Resin composites top/bottom hardness and different light cure

Purpose: The aim of this study was to evaluate microhardness and hardness ratio of two type hybrid resin composites using different irradiance light sources. Materials and Methods: Light curing units were an LED light-curing unit and an experimental quartz-tungsten-halogen (QTH) light-curing unit. The light-cured resin composites were Clearfil AP-X (shade A3) and Estelite ∑Quick (shade A3). Composite specimens of 2-mm thickness were polymerized in Teflon molds using an energy density of 24,000 mJ/cm 2 . Light curing methods were LED 1,200 mW/cm 2 for 20 s and QTH 600 mW/cm 2 for 40 s. Just after light curing, the Knoop hardness was measured at the top and bottom surfaces of each specimen. The hardness ratio was calculated as follows: Knoop hardness of bottom surface/Knoop hardness of top surface. Results: Immediately after light curing, the Knoop hardness at the bottom surfaces of resin composites was significantly lower than that at the top surfaces with 1,200 mW/cm 2 20 s for both resin composite ( p < 0.05). There is no significant difference between the Knoop hardness at the top surfaces and the bottom surfaces with 600 mW/cm 2 40 s for both resin composite ( p > 0.05). The 600 mW/cm 2 40 s showed significantly greater hardness ratio compared with that of the 1,200 mW/cm 2 20 s for both resin composite ( p < 0.05). Conclusion: The polymerization of resin composites at the bottom surface was inhibited compared with that of at the top surface using the regular irradiance of LED light.