Effect of different modes of light modulation on the bond strength and Knoop hardness of a dental composite

Abstract

This study evaluated the bond strength and Knoop hardness of Z250 composite resin, light activated with XL2500 curing unit, using different protocols: continuous mode - high intensity (CH) (700 mW/cm2) for 20 s; continuous mode - low intensity (CL) (150 mW/ cm2) for 20 s; and pulse-delay with 150 mW/cm2 for 2(P2), 3(P3), 5(P5), 10(P10) or 15 s (P15), with a 1-min delay, followed by 700 mW/cm2 for 20 s. For the push-out test (n=10), the bond strength values were obtained using a universal test machine at a crosshead speed of 0.5 mm/min. For Knoop hardness (n=5), the specimens were made using the same light-activation protocols. The hardness measurements were made with a hardness tester at six depths (top, 1, 2, 3, 4, and 5 mm). The data were subjected to ANOVA and Tukey's test at 5% significance level. For bond strength, CL and P5 presented significantly higher mean values (p<0.05) than the other groups. There was no statistically significant difference (p>0.05) between P2, P3, P10, P15 and CH. For Knoop hardness, CH and P15 presented the highest mean values from top surface up to 4 mm depth, while CL presented the lowest hardness mean values (p<0.05). From the depth of 3 mm to 5 mm, the mean values of all groups were significantly lower (p<0.05) than those recorded on top surface. In conclusion, for the pulse-delay method, the initial exposure time can influence bond strength and Knoop hardness of composites.