THE EFFECT OF HUMIDITY ON THE PHYSICAL AND CHEMICAL-STABILITY OF SPRAY-DRIED ALUMINUM HYDROXYCARBONATE

The moisture sorption isotherm of amorphous spray-dried aluminum hydro xycarbonate (SDAHC) at 25 degrees C revealed that moisture sorption in creased sharply when the relative humidity exceeded 80%. The physical and chemical stability of SDAHC was studied for 1 year at 25 degrees C and 0, 11, 54, 84, or 100% RH. The results were interpreted to reveal that two mechanisms affect the properties of SDAHC at 25 degrees C. T he sample stored at 0% RH did not exhibit any significant changes duri ng the 1-year study period. The surface area and rate of acid neutrali zation decreased when samples were stored at 11 or 54% RH, but the mat erial remained amorphous. This change is believed to be due to aggrega tion and cementation of particles. Much larger changes in surface area and the rate of acid neutralization occurred in the samples aged at 8 4 or 100% RH. Under these humidity conditions, polymorphic transformat ions occurred in addition to aggregation and cementation. X-ray diffra ction indicated that microcrystalline boehmite was an intermediate pha se. The stable polymorph was bayerite, which is believed to form due t o the high surface pH produced by water sorption at 84 or 100% RH. The moisture sorption isotherm was useful in identifying the humidity con ditions which resulted in high moisture sorption. Polymorphic transfor mations were only detected when the SDAHC was aged under these humidit y conditions. (C) 1996 Academic Press, Inc.