**1. Introduction**

Denture teeth are currently made of either methacrylate-based resins (acrylic resin) or porcelain, but acrylic teeth have nearly eliminated porcelain teeth from the market [1] due to a number of advantages, including their chemical bonding to the denture base [2,3], lower susceptibility to fracture [4,5] and decreased clicking [6,7]. Acrylic resin tooth wear is a serious complication during denture service and can change the vertical dimension of dentures. This process harms the denture and exerts an impact on facial aesthetics and the function of the masticatory muscles, resulting in less efficient mastication. This can result in temporomandibular disorders, digestive disturbances and decreased patient comfort [6–9]. Efforts were made to enhance the wear resistance of the acrylic resin denture teeth, such as the formation of cross-linked polymer teeth (interpenetrated polymer network) [8,9]. Another possible solution is to add nanofillers to enhance mechanical properties. Despite this, these materials are softer than composite resin or porcelain teeth [8–10]. Nano-fillers, such as metal oxides, carbon, and glass fibers have also been

**Citation:** El-Tamimi, K.M.; Bayoumi, D.A.; Ahmed, M.M.Z.; Albaijan, I.; El-Sayed, M.E. The Effect of Salinized Nano ZrO<sup>2</sup> Particles on the Microstructure, Hardness, and Wear Behavior of Acrylic Denture Tooth Nanocomposite. *Polymers* **2022**, *14*, 302. https://doi.org/10.3390/ polym14020302

Academic Editors: Domenico Acierno and Antonella Patti

Received: 20 November 2021 Accepted: 10 January 2022 Published: 12 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

used to improve the mechanical behavior of the acrylic resin denture base materials [11–16]. Recently, zirconium oxide nanoparticles (ZrO<sup>2</sup> NPs) have been recognized for their high biocompatibility. However, because they are white in color, they are thought to be less likely to change aesthetics than other metal oxide nanoparticles [17]. ZrO<sup>2</sup> NPs are not only biocompatible but are also resistant to wear and corrosion. In addition to these characteristics, ZrO<sup>2</sup> NPs offer high toughness and mechanical strength [18–20]. ZrO<sup>2</sup> NPs are frequently used to mechanically reinforce polymers [12]. Saline coupling agents are applied to the surface of ZrO<sup>2</sup> NPs to decrease the risk of aggregation and enhance compatibility with the polymer matrix [21]. The stresses caused by the dispersion of ZrO<sup>2</sup> NPs are transferred from the weak Polymethylmethacrylate (PMMA) matrix to the strong nanoparticles [20]. Ayad et al. [18] reported a minor increase in the surface hardness and the impact strength of zirconium dioxide acrylic composite when compared to the unreinforced resin (control specimen). Another study reported a reduction in impact strength as well as surface hardness when 10 wt.% ZrO<sup>2</sup> and 20 wt.% ZrO<sup>2</sup> were added [22]. Alternatively, a study found that increasing the amount of modified (ZrO2) added to nano-ZrO2/PMMA composite increased the hardness by up to 10%, and increased the hardness of groups 5% and 10% (water storage groups) compared with the zero and 20% (*p* < 0.01). The highest hardness value was observed in group 5% (TC− 36.8, TC+ 35.2) [23,24]. Furthermore, after incorporating salinized zirconia NPs in acrylic resin, there was a significant increase in hardness and a minor improvement in surface roughness, as well as a decrease in porosity [25,26]. However, investigations into the addition of salinized nano-ZrO<sup>2</sup> powder to acrylic resin teeth are lacking. The overall aim of this study is to develop nanocomposite denture teeth with high wear resistance. To this end, the effect of incorporating salinized nano-ZrO<sup>2</sup> particles into acrylic resin on the acrylic denture tooth microstructure, hardness, and wear resistance was investigated.
