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09/09/2014
IT IS EASY TO SAY RUBBER...

Compound materials

 


The features that make up the tread and sidewalls of a tire depend largely on the fillers being used, their type and size


Massimo Clarke

When we talk about compounds, we generally refer to elastomers used in making a tire’s tread, the mixture of “rubbers”, and not what is added to them. Yet, fillers present within a compound can make up to 40% of the finished product, and their contribution is absolutely fundamental. Then there are additives: coagulants, antioxidants, plasticizers, anti-ozonants, waxes, mineral oils etc. And, finally, products necessary for quick curing and the correct cross-linking of polymer chains: sulfur, activators and accelerators.

 

The final characteristics of a compound depend not only on the quantity of elastomers present, and from the type of curing chosen, but also by the fillers present in the compound. Talking about the latter, it is important to consider, besides the different kinds being used, the structures and particles that form them. It must be observed that tire compound can differ from traditional composite material, reinforced with fibers or granules, since these interact chemically with the matrix (formed by a mixture of elastomer molecules) and even with other particles.

 

In fact, these particles or fibers connect to each other in forming a space frame structure that literally absorbs a certain quantity of elastomers. In other words, polymer chains are literally trapped by an aggregate formed by particles of filler. These physical connections, added to surface absorption, are further influenced by the Van der Waals forces. Another important feature is the relative permeability of fillers by polymers, and the ratio between exposed surface (available to bond with an elastomer) and filler mass. Smaller particles have an advantage in this regard (not as far as bonding with the matrix, though). On average, the most common fillers have a superficial area between 10 to 250 m2/g. Not to be overlooked is the structure of the particles.

 

For many decades, carbon black has been the most widely used filler, and as technology evolved, a number of different kinds have been developed. Produced by the incomplete combustion of crude oil, while very little use is made of production techniques involving the use of natural gas.

 

Depending on particle size, in relation to the production mode (fundamental in determining the size), carbon blacks are divided into several “series”. They can be of different types, each of which contributes to a greater or lesser extent in determining certain characteristics of the final compound.  

 

As far as the effects resulting from adding this filler, as the quantity of carbon black increases, so does also hardness, hysteresis and traction resistance of the tire, that becomes more rigid and increases its resistance to wear. The primary particles used in the tire industry generally have sizes ranging between 10 and 200 nm, corresponding to 0,01 – 0,2 micron. These are extremely small sizes, but then we are talking about soot. These particles are formed by carbon crystallites, which unite forming aggregates of carbon black.

 

During the last years of the twentieth century, another filler became as popular as carbon black in the industry, gaining widespread recognition due to its excellent characteristics widely used in producing tire compounds. It is amorphous silica. The use of this filler improves resistance to abrasion, heat and damages as well as to impacts, while improving the hardness and rigidity of the compound.

These remarkable qualities are comparable, for the most part, to what can be obtained through carbon black (although we must point out that with the same polymer different results can be obtained using different fillers!). Practically speaking, as far as motorists are concerned, the presence of a certain amount of silica in the compound can produce an appreciable reduction in rolling resistance (consequently of fuel consumption) and an improvement in grip on wet roads and at low temperatures.

 

To embed the silica in the matrix formed by polymers, it is necessary to "activate" it using silanes, which cover the filler particles and put them in a position to stick to the chains of elastomers. Silanes, which are to silica what hydrocarbons are to carbon, have proved advantageous also for the reactions that take place during tire curing.

Silica is silica oxide (Si O2), and in this case is generally obtained by a precipitation process, starting from sodium silicate. The factors that determine the characteristics of the final product are the temperature at which the process is carried out, the time of precipitation and the degree of acidity of the starting solution. In tire compounds, silica is normally used in conjunction with carbon black.

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