Viscosity is the resistance to
flow. A practical definition of rheology is the study of
flow. Coatings’ viscosity and rheology are important in ensuring acceptable
products. In this article, we present data from the analysis of three
commercially available, white interior house paints. Shear rate is the rate at which
a material is sheared, that is, a force is transmitted perpendicular to the
direction of flow, because of friction transmitted between infinitesimally-thin
layers of fluid. Shear rate is determined by a combination of (1) geometry and
(2) testing or application speed.
Paint sitting undisturbed in a can, on a shelf, seems
to be unsheared. That is, there seems to be no motion imposed upon that paint.
Therefore, the shear rate is essentially zero, and the corresponding viscosity
is the “zero-shear-rate” viscosity. Solids settling, due to gravity, may occur
over time, however. It is desired to have a viscosity high enough, under these
conditions, so that either settling does not occur or at least occurs extremely
slowly. QA/QC measurements would take a very long time to obtain if they ran at
nearly zero shear rates, so they are run at somewhat faster rates,
instead.