PRODUCTION AND CHARACTERIZATION OF COLORING LIQUIDS FOR DENTAL ZIRCONIA FRAMEWORKS

1. Kütahya Dumlupınar University, Evliya Çelebi Campus, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Kütahya 43100, Turkey. 2. Eskişehir Technical University, İki Eylül Campus, Faculty of Engineering, Department of Materials Science and Engineering, Eskişehir 26555, Turkey. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 01 April 2020 Final Accepted: 05 May 2020 Published: June 2020

The coloring of the dental zirconia prostheses is performed by immersing the pre-sintered zirconia framework in the coloring liquid and waiting for a while. There are various problems that dental prosthesis laboratories experience regarding coloring processes. The most important of these is that the desired color can not be obtained in the dental zirconia framework. This may be due to the different density and porosity values of the zirconia frameworks prepared from different commercial zirconia blocks, the thickness of the framework or the unstable coloring liquids. In the present study, it was aimed to produce coloring liquids suitable for Zirmax brand dental zirconia blocks and characterization of zirconia samples that were colored using these liquids. For this purpose; coloring liquids were produced by dissolving different metal salts, pure water and two different organic solvents. Zirconia samples are colored by immersing the samples into prepared coloring liquids. The colored zirconia samples were sintered at 1500 °C for 2 hours and then the colors of the samples were visually determined by a specialist dental technician. Crystallographic analyzes of zirconia frameworks were performed with X-ray diffraction (XRD) technique. Microstructures of zirconia samples were examined by scanning electron microscope (SEM). According to the color results of the zirconia samples that were analyzed visually, the coloring liquids were successfully produced in accordance with the Ivoclar Vivadent IPS e.max Ceram Dentin A-D color scale.
64 of the teeth [4]. The most common coloring method is the immersion of the ZrO 2 prosthesis, which is milled from a white and pre-sintered zirconia block, into the coloring liquid or application of the coloring liquid on the zirconia framework using a brush. This coloring process is an advantage in terms of time and aesthetics [5-6-7].
In the study of Tabatabaian et al., the effects of the thickness values of monolithic zirconia ceramics on the final product color had been investigated, and therefore, the pellets produced by transparent monolithic zirconia powders at different thickness values (0.7, 0.9 and 1.1 mm) obtained from two different manufacturers (DDcubex, CopraSmile), were colored in a coloring liquid corresponding to the A4 scale and CIE L*a*b* values were measured. Based on their study findings, they stated that the thickness of zirconia was effective on the color of the final product. Researchers have determined that zirconia ceramic thickness should be 0.9 mm in order to obtain a color that is acceptable in terms of application (8). Kaplan et al. produced samples of dental zirconia powders, which were made semi-stabilized with mol 3% yttria, in disc form, and they colored these samples in coloring liquids that were prepared using NiCl 2 , MoCl 3 and NiCl 2 + MoCl 3 metal salts in different concentrations (Weight. 0.1%, 0.25% and 0.5%) for different waiting times (5, 30, 60 seconds), and they examined the effects of the applied process on the mechanical properties of the discs, such as micro hardness, fracture toughness, flexural strength and rate of attrition, and their final colors. Their results revealed that the concentration of coloring liquids had an effect on the final color, while the waiting time in these liquids did not have a significant effect on the final color formation [9].In the study of Ozturk et al., it was observed that the products with different sample thicknesses exposed to coloring liquid had different colors obtained after repeated firing process. They used DC-Zircon and IPS e.max Press branded zirconia framework in their study. With the increase in the thickness of the ceramic, there was a significant decrease in the L* value of the DC-Zircon branded framework and an increase in the a* value, while there was no significant change in the b* value. While IPS e.max Press branded framework showed significant decreases in L* value, they found an increase in a* and b* values [10]. In the study of Kao et al., they observed that coloring liquids containing different amounts of iron oxide formed a color spectrum on the zirconia frameworks, ranging from yellow to red [11]. In the study of Wen et al. using CeO 2 and Er 2 O 3 colorants, they found that these colorant species went from yellow to green and yellow to red on the zirconia framework [12]. In their study, Lee et al. demonstrated that the brand of the zirconia block used was effective on the color of the final product [13]. In the study conducted by Son et al, it was noted that the color differences in the final product depended on the thickness of the framework, as well as the block brand [14]. Due to the different porosity and density values of different brand zirconia frameworks, even if coloring is performed with the same coloring liquid, obtaining different results creates a complicated situation in terms of coloring zirconia frameworks, and this situation is a factor that limits the use of coloring liquids in practice. In the present study, the coloring liquids suitable for Zirmax brand dental zirconia blocks were produced and it was aimed to characterize of zirconia samples colored using these liquids.

Materials and Method:-Preparation of Zirconia Pellets
Ready-to-press 3Y-TZP zirconium oxide powder and a 13 mm diameter press mold were used for preparing zirconia pellets. In order to see the effects of zirconia samples with different thickness on the final color, 0.15, 0.4 and 1.5 g zirconia powder were pressed at 500 PSI, respectively. In addition, 0.4 g zirconia powder was shaped by applying 250 PSI pressure to determine the effect of the forming pressure difference on the final color. After the forming process, the final pressing of all samples was done with cold isostatic press (CIP) at 2000 bar. According to the firing program presented in Table 1, zirconia pellets with CIP treatment were subjected to binder removal and presintering processes and they were made ready for coloring.

