THE EFFECT OF STORING CONDITIONS ON PHYSICAL PROPERTIES OF PHOSPHATE-BONDED INVESTMENT MATERIAL AND THE STUDY OF CHANGES IN POWDERY COMPOSITION OWING TO MOISTURE
*M. Carlevaris, **V. Kauèiè, **R. Gabrovšek
*Polident d.d., Volèja Draga 42, SI-5293 Volèja Draga, Slovenia **National Institute of Chemistry, P.O.Box 3430 SI-1001 Ljubljana, Slovenia
Changes in the powdery composition and physical characteristics of phosphate-bonded investment material on exposure to 90% relative humidity as well as storing conditions and suitable packing material were investigated by means of thermogravimetry (TG), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The setting agents (NH4H2PO4 and MgO) reacted due to moisture. They were converted into MgNH4PO4*6H2O. The quantity of NH4H2PO4 decreased as the exposure time was prolonged. NH4H2PO4 was not detected in the mixture of NH4H2PO4 and MgO, which was exposed to moisture for 51 days. The quantity of NH4H2PO4in the powder seems to be related to the initial setting time and setting expansion. The quantity of MgNH4PO4*6H2Oin the powder may contribute to the extreme delay of the setting reaction. Changes in powdery composition of setting agents due to moisture are similar to the reaction of setting the of NH4H2PO4- MgO mixture.