A Simple Method for Determining Resin Composites’ Elastic Modulus during Curing

Objectives: In assessing the mechanical characteristics of resin composites, in particular the level of shrinkage stress, it is essential to determine the temporal changes of the elastic modulus in addition to its final maximum value. Previous attempts using an oscillatory shear test have not been fully successful due to detachment of the specimen from the loading plate as its stiffness and resistance to motion increases. An alternative method is presented here which can capture the temporal changes of a resin composite’s elastic modulus during curing in a simple manner.
Methods: Using a recently developed analytical shrinkage-stress model, the elastic modulus (E) of a resin composite at any given time can be expressed as:
E=1/(de/ds-A/kl),
where de/ds is the rate of change of linear shrinkage strain with respect to shrinkage stress, A and l are the cross-sectional area and thickness of the disc specimen used in common static shrinkage stress measurement, and k is the stiffness of the stress-measuring device. The above equation was applied to experimental data found in the literature.
Results: The elastic modulus was found to follow the typical S-shaped curve with time. The final maximum value ranged from 10 to 15 GPa, similar to those found by beam-bending test, while the characteristic time ranged from 3 to 10 s.
Conclusions: The recently developed analytical shrinkage-stress model can be adopted to determine the temporal changes of resin composites’ elastic modulus during curing using shrinkage strain and stress data.