The rheology of aqueous polyurethane dispersions

The rheological properties of aqueous polyurethane dispersions are reported from the dilute regime up to mass concentrations in excess of 40 wt.%. Par ticle size decreases and the particles are more resistant to shear induced deformation with increasing ionic strength and, at very low ionic strength, the reduced viscosity passes through a minimum with increasing concentrati on. As the mass fraction of polyurethane increases, the dispersions shear t hin. The zero shear rate viscosity diverges with the development of an appa rent yield stress at the highest concentrations probed. Stress sweep experi ments show that deviation from linear viscoelasticity occurs at strains of less than 0.05% for the moderately concentrated dispersions. The Cox-Merz r ule is obeyed by these dispersions only at strains in the linear viscoelast ic region and at concentrations below the gel point. As the ionic strength is increased at fixed particle concentration, viscosities pass through a mi nimum. These phenomena are discussed in terms of particle deformability and an interplay of electrostatic and steric repulsive forces. (C)1999 Elsevie r Science B.V. All rights reserved.