Biomedical Engineering Reference
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10000
1000
100
10
145
MJ/m
2
290
MJ/m
2
435
MJ/m
2
834
MJ/m
2
435
MJ/m
2
a
a
a
b
c
FIGURE 14.6
Quantity of release induced by immersion after weathering. PA released
fragments in the size range below 150 nm by immersion and shaking with increasing UV
irradiance, in different labs collapsed in one graph. Gray: neat PA, black: PA with 4%
SiO
2
with weathering conditions a: dry_60W/m²_immersed; b: dry_140W/m²_immersed;
c: 15%rain_60W/m²_immersed (ISO4892-2).
fragments and small fragments with diameters below 150 nm (Figure 14.5a and b).
Their ratio shifts towards smaller sizes with increasing shear energies, suggesting
not only increased release but also possibly further reduction of fragment diameters
in the immersion fluids after detachment, thus changing the released entities. Only
at the worst-case shear forces (ultrasound, bath, or probe), structures appeared that
were rather individualized MWCNTs with attached polymer than a polymer frag-
ment with protruding MWCNTs. Identical results were obtained for induced release
after wet weathering.
Size-selective results showed that fragments from PA with SiO
2
released sponta-
neously with only 50% increase for ultrasound versus unagitated immersion, whereas
fragments from TPU with MWCNTs required strong shear with 160% increase for
sonication (Hirth et al. 2013). In contrast, the total release rates are primarily deter-
mined by the photodegradation rate of the polymer matrices, as evidenced by the
well-differentiated release levels on the order of 100 mg/m² from PA versus 10 mg/
m² from TPU (from neat polymers and nanocomposites alike) after 435 MJ/m² UV
energy.
The UV + immersion + centrifugation method is hence capable of ranking mate-
rials with regard to their probability of release under UV irradiation, where particles
release spontaneously, whereas MWCNTs do not, and where PA releases more ultra-
fine fragments (incl. nanofillers) than TPU. To verify that the ultrasound bath is truly
a worst case, the energy input was further increased by ultrasonic probes, resulting
in no significant increase of released amounts, but qualitatively decreasing the entan-
glement of the released MWCNTs (Hirth et al. 2013). Considering that MWCNTs
were found to collapse into a dense layer after degradation of the polymer matrix
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