Environmental Engineering Reference
In-Depth Information
of this property that protein is readily biodegradable. However, synthetic polymers
are just the opposite to this. They have short repeating units, which enhances their
crystallinity, by making the hydrolysable groups inaccessible to the enzyme.
Synthetic polymers with long repeating units are less likely to crystallise and so might
be biodegradable. Whereas a series of poly(amide-urethanes) are readily degraded
by subtilisin [7].
Many studies have been carried out to correlate polymer structure (including parameters
such as temperature, glass transition, MW and so on) with biodegradability. However,
so far only a few general relationships between structure and biodegradability have
been formulated. There is a complex interplay between some of the different factors
that occur simultaneously, causing dificulty in sorting out the primary effects and
correlations.
3.2.2 Composition
Polymer characteristics and properties are changed when a new monomer is introduced
into the polymer chain. Changes in T
g
and crystallinity have been observed in
polyanhydrides and these changes depend on the copolymer composition, which
in turn affects their ability to degrade. Time of erosion is signiicantly improved
by increasing the content of aromatic monomers. The type of bond determines
the degradation rate of polymers. Thermoplastic polyoleins are not susceptible
to degradation by microorganisms because of the presence of long carbon chains.
The incorporation of hetero groups including oxygen (O
2
)
in the polymer chain
makes it accessible for thermal degradation and biodegradation. The strength of the
neighbouring C−H bonds of the polymer is affected by the presence of the heteroatom.
Naturalrubberismoresusceptibletodegradationcomparedtopolyethylene(PE)
because of the presence of unsaturation in the polymer chain. LDPE has low packing
of polymer chains because of the randomness of the branching, whereas HDPE has
minimal branching and linearity, which is responsible for its high packing density [9],
so LDPE degrades faster than HDPE. The different orientation of the methyl group
in PP results in three possible conigurations: isotactic, syndiotactic and atactic [9].
Isotactic is the most crystalline while atactic is the least crystalline. The high crystalline
nature gives stiffness, hardness, high tensile strength and makes it highly resistant to
natural degradation or biodegradation [10].
3.2.3 Molecular Weight
Increase in the MW decreases the rate of degradation of the plastic [11].
