Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-12-20
2004-04-20
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S328000, C428S690000
Reexamination Certificate
active
06723456
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to hematite particles aggregates, a non-magnetic undercoat layer for magnetic recording medium using the hematite particles aggregates, and a magnetic recording medium having the non-magnetic undercoat layer, and more particularly, to hematite particles aggregates as non-magnetic particles capable of forming a non-magnetic undercoat layer of a magnetic recording medium having a more excellent surface smoothness, which can exhibit not only an excellent dispersibility but also have an improving ability of a surface smoothness of a coating film produced therefrom by calendering treatment; a non-magnetic undercoat layer for magnetic recording medium which contains the hematite particles aggregates and exhibits a more excellent surface smoothness; and a magnetic recording medium having the non-magnetic undercoat layer, which exhibits a more excellent surface smoothness.
With recent tendency toward long-time recording, miniaturization and weight-reduction of video or audio magnetic recording and reproducing apparatuses, magnetic recording media such as magnetic tapes and magnetic discs have been increasingly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, improved frequency characteristics and a lower noise level.
In particular, video tapes have 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 wavelength region have come to be used and as a result, the magnetization depth from the surface of the magnetic tape has come to be remarkably small.
With respect to short wavelength signals, 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. In order to achieve the reduction in thickness of the magnetic recording layer, it is required to smoothen the surface of the magnetic recording layer and eliminate unevenness in thickness thereof. For this purpose, the base film is also required to have a smooth surface.
In ordinary magnetic recording media, the surface of the magnetic recording layer has been smoothened by forming a magnetic recording layer containing magnetic particles and a binder resin on a non-magnetic base film, and then subjecting the magnetic recording layer to calendering treatment.
In recent years, with further reduction in thickness of the magnetic recording layer, there has been proposed such a method of forming one undercoat layer comprising a binder resin and non-magnetic particles such as acicular hematite particles dispersed therein (hereinafter referred to as “non-magnetic undercoat layer”) on a non-magnetic base film in order to solve problems such as deterioration in surface properties and electromagnetic performance of the magnetic recording layer, and magnetic recording media having such a non-magnetic undercoat layer have been already put into practice (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)).
In the case of such magnetic recording media having the non-magnetic undercoat layer, the non-magnetic undercoat layer comprising non-magnetic particles and a binder resin, and the magnetic recording layer comprising magnetic particles and a binder resin are successively formed on the non-magnetic base film, and then the obtained medium is subjected to calendering treatment to absorb irregularities on the non-magnetic base film by the non-magnetic undercoat layer, thereby smoothening the surface of the magnetic recording layer. For example, in Japanese Patent Application Laid-Open (KOKAI) No. 5-12650(1993), it is described that “. . . in the case where a non-magnetic layer is provided, when the layer containing hexagonal system ferrite-based magnetic particles is subjected to surface-smoothing treatment, the non-magnetic layer formed immediately beneath the magnetic layer is collapsed as a buffer layer. At this time, the underlying non-magnetic layer acts as an absorbing layer, so that the surface of the upper magnetic recording layer containing hexagonal system ferrite-based magnetic particles can be smoothened . . . ”.
Hitherto, in order to improve properties of magnetic recording media, various attempts for non-magnetic particles used in the non-magnetic undercoat layer have been conducted. For example, in Japanese Patent Application Laid-Open (KOKAI) No. 6-60362(1994), there is described a non-magnetic undercoat layer for magnetic recording media, which contains non-magnetic particles composed of acicular hematite particles coated with an Al compound. Also, in Japanese Patent Application Laid-Open (KOKAI) No. 10-334450(1998), there is described a magnetic recording medium having a non-magnetic undercoat layer containing fine acicular goethite particles in which three or less particles are overlapped at the same crystal planes and adhered with each other along the crystallographical a-axis direction thereof.
At present, it has been strongly required to provide acicular hematite particles as non-magnetic particles capable of forming a non-magnetic undercoat layer for a magnetic recording medium having a more excellent surface smoothness. However, such acicular hematite particles have not been obtained conventionally.
Namely, the hematite particles described in Japanese Patent Application Laid-Open (KOKAI) No. 6-60362(1994) do not have such a structure as oriented in the major axis direction thereof. Therefore, it is difficult to improve a surface smoothness of a coating film produced using such hematite particles by calendering treatment.
Also, even though the goethite particles described in Japanese Patent Application Laid-Open (KOKAI) No. 10-334450(1998) are used as non-magnetic particles for non-magnetic undercoat layer, it is difficult to attain the aimed dispersibility because of poor compatibility with binder resins or solvents due to a large amount of crystal water contained in the goethite particles.
As a result of the present inventors earnest studies, it has been found that when hematite particles aggregates having an average length of 0.005 to 0.6 &mgr;m and an average width of 0.001 to 0.40 &mgr;m, which are obtained by subjecting acicular goethite particles to milling treatment and then heat-dehydrating the treated particles at a temperature of 200 to 540, are used for forming a non-magnetic undercoat layer for magnetic recording medium, the obtained non-magnetic undercoat layer can be considerably improved in surface smoothness. The present invention has been attained on the basis of this finding.
SUMMARY OF THF INVENTION
An object of the present invention is to provide hematite particles aggregates as non-magnetic particles capable of forming a non-magnetic undercoat layer for magnetic recording medium having a more excellent surface smoothness, which can exhibit not only an excellent dispersibility but also have an improving ability of a surface smoothness of a coating film by calendering treatment.
Another object of the present invention is to provide a non-magnetic undercoat layer for magnetic recording medium having a more excellent surface smoothness, which contains the above hematite particles aggregates.
A further object of the present invention is to provide a magnetic recording medium having the non-magnetic undercoat layer, exhibiting a more excellent surface smoothness.
To accomplish the aims, in a first aspect of the present invention, there are provided hematite particles aggregates comprising aggregates of acicular hematite particles oriented in a major axis direction thereof, said acicular hematite particles having an average major axis diameter of 0.005 to 0.3 &mgr;m and an average minor axis diameter of 0.0005 to 0.10 &mgr;m,
Hayashi Kazuyuki
Ishitani Seiji
Iwasaki Keisuke
Morii Hiroko
Le H. Thi
Toda Kogyo Corporation
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