Magnetic Recording Medium

1. Field of the Invention

The present invention relates to a magnetic recording medium having excellent electromagnetic conversion characteristics and running durability with minimum clogging or dropout and without abrasion of the back coating layer. More particularly, the present invention relates to a magnetic recording medium having a feature in the composition of the coating layer (i.e. the back coating layer) formed on the resin side of the substrate opposite to the side on which a magnetic recording layer is provided.

2. Description of the Prior Art

Recently, magnetic recording media have been widely used in the fields of audio and video recording, computers, magnetic discs, etc. Accordingly, the amount of information to be recorded on magnetic recording media increases year after year, and there is an increasing demand for the improvement of the recording density of magnetic recording media.

As one of the conditions required for a magnetic recording medium for high density recording, it has been suggested to improve the coercive force and to make the medium thinner both from the theoretical and experimental points of view. There have been various proposals with respect to the compositions of the binder (binding agent), inorganic filler or lubricant for the back coating layer. However, conventional inorganic pigments have relatively large particle sizes, and they used to bring about drawbacks such that hard ones such as alumina or silica, tended to abrade e.g. tape guides during the tape running, or they were likely to impair the surface properties and to deteriorate the electromagnetic conversion characteristics.

The present inventors have conducted extensive researches to overcome the above-mentioned drawbacks, and they have found it possible to solve the above problems by incorporating into the back coating layer an inorganic oxide pigment having a certain average particle size and a certain specific surface area. Thus, the present invention has been accomplished.

Namely, the present invention provides a magnetic recording medium comprising a non-magnetic substrate, a magnetic recording layer formed on one side of the substrate and a back coating layer formed on the other side of the substrate, characterized in that the back coating layer is a radiation-cured coating layer containing a fine particulate pigment having an average particle size of the primary particles of at most 50 m.mu.m and a specific surface area of at least 35 m.sup.2 /g as measured by BET method.

FIG. 1 is a graph illustrating the relation between the surface roughness of the magnetic layer and the back coating layer of a magnetic recording medium and the S/N ratio.

FIG. 2 is a graph illustrating the relation between the BET specific surface area of an alloy magnetic powder and the S/N ratio.

By the use of an inorganic oxide pigment having an average particle size of the primary particles of at most 50 m.mu.m and a specific surface area of at least 35 m.sup.2 /g as measured by BET method, the surface roughness of the back coating layer can be improved, and it is thereby possible to improve the envelope property, lower the friction and minimize the abrasion without substantially deteriorating the electromagnetic conversion characteristics.

With respect to the incorporation of an inorganic oxide pigment into the back coating layer, it used to be believed that a hard pigment such as alumina or silica is undesirable since it damages tape guides, etc. during the running of the tape. It has now been found that even such an inorganic pigment having relatively great hardness can be used without bringing about the undesirable abrasion of tape guides, probably because a fine particulate inorganic oxide pigment having a small average particle size may be soft as the pigment.

The particle size can be freely reduced to a level of at most 50 m.mu.m (500 .ANG.). When the above fine particulate inorganic oxide is used in combination with carbon black, the surface property can substantially be improved. Particularly when both of them have substantially the same ranges of the particle sizes, the most effective results are obtainable.

If the specific surface area of the inorganic oxide is less than 35 m.sup.2 /g as measured by BET method, when it is incorporated into the back coating layer, the hardness increases and the abrasion of the tape guides will be brought about for some uncertain reasons, such being undesirable.

In order to improve the dispersibility of the fine particulate inorganic oxide of the present invention, it is preferred to use a dispersant such as a surface active agent, a fatty acid or a fatty acid ester.

As the fine particulate inorganic oxide pigment to be used in the present invention, there may be mentioned, for instance, ZrO.sub.2, Cr.sub.2 O.sub.3, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, CeO.sub.2, Fe.sub.3 O.sub.4, Fe.sub.2 O.sub.3, ZrSiO.sub.4, Sb.sub.2 O, SnO.sub.2 and TiO.sub.2. It is surprising that these materials are useful irrespective of their hardness so long as they are in the form of fine particulate pigments. These fine particulate pigments have a particle size of at most 50 m.mu.m (500 .ANG.), preferably at most 300 .ANG..

If the particle size of the fine particulate pigment exceeds 500 .ANG., a pigment having great hardness brings about the undesirable abrasion of tape guides, such being undesirable.

