Method for making hollow porous microspheres

A method for making hollow porous microspheres of 200 to 10,000 microns diameter and 1 to 1000 microns wall thickness from a stable dispersion of a dispersed particle film forming composition, said composition comprising uniformly dispersed particles in a continuous liquid phase, said method comprising feeding said dispersed particle compositon and a blowing gas to a coaxial blowing nozzle, said coaxial blowing nozzle having an inner coaxial nozzle for said blowing gas and an outer coaxial nozzle for said dispersed particle composition and a coaxial blowing nozzle orifice, feeding said blowing gas to said inner nozzle, feeding said dispersed particle composition to said outer nozzle to form and blow, at said coaxial blowing nozzle orifice, hollow dispersed particle microspheres having stable film walls, continuously feeding said dispersed particle composition to said outer coaxial nozzle while said microspheres are being formed, directing an entraining fluid at sadi coaxial nozzle at an angle relative to a line drawn through the center axis of coaxial blowing nozzle, said entraining fluid passing over and around said coaxial blowing nozzle to fluid dynamically induce a pulsating or fluctuating pressure field having periodic oscillations at the opposite or lee side of the blowing nozzle in the wake or shadow of said blowing nozzle, said entraining fluid acting on the microspheres to pinch and close-off the microspheres at a point proximate to the coaxial blowing nozzle and said entraining fluid acting to detach the microspheres from the coaxial blowing nozzle and move the microspheres away from the coaxial nozzle, surface tension forces acting on said hollow microspheres to cause said hollow microspheres to form a spherical shape, treating said detached hollow microspheres to bring the dispersed particles into point to point contact and to harden them to obtain hollow green microspheres;

• and subjecting said hollow green microspheres to a sufficiently high temperature for a sufficient period of time to remove the continuous liquid phase from the hollow green microspheres and to sinter the dispersed particles at their points of contact and to form within the walls of said hollow microspheres interconnecting voids that are continuous from the outer wall surface to the inner wall surface of the hollow microspheres, and to obtain hollow porous microspheres that are substantially spherical, having substantially uniform diameters, substantially uniform wall thickness and substantially uniform distribution of interconnecting voids in the walls of the microspheres.