MAGNETIC RECORDING MEDIUM

Abstract: To improve electromagnetic conversion characteristics by lowering a friction coefficient of a coating type magnetic recording medium and improving travelling stability, a magnetic layer contains carbon black having a mean primary particle diameter of 25 to 65 mµm and a standard deviation σ of a particle diameter distribution of 12 to 24 mµm.

Description: TECHNICAL FIELD This invention relates to a coated type of magnetic recording media such as various magnetic tapes or various floppy disks. BACKGROUND ART Various magnetic recording media including on a nonmagnetic support substrate a coated type magnetic layer obtained by dispersing ferromagnetic powders in a binder are generally used. In order to achieve an improvement in the recording density, however, it is naturally necessary to record high-frequency signals having short recording wavelengths. This, however, makes a lowering of the reproduction output unavoidable. In order to prevent such an output drop, it is necessary to bring a medium in as microscopically close contact with the associated head as possible, thereby reading signals from the medium with a reduced spacing loss and without waste. For this purpose, the magnetic layer is required to be much more smoothed on its surface than ever before. However, smoothing the surface of the magnetic layer gives rise to an increase in the area of contact of the magnetic head with the medium, which in turn makes their coefficient of friction too large to disturb the stable running of the medium and decrease the wear resistance of the medium or head. It has been known so far in the art that improving the running stability of a magnetic recording medium is achieved by adding various lubricants to the magnetic layer, thereby lowering the coefficient of friction. However, excessive use of the lubricants so as to improve the surface slip characteristics of the magnetic layer causes the magnetic layer to bloom out on the surface, rendering the running of the medium unstable, permits the associated magnetic head to be clogged up, dusted or otherwise have trouble, and gives rise to output and S/N ratio drops. When the amounts of the lubricants added are too small, on the other hand, their lubricating action becomes insufficient, failing to improve the running characteristics of the medium. Thus, not only is it very difficult to use the magnetic recording medium, but much difficulty is involved in designing it. Apart from this, it has been known to use an MT class of low-structure carbon black so as to achieve a lowering of the coefficient of friction (JP-P-1-32573, JP-Kokai-58-133626, etc.). However, such carbon black having a large particle diameter in a matter of 200 mµm have been found to be less effective, because its unfavorable dispersibility gives rise to considerable output and S/N ratio drops. From US-A-4,772,522 a magnetic recording medium is known comprising a substrate, an optional undercoat layer and a magnetic layer formed thereon containing a hexagonal plate magnetic powder, an electroconductive material and a binder. The carbon black used should be in the range of 10 to 100 nm and should have a BET specific surface area of 20 to 300 m 2 /gram. The magnetic powders disclosed have a particle size of 100nm. JP-A-63-55721 discloses a magnetic recording medium comprising a magnetic layer on a substrat comprising at least 2 carbon blacks, contrary to the present application. DISCLOSURE OF THE INVENTION It is a major object of this invention to provide a magnetic recording medium which has a low coefficient of friction and a high running stability, is improved in terms of electromagnetic conversion characteristics and is capable of achieving high-density recording. This object is attained by the following aspects (1) and (2) of this invention. (1) A magnetic recording medium consisting essentially of a nonmagnetic support substrate and one magnetic layer in which ferromagnetic fine powders and one carbon black are dispersed in a binder, wherein: said carbon black has a mean primary particle diameter lying in the range of 30 to 60 nm with the standard deviation σ of the particle size distribution lying at 14 to 20 nm, an oil absorption in DBP of 40 to 90 ml/100 g and a BET specific surface area of 30 to 80 m 2 /g. (2) A magnetic recording medium recited in the above-mentioned (1), in which the sliding speed relative to a magnetic head is at least 0.5 m/sec when used for recording with a magnetic head. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a graphical representation showing the particle size distribution of carbon black added to the magnetic layer of the magnetic recording medium according to this invention, and FIGURE 2 is a graphical representation showing the particle size distribution of two carbon blacks when mixed together for use. OPERATION AND ADVANTAGES OF THE INVENTION In the ensuing description, the illustrative construction of this invention will be explained at great length. For a coated type magnetic recording medium, it has been known that in order to prevent the medium from being injured or otherwise damaged by head clogging-up or dusting and reduce the coefficient of friction, relatively large, hard particles such as those of low-structure carbon black or graphite are provided in the vicinity of the surface of the magnetic layer. In this case, graphite is of large size and so is less effective than carbon black, if they are used at the same amount. As mentioned above, however, the low-structure carbon black used so far to reduce the coefficient of friction is too large in diameter to stand up to practical use in view of electromagnetic conversion characteristics. For this reason, the present invention makes use of a carbon black having a mean primary particle diameter, d, lying in the range of 