Results

A Typical Example of the Measurement of the Visco-elastic Changes During Curing of dental restorative materials

M.P. Aarnts, PhD

Figure 1 Volumetric shrinkage

Figure 1 shows the volumetric shrinkage (%) of a highly filled (30 % resin / 70 % glass filler) and a less filled (50% resin 50% glass filler) restorative material as a function of time. After two hours the volumetric shrinkage of the low filled material is higher than for a high filled material. This is not surprising because the amount of resin, which is responsible for the volumetric shrinkage, is much higher in the low  filled material.

Figure 2 Contraction stress and stress response to the applied sinusoidal deformation

Development of the contraction stress and response to an applied sinusoidal deformation of the high filled and low filled material during light curing. The measured stress response on both the volumetric shrinkage and the applied sinusoidal deformation. This signal can be split into the stress response on the volumetric shrinkage (Figure 3) and the response on the sinusoidal deformation (Figure 4).

Figure 3 Contraction stress

Figure 3 shows the development of the contraction stress, without the stress response on the applied sinusoidal deformation (derived form previous graph). The contraction stress is higher for the high filled material. Thus although the high filed material has a smaller volumetric shrinkage, it can develop a higher contraction stress. The origin of this effect can be found in the different development of the E modulus for both materials depicted in figures 4 and 6. The E modulus of the high filled material reaches already in an early stage of the reaction a considerable value.

Figure 4    Response to an applied sinusoidal deformation

Stress response of the curing materials on the applied 1 Herz sinusoidal strain, without the stress response on the volumetric contraction. (derived from figure 2 and 3)

Figure 5    Measured sinusoidal strain

Figure 5 shows the strain as a function of time, measured as close as possible to the sample to avoid influence of the compliance of loadcell of tensilometer. The compensation for the volumetric shrinkage is very good since there is hardly any deviation from the 0 baseline. The small decrease of the amplitude is caused by the increasing stiffness of the sample.

Figure 6    Dynamic moduli

Figure 6 shows the complex dynamic elastic modulus (E*), Loss (E’’) and storage (E’) modulus calculated for each individual stress/strain cycle. By combining the results of figure 4 and 5 we can calculate several properties. Most relevant for our purpose is the complex E modulus (E*) and the loss (E’’) and storage modulus (E’).

Conclusion

We have shown that it is possible to measure in one experiment the contraction stress development and the viscoelastic changes during light curing.