Polymerization behavior of resin composites using different irradiance lights

Purpose: This study aimed to evaluate the microhardness and hardness ratios of hybrid resin composites using different irradiance lights. Materials and Methods: Light-curing units were light emitting diode (LED) light-curing unit (Demi Ultra). The lightcured resin composites were Clearfil AP-X (shade A3) and Clearfil Photo Bright (shade US). Composite specimens of 2mm thickness were polymerized in Teflon molds using an energy density of 24,000 mJ/cm2. Light irradiances were 1,200 mW/cm2 20 s and 600 mW/cm2 40 s. Just after light curing, the Knoop hardness (KHN) was measured at the top and bottom surfaces of each specimen using a hardness tester. The hardness ratio was calculated as follows: KHN of bottom surface/KHN. All experiments were performed at room temperature of 23 ± 2 ̊C with humidity of 50 ± 10%. Results: Immediately after light curing, the KHN at the bottom surfaces of resin composites was significantly lower than that at the top surfaces for every group (p < 0.05). The 1,200 mW/cm2 20 s showed significantly smaller hardness ratio compared with that of the 600 mW/cm2 40 s for both Clearfil AP-X and Clearfil Photo Bright resin composites (p < 0.05). Conclusion: When the energy density was the same, the polymerization of resin composites at the bottom surface was more inhibited than that of at the top surface using high-irradiance light. (Asian Pac J Dent 2020; 20: 25-28. doi: 10.47416/apjod.20-0275)