Hardening of Flowable Bulk-fill Resin Composites in Large Increments

Methods: Three flowable bulk-fill materials [Surefil SDR flow (SDR), x-tra base (XB) and Venus Bulk Fill (VB)] and one conventional flowable control resin composite [Tetric EvoFlow (TEF)] were filled in Teflon molds (diameter: 10 mm, height: 4 mm) and irradiated for 20 or 30 s with a LED light-curing unit at 1170 mW/cm². After storing the specimens for 24 h in the dark at 37°C, Knoop hardness (KHN) was measured at the top (0 mm) and bottom (4 mm) specimen surface (n = 8). For each specimen, four measurements were performed at each surface. Data were analyzed using paired and unpaired t-tests (p < 0.05).

Results: Longer irradiation time (30 vs. 20 s) significantly increased bottom KHN of TEF, but had no effect on either top or bottom KHN of the tested bulk-fill resin composites. With both irradiation times, TEF attained significantly lower KHN at the bottom surface than at the top surface. Nevertheless, an acceptable bottom-to-top KHN ratio of 86% was achieved for TEF with 30 s of irradiation. No significant differences between top and bottom KHN were observed within the bulk-fill materials XB and VB, irrespective of irradiation time. SDR even reached significantly higher KHN at 4-mm depth (bottom surface) than at the top surface with 20- and 30-s irradiation time.

Conclusion: The investigated flowable bulk-fill resin composites achieved adequate curing at 4-mm incremental thickness with 20 s of irradiation (> 1000 mW/cm²).