Progress in Organic Coatings 40 (2000) 111–117 Rheology of sterically stabilized dispersions and latices Jan Mewis ∗ , Jan Vermant Department of Chemical Engineering, de Croylaan 46, Katholieke Universiteit Leuven, 3001 Leuven, Belgium Abstract Steric stabilization is a method that is often used to properly disperse small particles. It can be applied in aqueous as well as non-aqueous media. The rheological properties of sterically stabilized dispersions are discussed here. The various controlling parameters and the physical mechanisms involved are reviewed. Brownian hard spheres are used as a reference. Scaling relations are presented that make it possible to reduce data sets and to predict properties. Viscosity, yield stress, shear thickening and viscoelasticity are included. The rheological properties are also related to the fundamental colloidal properties of the dispersions under consideration. Quantitative results are available for monodisperse spherical particles, although the effects of particle size distribution can sometimes be predicted also quite well. In other cases the procedures presented here can be used qualitatively to predict viscosities. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Rheology; Latex; Suspensions; Steric stabilization; Viscosity predictions; Viscosity scaling; Dynamic moduli 1. Introduction Because of the presence of small particles most liquid coatings can be considered colloidal suspensions. The col- loidal stability of the system then determines whether the particles will remain well dispersed or whether they will flocculate. When flocculation is not desirable stability can be induced by electrostatic repulsion between the particles, its application is mainly restricted to aqueous media. Steric repulsion between layers attached to the particle surface can provide stability in any suspending medium. Electrostatic stabilization is very effective but makes the structure and the rheology sensitive to variations in pH or ionic strength. Steric, or polymeric, stabilization is not affected by these parameters but, if the suspending medium is not a good sol- vent for the stabilizer molecules, the stability might change with temperature. Steric stabilization is frequently used as a suitable and ro- bust way of ensuring proper dispersion of the particles. The stabilizer layer can be chemically grafted on the particle surface or, more often, physically adsorbed. The formula- tion of such materials would obviously be accelerated if the rheology could be predicted or estimated rather accurately on the basis of the composition. This is the problem which is addressed here. Only stabilizer layers of grafted polymers and adsorbed blockcopolymers or surfactants are consid- ered, not homopolymers or statistical copolymers as such. In the latter case the stabilizer layer consists of a complex ∗ Corresponding author. mixture of “tails” and “loops” of the stabilizing polymer, whereas only “tails” are present in the other two cases. The suspending medium can be aqueous or non-aqueous. 2. Brownian hard spheres Various parameters affect the rheology of stable colloidal suspensions. In the limiting case of spheres without any interparticle interactions, only Brownian (thermal) forces and hydrodynamic forces affect the flow behaviour. The case of “Brownian hard spheres” is quite well documented, experimentally and theoretically, at least for monodisperse particles (e.g. Ref. [1]). At sufficiently low shear rates Brow- nian motion will dominate the convective motion caused by the flow. Under these conditions the equilibrium structure of the particles that exist at rest is preserved during flow. As a result the viscosity does not change with shear rate and the contribution of the Brownian forces to the viscos- ity is at its maximum. When increasing the shear rate the Brownian motion will, at a certain stage, become slower than the convective motion. From then on the contribution of the Brownian motion to the viscosity will gradually decrease with increasing shear rate, whereas the hydrody- namic contribution remains relatively constant. This causes the viscosity to drop; a shear thinning region develops. At still higher shear rates the Brownian contribution levels off and becomes negligible, but an increase in hydrody- namic effects can either compensate the decrease, causing a pseudo-Newtonian high shear plateau, or overcompensate, producing a shear thickening zone [2]. 0300-9440/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0300-9440(00)00142-9