The Effect Of Concentrations And Surface Treatments Of Nanozirconia Fillers On Dental Bisphenol A-glycidyl Methacrylate-based Resin Composites

ObjectivesZirconia has been added to silica-based filler systems owing to its high strength,fracture toughness,hardness and chemical stability.As zirconia-filler surfaces lack polar bonds and cannot form chemical bonds with silane hydroxyl groups,it is hard to bond with organic resin matrix.Therefore,the mechanical properties of resin products would perhaps be further enhanced if suitable surface treatments can be adopted in order to activate zirconia filler surfaces to chemically bond with the resin matrix.In the present study,the authors evaluated effects of several surface treatments for nano-scale zirconia fillers with different concentrations on the mechanical properties of dental resin composite.These surface treatments included conditioning with the phosphate ester monomer MDP,with and without precoating with nano-zirconium hydroxide,with an aim to mediate chemical bonding between zirconia fillers and resin matrix.MethodsWe obtained nanozirconia fillers coated with different concentrations of Zr（OH）₄using 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 and Vickers hardness with or without addition of different concentrations of zirconia using untreated zirconia fillers as controls.ResultsWe 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）₄coating 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.Conclusions1.Prior Zr（OH）₄ coating（obtained by adding 0.5 g zirconia particles to 5mmol/L zirconium chloride）and MDP conditioning is recommended for treating zirconia fillers of resin composites,and resin composites with 5 wt%and 7.5 wt%MDP-conditioned zirconia fillers with prior Zr（OH）₄ coating achieved the best three point bending strength.2.Zr-O-P bonds between MDP and zirconia improved bonding between zirconia fillers and resin matrix.Zr（OH）₄ coatings improved the bonding of 10-MDP to zirconia.