Environmental Engineering Reference
In-Depth Information
1.5 Differences between Various Plastics and their Degradability
Degradation of plastics is very slow in the natural environment [14]. Most of the
plastics that we use are resistant to biodegradation, i.e., the process which occurs
with help of microorganisms ( Table 1.2 ). PE and PP are the two abundant polymers
that are highly resistant to biodegradation.
Table 1.2 Degradation rates of common plastic materials
Material
Degradation rate (years)
Plastic bag
10−20
Commercial netting
30−40
Foamed plastic buoy
80
Plastic bottle
450
Aluminium drink can
200−400
Monoilament ishing line
600
Glass bottle
1 million
The primary reason behind this is the high stability and durability of plastic.
Degradation is a chemical process that results in the drastic reduction of average
molecular weight (MW) of the polymer, and its mechanical strength. This makes the
material brittle and results in fragments, which may not be visible to the naked eye.
Degradation of plastics depends on various factors, such as physical, chemical and
environmental factors some of which include:
• Thedensityoftheplasticsdetermineswhetheritisloatingorsinking.Thisinturn
determines the amount of sunlight it receives (see Table 1.3 ). In the ocean, some
of the plastics, including: PC; polystrene (PS); and PET sink, whereas low-density
polyethylene (LDPE); HDPE; PP; and foamed plastics loat on the ocean surface.
Sunlight and wave action cause these loating plastics to fragment, breaking into
increasingly smaller particles, but never completely disappearing. Whereas, those
that sink remain intact for long periods of time. The broken ones may end up in
the gyres and get further broken up.
• Thetypeoftheplastic(structure,morphologyofthematerial,hydrophobicity
and method of synthesis).
• Thetemperatureoftheoceanwater(degradationisfavouredbyhotwater).
 
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