JOURNAL OF MATERIALS SCIENCE LETTERS 14 (1995) 1380-1382

Effect of magnesia content on expansion, strength and roughness in dental ethyl silicate-bonded investment Y. NOMURA, K. WAKASA, M. YAMAKI

Hiroshima University School of Dentistry, Department of Dental Materials, Kasumi I chome, Minamiku, Hiroshima City, 734 Japan

Dental castable glass ceramics have been cast into investment moulds, including refractory materials of alpha cristobalite and quartz and also a binder [1-5]. The clinically used glass ceramics were calciumphosphate based apatite, mica-based ceramic and apatite/diopside/beta-tricalcium phosphate based ceramic [1-3]. To cast it into an investment mould, the refractory material requires higher thermal resistance because of mould temperature (heated to more than 800 °C) and melting temperature (more than 1100 °C) of glass ceramics [1-3]. An ethyl silicate-bonded investment was thus developed to compensate for the greater shrinkage value of glass ceramics [6-8]. Castings having higher melting temperatures exhibited greater shrinkage values, and it was possible to control the expansion value of dental investments by changing the fraction of alpha cristobalite and/or quartz, or by mixing magnesia and alumina powder with the refractory material [9]. After casting or firing at high mould temperatures, the strength and surface roughness of the mould surface might be affected by the refractory materials of the investment. It is thus important to examine how the magnitudes of the constituents of investment moulds are control led. In magnesia/alumina based investment, use of a magnesia fraction as refractory material strengthened the investment matrix because of the higher strength of magnesia powder compared with silica powders [9, 10]. When setting or heating, the strength of the investment mould was required to be the same as the mould. Expansion of investment is needed because melted glass ceramics shrink during cooling, and the compressive strength expresses the resistance against the casting of glass ceramics. Surface roughness is the roughness of the glass surface after casting and removing the investment powders. In this study, only magnesia powder was added to investment powders in order to control the setting and thermal expansion

T A B L E I Materials tested in this study Glass

Investment

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Apatite-based castable glass-ceramic Powder a-quartz a-cristobalite Liquid A Si(OC2Hs)4, He1 B NH 3, It20

Nippon Electric Glass Co.

properties and compressive strength and surface roughness of the moulds. The investment powders used in this study, which were composed of alpha cristobalite and quartz, were developed by us [3-7]. Alpha cristobalite and quartz were, respectively, of 15 and 5 gm median size (SALD-2000, Shimadzu Co, Kyoto), which was measured within distilled water by a laser diffraction method. The contents were, respectively, 55 and 45 wt % as a refractory material. To this investment powder, magnesia powder was added in the range of 10 to 70 wt% in 10 wt% steps (Magnesia refractory cement, M-l; Nippon Kagaku Tougyou Co, Tokyo). The manufacturers specification indicated MgO 96%, and less than 0.85% of CaO, SiO2, A1203, Fe203 and 400 ppm of boron. Moulds were mixed in solutions of ethyl silicate and ammonium aqueous solution. The mixture (pH = 8,8) was composed of 16 ml of pH-adjusted silica sol (pH=3.5) and 1 ml of ammonium aqueous solution (pH = 9.4) [6, 7, 11]. Mechanical properties, such as setting and thermal expansion, compressive strength and surface roughness value were as follows. To measure setting and thermal expansion values of the dental ethyl silicatebonded investment, the setting behaviour wag defined as the linear dimensional change that occurred in an investment setting within a stainless steel tube (25 mm in diameter and 5 mm long) lined with kaolin material (Dentsply Co, York, ME, USA). Measurements were made with a travelling microscope (Seiki-shya, Tokyo). The thermal expansion behaviour of samples (5 mm in diameter and 12 mm long) was measured using a thermal analyser(Rigaku Thermoflex, Rigaku Co, Tokyo), under the following conditions: heating rate, 10°C/min and heating ranges 20 to 800°C. Compressive strength was measured on test samples 5 mm in diameter and 10 mm long, using DCS-500 (Shimadzu Co, Kyoto) under the following conditions of test sample: crosshead speed 0.5 mm/min; chart speed 50 mm/ rain. The strength value was calculated as the load at fracture of thetest sample divided by the area of sample (diameter = 5 mm). Surface roughness values were measured as a maximum surface value, obtained by the difference between maximum and minimum roughness values on the surface roughness curve, using a surface roughness tester under the following conditions: transverse measuring length 4 mm; transverse speed 2 mm/s (SE-3; Kosaka Lab, Tokyo). All tests were carried out using ten samples, 0261-8028 @ 1995 Chapman & Hall

