Miniemulsion polymerization

Abstract

Miniemulsions have some unique and desirable properties. They are far more robust to variations in the recipe or contaminant levels than conventional emulsions. Particle number was found to be less sensitive by at least an order of magnitude, to changes in initiator, water-phase retarder, and oil-phase inhibitor concentrations than macroemulsion polymerizations. This is because, unlike macroemulsion polymerization, there is no competition between particle nucleation and particle growth for the available surfactant. Miniemulsions tend to have a greater shear stability than macroemulsions, probably due to the lack of the small number of polymerized monomer droplets found in macroemulsions. Hydrophobic co-monomers have been used successfully as co-surfactants. It has been found that such systems will give a more uniform co-polymer composition since the supply of hydrophobic co-monomer is not mass transfer limited as in macroemulsion polymerization. Miniemulsion polymerization has been used to produce alkyd-acrylate graft co-polymers which are being evaluated as resins for low VOC alkyd coatings. Since alkyd, like hydrophobic co-monomers, is transported only slowly across the aqueous phase of a macroemulsion, the grafting will not take place to any significant degree in a macroemulsion. This same technique has been applied to polyester and urethane grafting to produce resins for water-borne specialized coatings. Continuous macroemulsion polymerization is subject to sustained conversion oscillations and multiple steady states in CSTR's. Conversion oscillations result from competition between micelles and growing polymer particles for available surfactant. It has been shown that oscillations are eliminated when droplet nucleation predominates since the droplet size changes very little during the reaction.