A simple model to describe the thixotropic behavior of paints

A simple rheological model is proposed to describe the thixotropic behavior of paints since the classical hysteresis area, which is usually employed, is not enough to evaluate thixotropy. The model, which is based on the assumption that viscosity is a direct measure of the structural level of the paint, depends on two equations: the Cross-Carreau equation to describe the equilibrium viscosity and a second order kinetic equation to express the time dependence of viscosity. Two characteristic thixotropic times are distinguished: one for the net structure breakdown, which is defined as a power law function of shear rate, and another for the net structure buildup, which does not depend on the shear rate. The useful information provided by these two kinetic processes can be potentially used to improve both the quality and applicability of paints. Five representative commercial protective marine paints were tested. These are based on chlorinated rubber, acrylic, alkyd, vinyl, and epoxy resins. Furthermore, the influence of the temperature in the rheological behavior was also investigated. Results indicate that the paints exhibit both shear thinning and thixotropic behavior. The proposed model fits satisfactorily the thixotropy of all the studied paints and, additionally, it is also able to predict the thixotropy dependence on temperature. Both the viscosity and the degree of thixotropy increase as the temperature decreases. (c) 2006 Elsevier B.V. All rights reserved.