Methods for the Measurement of Release of MWCNTs from MWCNT-Polymer Composites
Debra L. Kaiser, Aleksandr B. Stefaniak, Keana C. Scott, Tinh Nguyen, Jurg Schutz
Multi-wall carbon nanotube (MWCNT)/polymer composites hold great promise as advanced materials for consumer and industrial applications. Throughout the life cycle of these composites, there is opportunity for release of particles that may contain MWCNT and/or free MWCNT. To fully understand the potential for exposure from release events requires validated sample collection, preparation, and analysis techniques. The current state-of-science is limited to adaptation of techniques that were originally developed for studies of micron-scale particles and none have been validated for nanomaterials. The most commonly utilized sample collection techniques involve capture of airborne particles on filters or other substrates such as microscopy grids. The exact sample preparation method is dependent upon the chosen analytical technique, though it is critical that any manipulation preserves the native properties of the MWCNTs as they were in the media to which they were released. Sample analysis is a two-step process. The first step involves detection - a qualitataive method to answer (yes or no) whether MWCNTs are released from MWCNT-polymer composites. The second step involves quantification, which is an enumeration of the number or mass concentration of MWCNTs in or released from MWCNT-polymer composites per unit volume or area of polymer matrix. Quantification of released MWCNTs, particularly those in polymer fragments, is challenging and available methods that can only yield a qualitative measure of concentration are limited to electron microscopy. Though quantitative, electron microscopy is tedious and expensive and suffers from various limitations. Going forward, there are several critical research gaps that need to be filled to enable accurate measurement of MWCNTs released from polymer composites and include improved (or new) measurement methods, inter-laboratory studies of release scenarios, development of test protocols, and standardization of methods.