Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2000-08-29
2002-08-27
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S329000, C428S332000, C428S690000, C428S690000, C423S625000, C423S629000
Reexamination Certificate
active
06440552
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a boehmite and a base coat layer for magnetic recording medium, which is obtained by using the boehmite.
BACKGROUND OF THE INVENTION
In order to improve high density recording performance of magnetic recording medium such as magnetic tape and running durability thereof, a method has been known in which a base coat layer, that is a non-magnetic layer formed by dispersing a non-magnetic pigment in a binder, is placed between a substrate and a magnetic layer to obtain a magnetic recording medium.
Various proposals have been made for improving a base coat layer. For example, a needle-like iron oxide (&agr;-Fe
2
O
3
) has been utilized as a non-magnetic pigment in a base coat layer (as described in JP-A-10-198948).
The base coat layer obtained by using the needlelike iron oxide, however, does not have a satisfactory surface smoothness. As a result, the base coat layer needs a thick magnetic layer to be placed thereon. It is known that a thinner magnetic layer can provide an excellent magnetic recording medium with a lower noise and a higher output than a thick layer. Therefore, a base coat layer having a satisfactory surface smoothness has been demanded.
SUMMARY OF THE INVENTION
Objects of the present invention are to provide a non-magnetic pigment which can provide a base coat layer having an excellent surface smoothness in a magnetic recording medium and to provide a base coat layer for magnetic recording medium which comprises the nonmagnetic pigment.
The present inventors have conducted extensive studies on non-magnetic pigments for base coat layer having an excellent surface smoothness for use in magnetic recording medium. As a result, it has been found that a boehmite (alumina monohydrate) having a specific crystallite diameter can provide a base coat layer having excellent surface smoothness. Thus, the present inventors have completed the present invention.
The present invention provides a boehmite in which (i) a crystallite diameter in the direction perpendicular to the plane (
020
) is about 65 angstrom or longer and (ii) a ratio of a crystallite diameter in the direction perpendicular to the plane (
002
) to a crystallite diameter in the direction perpendicular to the plane (
200
) is about 1.8 or more.
The present invention also provides a base coat layer for magnetic recording medium, which comprises a binder and a boehmite in which (i) a crystallite diameter in the direction perpendicular to the plane (
020
) is about 65 angstrom or longer and (ii) a ratio of a crystallite diameter in the direction perpendicular to the plane (
002
) to a crystallite diameter in the direction perpendicular to the plane (
200
) is about 1.8 or more.
DETAILED DESCRIPTION OF THE INVENTION
A boehmite of the present invention has a crystallite diameter in the direction perpendicular to the plane (
020
) of about 65 angstrom or longer, preferably about 70 angstrom or longer.
A boehmite of the present invention has a ratio of a crystallite diameter in the direction perpendicular to the plane (
002
) to a crystallite diameter in the direction perpendicular to the plane (
200
) of about 1.8 or more, preferably about 2.0 or more.
The crystallite diameter is the value which is measured from an X-ray diffraction of the crystals using the Scherrers's equation(described in Example).
When a crystallite diameter in the direction perpendicular to the plane (
020
) is less than about 65 angstrom, and/or a ratio of a crystallite diameter in the direction perpendicular to the plane (
002
) to a crystallite diameter in the direction perpendicular to the plane (
200
) is less than about 1.8, a base coat layer obtained therefrom does not have excellent surface smoothness.
The boehmite of the present invention may have a sodium content of 100 ppm or less, preferably 50 ppm or less. When the sodium content is more than 100 ppm, a magnetic recording medium which has a base coat layer containing such a boehmite may have a large friction coefficient, or have a lower lubricating or running property due to a reaction between sodium and a lubricant agent when a lubricant agent is contained in the base coat layer.
The boehmite of the present invention may have a BET specific surface area of from 50 m
2
/g to 150 m
2
/g, preferably from 60 m
2
/g to 130 m
2
/g. When the BET specific surface area is larger than 150 m
2
/g, dispersion of the boehmite in the binder tends to be difficult when a base coat layer is produced using the boehmite.
The boehmite of the present invention may be produced, for example, by a method in which aluminum hydroxide as a raw material is subjected to hydrothermal treatment in an acidic or neutral aqueous solution, a method in which pH of aqueous solution containing aluminum is swung from acidic to alkaline or from alkaline to acidic to crystallize a boehmite (pH-swing method), a method in which aluminum hydroxide is subjected to steam treatment under pressure.
In the following, the method in which aluminum hydroxide is subjected to hydrothermal treatment in an acidic aqueous solution is described in detail.
Aluminum hydroxide as a raw material can be obtained by known methods. Examples thereof include a method in which an aluminum alkoxide is hydrolyzed, a method in which an aluminum salt such as sodium aluminate is neutralized, a method in which an intermediate alumina such as &rgr;-alumina is re-hydrated, a method in which an aluminum salt such as aluminum nitrate is hydrolyzed in supercritical water and the like. Amongst them, the method in which aluminum alkoxide is hydrolyzed can be preferably applied, since the sodium content of the resulting aluminum hydroxide can be reduced.
Examples of the acidic aqueous solution utilized for the hydrothermal treatment of aluminum hydroxide include an aqueous solution of organic acid such as acetic acid, and an aqueous solution of inorganic acid such as hydrochloric acid. An organic acid such as acetic acid is preferably utilized, since corrosion of apparatus can be prevented. The concentration of acid in the acidic aqueous solution may be about 0.1% by weight or more, preferably from about 0.5% by weight to about 10% by weight, more preferably from about 1% by weight to about 8% by weight.
The hydrothermal treatment may be effected by placing an acidic aqueous solution and aluminum hydroxide (in an amount such that the concentration of aluminum hydroxide falls within a range of from about 2% by weight to about 32% by weight based on the acidic aqueous solution) in a pressure vessel equipped with a stirrer and heating, followed by heating. The crystallite diameter may vary depending on concentration of acid in the acidic solution, temperature for treatment, and period of time for treatment and, therefore, these conditions are adjusted so that the resulting boehmite has a desired crystallite diameter in the direction perpendicular to the plane (
020
) and a ratio of a crystallite diameter in the direction perpendicular to the plane (
002
) to a crystallite diameter in the direction perpendicular to the plane (
200
).
For example, the hydrothermal treatment may be carried out at the temperature of from about 150° C. or more, preferably from about 180° C. to about 230° C. in acidic aqueous solution in which the concentration of acid is from about 1% by weight to about 8% by weight. The time of the hydrothermal treatment become shorter as the temperature become higher. It is not preferable to carry out the hydrothermal treatment at the temperature of about 230° C. or more because an apparatus for the treatment becomes more expensive. It requires the time of about 3 hours or more to carry out the hydrothermal treatment at the temperature of less than about 180° C. The time of the hydrothermal treatment is usually selected from the range of about 0.5 hours to about 6 hours.
When the hydrothermal treatment is carried out at a higher temperature and/or for a longer period of time, the crystallite diameter tends to be greater.
After the hydrothermal treatment, the resulting boehmite may be collecte
Kajihara Kazuhisa
Takeuchi Yoshiaki
Resan Stevan A.
Sumitomo Chemical Company Limited
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