Semibatch surfactant-free emulsion polymerization of butyl acrylate in the presence of carboxylic monomers

Incorporation of a small amount of acrylic acid (AA) into the polymer particles can depress the limited flocculation process and, therefore, result in a decrease in the particle size during the semibatch surfactant-free emulsion polymerization of butyl acrylate. The final latex particle size first increases to a maximum around 2% AA and then decreases with increasing concentration of AA, which is due to the polyelectrolyte effect during the particle nucleation period. The final particle size increases significantly with increasing concentration of sodium bicarbonate (a neutralizing agent) because of the ionic strength effect. The run with the highest concentration of sodium bicarbonate shows the worst latex stability, whereas the run with 0.2% sodium bicarbonate leads to the best latex stability. The type of carboxylic monomers has an insignificant effect on the particle size. Both the pH and conductometric titration data suggest that a significant proportion of the AA unit is present near the particle surface layer, whereas the more hydrophobic methacrylic acid (MAA) unit can be distributed more uniformly in the particles. The critical flocculation conentration data show that incorporation of 5% AA or 5% MAA into the emulsion polymers can greatly improve the chemical stability of the finished latex products, especially when the pH is high. Furthermore, AA is more effective in promoting the chemical stability of the finished products as compared to MAA.