Mechanical Properties of Nanohybrid Resin Composites Containing Various Mass Fractions of Modified Zirconia Particles

Abstract

Purpose: The aim of this study was to investigate the effect of various mass fractions of 10-methacry-loyloxydecyl dihydrogen phosphate (MDP)-conditioned or unconditioned zirconia nano- or micro-particles with different initiator systems on the mechanical properties of nanohybrid resin composites.

Methods: Both light-cured (L) and dual-cured (D) resin composites were prepared. When the mass fraction of the nano- or micro-zirconia fillers reached 55 wt%, resin composites were equipped with dual-cured initiator systems. We measured the three-point bending-strength, elastic modulus, Weibull modulus and translucency parameter of the nanohybrid resin composites containing various mass fractions of MDP-conditioned or unconditioned zirconia nano- or micro-particles (0%, 5 wt%, 10 wt%, 20 wt%, 30 wt% and 55 wt%). A Cell Counting Kit (CCK)-8 was used to test the cell cytotoxicity of the experimental resin composites. The zirconia nano- or micro-particles with MDP-conditioning or not were characterized by transmission electron microscopy (TEM), Fourier infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS).

Results: Resin composites containing 5-20 wt% MDP-conditioned or unconditioned nano-zirconia fillers exhibited better three-point bending-strength than the control group without zirconia fillers. Nano- or micro-zirconia fillers decreased the translucence of the nanohybrid resin composites. According to the cytotoxicity classification, all of the nano- or micro-zirconia fillers containing experimental resin composites were considered to have no significant cell cytotoxicity. The FTIR spectra of the conditioned nano- or micro-fillers showed new absorption bands at 1719 cm^-1 and 1637 cm^-1, indicating the successful combination of MDP and zirconia particles. The XPS analysis measured Zr-O-P peak area on MDP-conditioned nano- and micro-zirconia fillers at 39.91% and 34.89%, respectively.

Conclusion: Nano-zirconia filler improved the mechanical properties of nanohybrid resin composites, but cannot be the main filler to replace silica filler. The experimental dual-cured composites can be resin cements with better opacity effects and a low viscosity.

Keywords: MDP; initiator system; mechanical property; nano-zirconia filler; resin composite; translucence.

Figure 1

(A) FTIR spectra of the MDP-conditioned and unconditioned nano-zirconia fillers in the range of 3500–500 cm⁻¹ and peak-fitted FTIR spectra of MDP-conditioned nano-zirconia fillers in the range of 1200–950 cm⁻¹. The peaks corresponding to the P–O bond were located at 985.5 and 1082 cm⁻¹; (B) FTIR spectra of the MDP-conditioned and unconditioned micro-zirconia fillers in the range of 3500–500 cm⁻¹ and peak-fitted FTIR spectra of MDP-conditioned micro-zirconia fillers in the range of 1200–950 cm⁻¹. The peaks corresponding to the P–O bond were located at 972.6 and 1083.7 cm⁻¹. The absorption band of MDP-conditioned nano- and micro-zirconia fillers at 2930 and 2850 cm⁻¹ correspond to the –CH₂– stretching vibration. The peaks located at 1719 and 1637 cm⁻¹ were attributed to C=O bond and C=C bond, respectively.

Figure 2

(A) Wide-scan X-ray photoelectron spectroscopy spectra of the unconditioned and MDP-conditioned nano-zirconia fillers; (B) wide-scan X-ray photoelectron spectroscopy spectra of the unconditioned and MDP-conditioned micro-zirconia fillers; (C) narrow-scan O1s spectra of the unconditioned nano-zirconia fillers; (D) narrow-scan O1s spectra of the unconditioned micro-zirconia fillers; (E) narrow-scan O1s spectra of the MDP-conditioned nano-zirconia fillers; and (F) narrow-scan O1s spectra of the MDP-conditioned micro-zirconia fillers. I (C–O), II (P–O–H), III (Zr–O–P), IV (Zr–O–Zr), V (OH–) represent the different deconvoluted peaks within the main peak.

Figure 3

(A) SEM image of the unconditioned micro-silica fillers; (B) SEM image of the unconditioned micro-zirconia fillers; (C) transmission electron micrograph of the unconditioned nano-silica fillers; (D) electron diffraction pattern of the unconditioned nano-silica fillers; (E) transmission electron micrograph of the unconditioned nano-zirconia fillers; and (F) electron diffraction pattern of the unconditioned nano-zirconia fillers.

Figure 4

(A) SEM images and EDS maps of light-cured resin composites of L-55si+5zr, L-55si+5Mzr, L-30si+30zr, L-30si+30Mzr; (B) SEM images and EDS maps of dual-cured resin composites of D-55si+5zr, D-55si+5Mzr, D-30si+30zr, D-30si+30Mzr, D-5si+55zr, D-5si+55Mzr. The arrows represent the pits left by the fillers splitting away from the resin surface.

Figure 5

(A) Mean values of three-point bending strength of the light-cured resin composites; (B) mean values of three-point bending strength of the dual-cured resin composites. Same superscript letters (a–d) indicates no significant difference between the groups (P > 0.05).

Figure 6

(A) Weibull distribution plot with 95% confidence intervals of light-cured resin composites; (B) Weibull distribution plot with 95% confidence intervals of dual-cured resin composites.

Figure 7

(A) Mean values of translucency parameter of the light-cured resin composites; (B) mean values of translucency parameter of the dual-cured resin composites. Same superscript letters (a–e) indicates no significant difference between the groups (P > 0.05).

Figure 8

Relative cell proliferations of the experimental groups and the control group.

Figure 9

Cell morphology in each group after 1 (A, D, G, J, M), 3 (B, E, H, K, N) and 5 (C, F, I, L, O) days of cell culture. (A–C): L-55si+5Zr group; (D–F): L-55si+5Mzr group; (G–I): D-50si+10zr; (J–L): D-50si+10Mzr; and (M–O): control group.