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

Endpoint summary

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.

Hydrolysis:

Due to their structure, MWCNT are not expected to be hydrolysed in water.

Biodegradation:

A biodegradation test is not suitable for MWCNT. The substance is inorganic and will not be degraded to carbon dioxide. However for research purposes studies have been performed.

Bioaccumulation:

Following current ECHA guidance chapter R7c, a substance is unlikely to be considered as bioaccumulative when it has a molecular weight > 1100 g/mol and a diameter Dmax average of > 1.7 nm. This is based on substances with high molecular weights (typically > 700 g/mol) and/or large molecular diameters (typically > 1.5 nm) being unlikely to be absorbed and to bioconcentrate.

The structure of Carbon Nanotubes (CNT) is a carbon layer similar to graphite which is additionally rolled to form tubes. Multi-Walled Carbon Nanotubes (MWCNT) are large molecules consisting of typically several million carbon atoms with a high molecular weight.

The MWCNT covered in this registration dossier of the form defined in IUCLID section 4.5 exceed the above referenced value for molecular weight by several orders of magnitude and the molecular size criterion by over an order of magnitude for the individual tube strands and the molecular size criterion by several orders of magnitude based on the tangled tube networks. Therefore, the potential for adsorption and bioaccumulation is negligible. Consequently, testing for bioaccumulation is unnecessary.

Adsorption / desorption:

Substances tested with OECD 106 or 121 should have a certain water solubility and an analytical method which is sensitive enough is required. The water solublity of carbon nanotubes is very low (< 2 mg/L; limit of quantification). Additionally and although research efforts are ongoing, no standard analytical method is available to sensitively identify and quantify carbon nanotubes in aqueous solution or on soil.