Magnetic Recording Medium

1. Field of the Invention

The present invention relates to a magnetic recording medium. More particularly, it relates to a composition which constitutes a superior magnetic recording layer in a magnetic recording medium such as a video tape or a computer tape for which durability is required.

2. Description of the Prior Art

The magnetic recording medium such as a video tape or computer tape is brought in contact and frictional engagement with a magnetic head under a high speed, and the magnetic coating layer is likely to be damaged. Accordingly, a magnetic recording medium having superior durability is required. To meet this requirement, a magnetic recording medium has been proposed which has a magnetic coating layer containing from about 1 to about 25% by weight, based on the magnetic powder, of non-magnetic powder having a particle size of at most 5 .mu.m and a Mohs' hardness at least 6. With this magnetic recording medium, the abrasion resistance of the magnetic coating layer is sufficiently improved, but the abrasion of the magnetic head and the decrease in the S/N ratio become substantial, whereby it becomes very difficult to control the abrasion of the magnetic head or the decrease of the S/N ratio and in an extreme case, it is likely that the magnetic head is damaged or the surface of the magnetic head is roughened. Further, when such non-magnetic powder is used in a substantial amount, unwanted substances derived from the abrasion of a dispersing machine during the dispersing operation, are likely to be incorporated, whereby in many cases, the physical property of the binder will be modified and in an extreme case, the coating material tends to be gelled. Thus, when applied to a substrate such coating material has often led to an inferior finish of the surface or deterioration of the electromagnetic conversion characteristics.

Under the circumstances, the present inventors have conducted extensive researches to overcome such difficulties and as a result have found that when two specific types of non-magnetic powders having specific average particle sizes and surface areas are used as a mixture, it is possible to obtain far superior effectiveness as compared with the case where various non-magnetic powders heretofore are used.

The, the present invention provides a magnetic recording medium comprising a magnetic layer coated on a substrate and composed of ferromagnetic powder dispersed in a binder, which is characterized in that the magnetic layer contains TiO.sub.2 having an average particle size of at most 0.8 .mu.m and a surface area of at most 25 m.sup.2 /g as measured by a nitrogen adsorption method, and Al.sub.2 O.sub.3 having an average particle size of at least 0.1 .mu.m and a surface area of at most 30 m.sup.2 /g as measured by a nitrogen adsorption method.

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

The present invention is characterized in that among various non-magnetic powers, TiO.sub.2 having an average particle size of at most 0.8 .mu.m and a surface area of at most 25 m.sup.2 /g as measured by a nitrogen adsorption method Al.sub.2 O.sub.3 having a particle size of at least 0.1 .mu.m and a surface area of at most 30 m.sup.2 /g as measured by a nitrogen adsorption method are used in combination. By the combined use of the two specific types of non-magnetic powders, it is possible, with an addition of a relatively small amount of the non-magnetic powders, to minimize the abrasion of the magnetic head and to prevent the discoloration of the surface layer of the head and the scratching on the head surface which are likely to be caused by the high speed frictional engagement of the magnetic recording tape with the magnetic recording head, whereby it is possible to control the abrasion of the head within an optional limited while maintaining the S/N ratio and the frequency characteristic at high levels. Thus, it is possible to provide a magnetic recording medium having superior durability and capable of ensuring that the head surface be maintained to be smooth and in good condition.

The use of TiO.sub.2 powder or Al.sub.2 O.sub.3 powder has already been proposed. However, it has never been proposed to use them in combination taking into account their surface areas and particle sizes. According to the present invention, unexpectedly superior effects can be attained by the combined use of TiO.sub.2 and Al.sub.2 O.sub.3 powders having the above-mentioned specific surface areas and particle sizes.

As TiO.sub.2 to be used as one of the non-magnetic powders of the present invention, those known by the trade names R-820, R-580 and TY-50 manufactured by Ishihara Sangyo Kaisha Ltd. are particularly useful. As Al.sub.2 O.sub.3, those known by the trade names AKP-20 and AKP-30 manufactured by Sumitomo Chemical Co., Ltd. are useful.

