Thermally foamable microsphere and production process thereof

1. A thermally foamable microsphere having an average particle diameter of 3 to 100 μm, comprising (i) a shell formed of a polymer which is a copolymer of a polymerizable monomer and an organosilicon compound, (ii) a foaming agent wrapped up and encapsulated in the shell formed of the polymer, and (iii) inorganic fine particles having an average particle diameter of up to 1 μm adhered uniformly and firmly to a surface of the shell via the organosilicon compound, wherein the shell formed of the polymer is formed by suspension polymerization of a polymerizable mixture containing at least the foaming agent and the polymerizable monomer in the presence of the organosilicon compound in an aqueous dispersion medium containing a dispersion stabilizer comprising the inorganic fine particles in an amount of 0.1 to 20 parts by weight per 100 parts by weight of the polymerizable monomer in an acidic environment, wherein

a) the foaming agent is a low-boiling organic solvent that gasifies at a temperature lower than a softening point of the shell-forming polymer,

b) the polymerizable monomer is selected from the group consisting of (i) a mixture of vinylidene chloride and at least one vinyl monomer copolymerizable with vinylidene chloride and selected from the group consisting of acrylonitrile, methacrylonitrile, acrylic ester, methacrylic ester, styrene and vinyl acetate, and (ii) a mixture of (meth)acrylonitrile and at least one a vinyl monomer copolymerizable with (meth)acrylonitrile and selected from the group consisting of vinylidene chloride, acrylic ester, methacrylic ester, styrene and vinyl acetate,

c) the organosilicon compound is a silane coupling agent represented by the following formula (1)

YSi(CH 3 ) 3-n X n   (1),

in which

n is 1, 2 or 3;

Y stands for an organic functional group selected from the group consisting of a vinyl group, a methacryl group, an acryl group, and an allyl group; and X stands for —OR or —NR, wherein R is a hydrogen atom or a lower alkyl group,

d) the organosilicon compound is contained in an amount of 0.015 to 1% by weight on the basis of a total weight of said thermally foamable microsphere, and

e) the inorganic fine particles are colloidal silica particles.

2. The thermally foamable microsphere according to claim 1 , wherein said shell-forming polymer comprises a vinylidene chloride copolymer.

3. The thermally foamable microsphere according to claim 1 , wherein the foaming agent comprises isobutane, n-butane, n-pentane, isopentane, n-hexane, isohexane, heptane, 2,2,4-trimethylpentane, petroleum ether, or a mixture of two or more thereof.

4. The thermally foamable microsphere according to claim 1 , wherein said shell-forming polymer comprises a (meth)acrylonitrile copolymer.

5. The thermally foamable microsphere according to claim 1 , wherein said microsphere has an average particle diameter of 5 to 50 μm.