Barrier Properties of Biodegradable Bacterial Polyester Films - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 61

properties is essential to ascertain their potential with those of conventional polymers
commonly used such as polyethylene, polypropylene and with those of polyesters
available in industry such as poly(lactic acid), polycaprolactone, for instance, that at
the present time present the highest potentials due to their availability and low price.
For that, an introduction of these polyesters is thereafter proposed.
Other Bio-resources and Petroleum Based Polyesters
PLA - Lactic acid is an organic acid found in many products of natural origin such as
animals, plants and microorganisms (Sodergard and Stolt, 2002). Industrially, lactic
acid can be derived from intermediates with renewable origin or from chemical mole-
cules based on coal or oil. Lactic acid (2-hydroxypropanoic acid) is one of the smallest
optically active molecules which can be either of L(+) or D(-) enantiomer monomers.
The properties of lactic acid based polymers depended on the ratio and distribution
of the two enantiomer monomers and can be varied to a large extent: 100% L-PLA
present a high crystallinity and copolymers of poly(L-lactic acid) and poly(D,L-lactic
acid) are rather amorphous (Perego, 1996).
These polymers with high molecular weight (Albertsson and Varma, 2002) can be
obtained by different routes such as the dimerization of polycondensated lactic acid
into lactide (cyclic dimmer) or by ring-opening polymerization (ROP), as reported by
Carothers and co-workers (Carothers, 1932). The polymers derived from lactic acid by
polycondensation are generally referred to as poly(lactic acid) and the polymers result-
ed to lactide by ROP as poly(lactide). Both types are generally referred to as PLA. The
PLA is presumed to be biodegradable, although the role of hydrolysis versus enzymatic
depolymerization in the biodegradation process remains open to debate according
to Bastioli (Bastioli 1998). But, according to Tuominen and co-workers (Tuominen,
2002), PLA does not exhibit any eco-toxicological effect during its biodegradation.
Indeed, a large number of biodegradable polyesters are based on petroleum re-
sources, obtained chemically from synthetic monomers and polycaprolactone, poly-
esteramide, or aliphatic/aromatic copolyesters can be distinguished. Generally, these
petroleum-based polyesters are soft at room temperature since their glass transition
temperatures are lower.
PCL - The polycaprolactone is derived from ε-caprolactone by ring-opening po-
lymerization catalyzed by transition metal compounds. Tokiwa and Suzuki (Tokiwa
and Suzuki, 1977) have reported that PCL can be enzymatically degraded in pres-
ence of fungi and the biodegradation process has been discussed by Bastioli (Bastioli,
1998).
PEA - The highest polar polyester is represented by polyesteramide which is syn-
thesized from statistical polycondensation between polyamide monomers with adipic
acid. Different commercial grades, named BAK ® , have been developed by Bayer but
the production has been stopped in 2001. Contrary to PLA, PEA polymers have exhib-
ited a negative eco-toxicological effect during composting (Averous, 2004).
Aliphatic and aromatic copolyesters - Large grades of copolyesters are obtained
from the polycondensation of diols with dicarboxylic acid. Depending on the dicar-
boxylic acid used (terephtalic acid, adipic, or succinic acid), aromatic or aliphatic