Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2002-02-04
2003-05-27
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S404000, C428S405000, C428S407000, C428S428000, C428S328000, C428S329000, C428S690000, C428S690000, C428S690000
Reexamination Certificate
active
06569530
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic recording medium and magnetic composite particles for magnetic recording medium, and more particularly, to a magnetic recording medium having excellent strength, surface smoothness and durability as well as low light transmittance, and magnetic composite particles used for such a magnetic recording medium.
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.
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, the S/N ratio of a magnetic recording medium.
With a recent tendency toward reduction in thickness of the magnetic recording layer, magnetic recording media themselves tend to be poor in durability. Therefore, the magnetic recording media have been strongly required to have an improved strength and durability in themselves.
As to this fact, in Japanese Patent Application Laid-Open (KOKAI) No. 5-298679(1993), it is described that “. . . With recent remarkable development of magnetic recording techniques, higher image quality and higher sound quality have been required. For satisfying these requirements, it has been attempted to improve the electromagnetic performance of magnetic recording media, especially to reduce the size of ferromagnetic particles and increase the density of the particles in the magnetic recording layer. In addition, the magnetic tape has been required to have a smoother surface in order to reduce its noise level and improve its C/N ratio . . . . However, during running of the magnetic tape within a magnetic recording/reproducing device, the friction coefficient of contact between the magnetic recording layer of the magnetic tape and the device is considerably increased, so that the magnetic recording layer tends to be severely damaged or peeled from the magnetic tape even after used for a short period of time. In particular, in the case of video tapes, since the magnetic recording medium is traveled at a high speed while contacting with a video head, ferromagnetic particles contained therein tend to be fallen off from the magnetic recording layer, resulting in clogging of the magnetic head. Therefore, the magnetic recording medium has been strongly required to show an improved running durability. . . . ”.
Hitherto, in order to enhance durability of magnetic recording media themselves, oxides particles such as alumina have been added as abrasives into a magnetic recording layer thereof. However, it is known that when a large amount of alumina having a poor dispersibility is added to the magnetic recording layer, the obtained magnetic recording media suffer from not only increased drop-out, but also deteriorated surface smoothness. Therefore, it has been required to provide magnetic particles capable of maintaining a good durability of magnetic recording media even though the amount of abrasives such as alumina added is reduced.
In the progress of reduction in thickness of the magnetic recording layer, such a magnetic recording medium having at least one undercoat layer formed on a non-magnetic base film which comprises a binder resin and non-magnetic particles dispersed in the binder resin such as acicular hematite particles (hereinafter referred to as “non-magnetic undercoat layer”), has been proposed and practically used in order to solve problems such as poor surface properties, deteriorated electromagnetic performance, etc. of the magnetic recording layer (Japanese Patent Publication (KOKOKU) No. 6-93297(1994) and Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338(1987), 63-187418(1988), 4-167225(1992), 4-325915(1992), 5-73882(1993) and 5-182177(1993)).
However, when the magnetic recording layer is formed on such a non-magnetic undercoat layer, the obtained magnetic recording medium is deteriorated in durability though the surface smoothness thereof is improved.
As to this fact, in Japanese Patent Application Laid-Open (KOKAI) No. 5-182177(1993), it is described that “. . . Although the adverse influence of rough surface of the base film is avoided by forming a thick non-magnetic undercoat layer on the base film and then forming the magnetic layer as an upper layer thereon, there still remain problems such as head abrasion and poor durability. The reason therefor is considered to be that since thermosetting resins have been conventionally used as the binder resin of the non-magnetic undercoat layer, the frictional contact between the magnetic layer and the magnetic head or other members is performed under a non-buffering condition because of the hardened undercoat layer, or the magnetic recording medium having such a undercoat layer shows a somewhat poor flexibility. . . . ”.
Thus, it has been required to provide magnetic particles for magnetic recording media which are capable of maintaining and improving strength and durability of the magnetic recording media even when the magnetic recording layer is formed on the non-magnetic undercoat layer.
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 production of a 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 fine particles 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 fine particles impairs not only the enhancement of the magnetic recording density but also the development of a thinner recording layer. 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 fine particles which are added to a magnetic recording layer.
Consequently, it has been strongly demanded to provide a magnetic recording medium capable of exhibiting a low light transmittance even when the amount of carbon black fine particles added to a magnetic recording layer thereof is reduced as low as possible.
It is known to use magnetic particles on which carbon black is adhered in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of the magnetic particles, in order to not only reduce an amount of fine carbon black particles incorporated into the magnetic recording layer but also lower a light transmittance of the obtained magnetic recording media (Japanese Patent Application Laid-Open (KOKAI) Nos. 2000-36404, 2000-138115, 2001-2424, 2001-2425, 2001-15317 and 2001-15318).
At present, it has been strongly required to provide magnetic particles capable of producing magnetic recording media having excellent strength and surface smoothness as
Hayashi Kazuyuki
Iwasaki Keisuke
Kamigaki Mamoru
Morii Hiroko
Nixon & Vanderhye P.C.
Resan Stevan A.
Toda Kogyo Corporation
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