Supermicron monodisperse styrene/butyl acrylate latexes by a novel aggregative emulsion polymerization

A novel aggregative surfactant-free emulsion polymerization technique has been developed. It allows the synthesis, in one step or in sequential semi-continuous procedures, of poly(styrene-co-butyl acrylate) and poly(styrene-co-butyl acrylate-co-acrylic acid) monodisperse polymer particles with diameters in excess of three microns and with high solids contents.; In order to achieve stable (80:20) styrene/butyl acrylate (St/BA) micronsized particles, the one step reactions must be carried out with an ionic initiator in presence of electrolytes, while the polymerization system has to be under monomer flooded conditions.; The experimental data revealed that: (i) The increase of ionic strength by salt addition: (a) slowed the polymerization rate; (b) favored aggregative processes; (c) increased the particle site; (ii) The increase of ionic strength by increasing the initiator concentration: (a) increased the polymerization rate; (b) limited aggregative processes; (c) decreased the particle size.; The largest particles resulted when the polymerizations were carried out at an initial pH {dollar}<{dollar} 8.0.; The styrene/butyl acrylate/acrylic acid (St/BA/AA) copolymerizations showed that monodisperse, stable, micronsized latex particles can be obtained in a one step synthesis if (i) A semi-continuous monomer feeding procedure, with the AA comonomer completely distributed in the monomer phase, was employed. (ii) The polymerizations were performed in the presence of salts, under monomer flooded conditions, provided that the initial pH of the reaction medium (before the addition of the monomer mixture) was less than 8.5.; It was also found that the addition of acrylic acid (as an active comonomer with potential surface-activity) to the styrene/butyl acrylate copolymerization system led to: (i) A significant increase in the colloidal stability of the latex particles, due to an electrosteric stabilization mechanism. (ii) An increase in the polymerization rate, if the AA composition in the monomer mixture was higher than 1 wt %. (iii) No influence on the particle size of the final latex particles, when the AA composition was between 1 wt % to 5 wt %, since the polymerization was carried out salt-free. (iv) A change of the particle surface morphology.; All the reaction parameters investigated suggested that the St/BA/AA surfactant-free emulsion copolymerizations proceeded via a similar aggregative-polymerization mechanism as that found for the AA-free surfactant-free emulsion copolymerization of St/BA.; Different scenarios, based on the seed type, initiator type, composition of the monomer mixture feed and monomer feeding rate were investigated to find the best reaction parameters for growing St/BA and St/BA/AA micron particles from seed. Using a semi-continuos surfactant-free technique, stable, monodisperse latexes with a final particle size of up to 4 {dollar}mu{dollar}m and solids content around 40 wt % could be produced. The experimental results strongly supported the limited capacity of the method to grow St/BA or St/BA/AA particles with diameters higher than 4 {dollar}mu{dollar}m.