Stock material or miscellaneous articles – All metal or with adjacent metals – Composite powder
Reexamination Certificate
2000-07-05
2002-05-14
Mai, Ngoclan (Department: 1742)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite powder
C427S212000, C427S213300, C427S214000, C427S216000, C427S217000, C427S219000
Reexamination Certificate
active
06387532
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a technique for producing a powder performing a combination of functions by coating the surface of a powder particle with plural layers of other substance. More particularly, the present invention relates to a film-coated powder comprising a base particle having thereon a film formed by reaction in an aqueous solvent, and to a process for producing the same.
BACKGROUND ART
A technique comprising coating the surface of a powder with another substance to improve the properties of the powder or impart a variety of properties thereto is known. Various means therefor have been conventionally proposed.
For example, many coating techniques for forming a film on the surface of an object for the purpose of protection or decoration are known, such as spreading method, precipitation method, sputtering, vapor deposition, electrodeposition, anodization, and the like. However, in the spreading method and the precipitation method, it is difficult to form a film in an even thickness. In the sputtering and the vapor deposition, it is difficult to obtain a thick coating film. Furthermore, the electrodeposition and the anodization have a problem that these techniques are unsuitable for the treatment of a powder because the treated material should be used as an electrode.
With progress in various technical fields, a powder having unique properties, especially a metal or metal compound powder, is desired. Namely, a powder which combines properties inherent only in a powder, especially a metal or metal compound powder, with other properties so as to have a combination of functions is desired. Such powders were thought to be produced by forming plural layers of a metal oxide film etc. having an even thickness on a base particle.
As a useful method of metal oxide formation for providing a powder, especially a metal or metal compound powder, which has a combination of properties capable of meeting new requirements and can perform a combination of functions such as described above, the present inventors invented a powder comprising a base of metal or a metal compound having thereon a metal oxide film having a thickness of 0.01 to 20 &mgr;m and containing a metal component which is different from the metal constituting the base by dispersing metal powders or metal oxide powders in a metal alkoxide solution and hydrolyzing the metal alkoxide to form a coating film of a metal oxide (JP-A-6-228604).
When plural layers of the metal oxide film are formed on the powder, a special function can be imparted by regulating the thickness of each layer of the film. For example, when coating films which are different in refractive index are formed on the surface of a base in a thickness corresponding to one-fourth the wavelength of light, all the light is reflected. When this means is applied to a powder comprising a magnetic material as the base, such as a powder of a metal, e.g., iron, cobalt, nickel etc., a powder of a metal alloy, or a powder of iron nitride, then a magnetic powder for magnetic toners can be obtained which totally reflects light and has a shining white color. The reference further discloses that when a colored layer is formed on the powder and a resin layer is formed on the surface thereof, then a magnetic color toner is obtained.
The present inventors further improved the powder described above and disclosed also a powder having not one or more metal oxide films alone but two or more metal oxide films arranged alternately with two or more metal films (JP-A-7-90310). This powder has excellent properties when used as a magnetic color toner or the like.
As a method for forming a metal oxide coating film on a powder in water, a process for producing a magnetic metal powder is disclosed, which comprises adding an aqueous nickel compound solution to an alkaline suspension of a powder mainly comprising iron oxyhydroxide or iron oxide, subsequently adding an aqueous silicon compound solution, depositing the nickel compound as nickel hydroxide on the surface of the powder particles, and then neutralizing the liquid to further deposit a silicon compound on the surface of the powder particles, whereby the nickel compound and the silicon compound are successively deposited on the surface of the powder particle (JP-A-59-31003).
The present inventors directed attention to the fact that the metal oxide film formed by the hydrolysis of a metal alkoxide is extremely dense and inert and is also preferred from the standpoint of durability. Namely, even when the precipitation method is used to deposit a film on a powder particle by precipitation from an aqueous metal salt solution, the powder particle is protected under the reaction conditions for the precipitation method so long as the powder particle is coated with the metal oxide film. The present inventors thus disclosed a multilayer-coated powder characterized in that the multilayered film comprises at least one layer comprising a metal hydroxide or metal oxide film formed by the hydrolysis of a metal alkoxide and, as a layer disposed on the layer, a coating film comprising a metal hydroxide or metal oxide film formed by reaction of a metal salt in water (Japanese Patent Application No. Hei. 8-147422).
Furthermore, the present inventors found that the reflected-light interference waveform for a multilayered film can be adjusted by regulating a combination of materials for the multilayered film and regulating the film thickness. The inventors thus disclosed a pigment for color ink giving a single color, such as blue, green, yellow, and the like, and a filler for plastics or paper can be designed by coloring a powder comprising a base particle of a specific gravity of from 0.3 to 2.8 g/cm
3
, such as an acrylic resin particle, an inorganic hollow particle, and the like, having provided thereon thin coating films which are different in refractive index (titanium dioxide film, titania film, polystyrene film, silver metal film, etc.), followed by dispersion to a fluid, even without using a dye or pigment, and that a pigment powder having a stable color tone even when stored over long is provided (WO 96/28269).
However, the method in which a metal alkoxide is hydrolyzed in order to form a coating film of a metal oxide on the surface of a metal powder or metal compound powder necessitates use of a highly flammable organic compound as a solvent and use of a metal alkoxide, which is an expensive compound, as a starting material. In order to use a highly flammable organic solvent, it is necessary to use an explosion-proof production equipment and to strictly control temperature and humidity. The overall cost of the product produced using the equipment is, of course, high.
On the other hand, the method in which a coating film is formed by the precipitation from an aqueous metal salt solution caused by reacting the metal salt has the following drawback. When a base particle made of a metal or the like is directly used in the reaction, the base is attacked by an acid or alkali and is hence dissolved due to that the solvent is in strongly acidic or alkaline conditions. As a result, a preferred film-coated powder is not obtained.
Consequently, a base particle made of a metal or the like cannot be used as it is, and a coating film inert to acids or alkalis must be first formed on the surface of the base particle, for example, by the hydrolysis of a metal alkoxide in the manner described above, or the base to be dispersed in a solvent must be limited to oxides and the like having resistance to acids, alkalis, and the like.
Accordingly, there is a problem that it is inevitably necessary to use the method based on the hydrolysis of a metal alkoxide, which is costly and highly dangerous, or to limit the material of the base particle in production.
Furthermore, in the coating film formation by the reaction of a metal salt, a solid phase ingredient may excessively precipitate depending on conditions for acid or alkali neutralization or for heating. As a result, the film deposited on the base surface is uneven in thickness, and there
Atarashi Takafumi
Kishimoto Akira
Nakatsuka Katsuto
Mai Ngoclan
Nittetsu Mining Co., Ltd.
Sughrue & Mion, PLLC
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