Elastomeric particles having polymerizable surface layer

Claim 1

A heterogeneous elastomeric particle comprising an elastomeric core whose primary glass transition temperature is below the lowest temperature to which the polymeric composition is likely to be subjected surrounded by a surface layer formed from a polymerizable compound, said surface layer having a glass transition temperature above room temperature and being capable of forming free radicals and being covalently bonded to the elastomeric core, the elastomeric core containing finely divided particles chosen from the group consisting of titanium oxides and iron oxides wherein the finely divided particles are in the size range of 0.01 to 1 micron and the proportion of finely divided particles in the elastomeric core is in the range of 0.1 to 50% W/W and being substantially inert such that the core will not readily generate free radicals under the conditions in which the surface layer readily forms free radicals and will absorb less solvent than 150% w/w of the core.