RHEOLOGICAL PROPERTIES OF ELASTOMERIC IMPRESSION MATERIALS BEFORE ANDDURING SETTING

In this study, we examined the rheological properties of elastomeric i mpression materials, both before and during setting, to assess the cli nical significance of certain key characteristics such as viscosity, p seudoplasticity, and the rate of development of elasticity. The hypoth esis to be tested was that monitoring the change in tan delta is the m ost appropriate means of monitoring the setting characteristics of ela stomers. The loss tangent (tan delta) and the dynamic viscosity (eta') 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 a t 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 fr equency (1 Hz) and torque (3 x 10(-3) Nm). Most base and catalyst past es were pseudoplastic before being mixed. Immediately after being mixe d, the polyether (tan delta = 9.85) and polysulfide (tan delta = 9.54) elastomers showed tan delta markedly higher than those of other mixed materials (tan delta = 4.96 to 3.01). The polyvinylsiloxane elastomer s showed lower initial tan delta, which rapidly reduced even further w ith time. This suggests that these materials should be used as soon as possible after being mixed. The polyether elastomer had a comparative ly long induction period during which the tan delta remained at a high value. These characteristics are thought to be key factors in control ling clinical efficacy and therefore support the hypothesis that monit oring tan delta is an appropriate method for evaluating the setting ch aracteristics of elastomers. One limitation was that the controlled-st ress rheometer was unable to monitor rheological properties through to completion of setting.