If the ‘bio’ components in liquid fuels are not well understood – as discussed in our last newsletter – the composition of vehicle tyres verges on mysterious. Fuels have attracted close focus over the last few decades primarily because of the pollutants, especially particles, released when the fuel is combusted in an engine. More recently, the subject of upstream carbon dioxide emissions from the production of fuel has risen up the agenda with concern about climate change. In contrast, although tyres clearly shed significant amounts of material into the environment, their wear has not been regulated and, therefore, where the tyre particles go and what they do to humans and the environment has generated little interest.
Tailpipe emissions continue to fall and, in respect of particles, leave tyre wear emissions somewhere between ten and a thousand times greater than the tailpipe emissions from a typical car, according to earlier Emissions Analytics’ research. This, together with the increasing weight and torque of new vehicles, has led at last to a focus on tyre wear emissions. As a result, a number of important questions are now being posed.
First, what is the rate of tyre wear emissions in real-world driving, quantified either as mass or number – the latter being important to capture potential large numbers of ultrafine, near-massless particles? Second, to what extent do particles measured in the environment come from tyres? Third, what is the chemical composition of tyres, and how does that differ between brands? Fourth, what pollutants do tyre wear particles leech into the environment once released? This newsletter considers approaches that could be used to build a better understanding on these points.
From previous Emissions Analytics' newsletters – for example, https://www.emissionsanalytics.com/news/whats-in-a-tyre – the total mass loss from the four tyres of a vehicle averages around 64 mg/km when the tyres are new, falling to about 32 mg/km over the full life of the tyres. Approximately 90% of the mass shed is from larger particles up to at least 10 µm in diameter, whereas around 90% of the particle number is ultrafines down to 10 nm and below. Wear rates differ by a factor of around two to three times between the fastest and slowest wearing.
While we have a good understanding of the rate at which tyre wear particles are shed from vehicles, we have little idea where the particles go. Broadly, they will end up in soil or water, although some maybe trapped in storm water filtration systems. The distance the particles travel from the carriageway depends on factors such as the particle mass and they can therefore accumulate on the verge or be blown to neighbouring fields and gardens. Particles that fall to the carriageway typically get washed into drains and end up in rivers and the sea. According to a 2017 report, 28% of primary microplastics in the ocean are from tyres¹. The proportion that become airborne eventually settles on soil or water. Therefore, it should be expected that tyre material is observed in samples of water and soil taken in almost any location.
A potential method for estimating the amount of tyre material in a sample from the environment is to use fingerprinting. The complicating factor is that a tyre particle released into the environment will get mixed up with many other sorts of material. By using a pyrolysis technique – burning a sample in the absence of oxygen – leads the tyre material to break down in a predictable way, creating a ‘pyrogram’. Emissions Analytics has been building a reference database, or ‘library’, of the pyrolysis products from a wide range of new tyres. By matching the patterns from the environment samples, it is possible to start to understand what fraction of the sample is from tyres – a process that is called ‘source apportionment’.
In principle, this can be used not just to see tyre material generically, but also the presence of wear from particular tyre brands, where they offer unique chemical fingerprints. It should be noted that pyrolysis causes some of the compounds to break down – albeit in predictable ways – and therefore chemicals observed after pyrolysis are not necessarily the same as those in the tyre when new, although they may indicate compounds that will be released as tyres naturally degrade over time.
To illustrate the approach, Emissions Analytics tested samples from ten different tyre brands using a pyrolysis front end on its two-dimensional gas chromatography and time-of-flight mass spectrometry laboratory, supplied by Markes International and SepSolve Analytical. This allows excellent separation, identification and quantification of organic compounds in the sample.
The table below shows the top five most prevalent compounds from the pyrolysis of the ten samples. The metric is the ‘peak area’ percentage, which is the area under the peak for each compound as a share of the area under all the peaks on the chromatogram added together. It is not an explicit quantification, but nevertheless a good indication of relative importance.
A number of the most prevalent compounds offer a characteristic odour, including limonene², β-guaiene and longifolene. These come from natural sources and include sweet, woody, dry, guaiacwood, spicy and powdery smells. Most of these compounds are not clearly toxic, although cyclohexane‚ 1‚2‚4-triethenyl- is an irritant to eyes and skin, and desogestrel is a hormone. Two of the most prominent, androstan-17-one‚ 3-ethyl-3-hydroxy-‚ (5α)- and 2-[4-methyl-6-(2‚6‚6-trimethylcyclohex-1-enyl)hexa-1‚3‚5-trienyl]cyclohex-1-en-1-carboxaldehyde, are complex compounds that have unknown characteristics. Overall, these compounds are relatively high-order, complex, heavy hydrocarbons and oxygenates.
