| Driven by environmental regulations, water-borne coatings and adhesives are of steadily growing importance. One system being developed to satisfy this need is aqueous polyurethane. Aqueous polyurethane particles have a very unique structure, which lies somewhere between that of polymer chains and traditional colloidal particles. In this fundamental study, we attempt to characterize and understand the flow behavior of these unique electrostatically stabilized polymer colloids from a colloidal perspective. We examine the rheological behavior of anionic aqueous polyurethane dispersions from the dilute to the concentrated regime. At dilute to intermediate concentrations these systems behave as standard electrostatically stabilized particles. However at higher concentrations, standard models for electrostatically stabilized hard particles fail at predicting both the magnitudes and the trends exhibited by these systems. The flow behavior observed for concentrated aqueous polyurethane systems is attributed to a complex interplay of electrostatics, sterics, and particle deformation. This study represents the first attempt to fully characterize and understand the rheological behavior of these systems from a fundamental perspective. The results of this study lay the ground work for future work, which should ultimately result in a more concise picture of the particle morphology and better more advance models for predicting the observed behavior. |
