The Effect of Polymer Components on the Rheological Behavior of Coating Colors

Abstract

Coating color is a complicated system consisting of clay, calcium carbonate, starch, styrenebutadiene latex (SBL), water, etc. At rest the system forms a three dimensional network structure through the interactions between the components, however, the structure changes at high shear rates imposed upon the system during the blade coating process, only to recover its original structure after the shear is removed. Although the flow properties of coating color system, especially the viscosity at high shear rate, are known to affect the stability of blade coating process, the viscoelastic properties seem to be more important than the flow properties in steady shear because the former is sensitive to the structure change. To elucidate the process of the formation of the fluid structure of the coating color system, we measured the dynamic viscoelasticity as a function of the passage of time after the system was prepared. By estimating the increase in the storage modulus and decrease in the dynamic loss tangent, we proposed a model that the structure formation proceeds in three steps. In the first step, the formation of local aggregation of starch and SBL takes place. In the second step, a three dimensional network structure is formed and, in the third step, finally flocculation of pigments occurs. The structures of the first and second steps are strongly controlled by the polymer components. We also discuss the critical concentration of starch and SBL needed to form the network structure.