DENTAL INVESTMENT BASED ON NEW EXPANSION MECHANISM

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Dental investments, gypsum-bonded and phosphate-bonded investments were developed using the expansion originating form thermal decomposition and crystal growth of decomposed phases of a new aluminum phosphate compound, aluminum phosphite Al_2(PHO_3)_3. The thermal decomposition of aluminum phosphite that was added as a refractory was investigated thermal analytically together with x-ray diffraction and gas-chromatography. The aluminum phosphite, which was amorphous at room temperature, expanded as the temperature increased, while transforming to crystal growth of aluminum orthophosphate and aluminum methaphosphate, which took place when heated further, contributed to a large expansion of the investments. Since the amount of thermal expansion could be regulated without any difficulty, it would be possible to develop dental investments that employ expansion alone.Gypsum-bonded investment was prepared by addition of aluminum phosphite as a refractory and some physical properties were investigated. Also adaptability of castings made of Ag-Pd-Au alloys was examined with MOD metal mold. It was possible to regulate amounts of heating expansion without changing handling properties such as water-to-powder ratio, flow and setting time.Nevertheless, the castings obtained with this trial investments showed good adaptability and their outer surfaces were smooth. Also the compressive strength was shown to be high enough for use as dental investments and the setting expansion could be lowered to less than 0.2% by controlling the amount of Al_2(PHO_3)_3 added. With a dental investment, in which SiO_2 was added as an additional refractory, the thermal expansion increased almost linearly during the heating when the amounts of Al_2(PHO_3)_3 and SiO_2 were controlled.