvent Methylethyl ketone/toluene (50/50) 100 parts by weight The above composition was mixed in a ball mill for 3 hours to thoroughly wet the needle-like magnetic iron oxide #y-Fe2O3 with the dispersing agent.

Then, the following mixture was thoroughly mixed and dissolved.

Vinyl chloride-vinyl acetate copolymer (VAGH manufactured by Union Carbide Co.O 15 parts by weight Thermoplastic urethane resin (Nipolan 3032 manufactured by Nippon Polyurethane Co.) 15 parts by weight Solvent Methylethyl ketone/toluene (50/50) 200 parts by weight Fatty acid ester X-type 2 parts by weight Fatty acid Y-type 1 part by weight In this case, the fatty acid ester X-type and the fatty acid Y-type were selected in the same manner as in Example 1. The dissolved mixture was introduced into the ball mill in which the magnetic material was previously treated, and the mixing and dispersing were again conducted for 42 hours.Then, 5 parts by weight (as calculated as solid content) of an isocyanate compound (Desmodule L manufactured by Bayer Co.) which is reactive with the functional groups i.e. mainly hydroxyl groups, of the binder in the magnetic coating material and which is cross linkable, was added to the coating material in the ball mill, and the gas atmosphere at a acceleration voltage of 150 KeV at an electrode current of 10 mA for an absorbed dose of 5 Mrad by means of Electro Curtain Type electron ray accelerator. The tape thereby obtained was cut in a width of 1/2 inch to obtain a video tape. This video tape was evaluated in the same manner as in Example 1, whereby similar results were obtained.

Example 3 Needle-like magnetic powder of Fe alloy (long axis: 0.3,am, short axis: 0.04,am, Hc: 1100owe) 120 parts by weight Carbon black (Mitsubishi Carbon Black MA-600) 5 parts by weight o-AI203 (particles of 0.5 m) 2 parts by weight Dispersing agent Oleic acid 2 parts by weight Solvent Methylethyl ketone/toluene (50/50) 100 parts by weight The above composition was mixed for 3 hours to thoroughly disperse the fine magnetic alloy powder in the dispersing agent.

Then, the following mixture was mixed together with the previously treated magnetic material for 1 hour and 10 minutes by a high speed mixer and then the dispersing was conducted for 4 hours by means of a sand mill.

Saturated polyester resin (L-411 manufactured by Dynamit Nobel A.G.) 15 parts by weight Acrylic double bond-introduced poly caprolactam urethane prepolymer 15 parts by weight Solvent Methylethyl ketone/toluene (50/50) 200 parts by weight Amylpalmitate 1.5 part by weight Myristic acid 0.5 part by weight Thereafter, the formation of the magnetic coating layer on the substrate, the curing treatment by irradiation and the preparation of a tape were conducted in the same manner as in Examples 1 and 2, to obtain a similar video tape. With respect to the video tape thus obtained, the evaluation tests were conducted in the same manner as in Examples 1 and 2, whereby similar results were obtained.

Having thus described the present invention, it should be understood that the present invention provides a magnetic recording medium having a magnetic recording layer composed of a binder containing a fatty acid ester having an alkyl group and a fatty acid, wherein the binder is a radiation sensitive resin. Accordingly, when it is intended to improve the property by using a fatty acid ester having an alkyl group and a fatty acid as the additives, the useful ranges of these additives are enlarged according to the present invention.

Further, the present invention is applicable not only to the magnetic tapes for recording but also to a wide range of magnetic recording media having a construction wherein a magnetic recording layer comprising a binder and magnetic powder material is formed on a non-magnetic substrate.

Claims (6)

1. A magnetic recording medium having a magnetic recording layer composed of a binder containing a fatty acid ester having an alkyl group and a fatty acid, characterized in that said binder is a radiation sensitive resin.

2. The magnetic recording medium according to Claim 1, wherein the fatty acid ester has an alkyl group having at least 9 carbon atoms.

3. The magnetic recording medium according to Claim 1, wherein the fatty acid has a melting point of from 32 to 810C.

4. The magnetic recording medium according to Claim 1, wherein the magnetic recording layer contains at least one kind of magnetic particles selected from the group consisting of cobalt-modified magnetic iron oxide particles and magnetic alloy particles.

5. The magnetic recording medium according to Claim 1, wherein the binder is irradiated in an inert gas stream.

6. A magnetic recording medium as claimed in claim 1, substantially as described.