In the case of SiO.sub.2, the fine particulate pigment is available in the form of (1) a colloidal solution of super-fine particles of silicic anhydride (Snowtex (trademark), an aqueous system, a methanol silica sol, etc., manufactured by Nissan Chemical Industries, Ltd.) or (2) super-fine particulate anhydrous silica (Standard product: 100 .ANG.) produced by burning purified silicon tetrachloride (Aerosil (trademark), manufactured by Nippon Aerosil Co., Ltd.).

Further, it is possible to use super-fine particulate aluminum oxide and titanium oxide which can be produced by a vapour phase method in the same manner as for the above-mentioned colloidal solution of super-fine particles (1) and the super-fine particulate anhydrous silica (2), as well as the above-mentioned fine particulate pigments.

As the carbon black to be used for the back coating layer of the present invention, there may be employed any carbon black produced by any one of conventional methods, such as furnace, channel, acetylene, thermal or lamp. However, acetylene black, furnace black, channel black, roller and disc black and German naphthalene black are preferred. The particle size of the carbon black to be used in the present invention is preferably at most 50 m.mu.m, more preferably at most 30 m.mu.m. If the particle size exceeds 50 m.mu.m, when such carbon black is used in combination, the surface roughness of the back coating layer increases, the electromagnetic conversion characteristics tend to be inferior, and the dispersibility will be poor.

Other additives which are commonly used for the back coating layers of this type, such as organic binders, lubricants, dispersing agents or antistatic agents, may be incorporated in the back coating layer of the present invention in a usual manner.

The organic binder to be used for the back coating layer of the present invention is preferably a radiation curable resin which is composed of an acryl-modified vinyl chloride-vinyl acetate-vinyl alcohol copolymer (including the one containing a carboxylic acid) and a urethane acrylate. As the radiation curable resin, there may be employed, in addition to the above-mentioned preferred combination, a thermoplastic resin which contains in its molecule or is incorporated with radiation curable or polymerizable groups such as acrylic double bonds of acrylic acid, methacrylic acid or their ester compounds containing unsaturated double bonds having radical polymerizability, allyl-type double bonds of e.g. diallyl phthalate, or unsaturated bonds of e.g. maleic acid or a maleic acid derivative. As other useful binder components, there may be mentioned acrylic acid, methacrylic acid and acrylamide as monomers. A binder containing double bonds may also be obtained by modifying various polyesters, polyols, polyurethanes or the like with a compound having an acrylic double bond. Further, it is also possible to obtain resins having various molecular weights by incorporating a polyhydric alcohol and a polybasic carboxylic acid, as the case requires. The above-mentioned specific examples are only a part of useful radiation sensitive resins, and they may be used in combination as a mixture. Particularly preferred is a combination comprising:

(A) a plastic compound having a molecular weight of from 5,000 to 100,000 and containing at least two radiation-curable unsaturated double bonds;

(B) a rubber-like compound having a molecular weight of from 3,000 to 100,000 and containing at least one radiation-curable unsaturated double bond or containing no such double bond; and

(C) a compound having a molecular weight of from 200 to 3,000 and containing at least one radiation-curable unsaturated double bond, in the proportions of from 20 to 70% by weight of the compound (A), from 20 to 80% by weight of the compound (B) and from 10 to 40% by weight of the compound (C). It is advantageous to use a radiation curable resin, since the curing time is short and there will be no transfer of e.g. fillers from the back coating surface to the magnetic layer after the winding up operation. Whereas, in the case of thermosetting resin, there will be a problem that the electromagnetic conversion characteristics differ as between the inner side and the outer side of the jumbo roll during the course of the thermosetting due to the transfer of the rear side pattern of the back coating surface caused by the tightening of the winding of the jumbo roll at the time of the thermosetting.

As the lubricant to be used for the back coating layer of the present invention, there may be employed e.g. silicone oil, fluorine oil, a fatty acid, a fatty acid ester, a paraffin, a liquid paraffin or a surfactant as a lubricant which has been commonly used for the back coating layer of this type. However, it is particularly preferred to use a fatty acid and/or a fatty acid ester.

