Methods for producing hollow microspheres made from dispersed particle compositions

A method for making hollow microspheres of substantially uniform diameter of 500 to 6000 microns and of substantially uniform wall thickness of 5 to 400 microns 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 composition 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 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 said coaxial nozzle at an angle relative to a line drawn through the center axis of the 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, said dispersed particle composition forming the walls of said hollow microspheres and said dispersed particles being uniformly dispersed in said continuous liquid phase, treating said detached hollow microspheres to remove a portion of the continuous liquid phase to bring the dispersed particles into point to point contact and to harden them, said dispersed particles being of sufficient size and in a sufficient amount such that the dispersed particles in the walls of the hollow microspheres link up and form a relatively rigid latticework of dispersed particles to obtain hollow green microspheres that have substantially uniform distribution of the dispersed particles in the continuous phase of the walls of the microspheres;

• 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 continuing to heat said hollow microspheres for a sufficient period of time to close and seal said interconnecting voids to obtain hollow microspheres in which the interconnecting voids have been closed and sealed.