Abstract
This study investigated the thermal stability and top-to-bottom hardness ratio of six bulk-fill composites (STARK® Bulk Fill Composite, Filtek™ One Bulk Fill Restorative, SDR® Plus Bulk Fill Flowable, Tetric® N-Ceram Bulk Fill, X-tra fil, and Beautifil-Bulk Restorative) following acidic softening. Discs-shaped specimens were fabricated and photo-cured for 20 s. Vickers hardness was sequentially measured, immediately after irradiation (VH0), after 24 h of dry storage (VHdry) and after 24 h of acidic storage (VHacidic) at 37°C. Network integrity was assessed by comparing the relative hardness decrease (VHRD) after acidic softening with the initial maximum rate temperature (IRT) of mass loss. Compared to VH0, VHdry significantly increased, whereas VHacidic significantly decreased. The VHRD of top surface ranged from 16.62% for FBF to 62.84% for SDR. IRT primarily reflects resin composition and residue is a filler-based value. Relative network integrity estimated by IRT revealed a negative correlation with VHRD. Consequently, acidic erosion hardness development can be indirectly employed to assess network integrity in bulk-fill composites.