As the fatty acid, there may be mentioned a fatty acid having at least 12 carbon atoms (RCOOH where R is an alkyl group having at least 11 carbon atoms) such as caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linolic acid, linolenic acid or stearolic acid. As the fatty acid ester, there may be employed a fatty acid ester of a monobasic fatty acid having from 12 to 16 carbon atoms with a monohydric alcohol having from 3 to 12 carbon atoms or a fatty acid ester of a monobasic fatty acid having at least 17 carbon atoms with a monohydric alcohol having carbon atoms in a total of from 21 to 23 as added with the number of carbon atoms of the fatty acid.

As the silicone, there may be employed a fatty acid-modified silicone or a partially fluorinated silicone may be employed. As the alcohol, there may be employed a higher alcohol. As the fluorine oil, there may be employed those produced by electrolytic substitution, telomerization or oligomerization.

If no lubricant is incorporated, the back coating layer will have a high frictional coefficient, whereby flickering of the image or jitters are likely to be brought about. Further, since the frictional coefficient is especially high under a high temperature running condition, abrasion of the back coating layer is likely to be led, and the tape winding tends to be irregular.

As the dispersing agent, there may be employed an organic titanium coupling agent, a silane coupling agent or a surfactant. As the antistatic agent, there may be employed a natural surfactant such as saponin; a nonionic surfactant such as an alkylene oxide-type, a glycerine-type or a glycidol-type; a cationic surfactant such as a higher alkyl amine, a quaternary ammonium, pyridine or other heterocyclic compounds, phosphonium or a sulfonium; an anionic surfactant containing acid groups such as carboxylic acid groups, sulfonic acid groups, phosphoric acid groups, sulfate groups or phosphate groups; or an amphoteric surfactant such as an amino acid, an amino sulfonic acid or a sulfuric acid or phosphoric acid ester of an amino alcohol.

In the present invention, the fine particulate inorganic oxide pigment, carbon black, the organic binder, the lubricant and other additives are usually used in the following proportions for the back coating layer. With respect to the fine particulate pigment incorporated in the organic binder and the lubricant in the back coating layer, the weight ratio of the fine particulate pigment to the organic binder is preferably from 4/1 to 1/4. If the amount is excessive, the pigment tends to be hardly dispersed since it is very fine. On the other hand, if the amount is too small, the back coating layer tends to be brittle. The ratio is more preferably from 3/1 to 1/3.

The weight ratio of the organic binder to the lubricant is preferably 100:20.

With respect to the carbon black, the weight ratio of the carbon black to the fine particulate pigment is preferably from 1/9 to 8/2, more preferably from 1/9 to 5/5.

If the amount of the binder is excessive, blocking tends to occur. On the other hand, if the binder is insufficient, adhesion is likely to occur during the calender treatment, such being undesirable.

The thickness of the back coating layer of the present invention is usually within a range of from 0.3 to 10 .mu.m after being coated and dried.

As the active energy rays to be used for crosslinking in the present invention, there may be employed electron beams generated from a radiation accelerator as the source of radiation, .gamma.-rays generated from Co.sup.60 as the source of radiation, .beta.-rays generated from Sr.sup.90 as the source of radiation, or X-rays or ultraviolet 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.

On the other hand, as the magnetic layer of the present invention, there may be employed a coating type composed of a coating layer comprising fine ferromagnetic particles and a binder, or a thin metal film type composed of a thin layer of ferromagnetic metal. As the ferromagnetic material, there may be mentioned .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3 -Fe.sub.3 O.sub.4 solid solution, Co compound-adsorbed .gamma.-Fe.sub.2 O.sub.3, Co compound-adsorbed Fe.sub.3 O.sub.4 inclusive of an intermediately oxidized state with .gamma.-Fe.sub.2 O.sub.3 (the Co compound here is meant for e.g. cobalt oxide, cobalt hydroxide, cobalt ferrite or cobalt ion-adsorbed substance, whereby the magnetic anisotropy of cobalt is utilized for the improvement of the coercive force), or iron, cobalt, nickel, other ferromagnetic metal or a magnetic alloy such as Fe-Co, Fe-Ni, Co-Ni, Fe-Rh, Fe-Cu, Fe-Au, Co-Cu, Co-Au, Co-Y, Co-La, Co-Pr, Co-Gd, Co-Sm, Co-Pt, Ni-Cu, Fe-Co-Nd, Mn-Bi, Mn-Sb, Mn-Al, Fe-Co-Cr or Co-Ni-Cr. Further, a ferrite magnetic substance such as Ba ferrite or Sr ferrite may be mentioned.

