Investigation of Mechanical Properties of Experimental Bis-GMA/TEGDMA Dental Composite Resins Containing Various Mass Fractions of Silica Nanoparticles

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 mu m to 40 mu m 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.