INTRODUCTION

Titanium and its alloy castings have been widely used in dentistry because of their excellent biocompatibility1). However, there are many difficulties associated with titanium casting using conventional dental investments because of titanium’s high melting point and considerable chemical reactivity at high temperatures. Thus, mold materials represent a very important factor for precision titanium casting. Although some investments for titanium castings have been available on the market, they are not always optimal with respect to manipulation, mold expansion, and compressive strength.

Yttria2), calcia3-6), magnesia7-10), alumina11,12), and zirconia13), which are more stable than silica for titanium with the exception of titanium’s melting point, have been investigated. Particularly, magnesia and alumina are popular as principal components for titanium, and various products have already been commercialized. According to the literature, calcia, zirconia, and yttria have been expected to have hidden potential; however, suitable binder materials were not easily found. Normally, dental casting investments are roughly divided into gypsum-bonded and phosphate-bonded investments. Both are greatly compatible with titanium, which has posed a problem.

Incidentally, it is well known that the ability of casting mold is influenced by the water/powder (W/P) ratio, mixing liquid, and particle size of the refractory materials12,14). In particular, the W/P ratio can be artificially controlled. A higher rate will result in low expansion, low strength, and longer operation time.

On the other hand, to suppress the reaction at high temperatures, the casting temperature of the mold tends to be lower. As a result, the fired strength and thermal expansion of the mold seem to be insufficient. Generally, thermal expansion of the mold is caused by a refractory material, but may depend on additives3,15,16).

We previously investigated mold materials for titanium casting8,17,18). In this study, the development of an investment with high performance for titanium was reviewed. In the case of titanium casting, inert refractory and hydraulic binders are helpful for an operator to fabricate high-precision titanium casting. As mentioned, zirconia and alumina have been considered as candidates for stable mold materials for titanium. Alumina is an excellent refractory material, and alumina cement is used as a binder. The expansion is larger when metal zirconium is oxidation on heating, metal zirconium is added to some dental investment products as an expanding agent.

The purpose of this study was to design an investment for titanium casting based on both alumina and zirconia, which are mixed with a special mixing liquid to reduce the W/P ratio for improving eapansion and strength as a new attempt. The properties of their molds were evaluated.

MATERIALS AND METHODS

Experimental investmentsThe experimental investments consisted of zirconia (ZrO2, 60-mesh; Tianjin Growth Zirconia Co., Ltd., China) and alumina (Al2O3, ≤2 µm; KaiFeng GaoDa, China). Aluminous cement AL70 (≤2 µm; KaiFeng GaoDa) as a binder, lithium carbonate (LiCO3; Beijing

Properties of experimental titanium cast investment mixing with water reducing agent solutionZutai ZHANG1, Ning DING1, Yukimichi TAMAKI2, Yasuhiro HOTTA2, Cho HAN-CHEOL2 and Takashi MIYAZAKI2

1 Beijing Institute of Dental Research, Capital Medical University, School of Stomatology, 4 Tian Tan Xi Li, Dongcheng District, Beijing 100050, PR China

2 Department of Oral Biomaterials and Technology, Showa University School of Dentistry,1-5-8 Hatanodai, Shinagawa-ku, Tokyo 142-8555, JapanCorresponding author, Zutai ZHANG; E-mail: showazhang@hotmail.com

This study aimed to develop a dental investment for titanium casting. ZrO2 and Al2O3 were selected as refractory materials to prepare three investments (Codes: A-C) according to the quantity of Zr. Al2O3 cement was used as a binder at a ratio of 15%, they were mixed with special mixing liquid. B1 was used as a control mixed with water. Fundamental examinations were statistically evaluated. A casting test was performed with investment B. Fluidities, setting times, and green strengths showed no remarkable differences; however, they were significantly different from those of B1. Expansion values for A, B, C, and B1 at 850°C were 1.03%±0.08%, 1.96%±0.17%, 4.35%±0.23%, and 1.50%±0.28%, respectively. Castings were covered by only small amounts of mold materials. The hardness test showed no significant differences between castings from B and the ones from commercial investments. The experimental special mixing liquid effectively reduced the water/powder ratio and improved the strength and thermal expansion.

