[Thermal behavior of phosphate-bonded investment. Part 1. Thermal behavior of MgNH4PO4.6H2O (author's transl)].

Abstract

Thermal behavior of MgNH4PO4.6H2O was investigated by means of TG-DTA and X-ray diffraction. Two endothermic peaks (90 degrees C and 230 degrees C) and a exothermic peak (660 degrees C) were observed in a micro-DTA curve. Weight loss occurred at two stages corresponding to the two endothermic peaks. Values of weight loss were about 38% and 53%, respectively, at the end of each stage. These values were nearly equal to theoretical weight losses in the formation of Mg2P2O7 from MgNH4PO4.6H2O via MgNH4PO4.H2O. The intermediate, MgNH4PO4.H2O was identified by X-ray diffraction. The second weight loss continued till about 600 degrees C. The product during the second stage was amorphous and Mg2P2O7 began to crystallize above about 600 degrees C. The exothermic peak was thought to correspond to the crystallization. On the other hand, the first endothermic peak separated into two peaks in a macro-DTA curve. The first corresponded to removal of crystal water from MgNH4PO4.6H2O and the second to evaporation of the condensed water. Moreover the exothermic peak also separated into two peaks and the peak temperatures lowered. It was considered that the crystallization was affected by the atmosphere.