Heretofore, there have been commonly used as the ferromagnetic powder, for instance, .gamma.-Fe.sub.2 O.sub.3, Co-containing .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing Fe.sub.3 O.sub.4 or CrO.sub.2. However, the magnetic characteristics such as the coercive force and the maximum residual magnetic flux density of these ferromagnetic powders, are inadequate for high sensitivity high density recording, and they are not suitable for use for magnetic recording of a signal having a short recording wavelength of a level of at most about 1 .mu.m or for the magnetic recording with a narrow track width.

As the requirements for the magnetic recording media have become severe, there have been developments in or proposals for ferromagnetic powders having characteristics suitable for high density recording. Such ferromagnetic powders include metals or alloys such as Fe, Co, Fe-Co, Fe-Co-Ni or Co-Ni, and alloys thereof with Al, Cr or Si. For the purpose of high density recording, the magnetic recording layer using such an alloy powder is required to have a high coercive force and a high residual magnetic flux density. Therefore, it is desirable that various methods for the production or alloy compositions are selected to meet these requirements.

The present inventors have prepared magnetic recording media by employing various alloy powders, and have found that a magnetic recoridng medium with a noise level being sufficiently low and suitable for high density short wave recording is obtainable when the specific surface area as measured by BET method is at least 48 m.sup.2 /g, the coercive force of the magnetic layer is at least 1000 Oe, and the surface roughness of the magnetic layer is at most 0.08 .mu.m as a R.sub.20 value (an average value of 20 values) with a cut off of 0.17 mm by Talystep method. When such a magnetic layer is combined with the back coating layer of the present invention, the cinching phenomenon (the loosening of the tape winding when the tape has been stopped abruptly), dropouts and friction can be reduced. Further, there is a trend that as the base for the magnetic tape, a plastic film made of e.g. polyethylene terephthalate, polyethylene naphthalate, polyimide or polyamide has a thickness as thin as about 11 .mu.m or less, and accordingly there is an increasing tendency for greater tightening of the tape winding, thus leading to the transfer of the surface roughness of the back coating surface to the magnetic layer and the consequential decrease in the output. Such drawbacks are also overcome by the above-mentioned combination of the magnetic recording layer and the back coating layer according to the present invention. Further, when a ferromagnetic metal is used as the major component of the ferromagnetic material, it is likely that the electric resistance of the layer is high, and dropouts are likely to result, and accordingly it is necessary to take a certain antistatic measure. However, such a problem can be solved also by the combination with the back coating layer of the present invention.

The preferred range of the coercive force of the above-mentioned magnetic recording layer is from 1,000 to 2,000 Oe. If the coercive force exceeds this range, the magnetic head tends to be saturated at the time of recording, or it becomes difficult to erase the magnetization. In general, the greater the specific surface area of the magnetic powder, the more the S/N ratio will be improved. However, if the specific surface area is too large, the dispersibility of the magnetic powder into the binder tends to be poor, or the effectiveness tends to be saturated. On the other hand, the surface roughness of the magnetic recording layer affects the recording sensitivity. If the surface roughness is small, the recording sensitivity for a short wavelength will increase.

As a ferromagnetic alloy satisfying the above characteristics, there may be employed a fine powder of Co, Fe-Co, Fe-Co-Ni or Co-Ni, or such a fine powder mixed with Cr, Al or Si. It may be a fine powder obtained by wet-reducing a metal salt with a reducing agent such as BH.sub.4, a fine powder obtained by coating the surface of iron oxide with a Si compound and dry-reducing the product in H.sub.2 gas, or a fine powder obtained by evaporating an alloy in a low pressure argon atmosphere, and it has an axial ratio of from 1:5 to 1:10 and a residual magnetic flux density Br of from 2,000 to 3,000 gauss, and satifies the above-mentioned conditions for the coercive force and the specific surface area.

Various binders may be employed in combination with the alloy magnetic powder to prepare magnetic coating compositions. It is usually preferred to employ a thermosetting resin binder or a radiation curable binder. As other additives, there may be employed a dispersant, a lubricant or an antistatic agent in accordance with the conventional method. Since there is a problem in the dispersibility because of the employment of the magnetic powder having a BET specific surface area of at least 48 m.sup.2 /g, it is advisable to employ a surfactant, an organic titanium coupling agent or a silane coupling agent as the dispersant. As the binder, there may be employed a binder comprising a vinyl chloride-vinyl acetate-vinyl alcohol copolymer, a polyurethane prepolymer and a polyisocyanate, such a binder which further contains nitrocellulose, other known thermosetting binders, or a radiation curable binder containing resin groups such as acrylic double bonds or maleic double bonds sensitive to ionized energy.

