Surface hardness ratios and surface roughness of bulk-fill resin composites following different heating protocols

Abstract

Purpose: To evaluate the impact of preheating on the physical properties of bulk-fill resin composites, with a particular focus on microhardness and surface roughness. Methods: In this study, seven bulk-fill resin composites (SonicFill 3, Opus Bulk-Fill Flow, Opus Bulk-Fill, Metafil Bulk-Fill, Tetric PowerFlow, Tetric N-Ceram, and Estelite Bulk-Fill Flow) and one conventional resin composite (Filtek Z250) were tested. Specimens were divided into two main groups: Group 1 (preheated 68°C for 10 minutes) and Group 2 (stored at room temperature). For each group, five disc-shaped resin composites (4 mm thickness x 5 mm diameter) were prepared. Each bulk-fill resin composite was applied in a single increment, whereas Filtek Z250 was placed in the molds in 2 mm layers. To complete the polymerization, the samples were kept in distilled water at 37°C for 24 hours. Then, the surfaces of the samples were polished with Twist Dia (Clearfil) polishing discs to imitate the finishing and polishing processes. Baseline microhardness and surface roughness values were measured. Subsequently, specimens underwent artificial aging simulating 2 years of clinical use (10,000 brushing cycles + 1,200 thermal cycles). Post-aging measurements were repeated, and statistical analyses were conducted. Results: Significant differences in microhardness values were observed among the experimental groups for all restorative materials, except for Estelite Bulk-Fill Flow (P> 0.05). The highest bottom-to-top hardness ratio was found in both preheated and non-preheated Opus Bulk-Fill Flow, while the lowest was observed in preheated Tetric PowerFlow. Microhardness generally decreased with depth, and the impact of preheating varied among materials. Post-aging surface roughness increased in all groups. Preheated Metafil Bulk-Fill exhibited the highest surface roughness, whereas Estelite Bulk-Fill Flow (both preheated and non-preheated) showed the lowest. Preheating reduced viscosity and enhanced microhardness in certain resin composites but also increased surface roughness over time. The effects of preheating are material-dependent due to compositional differences.