Viscosity and Consistency of Impression Rubbers

¹ Dental School of The London Hospital Medical College, University of London, London, England

Most silicone and polysulfide impression rubbers exhibit non-Newtonian viscosity characteristics, viscosity decreasing with increasing rate of shear. The higher the general level of viscosity, the more pronounced is the shear rate dependence. Hence, differences between matrials are most pronounced at low rates of shear, as for example when the material flows under its own weight. Consequently, subjective impressions of consistency under such conditions are no guide to behavior at the high rates of shear which occur in various aspects of usage.

Standard consistency tests involve shear rates that are, in general, appropriate to those encountered in practice. The results obtained from such tests are in accord with hydrodynamical theory, and viscosity values obtained by the cone and plate viscometer.

Shear failure could, in principle, occur at the shear rates involved in the practical handling of the materials studied. (Silicone materials exhibit these phenomena at much lower rates of shear than the polysulfides.) It is not yet clear whether this is an important factor in the use of these materials.