In accordance with a usual method, the alloy magnetic powder is mixed with the binder and a predetermined solvent and various additives to prepare a magnetic coating material. The coating material is then coated on a substrate such as a polyester base film and then subjected to thermosetting or radiation curing to form a magnetic layer, and further to calender treatment.

In a case where a radiation curable binder is used for both the magnetic layer and the back coating layer, a continuous curing is possible, and no transfer of the roughness of the rear side to the magnetic layer is likely to take place, whereby dropouts can be prevented. Besides, the radiation curing can be conducted on-line, thus serving for the energy saving and manpower saving for the production, which in turn results in the reduction of the costs. From the property point of view, it is possible not only to prevent dropouts due to the tightening of the winding during the heat curing operation but also to prevent the difference in the output in the longitudinal direction of the magnetic tape attributable to the difference in the pressure of the respective portions in the direction of radius of the rolled tape. When the base thickness is as thin as 11 .mu.m or less and the surface hardness of the magnetic layer is small as the hardness of the magnetic metal powder is smaller than the magnetic oxide such as .gamma.-Fe.sub.2 O.sub.3, the magnetic recording medium is susceptible to the influence of the tightening of the tape winding. However, by means of the radiation-cured back coating layer, this adverse effect can be eliminated, and the output difference as between the outside and the inside of the wound tape, or the difference in the dropouts can be eliminated. Thus, it is particularly preferred to employ a radiation curable binder for the back coating layer.

In addition to the above combination, a combination of the back coating layer of the present invention with a thin ferromagnetic metal film as the magnetic recording layer provides excellent electromagnetic conversion characteristics and good surface roughness and serves to prevent curling and minimize the dropouts.

As described in the foregoing, in the present invention, a fine particulate pigment having an average particle size of primary particles of at most 50 m.mu.m and a specific surface area of at least 35 m.sup.2 /g as measured by BET method, is incorporated in the back coating layer, whereby the surface roughness of the back coating layer is improved, the deterioration of the electromagnetic conversion characteristics is avoided, the envelope property is improved, the friction is minimized, and the damage to the tape guides during the tape running is avoided. When the average particle size of the pigment is 50 m.mu.m or less, the pigment acts as if it were soft, whereby even a pigment having great hardness does not damage the tape guides, and its desirable characteristics can be utilized. Thus, it is possible to obtain a superior magnetic recording medium.

The magnetic recording medium of the present invention can be used in a wide range of various fields such as audio tapes, video tapes, computers and magnetic discs.

Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted by these specific Examples.

In the following Examples, different kinds of magnetic layers and back coating layers were formed. Magnetic tapes were prepared with various combinations of such magnetic and back coating layers and examined for the effects of the present invention.

The mixture having the above composition was mixed in a ball mill for 3 hours to adequately wet the acicular magnetic iron oxide with the dispersant.

Then, the following mixture was thoroughly mixed and dissolved.

The mixture thus obtained was introduced into the ball mill where the magnetic powder was treated, and the mixture was dispersed again for 42 hours. Ater the dispersing, 5 parts by weight (as solid content) of an isocyanate compound (Desmodule L, manufactured by Bayer AG) reactive and cross linkable with functional groups composed mainly of hydroxyl groups of the binder in the magnetic coating material, was mixed with the above coating material in the ball mill for 20 minutes.

The magnetic coating material thus prepared was applied onto a polyester film having a thickness of 15 .mu.m, oriented on a parmanent magnet (1600 gauss), and the solvent was evaporated by means of an infrared lamp or hot air. Then, the coated film was subjected to surface smoothing treatment, and then kept in a rolled state in an oven maintained at 80.degree. C. to promote the cross linking reaction by the isocyanate.

The mixture having the above composition was mixed in a ball mill for 3 hours to adequately wet the acicular magnetic iron oxide with the dispersant.

Then, the following binder composition was thoroughly mixed and dissolved.

The binder mixture was introduced into the ball mill wherein the magnetic powder was previously treated, and the mixture was again mixed and dispersed for 42 hours.

