Post-curing in dental resin-based composites

Objective

To determine the post-curing in six commercial contemporary resin-based composites (RBCs) using axial shrinkage, the degree of conversion, and Vickers hardness.

Methods

Five Bulk Fill and one conventional RBCs from three companies were selected with a wide range of filler volume content. The axial shrinkage of samples that were 1.00 mm thick by 9–10 mm diameter was measured using a modified bonded disk method over a time between 15 h and 19 h at temperatures of 26 °C and 34 °C (mouth temperature). The degree of conversion (DC) was collected continuously for 10 min using mid-infrared spectroscopy in the attenuated total reflectance geometry. Vickers hardness was measured at 1 h post-irradiation using a load of 300 gf. For all three tests, the samples were irradiated at five exposure times, 20, 5, 3, 1.5 and 1 s with a light curing unit radiant exitance of 1.1 W/cm². Three samples (n = 3) were used for each experimental condition.

Results

After light exposure, the axial shrinkage and degree of conversion exhibited a functional time dependence that was proportional to the logarithm of time. This suggests an out-of-equilibrium polymer composite glass that is transitioning to thermal equilibrium. At a sufficiently long time and among the RBCs investigated, the shrinkage related physical aging rate was found to vary between 1.34 and 2.00 μm/log(t). The rate was a function of the filler content. Furthermore, 15 h after light exposure, the post-curing shrinkage was estimated to be an additional 22.5% relative to the shrinkage at 100 s for one RBC at T = 34 °C. The hardness in the photo-cured RBC was varied by using different light exposure times. The first two experimental techniques show that the higher the initial DC 10 min after light exposure, the smaller is the post-curing shrinkage related and DC related physical aging rates. A direct correlation was observed between the shrinkage related and the DC related physical aging rates.

Significance

Post-curing shrinkage should be evaluated for longer than 1 h. The post-curing shrinkage 15 h after light exposure in dental RBCs can be appreciable. The long-term development of built-in stress within the tooth wall structure may shorten the restoration’s lifespan.