housands of chemicals derived from crude oil, many of them volatile organic compounds, apparently.
This matters because tyres shed a lot of material into the environment. Emissions Analytics’ estimates suggest that around 300,000 tonnes of ‘rubber’ are released every year from passenger cars in Europe and US alone, the equivalent of over forty million brand new, entire tyres. These particles go into the air, soil and watercourses. If you were to stack all the tyres manufactured in the world in a year on their side, it would reach the moon.
While there is a tyre labelling scheme in the EU, it only rates rolling resistance, wet grip and noise. These are clearly vital to the performance and safety of tyres, but it leaves the ratings blind to the environmental consequences of the tyre wear emissions. There are restrictions on toxic chemicals that can be included in the manufacture of tyres under the European REACH regulations, but the number of chemicals affected is limited.
While we have been occupied with reducing exhaust emissions to control air quality problems, other sources of pollutants have not received the same attention historically. Now that tailpipe emissions of modern internal combustion engines (ICEs) in both Europe and US are generally well below regulated limits for pollutants, focus is now turning to ‘non-exhaust emissions’, which cover tyres. Emissions Analytics’ testing shows that, in normal driving, tyre wear emissions are about one hundred times greater than tailpipe particle mass on a modern ICE vehicle. In legal but extreme driving, enough to reduce significantly the lifespan of a vehicle’s tyres, that factor increases to around one thousand.
In addition to tyres, non-exhaust emissions cover material from brake and road wear, as well as resuspended solids, whipped up from the carriageway by the moving vehicle. Of these, tyre wear emissions are probably the largest and fastest-growing component. Brake wear emissions are forecast to fall as regenerative braking becomes more widespread. Road wear and resuspension rates are only partly related to the passing vehicle, including its weight, but are probably more determined by the road material and condition, and what particles are blown onto the road from multiple surrounding sources. Tyre wear emissions are likely to grow as vehicles continue the long-term trend of becoming heavier, although this may at some point be offset by using more lightweight construction materials.
Understanding tyre wear emissions provides a challenge as they are heterogenous. Unlike, for example, nitrogen oxide (NOx), which is a unique compound that can be measured as a mass or volume, particles from tyres come in an infinite combination of shapes, sizes and densities. Moreover, the particles are made up of a wide array of chemical compounds, and these chemicals may also stick – or adsorb – to the surface of the particle. In this way, particles can act as the distribution vector for other compounds.
An emerging approach to characterising tyre wear emissions is, therefore, to measure both the wear rates and chemical make-up of the particles. This enables a quantification of the amount of individual chemicals that are released into the environment. This information can then be put together with toxicity ratings to assess the potential effect on human health, wildlife and biodiversity. For the semi-volatile organic compounds particularly, the effect of these on secondary organic aerosol formation – in other words, gaseous emissions that condense to become airborne particles over time – can be evaluated.
To begin to understand the degree and nature of the tyre wear emissions problem, Emissions Analytics recently tested a range of different tyres. Full sets of tyres of eight different brands and types were selected and installed on the same test vehicle, a 2012 rear-wheel drive Mercedes C-Class. The wheel alignment and tyre pressures were checked. Each set of tyres was driven for over 1,000 miles, around 90% by distance being conducted on the motorway. The four wheels – i.e. leaving the tyres on the rims to avoid damage – were weighed at the start and end, and the distance-specific loss of mass was calculated. The results are shown in the chart below.
Across the brands, the average mass loss was 64mg/km for the vehicle, adding all four tyres together. Wear on the rear tyres was greater, accounting for 71% of the total on average, strongly influenced by this being a rear-wheel drive car. The wear rate on the fastest abrading tyre was 2.3 times higher than the slowest. Therefore, tyre choice by manufacturers and consumers can have a material impact on emissions rates.
The wear rate is faster when tyres are new, for the first few thousand miles. Thereafter, the wear rate appears to decline at an approximately logarithmic rate. Over a lifetime, therefore, the average wear rate may be half the figures above. If we assume that the average vehicle travels around 16,000km per year, the rates above mean that each car sheds around 0.5kg per year on average over its lifetime. As there are almost 600 million vehicles in Europe and the US, this is equivalent to 300,000 tonnes of particles. An average tyre weights around 8kg, hence the total amount shed is equivalent to almost 40 million whole tyres. These figures do not include tyre wear from heavy-duty vehicles, which would also be significant.
