Biomedical Engineering Reference
In-Depth Information
requirements need to be met and there is no acceptable substitute. Therefore,
innovative technology is required.
The design, production, and consumption of polymeric materials for commod-
ity and specialty plastic items must surely contend with all the constraints and
regulations already in place or predicted to deal with the management of primary
and postconsumer plastic waste. This is certain to involve the formulation of
environmentally sound degradable polymers. Technologies based on the recovery
of free energy content through recycling and from incineration with heat recovery
will be fl anked by the increasingly attractive option of environmentally degradable
macromolecular materials. These latter polymers should be considered as pre-
ferred replacements for conventional commodity plastics in those product seg-
ments for which recycling is not a practical option. The strategies that are
nowadays receiving a great deal of attention at both fundamental and applied
levels include the design of some bio-based polymers, the introduction of hybrid
polymeric formulations, and the reengineering of well-established polymers of
synthetic and natural origin.
16.2
Controlled - Lifetime Plastics
During the past 20 years, science and technology have been developed for poly-
mers that can biodegrade after being used and discarded. Everything from shop-
ping bags to agricultural mulch fi lms to daily landfi ll covers to food packaging as
examples can be made to disintegrate after disposal and to yield thereby molecular
fragments that are susceptible to mineralization by naturally occurring microor-
ganisms. The carbon in these polymers is returned to the biocycle, and there are
no harmful residues. These are the oxo-biodegradable polyolefi ns, as defi ned
below.
According to the ASTM defi nition [3], a biodegradable plastic is “
a degradable
plastic in which the degradation results from the action of naturally occurring microor-
ganisms such as bacteria, fungi and algae.
” Rather more informative is the ASTM
defi nition [4] of the environmental degradation of a plastic:
“abiotic or biotic degra-
dation process or both that occurs in a given environment and includes photodegrada-
tion, oxidation, hydrolysis and biodegradation. Living organisms affect biotic degradation
processes and abiotic degradation processes are not biological in nature.”
Two principal
types of commercially viable biodegradable plastics have been developed and are
fi nding a variety of applications in many mercantile segments and consumer
products: (i)
oxo - biodegradable polymers
for which degradation is the result of oxida-
tive and cell-mediated phenomena, either simultaneously or successively and (ii)
hydro - biodegradable polymers
for which degradation is the result of hydrolytic and
cell-mediated phenomena, either simultaneously or successively. Both types of
biodegradable polymer feature a two-stage sequential molar mass reduction in the
environment with the fi rst stage being abiotic. Since the objective is to reduce the
amount of plastic with minimum effect on the environment, the second stage is
