Where and how are tire particles produced?
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As with almost everything related to these particles, the answer is complicated. The easy part is where they are produced.
It is in the interface between tire and road and especially when the tire is exerting forces on the road to help the vehicle change either direction or speed.
This means there are hot spots in the road layout – such as junctions, sharp bends, slopes – where the tires have to work harder to keep the vehicle on track. Tire particle emissions are higher in these hotspots. Emissions tend to be lower on flat, straight roads with a constant speed limit.
The question of how they are produced requires a deep dive into tire science.
When the driver asks the vehicle to change direction or speed, it requires a force between the tires and the road surface. That force is created when the rubber and carcass material of the tires distort. This distortion might be large – such as when the tire is cornering hard – or it can be small, when the tire is decelerating gently.
When the vehicle is interacting with the road in this way, the tire’s structure and materials distort, building up strain energy in the rubber and some of the reinforcement cords.
Larger forces (such as hard cornering) lead to more distortion and more strain energy.
This energy needs to be dissipated. One way to do that is to make new surface (that is, extend small cracks and tears). Another way is to break some of the carbon-carbon bonds in the rubber. Another way is to create heat.
Thus, hard cornering and hard braking generate heat in the rubber, and extend cracks through the rubber and even starts to break molecular bonds within the rubber. These cracks and tears eventually merge and lead to the creation of separate, small particles. That is to say, small particles of the tread material separate from the main bulk of the tire.
It is important to note that these particles are a necessary consequence of providing grip during hard braking or cornering.
They are not, however, the only mechanism for producing grip.
More gentle braking, acceleration and cornering employ other mechanisms of grip. One mechanism is the mechanical interaction of a rough road surface with an elastically-deforming tread.
In this case, the tread deforms around surface asperities in the road surface, and the energy is used to stretch the rubber. As the tire moves on, the rubber recovers, with only a little of the energy lost to heat. In this situation, the car can brake or corner with minimal emissions of particles.