FRACTURE-TOUGHNESS OF EXPERIMENTAL DENTAL COMPOSITES AGED IN ETHANOL

Fracture toughness (K-Ic) is an intrinsic property which may be relate d to the ability of a restorative material to resist fracture and abra sion. This property may change for a dental composite restorative due to the effects of various oral solvents. The hypothesis to be tested w as that aging in ethanol would cause a reduction in the fracture tough ness of dental composites, and that the extent of this reduction might be dependent upon certain compositional variables. The fracture tough nesses of three series of experimental. composites with various degree s of conversion, filler volume, and percent of silane-treated fillers were compared after the composites were aged for periods of one month and six months in 75% ethanol/water, a solvent which serves as a food- simulating liquid. An unfilled Bis-GMA/TEGDMA resin served as the cont rol. All composites, with the exception of one subjected to a post-lig ht-curing heat treatment, experienced a significant reduction (from 30 to 56%) in K-Ic after being aged in 75% ethanol for six months. A sim ilar reduction in K-Ic of 58% for the unfilled resin suggested that th e reduction for the composites was due to a weakening of the resin mat rix, which facilitated crack propagation. A simultaneous reduction in microhardness was also demonstrated. One month of aging in ethanol als o produced large reductions in K-Ic for specimens with insufficient cu re and minimal filler volume, suggesting that the properties of the re sin matrix predominated for these composites. Aging in ethanol for one month increased K-Ic by from 5 to 10% for three partially silanated c omposites, but further aging caused a large reduction in their resista nce to crack propagation. This study verified that the K-Ic and hardne ss of Bis-GMA-based composites was reduced after long-term aging in an effective solvent. In addition, insufficient curing and severely comp romised filler/matrix adhesion, which caused the properties of the res in matrix to dominate, contributed to a reduction in the fracture toug hness of composites.