Polymerization process

Claim 1

A coating composition comprising a blend of: (A) composite polymer microparticles having a diameter of from 0.1 to 10 microns, each of which comprises a core of crosslinked polymer surrounded by and grafted to a layer of non-crosslinked polymer, the microparticles being produced as a dispersion in a non-aqueous liquid medium as hereinbelow defined by a process comprising the steps of (1) polymerising one or more ethylenically unsaturated monomers, including at least one crosslinking monomer, from which the crosslinked core polymer of the microparticles is to be derived, in an aqueous medium comprising (a) at least 30% by weight of water and (b) not more than 70% by weight of a second constituent which is miscible with water, the nature and proportion of which are such that the mixture as a whole is capable of dissolving the monomer or monomers being polymerised to the extent of at least 3% by weight but is a non-solvent for the polymer formed, the polymerisation being carried out in the absence of ionic species and at a temperature at least 10° C. higher than the glass transition temperature of the core polymer, in the presence in the aqueous medium as steric stabiliser of a block or graft copolymer which contains in the molecule a polymeric component which is solvatable by the aqueous medium and another polymeric component which is not solvatable by the aqueous medium and is capable of becoming associated with the polymer particles formed, the concentration of free monomer in the polymerisation mixture being maintained throughout this process step at a level such that at no time does the free monomer form a separate phase and the total amount of monomer polymerised being such that the resulting dispersion contains at least 20% by weight of microparticles, (2) polymerising, in the dispersion obtained from step (1) and again in the absence of ionic species, one or more further ethylenically unsaturated monomers, not including any crosslinking monomer, from which the non-crosslinked polymer of the outer layer is to be derived, optionally in the presence of additional block or graft copolymer stabiliser, the concentration of such further monomer in the free state in the polymerisation mixture being maintained at a level such that at no time does that free monomer form a separate phase, and (3) transferring the polymer microparticles from the resulting dispersion into a non-aqueous liquid medium which is a solvent for the non-crosslinkable polymer under such conditions that the particles become stably dispersed therein, with (B) a main film-forming constituent which is compatible with the non-crosslinked polymer component of the microparticles.