The magnetic coating material thus obtained was applied onto a polyester film having a thickness of 15 .mu.m, and oriented on a parmanent magnet (1600 gauss). After evaporating the solvent by means of an infrared lamp or hot air, the coated film was subjected to surface smoothing treatment, and then electron beams were irradiated by means of an electro curtain-type electron beam accelerator manufactured by ESI Company at an acceleration voltage of 150 KeV at an electrode current of 20 mA and at a total dose of 5 Mrad in a nitrogen atmosphere, to cure the coating layer.

The methods for the preparation of the radiation sensitive binders in the above mixture will be described as follows:

(a) Synthesis of an acryl-modified product of a vinyl chloride-vinyl acetate-vinyl alcohol copolymer:

750 Parts by weight of S-lec A, 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.degree. C., 6.14 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 80.degree. C. in a nitrogen (N.sub.2) 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.degree. C. in a four-necked flask of one-liter capacity in a nitrogen (N.sub.2) stream. Thereafter, 260 parts by weight of 2-hydroxyethyl 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.degree. to 85.degree. C. After completion of the dropwise addition, the mixture was stirred for three hours at 80.degree. 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 urethane elastomer:

250 Parts by weight of an isocyanate-terminated diphenylmethane diisocyanate (MDI) type urethane prepolymyer, "Nipporan 3119", manufactured by Nippon Polyurethane Industry Co., 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.degree. 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.degree. to 90.degree. C. After completion of the dropwise addition, the reaction was conducted at 80.degree. C. until the conversion of NCO reached at least 95%.

Each of these two back coating layers was applied, respectively, to the side of the substrate opposite to the side on which the magnetic recording layer was previously formed, so that the dried thickness would be 1.0 .mu.m, then dried and subjected to calender treatment for smoothing the surface. Then, electron beams were irradiated on the back coating layer by means of an electron curtain type electron beam accelerator at an acceleration voltage of 150 KeV, at an electrode current of 10 mA and at an absorption dosage of 3 Mrad in a nitrogen atmosphere to cure the back coating layer.

Various characteristics of the magnetic tapes obtained from various combinations of the above magnetic layers and back coating layers are shown in Table 1. The SiO.sub.2 used for the back coating layer 2 had an average particle size of 16 m.mu.m and a BET specific surface area of 130 m.sup.2 /g. As a Comparative Example, TiO.sub.2 having a BET specific surface area of 20 m.sup.2 /g and an average particle size of 0.25 .mu.m was used instead of TiO.sub.2 in the back coating layer 1 (see Comparative 1').

From Table 1, it is evident that Sample Nos. 2, 3 and 4 where at least one of the layers is made of a radiation curable type resin, have better electromagnetic conversion characteristics, and particularly that those where the organic binder of the back coating layer is composed of (A), (B) and (C) (i.e. the back coating layer 2) are superior without abrasion of the back coating layer. Further, when the average particle size is at most 50 m.mu.m, good results are obtained without abrasion of the tape guides. Whereas, in the Comparative Example, the abrasion of the tape guides is substantial, the tape winding appearance is consequently inferior, and foreign matters from the abrasion of the tape guides deposit on the back coating surface, whereby the friction increases and the dropouts likewise increase.

Now, with respect to the magnetic tape prepared by the combination of the above magnetic layer 2 and back coating layer 2, the changes of various characteristics due to the changes in the particle size of the fine particulate pigment are shown in Table 2. As the fine particulate pigment, SiO.sub.2 prepared by the above-mentioned method (2) was used instead of the colloidal silica of the back coating layer 2. Further, as a Comparative Example, the pigment having a particle size outside the scope of the present invention was used for the back coating layer.

From Table 2, it is evident that when the fine particulate pigment of the present invention is employed, even if its hardness is high, the obtained magnetic recording tape is surprisingly excellent without abrasion of the back coating layer and with superior electro-magnetic characteristics so long as the average particle size is not greater than 50 m.mu.m. The friction is low from the initial stage and remains to be low after the running operations. Accordingly, the winding appearance of the tape is good, and no image flickering is observed. On the other hand, when the average particle size is 100 m.mu.m, the friction increases.

Different alloy magnetic layers were formed in the following manner, and magnetic recording media were prepared by combining them with the back coating layers of Example 1 and examined for the effects of the present invention.

