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

Purpose: The physical and mechanical properties of resin composites are highly affected by the extent of conversion obtained by polymerization. The hardness test can be used as an indirect method to evaluate degree of conversion.The purpose of this study was to evaluate the effect of three preheating temperatures on microhardness of three different nanohybrid resin based composites. Methods: The 30 specimens for each commercial resin composite [Grandio (Voco), Simile (Pentron) and Tetric N-Ceram (Ivoclar Vivadent)] were randomly subdivided in 3 subgroups which 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 specimens were photopolimerized with QTH light- curing unit for 20 s following the preheating process. Vickers microhardness test was performed for the top and bottom surfaces of each specimen. Three random indentations were taken for each surface and a mean value was calculated. Results: The microhardness values in Grandio group were significantly different between all three subgroups (p value ≤ 0.001). In Simile group the only significant difference was between 21˚ C and 54˚ C (p value ≤ 0.005) and in Tetric N-Ceram group the difference between 21˚ C and 54˚ C (p value ≤ 0.001) and also between 21˚ C and 37˚ C (p value ≤ 0.01) were considered as statistically significant. Conclusion: Regardless of the resin composite material used, surface hardness was considerably improved by increasing temperature. The microhardness values were influenced significantly by resin-based composite brand