Influence of the Processes Parameters on the Properties of The ...

Chapter 1.
Polylactide Based Bio-Materials
The cell volume changes with composition in agreement with <strong>the</strong> equation:
V=3507 + 4.9xÅ 3 = 13 Å 3
So, in our case <strong>the</strong> volume <strong>of</strong> <strong>the</strong> unit cell is: V = 3556 Å 3
3.2.2.4 Physicochemical Properties <strong>of</strong> TCP Phases
The solubility in water <strong>of</strong> α and βTCP has been fully investigated and reported in <strong>the</strong> literature
[Fowler and Kuroda, 1986; Gregory et al., 1974]. As a general trend, <strong>the</strong> solubility <strong>of</strong> <strong>the</strong>se phases was
found to decrease in <strong>the</strong> order α TCP > βTCP > Ca-deficient apatites > HAp [Ducheyne et al., 1993]. It may
exhibit metastable solubility equilibrium, as recorded by Baig et al. [1999] in <strong>the</strong> case <strong>of</strong> bone mineral and
non-stoichiometric apatites. Table 1.5 ga<strong>the</strong>rs <strong>the</strong> solubility products <strong>of</strong> <strong>the</strong> different components.
Table 1.5: Solubility products <strong>of</strong> TCP phases in water at 25°C.
Phase Chemical Formula PK sp (25 °C) References
αTCP α-Ca 3 (PO 4 ) 2 25.5 [Fowler and Kuroda, 1986]
β-Ca 3 (PO 4 ) 2
[Gregory et al., 1974]
28.9−85.1
Ca 10−x (PO4) 6−x (HPO4) x (OH) 2−x
[Ratner, 2004]
ATCP Ca 3 (PO 4 ) 2·n(H 2 O) 24.2 [Somrani et al., 2005]
HAp Ca 10 (PO 4 ) 6 (OH) 2 117.2 [Ratner, 2004]
βTCP
Ca-deficient apatite
3.2.2.5 Thermal Treatment in Air <strong>of</strong> <strong>the</strong> TCP Phases
No variation <strong>of</strong> <strong>the</strong> Ca/P ratio <strong>of</strong> TCP phases can occur on heating in air. Among <strong>the</strong> tricalcium
phosphate phases, α TCP is known as <strong>the</strong> high-temperature stable form. Its stability region ranges from
1125 to 1430°C [Welch and Gutt, 1961]. Under 1125°C, βTCP is <strong>the</strong> stable tricalcium phosphate phase.
Rapid quenching from temperatures higher than 1125°C, however, permits <strong>the</strong> preservation <strong>of</strong> <strong>the</strong> α-TCP
phase at room temperature. The transition temperature between <strong>the</strong> β and α TCP phases may vary depending
on ion impurities such as Mg, Zn and Fe which stabilise βTCP. Apatitic TCP can be considered as <strong>the</strong> lowtemperature
crystalline form <strong>of</strong> ATCP (e.g. upon drying at 80°C). On heating at temperatures higher than
800°C, this phase transforms into βTCP [Destainville et al., 2003]. Pure ATCP remains amorphous when
heated up to 630°C [Eanes, 1970]. Above this temperature, it crystallises first into <strong>the</strong> metastable α TCP
generally associated with small fractions <strong>of</strong> βTCP and, around 850°C, into pure βTCP. However, ATCP
containing Mg ions (or containing o<strong>the</strong>r elements stabilising <strong>the</strong> βTCP phase such as Fe and Zn) transforms
directly into <strong>the</strong> βTCP phase without <strong>the</strong> intermediary formation <strong>of</strong> α TCP. It has been suggested, based on
<strong>the</strong>rmodynamic and nuclear magnetic resonance studies, that α TCP formed by <strong>the</strong>rmal crystallization <strong>of</strong>
ATCP could be more stable and contains fewer defects than <strong>the</strong> α TCP phase obtained by quenching from
temperatures above <strong>the</strong> β to α TCP transition [Somrani et al., 2003; Belgrand, 1993].
Thermal treatment in air <strong>of</strong> <strong>the</strong> low-temperature phases, ATCP and Ap TCP, at 900°C, for several
hours does indeed lead to <strong>the</strong> formation <strong>of</strong> βTCP, and this is a way to prepare this phase with high purity.
When TCP’s Ca/P atomic ratio is not exactly 1.5, impurities appear. The main impurities, hydroxyapatite
(corresponding to a Ca/P atomic ratio above 1.5) and β calcium pyrophosphate (corresponding to a Ca/P
atomic ratio under 1.5), can be detected, respectively, by XRD and FTIR spectroscopy. α TCP is generally
obtained by heating βTCP above 1125°C (<strong>the</strong> allotropic transition temperature for β → α), followed by rapid
quenching. It is interesting to note that α TCP can also be obtained transitorily by heating ATCP at
temperatures lower than 1125°C (between 630 and 850°C), but this method generally leads to a product
containing traces <strong>of</strong> βTCP [Somrani et al., 2003; Eanes, 1970].
Both α and βTCP can also be prepared from o<strong>the</strong>r starting powders and, more conveniently, from
mixtures <strong>of</strong> Ca-P phases with <strong>the</strong> adequate global Ca/P ratio <strong>of</strong> 1.5. For example, <strong>the</strong>y can be obtained by
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