One of the brands has a noticeably different mix of chemicals in its composition: Tyre 10. As can be seen in the above table, it only has three of the top compounds present, whereas most of the compounds are present in the other brands. Furthermore, the Tyre 10 sample contained a number of prominent compounds that were unique to it. These included cis-Thujopsene – a terpene that is an essential oil found in conifers – linolenyl alcohol³, eicosapentaenoic acid⁴, sorbic alcohol⁵ and the unknown bicyclo[3.1.1]hept-3-ene-spiro-2‚4'-(1'‚3'-dioxane)‚ 7‚7-dimethyl-.
Therefore, using this approach, the pyrolysis results from environmental samples can be compared to these reference measurements to estimate both the generic presence of tyre wear material, but also potentially the approximate split between different types of tyre. As relatively complex hydrocarbons, they do not generally occur naturally, although it should be noted that there are potentially other sources of some of them, such as from industry. It is therefore important to get as much detail on the tyre composition as possible, in order to uncover specific chemical markers that can be used for robust source apportionment.
The same analyses can be used to probe into the chemical composition of tyres and understand the differences between different tyre brands. The chart below is a plot using ‘principal component analysis’ (PCA) to understand what groups of compounds – the principal components – account for the most difference between the tyres. The aim of the method is to reduce the number of dimensions to something we can easily understand from samples that typically contain over one thousand organic compounds. It trades a little accuracy to aid understanding.
The first principal component accounts for 80% of the observed differences. The second component gives a further 12%, with 4% from the third principal component. In these dimensions, there are three broad groupings: Tyres 1, 2 and 3 are separated from the rest in the first dimension, Tyres 5-10 in the second dimension, while Tyres 4, 5 and 10 separate in the third dimension.
Tyres 1, 2 and 3 are closely clustered and share a number of compounds in common that are less prevalent in the others, including toluene, 1,3-pentadiene and methylenecyclopropane. The first two of these present potential toxic danger if swallowed or inhaled. Tyres 5 and 10 occupy a similar position distinct from the other samples and disproportionately share 2-pentene and 1,4-cyclohexadiene, 1-methyl- in common, which present similar toxic risks. A compound with a leathery smell that can be a respiratory irritant is phenol, 2-(1,1-dimethylethyl)-4-methyl-, which stands out in Tyre 8.
Interestingly, a particular compound, 1,4-benzenediamine, N-(1,3-dimethylbutyl)-N'-phenyl, was present in relatively high concentrations in Tyre 4 and present in all the others except Tyre 5, from which it was absent. This compound is better known at 6PPD. A study published in 2020 in Science6 linked 6PPD-quinone to the widespread deaths of coho salmon in California. 6PPD-quinone is produced by 6PPD reacting with ozone in the air.
The fourth question originally posed was what tyre wear particles released may be leeching into the environment over time. While a microchamber or liquid extraction method may be more appropriate, we can nevertheless get some idea of what could be occurring from the pyrolysis results. The two-dimensional chromatogram below gives a visual idea of the large number or organic compounds in a tyre, with each peak representing one compound.
These compounds can be put into generic groups to assist understanding. In this case, we used two broad categories: aliphatics and aromatics. The aliphatics contain ‘saturated’ (lots of hydrogen atoms compared to carbon) alkanes, alkenes, alkynes, carbonyls and acids. The aromatics are mainly cyclic, based on benzene rings.
The table below shows the relative prevalence of compounds in these two groups. As a broad statement, the alkane group contains many compounds that are often irritants to human organs, whereas the aromatics group includes many suspected carcinogens. Therefore, at a simple level, the lower the proportion of aromatics the lower the human toxicity is likely to be. To be more precise, the individual compounds contained in each group can be identified using this method, which may identify compounds toxic even at very low concentrations.
Overall, this initial data suggests there is good cause for significantly accelerated research on tyre wear emissions. The rate of particle loss into the environment is orders of magnitude higher than from the tailpipe. Knowledge of where the particles end up is currently limited. As tyre manufacturers are naturally commercially coy about the chemical make-up of their tyres, it has been historically hard to assess whether the particles present a material problem of human and wildlife toxicity, as they sit and leech into the water and soil. The method described above is one approach that is emerging that could help accelerate the understanding so badly needed.
References
1. Portals.iucn.org/library/node/46622
2. cyclohexene‚ 1-methyl-4-(1-methylethenyl)- ‚ (S)-
3. 9‚12‚15-Octadecatrien-1-ol‚ (Z‚Z‚Z)-
4. cis-5‚8‚11‚14‚17-Eicosapentaenoic acid
5. 2‚4-Hexadien-1-ol
6. Science.org/doi/10.1126/science.abd6951