DETAILED DESCRIPTION OF THE INVENTION Silica-type microparticles useful for the preparation of the fluidity improver of the present invention are, for example, fumed silica, silica aerogel, precipitated silica, and the composite microparticles (silica+another metal oxide) prepared from silicon tetrachloride and another metal halide such as aluminum trichloride, titanium tetrachloride and the like.
Fumed silica is most preferred.
Considered from the standpoint of the properties as a fluidity improver for positively-charging resin powder, the silica-type microparticles preferably have a BET specific surface area of 130 to 400 m2/g.
In order to increase the effects of treatment, the silica-type microparticles preferably contain a small quantity of water rather than being completely anhydrous.
For this purpose, the silica-type microparticles preferably have a water content of 0.
3 to 5 wt %.
It is thought that the water accelerates the condensation reaction between the alkoxy groups in the silane under consideration and the silanol groups on the silica surface.
Such silica-type microparticles include the products available commercially under the following brandnames: Aerosil 130, Aerosil 200, Aerosil 300, Aerosil 380, Aerosil MOX80, and Aerosil MOX170 from Nippon Aerosil Kabushiki Kaisha (Japan); Cab-O-Sil M-5, Cab-O-Sil MS-7, Cab-O-Sil MS-75, Cab-O-Sil HS-5, and Cab-O-Sil EH-5 from Cabot Corporation (USA); and HDK N20, HDK V15, HDK T30, and HDK T40 from Wacker Chemie (West Germany).
With regard to the silane with general structural formula (I) which is used for the treatment of such silica-type micropowders, the alkyl groups which can comprise R are exemplified by methyl, ethyl, propyl, butyl, and decyl; the alkyl groups which can comprise R.
sup.
1 are exemplified by the alkyl groups which can comprise R; R.
sup.
2 is exemplified by ethylene, propylene, butylene, and hexylene; the alkyl groups which can comprise R.
sup.
3 are exemplified by the alkyl groups which can comprise R; and R.
sup.
4 is exemplified by methyl, ethyl, butyl
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Molecular Biology 
