Influence of Polymerization Mechanism on Surface Roughness, Microhardness, and Polymerization Efficiency of Bulk-Fill Resin Composites

Objective: To investigate how polymerization mechanism influences surface roughness, microhardness, and curing homogeneity of bulk-fill resin composites. Materials and Methods: Two light-cure bulk-fill composites (Estelite Bulk Fill Flow, VisCalor Bulk) and two self-cure bulk-fill composites (Stela Automix, Stela Capsule) were evaluated (n = 12 per material). Cylindrical specimens (6 mm diameter × 4 mm thickness) were fabricated. Surface roughness (Ra) was measured using contact profilometry. Vickers microhardness (VHN) was determined on both top and bottom surfaces. Curing homogeneity was assessed using the bottom-to-top hardness ratio. Data were analyzed using two-way ANOVA and Tukey post hoc tests (α = 0.05). Results: Self-cure composites demonstrated significantly higher surface roughness than light-cure materials (p < 0.001). Light-cure composites showed significantly greater top and bottom surface microhardness (p < 0.001). However, light-cure materials exhibited a significant reduction in hardness at the bottom surface, resulting in lower hardness ratios (0.83–0.85). In contrast, self-cure materials showed no significant top–bottom difference and presented significantly higher hardness ratios (0.95–0.96; p < 0.001). Conclusions: The polymerization mechanism appears to influence the surface characteristics and curing behavior of bulk-fill resin composites. Light-cured materials tended to exhibit higher hardness values, whereas chemically cured materials demonstrated a more uniform polymerization profile throughout the material thickness. These findings suggest that the type of polymerization may be a factor worth considering during material selection, particularly in deep restorations where light transmission may be limited.