Microhadness change of bulk-fill resin composites in different increment thickness

Objectives: To evaluate the top/bottom surface microhardness change (%) of a conventional and three bulk-fill resin composites in different thickness.
Methods: Microhardness evaluations and hardness change were determined on specimens of the packable resin composite Solitare 2 (SOL, control group) and the bulk-fill resin composites Sonicfill (SNF), Filtek Bulk Fill (FBF), and Tetric EvoCeram Bulk Fill (TEBF). Seven specimens (n=7) of each material type were prepared in cylindrical stainless steel molds in different thickness (8-mm in diameter and 2-, 4- and- 5-mm in thickness) and the specimens were light cured from top surface. All specimens were stored at 37◦C for 24 h, before measurement. Microhardness was measured at the top and bottom surface of the 2-, 4- and 5-mm thick resin composite specimens. Data were analyzed by one-two way ANOVA using Bonferroni post hoc tests at p<0.005.

Results: When top/bottom surface hardness change of materials were evaluated for 2-mm thickness, significantly lowest hardness change was observed in SOL group (p<0.001) which was not significantly different from SNF (p=0.143). Significantly different hardness change was observed between SNF and TEBF (p<0.005) for 4-mm thickness. No significant hardness change was observed between other materials tested (p>0.005). When hardness changes were evaluated in 5-mm thickness, highest hardness change was observed in TEBF which was significantly different from FBF and SNF (p<0.001).
Conclusions: Increasing the increment thickness generally decreased the bottom surface microhardness values of the bulk-fill resin composites. But SNF and FBF had the lowest top/bottom surface hardness change with the increased increment thickness among the composites examined.