Keywords: Titanium, Investment, Cast, Special mixing liquid

Color figures can be viewed in the online issue, which is avail-able at J-STAGE.Received Jan 30, 2012: Accepted May 4, 2012doi:10.4012/dmj.2012-030 JOI JST.JSTAGE/dmj/2012-030

Dental Materials Journal 2012; 31(5): 724–728

Table 1 Experimental investments in this study

Code Al2O3 cement (%) Al2O3 powder (%) ZrO2 powder (%) Zr powder (%) LiCO3 (%) Liquid L/P

A 15 15 67 3 0.2 Special 0.17

B 15 15 65 5 0.2 Special 0.17

B1 15 15 65 5 0.2 Water 0.24

C 15 15 62 8 0.2 Special 0.17

Chemical Reagent Co., China) as an accelerator agent8,19,20), and Zr powder (200-mesh; Beijing Haoyung Co., Ltd., China) as a thermal expansion agent were added based on a previous report. The experimental special mixing liquid(S) was deionized water mixed with a 2% polycarboxylic type high-performance water reducer (Sika ViscoCrete 3301C; Sika, Switzerland). Three types of experimental investments (Codes: A, B, and C) were produced as shown in Table 1. As a control, B mixed with water was also examined (Code: B1).

Fundamental tests1. FluidityThe fluidity test as instructed by the International Standard Organization (ISO: 9694)21) was performed. Each experimental investment was mixed at its W/P ratio for 30 s by hand and then automatically mixed for a further 30 s under a vacuum using a mixing machine. The mixed investment slurry was poured into a steel ring (28 mm in diameter and 50 mm in height), which was set on a glass slab. The ring was pulled up 2 min after mixing. Both the maximum and minimum diameter of the spread slurry was measured 3 min after mixing, and the average value was calculated. Tests were repeated three times for each investment.

2. Setting timeThe Vicat needle penetration test was performed to measure the setting time of each investment according to ISO: 969421). Three repetitions were performed for each experimental investment.

3. Compressive strengthGreen compressive strengths were measured after 24 h of mixing for each investment (diameter: 10 mm; height: 20 mm) using a universal testing machine (Instron MD-1125; Instron, Japan) with a crosshead speed of 1.0 mm/min. Fired strengths were also tested using specimens that were heated to 850ºC at a rate of 10ºC/min, held at that temperature for 1 h, and then cooled to room temperature in the furnace. Five specimens were prepared for each investment.

4. Thermal expansionThermal expansion curves were drawn using a thermal dilatometer (Thermo Plus TMA 8310; Rigaku Co.,

Japan). The specimen (diameter: 6 mm; height: 12 mm) was heated to 850ºC at a rate of 10ºC/min and then cooled to room temperature. Thermal analysis was performed three times for each investment.

5. Evaluation of the titanium castsA pure titanium cast test was also performed using acrylic plates (10×10×1 mm) with investment B. Twenty-four hours after investing, the molds were heated to 850ºC at a rate of 10ºC/min and then cooled to 200ºC for casting. A commercial pure titanium ingot (KS-50, JIS Grade 2; purity, 99.5%; Selec Co., Ltd., Osaka, Japan) was melted in a copper crucible and cast into the mold using an argon arc melting and gas pressure type with one chamber casting machine (Ti Cascom; Denken Co., Ltd., Kyoto, Japan). Casts were carefully removed from the mold.

6. Vickers hardness testUsing a Vickers microhardness indenter (FM-700; Future-Tech Corp., Tokyo, Japan), Vickers hardness of the titanium casting obtained from investment B was measured. As a control, a commercial magnesia-based investment (Titan Cascom CB; Denken Co., Ltd., Kyoto, Japan) was selected. The titanium casting procedure was the same as that used for evaluation of the titanium casts. The exposed cross-section was mounted in epoxy resin and polished with 600-mesh emery paper. Microhardness values of these specimens were measured at 25-, 75-, 125-, 175-, and 225-µm points from the outer surface investing (n=5).

7. Statistical analysisThe data for the fluidity, setting time, compressive strength, and thermal expansion among experimental investments were statistically evaluated by one-way ANOVA and Scheffe’s multiple comparison test at a significance level of α=0.05.

RESULTS

The fluidities of A, B, and C were within the range of 136.60–141.20 mm. There were no remarkable differences among them; however, they were significantly different from that of B1 (71.20±6.5 mm) (Fig. 1).

