Network integrity of bulk-fill composites: Thermal stability, post-curing hardness development and acidic softening

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 (VH_dry) and after 24 h of acidic storage (VH_acidic) at 37°C. Network integrity was assessed by comparing the relative hardness decrease (VH_RD) after acidic softening with the initial maximum rate temperature (IRT) of mass loss. Compared to VH₀, VH_dry significantly increased, whereas VH_acidic significantly decreased. The VH_RD 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 VH_RD. Consequently, acidic erosion hardness development can be indirectly employed to assess network integrity in bulk-fill composites.