Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
2001-01-12
2002-02-26
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S329000, C428S336000, C428S403000, C428S690000, C428S690000
Reexamination Certificate
active
06350505
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to high-density acicular hematite particles, a non-magnetic undercoat layer containing the high-density acicular hematite particles and a magnetic recording medium having the non-magnetic undercoat layer. More particularly, the present invention relates to high-density acicular hematite particles suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium using magnetic particles containing iron as a main ingredient, which have an excellent dispersibility in binder resin, a pH value of not less than 8, a less content of soluble sodium salts and soluble sulfates, and a high surface conductivity; a non-magnetic undercoat layer containing the high-density acicular hematite particles and suitably used for a magnetic recording medium using magnetic particles containing iron as a main ingredient; and a magnetic recording medium having the non-magnetic undercoat layer.
With a development of miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.
Various attempts have been made at both enhancing the properties of magnetic particles and reducing the thickness of a magnetic recording layer in order to improve these properties of a magnetic recording medium.
The enhancement of the properties of magnetic particles will first be described.
Magnetic particles are required to have, in order to satisfy the above-described demands on a magnetic recording medium, properties such as a high coercive force and a large saturation magnetization.
As magnetic particles suitable for high-output and high-density recording, acicular magnetic particles containing iron as a main ingredient which are obtained by heat-treating acicular goethite particles or acicular hematite particles in a reducing gas are widely known.
Acicular magnetic particles containing iron as a main ingredient have a high coercive force and a large saturation magnetization, since the acicular magnetic particles containing iron as a main ingredient used for a magnetic recording medium are very fine particles having a particle size of not more than 1 &mgr;m, particularly, 0.01 to 0.3 &mgr;m. Therefore, such particles easily corrode, and the magnetic properties thereof are deteriorated, especially, the saturation magnetization and the coercive force are reduced.
In order to maintain the characteristics of a magnetic recording medium which uses magnetic particles containing iron as a main ingredient as the magnetic particles, over a long period, it is strongly demanded to suppress the corrosion of the acicular magnetic particles containing iron as a main ingredient as much as possible.
A reduction in the thickness of a magnetic recording layer will now be described.
Video tapes have recently been required more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
With respect to short wavelength signals, a reduction in the thickness of a magnetic recording layer is also strongly demanded in order to improve the high output characteristics, especially, an S/N ratio of a magnetic recording medium. This fact is described, for example, on page 312 of
Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder,
published by Sogo Gijutsu Center Co., Ltd. (1982), “ . . . the conditions for high-density recording in a coated-layer type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force Hc and residual magnetization Br, . . . and the coating film has a smaller thickness . . . ”.
Development of a reduction in the thickness of a magnetic recording layer has caused some problems.
Firstly, it is necessary to make a magnetic recording layer smooth and to eliminate the non-uniformity of thickness. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the substrate must also be smooth. This fact is described on pages 180 and 181 of
Materials for Synthetic Technology-Causes of Friction and Abrasion of Magnetic Tape and Head Running System and Measures for Solving the Problem
(hereinunder referred to as “
Materials for Synthetic Technology” (
1987), published by the Publishing Department of Technology Information Center, “ . . . the surface roughness of a hardened magnetic layer depends on the surface roughness of the substrate (back surface roughness) so largely as to be approximately proportional, . . . , since the magnetic layer is formed on the substrate, the more smooth the surface of the substrate is, the more uniform and larger head output is obtained, and the more the SIN ratio is improved.”
Secondly, there has been caused a problem in the strength of a non-magnetic substrate such as a base film with a tendency of the reduction in the thickness of the non-magnetic substrate in response to the demand for a thinner magnetic layer. This fact is described, for example, on page 77 of the above-described
Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder,
“ . . . Higher recording density is a large problem assigned to the present magnetic tape. This is important in order to shorten the length of the tape and to miniaturize the size of a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a substrate . . . With the tendency of reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth travel in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the machine direction and in the transverse direction is now strongly demanded . . . ”
The end portion of a magnetic recording medium such as a magnetic tape, especially, a video tape is judged by detecting a portion of the magnetic recording medium at which the light transmittance is large by a video deck. If the light transmittance of the whole part of a magnetic recording layer is made large by the thinner magnetic recording medium or the ultrafine magnetic particles dispersed in the magnetic recording layer, it is difficult to detect the portion having a large light transmittance by a video deck. For reducing the light transmittance of the whole part of a magnetic recording layer, carbon black or the like is added to the magnetic recording layer. It is, therefore, essential to add carbon black or the like to a magnetic recording layer in the present video tapes.
However, addition of a large amount of non-magnetic particles such as carbon black impairs not only the enhancement of the recording density but also the development of a thinner recording layer. Therefore in order to reduce the magnetization depth from the surface of the magnetic tape and to produce a thinner magnetic recording layer, it is strongly demanded to reduce, as much as possible, the quantity of non-magnetic particles such as carbon black which are added to a magnetic recording layer.
It is also strongly demanded that the light transmittance of a magnetic recording layer should be small even if the carbon black or the like which is added to the magnetic recording layer is reduced to a small amount. From this point of view, improvements in a substrate are now in strong demand.
Further, in order to reduce not only the above-mentioned optical transmittance but also surface resist
Hayashi Kazuyuki
Iwasaki Keisuke
Morii Hiroko
Tanaka Yasuyuki
Nixon & Vanderhye
Resan Stevan A.
Toda Kogyo Corporation
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