Mechanisms of Aging and Release from Weathered Nanocomposites - Safety of Nanomaterials along Their Lifecycle - page 320

The photodegradation improvement by TiO 2 nanoparticles was also demonstrated in
a PE-oxidized wax-nanoTiO 2 system, which revealed a higher degradation efficiency
than either PE/TiO 2 nanocomposite or neat PE materials (Fa et al. 2010).
Similar to TiO 2 , ZnO nanoparticles also exhibited both photocatalytic and
photostabilizing behavior in polymer nanocomposites. The enhanced photosta-
bility due to ZnO was observed for composites made with polypropylene (PP)
(Chandramouleeswaran et al. 2007; Zhao et al. 2006), linear-low-density PE (Yang
et al. 2005), PU (Hegedus et al. 2008), and wood polymer composite (Dekaand Maji
2012). The photostability was attributed to the superior UV radiation screening
effects of the ZnO nanoparticles. However, other studies reported the catalytic effect
of ZnO nanoparticles. For example, the degradation of poly(vinyl chloride) (PVC)/
ZnO nanoparticle composite was found to be greater than that of neat PVC (Sil et al.
2010). Similar accelerated degradation was observed by Gu et al. (2012a, 2012b) for
PU/ZnO nanoparticle composite during its exposure to 295-400 nm UV light in the
NIST SPHERE under dry and humid conditions. The rate of acceleration depended
strongly on ZnO concentration and relative humidity of the exposure. This study also
noted that AFM in the phase imaging mode is useful for revealing the preferential
degradation of the polymer layer near the ZnO nanoparticle surface. Using the same
UV source, Nguyen and coworkers (Nguyen 2012; Nguyen et al. 2012; Nguyen et al.
2011; Gorham et al. 2012) showed that the epoxy composite containing 5% mass frac-
tion of silane-coated nanosilica degraded at a higher rate than the neat epoxy (e.g.,
Figure 14.2a). However, there is little difference in the oxidation rate (Figure 14.2a)
between the nanocomposite and the neat epoxy. Further, the matrix degradation
resulted in an increasing accumulation of silica nanoparticles on irradiated surfaces
with exposure time, as demonstrated by AFM imaging displayed in Figure 14.3.
Before exposure (day 0), the epoxy/nanosilica composite surface appeared smooth
and nearly featureless, with little evidence of nanofillers, similar to that observed for
MWCNT composites. After seven days of UV exposure, silica nanoparticles started
to appear on the surface, and after 62 days, silica nanoparticles had covered almost
the entire composite surface. The presence of silica nanoparticles on the surface was
also supported by XPS and IR results (Nguyen et al. 2012; Gorham et al. 2012).
Clay nanofillers are currently incorporated in many commercial polymers to
improve their mechanical strength, fire retardancy, gas barrier, and dimensional sta-
bility (Pavlidou and Papaspyrides 2008). The weathering performance of polymer/
clay nanocomposites has received some attention (Kumar et al. 2009). Depending
on material type, nanoclays displayed both catalytic and stabilizing effects on
photodegradation of polymers. For example, nanocomposites made with montmo-
rillonite (MMT) and PP(Morlat-Therias et al. 2004 and Mailhot et al. 2003), PE
(Quin et al. 2003), and EPDM (Morlat-Therias et al. 2005b) were found to have a
higher photo-oxidation rate than their neat respective polymers. The acceleration
of photo-oxidation was attributed to the degradation of the alkyl-ammonium cation
exchanged in natural MMT and the catalytic effect of iron impurities in the chemi-
cally modified MMT. The catalytic active sites in the nanoclays can accept electrons
from donor molecules of the polymer matrices and induce the formation of free
radicals in the polymer chains upon UV irradiation (Qin et al. 2004), leading to
degradation of the polymers. In the presence of clay nanofillers, the efficiency of UV