Effect across abrasion cycles on a polyurethane coating containing multi-walled carbon nanotubes (MWCNTs). The abrasion process generates multiple release fragments.

Released to

WWTP

Soil

Release rate and forms

Depending on the specific NEP and the kind of stress that is being simulated, very different release rates and forms could be generated, so there is a need to assess each scenario in detail. Due to the large-scale physical effect of the abrasion process, ENMs released from the nano-enabled product are generally still embedded in the matrix fragments. A thermal effect due to friction should also be considered, which could be more or less relevant according to the material under study.

Measurement protocol

In order to evaluate the abrasion impact on ENMs release from nano-enabled products, the process can be simulated in an experimental setting. Depending on the type of abrasion that needs to be simulated different protocols can be followed. Most  protocols were originally developed to test products’ resistance to abrasion, but the same principle can generally be used to evaluate the release. Two of the instruments most frequently used to simulate mechanical stresses are the Martindale and the Taber.

The Martindale produces abrasion by rubbing an abrader (of either worsted wool or wire mesh) in an oscillating circle on the sample. The Taber involves mounting a flat sample on a turntable platform that rotates on a vertical axis at a fixed speed; two abrasive wheels are then lowered onto the sample surface and as the turntable rotates abrasion is produced.  In both techniques, the mechanical stress can be adjusted by changing the abrasion material and the number of cycles. Multiple standardized protocols provide indications on how to perform the assays depending on the product of interest that is being tested and the instrument used.

Apart from the Martindale and Taber, which are commonly used, other instruments also simulate in a controlled way relevant stresses like cutting, drilling, mechanical impact  or linear abrasion. For linear abrasion Crockmeters are typically used, applying a standard pressure and rubbing motion to provide reliable and reproducible test results, each of them having a different effect on the nano-enabled product under study, as well as the abrasion material and the number of abrasion cycles. During abrasion, release effects can generally be observed relatively fast compared with other scenarios (e.g. weathering).

Adaptations can be added to the methodology to provide a higher value to the abrasion release studies.

1. Collecting the released fragments: an analysis of the released fragments (by electronic microscopy for example), can provide information on release forms (e.g. ENM distribution inside released fragments). Elemental analysis, in some cases, may also provide information on the ENMs’ concentration in the released fragments.
2. Performing air measurements: with specialized air instruments, the number concentration of airborne particles can be determined (at multiple size ranges), which provides information on release rates and forms. Fragments released to air during abrasion can also be collected for further analysis.  
3. Simulating wet conditions: Adding a liquid (e.g. water or even artificial sweat) during abrasion may change the release behaviour. In cases where wet conditions may be relevant, by simulating different scenarios (with different liquids and in different amounts) a more comprehensive release evaluation can be obtained.

Read more

Read also

Visit the NanoFASE Library to read summaries:

NanoFASE Report D4.1 Inventory of estimates of ENMs and nano-enabled products value chain

NanoFASE Report D4.2 Release estimations during ENMs and nano-enabled products value chain

ISO 5470-1:2016 Rubber- or plastics-coated fabrics -- Determination of abrasion resistance -- Part 1: Taber abrader.
ISO 5470-2:2003 Rubber- or plastics-coated fabrics -- Determination of abrasion resistance -- Part 2: Martindale abrader.
ISO 21546:2019 Paints and varnishes -- Determination of the resistance to rubbing using a linear abrasion tester (Crockmeter).
Koivisto AJ, Jensen ACØ, Kling KI, et al (2017) Quantitative material releases from products and articles containing manufactured nanomaterials: Towards a release library. NanoImpact 5:119–132. doi: 10.1016/j.impact.2017.02.001