RESUMO
PURPOSE: Mechanical properties of dental composite resins need to be improved in order to enhance their performance for applications in direct restorations. Application of nanoparticles in this field is a recent development. The aim of this study was to investigate the mechanical properties of experimental composites containing various mass fractions of silica nanoparticles. MATERIALS AND METHODS: Experimental composites were composed of a visible-light-curing monomer mixture (70 wt% Bis-GMA and 30 wt% TEGDMA) and silica nanoparticles of a size ranging from 20 nm to 50 nm modified with gamma-methacryloxy propyl trimethoxy silane (gamma-MPS) as reinforcing filler. The composites were classified into four groups according to their filler mass fractions ranging from 20% to 50%. Following the same preparation procedure, a conventional composite was also fabricated consisting of a mass percentage of 60% silica fillers having particle sizes ranging from 10 microm to 40 microm in the same organic matrix, which served as control. Ten specimens were prepared of each experimental group and also of the control. Fracture toughness was measured using single-edge notched bend (SENB) specimens. Specimen fracture surfaces were mounted on aluminum stubs with carbon cement, sputter-coated with gold and examined under scanning electron microscopy (SEM). Flexural strength was evaluated through a standard three-point bending test and Vickers microhardness test was performed to investigate the hardness of the samples. RESULTS: Filler mass fraction had a significant effect on composite properties. Fracture toughness, flexural strength, and hardness of composites at filler mass fraction of 40% of silica nanoparticles were (mean +/- SD) 1.43 +/- 0.08 MPa.m(1/2), 149.74 +/- 8.14 MPa, and 62.12 +/- 3.07 VHN, respectively; relevant values for composites at 50% mass fraction of silica nanoparticles were 1.38 +/- 0.07 MPa.m(1/2), 122.83 +/- 6.13 MPa, and 70.69 +/- 3.67 VHN, respectively, all of which were significantly higher than 1.07 +/- 0.06 MPa.m(1/2), 104.61 +/- 8.73 MPa, and 52.14 +/- 4.02 VHN of the control, respectively (Tukey's multiple comparison test; family confidence coefficient = 0.95). Measured values for composites at 20% mass fraction of silica nanoparticles were 0.94 +/- 0.06 MPa.m(1/2), 103.41 +/- 7.62 MPa, and 42.87 +/- 2.61 VHN, respectively; relevant values for composites at 30% mass fraction of silica nanoparticles were 1.16 +/- 0.07 MPa.m(1/2), 127.91 +/- 7.05 MPa, and 51.78 +/- 3.41 VHN, respectively. CONCLUSIONS: Reinforcement of dental composite resins with silica nanoparticles resulted in a significant increase in the evaluated mechanical properties in comparison with the conventional composite. The filler mass fraction played a critical role in determining the composite's mechanical properties.
