Purpose: To investigate the effects of nanozirconia fillers conditioned with 10-methacryloyloxydecyl dihydrogen phosphate (MDP) with or without zirconium hydroxide precoating on bending strength, Vickers hardness, and translucence of dental resin composites.
Methods: We obtained nanozirconia fillers coated with different concentrations of Zr(OH)₄ using wet-chemical synthesis. We analyzed coating quality by observing electron-diffraction patterns using transmission electron microscopy. We conditioned zirconia fillers, with or without prior Zr(OH)₄-coating, using MDP-containing primers and evaluated the formation of chemical bonds using nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS). We then performed three-point bending-strength tests, Weibull analysis, Vickers hardness, and translucence-parameter analysis with or without addition of different concentrations of zirconia using untreated zirconia fillers as controls.
Results: We achieved desirable Zr(OH)₄ coating using 5 mmol/L zirconium chloride. NMR and XPS analysis detected stronger Zr–O–P peaks on MDP-conditioned zirconia fillers with prior Zr(OH)₄-coating compared with MDP-conditioned fillers alone, suggesting that MDP bonding with zirconia was enhanced by zirconium hydroxide. Our three-point bending-strength tests revealed that increasing levels of untreated zirconia fillers decreased the three-point bending strength of the resin composites, while MDP-conditioned zirconia fillers with or without prior Zr(OH)₄ coating improved three-point bending strengths. Adding 5 wt% and 7.5 wt% MDP-conditioned zirconia fillers with prior Zr(OH)₄ coating achieved the highest three-point bending strength. Furthermore, addition of zirconia fillers decreased the translucence of silica-based resin composites.
Conclusion: MDP conditioning with prior Zr(OH)₄ coating is recommended for treating nanozirconia fillers of resin composites.
Keywords: resin composite, zirconia, surface treatment, phosphate ester monomer, MDP, filler