Thermo-expansive microspheres, their production process and their application

Volume-retaining material produced by filling pressure vessel with expanded hollow microspheres which are produced by heating thermo-expansive microspheres at a temperature not lower than the softening point of their thermoplastic resinous shell to expand to a volume of 10 or more of expansion ratio, and which are characterized by their true specific gravity of 0.1 or lower and particle size distribution with a CV, or coefficient of variation, of 30% or less,

• wherein the thermo-expansive microspheres comprise a shell of thermoplastic resin and a blowing agent encapsulated in the shell, and have a retaining ratio of blowing agent encapsulated being 90% or more;
• said blowing agent including at least one member selected from the group consisting of C3F7OCH3 and C7F15OC2H5, and gasifying at a temperature not higher than the softening point of said thermoplastic resin;
• the surface of said microspheres is coated with 0.1 to 95 weight percent of a fine-particle coating agent comprising at least one selected from the group consisting of organic compounds having a melting point of 90° C. or higher and inorganic compounds formed into layer lattice;
• and said thermoplastic resin comprises a polymer of monomeric mixture containing 0.01 to 0.43 weight percent of a polymerizable monomer having at least two polymerizable double bonds.