Influence of simulated gastric acid exposure on surface characteristics and color stability of flowable and packable bulk-fill composites

This study aimed to evaluate the effects of simulated gastric acid exposure on the surface roughness, gloss, and color stability of flowable and packable bulk-fill composite resins. Five composite materials were evaluated: a universal composite—Estelite Σ Quick (Tokuyama Dental, Japan) [EQ]; two packable bulk-fill composites—Filtek One Bulk Fill Restorative (3M ESPE, USA) [FR] and Tetric N-Ceram Bulk-Fill (Ivoclar Vivadent, Liechtenstein) [TN]; and two flowable bulk-fill composites—Estelite Bulk-Fill Flow (Tokuyama Dental, Japan) [EF] and Hi-Fill Bulk-Flow (FSM Dental, Türkiye) [HF]. Standardized disc-shaped specimens (n = 12 per group) were fabricated using Mylar strips; no finishing or polishing procedures were performed on the specimen surfaces. Baseline measurements of surface roughness (Ra), gloss (GU), and color parameters (for ΔE₀₀ calculation) were obtained prior to exposure using a contact profilometer, glossmeter, and spectrophotometer, respectively. The specimens were then subjected to simulated gastric acid (pH 1.2) for durations corresponding to 1 year (3 h) and 5 years (15 h) of cumulative exposure. Following each exposure period, all measurements were repeated. Color change (ΔE₀₀) were calculated according to CIE2000 formula. Simulated gastric acid exposure resulted in a significant increase in surface roughness and color change, along with a reduction in gloss values in most materials (p < 0.05). Surface roughness increased significantly in all groups except EQ (p > 0.05), with the highest values observed in the TN group in a 5-year simulation (0.43 ± 0.25 μm; p = 0.037). Gloss values significantly decreased in all materials (p ≤ 0.007), although FR exhibited a more limited reduction, reaching significance only after prolonged exposure (p = 0.017), and maintained the highest gloss after 5 years of simulation (74.9 ± 8.9 GU). ΔE₀₀ increased significantly in all groups over time (p < 0.001), with the greatest change observed in EQ (3.18 ± 0.7). Simulated gastric acid exposure compromised the surface and optical properties of all tested composites, with flowable bulk-fill materials demonstrating greater susceptibility than packable counterparts. FR exhibited superior performance, preserving gloss and maintaining color changes within clinically acceptable limits (ΔE₀₀≤1.8). These findings highlight the pivotal influence of filler content, resin matrix composition, and photoinitiator system on composite resistance to acid-induced surface degradation and optical instability.