ABSTRACT
The rheology or flow behavior of a coating is of crucial importance to its pro cessing (mixing, stirring, pumping), ease of application (brushing, roll or spray coating), and to phenomena such as splatter, leveling, and sagging, which influ ence the final appearance. An enormous amount of literature exists on the rheo logical behavior of coatings. It is well understood that the flow behavior over a wide range of shear and extensional deformation rates is pertinent to optimal performance of coatings [1] (see Table 1). Thus, an ideal coating should exhibit strong shear-thinning characteristics, such that it flows easily at higher shear rates relevant to application conditions, and a relatively high, although not too high, viscosity at low shear rates to facilitate leveling and avoid sagging [2]. An inordi nately high extensional viscosity may result in excessive splattering of coatings in roll applications [3]. Also, waterborne coatings may exhibit time-dependent rheological behavior due to interparticle structure buildup (rheopexy-viscosity increase) or loss of interparticle structure (thixotropy-viscosity decrease) [4]. Under such circumstances, the flow characteristics may become dependent on the geometry of the rheometer (e.g., gap size, parallel plate versus cone and plate, etc.). This is generally an indication that macroscopic structures are being created and destroyed in the flow cell. Such an effect can lead to flow-instability behavior, which may adversely affect coating deposition.
