Abstract
The aim of this study was to evaluate the influence of different light-curing units on the tensile bond strength and microhardness of a composite resin (Filtek Z250 – 3M/ESPE). Conventional halogen (Curing Light 2500 – 3M/ESPE; CL) and two blue light emitting diode curing units (Ultraled – Dabi/Atlante; UL; Ultrablue IS – DMC; UB3 and UB6) were selected for this study. Different light intensities (670, 130, 300, and 600 mW/cm2, respectively) and different curing times (20s, 40s and 60s) were evaluated. Knoop microhardness test was performed in the area corresponding to the fractured region of the specimen. A total of 12 groups (n=10) were established and the specimens were prepared using a stainless steel mold composed by two similar parts that contained a cone-shaped hole with two diameters (8.0 mm and 5.0 mm) and thickness of 1.0 mm. Next, the specimens were loaded in tensile strength until fracture in a universal testing machine at a crosshead speed of 0.5 mm/min and a 50 kg load cell. For the microhardness test, the same matrix was used to fabricate the specimens (12 groups; n=5). Microhardness was determined on the surfaces that were not exposed to the light source, using a Shimadzu HMV-2 Microhardness Tester at a static load of 50 g for 30 seconds. Data were analyzed statistically by two-way ANOVA and Tukey's test (p<0.05). Regarding the individual performance of the light-curing units, there was similarity in tensile strength with 20-s and 40-s exposure times and higher tensile strength when a 60-s light-activation time was used. Regarding microhardness, the halogen lamp had higher results when compared to the LED units. For all light-curing units, the variation of light-exposure time did not affect composite microhardness. However, lower irradiances needed longer light-activation times to produce similar effect as that obtained with high-irradiance light-curing sources.
Highlights
The properties of resin-based composites materials are frequently reevaluated because they may be influenced by several factors
Light emitting diodes (LEDs) produce a narrow band of wavelengths (450-490 nm) that is conveniently situated in the absorption spectrum of camphorquinone, which is the photoinitiator present in most light-activated dental materials
No filters are required in LED lightcuring units (LCUs)[11,16]
Summary
The properties of resin-based composites materials are frequently reevaluated because they may be influenced by several factors. It is known that degree of conversion is related to some properties, such as microhardness and intrinsic strength of the material depending on composite resin shade, curing time, light-curing unit, irradiance, emitted light spectrum and increment thickness[3,5,8,9,14,17,21,23]. No filters are required in LED lightcuring units (LCUs)[11,16] This feature allows total use of the emitted light, resulting in minimal heat generation, differing from halogen curing units (QTH). Another difference between these light-curing units is their durability. Compared to the first generation, the generation one offers better performance at shorter curing times
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