Colloidal stability of aqueous polymeric dispersions: Effect of water insoluble excipients

An important issue in the aqueous coating process is dispersion stability. An unstable dispersion results in aggregation of the colloidal particles, thereby affecting the film coating process. In the coating suspension containing pigment, a latex for aqueous film coating might interact with pigment, resulting in unstable dispersion. We therefore conducted a stability investigation in a mixed dispersion including latexes, EudragitL30D-55 (A-latex), EudragitRL30D (C-latex) and EudragitNE30D (N-latex) and pigments, titanium dioxide and iron oxide yellow. An aggregation of the dispersion containing A-latex was observed at pH 2. Regarding the dispersions with C-latex and N-latex, no aggregation was observed in the range pH 2-11. We calculated total interaction energy between latex-latex particles, pigment-pigment particles and latex-pigment particles based on DLVO theory. The calculated results explained two mechanisms of the stable mixed dispersion. The first was that the individual latex particle and the pigment particle dispersed without aggregation in the mixed dispersion because of the electrostatic interaction. The next was that the latexes adsorbed onto the surface of the pigments, making electrostatically stable heterocoaggregates. We also calculated the binding constant of iron oxide yellow for C-latex at pH 10. The value of the constant was determined to be 1.1 x 10(-2).