30 to 60 nm, with the standard deviation σ of the particle size distribution lying at 14 to 20 nm. A large particle diameter type of carbon blacks represented by conventionally used MT and FT classes of low-structure carbon blacks possibly contain particles larger than expected, because they have too large a mean particle diameter with the σ of the particle size distribution being relatively large; that is, considerable deterioration of electromagnetic conversion characteristics is unavoidable. By contrast, a high-structure carbon black having a small mean primary particle diameter in a matter of 20 nm has some antistatic effect and relatively favorable electromagnetic conversion characteristics, but has so far been considered incapable of making any contribution to a lowering of the coefficient of friction. According to this invention, it has now been found that a carbon black having such a mean primary particle diameter d and a given particle size distribution σ corresponding to this d gives a suitable degree of smoothness on the surface of a magnetic layer and so is effective for lowering the coefficient of friction, due to containing no carbon black particles having an extremely large particle diameter. Then, such action is further effectively exerted, while the magnetic medium runs at high speed. In addition, this carbon black is well dispersible in the binder, so that the electromagnetic conversion characteristics required for the magnetic recording medium can be imparted to the magnetic layer. More specifically, when d<30 nm, there is an increase in the coefficient of friction with some deterioration of running stability. This is true even when σ=14 to 20 nm. When d>60 nm, on the other hand, there is some deterioration of electromagnetic conversion characteristics. This is again true even if σ=14 to 20 nm. By contrast, when d=25-65 mµm but σ<14 nm, there is some deterioration of electromagnetic conversion characteristics with deteriorations of running sbability and hence C/N ratios as a result. And when σ>20nm, there is some deterioration of electromagnetic conversion characteristics. These effects are critically improved at d=30 to 60nm and σ=14 to 20 nm. Measurement of the particle diameter of such carbon black, for instance, may be achieved with electron microscope photographs. More illustratively, transmission type electron microscope (TEM) photographs were taken of carbon black particles at magnifications of 10,000-100,000, from which some, say, about 1,000 carbon black particles were sampled at random for actual primary particle diameter measurements. By calculation, they were averaged to find the mean primary particle diameter. From the particle diameters thus determined, the standard deviation σ of the particle diameter deviation σ is found by statistical technique. In the present invention, it is preferred that such carbon black have an oil absorption in DBP of 40-90 ml/g and a specific surface area of 30-80 m 2 /g, as measured by the BET adsorption method using nitrogen adsorption. When the oil absorption in DBP and the BET specific surface area assume such values, the effect of this invention is further improved. It is generally understood that, in a magnetic layer, such carbon black has often a low structure in which one to a few primary particles exist. One or two or more such carbon blacks having such particle diameters and particle size distributions as mentioned above may be used to achieve similar particle diameters and size distributions. In the present invention, no special other limintation is imposed on the carbon black used, if it has such d and σ as mentioned above. To this end, use may be made of different types of carbon black obtained by different production processes such as thermal black, channel black, acetylene black, furnace black and lamp black. In the present invention, it is preferred that the amount of such carbon black used be 15 parts by weight or lower, esp., 1-10 parts by weight per 100 parts by weight of ferromagnetic powders. By allowing the content of the carbon black to lie in this range, it is possible to obtain much more improved running stability and electromagnetic conversion characteristics than ever before. The magnetic layer containing such carbon black is obtained by dispersing ferromagnetic powders in a binder, and is then coated on the nonmagnetic support substrate. The ferromagnetic fine powders used, for instance may be known ferromagnetic fine powders, for instance, those of oxides such as γ-Fe 2 O 3 , Co-containing γ-Fe 2 O 3 , Fe 3 O 4 , Co- containing Fe 3 O 4 , CrO 2 , barium ferrite and strontium ferrite, and those of Fe, Co and Ni or their alloys. As the binder in which these ferromagnetic fine powders are dispersed, use may be made of those generally used for magnetic layers which may or may not be modified or otherwise denatured by radiation sensitization and which may be used alone or in combination of two or more. For instance, mentioned are vinyl chloride resin, vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinyl acetate-vinyl alcohol copolymers, polyvinylidene chloride-acrylonitrile copolymers, phenoxy resin, cellulosic conductives, acrylonitrile-butadiene rubber, styrene-butadiene rubber, epoxy resin, polyurethane resin, polyester resin, polyisocyanate and polyamide. Preferably, these binders have been thermally set by the addition of curing agents or by radiations. If required, the magnetic layer of this invention may contain an abrasive. As the abrasive, use may be made of generally used ones such as alumina, chromium oxide and silicon carbide, but particular preference is given to an abrasive having a Mohs hardness of 6 or higher. If required, the magnetic layer of this invention may additionally contain lubricants, dispersants, antistatics or other additives, for instance, paraffin, silicone