Online monitoring and control of the (co-)polymer encapsulation of TiO2 in aqueous emulsion systems

Titanium dioxide (TiO2) was encapsulated with polymer by a two-step process consisting of: (1) modification of the pigment surface with titanates in order to render the surface hydrophobic, followed by (2) an emulsion polymn.-like reaction leading to the actual polymer encapsulation. The emulsion homo-polymn. reactions were carried out with either Me methacrylate (MMA) or styrene (STY). Compared to reactions carried out with MMA, encapsulation reactions performed with STY generally showed substantial coagulation occurring both during and at the end of the reaction. Online cond. measurements are related to surfactant migration because the mobility of the surfactant mols. strongly depends on the state they are in (adsorbed on a surface, dissolved in the aq. phase, in micelles). These measurements were used to obtain qual. information concerning the course of the reaction by visualizing: the moment of initiation, the moment of droplet disappearance during batch reactions, whether or not monomer starved conditions are obtained during semi-continuous reactions, and the occurrence of coagulation during encapsulation reactions. Furthermore, the cond. measurements helped to clarify various reaction mechanisms taking place during encapsulation reactions and in principle can serve as a basis for surfatcant addn. during the reaction in order to prevent coagulation from taking place. Also the possibility of creating multi-layered shells around modified TiO2 was investigated using MMA and STY as (co-) monomers. The addn. of a chain transfer agent (1-dodecyl mercaptan) did not seem to influence the course of the reaction very much, but made removal of the surface polymer (with THF, THF), for the purpose of anal., easier