Environmental Engineering Reference
In-Depth Information
to a few select plastics such as poly(lactic acid) (PLA) (Höglund et al., 2012;
Muller et al., 1998). Common plastics such as PE, polypropylenes (PP), PS
(polystyrene), and polyvinyl chloride (PVC) do not hydrolyze appreciably
under ambient environmental conditions.
Figure 6.1
Principal agents of plastics degradation in the environment.
Photo-oxidation is the common mode of degradation for plastics exposed
outdoors and is due to two processes acting in concert, photoinitiation, and
thermal oxidation. In many instances, it is common to have several of these
agents operate together, or sequentially, to degrade the plastic in a given
environment.
Regardless of the mechanism involved, the extent of degradation increases
with the duration of exposure of the material to the agent. Over very long
durations, all polymers will invariably be completely degraded, both
abiotically from thermo-oxidation followed by environmental
biodegradation of the oxidized polymer. Complete biodegradation amounts
to mineralization where the entire sample is converted into small molecules
such as CO
2
and water, allowing the carbon sequestered in the plastic to
join the nature's carbon cycle. At intermediate durations of exposures, a
partially degraded plastic with a reduced average molecular weight
Mn
(g/
