*Article* **Effects of UV Absorber on Zirconia Fabricated with Digital Light Processing Additive Manufacturing**

**Jin-Ho Kang 1,†, Kumaresan Sakthiabirami 1,2,†, Hyun-Ah Kim <sup>1</sup> , Seyed Aliakbar Hosseini Toopghara <sup>3</sup> , Mee-Jin Jun <sup>4</sup> , Hyun-Pil Lim <sup>1</sup> , Chan Park <sup>1</sup> , Kwi-Dug Yun 1,\* and Sang-Won Park 1,2,\***


**Abstract:** This study evaluated the effect of UV absorbers on the dimensional accuracy of zirconia specimens fabricated by additive manufacturing using a digital light process. Zirconia suspension for additive manufacturing was prepared by setting the volume fractions (0, 0.005, 0.05, and 0.1%) of various UV absorbers. The effect of UV absorber content was evaluated through curing thickness, geometric overgrowth model design, linear deviation, and microstructure evaluation before and after sintering. Statistical analysis was performed by Kruskal–Wallis H and post-tested by the Bonferroni correction method. There was no significant difference in the cure depth according to the presence or absence of the UV absorber, the difference in geometric overgrowth was from 2.1 to 12.5%, and the overgrowth significantly decreased as the amount of added UV absorber increased. This result may contribute to improved precision of 3D multilayer ceramic products.

**Keywords:** UV absorbers; zirconia; additive manufacturing; cure depth; geometric overgrowth

## **1. Introduction**

With the development of digital dentistry, computer-aided design/computer-aided manufacturing (CAD/CAM) system zirconia prostheses have gained attention for satisfactory esthetics, high biocompatibility, and improved mechanical properties compared to existing dental ceramics [1].

Zirconia ceramic also has a variety of different applications other than dental implants and frameworks [2–4], such as electrolyte [5,6] and monolithic support for solid oxide fuel cells [7], as a part of cutting tools and blades [8], for elaborate molds [9] and other precision components in thermal and mechanical applications [10].

Currently, zirconia prostheses are fabricated by subtractive manufacturing using a CAD/CAM system. A completely sintered zirconia prosthesis is obtained by milling and sintering a pre-sintered zirconia block. Studies show that 15–30% (approximately 20%) of linear shrinkage occurs due to sintering [11]. However, the exact shrinkage rate is compensated for by setting the enlargement rate according to the manufacturer's instructions for the zirconia block. Prosthesis fabrication through subtractive manufacturing has certain disadvantages, including the consumption of materials from milling burs or remaining blocks, the possibility of microcracking due to surface roughness or defects, and difficulty reproducing complex structures [12–15].

Three-dimensional printing techniques (or additive manufacturing) have gained popularity in various fields, such as temporary prostheses, splints, and model manufacturing,

**Citation:** Kang, J.-H.; Sakthiabirami, K.; Kim, H.-A.; Hosseini Toopghara, S.A.; Jun, M.-J.; Lim, H.-P.; Park, C.; Yun, K.-D.; Park, S.-W. Effects of UV Absorber on Zirconia Fabricated with Digital Light Processing Additive Manufacturing. *Materials* **2022**, *15*, 8726. https://doi.org/10.3390/ ma15248726

Academic Editors: Csaba Balázsi and Emilio Jiménez-Piqué

Received: 25 October 2022 Accepted: 5 December 2022 Published: 7 December 2022

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due to the ability to consume less material than subtractive manufacturing and reproduce complex structures [16–18]. With the recent development of additive manufacturing, studies have aimed to apply these techniques to prosthetic manufacturing in dentistry. Studies using various 3D printers, such as photopolymerization digital light processing (DLP), stereolithography apparatus (SLA), selective laser sintering (SLS), and spraying polyjet printers, are being introduced for zirconia additive manufacturing [19–23].

The photopolymerization method shows relatively higher precision, faster manufacturing speed, and higher surface roughness among these techniques. Therefore, it is more likely to be used in manufacturing dental prostheses [24]. The DLP uses a projector, modified to produce a specific wavelength, and a digital micromirror device to cure the liquid resin in layer units. Although the precision is lower than the SLA method, using a laser scanner, DLP shows excellent accuracy in small prints, such as crowns, and is advantageous due to its high manufacturing speed and relatively inexpensive equipment cost [7,25].

