Release from Composites by Mechanical and Thermal Treatment - Safety of Nanomaterials along Their Lifecycle - page 256

the abrasion wheels and wear rate is often measured as a function of mass loss by
the sample.
Vorbau et al. (2009) developed a method based on the Taber Abraser (Model 5131,
Taber Industries, USA) for the characterization of abrasion-induced nanoparticle
release into air from surface coatings due to its wide spread use and description in
many national and international standards. The stress of the Taber test corresponds
to the typical stress applied to surface coatings in a domestic setting, for example,
when walking with sandy shoes on a floor surface. The Taber Abraser consists of two
abrasion wheels, which are moved by the rotation of the sample (Figure 12.6). The
abrasion is caused by the friction at the contact line between the sample surface and
the abrasion wheels. The samples are rotated at 60 rpm with a normal force of 2.5 N
and the stressing cycles were 3 × 100 revolutions. The abraded zone is an annular
area and the abraded material is removed continuously with a stationary exhaust
device. The wear rate is determined by weighing the clean sample at the beginning
and at the end of the Taber test.
A sampling hood was realized as a small box, which almost completely encap-
sulated the sample suction zone behind the abrasion wheel on the surface coating
(Figure 12.6, right). A CPC (Model 3022, TSI, Inc.) and an SMPS (Model 3934,
TSI, Inc.) were connected to the sampling aerosol flow from the sampling hood.
The test objects were sample carriers with three different coatings that contained
nanoparticles (ZnO) or no nanoparticles. For each coating four different samples
were examined. The results showed that during the abrasion tests the released par-
ticle mass depends on substrate and coating, but there is no significant correlation to
nanoparticle content. Furthermore, the transmission electron microscope (TEM) and
energy dispersive X-ray spectroscopy (EDX) analysis showed that the particles <100
nm remain embedded in the coarse wear particles.
Schlagenhauf et al. (2012) investigated the released particles and the possibility
of releasing CNTs from an epoxy-based nanocomposite (with 0, 0.1, and 1 mass%
CNT) by simulating a sanding process with a commercial Taber Abraser. A setup
was developed where the abraded particles are characterized by aerosol measure-
ment devices APS, FMPS, and SMPS directly after the abrasion. After collection on
filters or grids, the abraded particles were analyzed by SEM, TEM, and EDX. For the
particle measurements and collection only one of the two abrasive wheels was used
whereon a small chamber was designed (Figure 12.7). During the tests the samples
rotated at 60 rpm with a normal force of 7.5 N.
In this study, release of particles smaller than 100 nm was not observed. The
detected smallest particle sizes were 300-400 nm. A trend of increasing size with
increasing nanofiller content was observed. Imaging by TEM revealed that free indi-
vidual CNTs and agglomerates were emitted during abrasion.
A similar setup was chosen by Guiot et al. (2009) (Figure 12.7, right), which was also
tested for its measurement efficiency depending on the abrasion speed (0.05-0.2 m/s)
using a CPC. They injected 6 nm Pt particles into the abrasion zone and defined 100%
as the number concentration at a rotation speed of 0. The rotation speed was stepwise
increased to 0.2 m/s and they found a decrease in the concentration to about 80% of
the initial concentration. This study reveals that evaluations of test setups for different
conditions are needed in all cases to ensure that comparability is achieved.