The setting times showed no remarkable differences

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among the investments with the exception of the control. The range was 19.8–23.2 min, and there were significant differences between B1 (32±2.55 min) and the others (Fig. 2).

The results for the compressive strengths are shown in Fig. 3. The average green strengths of A, B, B1, and C were 10.89±1.77, 10.12±0.69, 6.49±0.78, and 9.84±1.80 MPa, and fired strengths were 3.92±0.37, 3.43±0.79, 1.73±0.14, and 1.73±0.15 MPa, respectively. There were obvious differences in green strength between B1 and A,

B, and C. After firing, the strength of each investment was significantly decreased (p<0.05).

The thermal expansion rates for A, B, C, and B1 at 850°C were 1.03%±0.08%, 1.96%±0.17%, 4.35%±0.23%, and 1.50%±0.28%, respectively. There were statistical differences among them. Typical thermal expansion curves for these investments are shown in Fig. 4.

Cast titanium specimens indicated that castings were covered by only small amounts of mold materials, and no cracks or defects were observed (Fig. 5).

Fig. 1 Fluidity of experimental investments. Vertical bars indicate standard deviation.

Fig. 2 Setting time of experimental investments. Vertical bars indicate standard deviation.

Fig. 3 Compressive strength of experimental investments. Vertical bars indicate standard deviation.

Fig. 4 Thermal expansion curves of experimental investments.

Fig. 5 Castings of titanium from B: castings were covered by only small amounts of mold materials.

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Microhardness values indicated no significant differences between castings from B and the control (Fig. 6).

DISCUSSION

It is well known that phosphate-bonded investments are popular in dentistry for high-temperature casting at 1,000°C or more, however, the surface of titanium castings exhibits a complex layer that is formed by reacting between titanium and phosphate-bonded or silica-based investments. Therefore, some non-phosphate-bonded and non-silica investments for titanium have been successfully examined in dentistry3-13,15,16,22). Some of them have been commercialized, and some have remained at an experimental level. In this experiment, zirconia and alumina were coupled, which provides stable ceramics with alumina cement to enable high-precision, high-quality titanium casting.

Sufficient thermal expansion of the investment is critical to the accuracy of dental castings. Nishimura et al.23) estimated that the casting shrinkage of the titanium crown was approximately 1.8–2.0%. The investment designed in this study had difficulty in compensating the amount of the aforementioned shrinkage. Therefore, improvement of the dimensional accuracy depended on utilizing the oxidizing expansion of zirconium metal powder mixed in the investment.

Although the expansion was adjustable with the additive, increasing the mold strength after firing was not simple. Mold strength is mandatory to resist at least the casting pressure. In this study, the experimental special mixing liquid contained a polycarboxylic type high-performance water reducing agent so that it could reduce the W/P ratio in A, B, and C compared with B1. Meanwhile, it improved the fluidity and compressive strengths (green and fired) and shortened the setting time24). The lower W/P ratio of A, B, and C compared with that of B1 resulted in closer contact of experimental investment particles; so their thermal expansion values

became larger, especially those of B compared with B1. Furthermore, with the increase in the contents of the thermal expansion agent Zr powder, the thermal expansion was increased. The larger thermal expansion reduced the fired compressive strengths; the fired strengths decreased with increases in the thermal expansion agent. On the other hand, the water reducing agent mainly contained organic subjects, and it could not affect the titanium castings because the water reducing agent would have been gone after the casting mold was fired.

Within this limited study, the fluidities, setting times, and strengths of the experimental investments A, B, and C were acceptable in the laboratory for dental use. Investment C had the maximum thermal expansion rate of 4.35%±0.23% at 850°C; even at 250°C, its expansion rate reached 3.92%±0.26%. The mean value can probably compensate for the titanium crown of frame casting shrinkage through adjustment of the contents of the thermal expansion agent Zr powder.

However, further studies are needed to elucidate the effects of the experimental special mixing liquid on the titanium casting surface and the mechanisms of action of the experimental special mixing liquid.

CONCLUSIONS

The experimental investment powders were stable for titanium cast, and the special mixing liquid effectively reduced the W/P ratio to improve the fluidity, compressive strength, and thermal expansion of the experimental investments. It is possible to use these experimental investments for clinical titanium casting.

ACKNOWLEDGMENT

This study was supported by the National Natural Science Foundation of China (Grant No. 30872909).

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