| Abstract: | The changes of shear stress with time under constant shear rate and temperature of several pigmented acrylic polymer gels were examined. This behavior, known as thixotropy, was interpreted based on a model which separates the stress into two regions. Region I covers the growth of the stress up to the yield point, represented by a peak stress, F0. Region II covers the decay of the stress from the peak to the point it attains an asymptotic value, F∞. The material in region I is assumed to be elastic and in region II is represented by a dynamic equilibrium between structure A, which is non-Newtonian and structure B, which is Newtonian. The transitions between A and B represent structure degradation and structure recovery. The concentrations of A and B are solved explicitly. The total shear stress is then evaluated following a theory proposed by Ree and Eyring and is shown to give relationship between shear stress–shear rate–shearing time in good qualitative agreement with experimental observations. The ratio (F0 ? F∞)/F∞, defined as the coefficient of thixotropy, is sensitive to the fundamental physical chemical parameters of the system and is easy to measure. It is shown to be useful in the characterization of thixotropic materials such as gels. Methods for evaluating the elastic modulus and yield stress of the material in the gel state are also illustrated. |
