Effect of radical amplified photopolymerization (RAP) in resin-based composites

The objective of this study was to evaluate the effect of irradiation time and specimens thickness on the polymerization kinetic and variation in micro-mechanical properties of two commercial resin-based composites (RBCs) based on radical amplified photopolymerization (RAP) technology™, and to compare them with four camphorquinone (CQ)/amine-based RBCs.

The materials were analysed by assessing the polymerization kinetic and the degree of cure (DC) at 0.1 mm and 2 mm depth during 5 minutes after photoinitiation, after curing for 10 s, 20 s and 40 s (Elipar Freelight2). The variation in micro-mechanical properties (Vickers hardness (HV), indentation modulus (E), and depth of cure (DOC)) was assessed in 100 μm steps on 6-mm-high specimens irradiated as above and stored in the water for 24 h at 37 °C.

The results were statistically compared using one-way ANOVA with Tukey HSD post hoc test (α = 0.05) and a general linear model. The parameter material exerted the strongest effect on DC (partial eta-squared η _p ² = 0.83), followed by irradiation time (η _p ² = 0.27), and depth (η _p ² = 0.09). The polymerization kinetic, well described by an exponential sum function, showed in all materials a faster decrease in carbon–carbon double bonds at 0.1 mm than at 2 mm depth. The materials based on RAP achieved the highest DC values and a faster polymerization at both depths. The irradiation time exerted the strongest effect on the mechanical properties (DOC, η _p ² = 0.96; HV, η _p ² = 0.89; E, η _p ² = 0.86), followed by depth (HV, η _p ² = 0.63; E, η _p ² = 0.54) and material (HV, η _p ² = 0.40; E, η _p ² = 0.67). At the most favorable curing conditions (40 s, surface), the mechanical properties of the analyzed materials varied between 11.38 (0.80) GPa in Estelite® Sigma Quick and 20.80 (1.42) GPa in Estelite® Posterior for E, and between 74.33 (3.56) N/mm² in Tetric EvoCeram® and 120.71 (6.24) N/mm² in Estelite® Posterior for HV.

RAP-initiated material demonstrated a higher increase in DOC with prolonged irradiation time than the analyzed CQ/amine based materials.

An irradiation time of 20 s is also recommended for RAP-initiated RBCs.