Preparation and characterization of fine shirasuballoons

Hollow glass microspheres named Shirasuballoons have been prepared from vitric volcanic materials named Shirasu or Hakudo. Fine Shirasuballoons were desired to improve their physical and chemical properties. Preparation conditions of fine Shirasuballoons are examined for five kinds of vitric volcanic materials (Shinjo-Shirasu, Kakuto-Shirasu, Yoshida-Shirasu, Biei-Hakudo and Nakano-Hakudo), Fine Shirasuballoons with particle sizes under 20 mu m and bulk densities under 0.5 g/cm(3) were prepared by heating rapidly the milled powders except Shinjo-Shirasu using a fluidized bed furnace. Foaming conditions are discussed by water contents and foaming temperatures, Evolved gases from the raw materials on heating in vacuum were almost water. Two water desorption peaks are observed; a large amount of water desorption peak at temperature ranges of 200-300 degrees C and a small amount of water desorption peak around 700 degrees C, where the latter peak was not observed for the Shinjo-Shirasu with a low water content. Existence states of water are interpreted by infrared absorption spectrum. The shell thickness of fine Shirasuballoons produced is closely related to the water content of the raw material, which was interpreted by the phenomenon that an increase in water content of glass results in a decrease in viscosity at a foaming temperature.