Water-Based\nPolyurethane Dispersions: A Detailed Analysis\nof the Particle Charge Using Soft and Hard Particle Model

Water-based polyurethane\ndispersions (PUDs) show a characteristic\ndependency of the electrophoretic mobility on the electrolyte concentration,\nwhich can be investigated by hard and soft particle models. For this\npurpose, additional information can be obtained by determining particle\ncharges and electrostatic potentials. PUDs with different contents\nof an intrinsic ionic stabilizer and various polyol components were\nsynthesized according to the acetone process. The particle charge\nwas characterized by potentiometric acid–base titration, and\nthe data of the titration curve were fitted assuming multiple functional\ngroups with adaptable acid strengths. To investigate the electrostatic\npotentials, electrophoretic mobilities were measured as a function\nof electrolyte concentration and analyzed by soft and hard particle\ntheory. Acid–base titration experiments indicated that not\nall detected ionic groups are located on the surface but are partly\narranged inside the polymer particle, as evidenced by a significant\ndecrease of the corresponding effective dissociation constant. The\nevaluation of the titration data and the electrokinetic experiments\nshowed that the soft particle model of Ohshima is not suitable to\nreflect the actual particle charge. In contrast, the hard particle\nmodel can describe the measured electrophoretic mobility of the dispersions\nvery well if the relaxation effect is taken into account. The dependency\nof the corresponding zeta potentials on the electrolyte concentration\ncan be excellently modeled assuming a constant surface potential ψ<sub>0</sub> and distance of the shear plane <i>x</i><sub>s</sub>. Reasonable results are obtained for both parameters with only minor\ndifferences between the PUD series. Nonetheless, the electrokinetic\nsurface charge densities calculated with respect to the surface potential\nare lower than expected from the titration results. Although our results\nindicate a more complex charge distribution in a peripheral layer,\nthe hard particle model currently shows the best description of the\nelectrokinetic behavior of the PUDs.