Visco-elastic properties of dental elastomeric impression materials.

The visco-elastic properties of a number of set elastomeric impression materials were determined using a torsional pendulum apparatus, i.e. the real (G1) and imaginary (G2) parts of the complex shear modulus and the mechanical loss tangent (Tan delta). Comparison with published data on tension and compression set for these materials showed that whilst there was no correlation between Tan delta and tension set, there was significant (greater than 99%) correlation with compression set. Treating a permanent set test as the application of a rectangular stress pulse of duration tau, and assuming (i) linear visco-elasticity theory and (ii) that G1 and G2 are independent of frequency. Fourier integral analysis predicts that Tan delta should be proportional to permanent set. Tension set does not conform to this prediction because the deformation is nonlinear. The compression set test is shown to involve shear strains and elastomers generally are Hookean in shear. Whilst the theoretical treatment applies qualitatively, there are major quantitative disparities, the reasons for which are discussed.