Water-Based Polyurethane Dispersions: Analysis of the Dispersion Stability in Terms of DLVO and XDLVO Theories

Although water-based polyurethane dispersions (PUDs) are predominantly stabilized through the incorporation of ionic groups, a comprehensive investigation of the dispersion stability of PUDs is still pending. For this reason, the structure-stability relationship of a series of PUDs with different soft segments (polyether, polyester, and polycarbonate polyols) and variable contents of sulfonate groups is examined in detail. The dispersion stability was characterized by time-resolved photometric determination of the critical coagulation concentration (ccc) and analyzed in terms of DLVO and XDLVO theories. Hamaker constants were determined based on contact angle data of PUD films with an apolar test liquid using the van Oss-Chaudhury-Good theory. As expected, it was found that the ccc increases with increasing 2-((2-aminoethyl)amino)ethanesulfonate (AAS) content. Moreover, the results indicate a strong influence of the nature of the polyol component on dispersion stability. The polycarbonate-based PUDs have the highest electrolyte stability, followed by polyester- and polyether-based PUDs. The apparent Hamaker constants A₁₃₁^app calculated according to the DLVO theory show nonconstant and unexpected high values for water-based polymer dispersions. Therefore, the observed dispersion stabilities cannot be adequately described by electrostatic and van der Waals interaction energies alone, so additional attractive interaction forces are likely to occur. On the other hand, Hamaker constants A₁₃₁^vOCG determined from contact angle data are found to be in the range of 0.6 × 10^-20 J for all samples, which is in good agreement with typical values for water-based polymer dispersions. Using this Hamaker constant and extending the DLVO theory with an additional, exponentially decaying interaction term, the observed dispersion stabilities can be explained very well. This additional interaction is attractive, as the negative sign of the pre-exponential factor shows, and is presumably hydrophobic in nature. The magnitude of this factor also reveals clear differences between the different polyol components and the AAS contents of the PUDs.