AnalysisTire (& Road) wear particles

Industry briefing on T(R)WP

Because the issue of tire (& road) wear particles (“T(R)WP”) is now such a hot topic across the industry, we have prepared some briefing notes on the subject.

We felt this was necessary because a lot of the commentary in the public domain contradicts the latest science and appears to be confusing – and possibly meant to confuse.

In preparing these notes, we spoke to a series of scientists working for academic institutions, government research agencies and specialised commercial research companies as well as tire industry representatives.

There is a strong view among the more independent researchers that the tire industry is responsible for much of the confusion and misunderstandings in this field.

The tire industry position is set out clearly at this website: Be aware that this project is funded entirely by tire makers and that independent researchers dispute many of the claims made there.

A less controversial perspective from the ETRMA is set out at this website.

We set out to answer these key questions

Click on any of the links above, or below to jump straight to that article:

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Before getting into those answers, it may be useful to offer some background information.

What is a tire tread?

A typical car tire will emerge from the factory with a tread package about 10mm thick around the outside of the tires. That is made up of the outer tread that has a pattern made up of grooves 7mm – 8mm deep and a further thin layer of under-tread. Typically this tread package comprises some polymers: styrene-butadiene rubber (SBR), butadiene rubber (BR), and some natural rubber (NR). Other ingredients include fillers such as carbon black and silica and a range of other chemicals.

In a truck tire, the tread package is deeper and wider and comprises some 15kg of rubber compound. That is replaced typically every 18-24 months either in a retread procedure, or when fitting a new tire

The tread is designed to do a couple of things. The first is to clear water away from the contact patch, so that the rubber can directly contact the road surface. That is the job of the pattern of grooves moulded into the tread.

The most important job of the tread, however, is to transmit forces between the wheel and the road surface. Those forces allow the vehicle to change its momentum. That is seen either as a change of direction in a bend (lateral grip), or a change of speed (accelerating and braking) (in-line grip), or a combination of the two.

Because of the nature of tires and roads, these forces result in the gradual abrasion of the tread rubber over time. So a third aspect of the tread compound is that it is designed as a sacrificial surface. It is designed to wear away as a key part of its function of delivering the vehicle’s ability to steer, accelerate and brake.

Precisely how these forces cause the rubber to wear away is still not fully understood, but we attempt a simplified explanation in this related article.

An additional factor is that during the interactions between tire and road, the road also wears away. Particles produced by this combined wear activity vary in composition between rubber (mostly from tires) and minerals (mostly from the road surface). Some are more or less pure rubber; others are more or less pure mineral, with every compositional distribution in between.

Road surfaces tend to be composed either of concrete, or of asphalt. Asphalt is made up of a heavy oil fraction, known as bitumen, mixed with stones and other minerals in the form of aggregate.

This situation is further complicated by the fact that some road surfaces use a type of asphalt in which the bitumen is mixed with a styrene-butadiene-styrene (SBS) material designed to improve the life and drainage of the road surface. This SBS material is indistinguishable from the SBR used in tire treads – at least when using the current generation of analytical equipment.

Once these particles have been produced at the tire-road interface, they enter the environment. Small particles (typically below about 10µm) become airborne, while most of the larger particles fall onto the road surface.

On urban roads, where speeds are low, the particles usually remain close to where they were shed. On highways where speeds are higher and there is more turbulence from passing vehicles, they move more quickly to the side of the road.

Not all the particles that fall onto the road surface remain there. A following vehicle might drive over the particle, creating a new interaction with the road surface, often giving the particle impetus to fly up into the air once more (called re-suspension).

This process can happen multiple times and each new interaction can lead to either further fragmentation of the particle, or a change in composition (often adding more minerals), or both.

There appears to be no good research on how often previously-shed particles are driven over, but one might expect it to happen more in urban streets with high traffic densities.

One area for research, therefore, might be the variation in composition of these particles in urban environments, compared with highway sites and how that composition and mass distribution varies over time.
Once in the environment, however, it becomes difficult to keep track of them. One estimate suggests that these T(R)WP make up around 5% of all the roadside debris.

However, given the number of vehicles on our roads, these T(R)WP are one of the biggest sources of microplastics in the environment.

A range of research reports offer emissions rates of 0.5kg/person/year up to 5.5kg/person/year. Even at the lowest estimate, that translates to some 3.5 million tonnes worldwide, each year.

However, there is a consensus emerging that the amount is closer to 5 million tonnes per year.

That is a lot of micro-plastic pollution. The European Commission puts them as the second biggest source.
The smaller particles become airborne and can be breathed in. The heavier particles are eventually washed away down drains, or end up as debris at the sides of roads, or find their way into cracks in the road and remain there, contributing to the road surface.

Once the particles find their way into the drainage system, they might follow a number of different paths. Across Europe and in other regions, highway engineers tend to build settlement ponds close to the highway where road run-off (typically from rainfall) is directed. In urban environments, the road run-off, together with particles finds its way into the sewage system. Where there is no water management system, they can also find their way into fields and streams surrounding the road.

From there some – though it is not clear how much – finds its way into rivers and oceans.

The tire industry says the particles are dense and so sink into the river beds and are not transmitted to oceans.
That appears to contradict research in those settlement ponds. That research shows particles are still in the water column many days after being introduced to the ponds.

There is currently a great deal of discussion about the toxicology of these particles.

The most damning evidence is the poisoning of large numbers (up to half the total population) of Coho salmon and other salmonid species in the Pacific North-West of the United States due to the negative effects of 6PPD-quinone on the blood-brain barrier. This 6PPD-quinone is an oxidation derivative of 6PPD, an anti-degradant used extensively in tires.

Recent research has found 6PPD-quinone in human urine. Additionally, more evidence is emerging of the load they add to the environment (see related article).

One of the great challenges of this field is that there are very few agreed procedures and metrics.

When a research team seeks to compare datasets they often develop their own metrics for doing that. A different team might develop a similar set of metrics, but the two cannot necessarily be compared easily.
All the researchers we spoke to said this is something of an issue at present, but that in reality, the range of techniques is quite limited, so it is possible to compare the results of different datasets with some degree of confidence, even if the actual numbers are not directly comparable.

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