Nanomaterials, a tire's "biggest" friends
The use of this technology represents a challenge for many manufacturers in their quest for high-performance tires.
As we know, tires are much more than just a few elastomers, natural or synthetic, put together: in fact, scores of other materials and substances are involved in the making of a tire.
Some are visible to the naked eye due to their significant size. Belts, plies (which are now practically always made of synthetic materials rather than natural fibres) and steel beads can be clearly seen when dissecting a tire.
However, even though some elements are too small to be seen, they are still of the utmost importance and we are not just talking about the elastomers’ macromolecules.
Small is beautiful
Fillers, in fact, play a vital role in transforming a very soft material into a compound capable of ensuring high performance in terms of grip, comfort and wear resistance.
Mineral fillers, whether synthetic or natural, are universally used for their ability to improve strength and stiffness, but the effectiveness of their performance depends on several factors such as the size and shape of the particles, their ability to disperse (and the direction they take) in the rubber matrix, how they stick to polymeric chains and filler-filler interactions.
Scaling down the size, we enter the field of nanomaterials and here we find particles with an average sizes ranging between 1 and 100 nm (1nanometer = 1 nm = 1 millionth of a millimetre) are very useful. Filler particles of this size can in fact substantially improve the properties of a compound even in small amounts. Some nanomaterials also display additional or different properties compared to larger particles of the same substance.
Bounce or cushion?
While carbon black has been around since the dawning of modern industry, silica is a more recent introduction and is successfully used in many industrial sectors.
Carbon black, in fact, proves very useful in reinforcing rubber but dynamic properties far from impeccable are the definite minus. These can be improved with the use of silica, able to reduce rolling resistance compared to the sole use of carbon black. Silica improves the elastic hysteresis of the rubber used in making tires.
We can think of hysteresis as a gauge of a tire’s internal friction, a way to dissipate the elastic energy stored within the material. If compressed, a steel coil returns, once released, practically all its elastic energy at once, rubber, on the other hand, releases less Energy having a much higher hysteresis.
The rolling motion of a tire involves large low-frequency deformations (a 195/50 R15 for example rotates at about 1,000 rpm when travelling at 109 km/h), connected to the flexibility of the sidewalls around the contact area. The tread’s micro deformations as the tire travels over a rough surface, absolutely vital in ensuring adequate grip, has a much higher frequency. A tire should ideally have a low hysteresis at low frequency, to reduce the energy dissipation – and friction - connected to the rolling motion. Hysteresis should instead be higher at high frequencies as this would guarantee higher grip and silica-based fillers play a vital role in this process.
Fillers, though, are far from perfect as they are difficult to disperse in a compound; hence the use of nanosilica, able to mitigate this inconvenience.
Carbon nanotubes, on the other hand, seem to be potentially able to overcome these limits, given that their adhesion of the rubber to their surface is extremely high; their surface/weight ratio can reach 1,300 m2/gram compared to the about 145 of carbon black. In addition these components boast high tensile strength and high heat conductivity, which allows a higher thermal stability eliminating localized overheating; furthermore, carbon nanotubes’ superior electrical conductivity guarantee a higher discharge of static electricity.
Nanomaterials in the spotlight
Curing time is greatly reduced and the formation of bonds between elastomers is uniformed, a very useful property in the case of thick rubber objects that are thus vulcanized evenly.
The use of nanotubes goes a long way in reducing the weight of the tires, creates stronger bonds between the molecules of the elastomers and improves 2 apparently diametrically opposed characteristics such as grip and wear resistance.
In general, it can be said that nanomaterials facilitate the formation of chemical bonds and changes in the shape, structure and direction of polymeric chains.
Nanoclays, natural materials of various types, are another promising class of fillers. They are widely used to reduce air permeability but are not always very compatible with some elastomers. There are a number of treatments to overcome the problem but these make the production cycle a little more complicated.
Another nanomaterial that seemed promising was zinc oxide, able to facilitate the rubber making process and the physical, mechanical and thermal properties of compounds. Concerns on environmental issues and human health, however, have led the industry to limit its use.
Biology, the new frontier
On-going research in the field of nanomaterials brought to the discovery of a compound with a name that is a real mouthful, Polyhedral Oligomeric Silsesquioxane (Poss). These molecules are considered the smallest particles of silica and have a "cage-like" structure that can contain 8, 10 or 12 silicon atoms. The diameter is in any case less than 3 nm emphasize even more the qualities of silica.
Equally promising are fillers of biological origin, with low environmental impact and independent from oil prices.
This category includes, for example, derivatives of polysaccharides (compound sugars) such as starch, chitin, which is found, for example, in the exoskeleton of insects and cellulose and can be used in developing of bionanocomposites. These appear as promising substitutes for carbon black in reinforcing tire compounds even if high loads are involved; their production, furthermore, appears commercially feasible.
The combined effect of the infinitesimal size of these fillers and their constitution improves the bond of the polymeric matrix and the characteristics of the product.
Cellulose, chitin and starch are abundant, natural, renewable and biodegradable polymers that lend themselves to the preparation of nanofibres (cellulose and chitin) and starch nanocrystals.