Recycling Dental Zirconia Wastes: Is It Feasible?

Abstract

Objectives: Dental yttria-stabilized zirconia is a CAD/CAM material that has a good mechanical and aesthetic properties for dental restoration. However, after the milling, a lot of milling dusts and residual of the blocks remains. This is essentially not environmentally friendly and waste of money. Thus, the objective of this study was to design a easy procedure that could recycle pre-fabricated zirconia blocks with optimum mechanical strength. Methods: Used commercial zirconia blocks (Lava Plus High Translucency, 3M ESPE) were pulverised (10,000rev/min) into powders and screen-sieved (300-mesh). Then, the screened powder was pickled by acid (0.5-mol/L HNO3, 55°C for 5 minute). Next, the powder was dry pressed into green body blocks (80kN) in a mould (35.0×30.0×3.0-mm3). Finally, the green body was pre-sintered in different temperatures in 800, 850, 900, 950, 1000, 1050 and 1100°C. These pre-sintered blocks, together with the control block, i.e. new zirconia block with the same batch, were then fully sintered at 1450°C according to manufacturer instruction. XRD was used to evaluate the crystalline structure. SEM/EDX was used to study the element composition, surface and particle topography. Relative density and knoop microhardness were tested before and after full sintering. Four-point bending were tested for flexural strength of fully sintered beams (n=7, 24.0×4.0×2.0-mm3). One-way ANOVA was used to analyze the results (α=0.05). Results: Only tetragonal phase was detected by XRD for both new and recycled zirconia blocks, and similar elemental contents were confirmed by EDX. SEM results showed the pre-sintered samples at 900-1000°C had a smooth and flat surface and evenly distributed particles. No statistical significant difference (p>0.05) was found between new and 900°C to 1050°C recycled fully sintered zirconia for relative density. In terms of hardness, only green blank that was pre-sintered at 1000°C showed no statistically significant difference (p=0.586) than new. Four-point bending test results showed that recycled zirconia pre-sintered at 950-1050°C had no statistical significant difference (p>0.05) than new one . Conclusions: Recycling of commercial zirconia block deemed to be feasible, and 1000oC seems to be the best pre-sintering temperature for such a recycling process.