Statement of problem: Rheological tests of elastomeric impression materials during setting have been most often conducted at room temperature rather than at intraoral temperature. Because temperature may affect properties and the setting kinetics, clinically relevant inferences may not be accurate with studies conducted at room temperature.
Purpose: The purpose of this study was to determine the viscoelastic properties of new low- and medium-viscosity elastomeric impression materials during setting at 33 degrees C and to evaluate the medium-viscosity materials at 3 additional temperatures.
Material and methods: The impression materials investigated at 33 degrees C were 2 polyvinylsiloxanes (PVS) (Aquasil Deca and Aquasil LV) and 5 polyethers (PE) (Impregum Penta, Impregum Penta Soft H, Impregum Penta Soft L, Impregum Garant Soft L, and Permadyne Garant L). Three impression materials (Aquasil Deca, Impregum Penta, and Impregum Penta Soft H) were also investigated at 25 degrees, 29 degrees, and 37 degrees C. Time-dependent oscillatory rheometry was carried out on these materials (n=3) with a rheometer with a 25-mm diameter parallel plate cell. The storage modulus (G') and the loss tangent (tandelta) were determined as functions of time over a period from 0 seconds to 900 seconds, commencing 40 seconds after mixing. Induction time (t(ind)) or initial setting time and tandelta, the relative liquidlike behavior, were also computed. A single-factor analysis of variance (ANOVA) was used for the properties determined at 33 degrees C and a 2-factor ANOVA was used for the temperature studies, with hypothesis testing at alpha=.05.
Results: The G'(t) curves for all materials displayed the expected sigmoidal shape with time, with the solid-like behavior rising slowly, then more rapidly, and again slowly to final set. The initial setting time (t(ind)) was found to be approximately 2.8 minutes for the PVS materials and for Impregum Penta and Impregum Penta Soft H, but was significantly longer for the remaining 3 PE low viscosity materials. The solid-like behavior (G') at final set or shear modulus differed among all materials, ranging from 1.0 MPa for Aquasil Decca, 1.69 for Impregum Penta Soft H, and 1.8 MPa for Impregum Penta. G' for low-viscosity materials ranged from 0.66 MPa for Aquasil LV and 0.79 MPa for Permadyne Garant L to 1.2 MPa for Impregum Penta Soft L. The loss tangent at 40 seconds tandelta (t(0)) varied among medium and low viscosity materials, ranging from liquid-like behavior of 4.3 for Permadyne Garant and less than unity or significant solid-like behavior for Impregum Penta. All materials showed tandelta values less than unity at their setting times. The temperature studies revealed significant changes in the kinetics of setting, with the setting time decreasing more than 3-fold between 25 degrees and 37 degrees C for the Aquasil Deca and the Impregum Penta Soft H and more than 2-fold for the Impregum Penta. At room temperature of 25 degrees C, the storage modulus for Impregum Penta Soft H and Aquasil Deca was 1.1% and 37.5% lower than Impregum Penta, respectively.
Conclusion: The development of the viscoelastic rheological properties with time for 3 medium- and 4 low-viscosity impression materials at 33 degrees C showed significant differences in the setting time and the magnitude of the storage modulus. Compared with Impregum Penta, the new Impregum Penta Soft H was 6% less stiff when set, compared with 44% lower stiffness for the medium viscosity addition silicone. Temperature studies between 25 degrees C and 37 degrees C revealed strong temperature sensitivity of the kinetics of setting and evolution with time of their rheological properties.