Classification Strategies for Regulatory Nanodefinitions - Safety of Nanomaterials along Their Lifecycle - page 53

(Bleeker et al. 2013): The “best possible dispersion” achievable during the intended
manufacture and use could be the basis of the measurement. For example, pigments
would be dispersed in the coating where they achieve performance, but indispersible
aggregates would be treated as constituent particles in the sense of the ISO work-
ing item of TC 24/SC 4 which defines “Constituent particles of agglomerates […]
are aggregates of fused together or elsewise combined former primary particles.”
However, an application of this approach to organic and inorganic pigments reveals
that the successful dispersion in Figure 3.2c is an exception, whereas most engi-
neered nanomaterials remain aggregated to diameters factors 2 to 10 above the pri-
mary particle size, and are no longer classified as nanomaterials. In summary, the
“best possible dispersion” approach appears like a pragmatic technical guidance,
but is in fact a major restriction of the scope of the nanodefinition. The best possible
dispersion approach as very first step of risk screening (Chapter 16) is defendable
if a low degree of dispersability is sufficient to suppress other effects due to the
nanoscale structure, such as those from solubility, surface, or reactivity.
3.5 TIERED TESTING FOR COST-EFFECTIVE
SCREENING AND CONFIRMATION
Calzolai et al. have recognized that “At the moment there is no single technique that
can by itself provide a robust analytical method” (Calzolai, Gilliland, and Rossi
2012). Guidance on sample preparation, method selection, and evaluation is required.
A rudimentary scheme of a tiered measurement strategy has been proposed recently
(Figure 3.3) (Brown et al. 2013), and a slightly refined variant has been introduced
in ISO as a new working item (Hayashi 2013). The key to validity is a quick valida-
tion of dispersion quality before measurement, benchmarked on expectations from
VSSA or simple EM. If the size distribution in number metrics is dominated by
constituent particles, an agglomeration-tolerant measurement technique will give
the correct classification even if agglomerates are a significant fraction of the total
mass. Alternative to the validation iteration, a future guidance could refer to the
lowest degree of dispersion that is obtainable in the commercial application, as dem-
onstrated by the example of pigments in coatings (Figure 3.2) and could establish
standardized, sector-specific dispersion protocols.
Note that these schemes aim at the EC nanodefinition recommendation, and hence
do not allow a direct path from VSSA <60 m²/cm³ to “not nano.” Given that for
a wide range of materials the VSSA criterion performs well to identify non-nano
substances, it deserves a more prominent place as a refining criterion to make the
EC nanodefinition practical. Another refining criterion could specify the volume%
below 10 µm or any other cut-off (Brown et al. 2013). At the very least, these refining
criteria can be used to group materials. As an example, one could group all CaCO 3
fillers with less than 50% in volume metrics below 1 µm diameter; only the CaCO 3
substance with the lowest average diameter would be measured by EM to confirm
that it is not a nanomaterial, and this classification applies to the entire group then.
Alternatively, also the VSSA will serve as an excellent grouping criterion since it has
a proven sensitivity to the primary particle size (Gilliland and Hempelmann 2013)
and proven precision (Hackley and Stefaniak 2013). A much more refined decision