Mechanisms of Aging and Release from Weathered Nanocomposites - Safety of Nanomaterials along Their Lifecycle - page 321

stabilizers and antioxidants for photodegradation of polyolefins was also substan-
tially reduced (Chmela 2005; Mailhot et al. 2003). Further, clay nanofillers and their
compatibilizers did not change the photo-oxidation mechanism, but they strongly
affected the oxidation rates of these polymers (Morlat 2004; Hrdlovic 2004). Similar
enhanced photodegradation was also observed for PP/TiO 2 -organoclay, polycarbon-
ate/layered silicate (Sloan et al. 2003), and polystyrene/layered double hydroxide
(LDH) nanocomposites (Peng and Qu 2005). However, the rate of PP photodegrada-
tion was reduced when LDH (Ding and Qu 2006) or clay nanofiller modified with
dopamine (Phua et al. 2013) was used.
14.3 RELEASE OF FRAGMENTS FROM MATRIX
DEGRADATION: SPONTANEOUS OR INDUCED?
Once the matrix is degraded, the forces occurring in certain lifecycle scenarios
(Le Bihan et al. 2013) may be sufficiently strong to overcome the remaining cohe-
sion within the matrix and/or the adhesion between the matrix and nanofiller. As a
consequence, fragments of the nanocomposite and/or individual nanofillers may be
released into the atmosphere, soil, or liquids. If the physicochemical interactions
with external media dominate, we designate the release as “spontaneous”; if external
forces such as mechanical stress contribute substantially, we designate the release as
“induced.” Both can occur in real life and in laboratory-scale simulations.
Spontaneous release of nanofillers into run-off water was investigated by the
NanoPolyTox project, and first results on the degradation of the nanocompos-
ites (Vilar et al. 2013) and the released fragments in run-off waters (Busquets-Fite
et al. 2013) have been reported. The tests were performed for 1,000 h in a climatic
chamber (Suntest XXL+, Atlas) according to ISO 4892/06 with 0.5 W/m² nm at
340 nm (60 W/m² integrated from 300 nm to 400 nm). In the adapted protocol, after
29 min dry irradiation, rain was simulated for 1 min and the run-off waters were
collected. The characterization was performed using a multitude of techniques, but
specifically the solid content of fragments released into the run-off was assessed after
lyophilization using gravimetry (Figure 14.4), and further identified using TEM and
energy-dispersive X-ray spectroscopy (EDXS). For many other established colloidal
techniques the total amount of fragments (on the order of 10 mg), dispersed in several
liters of run-off, is prohibitively low.
By comparison of 12 nanocomposites (MWCNT, SiO 2 , TiO 2 , and ZnO nanofillers
compounded in polyamide (PA), PP, and EVA matrices), Busquets-Fité et al. cor-
related the compatibility between nanofiller and matrix, as evidenced by the degree
of nanofiller dispersion in the composite, with a significantly reduced probability of
release during weathering. For good matrix/filler compatibility systems, for example,
pristine MWCNTs in PP, the total release remained as low as 0.02% of the total spec-
imen mass, corresponding roughly to 200 mg/m² release per irradiated surface at an
UV dose of 216 MJ/m². No evidence of free MWCNTs was observed. In contrast,
a sample with substantial aggregation of nanofillers in the matrix such as propyl-
functionalized SiO 2 in PP led to 10-fold higher release, and an over-representative
accumulation of SiO 2 nanofiller in the run-off waters (Figure 14.5h) (Busquets-Fite
et al. 2013).