Mechanical performance of a conventional resin composite and its bulk-fill restorative counterpart after long-term accelerated aging

Background. The long-term mechanical properties and stability of various resin composites in areas under stress are questionable.Objectives. The aim of the study was to determine the effects of long-term hydrothermal aging on the mechanical properties of a microhybrid conventional resin composite and its bulk-fill counterpart.Material and methods. We used a conventional and high-viscosity restorative bulk-fill resin-based composites (RBCs) from one company. Bar-shaped specimens of each type of resin composite were fabricated using steel molds and divided into 2 groups. The specimens were stored at 37degree celsius for 24 h, and half of the specimens in each group were subjected to the 3-point bending flexural test and microhardness measurement. The remaining specimens were aged for 10,000 thermal cycles between 5degree celsius and 55degree celsius, and then subjected to flexural testing and microhardness measurement. A Vickers microhardness tester was used to estimate the surface microhardness of the specimens. Data was analyzed using an independent ttest and the Mann-Whitney Utest. The statistical significance level was set atp <= 0.05. Scanning electron microscopy (SEM) was used to investigate the surface of each material.Results. The bulk-fill RBC showed similar flexural strength and modulus to its conventional counterpart before aging. The flexural strength of both resin composites was significantly decreased after thermocycling (p < 0.001). Hydrothermal aging had no significant effects on the flexural modulus (p = 0.84). There was a significant decrease in the surface microhardness of the bulk-fill RBC. Scanning electron microscopy photomicrographs showed several pits as a result of the exfoliation of the filler particles on the surface of the bulk-fill RBC after aging.Conclusions. The flexural strength of both resin composites decreased significantly after aging. The flexural properties, surface changes and microhardness of the bulk-fill type were additionally affected by the aging process.