Role of reactive surfactants in miniemulsion polymerization

Reactive surfactants which contain a polymerizable group that can be irreversibly bound to polymer particles are used to reduce the negative effects of conventional surfactants in latex films, greatly increasing both the latex stability and the water resistance of the film. Miniemulsion polymerization is proposed as an alternative to conventional emulsion polymerization to increase the amount of surfactant incorporated on the particle surfaces and to minimize the amount of buried surfactant. Two reactive surfactants, HITENOL BC-20 (polyoxyethylene alkylphenyl ether ammonium sulfate) (anionic) and HITENOL KH-10 (polyoxyethylene-1-(allyloxy methyl alkylether sulfate) (anionic), have been studied. The critical micelle concentrations (cmc), and their partitioning behavior were determined. The stability of miniemulsions prepared using reactive surfactants HITENOL BC-20 and HITENOL KH-10 was studied and compared with miniemulsions prepared using a conventional surfactant (SLS). Results indicate that Ostwald ripening is occurring for all three surfactants, but it is more prominent in styrene miniemulsions prepared with a conventional surfactant (SLS).The kinetics of conventional emulsion polymerizations of styrene using reactive surfactants HITENOL BC-20 and HITENOL KH-10 were found to be similar to the conventional surfactant. However, a decrease was observed in the reaction rate when HITENOL KH-10 was used in comparison with HITENOL BC-20 due to the retardation effect caused by the resonance stability of the allylic radical. In the kinetics of the miniemulsion polymerization of styrene using the reactive surfactants HITENOL BC-20 and HITENOL KH-10, similar kinetics were found for the two reactive surfactants. Further latex characterization by TEM showed that the number of particles increased with increasing monomer conversion up to 10 % and then remained constant till the end of the reaction for both reactions indicating a short nucleation period.To characterize the surfactant incorporation in the latexes prepared by miniemulsion and conventional emulsion polymerization, ion exchange/conductometric titration combined with UV analysis was applied. The results show that for miniemulsion polymerization carried out with 5 mM HITENOL BC-20 (20 % solids), 82 % of the BC-20 was chemically bound to the polymer particles out of which 68 % was bound on the surface while 14 % was buried. In a parallel study using conventional emulsion polymerization, of the 77 % that was chemically bound, 55 % was on the surface and 22 % was buried inside the particles. In styrene miniemulsion and conventional emulsion polymerizations using HITENOL KH-10, the amounts incorporated on the surface of the particles were found to be higher than in the BC-20 reactions. Results from control experiments and molecular weight studies show that major incorporation of HITENOL BC-20 is through copolymerization, and chain transfer occurs mainly during nucleation period. For HITENOL KH-10, chain transfer combined with copolymerization resulting in intermolecular branching acts as a primary mode for incorporation. AFM studies of latex morphology showed that reactive surfactants can only be partially removed from latexes. Freeze-thaw cycles and contact angle studies show that latexes prepared with reactive surfactants by miniemulsion polymerization showed improved stability and little surfactant migration.