Environmental Engineering Reference
In-Depth Information
Biodeterioration is observed because of the effects of the ocean currents.
Figure 3.1
shows the surface degradation observed on various polymers immersed in surface
waters for year. Several factors lead to the biodegradation/biodeterioration and these
are discussed in the next sections.
3.2.1 Polymer Structure and Morphology
A biological system generally degrades natural macromolecules (cellulose, protein and
starch) by hydrolysis or by oxidation. Most of the synthetic biodegradable polymers
contain a linkage along the polymer chain (amine, ester, urea, urethane linkage) that
are susceptible to biodegradation by the microorganisms present in the environment.
Thus, most of the proteolytic enzymes speciically catalyse the hydrolysis of peptide
linkages. So polymers with benzyl, carboxyl, hydroxyl, methyl, and phenyl groups
have been prepared hoping that the presence of these groups might increase the
biodegradability [7]. Among the benzylated polymers, polyamide (PA) shows mixed
results. Achiral poly(hexamethylene-α-benzylmalonamide) is readily hydrolysed
by chymotrypsin whereas poly(alkylene D,L-α-benzyladipamide) shows very low
biodegradability. Chymotrypsin is also found to be more effective in degrading
benzyl-substituted poly(ester-ureas) derived from phenylalanine. It does not degrade
unsubstituted poly(ester-ureas) that are derived from glycine. The synthetic polymer
chain must be lexible enough to it into the active site of the enzyme in order to be
degraded (
Table 3.1
). Thus, lexible aliphatic polyesters are more readily degraded
by bacteria, whereas the more rigid aromatic PET is generally bioinert [8].
Table 3.1 Chemical structures of aliphatic polyester, PC, PU and polyamides
with their T
m
Name
Chemical structure
T
m
(
o
C)
Polyester
-O-(CH
2
)6-O-CO-(CH
2
)4-CO-
60
PC
-O-(CH
2
)4-O-CO-O-(CH
2
)4-O-CO-
65
PU
-NH-(CH
2
)6-NH-CO-O-(CH
2
)4-O-CO-
180
PA
-NH-(CH
2
)6-NH-CO-(CH
2
)6-CO-
240
One of the major differences between protein and a synthetic polymer is that the
latter has equivalent repeating units along the polypeptide chain which the former
does not have. This property results in a less crystalline protein chain. It is because
