Patent ID: 8388809

Claim:
Thermally expandable thermoplastic microspheres comprising a polymer shell made from ethylenically unsaturated monomers encapsulating a propellant, said ethylenically unsaturated monomers consisting essentially of from 20 to 70 wt % of acrylonitrile, from 20 to 80 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, from 0 to 10 wt % of methacrylonitrile, from 0 to 25 wt % of monomers selected from the group consisting of esters of methacrylic acid, and optionally a small amount of one or more crosslinking multifunctional monomers, the total amount of acrylonitrile and acrylate monomers constituting from 50 to 100 wt % of said ethylenically unsaturated monomers, and said propellant comprising isobutane.