Multi-Walled Carbon Nanotubes (MWCNT), synthetic graphite in tubular shape

Particle size distribution (Granulometry)

Description of key information

Electron microscopy images show that the registrants’ Multi-Walled Carbon Nanotubes (MWCNT) consist of tightly bound agglomerates consisting of tangled tubes. In dispersions, the median diameters of these agglomerates are in the range of 533 - 569 µm with a D90 in the range of 679 - 945 µm (analysis with laser diffraction) (Voetz, 2010a; Voetz, 2011a). These data correspond to results obtained by digital microscopy  where the median diameter was 842 µm (D90=995 µm (Voetz 2011d).

The size of the agglomerates is not significantly reduced when MWCNT is aerosolised in the dry state and analysed with a Malvern particle size analyzer at pressures of 1 and 4 bar. Depending on pressure, the mean particle diameters are in the range of 85 to 427 µm (D90: 228-1172 µm) and the inhalable fraction (under 10 µm) is very small (0 % at 1 bar and 0.19 % at 4 bar) (Lecloux, 2011b).

The tubes within the MWCNT agglomerates can be described as short, thin and tangled. Specifically, they display an outer tube diameter distribution of at least 90 % under 30 nm (D90 <= 30 nm). Single results were D90 = 18 nm, 24 nm and 12.7 nm. The mean outer diameter range was 10 nm, 13.4 nm and 9.2 nm (Voetz, 2010b; Voetz, 2011b; Lecloux, 2011c).

The tube length distribution of MWCNT was measured by Transmission Electron Microscopy after dispersion in aqueous medium after sonication. According to the World Health Organization (WHO) any particle > 5 μm long with an aspect ratio equal to or greater than 3:1 and a diameter < 3 μm is considered a fiber. The mean tube length was 380 - 902 nm (D90 = 980 - 1820 nm). In all cases length D90 was lower than 5 µm (Voetz, 2010c; Voetz, 2011c; Voetz, 2010d; Cox, 2008).

Additional information

Only studies performed with Multi-Walled Carbon Nanotubes (MWCNT) meeting the form described in section 4.5 of the IUCLID dossier are considered to be relevant for this submission.

Although a large number of studies on MWCNT have recently been published, clear characterisation of the test materials including sample preparation is often missing although this is quite essential for ensuring reproducibility and reliability of a study. Importantly, modifications of some specific characteristics of the MWCNT tested such as variations in tube diameter or length distribution might give rise to differences in the (eco-)toxicological profile. 

Effects of different MWCNT seem to depend on the form (length) and physico-chemical properties (metal content, aggregation/agglomeration, surface chemistry, and functionalisation). Thus, for the time being a case-by-case approach is appropriate and the hazard assessment for MWCNT meeting the form described in section 4.5 of the IUCLID dossier is based solely on the information collected with this form of MWCNT. That is the reason why a comprehensive documentation is restricted to all available and relevant data on MWCNT meeting the form described in section 4.5 of the IUCLID dossier. That means on the other hand that the values used for any risk assessment should not be used for all MWCNT or for CNT in general, as the results obtained with one particular type of MWCNT may not necessarily be relevant for other CNT with other dimensions and properties.

However, the information on MWCNT not meeting the form described in section 4.5 of the IUCLID dossier (because the differences can be tracked back to the original papers or because the characterisation of the materials was unclear or incomplete) is also included in the IUCLID dossier but the entries are restricted to information on the data source.