Miniemulsion polymerization

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 or der of magnitude, to changes in initiator, water-phase retarder, and oil-ph ase inhibitor concentrations than macroemulsion polymerizations. This is be cause, unlike macroemulsion polymerization, there is no competition between particle nucleation and particle growth for the available surfactant. Mini emulsions tend to have a greater shear stability than macroemulsions, proba bly due to the lack of the small number of polymerized monomer droplets fou nd in macroemulsions. Hydrophobic co-monomers have been used successfully a s co-surfactants. It has been found that such systems will give a more unif orm co-polymer composition since the supply of hydrophobic co-monomer is no t mass transfer limited as in macroemulsion polymerization. Miniemulsion po lymerization has been used to produce alkyd-acrylate graft co-polymers whic h are being evaluated as resins for low VOC alkyd coatings. Since alkyd, li ke hydrophobic co-monomers, is transported only slowly across the aqueous p hase of a macroemulsion, the grafting will not take place to any significan t degree in a macroemulsion. This same technique has been applied to polyes ter and urethane grafting to produce resins for water-borne specialized coa tings. Continuous macroemulsion polymerization is subject to sustained conv ersion oscillations and multiple steady states in CSTR's. Conversion oscill ations result from competition between micelles and growing polymer particl es for available surfactant. It has been shown that oscillations are elimin ated when droplet nucleation predominates since the droplet size changes ve ry little during the reaction. (C) 1999 Elsevier Science B.V. All rights re served.