Biomedical Engineering Reference
In-Depth Information
bioassimilation of the molecular fragments that are generated in the fi rst stage.
Abiotic mechanisms are generally regarded as too slow by themselves to be ade-
quate in a variety of disposal environments.
There are several applications in which really quite rapid degradation of plastics
after use is required. For example, plastics that end up in water- or sewage-
treatment systems are an example of situations in which they need to lose integrity
relatively rapidly so as to avoid plugging pumps, fi lters and the like. Hydrolytically
unstable biodegradable plastics can provide an answer here. In many other uses
(e.g., food packaging), however, hydrolytic instability is a disadvantage. Overall
stability is required during shelf storage and use, but this should be followed by
relatively rapid abiotic degradation within a specifi c time, depending on the dis-
posal environment. The avoidance of the accumulation of plastic fragments
requires that these be consumed through biodegradation by microorganisms in
virtually all disposal environments. Effective biodegradation of such residues can
be achieved when originally hydrophobic plastics acquire water-wettable
(hydrophilic) surfaces and a relatively low molecular weight so that there is a
signifi cant number of molecular “ends” accessible at the surface. The science and
technology of the development of commercially viable commodity plastics that can
meet these criteria are the topics addressed in this chapter.
Of the current worldwide production of synthetic polymers, nearly 90% is rep-
resented by full-carbon-backbone macromolecular systems (polyvinyl and polyvi-
nylidenics [5]), and 35% to 45% of production is for one-time use items (disposables
and packaging).Therefore, it is reasonable to envisage a dramatic environmental
impact attributable to the accumulation of plastic litter and other plastic waste
from discarded full-carbon-backbone polymers, which are conventionally recalci-
trant to physical, chemical, and biological degradation processes. The mechanism
of biodegradation of full-carbon-backbone polymers requires an initial oxidation
step, mediated or not by enzymes, followed by fragmentation with a substantial
reduction in molecular weight. The functional fragments then become vulnerable
to microorganisms present in different environments, with production (under
aerobic conditions) of carbon dioxide, water and cell biomass. Figure 16.1 outlines
the general features of environmentally degradable polymeric materials, which are
classifi ed as hydro-biodegradable and oxo-biodegradable. Typical examples of
oxo-biodegradable polymers are PE, poly(vinyl alcohol) [6], natural rubber (poly-
cis -
1,4 isoprene) [7], and lignin, a naturally occurring structurally complex
heteropolymer.
The prodegradant added to polyolefi ns to convert them to oxo-biodegradable
status does not cause any oxidation or other degradation as long as antioxidants
are present. Thus, the shelf life and use life of the plastics are maintained for a
period that is controlled by the amount of antioxidant (or other stabilizing addi-
tives) present in the formulation. Once the stabilizers have been depleted, the
prodegradant catalyses the oxidative degradation of the polymer, with the rate of
degradation related to the concentration of prodegradant. By controlling the con-
centrations of these two classes of additive, one practically controls the “lifetime”
of the plastic.
