Effect of loading chemically and mechanically pre-treated fumed silica as filler on an etch & rinse model dental adhesive

Fumed silica is one of the most commonly used fillers for dental resin adhesives. However, fumed silica produced by flame hydrolysis of chlorosilanes are typically agglomerated aggregates instead of isolated primary particles. The fumed silica aggregates may affect the dispersion of fillers in the resin matrix, thus affect the bonding performance of the dental adhesives. In this study, fumed silica fillers chemically treated with methacrylsilane were loaded into an etch & rinse model dental adhesive. Filler 1 was only chemically pre-treated, while Filler 2 was combined chemically and mechanically pre-treated to break the agglomerated aggregates. The experiment result indicated that Filler 1 has faster rheology control effect than Filler 2 on the handling property of the filled resin adhesive showing as the rapid increased viscosity, which may due to the fast inorganic phase growing in the Filler 1 filled resin matrix. As for the mechanical properties, neither Filler 1 nor Filler 2 has an positive effect on the micro-tensile bond strength in short term and long term, despite of the fact that lower weight percent of Filler 2 loading is favourable to the penetration of the experimental adhesive into dentin substrate. In conclusion, mechanically pre-treatment to break the agglomerated fumed silica is helpful to increase the limit of filler loading percent while maintaining appropriate handling property, but the chemically and mechanically pre-treatment cannot provide positive effect on bonding performance within the range of this study.