The effect of particles on human health and the wider environment is an on-going and active area of research. It is complex to isolate the causal links. In terms of air pollution, it is generally accepted that there is a connection between particle mass emissions and diseases such as cancer and heart disease from prolonged exposure. The effects in terms of particle number are less clear-cut, although the EU regulates these at the tailpipe from a precautionary motive. The aim of this newsletter is not to review the evidence, but rather to contribute early findings on the chemical composition of tyres studied so far by Emissions Analytics.
Tyres are highly-engineered products and made up of a complex mixture of substances. For light-duty vehicles, the majority of the content of the tyre tread and walls comes from crude oil derivatives, with only a minority of natural rubber. Therefore, to understand the composition of tyres, it is necessary to employ a technique that can separate these out. We decided to focus on the organic compounds rather than metals, and employed our two-dimensional gas chromatography equipment coupled with a time-of-flight mass spectrometer (GCxGC-TOF-MS from SepSolve Analytical and Markes International, see https://www.emissionsanalytics.com/tyre-emissions). The gas chromatograph achieves separation by passing a sample through a long ‘column’, and the mass spectrometer does the compound identification. Two dimensions, both of time, are required to separate compounds that ‘elute’ in the same place in a one-dimensional chromatogram. Taking an example tyre, we heated samples to 100˚C and analysed the compounds released to obtain the following two-dimensional chromatogram.
Broadly, compounds cluster in different areas depending on common chemical characteristics. Some frequently used groups are illustrated above. Alkanes (e.g. pentane) typically affect the lungs, liver, kidney and brain. Cycloalkanes (e.g. cyclohexane) lead to headaches and dizziness. Terpenes (e.g. limonene) are generally less problematic and are responsible for aromas, unlike aromatics (e.g. benzo(a)pyrene), which are often carcinogens, as are nitrogen-containing compounds (e.g. quinoline). This is a significant simplification for the purposes of illustration.
Each shaded area on the chromatogram indicates a distinct chemical, with the intensity of the colour reflecting its abundance. The measurement breadth of the equipment is from compounds containing two carbon atoms (C2) to at least C44. This covers what are called volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs). Just this one sample contains well over a thousand distinct compounds of these types.
The next stage is to identify as many of the compounds as possible and understand if they are problematic for health or the environment. Zooming in on the nitrogen-containing part of the chromatogram, it is possible to identify a number of potentially problematic compounds, as shown below.
N-Phenyl pyrrole, quinazoline, 4-tert-butyl-2-chlorophenol at certain levels of exposure can lead to symptoms in humans including skin, eye and respiratory irritation. In addition to these effects, quinoline and 3-methyl-quinoline have potential carcinogenicity and mutagenicity in humans. Quinoline and 1,2-dihydro-2,2,4-trimethyl affect aquatic environments more than humans.
This suggests that potentially concerning compounds are present in tyres, but if we compare the composition of different tyre brands it is also possible to see that the mix of chemicals differs. This reflects the many formulations used by different producers, but also means that tyre selection can lead to different environmental and health effects. The chart below illustrates the point by comparing four different tyre types. Each sample was pyrolysed to release as many compounds in the underlying materials as possible, and then analysed using Principal Component Analysis.
Tyre Brand B is strongly differentiated from the other three tyres by the presence of 1-methyl-2-pentyl-cyclohexane – a cycloalkane. Although it does not have any particular toxic indications for humans, it is potentially possible to relate the presence of this defining compound to other characteristics such as rolling resistance, noise or wet grip.
Where toxic compounds are identified by this approach, it does not guarantee that they are present in amounts that could cause harm. Therefore, the final stage is to quantify each in the sample, so the total amount in a tyre can be worked out. However, even if the amounts are small in one tyre, due to the large amount of material released each year in total – as calculated above – even low concentrations could lead to deleterious effects at the macro level.
All in all, this initial testing has demonstrated that it is possible to measure tyre wear explicitly, without it being combined with brake or road wear, and the separation capability of the two-dimensional gas chromatograph can help identify thousands of constituent compounds. The results themselves then show that there are relevant and material differences in the wear rates and chemical make-up of different brands and models of tyre. Therefore, choices of tyre when the car is first sold and at subsequent tyre changes are directly relevant to the vehicle’s environmental impact, and requires deeper and urgent study.