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

Chapter 1.
Polylactide Based Bio-Materials
heating an intimate mixture <strong>of</strong> CaHPO 4 (di-calcium phosphate anhydrous) and CaCO 3 (calcium carbonate)
in <strong>the</strong> molar ratio 2/1, between 1150 – 1200°C, for at least one day. α TCP is <strong>the</strong>n obtained by quenching in
liquid nitrogen whereas prolonged heating at about 900°C (until complete disappearance <strong>of</strong> α TCP) leads to
<strong>the</strong> production <strong>of</strong> βTCP. Several authors also reported <strong>the</strong> syn<strong>the</strong>sis <strong>of</strong> pure βTCP by heating a di-calcium
phosphate dihydrate, calcium carbonate mixture in <strong>the</strong> molar ratio 2/1 at 930°C, for 2 hours [Yang and
Wang, 1998] or, at 900°C, for 14 hours [Vallet-Regí et al., 1997].
From a structural point <strong>of</strong> view, βTCP is a stable anhydrous tricalcium phosphate phase. It
crystallises in <strong>the</strong> rhombohedral system with 21 formula units Ca 3 (PO 4 ) 2 per hexagonal unit cell [Dickens et
al., 1974]. A thorough structural description <strong>of</strong> this phase is given in <strong>the</strong> literature based mainly on a
comparison with <strong>the</strong> phase Ba 3 (VO 4 ) 2 and it will <strong>the</strong>refore not be reported here [Elliott, 1994]. An
interesting structural feature <strong>of</strong> βTCP is, however, <strong>the</strong> presence <strong>of</strong> columns <strong>of</strong> ions (anions and cations)
which can be distinguished parallel to <strong>the</strong> c-axis.
3.2.2.6 Aqueous Evolution <strong>of</strong> TCP Phases
In aqueous medium, it is worthwhile reminding that <strong>the</strong> first precipitates obtained under given
conditions (pH, temperature and ion concentrations) from calcium phosphate solutions do not necessarily
correspond to <strong>the</strong> <strong>the</strong>rmodynamically most stable phase. For example, phases such as ATCP or
Ca 8 H 2 (PO 4 ) 6 ,5H 2 0 (octocalcium phosphate) may form transiently in solution, somewhat analogous to
precursor phases prior to <strong>the</strong>ir progressive hydrolysis into apatite [Tung, 1998]. In particular, ATCP is
generally <strong>the</strong> first solid phase that spontaneously precipitates upon mixing alkaline calcium and phosphate
solutions [Chow and Eanes, 2001].
When immersed in solution, <strong>the</strong> most soluble TCP phases, α TCP and ATCP, show a strong
tendency to evolve towards a more stable phase through a hydrolysis process. At alkaline and neutral pH,
ATCP progressively transforms into non-stoichiometric hydroxylated apatites [Eanes and Meyer, 1977].
According to Heughebaert [1977], in this process ATCP is generally found to remain amorphous up to half
hydrolysis corresponding to <strong>the</strong> composition Ca 9 (PO 4 ) 5 (HPO 4 )(OH) and apatite structure crystallisation. The
apatite obtained by hydrolytic conversion <strong>of</strong> ATCP may, however, evolve differently in <strong>the</strong> solution
depending on pH, temperature, maturation time and ion content. Generally, an increase in <strong>the</strong> Ca/P ratio <strong>of</strong>
<strong>the</strong> solid is noticed, associated with a decrease in HPO 4 2− content and an increase in OH − content [Somrani
et al., 2005]. It was found that an increase in <strong>the</strong> Ca/P ratio in <strong>the</strong> starting solutions led to faster
crystallisation into hydroxyapatite [Kim et al., 2004]. Interestingly, <strong>the</strong> addition <strong>of</strong> some ionic species such
as Mg 2+ , CO 3 2−, and P 2 O 7 4− which inhibit apatite crystal growth was shown to delay this hydrolysis process
[LeGeros et al., 2005; Boskey and Posner, 1974]. Considering heated ATCP, <strong>the</strong> conversion rate into apatite
has been shown to be related to <strong>the</strong> residual water content and rehydration <strong>of</strong> <strong>the</strong> powder appears as a
determining stage [Somrani et al., 2005].
α TCP also hydrolyses rapidly in aqueous solution [Monma et al., 1981; Monma, 1980]. The
hydrolysis products are generally Ca 8 H 2 (PO 4 ) 6 ,5H 2 0 (octocalcium phosphate) and non-stoichiometric
apatite, although CaHPO 4 (di-calcium phosphate anhydrous or monetite) and CaHPO 4 ,2H 2 O (di-calcium
phosphate dihydrate or brushite) can also form. The transformation mechanism is generally identified with
dissolution–reprecipitation reactions. Although less investigated than <strong>the</strong> hydrolysis <strong>of</strong> Am-CP, <strong>the</strong>
hydrolysis <strong>of</strong> α-TCP is believed to depend on <strong>the</strong> same physicochemical factors (temperature, pH, hydrolysis
time, solution composition and presence <strong>of</strong> ionic impurities).
In contrast, βTCP does not show a tendency to hydrolyse rapidly in solution, at physiological
temperatures. However, pure βTCP has been shown to hydrolyse completely into relatively well-crystallised
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