EFFECTS OF TEMPERATURE ON THE FORMATION OF HYDROXYAPATITE

The formation of hydroxyapatite by an acid-base reaction between solid calcium phosphates at temperatures from 5 to 60-degrees-C was examine d. The basic reactant used is Ca4(PO4)2O, while the acidic reactants i nclude CaHPO4, CaHPO4 . H2O, and Ca(H2PO4)2 . H2O. Rates of heat evolu tion during reaction were determined by isothermal calorimetry. The va riations in the proportions of reactants and hydroxyapatite and the fo rmation of intermediate products were assessed by x-ray diffraction. D evelopment of microstructure was observed. Generally, hydroxyapatite f ormation occurs by rapid initial reaction followed by a period during which reaction occurs slowly. Apparent activation energies calculated for the reaction when CaHPO4 is the acidic reactant show differing val ues depending on its surface area. When the acidic reactant is Ca(H2PO 4)2 . H2O, intermediate products are formed. At low temperatures the i ntermediate is CaHPO4 . 2H2O, while at higher temperatures it is CaHPO 4. Above 38-degrees-C, the rate during the period of slow reaction dec reases with increasing temperature. This appears to be related to the retrograde solubilities of the reactants, CaHPO4 . 2H2O and CaHPO4, an d of HAp.