Influence of Solvent Challenge on the Hardness and Toughness of Viscosity-Modified Composites

This study assessed the effect of solvent aging on Vickers microhardness (VHN) and fracture toughness (K_IC) of four resin composites: two high-strength flowable materials indicated for all cavity classes (GUF and CBF), a packable nanohybrid composite (XTE), and its flowable counterpart (XTEF).

Disk specimens (n = 6 per group) were photocured and stored at 37 °C in distilled water or in a 75%/25% ethanol/water solution for 1 or 30 days. Vickers microhardness was recorded with a 300 g load applied for 15 s. Single-edge-notched beam (SENB) specimens (32 × 6 × 3 mm; n = 6 per group) were loaded in three-point bending for K_IC determination after 1 and 30 days of storage in water or ethanol/water. Data were analyzed by three-way ANOVA followed by Tukey's post hoc test (α = 0.05).

At 1 day, the four composites separated into distinct VHN groups in the order XTE > GUF > CBF > XTEF (p ≤ 0.05). After 30 d, VHN decreased in all materials, with larger reductions in ethanol/water (17-30% relative to the 1 d water value) than in water alone (5-11%). At 1 day, K_IC values for XTE, GUF and CBF formed a single statistical group, all significantly higher than XTEF (p ≤ 0.05). Thirty-day water storage did not affect K_IC for any material (p > 0.05), whereas ethanol/water storage reduced K_IC by 21-26% in all four composites and produced four distinct material groups (p ≤ 0.05).

The high-strength flowable composites showed greater hardness and toughness than a conventional flowable composite but did not match the mechanical performance of the highly filled packable composite. Ethanol/water aging markedly softened the composite surfaces and reduced fracture toughness, whereas prolonged water storage had a smaller effect on hardness and no measurable effect on K_IC.