Environmental Engineering Reference
In-Depth Information
3.3.2.1 Thermal Degradation
This refers to the chemical changes taking place to the polymer exposed to an elevated
temperature without the involvement of any other compounds [23]. Floating polymer
debris or those dispersed along the shoreline may undergo thermal degradation.
But, once the debris sinks into the ocean the effect of heat and UV light becomes
negligible. These processes are divided into:
• Depolymerisation,and
• Substituentreaction
Thermal degradation of the polymers occurs at the T m of the polymers, e.g.,
137-169 °C for PHB- co -hydroxyvalerate (HV) depending on the percentage of HV,
175 °C for PHB, 159-178 °C for L-PLA depending on its MW [24]. Compared to the
T m of a thermoplastic polymer the environmental temperature is very low. However,
some polymers and composite materials exhibit a T m near to the environmental
conditions, namely, PCL (T m ≈ 60 °C) and Mater-Bi ® (T m ≈ 64 °C).
The macromolecular framework of the polymer is inluenced by the temperature.
L-PLA, PCL, polybutylene adipate, biodegradable polymers and cellulose are semi-
crystalline polymers. They have amorphous and crystalline regions. The structural
changes of the polymers take place at their T g . Depending on the polymeric nature
and percentage of monomer, industrial thermoplastics have different properties and
biodegradation of the thermoplastic polymers can be inluenced by their crystalline
region. Bikiaris and Karayannidis [25] reported that presence of carboxylic end groups
within the copolymeric thermoplastics, PET and polybutylene terephthalate help in
the formation of free radicals, which favours the thermochemical degradability of
these plastics.
3.3.2.2 Thermo-oxidative
Degradation at an elevated temperature because of slow oxidation of plastic is known
as thermo-oxidative degradation. Bikiaris and co-workers [26] reported that by adding
pro-oxidants(transitionmetalssuchas:Ag,Co,Cr,Cu,Fe,Mn,Ni,VandZn,)to
LDPE its thermo-oxidative biodegradability could be enhanced. Thermo-oxidative
degradation yields a variety of products, namely, acetaldehyde, acetic acid, carbon
dioxide, formaldehyde, formic acid, and water, and in small quantities: hydroxy acids
and aldehyde acids, and so on.
 
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