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

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Chapter 1. Polylactide Based Bio-Materials bone replacement is conditioned bone. Therefore, cortical bone has a smaller content in strontium that <strong>the</strong> trabecular bone. Although strontium at high doses can produce undesirable effects in <strong>the</strong> body as human bone alterations, rickets, mineralization defects and change pr<strong>of</strong>ile <strong>of</strong> mineralization in low doses poses no health risk. Indeed, ATSDR (Agency <strong>of</strong> Toxic Substances and Disease Registry) said explicitly in his paper Toxicological strontium that exposure to low doses <strong>of</strong> stable strontium affects not <strong>the</strong> health <strong>of</strong> adult men. In addition to hand administration <strong>of</strong> <strong>the</strong> compounds <strong>of</strong> strontium (strontium ranelate) as <strong>the</strong>rapeutic method for treatment <strong>of</strong> osteoporosis has been well studied in <strong>the</strong> past years and <strong>the</strong> results were positive in relation to bone regeneration. The in-vitro experiments showed that strontium produces an increase in training and decreased bone resorption. The influence <strong>of</strong> <strong>the</strong> addition <strong>of</strong> certain cations to materials based on <strong>the</strong> CPA to improve interfacial interaction <strong>of</strong> polymer composites has been studied for some number <strong>of</strong> cations (Ag + , Fe +2 , Zn +2 , Al +3 , Fe +3 ) [O’Donnell et al., 2009]. O<strong>the</strong>r studies have shown <strong>the</strong> role <strong>of</strong> divalent cations strontium and zinc as preventive agents <strong>of</strong> dental caries [Baig et al., 1999]. 3.2.2.3 Effect <strong>of</strong> Isomorphous Substitution <strong>of</strong> Strontium in <strong>the</strong> βTCP The effect <strong>of</strong> isomorphous substitution <strong>of</strong> strontium in calcium-deficient apatite was already studied [Baig et al., 1999]. These studies confirm that <strong>the</strong> crystal structure <strong>of</strong> beta phosphate can accommodate up to 80% <strong>of</strong> strontium atoms, causing a widening <strong>of</strong> <strong>the</strong> unit cell. This fact agrees with <strong>the</strong> largest ionic radium strontium relative calcium. We can define <strong>the</strong>se calcium-deficient apatite resulting from <strong>the</strong> substitution <strong>of</strong> formula nonstoichiometric following: Ca 9-x Sr x (HPO 4 ) (PO 4 ) 5 (OH) Fur<strong>the</strong>rmore, <strong>the</strong> incorporation <strong>of</strong> strontium causes <strong>the</strong> shift <strong>of</strong> <strong>the</strong> absorption bands <strong>of</strong> <strong>the</strong> phosphate group to minor frequencies. In addition, <strong>the</strong> presence <strong>of</strong> divalent ions which replace <strong>the</strong> calcium appears to play an important role in <strong>the</strong> competition between HA and βTCP just described in previous sections. When you submit substituted apatite strontium heat treatment acts as a stabilizer <strong>of</strong> <strong>the</strong> β phase agree with: Ca 9-x Sr x (HPO 4 ) (PO 4 ) 5 (OH) → 3(Ca (3-x/3) Sr x/3 ) (OP 4 ) + H 2 O (1.3) The presence <strong>of</strong> foreign ions can alter some structural and physicochemical properties <strong>of</strong> βTCP, such as lattice parameter and <strong>the</strong> crystallinity, solubility and <strong>the</strong>rmal stability. Through <strong>the</strong> study <strong>of</strong> Baig et al. [1999] and <strong>the</strong> resulting relationship between lattice parameters as concentration <strong>of</strong> strontium added (equations 1.4 and 1.5), we can estimate <strong>the</strong> effect <strong>of</strong> addition <strong>of</strong> strontium in βTCP on our experiment. Since <strong>the</strong> structures <strong>of</strong> βTCP and α (Sr 3 (PO 4 ) 2 are very similar. The lattice parameters follow <strong>the</strong> linear relationship: a = 10.434 + 0.00373xÅ = 8.3×10 -3 Å (1.4) c = 37.211 + 0.00250xÅ = 7×10 -2 Å (1.5) As in our case <strong>the</strong> atomic concentration <strong>of</strong> strontium is 10% atomic <strong>the</strong>n <strong>the</strong> lattice parameters are: a = 10.4713Å c = 37.236 Å - 22 -
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 - 23 -
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Institut National Polytechnique de
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