Page 34 - Biodegradable Polyesters

12 1 Biodegradable Polyesters: Synthesis, Properties, Applications

As a food-packaging material, PLA is safe for its intended use as a polymer for
fabricating articles that will hold and/or package food [70, 71]. A 70 wt% PLLA and
30 wt% polyethylene glycol (PEG, mol wt 18 500) can be compression-molded to
make a blend which was made into a highly transparent, exudation-free, flexible,
craze-free sheet [72]. PLA blended with biodegradable starch ester compound can
even make high-impact-resistant thermoplastic, biodegradable starch materials.
Such materials are highly desirable in the packaging industry [73]. A compostable
biodegradable coating of paper or paperboard consists of an outer layer contain-
ing polylactide and an adhesive layer , which binds the outer layer to the paper
or paperboard, of biodegradable polymer material (e.g., polyesters) that is coex-
truded with the polylactide. The coated paper is used as packaging for food stuffs
and for disposable dishes such as containers for frozen foods, disposable drinking
cups, heat-sealed cartons, and packaging wraps [74].

1.3.1.2 Polyglycolide or Poly(glycolic acid)
Poly(glycolide) or poly(glycolic acid) is a biodegradable, thermoplastic polymer
and the simplest linear, aliphatic polyester. In 1954 Higgins et al. [75] patented
a hydroxyacetic acid or its esters form homopolymers from which tough films
and fibers can be prepared. Since then PGA has been known for its thermal
processability and biodegradability but also for its hydrolyticity, which limited its
applications for years. PGA is degraded by enzyme [76] or is highly hydrolytic in
water with high pH ≥ 10. However, at near-neutral pHs, the hydrolyticity of PGA
is reduced markedly [77]. Currently, monomers of lactic acid, ε-caprolactone,
trimethylene carbonate, homopolymer or copolymers of poly(glycolide),
poly(lactic-co-glycolic acid) (PLGA), poly(glycolide-co-caprolactone), and
poly(glycolide-co-trimethylenecarbonate) are widely used as a material for
the synthesis of absorbable sutures and are being evaluated in the biomedical
field.

Synthesis As early as in 1949, Sporzynski et al. [78] published their synthesis
routes for poly(glycolide). Not only for synthesis, Sporzynski and his coworkers
also described the physical properties of PGA as the yellow powders with melting
∘
point at 217 C. They found PGA was depolymerized to 10% glycolide; the
presence of copper increased the yield of glycolide. In 1967, Chujo et al. [79]
systematically studied the ring-opening polymerization within which the poly-
merization behavior of glycolide in the presence of various catalysts and the
cationic copolymerization reaction of glycolide with various comonomers were
examined. A schematic representation is shown in Figure 1.5.
In homopolymerization, an anionic polymerization catalyst such as KOH gives
brittle and highly colored polymers in low yield. A Lewis acid such as SbF gives
3
a tough and colorless polymer almost quantitatively. Antimony oxide is almost
as good as SbF . The temperature has substantial effects on viscosity and the
3
time to reach 100% yield. Ferric chloride–propylene oxide complex also gives a
high-molecular-weight polymer in good yield. Glycolide is easily copolymerized