Callister - An introduction - 8th edition

22.5 Recycling Issues in Materials Science and Engineering • 883
as a packaging material, especially for beverages
and food products—transparency, resistance to attack
by moisture and grease, odorlessness, and
odor barrier characteristics. Furthermore, PLA is
also bioresorbable, meaning that it is assimilated
(or absorbed) in biological systems—for example
the human body. Hence, it has been used in a
variety of biomedical applications, including resorbable
sutures, implants, and controlled release
of drugs.
The primary obstacle to the widespread use of
PLA and other biodegradable polymers has been
that of high cost, a common problem associated
with the introduction of new materials. However,
the development of more efficient and economical
synthesis and processing techniques has resulted
in a significant reduction in the cost of this
class of materials, making them more competitive
with the conventional petroleum-based polymers.
Although PLA is biodegradable, it will degrade
only under carefully controlled circumstances—
that is, at elevated temperatures generated in commercial
composting facilities. At room temperature
and normal ambient conditions it is stable indefinitely.
The degradation products consist of water,
carbon dioxide, and organic matter. The initial
stages of the degradation process in which a highmolecular-weight
polymer is broken into smaller
pieces is not truly one of “biodegradation” as described
earlier; rather it involves hydrolytic cleavage
of the polymer backbone chain, and there is
little or no evidence of microbial action. However,
the subsequent degradation of these lowermolecular-weight
fragments is microbial.
Furthermore, poly(lactic acid) is also
recyclable—with the right equipment it can be
converted back into the original monomer and
then resynthesized to form PLA.
A number of other characteristics of PLA
make it an especially attractive material, in particular
for textile applications. For example, it
may be spun into fibers using conventional meltspinning
processes (Section 15.24). In addition,
PLA has excellent crimp and crimp retention, is
resistant to degradation when exposed to ultraviolet
light (i.e., resists fading), and is relatively inflammable.
Other potential applications for this
material include household furnishings such as
drapes, upholstery, and awnings, as well as diapers
and industrial wipes.
Examples of applications for biodegradable/biorenewable poly(lactic acid):
films, packaging, and fabrics. (Photograph courtesy of NatureWorks LLC and
International Paper, Inc.)