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07/11/2014
TIRES AND NANOTECHNOLOGY, A PERFECT MATCH

Small items for major improvements


Nanomaterials, already found in tires, promise to improve all the tire’s characteristics in ways unthinkable nowadays   



Nicodemo Angì

Whether it is launching a GT car at breakneck speed, or moving a very efficient electric vehicle or taking an off-road car in places challenging even for an ibex, our tires are ready for any task that we want to entrust them with.

Vulcanization is now 175 years old and the industry has had time to develop sophisticated production techniques that have brought us to equip our cars with modern, highly efficient tires.
Yet the games are anything but over because nanotechnology promises to expand the horizons for many industries, including the rubber industry.

Many interesting news are found in a report produced by the OECD (the Organization of Economic Co-operation and Development) that studies how nanotechnology will affect, during the next few years, the tire industry.

Nanotechnology can be defined as technologies that can manipulate and build structures smaller than 100 nm (1 nm = 1 billionth of a meter).

 

From the infinitely small to large perspectives       

The research examines tires made for light and heavy commercial vehicles as well as cars, sectors covering about 90% of the total tire market; the focus is on Carbon Black, conventional highly dispersible (HD) silica and two emerging nanomaterials: nanoclays and new generation HD, high surface area (HD-HS) silica.

These four are the most promising nanomaterials because there are a lot of data concerning them, they have been studied by many different manufacturers, and will be used significantly in the coming decades since they promise the greatest improvement in performance.
It is hardly necessary to point out that tires must meet three equally important requirements (along with other lesser ones): low rolling resistance, high abrasion resistance and an appreciable grip even on wet roads.

Each tire is necessarily a compromise between these properties: high resistance to abrasion, for example, is difficult to reconcile with a good grip in difficult conditions; nanotechnology, instead, can improve them all at once.

In fact some nanomaterials are already used: for example, Carbon Black and Silica.

The particle size, structure and surface area of carbon black plays a significant role: in general, the smaller the particles the greater the performances since they have a higher surface to volume ratio.

Increasing Carbon Black, however, increases also the tire’s rolling resistance and weight, while improving traction and abrasion resistance.

It should be noted that carbon black comes in particles ranging from 5 to 100 nm, but these particles tend to aggregate into larger structures.

The use of silica, on the other hand, has made it possible to obtain an improvement in rolling resistance compared to carbon black alone, without compromising the tire’s wear resistance and grip.

To understand how this improvement was possible, a little digression is needed to talk about hysteresis, a property that affects many materials subjected to periodic stress.

 

Pressure and release are not the same      

Rubber is a viscoelastic material in the sense that it combines elasticity (the ability to store energy when it is compressed and then return it when released) with a non-negligible internal friction due to its partially viscous nature very different from the crystalline nature of metals.

A metal spring is made up by a regular structure of atoms, and is therefore able to return almost completely the energy stored during deformation, while a rubber element, rich in long and twisted polymers, dissipates a rather significant part of the energy received.

Hysteresis accounts for this non-symmetry between the compression and return energy cycle.

Rolling resistance is connected to the deformation of the tire at the point of contact with the ground: a rather large deformation, but with low frequency (a wheel’s rotational speed is in the order of hundreds of revolutions per minute), and in this case the hysteresis must be low to disperse as little energy as possible.

Grip, on the other hand, is linked to micro-strains formed between the tread and the roughness of the road surface: this is a high-frequency phenomenon, and in this case, it is a positive one because the high hysteresis allows the tread to adjust to all the small irregularities of the tarmac.

Silica is best suited for the two conditions: having, compared to carbon black, a greater hysteresis at higher frequencies and a smaller hysteresis at lower frequencies, is able to improve both rolling resistance and grip.

Well, today, Highly Dispersible (HD) Silica is used, with improved properties over standard silica but, as anticipated, it will probably have to give way to High Surface Area (HD-HS) Silica, which, due to its very low tendency to agglomerate and its higher compatibility with natural rubber, will also allow heavy vehicles to benefit from the improvements in terms of grip and rolling resistance while increasing wear resistance.

 

Future materials                                                 

Even "nanoclays" can have their say, as they increase the impermeability of the inner lining, greatly improving the stability of the tire’s pressure. Tires can thus work at the designed pressure, reducing wear, temperature, and rolling resistance; however, they are difficult to work with and, therefore, their introduction will be slower compared to HD-HS silica.

Even more futuristic are nanomaterials such as graphene, carbon nanotubes, silicon carbide, the NanoPro Tech (an extreme lightweight filler, lighter even than carbon black and silica), and even Nano-Diamonds.

These materials promise significant improvements (for nanotubes and graphene there are talks of life span comparable to those of the vehicle!), but their production processes are not reliable as yet, and - for some – there are negative implications on the environment.
In any case, the path is marked: we expect great improvements from these infinitely small materials. 

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