Effect of zirconium oxide nanoparticles addition on the optical and tensile properties of polymethyl methacrylate denture base material

Abstract

Background: Polymethyl methacrylate (PMMA) is widely used for the fabrication of removable prostheses. Recently, zirconium oxide nanoparticles (nano-ZrO₂) have been added to improve some properties of PMMA, but their effect on the optical properties and tensile strength are neglected.

Objective: The aim of this study was to investigate the effect of nano-ZrO₂ addition on the translucency and tensile strength of the PMMA denture base material.

Materials and methods: Eighty specimens (40 dumbbell-shaped and 40 discs) were prepared out of heat-polymerized acrylic resin and divided into four groups per test (n=10). The control group for each test included unreinforced acrylic, while the test groups were reinforced with 2.5, 5, and 7.5 wt% nano-ZrO₂. Acrylic resin was mixed according to manufacturer's instructions, packed, and processed by conventional method. After polymerization, all specimens were finished, polished, and stored in distilled water at 37°C for 48±2 hours. Tensile strength (MPa) was evaluated using the universal testing machine while the specimens' translucency was examined using a spectrophotometer. Statistical analysis was carried out by SPSS using the paired sample t-test (p≤0.05). A scanning electron microscope was used to analyze the morphological changes and topography of the fractured surfaces.

Results: This study showed that the mean tensile strength of the PMMA in the test groups of 2.5%NZ, 5%NZ, and 7.5%NZ was significantly higher than the control group. The tensile strength increased significantly after nano-ZrO₂ addition, and the maximum increase seen was in the 7.5%NZ group. The translucency values of the experimental groups were significantly lower than those of the control group. Within the reinforced groups, the 2.5%NZ group had significantly higher translucency values when compared to the 5%NZ and 7.5%NZ groups.

Conclusion: The addition of nano-ZrO₂ increased the tensile strength of the denture base acrylic. The increase was directly proportional to the nano-ZrO₂ concentration. The translucency of the PMMA was reduced as the nano-ZrO₂ increased.

Clinical significance: Based on the results of the current study, the tensile strength was improved with different percentages of nano-ZrO₂ additions. However, translucency was adversely affected. Therefore, it is important to determine the appropriate amount of reinforcing nano-ZrO₂ that will create a balance between achieved properties - mechanical and optical.

Keywords: PMMA; denture base; tensile strength; translucency; zirconium oxide nanoparticles.

Figure 1

Representative TEM images of nano-ZrO₂. Note: The average particle size was estimated to be ~40 nm. Abbreviations: nano-ZrO₂, zirconium oxide nanoparticles; TEM, transmission electron microscopy.

Figure 2

Schematic diagram of silanization of nano-ZrO₂. Abbreviation: nano-ZrO₂, zirconium oxide nanoparticles.

Figure 3

Specimens prepared for testing: disc shape obtained for translucency test and dumbbell shape for tensile strength test. Note: (A) Unreinforced specimen (n=20), acrylic resin specimens reinforced with (B) 2.5% nano-ZrO₂ (n=20), (C) 5% nano-ZrO₂ (n=20), and (D) 7.5% nano-ZrO₂ (n=20). Abbreviations: 2.5%NZ, 2.5 wt% nano-ZrO₂ reinforced heat-polymerized acrylic resin; 5%NZ, 5 wt% nano-ZrO₂ reinforced heat-polymerized acrylic resin; 7.5%NZ, 7.5 wt% nano-ZrO₂ reinforced heat-polymerized acrylic resin; nano-ZrO₂, zirconium oxide nanoparticles.

Figure 4

Representative SEM images. Note: (A) Unreinforced specimens, acrylic resin specimens reinforced with (B) 2.5%NZ, (C) 5%NZ, and (D) 7.5%NZ. Abbreviations: SEM, scanning electron microscopy; 2.5%NZ, 2.5 wt% nano-ZrO₂ reinforced heat-polymerized acrylic resin; 5%NZ, 5 wt% nano-ZrO₂ reinforced heat-polymerized acrylic resin; 7.5%NZ, 7.5 wt% nano-ZrO₂ reinforced heat-polymerized acrylic resin.

Figure 5

Possible explanation of translucency change of PMMA/ZrO₂ nanocomposite. Notes: (A) Higher refractive index difference between nano-ZrO₂ and resin matrix resulted in severe bending of the light at nano-ZrO₂/resin matrix interface, which reduced light transmittance. (B) Low refractive index difference between nano-ZrO₂ and resin matrix allowed incident light passing through the material with a little interference between nano-ZrO₂ and resin matrix. Abbreviations: PMMA, polymethyl methacrylate; nano-ZrO₂, zirconium oxide nanoparticles.