Effect of different initial light intensity by the soft-start photoactivation on the bond strength and Knoop hardness of a dental composite.

This study evaluated the bond strength (push-out method) and Knoop hardness of Z250 composite resin, photoactivated with XL 2500 curing unit, using different protocols: continuous mode (700 mW/cm(2) for 20s) (CO); soft-start (50 mW/cm(2) for 5 s, followed by 700 mW/cm(2) for 15 s) (SS1); soft-start (100 mW/cm(2) for 5 s, followed by 700 mW/cm(2) for 15 s) (SS2); soft-start (150 mW/cm(2) for 5 s, followed by 700 mW/cm(2) for 15s) (SS3); soft-start (200 mW/cm(2) for 5s, followed by 700 mW/cm(2) for 15s) (SS4); soft-start (250 mW/cm(2) for 5 s, followed by 700 mW/cm(2) for 15 s) (SS5); soft-start (300 mW/cm(2) for 5 s, followed by 700 mW/cm(2) for 15 s) (SS6). For the push-out test, the specimens were tested in a universal testing machine at a crosshead speed of 0.5 mm/min. For the hardness test, the specimens were polished for the hardness measurements, using a 50 g load for 15 s. Data were submitted to ANOVA and Tukey's test (alpha=5%). The results of bond strength showed that the SS3 group obtained the highest bond strength when compared to the CO group. There were no significant differences among the other modes in relation to the other groups. Regarding the other results in hardness, there were no significant differences among the groups in the surface region and up to 4 mm depth.