Latex Colloid Dynamics in Complex Dispersions

Coating colors are applied to the base paper in order to maximize the performance of the end product. Coating colors are complex colloidal systems, mainly consisting of water, binders, and pigments. To understand the behavior of colloidal suspensions, an understanding of the interactions between its components is essential. A colloidal particle exerts so-called Brownian motion due to thermal collisions with solvent molecules. With fluorescence microscopy and image analysis the behavior of small particles can be directly observed. By studying the Brownian motion, the self-diffusion coefficient of the particles can be determined. The fluorescence microscopy method has been evaluated at low and high volume fractions. For carboxylated polystyrene latex probes, at low volume fractions, the estimated diffusion coefficients correspond well with the theoretical predictions if thermal and electrostatical contributions are included in the discussions. It was also possible to discriminate latex interactions with an added polymer. At high volume fractions, up to 50%, the experimental results were compared to theoretical predictions for hard-sphere behavior. A comprehensive statistical analysis of the microscopy images establishes the Gaussian behavior, both for the single probe particles and for the averaged ensemble of particles. Cationic starches interact with the negatively charged probe, regardless of polyelectrolyte charge density or molecular weight. A higher starch molecular weight yields, at constant charge density, a looser polymer layer bound to the latex surface, expressed as a larger hydrodynamic radius. Adding unlabelled host particles to the systems increase the negatively charged total surface area. Under these conditions, the diffusion is solely determined by latex-latex interactions. Fluorescence microscopy can be used to study the dynamics of drying latex dispersions. The transport of particles can be studied as the water evaporates. Two latexes with Tg below and above the film forming temperature were studied. The latex colloids were modified by two cationic starches, differing in molecular weight. The cationic starch and anionic latex form aggregates. The presence of aggregates influences the film forming, giving a rougher surface.