Biomedical Engineering Reference
In-Depth Information
160
120
80
40
0
0
1
2
3
4
5
cov
Figure 16.3
Molecular weight versus carbonyl index (CO
i
) relationship in LDPE fi lm sample
thermally treated in air in oven at 70 °C.
The disintegration of fi lm samples occurred within 28 days, with fi lm debris
tending to sink to the bottom of the vessel. In parallel with the advancing oxidation
process as monitored by CO
i
, weight changes and wettability increase during
thermal degradation; a dramatic decrease in the molecular weight of the test sample
has been recorded after a few days of oven aging. The progressive shift toward
lower molecular weights as the CO
i
increases with the aging time can be observed
by HT-GPC analysis. The relationship between the
M
w
and CO
i
can be expressed
by a mono-exponential trend (Figure 16.3). Accordingly, CO
i
values may be used
in order to predict the
M
w
decrease as a function of the level of oxidation. Moreover,
the recorded trend is in agreement with a statistical chain scission mechanism, as
suggested for the thermal- and photodegradation of polyolefi ns [13] .
The feasibility to separate oxidized LDPE fi lms into high - and low - molecular -
weight fractions using a relatively simple extraction procedure with acetone has
also been demonstrated [16]. In particular, the level of oxidation, as related to the
carbonyl index, illustrates the increase in the amount of the solvent extractable
fraction in parallel with a signifi cant decrease in molecular weight. Accordingly,
from heavily oxidized test samples, more that 25% by weight of acetone extracts
can be obtained, thus also showing a very low
M
w (0.85 - 1.05 kDa) (Figure 16.2 ).
These data once more confi rm that LDPE containing prodegradant additives can
be effectively oxidized and massively degraded to low molar mass fractions which,
owing to their wettability and polar functionality, become vulnerable to
microorganisms.
16.4.3
Biometric Measurements
The second stage in the assessment of the environmental fate of “degradable”
polyolefi ns is the evaluation of the ultimate biodegradation of them under different
test conditions aimed at reproducing disposal or accidental littering environments.
