AnalysisTire (& Road) wear particles

What are they made from

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In terms of composition there is the fraction of tire material and minerals mentioned above. However, not all tires are made from the same materials. The basic polymers used in tire treads are varying proportions of natural rubber, butadiene rubber and styrene-butadiene rubber. Furthermore, there are fillers such as carbon black and silica as well as additives such as 6PPD. These are used in different proportions in tires for different applications and from different manufacturers.

This makes detecting and identifying particles that have their origin in tires especially difficult.

Many micro-plastics researchers use relatively simple detection methods such as shining light on a series of particles and measuring the spectra of the reflected or transmitted light.

This does not work with tire particles. First, these particles are always opaque, so no light is transmitted. Second, because of their nature (black and rough), very little light is reflected.

In the early research into plastics, the fraction that neither transmitted nor reflected light was assumed to be tire-related.

This is not accurate. Tire-related particles make up only around 5% of the debris on the roads, so assuming all opaque material is tire-related massively over-estimates the proportion of tire materials.

The preferred method for current tire particle researchers is to take the total mass of material under investigation and pyrolise it in very carefully-controlled conditions (this process is completely unlike the pyrolysis in a tire disposal plant) and look for specific markers that can be used to identify target chemicals in the input materials.
Typically, researchers might look for vinyl cyclohexane as a marker for butadiene monomer so it can be used as a marker for both butadiene rubber (BR) and for styrene-butadiene rubber (SBR).

Because natural rubber is a natural compound, the marker (D-Pentane) that has been proposed to identify NR is also found in decaying organic matter, so it becomes more difficult to identify NR in the roadside debris.

Rather than relying on a single marker, researchers led by Elisabeth Rødland at the Norwegian Institute for Water Research (NIVA) propose multiple markers for each of the different elastomers found in tires, and this gives a more accurate picture of the amount of tire-related material in a sample.

Just looking for multiple markers connected with butadiene or styrene, however, is still not enough. Researchers need to identify the markers and then see if the different markers are present in the right proportions to positively identify a tire elastomer, as opposed to some other organic compound.

This is known as finger-printing of a particular compound. Nick Molden, CEO and founder of Emissions Analytics, said, “Single markers just don’t work.”
He said that there is not one single organic compound that appears in every tire his team has tested in measurable amounts.

Even if such a single compound were present in every tire, he noted that the concentration of each compound varies hugely from one tire to another, because each tire maker – and each tire model – varies in its composition between manufacturers and models.

There is no single compound that appears at a given percentage in each tire, so no single compound can be used as a marker for ‘tire material’

It is better, he said, to have a handful of compounds that represents a typical tire and use that as a fingerprint. He said his technique of 2-dimensional GC-MS (gas chromatography – mass spectrometry) identifies over 400 individual compounds. Using single-dimension GC-MS, he said, offers a much narrower range of chemicals that are available to use as fingerprints. Besides, he noted, all the available markers are at the lower end of the molecular weight spectrum, while the defining compounds are usually at the heavier end of that spectrum. And 2-D GC-MS is better at distinguishing between the heavier compounds.

Molden said using a single marker identified in conventional GC-MS will lead to conclusions that have such a high uncertainty that it gives people a false sense of certainty – while a research team might be able to publish a headline number, the error bars around that data will be so wide that no reasonable conclusions can be drawn.
He commented, “it will give the impression that the research is conclusive, when in fact we can’t really draw any worthwhile conclusions from the experiment.”

He said Emissions Analytics is now finalising its procedures with very well-respected and independent metrology and measurement group and will produce results during 2023.

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