Effect of exposure intensity and post-cure temperature storage on hardness of contemporary photo-activated composites

Objectives: The effect of variation in post-exposure storage temperature (18 vs. 37°C) and light intensity (200 vs. 500 mW/cm2) on micro-hardness of seven light-activated resin composite materials, cured with a Prismetics Mk II (Dentsply) light activation unit, were studied. Methods: Hardness values at the upper and lower surfaces of 2 mm thick disc shaped specimens of seven light-cured resin composite materials (Herculite XRV and Prodigy/Kerr, Z100 and Silux Plus/3M, TPH/Dentsply, Pertac-Hybrid/Espe, and Charisma/Kulzer), which had been stored dry, were determined 24 h after irradiation with a Prismetics Mk II (Dentsply) light activation unit. Results: Hardness values varied with product, surface, storage temperature, and curing light intensity. In no case did the hardness at the lower surface equal that of the upper surface, and the combination of 500 mW/cm2 intensity and 37°C storage produced the best hardness results at the lower surface. Conclusions: Material composition had a significant influence on surface hardness. Only one of the seven products (TPH) produced a mean hardness values at the lower surface >80% of the maximum mean upper surface hardness obtained for the corresponding product at 500 mW/cm2 intensity/37°C storage temperature when subjected to all four test regimes. Despite optimum post-cure storage conditions, 200 mW/cm2 intensity curing for 40 s will not produce acceptable hardness at the lower surface of 2 mm increments of the majority of products tested.