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

Chapter 1.
Polylactide Based Bio-Materials
2 Polyesters Based Bio-materials
2.1 Polylactides (PLA)
2.1.1 Structure <strong>of</strong> Polylactic Acid
The term "Bio-polyester" can be understood in several different ways. Bio-polyesters can be
interpreted as polyesters <strong>of</strong> strictly biological origin. Poly(lactic acid) belongs to <strong>the</strong> family <strong>of</strong> aliphatic
polyesters commonly made from acid, and are considered as biodegradable and compostable [Kaplan,
1998]. One can also interpret bio-polyesters as polyesters that have been syn<strong>the</strong>sized by biological means,
for instance by enzyme-catalyzed polymerization reactions. Moreover, <strong>the</strong>re are hybrids between <strong>the</strong>se two
strict definitions <strong>of</strong> bio-polyesters. For example, monomer syn<strong>the</strong>sis for poly(lactic acid) came from a
biological process in which lactic acid is produced microbially by <strong>the</strong> fermentation <strong>of</strong> a renewable
polysaccharide-based resource, mostly corn. Lactic acid is subsequently polymerized chemically into
poly(lactic acid) by a condensation reaction. It is one <strong>of</strong> <strong>the</strong> few polymers in which <strong>the</strong> stereo-chemical
structure can easily be modified by polymerizing a controlled mixture <strong>of</strong> <strong>the</strong> L- or D-isomers to produce
high molecular weight amorphous or semi-crystalline polymers that can be used for food contact and are
generally recognized as safe [Conn et al., 1995].
The structural formulas <strong>of</strong> poly(lactic acid) are given in Figure 1.2. As PLA contains an
asymmetrical carbon atom in its structural unit, iso-tactic P L LA and P D LA polymers are optically active.
Consequently, <strong>the</strong> meso-lactide P D,,L LA is a syndio-tactically alternating D,L-copolymer or a copolymer
having L-units and D-units and is non optically active [Van de Velde and Kiekens, 2002]. Latter findings
have been ga<strong>the</strong>red under ‘PLAs’ and are thought to be mostly non-syndio-tactic P D,L LA. All polyesters, <strong>of</strong>
natural and syn<strong>the</strong>tic origins, are characterized by <strong>the</strong> common formula presented in Figure 1.2.
2.1.2 Syn<strong>the</strong>sis <strong>of</strong> Polylactic Acid
Figure 1.2: Stereo-forms <strong>of</strong> lactides.
[Madhavan Nampoothiri et al., 2010]
It can be easily produced in a high molecular weight form through ring-opening polymerization
(cf. Figure 1.3) using most commonly a stannous octoate catalyst (sometimes tin (II) chloride).
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