This SOP describes how to detect and quantify the release of nanoparticles from surface coatings into the air using a mechanical process that employs abrasion to simulate sanding. A material containing nanoparticles will be physically abraded and the materials released will be collected in a custom abrasion testing system. They will then be characterized by different methods such as Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM) and other methods.
Several recent studies have attempted to bridge the critical knowledge gap of nanomaterial release from nano-composite materials using mechanical abrasion techniques. Some showed that release of carbon nanotubes (CNTs) from a nanocomposite material required a combination of UV-induced weathering and physical abrasion. Other studies found some level of individual nanomaterial release under heavy mechanical stress, while others found no individual nanomaterials released under similar mechanical stress conditions. Many of these studies identified difficulty in generating reproducible results during these mechanical abrasion studies due to variability in material, abrasion and aerosol sampling as a significant issue in determining release of engineered nanomaterials (ENMs).
These difficulties point to the necessity of identifying a standardized procedure for abrasion testing of ENM-containing materials. One of the most common techniques relies on adaptations to the Taber abraser, a sanding simulation device. The standard test is performed with the sample being rotated while in contact with two abrasive wheels moving in the opposite direction. Particle release depends on the surface coating and substrate material used.
After particles are released, proper characterization is essential to determine the potential hazard of ENMs that may be included in the released material. The particle number density is characterized with a condensation particle counter (CPC), while a fast mobility particle sizer (FMPS) determines the particle size/mass distribution. Light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscope (TEM) are three techniques to further analyze the characterization of the wear particles. These procedures can also be coupled with energy dispersive x-ray spectroscopy (EDS) for chemical microanalysis.
One advantage of abrasion testing is it can provide data in a matter of minutes compared to the years that may be required by in-use testing. The results can be used to inform risk decisions based on the mass of material released, the number of nanoparticles, the size and composition of the particles, and the rate of their release. There are some limitations associated with abrasion testing, such as limited shear rates, potential clogging of the abrasion material due to pickup of test material, and potential contamination of testing due to interference by abrading material (e.g., an Al2O3 wheel). However, these limitations can be overcome significantly through test design and operational practices to ensure that the data represents realistic release scenarios.
The accompanying figure shows the general experimental setup. A 12 inch × 8 inch × 16 inch cabinet houses the abrasion testing system, which includes a commercial lathe with attached sample-holder, an aluminum disc with replaceable sandpaper attached to the face contacting the sample at the end of a rod with platform for adding mass (to increase normal force during abrasion), inlets for introduction of HEPA-filtered air into the cabinet, and outlets for particulate sampling/ collection. The inlet airflow is controlled to 20 lpm, matching or slightly exceeding the flow out to the instruments and collection filters. For this setup, the CPC has a flowrate of 0.7 lpm, the FMPS has a flowrate of 10 lpm, and the two in-line filters have flowrates of 4.5 lpm each, for a total sampling rate of 19.7 lpm. The airborne particles from the cabinet are passed through a large impaction chamber to allow settling of large particles, then into a 4-way distribution manifold that splits the flow to the instruments and filters.
It is recommended to run at least three tests on each material to help identify the presence of outlier abrasion tests and establish a more confident representation of the release of each material.
This work was done by Monica A. Ramsey, Jonathon Brame, Aimee R. Poda, Michael Cuddy, and Robert D. Moser for the Army Engineer Research and Development Center. ERDC-0001
This Brief includes a Technical Support Package (TSP).
Abrasion Testing of Products Containing Nanomaterials, SOP-R-2
(reference ERDC-0001) is currently available for download from the TSP library.
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