Mechanisms of Aging and Release from Weathered Nanocomposites - Safety of Nanomaterials along Their Lifecycle - page 327

the advantage (just like SMPS for atmospheres) to provide both size distributions
and size-classified quantification in mass metrics (Wohlleben 2012). Calcination/
thermogravimetry can be used to identify the nanofiller content in run-off waters
at the expense of losing information on fragment morphology (Vilar et al. 2013).
Experimental techniques to characterize nanofiller release from polymer nanocom-
posite weathering are included in Figure 14.1. Standardized methods for larger frag-
ments such as laser diffraction are not applicable to immersion or run-off waters due
to very limited amounts of particles.
14.5 CONCLUDING REMARKS
Polymer nanocomposites are stronger, tougher, lighter, more dimensionally stable,
less permeable, and more durable than neat polymers or traditional polymer com-
posites. In coming years, polymer nanocomposite products will enter the consumer
markets in large quantities, and they will be increasingly used in essentially every
segment of the industry from textiles, buildings, sporting goods, electronics to aero-
space. Whatever the application, both the long-term weathering performance of the
nanocomposites and the fate of the nanofillers in the matrices during the products'
lifecycle play a key role in the widespread uses of these products. In this chapter, we
have briefly presented recent advances in the weathering of polymer nanocomposites
and the weather-induced release of nanofillers. Extensive literature data indicated
that the presence of nanofiller either decreases or increases the degradation rate of
the host matrix. Regardless of the effect, the overwhelming conclusion is that UV
radiation of the weathering environments will degrade the matrix and the nanofillers
will be exposed on the composite surface, and eventually released to the environ-
ment spontaneously or induced by other external factors such as mechanical forces,
rain, hails, and snow.
The process of nanofiller surface accumulation and release is summarily illus-
trated in Figure 14.7 for, as two examples, polymer/nanosilica and polymer/MWCNT
composites exposed to solar UV. Under this weathering environment, the matrix
layer near the surface will first undergo photodegradation and is removed. The loss
of the matrix surface layer will result in a gradual increase in the number of nanofill-
ers on the composite surface with increasing exposure time or increasing radiation
dose. For some nanofillers, at a critical thickness/concentration, they will release
spontaneously, whereas for others such as MWCNTs (Kingston et al. 2013), where a
entangled network is formed on the surface (Figure 14.7b), additional forces such as
mechanical forces will be required to liberate the surface exposed nanofillers from
the nanocomposite surface.
The eroded surface layer of polymers without UV stabilizers was found to release
fragments on the order of 1 g/m²/year (corresponding to a 1-µm layer thickness),
depending primarily on the degradation of the polymer matrix and the nanofiller
shape, and also on the filler-matrix compatibility and on shear during use. If we
assume a mm-to-cm thickness of typical polymer structural parts in cars of consum-
ers products, the total nanoparticle release measured throughout a 10-year use phase
corresponds to 0.001% of the entire polymer product (Wohlleben et al. 2014).