Tire Treads, Green Tire, Precipitated Silica Versus Carbon Black, Payne Effect, Rolling Resistance, etc.

While all the elastomeric components of a tire use fillers, the tread is especially important and has the highest performance requirements. Quite complex properties are involved when one comes to consider the use of reinforcing fillers in tire treads. Notable among these are abrasion resistance, hysteresis and the related properties of heat build-up, grip (traction), noise, and rolling resistance.

Of particular interest is the relationship between rolling resistance, grip, and abrasion resistance. This is commonly referred to as the magic triangle (Fig. 6), with improvements in one of these usually being offset by deterioration in another. As a result, compromises usually have to be made.

Many of the published studies use dynamical mechanical analysis to predict the rolling resistance and grip of filled elastomer compounds. This is done by measuring the energy losses (tan delta) at two temperatures, usually 0 °C and 60 °C or 70 °C. This uses the temperature, strain rate equivalence referred to above, with the lower temperature being considered equivalent to high strain rates (grip), and the high temperature to low strain rates (rolling resistance). While a good approximation in most cases, this analysis is not infallible and must be treated with caution.

One of the major advances in recent years has been the development of effective passenger tire tread compounds based predominately on the use of precipitated silica as the filler rather than carbon black. As will now be explained, this change of filler,

Fig. 6 The magic triangle concept, showing the interrelationship between grip, tread wear, and rolling resistance

Table 6 Effect of improving dispersion of precipitated silica on abrasion resistance

together with the use of specialized coupling agents, allows an expansion of the “magic triangle” referred to above and a better balance to be achieved between tread wear, grip, and rolling resistance (fuel consumption) than was achieved with carbon blacks.

The potential for precipitated silicas to give lower rolling resistance than carbon blacks has been known for some time. The initial block to realizing this was the much poorer tread wear (abrasion resistance) achievable from the silica. This problem was associated with two factors. The first and most important is the relatively weak filler to elastomer bonding that silica achieves compared to carbon blacks. The second was the relatively poor dispersibility associated with the precipitated silicas available at the time. It was only when ways of overcoming these issues were found that the benefits of this filler could start to be fully realized.

The poor dispersibility was tackled by improving the production process, especially the filtration, washing, and drying steps. This led to the emergence of what has become known as easy dispersing (ED) and highly dispersing (HD) grades of precipitated silica. The contribution that improved dispersion can make to abrasion resistance is illustrated in Table 6.

The improvements in interfacial bonding that were required were realized through the development of specialized organosilane chemicals that can react with both the filler surface and, during vulcanization, with the elastomer and so strongly tie (couple) the two together. These chemicals belong to a class known as coupling agents. The first successful coupling agent introduced for this purpose was a polysulphidic tri-ethoxysilane known as TESPT. While other types have been developed since, this continues to be the dominant product today. In order to maximize the improvement in abrasion resistance, and achieve something similar to the tread life associated with carbon blacks, it is necessary to approach complete coverage of the filler surface with the coupling agent. Because of the high specific surface area of the fillers, this translates into quite high addition levels (between 5% and 10% w/w on the filler), making the coupling agent a significant factor in raw material costs.

The effectiveness of the present coupling agents is very much dependent on the type of elastomer they are used in, with solution polymerized synthetic elastomers giving better results than natural rubber or emulsion polymerized synthetics. This may be due to the presence of large amounts of surface active species able to compete with the coupling agent for the filler surface in the latter types.

Once the tread wear issue had been resolved, it was possible to tackle obtaining decreased rolling resistance, while maintaining good grip. For good grip, high hysteresis is required (i.e., the ability to dissipate energy), but the opposite is true for low rolling resistance (fuel consumption). For many years, it was believed that these two properties were inextricably linked and so a compromise had to be made. More recently, it has been established that this does not have to be so, as the conditions prevailing when grip is most required are quite different from normal driving where low rolling resistance is important. Fuel consumption is largely dominated by normal straight line travel, while grip is important when braking or cornering. The deformation conditions in these two cases are significantly different and hence it becomes possible to envisage systems where the hysteresis is maximized under one set of conditions and minimized under the other. It is this realization that has led to the evolution of new, improved tread technologies.

In order to understand how this separation can be achieved, we need to investigate the processes that lead to hysteresis, and how they can be controlled. While still a controversial subject, there are two main potential sources for filler induced hysteresis; these are filler transient structure breakdown and filler/polymer adhesion breakdown. It is now believed that the former (transient structure breakdown) plays the major role in hysteresis, while, as we have seen above, filler to polymer adhesion is mainly responsible for abrasion resistance.

The effects associated with the transient network are well illustrated by a long known phenomenon, which is called the Payne effect, and they manifest themselves as a significant change in energy loss (measured as a property known as tan delta) and elastic modulus with changing low amplitude frequency. The Payne effect can be used to compare fillers and throw some light on how they give different levels of rolling resistance. This is illustrated in Figs. 7 and 8. Figure 7 shows how tan delta (energy loss) changes with dynamic strain for similar elastomer compounds containing carbon black and precipitated silica, while Fig. 8 shows the effect of the same fillers on elastic modulus. It is apparent that the energy loss for the uncoupled precipitated silica is much less than the carbon black at low strains (under 10%) associated with normal driving conditions and peaks much later. This indicates that the precipitated silica has much a stronger filler-filler network. Precipitated silica also has the highest elastic modulus at low strains, indicating considerably more transient network formation than the carbon black. Finally, it is seen that the TESPT coupling agent treated precipitated silica has a broad shallow tan delta trace and low flat elastic modulus trace, indicating little

Fig. 7 Tan delta as a function of double strain amplitude for carbon black and precipitated silica filled tire tread compounds

Fig. 8 Elastic modulus as a function of double strain amplitude for carbon black and precipitated silica filled tire tread compounds

network formation. These features explain how precipitated silica and especially coupling agent treated precipitated silica is able to achieve lower rolling resistance than carbon black.

To summarize, precipitated silica without coupling agent has an extensive network, but this is too strong to cause significant energy losses under normal driving conditions. Carbon black, on the other hand has a less extensive network, but it is much weaker. Paradoxically to some, this means that it gives rise to greater energy losses under normal conditions. These differences explain the lower rolling resistance obtainable from precipitated silicas. Introduction of the coupling agent necessary to improve the lower abrasion resistance with precipitated silicas appears to destroy the strong network, but to form so little in the way of a weak one, that the rolling resistance advantage is maintained, although for a different reason.