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

Adequate polymerization is essential for the mechanical performance and longevity of resin-based composites; however, recently introduced chemically cured bulk-fill composites remains insufficiently investigated. This in vitro study compared the degree of conversion (DC) and surface microhardness (SMH) of a chemically cured bulk-fill composite (Stela, SDI) with those of a nano-hybrid light-cured composite (Filtek™ Z250, 3M). Sixty standardized disc shaped specimens (5 mm x 2 mm; n= 30 per material) were prepared according to the manufacturers instructions. The DC was measured using Fourier-transform infrared spectroscopy (FTIR) immediately after polymerization and after 5, 10, 20 minutes and 24 h. SMH was evaluated using Vickers microhardness tester before and after artificial aging by thermocycling (5,000 cycles). The light-cured composite exhibited significantly greater SMH then the chemical cured composite (95.01 ± 0.59 HV vs. 51.21 ± 0.50 HV, p < 0.001). Both materials demonstrated progressive increase in DC over time. However, the light-cured consistently achieved higher conversion values, reaching 62.39 ± 4.23% after 24 h compared with 47.58 ± 3.04% for the chemically cured composite (p < 0.001). These findings indicate that the DC and SMH alone should not be considered a predictor of composite performance.