In vitro aging of dental composites in water—Effect of degree of conversion, filler volume, and filler/matrix coupling

The purpose of this study was to evaluate the long-term effect of aging in water on the physical properties of experimental composites having systematically controlled differences in degree of conversion (DC), filler volume fraction (Vf), and percentage of silane-treated fillers. Composites were made with a 50% Bis-GMA:50% TEGDMA light-cured resin and a 1–2 μm (average size) strontium glass filler (+ 5 wt% SiO2 microfiller). For composites A–E, the DC was varied from 56–66% by changing the curing time; for D and F–I, the Vf was varied from 28–62 vol%; and for D and J–M, the percent of fillers with a silane coupling agent (γ-MPS) was varied from 20–100%. Fracture toughness (KIc), flexure strength (FS), elastic modulus (E), and hardness (KHN) were tested after soaking in water at 37°C for 1 day, 6 months, 1 year, and 2 years. The KIc was reduced 20–30% for all composites after 6 months, with minimal changes thereafter. The FS was reduced for several composites at 6 months, but only those with poor cure (A and B) were lower at 2 years than they were initially. The E was not reduced for most composites. Hardness was reduced for most composites after 6 months, but many returned to their original levels at 2 years. Long-term aging in water caused a reduction in the KIc, independent of composition, but had little effect on other properties, suggesting limited degradation of composites in water.