Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reissue Patent
1998-07-02
2003-03-25
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S323000, C428S336000, C428S690000, C428S690000, C428S690000, C428S690000, C428S690000, C428S690000, C428S690000, C428S690000, C428S900000, C428S328000, C428S332000, C428S928000, C428S329000
Reissue Patent
active
RE038048
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium with reduced noise at high density recording and high Output which are required in, for example, a small size video tape with high picture quality. In particular, the present invention provides a magnetic recording medium, such as a magnetic tape, which has a magnetic layer and a substrate, a backcoating layer having a total thickness of not greater than 14 &mgr;m used in a wound form, such that the noise does not increase after long term storage.
2. Description of the Related Art
In general, a magnetic recording medium comprises a substrate made of, for example, a polyester film and a magnetic layer formed on a surface of the substrate by applying a magnetic paint which comprises a magnetic powder, a binder, inorganic powder, a lubricant and the like dispersed in an organic solvent and drying it. In particular, in case of a video tape, a backcoating layer comprising a binder, inorganic powder, a lubricant and the like is formed on a back of the substrate to improve running properties.
To prevent abrasion during contact to a magnetic head or guide rollers, it is proposed to add inorganic powder having a Mohs' hardness of 5 or higher to both the magnetic layer and the backcoating layer.
The addition of the inorganic powder having Mohs' hardness of 5 or higher to the backcoating layer is disclosed in Japanese Patent Kokai Publication Nos. 112/1987, 38525/1987, 38526/1987, 38527/1987 and 38528/1987 and the addition of the inorganic powder having Mohs' hardness of 5 or higher to the magnetic layer is disclosed in Japanese Patent Publication Nos. 1857/1972, 15003/1973, 39402/1974, 28642/1977, 49961/1977 and 15771/1980.
To achieve a high density recording, it is necessary to shorten the recording wavelength. Therefore, the magnetic powder having a particle size smaller than the recording wavelength is used.
To obtain high output, many proposals have been made to improve magnetic characteristics. For example, Japanese Patent Publication No. 37761/1986 discloses the use of ferromagnetic metal powder having high saturation magnetization to achieve high output.
In addition, to decrease the noise, Japanese Patent Publication Nos. 17404/1977 and 12688/1985 disclose finishing of the surface of the magnetic layer by a super calendering treatment to smoothen the surface.
However, while the magnetic recording medium, which comprises the ferromagnetic metal powder as a magnetic recording element and is produced by surface finishing, has a low original noise, the noise increases after storage.
This problem is remarkable when the magnetic powder has a small particle size and a metal magnetic powder which is softer than iron oxide base magnetic powder is used, and it is found that the noise against a signal has a frequency corresponding to recording wavelengths not longer than about 0.8 &mgr;m. This problem is found in almost all prototype magnetic recording media, while the prototype magnetic recording media having no backcoating layer keeps the original low noise.
Therefore, the present inventors thinking that this problem might be caused by the backcoating layer have made extensive study to find that surface roughness of the backcoating layer could be transferred to the surface of the magnetic layer when the magnetic recording medium is wound and the transferred roughness might cause noise.
It was found that the increase of noise was remarkable in a winding type magnetic recording medium having a total thickness of 14 &mgr;m or less of the substrate, the magnetic layer and the backcoating layer. This is because such a thin magnetic recording medium is wound in a larger number of turns than a thick magnetic recording medium having a total thickness of about 20 &mgr;m in a unit winding diameter, and a near hub, winding force is larger.
Japanese Patent Kokai Publication No. 32122/1985 describes the similar phenomena and proposes to prevent flaws of tape guides by using a combination of inorganic powder having Mohs' hardness of 5 or higher and a particle size of not larger than 0.2 &mgr;m and a soft inorganic powder in an amount larger than the former inorganic powder.
Japanese Patent Kokai Publication No. 255584/1988 recognizes the decrease of S/N and C/N of the magnetic recording medium by the transfer of the surface roughness of the backcoating layer to the surface of the magnetic layer and describes that an amount of a solvent remained in the magnetic layer may relate to this problem.
However, the above patent literature relates to cobalt-containing iron oxide magnetic powder and a VHS type magnetic recording medium having the total thickness of about 20 &mgr;m of the substrate, the magnetic layer and the backcoating layer. Since in the VHS system, the shortest recording wavelength of the white level is only 1.3 &mgr;m, the above patent literature does not suggest the decrease of noise in the magnetic recording medium which comprises the ferromagnetic metal powder as the magnetic recording element and uses a center recording wavelength of 0.8 &mgr;m.
SUMMARY OF THE INVENTION
It has been determined that the affects on the magnetic and backcoating layer depends not only on the hardness of the particles contained in the layers but also on particle size, and that the reinforcing effect of the particles is decreased when the particle size is smaller than a specific size. In view of this fact, it has been further found that in order to prevent damage of the surface of the magnetic layer by the inorganic powder having a Mohs' hardness of 5 or higher, which is contained in the backcoating layer when the medium is wound, the inorganic powder in the backcoating layer should have a particle size smaller than a major axis of the metal magnetic powder.
Second, to improve the running properties, the backcoating layer should contain carbon black. This is disclosed in Japanese Patent Kokai Publication No. 8328/1987 which proposes improvement of running durability of the magnetic recording medium by the addition of fine carbon black having a particle size of from 10 to 30 m&mgr; and coarse non-magnetic powder having a particle size of 200 to 500 m&mgr; to the backcoating layer whereby increase of drop out is prevented.
However, this patent literature is silent on the relationship between the primary particle size or particle size of agglomerates of the carbon black and the major axis size of the ferromagnetic metal powder in the magnetic layer, or does not investigate the relationship between the the primary particle size or particle size of agglomerates of the carbon black and the noise against the signal having a frequency corresponding to the recording wavelength of 0.8 &mgr;m or less when the ferromagnetic metal particles have a small particle size.
Then, in the course of completing the present invention, the relationship between the primary particle size or particle size of agglomerates of the carbon black and the major axis size of the ferromagnetic metal powder in the magnetic layer has been studied, and it has been made clear that, when the primary particle size or particle size of agglomerates of the carbon black is larger than the ferromagnetic metal powder, the requirement for the low noise at the high output is fulfilled.
This is remarkable in the magnetic recording medium which is required to have a high recording density, in particular which comprises a magnetic layer having a thickness of not larger than about 3.0 &mgr;m for satisfying the requirement for a thinner magnetic layer.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, in order to provide a magnetic recording medium which comprises a magnetic layer and a backcoating layer on respective surfaces of a non-magnetic substrate, and to achieve high output and suppress an increase of noise in high density recording at a center recording wavelength of not longer than 0.8 &mgr;m even after long term storage, it is preferable that the magnetic layer have a squareness ratio of at lea
Kato Akira
Kawarai Seigi
Yoshida Ken-ichiro
Birch & Stewart Kolasch & Birch, LLP
Hitachi Maxell Ltd.
Kiliman Leszek
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