Preparation of Single-Unit Zirconia Frameworks in Tooth Form
Zirmax brand dental zirconia blocks (produced by Seramdent Dental Ceramics and Nanomaterials Ltd., Eskisehir / Turkey) were used for the preparation of single-unit zirconia frameworks in tooth form where coloring liquids were applied. Blocks were shaped using a milling device with CAD / CAM system.

Preparation of Coloring Liquids
In the preparation of coloring liquids, 5 different coloring salts (CS1-CS5) and distilled water, organic solvent-1 (OS1) and organic solvent-2 (OS2) were used (Table 2). Initially, the color after using only coloring salts in increasing amounts was determined. Then, three different liquids were prepared by dissolving 0.01 g/ml CS1 in three different solvents (distilled water, distilled water + OS1 and distilled water + OS2) in order to examine the effects of solvents used on liquid properties. Finally, 37 different coloring liquids were prepared for A series, 14 for B series, 118 for C series and 35 for D series using coloring salts, distilled water and OS2 according to Table 3. A.D.R. Group Table 3:-Salts and solvents used for the preparation of coloring liquids.

Receipt Code
Raw materials According to the recipes, organic solvents were added to the 20 ml beaker with a total liquid amount of 10 ml and mixed by the aid of a magnetic stirrer at 500 rpm. Colorized salts weighed separately were added into the beaker in the proportions specified in the recipe and mixed until a clear liquid was obtained. The coloring liquids obtained afterwards were stored in a plastic tube.

Coloring of Zirconia Samples
Coloring process was performed by immersing the samples in the coloring liquid and waiting for the desired time. In order to determine the effect of the holding time in the coloring liquid on the final color, the products in the form of pellets, which were subjected to pre-sintering, were colored in two different coloring liquids (A1 and A3) for 15 66 seconds and 60 seconds. The coloring of zirconia samples prepared in tooth form was done by keeping them for 60 seconds in the liquid. All the colored products were subjected to the final sintering process according to the program presented in Table 4. Color analysis of single crown zirconia samples produced by using Zirmax brand zirconia block were performed by Dt. Saim Güler in accordance with IPS e.max Ceram Dentin A-D color scale. Color analyzes were carried out between 12:00 and 14:00 hours, in sunny weather, on a gray ground and using a lighting device that provides 5500K daylight support.

Results:-
When three different liquids prepared to examine the effects of solvents used on liquid properties were examined after 15 days, precipitation was observed in distilled water and distilled water + OS1 containing liquids, while no precipitation was observed in the distilled water + OS2 containing coloring liquid. It was determined that the coloring liquids with precipitation were blurred, and the liquid containing OS2 preserved the clarity as in the day it was produced ( Figure 1). The shelf life of the coloring liquids used in dental prosthesis laboratories and their stability after preparation are extremely important in terms of color. Therefore, it was determined that it would be better to use OS2 in the preparation of coloring liquids. The colors of the zirconia pellets became darker, as expected, due to the use of 67 coloring salts in increasing amounts alone (Figure 2 -Figure 6). According to these results, it was possible to determine the amount of coloring salts that should be used to reach the IPS e.max Ceram Dentin A-D scale.

Amount of Coloring Salt
(g/ml) 0.001 0.01 0.05 0.1    When the cross section surface of zirconia pellets colored at 15 and 60 seconds and sintered is examined, the color absorption has been observed to increase due to the raised coloration time (Figure 7). Therefore, it was considered to color zirconia dental prosthesis materials for 60 seconds for a homogeneous coloring.

Colored and sintered pellets
. The densities of the pellets produced at different weights and pressure values and those subjected to pre-sintering are presented in Table 5. According to the density results, an increase in density value was observed with increasing press pressure, while the density did not change with increasing pellet thickness. This is due to the better packaging of zirconia powders due to the increase in press pressure.
Although they were colored at the same time (60 seconds), it was found that the colors of zirconia pellets with different weight and thickness were changed. The color obtained as a result of increasing the thickness of the sample had been darkened ( Figure 8 and Figure 9). Although all the samples had the same density (3.19 g / cm 3 ), the relevant color darkening was due to the increase in the distance traveled by the light in the zirconia sample.
It was determined that the color of samples was lightened with increasing pressure value used in shaping zirconia samples ( Figure 10 and Figure 11). This is due to the increase in the density of the sample with increasing press pressure, in other words, due to lower absorption of coloring liquid as a result of the decrease in the amount of porosity in the sample.
Color series B1 B2 A2 A3 Colored and sintered pellets Color series B1 B2 A2 A3 Colored and sintered pellets According to the findings of the visual analysis applied to the zirconia samples that were prepared in the form of teeth, it was determined that all the color series A, B, C and D in the IPS e.max Ceram Dentin AD scale could be obtained successfully with the coloring liquids prepared according to Table 3 ( Figure 12-Figure 15).   The results of XRD analysis applied to zirconia samples in colored tooth form are presented in Figure 16- Figure 19.
According to the results of XRD analysis, it was determined that the crystal phases in all samples were tetragonal zirconia and coloring liquids had no effect on the crystal structure of zirconia.    According to the results of SEM analysis, it is observed that uncolored and colored products are totally sintered and concentrated at 1500 °C. As a result, it was determined that the coloring liquids did not have a negative effect on the zirconia microstructure.