In dentistry, strength and precision are enormous challenges when manufacturing zirconia prostheses by DLP additive manufacturing. The process requires a zirconia photopolymer suspension, generally including zirconia powder, photosensitive resin, photoinitiator, and dispersant. For zirconia to have high strength, a high zirconia volume fraction is required, creating difficulty in obtaining an appropriate degree of dispersion and viscosity [26]. After mixing, it is crucial to have a well-dispersed suspension with relatively low viscosity to ensure precise additive manufacturing [27,28]. A study by Jang et al. [29] on the production of zirconia using DLP additive manufacturing showed that a volume fraction of zirconia of 58 vol% was the maximum range possible to achieve homogeneous mixing. As the volume fraction increases, the 3-point bending strength increases. However, the study reported that the viscosity increased rapidly to 56 vol%.

Geometrical overgrowth was introduced as a factor affecting manufacturing accuracy using DLP additive manufacturing zirconia [29,30]. Geometrical overgrowth refers to light scattering during the photopolymerization of the zirconia suspension, resulting in the over-curing of the surrounding. Light scattering occurs due to the high refractive index, polycrystalline grain structure, and relatively large grain size of zirconia [29–31]. A previous study [29,32] reported the evaluation of curing depth and geometrical overgrowth of additive manufacturing according to the volume fraction of zirconia and showed that the geometrical overgrowth by light scattering increased as the volume fraction increased, and a decreased curing time caused light scattering. A reduction in curing time could adversely affect cure depth. The photopolymerization method in ceramic additive manufacturing showed that overgrowth increased as the exposure time and area increased [30,33]. Based on the previous report, once the slurry and printing parameters were optimized, the layer lines played a minor role in the strength [34], which eventually highlights the importance of printing overgrowth optimization. Literature shows few laboratory studies on the geometrical overgrowth of additive manufacturing of zirconia. Mitteramskogler et al. [30,35] added a UV absorber to the suspension composition to suppress overgrowth due to light scattering during ceramic additive manufacturing. The UV absorber absorbs UV rays and converts them into thermal energy [36]. It can also control the penetration depth of UV rays and prevent proper dispersion and over-curing [37–39]. While the UV absorber is mainly studied as a polymer stabilizer by adding it to the photopolymer suspension, further studies on its effectiveness as a light scattering inhibitor and controlling overgrowth in zirconia additive manufacturing are needed.

Additive manufacturing using commercially available photopolymer resin for 3D printing not only controls but also compensates for the curing contraction of the resin and has shown clinically acceptable precision for manufacturing crowns and other prostheses [27]. It is necessary to compensate for the shrinkage due to sintering and evaluate and control geometrical overgrowth to obtain stability and precision in dental prostheses during the additive manufacturing of zirconia prostheses. Therefore, this study aimed to assess the degree of geometrical overgrowth based on the UV absorber of DLP additive manufacturing zirconia and also looks to increase the accuracy of additively manufactured

zirconia prostheses in dental practice. Moreover, we aimed to assess the influence of UV absorbers on the dimensional accuracy of zirconia shaping fabricated by DLP additive manufacturing.

#### **2. Materials and Methods**

#### *2.1. Zirconia Suspension Preparation*

Zirconia photopolymer suspension was prepared based on the acrylate series commercialized with zirconia powder (TZ-3Y; Tosoh, Tokyo, Japan), and Table 1 shows the mechanochemical properties of each material [29]. The other additive agents included a photoinitiator (Irgacure 819; Ciba Specialty Chemicals, Basel, Switzerland) and dispersant (BYK-180; BYK Inc., Wesel, Germany) added based on the previous study [29]. The volume fraction of the zirconia was calculated to be 54 vol% to prepare a suspension. Trimethoxysilane (MTMS; Sigma-Aldrich Inc., Saint Louis, MO, USA) was added for silane treatment, and hydroxyphenyl-triazine (Tinuvin-477; BASF, Ludwigshafen, Germany) was added as a UV absorber (Orange 3, Sigma-Aldrich, Saint Louis, MO, USA) [40] Table 2 shows the composition of zirconia suspension for each group. The UV absorber volume fraction was divided into four groups, with added 0, 0.005, 0.05, and 0.1 vol%, respectively. A planetary centrifugal mixer (ARV-310; Thinky Corp., Tokyo, Japan) was used for homogenous mixing.

**Table 1.** Materials and their properties.


\* 1,6-Hexanediol diacrylate, \*\* Isobomyl aceylate, \*\*\* Propoxylated neopentyl glycol diacrylatemtms, and \*\*\*\* Methyltrimethoxysilane.


**Table 2.** Composition of zirconia suspension.
