Biomedical Engineering Reference
In-Depth Information
Table 16.1
Structural changes recordable during the oxidation process of oxo-biodegradable
polyolefi ns.
Parameters to be monitored during the preaging of LDPE
Parameter
Meaning
Weight variation
Increase: oxygen uptake
Decrease: loss of volatile
intermediates
Carbonyl index
by FTIR
Formation of cleavable
oxidized groups in the
main chain
Preliminary characterization of
the oxidized functional groups
Wettability
Formation of an oxidized
polar group at the surface
Increase: raise of the
propensity to microbial
colonization
Molecular weight
Evaluation of the degree
of chain scissions
Decrease: raise of the
propensity to microbial attack
Fractionation by
solvent extraction
Estimation of the level of
degradation
Characterization of oxidized
intermediates
best described as an autocatalytic radical chain reaction leading to the oxidation
and scission of polymer molecules, with the concomitant formation of oxidized
low- molecular - weight fragments.
The parameters reported in Table 16.1 have been monitored during the preaging
step carried out at 70 °C in an air convection oven. After an induction period, an
appreciable weight increase according to a sigmoidal profi le, attributable to the
uptake of oxygen, is observed. Subsequently, as a result of prolonged treatment
time, the sample weight starts to decrease owing to the loss of volatile (i.e., low
molecular weight) degradation products. Since the carbonyl groups usually account
for most of the oxidation products of the thermooxidative degradation of PE, the
concentration of carbonyl groups on oxidation products, as determined from
the CO
i
, can be used in monitoring the progress of degradation [40]. In line with
the recorded weight increase, the CO
i
also shows a sigmoidal increasing shape.
In addition, FTIR spectroscopy shows progressive broadening in the 1700-
1780 cm
− 1
range for carbonyl groups with overlapping bands corresponding to
carboxylic acids (1712 cm
− 1
), ketones (1723 cm
− 1
), aldehydes (1730 cm
− 1
), and esters
(1740 cm
− 1
) during the aging period, thus indicating the formation of different
oxidation products as aging progresses, as reported previously [41].
The increase in surface wettability during the oxidation of PE fi lm containing
prodegradant is an important indicator of the loss in hydrophobic character of this
plastic. It has been shown that a few days of thermal treatment at 70 °C is suffi cient
to cause a dramatic decay in the contact angle of the LDPE fi lm surface as hydro-
phobicity decreases. In addition, as a result of thermal oxidation, bulk density
increases and fi lm disintegration is observed, by fl oating the test fi lm in tap water.
