Effects of Light Curing on the Mechanical Properties and Shade Stability of a Dual-Cure Bulk-Fill Resin Composite

Rationale: Despite the growing use of dual-cure bulk-fill resin composites (RCs), the influence of light-curing protocols on their polymerization and material behavior remains unclear. Understanding these effects is critical for optimizing clinical outcomes. Objective: This study investigated degree of conversion, flexural strength, flexural modulus, water sorption and solubility, and shade stability of a dual-cure bulk-fill RC, Bulk-EZ Plus (BZ), compared to a light-cured bulk-fill RC, Filtek-One Bulk-Fill (FB). Methods: Specimens were randomized into five groups: BZ self-cured (BZ-SC), BZ light-cured for 10-seconds (BZ-10), BZ light-cured for 20-seconds (BZ-20), FB light-cured for 20-seconds (FB-20), and FB light-cured for 40 seconds (FB-40). Degree of conversion was evaluated using FTIR spectroscopy, flexural strength and modulus, and water sorption and solubility were determined following ISO 4049:2019. Shade stability in coffee and distilled water was measured using a spectrophotometer after 24 hours, seven days, and 21 days. Two-way ANOVA assessed the effects of RC type and curing protocol. Shade stability was analyzed using repeated measures and three-way ANOVA (α = 0.05). Results: Light activation increased the flexural modulus of BZ (p = 0.012 and p = 0.001 for BZ-SC vs. BZ-10 and BZ-20, respectively) and flexural strength (p = 0.018 for BZ-SC vs BZ-20), and reduced water sorption (both p < 0.001) and solubility (p = 0.028 for BZ-SC vs. BZ-10 and p = 0.001 for BZ-SC vs BZ-20). Extending light exposure from 20 seconds to 40 seconds did not change the flexural strength and modulus, or water sorption and solubility of FB (all p > 0.05). Conclusion: Light activation improved the physical and mechanical properties of the dual-cure bulk-fill resin composite (Bulk-EZ Plus), but increasing the light exposure time did not improve the properties of the light-cured bulk-fill material (Filtek-One Bulk-Fill).