Different depth-related polymerization kinetics of dual-cure, bulk-fill composites.

The aim of this study was to evaluate the polymerization kinetics qualitatively and quantitatively for dual-cure bulk-fill composites in comparison with light-cure bulk-fill and traditional incremental composites at two clinically relevant depths. Five commercial dental composites were evaluated, including three dual-cure bulk-fill composites (BulkEZ, HyperFIL and Injectafil), one light-cure bulk-fill composite Filtek Bulk Fill Flowable (FBF) and one traditional incremental composite Filtek Z250 (Z250) as controls. Specimens were prepared in two different depths (0.5 mm and 5 mm) for 20 s light irradiation. Self-cure was also evaluated for the three dual-cure composites. The polymerization kinetics were measured continuously in real-time for at least 10 min using a Fourier-transform infrared spectroscopy (FTIR) with an attenuated total reflectance (ATR) accessory. The experimental kinetic data were fitted using two mathematical models — a sigmoidal function and a superposition of two exponential functions characterizing the gel phase and glass phase. The degree of conversion (DC) and the rate of polymerization were calculated for all test conditions. Both experimental FTIR measurements and mathematical modeling revealed distinct depth-related polymerization kinetics for BulkEZ compared to the other two dual-cure composites. Specifically, BulkEZ exhibited moderately-paced polymerization kinetics at both depths while HyperFIL and Injectafil exhibited faster polymerization at 0.5 mm and slower polymerization at 5 mm. The bulk-fill FBF and incremental Z250 exhibited relatively fast polymerization at both depths, a characteristic for light-cure. The DC values at the two depths were not significantly different for BulkEZ, but significantly higher at 0.5 mm than at 5 mm for the other four composites (α = 0.05). Polymerization kinetics and their depth variation for dual-cure bulk-fill composites are material dependent. The distinct depth-related polymerization kinetics revealed for BulkEZ compared to other composites may affect their contraction stress and clinical performance. [ABSTRACT FROM AUTHOR]