and statistical analysis was done using student's ttest. Fig. 1 shows the change of setting expansion with magnesia content, 10 to 70wt% in ethyl silicatebonded investment. The value (shrinkage) increased linearly with increasing magnesia content, ranging from -0.85 to -0.6%. Fig. 2 shows the change of thermal expansion value with increasing magnesia content, indicating that the value decreased linearly, with a range of 2.0 to 1.1%. The total value of setting and thermal expansion was from 1.15 to 0.5% in magnesia-added ethyl silicate-bonded investments. Fig. 3 shows the change of compressive strength of fired samples (900 °C for 30 rain) with a magnesia content of 10 to 70%; strength increased to more than 2 MPa. Fig. 4 shows surface roughness change at each level of magnesia content (10 to 70%) when tested after 1 h of mixing or 900°C; roughness decreased from 10 to 0.5 gm. In order to develop magnesia-added investments to cast glass ceramics, expansion behaviour, compressive strength and surface roughness were examined. Dental investment employed with gold casting alloys is composed of a hemihydrate of gypsum and a form

of silica, and also alpha hemihydrate is used because of greater strength [12]. The gypsum serves as a binder to the refractory materials, such as alpha cristobalite and, or alpha quartz. Setting occurs with the formation of dihydrate after mixing with water. In the investment used in this study, silica gel as the SiOa in hydrolysed silica sol solution was the binder between the refractory materials. The gelation process, which was described in [6, 7], was available for magnesia-added ethyl silicate-bonded investments. Because magnesia had no transition to another crystalline with great expansion within mould temperature, the addition of magnesia decreased the thermal expansion value (Fig. 2), as described for titanium castings [9]. Thus, thermal expansion depended on a change of crystalline form of alpha cristobalite or quartz (which occurred at transition temperatures between 200 to 270 °C and 575 °C) to beta cristobalite or quartz. The compressive strength was not less than 2.45 MPa when tested 2 h after setting, or firing at the constant mould test temperatures. Thus, in order to use ethyl silicate-bonded investments as the mould, the compressive strength of the investment mould is a factor to be considered,

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Figure 1 Setting expansion change with magnesia content 10 to 70 wt% added in 10 wt% steps to investment powders.

Figure 3 Compressive strength with increasing magnesia content. The investment mould was kept at 900 °C for 30 min.

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Figure 2 Change of thermal expansion value with magnesia content 10 to 70 wt%.

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Figure 4 Surface roughness values at each condition: 900 °C X 30 min and after mixing (1 h).

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in addition to the expansion behaviour. The fineness of the investment surface roughness value also affects the surface roughness of the casting. It is supposed that silica gel as the binder might be useful between silica refractory materials and/or magnesia powders. The smaller the surface irregularity on the castings, the more important is the role of silica gel in the silica sol solution used to mix the investment powders. Summarizing the results of this study, expansion values were controlled in the range 1.15 to 0.5% by the addition of magnesia. The magnesia contents tested ranged from I0 to 70wt% of the ethyl silicate-bonded matrix (Figs 1 and 2). Compressive strength values were more than 2.45 MPa (Fig. 3), showing that the greater strength of the magnesia effectively increased the strength of the investment mould. The decreased surface roughness approaches the smoothness of glass ceramic surfaces. It is recommended that magnesia-added investments are effectively applied to castable glass ceramics, provided that the controlled expansion value selected for each glass ceramic can compensate for the respective shrinkage values of glass ceramics used in the dental field.

Biomaterial Combined Analysis System, which was supported by the latest Grant-in-Aid (1993) from the Ministry of Education, Science and Culture, Japan, in Hiroshima University Graduate School.

References 1. P. J. ADAIR and D. G. GROSSMAN, Int. J. Perid. Restor. Dent. 2 (1984) 33. 2. S. KIHARA and A. WATANABE, J. Amer. Ceram. Soc. 67 (1984) C-100. 3. Y. NOMURA, M. TAIRA, K. WAKASA and M. YAMAKI, Hiroshima Daigaku Shigaku Zasshi 20 (1990) 363. 4. K. WAKASA and M. YAMAKI, J. Mater. Sci. Lett. 12 (1993) 1897. 5. Idem., Hiroshima Daigaku Shigaku Zasshi 25 (1993) 494. 6. K. WAKASA, A. IKEDA, Y. YOSHIDA and M. YAMAKI, J. Mater. Sci. Lett. 12 (1993) 1908. 7. ldem., ibid. 13 (1994) 258. 8. K. WAKASA and M. YAMAKI, Hiroshima Daigaku Shigaku Zasshi 26 (1994) 192. 9. Idem., J. Mater. Sci. Lett. 13 (1994) 416. 10. K. WATANABE, S. OKAWA, O. MIYAKAWA, S. NAKANO, N. SHIOKAWA and M. KOBAYASHI, Shika Zairyou Kikai 9 (1990) 623. 11. K. WAKASA and M. YAMAK1, J. Mater. Sci. Lett. 13 (1994) 1177. 12. R . W . PHILLIPS, "Sldnner's science of dental materials", 7th edn, (W. B. Sanders, Philadelphia, 1973) pp. 411-426.

Acknowledgements The authors would like to express the appreciation to the Central Research Laboratory for the use of the

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Received 18 November 1994 and accepted 1 May 1995