Examples of Polymer-Encapsulated Pigments and Fillers

Silica encapsulation has been widely studied both in the open literature and in the patent literature. The first studies were reported by Hergeth et ah, who described the elaboration of composite particles made from quartz powders and polyvinyl acetate through seeded emulsion polymerization [85]. These authors showed that the number of seed particles must exceed a minimal value to prevent formation of new particles and, thus promote seed particles growth. The polymerization was proved to take place in the vicinity of the surface according to the admiceUar polymerization mechanism described previously, and the so-produced interfacial polymer was shown to display physical properties different than those of the bulk polymer. 

Always in an attempt to compatibilize the core and shell materials, Yoshinaga and co-workers described the synthesis of a series of oHgomeric silane molecules and their use in encapsulation reactions [95]. However, as the polymerizations were performed in the absence of surfactant, the resulting composite particles were not colloidally stable. 

Nagai and co-workers [96] described, for example, the aqueous polymerization of a series of surface-active cationic monomers having different alkyl chain length 

Based on this same general idea, colloidal dispersions of nanocomposite particles made from silica cores and polymeric overlayers have been successfully prepared using appropriate cationic radical initiators, as described in a recent Japanese patent [100]. Recently, Luna-Xavier et al. also demonstrated the successful formation of nanosize siHca/PMMA composite colloids using AIB A as cationic initiator and a nonionic polyoxyethylenic surfactant (NP30) [63,91,101]. Composite particles made from silica beads surrounded by small heterocoagulated PMMA latexes or a thin polymer layer were produced, depending on the size of the silica beads (Fig. 4.11). 

The role of the suspension pH and the influence of the monomer, silica and initiator concentrations on the assembly process have been investigated in depth, and analyzed in a quantitative manner. Electrostatic attraction between the polymer end groups and the negatively charged silica surface proved to be the driving force of 

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