Biomedical Engineering Reference
In-Depth Information
wettability observed during the abiotic stage of degradation. The ready biodegrad-
ability of these compounds has been suggested previously because they disappear
once oxidized samples are incubated in the presence of hydrocarbon-degrading
microorganisms such as
Arthrobacter paraffi neus
[20, 40, 44, 45]. Additional support
for this hypothesis is obtained from the FTIR characterization of the LDPE fi lms
exposed for a few months to soil microorganisms during biometric tests. Indeed,
a signifi cant reduction of the absorbance in the carbonyl region with respect to
the values recorded at the beginning of the test has been observed repeatedly. In
contrast, the number of double bonds in the carbon-carbon polymer chains was
found to increase during the soil burial experiments, with a corresponding dra-
matic change in the fi ngerprint region of the IR spectra of the LDPE samples.
These observations suggest that, during the soil burial tests, preoxidized LDPE
samples undergo an ongoing degradation process, mediated by both abiotic and
biological factors, which leads to the formation of large amounts of oxidized
molecular fragments capable of being assimilated as carbon sources by soil micro-
organisms. This might explain the two-step biodegradation behavior that has been
observed repeatedly for preoxidized PE fi lms in soil.
The effect of different levels of oxidation as reached during the preaging (e.g.,
thermal degradation) step on the biodegradation propensity has also been evalu-
ated in respirometric tests carried out in an aqueous medium in the presence of
river water microbial populations. The complex biodegradation profi le character-
ized by the presence of alternating plateau and exponential phases has been
observed in this case also, which suggests that this behavior can be considered as
typical of the biodegradation of oxidized LDPE. A straightforward relationship
between the level of oxidation as determined by CO
i
and the biodegradation
behavior of thermally oxidized LDPE samples has been observed also in biodeg-
radation tests carried out in an aqueous medium [46]. In this experiment, heavily
oxidized fractions of thermally degraded LDPE fi lms as well as LDPE fi lms having
medium and high CO
i
values were supplied as the sole energy and carbon source
in a mineral salt medium to a microbial consortium obtained from a river water
sample. During incubation for 140 days at room temperature, degrees of miner-
alization ranging between 10% and 50% were recorded in the case of thermally
oxidized LDPE samples having CO
i
values between 4.6 and 20.7, respectively
(Figure 16.5). Negligible mineralization was observed in the case of lightly oxidized
LDPE with a CO
i
value of only 2.3. These data also suggest, therefore, that readily
biodegradable oxidized LDPE samples can be obtained, depending on the level of
oxidation reached during the abiotic pretreatment.
16.5
Processability and Recovery of Oxo-biodegradable Polyolefi ns
The use of activating additives of the types described above does not affect the
processing characteristics of conventional polyolefi n resins. These are “run” on
the usual equipment at normal speeds. The products are indistinguishable from
