Polymerization Behavior of New Stela Self-Cured Bulk-Fill and Light-Cured Composites: An In Vitro Comparative Study

Adequate polymerization is essential for the mechanical performance and longevity of resin-based composites; however, recently introduced chemical cure, bulk-fill composites remain underexplored. This in vitro study compared the degree of conversion (DC) of a chemical cure bulk-fill composite (Stela, SDI) with those of a nano-hybrid light-cured composite (Filtek™ Z250, 3M). Sixty standardized disc-shaped specimens (5 mm × 2 mm; n = 30 per material) were prepared according to the manufacturer's instructions. We measured DC using Fourier-transform infrared spectroscopy (FTIR) immediately after polymerization and at 5, 10, 20 min, and 24 h. Both materials showed a progressive increase in DC over time. However, the light-cured composite consistently achieved higher conversion values, reaching 62.39 ± 4.23% after 24 h compared with 47.58 ± 3.04% for the chemical cure composite (p < 0.001). These findings indicate that DC alone should not be considered predictors of composite performance.