EMULSION POLYMERIZATION PROCESS, POLYMER DISPERSION AND FILM-FORMING COMPOSITION

The present invention relates to a method to prepare a polymer dispersion using an aqueous substantially surfactant-free emulsion polymerization process comprising a seed and a feed stage, in which seed stage at least one ethylenically unsaturated monomer, having a water-solubility of at least 0.3 g/l at polymerization conditions, is polymerized in the presence of an addition fragmentation chain transfer agent and a hydrophilic free radical initiator to form a seed polymer that is substantially insoluble in water and in which feed stage at least one ethylenically unsaturated feed monomer is added to the seed polymer to form polymer particles. With this method a polymer dispersion can be obtained having at least 25 wt. % solid contents in combination with an average polymer particle size smaller than or equal to 300 nm. The invention further relates to a polymer dispersion obtainable by a surfactant-free emulsion polymerization process, a film-forming composition, preferably a coating composition, comprising such a polymer dispersion and coated articles coated with the coating composition. Further, the invention relates to a polymer particle powder obtained from the polymer dispersion and powder coating compositions.