Effect of Ultra-Fast Light Curing on the Irradiated-Surface Degree of Conversion and Polymer-Network Quality of Bioactive and Conventional Bulk-Fill Resin Composites: An In Vitro Study

Background: Ultra-fast high-irradiance curing shortens exposure time, but its effects on conversion and polymer-network integrity may vary among bulk-fill resin composites. This in vitro study was designed as a materials-level investigation of irradiated-surface polymerization behavior. It evaluated irradiated-surface degree of conversion and polymer-network quality while deliberately excluding assessment of depth of cure or polymerization throughout the 4 mm bulk increment. Methods: Eighty disk specimens (6 mm × 4 mm) of Filtek One Bulk Fill, SDR Plus, Beautifil Bulk Flowable, and Beautifil Bulk Restorative (n = 10/material/protocol) were cured with a polywave LED at 1200 mW/cm2 for 10 s or 3000 mW/cm2 for 3 s. Degree of conversion (DC) was measured by ATR-FTIR on the irradiated (top) surface only. Ethanol-induced softening (ES%, Knoop-hardness reduction at the irradiated surface after 24 h ethanol immersion) served as an inverse, indirect indicator of polymer-network quality (cross-link density). Data were analyzed by two-way ANOVA and Tukey tests (α = 0.05); the DC–ES% relationship was examined by Pearson correlation at the individual-specimen level (n = 80). Results: Ultra-fast curing yielded higher pooled DC (66.30 ± 10.06%) than conventional curing (59.37 ± 7.96%; p < 0.001), but the effect was material-dependent: DC increased significantly for Filtek One Bulk Fill (p < 0.001) and Beautifil Bulk Flowable (p = 0.035), but not for SDR Plus or Beautifil Bulk Restorative. ES% was influenced mainly by material (p < 0.001) and less by curing protocol (p = 0.009). At the specimen level (n = 80), DC and ES% showed a weak but statistically significant negative correlation (r = −0.31, p = 0.006). Conclusions: Within the conditions of this study, irradiated-surface polymerization behavior under ultra-fast curing depended primarily on composite formulation, underscoring the need to evaluate both conversion efficiency and polymer-network quality when optimizing dental restorative materials. Because conversion was assessed only at the irradiated surface, these findings describe irradiated-surface behavior and do not provide evidence of adequate cure at the bottom of the 4 mm increment.