DEGRADATION OF PLASTICS IN THE ENVIRONMENT - Plastics and Environmental Sustainability - page 225

generallyshowreduced Mn (g/mol)(Andradyetal.,1993b)atrelatively low
levels of oxidation. A particularly sensitive and practical bulk mechanical
property that reflects the extent of degradation is the tensile extensibility
of the plastic (Andrady, 2007). In semi-crystalline polymers, chain scission
occurs predominantly in the amorphous regions that oxygen can readily
diffuse into. Therefore, the percent crystallinity of the polymer (for instance
with PE) will generally increase with photo-initiated oxidation of these
polymers. But the presence of additives and fillers can modify the matrix
and this increase is not always observed in field exposure experiments.
6.2.2 Embrittlement and Fragmentation
In semi-crystalline polymers such as polyolefins, initial oxidation occurs in
theamorphoustiemoleculesbetweencrystallitesandsometimeseveninthe
inter-lamellar amorphous chains within crystallites. This allows a relatively
lowlevelsscissiontocauseadisproportionatelylargechangesinmechanical
properties due to brittle failure in the amorphous regions (Celina, 2013).
In PP for instance, beginning stages of bulk embrittlement corresponded to
only 0.01% of oxidation (Fayolle et al., 2004).
Plastics weathered outdoors or in the laboratory show surface cracking and
pitting because of extensive degradation at the surface layer (Küpper et al.,
2004; Qayyum and White, 1993; Yakimets et al., 2004; see Fig. 6.4 ). Solar
UV radiation can only penetrate a thin surface layer of the plastic due to
the high absorption coefficients of polymers at those wavelengths (Gulmine
et al., 2003). This will likely localize degradation and lead to cracking of
the surface. The particle size distribution yielded by fragmentation depends
on whether bulk or surface fragmentation is dominant. Surface layer
embrittlement leads to a large number of smaller fragments (Balabán et al.,
1969) with one dimension equal to the thickness of the surface layer. 5