Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-06-12
2004-04-13
Jones, Deborah (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S332000, C428S323000, C428S357000, C428S402000, C075S300000, C252S06251C, C252S062560, C148S105000
Reexamination Certificate
active
06720094
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to secondary agglomerates of magnetic metal particles for magnetic recording and a process for producing the same, and more particularly, to secondary agglomerates of magnetic metal particles for magnetic recording, which exhibit not only excellent handing property due to high storage efficiency, high transport efficiency and good flowability thereof, but also excellent kneading property when kneaded with various binder resins and organic solvents in a kneader, and excellent dilution-dispersibility when diluted with an additional amount of the organic solvent, upon the production of coating-type magnetic recording media, so as to more improve a surface smoothness and squareness of a magnetic coating film obtained therefrom, and a process for producing such secondary agglomerates of magnetic metal particles for magnetic recording.
In recent years, recording-time prolongation, miniaturization and lightening of audio, video or computer magnetic recording and reproducing apparatuses for various magnetic recording media such as digital audio tapes (DAT) for people's livelihood use, 8-mm video tapes, Hi-8 tapes, VTR tapes for business use, computer tapes or discs thereof have proceeded more rapidly. In particular, VTRs (video tape recorders) are now widespread, so that there have been intensely developed VTRs aiming at the transfer of analog recording types into digital ones in addition to the above recording-time prolongation, miniaturization and lightening thereof. On the other hand, with such recent tendencies, the magnetic recording media have been required to have high image quality and high output characteristics, especially high frequency characteristics. To meet these requirements, it is necessary to reduce noise due to the magnetic recording media themselves and enhance residual magnetic flux density, coercive force, dispersibility, filling property and tape-surface smoothness thereof. Therefore, it ha been further required to improve S/N ratio of the magnetic recording media.
These properties of the magnetic recording media have a close relation to magnetic particles used therein. In recent years, magnetic metal particles have been noticed because such particles can show a higher coercive force and a larger saturation magnetization as compared to those of conventional magnetic iron oxide particles, and have been already used as magnetic particles for magnetic recording media such as DAT, 8-mm video tapes, Hi-8 tapes, video tapes for business use, computer tapes or discs. The magnetic metal particles containing iron as a main component conventionally used in DAT, 8-mm video tapes, Hi-8 tapes or the like have been required to be further improved in output characteristics and weather resistance. In addition, the magnetic metal particles must fulfill applicability to existing format and good economy at the same time. Therefore, it has been strongly required to provide magnetic metal particles capable of satisfying the above requirements while minimizing amounts of various metals added thereto.
Various properties of coating-type magnetic recording media are detailed below.
In order to obtain high image quality in video magnetic recording media, it has been required to enhance S/N ratio and video frequency characteristics thereof. For this reason, it is important to improve a surface smoothness of the magnetic recording media. For improving the surface smoothness, it is also required to improve a dispersibility of magnetic particles in coating composition as well as orientation and filling properties thereof in coating film. In addition, in order to enhance the video frequency characteristics, the magnetic recording media have been required to exhibit not only a high coercive force and a large residual magnetic flux density, but also an excellent S.F.D. (Switching Field Distribution), i.e., a small coercive force distribution.
As known in the arts, the magnetic metal particles are produced by granulating as a starting material goethite particles, hematite particles obtained by heat-dehydrating the goethite particles, or particles obtained by incorporating metal elements other than iron into the above goethite or hematite particles, to form a granulated product having an appropriate size; and then heat-reducing the resultant granulated product. In this case, it is important to appropriately control the shape and size of the goethite particles as a starting material, and prevent occurrence of heat fusion between particles as well as deformation and shape breakage of each particle upon heat-treatments such as heat-dehydration and heat-reduction.
In general, the granulated product of magnetic metal particles is directly charged into a kneader and kneaded therein with various binder resins and organic solvents.
More specifically, in the production of coating-type magnetic recording media, after the granulated product of magnetic metal particles is kneaded with various binder resins and organic solvents in the kneader, the obtained kneaded material is mixed and diluted with an additional amount of the organic solvent and the obtained magnetic coating composition is coated on a non-magnetic substrate. As described above, since the dispersibility of the magnetic metal particles influences the surface smoothness of the magnetic coating film obtained therefrom, the poorer the dispersibility of the magnetic metal particles, the lower the squareness of the magnetic coating film. Therefore, it has been strongly required that the magnetic metal particles have an excellent dispersibility, and the granulated product of magnetic metal particles is readily deaggregated into the magnetic metal particles as primary particles.
On the other hand, it is preferred that the granulated product of magnetic metal particles has a bulk density as large as possible. This is because when the granulated product is filled in a container for temporary storage in warehouse or transport thereof, the size of the container used therefor can be reduced, thereby saving the storage space and reducing the transport cost. However, in general, particles having a large bulk density exhibit a deteriorated flowability, thereby causing such a clogging or bridging phenomenon that the discharge of the particles from an outlet of storage or transport container or receptacle is completely disturbed or frequently interrupted even though the outlet diameter is far larger than the size of the particles, i.e., causing considerable deterioration in handling property of the particles. Therefore, it has been strongly required to provide granulated product having an appropriate flowability in addition to other good properties.
It is generally known that the flowability of the granulated product varies depending upon particle diameter, particle density, particle shape and surface property thereof. Therefore, the present inventors' attention has been paid to the particle diameter and particle density.
As conventional techniques for improving magnetic properties, dispersibility, flowability, keeping properties or the like of magnetic particles for magnetic recording media, there are known those described in Japanese Patent Application Laid-Open (KOKAI) Nos. 62-275028(1987), 63-88807(1988) and 3-276423(1991), Japanese Patent Publication (KOKOKU) Nos. 1-52442(1989), 4-70363(1992) and 7-62900(1995), Japanese Patent Application Laid-Open (KOKAI) No. 8-172005(1996) or the like.
At present, it has been strongly required to provide granulated product of magnetic metal particles which can exhibit not only excellent kneading property when kneaded with various binder resins and organic solvents and excellent dilution-dispersibility when diluted with an additional amount of the organic solvent, upon the production of coating-type magnetic recording media, but also high storage efficiency and high transport efficiency as well as good flowability. However, such granulated product of magnetic metal particles cannot be obtained yet.
Namely, in Japanese Patent Application Laid-Open (KOKAI)
Okinaka Kenji
Uegami Masayuki
Jones Deborah
Koppikar Vivek
Nixon & Vanderhye P.C.
Toda Kogyo Corporation
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