Patent ID: 8486531

Claim:
A plurality of heat-expanded microspheres, each of the heat-expanded microspheres comprising a plurality of heat expandable microspheres that prior to heating include a shell of thermoplastic resin and a blowing agent encapsulated therein having a boiling point not higher than the softening point of the thermoplastic resin, the plurality of heat expandable microspheres having an average particle size from 1 to 100 micrometer, and after heating the heat-expanded microspheres have a void fraction not higher than 0.70, where the void fraction is a physical property value for evaluating packing efficiency of a bulk volume of the heat-expanded microspheres, the void fraction representing the ratio of void in the bulk volume of the heat-expanded microspheres, the heat-expanded microspheres having a repeated-compression durability not lower than 75 percent wherein the heat-expanded microspheres contain aggregated microspheres in an amount not higher than 5 weight percent and contain microspheres having a true specific gravity not lower than 0.79 g/cc at 25 degree. C. in an amount not higher than 5 weight percent.