THE FORMATION OF HYDROXYAPATITE-IONOMER CEMENTS AT 38-DEGREES-C

This study describes the formation of a calcium polyacrylate-hydroxyap atite cement. Our hypothesis was that calcium phosphates would rapidly hydrolyze in the presence of polyacrylic acid (PAA) to form a cement. PAA, tetracalcium phosphate (TetCP), and dicalcium phosphate (DCP) we re reacted together and formed calcium polyacrylate (CPA) and hydroxya patite (HAp) within 10 h at 38 degrees C, resulting in hardened masses . Reaction times increased with decreasing (HAp reactants)/PAA ratios. In the first of three reaction stages, the pH increased while CPA and dicalcium phosphate dihydrate (DCPD) formed. Two steady-state pH cond itions occurred during the second stage as TetCP reacted with DCPD and DCP. The first steady-state pH was the result of DCPD and TetCP react ing at near-equilibrium conditions. The second steady-state pH resulte d as the reaction became limited by DCP dissolution. The third, diffus ionally controlled, stage occurred as DCP and previously formed HAp re acted to produce calcium-deficient HAp (Ca/P=1.5). The emphasis of thi s investigation was to establish the mechanistic path involved and the rate-limiting steps of the reaction.