NUCLEATION OF EMULSION POLYMERIZATION IN THE PRESENCE OF SMALL SILICA PARTICLES

Several silica were used : either non functionalized silica from commercial origin or prepared according to the Stober method, or functionalized silica prepared upon surface coverage of non functionalized silica with functional coupling agents such as X(CH2)3Si(OR)3 where X is either a methacryloyl group or the precursor amino group for an azo compound. Some organophobic silica can be prepared via a sol-gel process from co-hydrolysis and co-condensation of tetraethoxysilane (TEOS) and the functional coupling agent with water in inverse microemulsion in the presence of a suitable surfactant. The average diameter of the silica particles is in the range 20-90 nm. When the silica is hydrophilic, encapsulation with polymer upon emulsion copolymerization of highly hydrophobic monomers (styrene, butyl acrylate) is not observed : a regular latex is produced independently from the silica, even if coverage of silica with surfactant has been previously carried out. Partial success has been obtained using limited amounts of methyl methacrylate. Very good results have been obtained for ethyl acrylate polymerization with a semi-continuous feed process, provided a suitable nonionic surfactant is used. The dispersability of functionalized silica is poor, but may be improved after a suitable treatment with ammonia, and a combination of an alcohol and a nonionic surfactant. Then, each latex particle is nucleated by the silica, again in the case of ethyl acrylate.