Evaluation of polymerization of light-curing hybrid composite resins

The quality of polymerization of hybrid composite resins was tested to explore their feasibility for dental restorations. For this, microhardness, polymerization shrinkage, the coefficient of thermal expansion, and surface morphology were evaluated during or after light curing in conjunction with the thermocycling process. Each product had different microhardness values. The repeated thermal stimulus has no specific effect on the change of microhardness. The difference of microhardness between the thermocycled specimens and specimens stored only in distilled water was minor. The measured microhardness had a linear correlation with the filler content (vol %) of the tested specimens. The polymerization shrinkage had rapidly increased only during the light curing, and then it reached a plateau. Among the specimens, Z250 showed the least amount of shrinkage for all tested thicknesses. Regardless of the product, the shrinkage values increased as the specimens became thick. The coefficient of thermal expansion of the control specimens ranged between 42 and 55 microm/degrees C in the temperature range of 30-80 degrees C. The coefficient showed an inverse correlation with the filler content. Through the thermocycling process, Palfique Estelite showed randomly propagating cracks on the surface. Larger fillers showed a more apparent detachment than the smaller fillers.