Deterioration of phosphate-bonded investment on exposure to 100% relative humidity atmosphere
Miyaji, T.; Utsumi, K.; Suzuki, E.; Shimizu, Y.
Bulletin of Tokyo Medical and Dental University 29(2): 53-62
1982
ISSN/ISBN: 0040-8921 PMID: 6961969 Document Number: 188174
Changes in the powdery composition and setting characteristics of the phosphate-bonded investment (Ceramigold) on exposure to 100% relative humidity atmosphere were investigated by means of thermogravimetry-differential thermal analysis (TG-DTA) and X-ray diffraction. In the powder, NH4H2PO4 and MgO, as setting agents, were converted into MgNH4PO4.cntdot.6H2O through Mg(NH4)2H2(PO4)2.cntdot.4H2O. As compared with the result of the powder stored in the thermohygrostatic room (control = not exposed), the setting time of the mixture of the exposed powder with the colloidal silica solution decreased temporarily, but increased after 4 days of exposure when the quantity of NH4H2PO4 in the powder became small and could not be measured after 10 days of exposure. The compressive strength and diametral tensile strength of the mixture at 2 h increased slightly but decreased after 4 days or more of exposure and could not be measured after 10 days of exposure. The thermal expansion of the mixture at 800.degree. C produced little difference between the control and after 8 days of exposure. As for the setting agents, the process of deterioration is similar to the setting reaction. The quantity of NH4H2PO4 in the powder seems to be related to the setting time and the strength of the set investment. The quantity of MgNH4PO4.cntdot.6H2O in the powder may contribute to the extreme delay of the setting reaction.