Biomedical Engineering Reference
In-Depth Information
way, in these compounds both antioxidant and prodegradant functionalities co-
exist. This characteristic has been used to fi nely control the lifetime before the
photooxidation of PE commences. This is the basis of the Scott- Gilead technology
[12] which led to the production and commercialization of controlled-lifetime
photodegradable mulching fi lms and analogous products, under the trade name
Plastor. Nowadays, several agricultural plastic items based on oxo-biodegradable
PE contain prodegradants comprising transition metal ions with organic ligands.
They have been developed by several companies for sale as master batches (for
blending on conventional equipment with normal resins) under different trade
marks (TDPA, EPI Environmental Plastics Inc.), or end products (Envirocare,
CIBA Specialty Chemicals).
Several studies have reported the signifi cant reduction of molecular weight after
thermal and photodegradation of PE samples containing prodegradants [20, 33]
as well as the extraction, isolation and identifi cation of oxidation products, includ-
ing carboxylic acids, ketones, esters, and low- molecular - weight hydrocarbons [34,
35]. The overall rate and extent of the abiotic peroxidation of polyolefi ns are related
to structural parameters such as chain defects and branching. The latter give rise
to facile oxidation due to the susceptibility to hydrogen abstraction from tertiary
carbon atoms through a vicinal hydrogen-bonded intermediate which can obvi-
ously be extensive in poly-
- olefi ns such as PP. The role of vicinal hydroperoxides
is of particular importance in carbon chain cleavage and it leads to the release of
small molecules carboxylic acids, alcohols, and ketones [20] even though random
chain scission is considered to be the predominant process initially. Taking into
account these considerations, it has been shown repeatedly that the decreasing
order of susceptibility of polyolefi ns to peroxidation is: iPP
α
>
LDPE
>
LLDPE
>
HDPE [36, 37].
The ultimate environmental fate of “degradable” polyethylenes has to be recog-
nized as the results of the combined action of abiotic factors and microorganisms.
This has suggested the defi nition of “ oxo - biodegradable ” materials in keeping with
the processes of biodegradation of lignin and natural rubber [11], for example,
since the evaluation of the extent and rate of peroxidation of these materials rep-
resents a powerful tool for the prediction of their biodegradation. Several studies
have therefore been carried out with the aim of determining the mechanisms of
the photo- and thermal oxidation of polyolefi ns containing prodegradant additives.
Nevertheless, most of these studies have been carried out under strictly controlled
laboratory conditions, including accelerated conditions that cannot be considered
as representative of natural environments. In fact, only a few investigations have
been performed by assessing the synergistic effects of temperature, humidity, and
sunlight exposure that are collectively involved in outdoor exposures. In the context
of the general mechanism of the radical oxidation of PE (Scheme 16.1), the fol-
lowing parameters can be monitored during abiotic degradation testing of LDPE
and LLDPE blown fi lms: (i) weight variation; (ii) CO
i
; (iii) surface wettability; (iv)
molecular weight changes; and (v) fractionation by solvent extraction. In particular,
gravimetric analysis can be used effectively to understand the weight changes as
a consequence of the oxygen uptake during the early stages of oxidation as well
