Biomedical Engineering Reference
In-Depth Information
• Hydro-Biodegradable
• Oxo-Biodegradable
O
2
- Uptake
Catalyst
natural or
synthetic
H
2
O - Uptake
Enzyme mediated
or not
• Functional Fragments
• Oxidized Fragments
Exo-Endo
Enzymes
Exo-Endo
Enzymes
• CO
2
, H
2
O, Cell Biomass
• CO
2
, H
2
O, Cell Biomass
• Polyesters
• Polymaides
• Polysaccharides
• Polyolefins
• Polyvinylalcohol
Lignin, Rubber
• Natural rubber
Figure 16.1
General features of the two classes of environmentally degradable polymers.
16.3
The Abiotic Oxidation of Polyolefi ns
The knowledge [8 - 10] of the thermal and photolytic peroxidation mechanisms of
PE and polypropylene (PP) constitutes the basis for the development of “reengi-
neered ” polyolefi ns susceptible to enhanced oxidation and fragmentation, when
exposed to heat or light, with the aim of overcoming the intrinsic recalcitrance of
polyolefi ns to biodegradation. It has been established that the beginning of the
sequence of reactions leading to polyolefi n peroxidation is the generation of sen-
sitizing impurities during the processing of these thermoplastics [8]. It has been
recognized that carbonyl [8, 11] and hydroperoxide [8, 12, 13] groups represent the
major sensitizing impurities formed during the processing of PE and of PP. At
this stage, the chemical structure of the polyolefi ns is considered to be the most
important parameter capable of infl uencing the oxidative degradation processes.
During subsequent use and disposal steps, the oxidation of both types of resin
appears to be mainly affected by structural parameters, such as the degree of
polymerization, chain conformation, degree of crystallinity, and geometry [13].
In the case of PE, the poor reactivity of the nonpolar C-C and C-H bonds mark-
edly constrains the degradation processes by radical reactions. These are generally
initiated by bond-breaking processes promoted by energy input in the form of
heat, UV radiation, mechanical stress, or some combination of these. Since the
susceptibility of saturated bonds to scission is dependent on bond energy, the
initial homolytic bond scission reactions are largely restricted to structural defects,
such as branch points and double bonds. The tertiary carbon-hydrogen bonds that
alternate with methylene groups along PP chains are obviously vulnerable. Fol-
lowing initial bond breakage, a complex series of radical reactions may lead to the
total degradation of the molecules.