In order to obtain superior characteristics for both the magnetic head and the magnetic recording medium, i.e. to minimize e.g. the head surface roughening, the head abrasion, the degradation of the S/N ratio and the reduction of the reproduction output, it is preferred that the above-mentioned two types of non-magnetic powders are mixed in a ratio of TiO.sub.2 to Al.sub.2 O.sub.3 within a range of from 7:3 to 3:7 weight. This mixture is preferably used in an amount of from 1 to 15 parts by weight, more preferably from 3 to 6 parts by weight, based on 100 parts by weight of the magentic powder. If the ratio of the twoo powders is outside the above range of from 7:3 to 3:7, the synergistic effect of the two types of the non-magnetic powders tends to decrease. Further, if the total amount of the two powders is lower than the lower limit of the above range of from 1 to 15 parts by weight, no adequate effects of the non-magnetic powders are obtainable and if the total amount exceeds the upper limit, the non-magnetic powders tends to adversely affect the magnetic property.

The representative characteristics of the above-mentioned commercial products are as shown in Tables 1 and 2.

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

The above components were introduced in a coating material dispersing machine and thorougly mixed and dispersed. Then, polyisocyanate as a crosslinking agent (Colonate L manufactured by Nippon Polyurethane Co.) was added in an amount of 12% by weight based on the resin components, and the mixture was thoroughly stirred to obtain a uniform coating material. This coating material was applied onto a substrate made of a polyester film having a thickness of 15 .mu.m to form a coating layer having a thickness of 5 .mu.m. Then, the coating layer was subjected to subjected to super calender surface treatment and then cured at a temperature of 60.degree. C. for about 24 hours to obtain a tape material. This tape material was cut in width of 12.65 mm to obtain a magnetic recording video tape. This tape was designated as Sample No. 1.

In the same manner as above, video tapes were prepared by incorporating the respective non-magnetic powders shown in Table 3 into the above-mentioned composition. These video tapes were designated as Sample Nos. 2 to 5.

Video tapes were prepared in the same manner as in Example 1 except that 21 parts by weight of each of AKP-20 and R-580 was used alone. These video tapes were designated as Sample Nos. 6 and 7.

Video tapes were prepared in the same manner as in Example 1 except that the non-magnetic powders shown in Table 4 were used in the same ratio as in Sample No. 3. These video tapes were designated as Sample Nos. 8, 9 and 10.

With respect to each of the video tapes thus obtained by Example 1 and Comparative Examples 1 and 2, the sensitivity at 5 MHz, the S/N ratio, the reduction of the reproduction output, the roughening of the head surface and the wearing of the magnetic head were measured. The results thereby obtained are shown in Table 5.

The sensitivity at 5 MHz and the S/N ratio were represented by relative values based on the respective values of Sample 1 being fixed as 0 (dB).

The reduction of the reproduction output was represented by the degree of the change in the sensitivity for the period of from the start of the reproduction to the end of the reproduction when the recorded video tape having an entire length of 245 m was run for reproduction.

The wearing of the magnetic head was represented by the degree of the wearing as measured after each video tape was continuously run for 10 hours.

It is evident from Table 5 that with the video tapes of Sample Nos. 2 to 5 wherein Al.sub.2 O.sub.3 and TiO.sub.2 were used in combination according to the present invention, it is possible to maintain the sensitivity, the S/N ratio and the reproduction output which are important for the magnetic tapes, in good conditions and at the same time good results are obtainable with respect to the surface roughening and abrasion of the magnetic head. Whereas, Sample Nos. 1 and 6 to 11 have a practical difficulty in at least one of the characteristics.

Thus, the present invention provides totally superior magnetic recording media in which various characteristics are all well balanced.

As the magnetic powder to be used in the present invention, not only .gamma.--Fe.sub.2 O.sub.3 used in Example 1 but also various other magnetic powders such as Co--.gamma.Fe.sub.2 O.sub.3, CrO.sub.2, Fe.sub.3 O.sub.4, dry process reduced metal power or wet process reduced metal powder may be used.