Rheological Properties of Elastomeric Impression Materials Before and During Setting

Abstract

In this study, we examined the rheological properties of elastomeric impression materials, both before and during setting, to assess the clinical significance of certain key characteristics such as viscosity, pseudo-plasticity, and the rate of development of elasticity. The hypothesis to be tested was that monitoring the change in tan 8 is the most appropriate means of monitoring the setting characteristics of elastomers. The loss tangent (tan 8) and the dynamic viscosity (η') for five impression materials (both unmixed pastes and mixed/setting materials) were measured by means of a controlled-stress rheometer in a cone/plate configuration. For unmixed pastes, tests were performed at various frequencies (0.1 to 10 Hz) and torques (from 1 to 50 x 10^-4 Nm), while testing on setting materials was performed at constant frequency (1 Hz) and torque (3 x 10-3 Nm). Most base and catalyst pastes were pseudoplastic before being mixed. Immediately after being mixed, the polyether (tan 8 = 9.85) and polysulfide (tan 8 = 9.54) elastomers showed tan 8 markedly higher than those of other mixed materials (tan 8 = 4.96 to 3.01). The poly-vinylsiloxane elastomers showed lower initial tan 8, which rapidly reduced even further with time. This suggests that these materials should be used as soon as possible after being mixed. The polyether elastomer had a comparatively long induction period during which the tan 8 remained at a high value. These characteristics are thought to be key factors in controlling clinical efficacy and therefore support the hypothesis that monitoring tan 8 is an appropriate method for evaluating the setting characteristics of elastomers. One limitation was that the controlled-stress rheometer was unable to monitor rheological properties through to completion of setting.