Shrinkage Stress, Polymerization Kinetics, and Hardness of Light and Self-Cured Bulk-Fill Resin-Based Composites

The polymerization shrinkage stress (SS), degree of conversion (DC), and Vickers hardness (HV) are properties that can affect the performance of resin-based composites (RBCs). This study tested four bulk-fill RBCs used in self-cured mode: Bulk EZ Plus (Zest Dental Solutions), Cention Forte (Ivoclar), Fill-Up! (Coltene), and Stela (SDI Limited), and two light-cured bulk-fill RBCs: Filtek One (Solventum) and SDR flow+ (Dentsply). The test specimens were 6 mm in diameter and 2 mm thick. Axial SS was measured in real time for 4000 s in the self-cured materials and for 1400 s after 10 s of light curing in the light-cured materials (n = 12 for self-cured RBCs; n = 11 for light-cured RBCs). To confirm that the RBCs were adequately polymerized, the DC was assessed using real-time ATR-FTIR spectroscopy, and the HV was measured on the top and bottom surfaces using a 300-gf load for 8 s (n = 5) after 24 h. The SS, DC, and HV differed significantly among the RBCs (p < 0.001). At 1400 s, Cention Forte developed the lowest stress (1.44 MPa), whereas Bulk EZ Plus and Fill-Up! produced the highest stress (3.77 MPa). The self-cured materials continued to develop measurable stress between 1400 s and 4000 s, while the light-cured RBCs had stabilized at 1400 s. Bulk EZ Plus and Stela produced the highest DC values, and Stela had the highest HV.