Environmental Engineering Reference
In-Depth Information
An often-posed question is how long it would take for a given plastic to
completely degrade (or mineralize) outdoors, in soil, or in the ocean.
3
Because mineralization rates depend on the type of polymer and biotic
characteristics of the environment it is placed in, this is a question that
cannot be answered without qualification. A valid question might be how
longdoeslowdensitypolyethylene(LDPE)takestomineralizecompletelyin
gardensoil?Directdataisnotavailabletoanswereventhisquestionreliably
for any plastic material. Estimates based on extrapolation of laboratory data
on accelerated biodegradation are sometimes quoted in response to such
queries because of the lack of good information. Such estimates of lifetimes
of common plastics are unreliable and inaccurate as extrapolations cannot
be justified given the long durations of exposure involved. However, the
process is slow enough to suggest that all common plastics ever produced
(and not incinerated) still survive somewhere in the environment,
undergoing slow biodegradation.
6.1 DEFINING DEGRADABILITY
At the outset, it is important to define the term “degradation” in the present
context; it is a chemical change that alters the properties of the material
(this change is called “degradation” because useful properties of interest
such as high strength or high stiffness of the plastics are compromised in
the process). For instance, polyolefins such as PE exposed to solar radiation
undergo photo-initiated oxidation, the principal degradation reaction
outdoors. In this case, oxidation is also accompanied by the scission of
long chain-like polymer molecules. It is the chain scission (rather than the
incorporation of oxygen into the molecular structure) that decreases the
useful mechanical properties of the material and lead to brittleness that
causes fracture. As discussed in
Chapter 3
, the unique, desirable properties
of plastics are a consequence of their long-chain architecture; any
shortening of the average chain length or DP markedly reduces the useful
properties of the material.
Common degradation processes are conveniently categorized according to
the principal agencies that bring about the chemical change.
Figure 6.1
is
self-explanatory and lists the different degradation mechanisms. Of these,
only light-induced degradation breaks down common plastics at rates that
are readily measurable. Biodegradation is perhaps the slowest of these.
Hydrolysis as a mechanism of environmental degradation is available only
