Effects of nano-zirconia fillers conditioned with phosphate ester monomers on the conversion and mechanical properties of Bis-GMA- and UDMA-based resin composites

Objectives: This study investigated the effects of the surface conditioning of nano-zirconia fillers with 10-methacryloyloxydecyl dihydrogen phosphate (MDP) and dipentaerythritol penta-acrylate phosphate (PENTA) on the degree of conversion and mechanical properties of Bis-GMA- and UDMA-based resin composites containing the fillers. Methods: MDP or PENTA conditioned nano-zirconia fillers were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The conditioned nano-zirconia fillers were then used to prepare Bis-GMA- and UDMA-based resin composites. The bending strength and elastic modulus of the prepared composites were evaluated, and Weibull analysis was adopted to evaluate the reliability of the bending tests. Degree of conversion (DC) was measured by FTIR. Results: FTIR and XPS analysis confirmed formation of Zr-O-P bonds in the MDP-conditioned and PENTA-conditioned zirconia fillers. MDP-conditioning and PENTA-conditioning increased bending strength of Bis-GMA-based composites (100.70 +/- 7.15 MPa, 98.65 +/- 4.76 Mpa) and UDMA-based composites (99.02 +/- 4.45 MPa, 99.92 +/- 5.78 MPa) compared to the groups with untreated nano-zirconia fillers (Bis-GMA-based, 84.98 +/- 5.13 MPa; UDMA-based, 88.24 +/- 4.61 MPa). The same trend was observed for the Weibull modulus (m) and elastic modulus. The MDP/PENTA conditioning did not affect the DC of the composites. Conclusion: MDP or PENTA conditioning of nano-zirconia fillers improved the mechanical properties of the composite compared to the composites containing untreated nano-zirconia fillers; however, it did not affect DC.