Permanent set and stress relaxation in elastomeric impression materials.

The permanent set of a range of elastomeric impression materials, used in dentistry, was investigated as a function of the materials' age and the duration and level of the deformation. The permanent set appears to be primarily controlled by the rate of polymerization at the time of deformation. However, stress relaxation experiments revealed that, for condensation silicone and in particular polysulfide elastomers, bond interchange reactions produce additional permanent set. The rate of recovery of the elastomers from deformation was very rapid and was consistent with the influence of entanglement in the network.