oil, fatty acids and fatty acid esters. Note that the magnetic layer is generally about 0.5-5 µm in thickness. The nonmagnetic support substrated used in this invention, for instance, may be plastics such as polyethylene terephthalate, polyethylene naphthalate and polypropylene, glass, ceramics and metals, which may have been surface-treated, if required. The media of this invention may be in the form of tapes, disks and drums. It is noted, however, that their effect is further improved by sliding them with magnetic heads at a running speed of at least 0.5 m/sec. esp., at least 2.5 m/sec. INDUSTRIAL APPLICABILITY According to the present invention, there is provided a magnetic recording medium which has a reduced coefficient of friction and is improved in terms of running stability and electromagnetic conversion characteristics. EXAMPLES In the following description, the present invention will be explained more specifically with reference to some illustrative examples. Example 1 The following carbon black containing composition was well dispersed together by means of a sand grinder mill, and the dispersant was stirred with 3 parts by weight of a binder curing agent polyisocyanate, followed by filtration. This product was then coated on both sides of a polyethylene terephthalate film at a thickness of 5 µm upon dried. The obtained magnetic layer was calendered to smooth its surface, and then punched out to form a magnetic disk of 2 inches in diameter. It is noted that the "parts" will hereinafter be given by weight. Carbon black A Oil absorption 70 ml/100g BET value 55 m 2 /g Mean primary particle diameter 45 mµm σ=17 mµm 5 parts σ-Alumina 5 parts Butyl stearate 1 parts Methyl ethyl ketone 250 parts Comparative Example 1 A magnetic disk was obtained by following Example 1 with the exception that the following carbon black B having a mean primary particle diameter d<25 mµm was used in place of carbon black A. Oil absorption 58 ml/100 g BET value 137 m 2 /g Mean primary particle diameter 24 mµm σ= 7 mµm Comparative Example 2 A magnetic disk was obtained by following Example 1 with the exception that carbon black C of σ<12 mµm was used for carbon black A. Oil absorption 65 ml/100g BET value 60 m 2 /g Mean primary particle diameter 40 mµm σ= 10 mµm Comparative Example 3 A magnetic disk was obtained by following Example 1 with the exception that carbon black D of σ<12 mµm was used for carbon black A. Oil absorption 76 ml/100g BET value 93 m 2 /g Mean primary particle diameter 28 mµm σ=10 mµm Comparative Example 4 A magnetic disk was obtained by following Example 1 with the exception that carbon black E of d>65 mµm and σ>12 mµm was used for carbon black A. Oil absorption 71 ml/100g BET value 25 m 2 /g Mean primary particle diameter 85 mµm σ=32 mµm Comparative Example 5 A magnetic disk was obtained by following Example 1 with the exception that carbon black F of d>>65 mµm and σ>>24 mµm was used for carbon black A. Oil absorption 41 ml/100g BET value 6 m 2 /g Mean primary particle diameter 390 mµm σ=170 mµm Example 2 A magnetic disk was obtained by following Example 1 with the exception that a mixture of 4 parts by weight of carbon black C with 1 part by weight of carbon black E was used instead of 5 parts by weight of carbon black A. Comparative Example 6 A magnetic disk was obtained by following Example 1 with the exception that a mixture of 2.5 parts by weight of carbon black C with 2.5 parts by weight of carbon black E was used instead of 5 parts by weight of carbon black A. Set out in Table 1 are the physical properties of carbon blacks A-F used in Examples 1 and 2 as well as Comparative Example 1-6 and the running properties and electromagnetic conversion characteristics of the magnetic disks containing these carbon blacks. Here the coefficient of friction was found by rotating each disc at a linear speed of 0.7 m/sec, measuring the load on the associated head by means of a strain indicator and calculating the value of µ from this load. The load applied was 30 g. The rotation torque was measured as a current loaded on the spindle motor of a magnetic disk drive. Measurement of the outputs and C/N ratios was done at the outermost peripheries by recording single signals having a recording frequency of 7.0 MHz and then measuring spectra of 7.0 MHz and 6.7 MHz and 6.0 MHz of the reproduced signals with a spectral analyzer. For C/N, the ratios were found. Whether or not there values are practically usable are estimated by ○ and X, which are again shown in Table 1. Note that Fig. 1 represents the particle size distributions of carbon blacks A-E used in Example 1 and Comparative Examples 1-4. Similarly, Fig. 2 illustrates the particle size distributions of a plurality of carbon blacks used in Example 2 and Comparative Example 6. From the results shown in Fig. 1, the critical effect of this invention will be clearly understood.

Claims: A magnetic recording medium consisting essentially of a nonmagnetic support substrate and one magnetic layer in which ferromagnetic fine powders and one carbon black are dispersed in a binder, wherein: said carbon black has a mean primary particle diameter lying in the range of 30 to 60 nm with the standard deviation σ of the particle size distribution lying at 14 to 20 nm, an oil absorption in DBP (dibutylphthalate) of 40 to 90 ml/100 g and a BET specific surface area of 30 to 80 m 2 /g. A magnetic recording medium as claimed in claim 1, wherein, when said recording medium is used for recording with a magnetic head, its sliding speed relative to a magnetic head is at least 0,5 m/sec.

Inventors: HITOMI YOHSUKE, IKARASHI TSUNEHIKO, HITOMI, YOHSUKE, IKARASHI, TSUNEHIKO, HITOMI, YOHSUKE TDK CORPORATION
Applicants: TDK CORP, TDK CORPORATION
Application year: 1992
Main classification: G11B5
IPC codes: C09D5, G11B5, C09D175, C09D127