Polymeric particles and their preparation

A process for the preparation of core/sheath polymer particles which comprises establishing a monomer system of at least one ethylenically unsaturated monomer with acid functionality, polymerizing said system in the form of an aqueous emulsion to form core particles, establishing an aqueous dispersion of said core particles and a mixture of a nonionic monoethylenically substituted aliphatic monomer, said nonionic monomer being free of functional acid groups and containing at least 2 weight per cent of an ester-containing unsaturated monomer selected from those monomers which on homopolymerization produce a polymer having a glass transition temperature not higher than 15.degree. C. and from 0 to 25 mole per cent of a nonionic monoethylenically unsaturated aromatic monomer and effecting co-polymerisation of the monomers to deposit on said core particles a hard sheath free of acid groups and having a glass transition temperature of at least 40.degree. C. and mixing said core/sheath particles so formed with a non-volatile fixed or permanent base in the absence of organic solvent to swell said cores at an elevated temperature to generate therein one or more voids.