The mechanical properties of monoliths of calcium-deficient and carbonated hydroxyapatite formed by dissolution-precipitation reactions at 38 °C have been determined. Particulate solid reactants were mixed at liquid-to-solid weight ratios of 0.11 and 0.2 and pressed into various configurations on which mechanical tests were carried out. Testing was performed on wet specimens which had been allowed to react for 8 h by which time phase-pure hydroxyapatite had formed. Calcium-deficient hydroxyapatite produced at a liquid-to-solids ratio of 0.11 exhibited a tensile strength as high as 18 MPa, an average compressive strength of 174 MPa and a Young's modulus of 6 GPa. These values were lower when a larger proportion of water (liquid-to-solid 0.2) was used in sample preparation. However, the compressive strengths of calcium-deficient hydroxyapatite prepared at 38 °C are comparable to the compressive strengths of sintered hydroxyapatite containing an equivalent total porosity. Carbonated hydroxyapatite showed mechanical properties inferior to those exhibited by calcium-deficient material. These differences appear to be related to the microstructural variations between these compositions.

The kinetics of hydroxyapatite (HAp) formation by direct hydrolysis of alpha-tricalcium phosphate (alpha-TCP) [alpha-Ca(3)(PO(4))(2)] have been investigated. Transformation kinetics were examined for reactions at 37 degrees C, 45 degrees C and 56 degrees C by isothermal calorimetric analysis. Setting times and morphologies of the resultant HAp were found to be strongly dependent on reaction temperature. XRD analysis accompanied by FTIR confirmed that phase pure calcium-deficient hydroxyapatite (CDHAp) [Ca(10-x)(HPO(4))(x)(PO(4))(6-x)(OH)(2-x)] was formed. Complete reaction occurs within 18, 11, 6.5 h at 37, 45 and 56 degrees C, respectively. The extent of HAp formation differs for particulate slurries and pre-shaped forms of reactant alpha-TCP. Formation of hydroxyapatite in pre-formed pellets was hindered due to limited water penetration, but enhanced with the presence of NaCl as a pore generator. Regardless of the precursor characteristics and temperature, HAp formation is characterized by an initial period of wetting of the alpha-TCP precursor, an induction period and a growth period during which the bulk transformation to HAp occurs. The microstructures of the resultant HAp at all temperatures were generally similar and are characterized by the formation porous flake-like morphology. Microstructural coarsening was observed for the CDHAp formed above the physiological temperature. The hardening generated by the hydrolysis reaction was demonstrated using diametrical compression tests. The original tensile strength of 56% dense alpha-TCP increased from 0.70+/-0.1 MPa to 9.36+/-0.4 MPa after hydrolysis to CDHAp at 37 degrees C, corresponding to a density of 70%.

Ultrasonic energy was used to accelerate the formation of hydroxyapatite (HAp). The experiments were carried out in aqueous systems on two different sets of reactants: (1) a mixture of Ca4(PO4)2O(TetCP) and CaHPO4 · 2H2O (brushite) and (2) α–Ca3(PO4)2 (α–TCP). The reaction systems were exposed to ultrasound of 20 kHz for various times ranging from 5 to 80 min. The products were characterized by XRD and SEM. Parallel experiments without ultrasound were carried out for calibration. The results show that the ultrasound substantially accelerates both reactions. With ultrasound, the time required for the TetCP-brushite system to complete the reaction forming HAp was reduced from 9 h to 25 min at 25 °C, and from 3 h to 15 min at 38 °C. At 87 °C, α–TCP does not hydrolyze within 1 h in de-ionized water unless the pH is adjusted. Hydrolysis of α–TCP was induced by sonication in less than 20 min, and longer treatment results in the formation of a homogeneous sol of HAp.

Ca 4 (PO 4 ) 2 O (TetCP) reacts with an acidic polyelectrolyte in the absence of a solvent to form a composite composed of Ca 10 (PO 4 ) 6 (OH) 2 (hydroxyapatite, or HAp) and the Ca salt of the polyelectrolyte. Mixtures of an acrylic copolymer and TetCP powders were hot-pressed, and the effects of temperature, pressure, and time on HAp formation were studied. Reaction starts when the copolymer is heated to above T g . Initial carboxyl site neutralization liberates water, continued TetCP hydrolysis, liberates Ca 2+ ions, which react with the copolymer forming its Ca salt. When 90% conversion to HAp was achieved, the composite had an average tensile strength of 51 MPa, a Vickers hardness of 145 kg/mm 2 and a T g ∼ 250°C.

This study describes the formation of a calcium polyacrylate-hydroxyapatite 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) were reacted together and formed calcium polyacrylate (CPA) and hydroxyapatite(HAp) within 10 h at 38 degrees C, resulting in hardened masses. Reaction times increased with decreasing (HApreactants)/PAA ratios. In the first of three reaction stages, the pH increased while CPA and dicalcium phosphate dihydrate (DCPD) formed. Two steady-state pH conditions occurred during the second stage as TetCP reacted with DCPD and DCP. The first steady-state pH was the result of DCPD and TetCP reacting at near-equilibrium conditions. The second steady-state pH resulted as the reaction became limited by DCP dissolution. The third, diffusionally controlled, stage occurred as DCP and previously formed HA preacted to produce calcium-deficient HAp (Ca/P = 1.5). The emphasis of this investigation was to establish the mechanistic path involved and the rate-limiting steps of the reaction.