Monomer conversion and shrinkage force kinetics of low-viscosity bulk-fill resin composites

Objective. To investigate the subsurface degree of conversion (DC) and shrinkage force formation of low-viscosity (flowable) bulk-fill composite materials. Materials and methods. Three flowable bulk-fill resin composites [SureFil SDR flow (SDR; Dentsply DeTrey), Venus Bulk Fill (VB; Heraeus Kulzer) and x-tra base (XB; VOCO)] and one conventional flowable control composite material [EsthetX flow (EX; Dentsply DeTrey)] were tested. The materials were photoactivated for 20 s at an irradiance of 1170 mW/cm(2) and the DC (n = 5) was recorded at 0.1-, 1.5- and 4-mm depth using Fourier transform infrared spectroscopy. Shrinkage forces (n = 5) of 1.5-mm-thick specimens were continuously recorded for 15 min using a custom-made stress analyzer. Data were statistically analyzed by ANOVA, Tukey's HSD and Bonferroni's post-hoc tests (alpha = 0.05). Results. SDR generated the significantly lowest shrinkage forces (22.9 +/- 1.4 N), but also attained the significantly lowest DC at 1.5-mm depth (67.5 +/- 0.8%). The conventional flowable composite EX generated the significantly highest shrinkage forces (40.7 +/- 0.7 N) and reached a significantly higher DC (74.4 +/- 1.3%) compared to SDR and XB at 1.5-mm depth. The shrinkage force values of VB (29.4 +/- 1.1 N) and XB (28.3 +/- 0.6 N) were similar (p > 0.05). All materials attained significantly higher DC at 4-mm depth than at the near-surface. Conclusion. The tested low-viscosity bulk-fill materials show lower shrinkage force formation than a conventional flowable resin composite at high levels of degree of conversion up to 4-mm incremental thickness.