Various alloy powders were prepared by a wet-reduction method. These powders were composed of acicular particles having an axial ratio (short axis/long axis) of from 1/5 to 1/10 and had a residual magnetic flux density of from 2,000 to 3,000 gauss, a coercive force of from 1,000 to 2,000 Oe and a specific surface area of from 45 to 70 m.sup.2 /g as measured by BET method. These magnetic powders were mixed in the following proportions in a usual manner to obtain the respective magnetic layers.

To this mixture, 30 parts by weight of polyisocyanate (Desmodule L manufactured by Bayer AG) was added to obtain a magnetic coating composition. The magnetic coating composition was applied onto a polyester film in a thickness of 3.5 .mu.m and then subjected to calender treatment.

The same magnetic alloy powder and base as used for the magnetic layer 3 were employed. The following mixture was applied onto a polyester film in a thickness of 3.5 .mu.m and subjected to electron beam curing and calender treatment.

These magnetic layers 3 and 4 were combined with the back coating layers 1 and 2 to obtain magnetic recording media. The characteristics of the magnetic recording media are shown in Table 3. For the back coating layer 2, SiO.sub.2 (average particle size: 12 m.mu.m, BET specific surface area: 200 m.sup.2 /g) was used. As Comparative Examples, SiO.sub.2 having an average particle size of 100 m.mu.m and a BET specific surface area of 25 m.sup.2 /g was used (back coating layer 2') as SiO.sub.2 for the back coating layer 2. However, in this Example, calender treatment was conducted following the formation of each layer.

In the Table, (1) and (2) represent the order for the formation of the respective layers.

From Table 3, it is evident that also in the case of the alloy magnetic powder, the surface roughness of the back coating layer was satisfactory, no deterioration of the electromagnetic conversion characteristics was observed, no abrasion of tape guides took place, and no image flickering was observed. Accordingly, even fine dropouts were not observed. It is very important that the coating layer is tough and free from abrasion, and dropouts are minimum. Whereas, when the average particle size was great, the abrasion of the tape guides was substantial, the winding appearance of the tape was inferior, and the friction was high.

Further, in the combination of the magnetic layer 3 and the back coating layer 1, when the back coating layer was first formed, followed by the formation of the magnetic layer, the electromagnetic conversion characteristics was 0.0 at the outside of the tape winding and -0.8 at the inside of the tape winding, thus indicating a substantial influence of the tightening of the tape winding during the thermosetting. Accordingly, in such a case, it is better to form the magnetic layer first to minimize the deterioration of the electromagnetic conversion characteristics.

Then, the surface roughness of the video tapes obtained by the above combination of the magnetic layer 4 and the back coating layer 2 was studied. FIG. 1 shows the S/N ratios (relative values) in the case where the video tapes were driven at a speed of 3.8 m/sec, and the recording and reproduction were conducted with a major frequency of 5 MHz. The numerical values allotted to the curves indicate the surface roughness of the magnetic layer. As is evident from the Figure, the S/N ratio can be kept at a high level when the surface roughness of the magnetic layer is at most 0.08 .mu.m, and the surface roughness of the back coating layer is at most 0.6 .mu.m. Similar results were obtained in the case of other combinations.

The relation between the BET specific surface area of the alloy powder and the S/N ratio was investigated with respect to the above video tapes wherein the surface roughness of the magnetic layer was at most 0.08 .mu.m and the surface roughness of the back coating layer was within a range of from 0.05 to 0.6 .mu.m, whereby the results as shown in FIG. 2 were obtained. However, 55 dB was employed as the reference. From FIG. 2, it is apparent that excellent characteristics are obtainable when the BET value is at least 48 m.sup.2 /g. Similar results were obtained in the case of other combinations.

Further, the tightening of the tape winding was measured, and at 40.degree. C. under a relative humidity of 80%, satisfactory results were obtained in all cases.

On one side of a polyethylene terephthalate base film having a thickness of 10 .mu.m, a Co-Ni alloy (Hc: 1100 Oe) was vapour-deposited by vacuum deposition in an average thickness of 0.2 .mu.m to form a thin magnetic layer.

Each of the back coating layers 1 and 2 of Examples 1 and 2 was applied, respectively, to the side of the substrate opposite to the side on which the magnetic recordihg layer composed of a thin metal film was thus formed, so that the dried thickness would be 1.0 .mu.m, then dried and subjected to calender treatment for smoothing the surface. Then, electron beams were irradiated on the back coating layer by means of an electron curtain type electron beam accelerator at an acceleration voltage of 150 KeV, at an electrode current of 10 mA and at an absorption dosage of 3 Mrad in a nitrogen atmosphere to cure the back coating layer. Various characteristics of these magnetic tapes were tested, and the results are shown in Table 4. For the back coating layer 2, SiO.sub.2 (average particle size: 30 m.mu.m, BET specific surface area: 80 m.sup.2 /g) was used.

