Effect of Preheating on the Microhardness of Nanohybrid Resin-based Composites

The physical and mechanical properties of resin composites are highly affected bythe extent of conversion obtained by polymerization. The hardness testcan be used as an indirectmethod to evaluate degree of conversion.The purpose of this study was to evaluate theeffect of three preheating temperatures on microhardness of three different nanohybrid resinbased composites. The 30 specimens for each commercial resin composite[Grandio (Voco), Simile(Pentron) and Tetric N-Ceram (IvoclarVivadent)] were randomly subdivided in 3 subgroupswhich 10 specimens were used for each one {subgroup 1=preheating at room temperature (21˚C), subgroup 2= preheating temperature 37˚ C and subgroup 3= preheating temperature 54˚C}.The specimenswerephotopolymerized with QTH light-curing unit for 20 s following thepreheating process. Vickers microhardness test was performed for the top and bottom surfacesof each specimen. Three random indentations were taken for each surface and a mean valuewas calculated. The microhardness values in Grandio group were significantly different between allthree subgroups(p value ≤ 0.001). In Simile group the only significant difference was between21˚ C and 54˚ C (p value ≤ 0.005) and in Tetric N-Ceram group the difference between 21˚ Cand 54˚ C (p value ≤ 0.001) and also between 21˚ C and 37˚ C (p value ≤ 0.01) were consideredas statistically significant. Regardlessof theresin composite material used, surface hardness was considerablyimproved by increasing temperature. The microhardness values were influenced significantlyby resin-based composite brand.