Patent ID: 7232607

Claim:
A thermally foamable microsphere having a structure in which a foaming agent is encapsulated in a shell formed of a polymer, wherein: (1) the shell formed of a polymer is formed of: (a) a vinylidene chloride (co)polymer obtained by polymerization of vinylidene chloride alone or a mixture of vinylidene chloride with a vinyl monomer copolymerizable therewith as a polymerizable monomer, and a tri- or poly-functional crosslinkable monomer having three or more polymerizable carbon-carbon double bonds, or (b) a (meth)acrylonitrile co(polymer) obtained by polymerization of (meth)acrylonitrile alone or a mixture of (meth)acrylonitrile with a vinyl monomer copolymerizable therewith as a polymerizable monomer, and a tri- or poly-functional crosslinkable monomer having three or more polymerizable carbon-carbon double bonds, wherein the crosslinkable monomer is used at a proportion of 1.1 to 5% by weight based on the polymerizable monomer, and (2) a maximum expansion ratio of the thermally foamable microsphere is at least 5.