Stability of polymer encapsulated titania in aqueous emulsion systems

The increasing need to reduce the use of organic solvents causes waterborne paints to grow in importance. In these paints the interaction between pigment (TiO2) and binder can be improved by polymer encapsulation of the pigment. This encapsulation process consists of two main steps, a modification with titanates, followed by an emulsion-like polymerisation. During the emulsion polymerisation on-line conductivity measurements provided valuable information about the course of the reaction, by visualising the start of the reaction and the disappearance of the monomer droplets. Indirectly, the reaction rate and surfactant migration can be derived from the conductivity data, giving information about the amount of surfactant still available for stabilisation of pigment and polymer. During 'monomer-starved' experiments (semi-continuous addition of monomer) the reaction mixture was found to be less stable when using styrene, as compared to similar reactions with methyl methacrylate. The number of oligomeric radicals in the case of styrene is probably insufficient for total stabilisation: a considerable coagulation took place. Coagulation may be prevented by adding surfactant at a rate (derived from conductivity measurements) so as to just compensate for the increase in surface area taking place during the reaction. The rate of addition can be determined from the conductivity data.