From Table 4, it is evident that also in the case of the tapes obtained by vapour deposition of metal, the surface roughness of the back coating layers was improved, whereby not only the electromagnetic conversion characteristics were improved but also the abrasion of the tape guides was eliminated, the running properties were good, and the problem of curling specific to metal vapour deposition tapes, was solved.

In the above back coating layer 2, x parts by weight of SiO.sub.2 (average particle size: 30 m.mu.m, BET specific surface area: 80 m.sup.2 /g) and y parts by weight of carbon black (30 m.mu.m) (x+y=200) were incorporated. The characteristics of a magnetic tape prepared by the combination of the back coating layer thus obtained and the magnetic layer 2, are shown in Table 5.

From Table 5, it is evident that the incorporation of carbon black is effective for the improvement of the surface properties and for the reduction of dropouts. Further, it is evident that the ratio of the inorganic pigment/carbon black is preferably from 9/1 to 1/9, more preferably from 7/3 to 3/7, since if it exceeds 9/1, the dropouts increase, and if it is less than 1/9, the winding appearance of the tape is inferior, and the abrasion is observed. Further, the difference in the specific gravity increases by the incorporation of carbon, whereby the friction is substantially reduced.

Further, similar results are obtainable when an alloy magnetic layer or a ferromagnetic thin film is used for the combination.

The molecular weights of the polymers and oligomers used for the back coating layers of the present invention, are number average molecular weights obtained by the following measuring method.

GPC (Gel Permeation Chromatography) is a method of separating molecules in a test sample depending upon their sizes in a mobile phase, wherein a porous gel functioning as a molecular sieve is filled in a column whereby liquid chromatography is conducted. For the calculation of an average molecular weight, a polystyrene having a known molecular weight is used as a standard sample, and a calibration curve is prepared based on its eluting time. An average molecular weight calculated as polystyrene is thereby obtained. ##EQU1## where Mn is a number average molecular weight, and Ni is a number of molecules having a molecular weight of Mi in a given polymer substance.

The above-mentioned various characteristics were measured or evaluated as follows:

1. Abrasion of the back coating layer

By means of a commercially available VHS system VTR, a tape was permitted to run 100 times at 40.degree. C. under a relative humidity of 80%, whereupon the scratches on the back coating surface were observed. Symbol .circleincircle. indicates a very clean surface condition, symbol .circle. indicates that no stain was observed, and symbol.times.indicates that substantial stains were observed.

2. Dropouts

By using a VHS deck at 40.degree. C. under a relative humidity of 80%, a single signal of 5 MHz was recorded and reproduced, whereby a number of samples wherein the reproduced signal lowers by at least 18 dB from the average reproduced level for at least 15 micro seconds, were counted with respect to 10 samples for 1 minute each, and the average number was taken as dropouts. The dropouts prior to the running test of the magnetic tape (initial) and the dropouts after 100 runs were measured.

3. Surface roughness

The surface roughness was obtained by 20 point average method from the chart obtained by means of Talystep (manufactured by TAYLOR-HOBSON CO.). A cut off of 0.17 mm and a needle-pressure of 0.1.times.2.5.mu. were employed.

4. Electromagnetic conversion characteristics

The recording and reproduction were conducted at a major frequency of 5 MHz and the S/N ratio (relative value) at the time of a departure by 0.7 MHz from 5 MHz was taken. The Comparative Example was used as the reference being 0 dB. A VTR of VHS was modified so that it was capable of measuring upto 5 MHz.

5. Electron microscopic method

(a) The average particle size was measured by a transmission electron microscope by selecting a sample from each tape.

(b) A cross sectional photographic method by means of a scanning type electron microscope.

6. Frictional coefficient

A magnetic tape was put around a polished aluminum cylinder having a diameter of 4 mm at an angle of 180.degree. C. so that the back coating layer is located inside, and permitted to run at a speed of 2 cm/sec, whereby the tensions at the dispensing side and at the winding-up side were measured, and the frictional coefficient was obtained by calculation from the measured values.