Effects of different photo-polymerization protocols on resin–dentine μTBS, mechanical properties and cross-link density of a nano-filled resin composite

ObjectivesTo evaluate the effects of four different light-curing protocols on the microtensile bond strength (μTBS), ultimate tensile strength (UTS), Knoop micro-hardness (KH) and cross-link density (CLD) of a nano-filled resin composite.MethodsFiltek Supreme XT (3M-ESPE) was used for the entire experiments following four light-curing approaches: ST, standard irradiance; HI, high irradiance; PD, pulse delay; SS, soft start. The specimens were submitted to different storage periods (24 h or 6 months), cut into match-sticks and subsequently submitted to μTBS testing. Hourglass specimens were also prepared for UTS and KH. Cylindrical specimens were prepared for the CLD evaluation after absolute ethanol challenge. The results were statistically analyzed with a two-way ANOVA and Tukey's test (α = 0.05).ResultsFor UTS and KH, continuous irradiance (PD and SS) induced statistically higher results (p < 0.05) both after 24 h and 6 months of water storage compared to ST and HI groups. However, a drop in UTS and KH was obtained after 6 months in all groups. The μTBS was not affected by the different light-curing approaches and, no statistical differences (p > 0.05) were observed between 24 h and 6 months storage. The CLD evaluation showed a statistical drop in KH after 24 h of ethanol storage for PD and SS (step-curing protocols) compared to those attained in continuous mode (ST and HI).ConclusionThe soft-start mode may improve the UTS and KH of nano-filled resin composites without compromising the resin–dentine μTBS. However, both the step-curing protocols may reduce the cross-link density of the composite polymeric network.Clinical significanceHigh irradiances photo-polymerization may be adequate for direct aesthetic restorations such as veneers and onlays. Meanwhile, the soft start protocol would seem more appropriate for the photo-polymerization of high c-factors class I and II restorations.