Water-Based Polyurethane Dispersions: New Insights into the Intrinsic Stabilization Using Carboxylate Groups

To achieve sufficient dispersion stability, ionizable groups such as dimethylolpropionic acid (DMPA) are often incorporated into the framework of water-based polyurethane dispersions (PUDs). As a weak acid, this group exhibits pH-dependent dissociation, which directly influences the degree of electrostatic repulsion between two particles. To gain deeper insights into the structure-stability relationship of PUDs, we synthesized numerous samples with different soft segments (polyether, polyester, and polycarbonate polyols) and varying contents of DMPA according to the acetone process. The dissociation behavior of the carboxyl groups and the particle charge were characterized by potentiometric acid-base titration, and the corresponding titration curves were fitted assuming multiple functional groups with adaptable acid strengths. To determine the surface potentials, electrokinetic measurements as a function of electrolyte concentration were performed and analyzed by the hard particle theory considering the relaxation effect. Dispersion stability was examined by means of the critical coagulation concentration (ccc) and evaluated according to the DLVO and XDLVO theory. Hamaker constants were determined based on contact angle data of the respective PU films with an apolar test liquid using the van Oss-Chaudhury-Good theory. The study shows that not all carboxyl groups incorporated into the polymer backbone can be detected by potentiometric titration. In addition, data from titration curves are best fitted when at least two functional groups with different acid strengths are assumed, indicating a nonuniform charge distribution in a peripheral layer and hindered dissociation. Interestingly, early DMPA incorporation during the prepolymerization step has a significant effect on the charge localization in the PUD particles. Compared to PUDs, in which the ionizable groups were incorporated during the final chain extension step, the proportion of charge groups in the particle interior increases significantly. However, these differences hardly affect the determined surface potentials and dispersion stabilities. Similar to a recent study, the surface potentials decrease slightly with increasing DMPA content, which is a result of the decreasing particle size. As expected, the ccc values increase with rising DMPA content, but in particular, they show a strong influence of the polyol component. The analysis of our data indicates that the DLVO approach should be extended by considering an additional attractive interaction energy, which is probably based on the hydrophobicity of the polymer components.