Hydrothermal-like synthesis of hydroxyapatite from calcium sulfates

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2010
Bingöl, Onur Rauf
Synthesis of hydroxyapatite (Ca10(PO4)6(OH)2, HAp) from commercial grade plaster of paris (CaSO4•0.5H2O, PoP) and gypsum (CaSO4•2H2O) has been performed. HAp synthesis was achieved by reacting 1 M of (NH4)2HPO4 (or 0.5 M of (NH4)2HPO4) solutions with solid calcium sulfate precursors under ambient pressure (1 atm) and hydrothermal-like (2 ± 0.2 atm, 120 °C) conditions. Under ambient conditions, HAp formation kinetics was investigated at 25 °C, 50 °C and 90 °C using 1 M of (NH4)2HPO4 solution. Conversion to HAp at such low temperature takes more than 21 days and it also promotes formation of additional calcium phosphate with HAp. At 25 °C, HAp formation started after 7 days accompanied with formation of brushite (CaHPO4•2H2O). At 50 °C no significant conversion was observed after 6 h. However, at 90 °C, phase pure HAp was formed after 2 h. On the other hand, under hydrothermal-like conditions, the HAp formation proceed much faster and it was also shown that HAp could be also synthesized from gypsum powders and bulk gypsum pellets. Using 1 M of (NH4)2HPO4 solution, HAp formation from PoP started 15 min and completed almost in 30 min, whereas 0.5 M of (NH4)2HPO4 reactant solution slowed down the conversion. The exact chemical identity of the HAp product of hydrothermal-like reaction was evaluated by post-synthesis calcinations and the thermal phase stability was related with the stoichiometry (Ca/P at ratio) of the HAp. The HAp phase was stable up to 600 °C and above 600 °C, β-tricalcium phosphate (β-Ca3(PO4)2, β-TCP) was formed, suggesting that the resultant HAp was calcium-deficient. Mechanical testing by diametrical compression was performed to the HAp samples produced from bulk gypsum pellets. The strength was measured 1.2 MPa with highest solid to liquid (s:l) ratio 3.33 and decreased with s:l ratio. This change was found to be related with the porosity differences due to differences in s:l ratio. Additional mechanical tests were applied to the polycaprolactone (PCL) coated bulk HAp pellets for which the tensile strength was doubled. This study presents an easy and feasible method for production of HAp from a cheap and abundant calcium source – PoP. In addition, the findings provide a potential processing route for developing irregularly shaped bulk porous HAp structures.

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Citation Formats
O. R. Bingöl, “Hydrothermal-like synthesis of hydroxyapatite from calcium sulfates,” M.S. - Master of Science, Middle East Technical University, 2010.