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

Abstract

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.