Contraction stress, caused by the shrinkage of resin containing restorative materials during curing is one of the important factors which has to be evaluated when designing new filling materials. Well-known problems with contraction stresses are the loss of adhesion and post-operative pain. The amount of developing contraction stresses is influenced by several factors, like volumetric shrinkage, adhesion, shape factors (cavity configuration) and visco-elastic behavior during curing. It is therefore important to determine these visco-elastic changes during curing..

A general method to determine the visco-elastic properties is applying a sinusoidal deformation on a material and registers the load response. A general method for determining the development of contraction stress is by keeping the displacement of the crosshead zero. By applying a sinusoidal deformation around a zero level both contraction stress and visco-elastic changes can be recorded in one experiment.

However, curing materials undergo a fast volumetric shrinkage during the polymerization reaction and a also tremendous change in elastic-modulus occurs. These two factors make it difficult to maintain a constant smooth sinusoidal deformation around a zero level, especially for light cured materials were these changes occur in ca. 40 s. Finally, because the limited light penetration depth, a maximum sample thickness for light cured materials is ca 2 mm. This means that also the deformation has to be kept small (in the order of microns), in order to stay in a linear visco-elastic region.

Results of a typical measurement can be found on this page

Our homebuild tensilometer the "ACTA Intense" is designed for small (mm) cyclic movements. Further more, we use a double feedback loop for keeping both the zero offset (shrinkage) and the amplitude (changing E-modulus) of the deformation constant.

Literature

M.P. Aarnts, B.S. Dauvillier, A.J. Feilzer and C.L. Davidson, Measurement of the Visco-elastic Changes During Curing of Resin Composites, CED, 23-09-1999 Montpellier.