Thermally expandable material, method for producing the same and soundproof sheet for automobiles

The invention provides a thermally expandable material comprising a foam composition containing: 100 parts by weight of (a) a crosslinkable polymer material; and 5 to 300 parts by weight of (b) a crystalline thermoplastic resin having oxygen in its molecule. The invention also provides a foam composition for obtaining a thermally expandable material that is expandable by heat from a compressed state, the composition comprising a crystalline thermoplastic resin formed in a predetermined shape and crosslinked.