Viscous properties of thixotropic materials

Measurements have been made on the viscous properties of thixotropic suspensions of titanium dioxide pigments in linseed stand oils. The effects of changes in concentration and temperature have been noted. The viscosity under steady flow conditions cannot be adequately defined by a single coefficient but it is convenient to display the results graphically, plotting shear stress against velocity gradient. It is shown that the shear stress required to maintain any steady velocity gradient has two components, one being independent of temperature and the other proportional to the viscosity of the medium as this changes with temperature. The samples studied took some time to attain steady conditions of flow and the approach to this state, the "thixotropic relaxation", occurs in a very complicated way. Two qualitative conclusions could be drawn: that the magnitude and speed of thixotropic change decreased with increasing values of shear stress, and that the magnitude and speed of thixotropic change (at the same shear stress) increased with increasing concentration.