Degree of conversion and microhardness of different composite resins polymerized with an advanced LED-curing unit

The aim was to evaluate the effects of different curing times (3s/10s/20s) of an advanced Light-Emitting Diode (LED) curing unit on the degree of conversion (DC%), microhardness, and hardness ratio of three different composite resin materials.

180 cylindrical specimens (3 × 4 mm; n = 60) were prepared from PowerFill (Ivoclar, Liechtenstein), PowerFlow (Ivoclar), and Omnichroma Flow Bulk (Tokuyama, Japan). Half of the samples were used for the degree of conversion analysis (nDC = 30), and the other half for the microhardness test (n VHN = 30). All specimens were irradiated using a Light-Emitting Diode (LED) curing unit (Bluephase PowerCure, Ivoclar). Based on polymerization times: 3-second rapid, 10-second conventional, and 20-second conventional; three sub-groups were formed for each material (n = 10). The specimens' degree of conversion (DC%) was analyzed using Fourier Transform Infrared Spectroscopy (FTIR). Microhardness values were determined using the Vickers microhardness test, and the bottom-to-upper surface hardness ratios were calculated. The degree of conversion and microhardness were analyzed with a paired t-test, ANOVA, and Tukey tests (p < 0.05).

Comparisons among all groups revealed significant differences in upper and bottom DCs (p < 0.05). PowerFill exhibited higher values on both surfaces after 3s polymerization, while PowerFlow required 10s and Omnichroma Flow Bulk 20s to achieve similar results (p < 0.05). When the upper surface microhardness values were compared, no significant difference was found between PowerFlow and Omnichroma Flow Bulk in the 3s group (p = 0.623) and after 20s polymerization, between PowerFill and Omnichroma Flow Bulk (p = 0.082). Material type and polymerization duration significantly affected bottom surface microhardness values (p = 0.000). As the polymerization time increased, the bottom microhardness values of all materials increased. The highest microhardness ratio (HR) was obtained in the PowerFill group at 20 s (p = 0.029). In the PowerFlow group, no difference was found between HR values of 10s and 20s (p = 0.238). For Omnichroma Flow Bulk group, all polymerization durations showed significant differences (p = 0.000).

The polymerization performed with an advanced LED curing unit in a shorter time (3s) resulted in varying polymerization efficiencies among different types of composite resins. For more homogeneous polymerization, it is advised to extend the duration to 10 s or longer.