Silica Versus Carbon Black in Tire Treads

The results in Table 7 give a comparison of a carbon black and silica of similar specific surface area in a typical low rolling resistance tread formulation (Solution SBR/BR blend) and provide a good illustration of the effects discussed in the previous section. It is seen that, even without coupling agent, the silica gives lower tan delta at the higher temperature, but markedly worse abrasion resistance. The use of silane coupling agent brings the abrasion resistance close to that of the carbon black, without any loss in the tan delta properties. (NB the laboratory abrasion and tan delta tests are only a guide to actual rolling resistance, grip, and road-wear, which themselves vary according to the severity of the test conditions. It is generally accepted that silica plus silane, while about equal to carbon blacks under modest road-wear conditions, are still inferior at the highest severity).

As expected, the use of TESPT markedly improves tread wear due to the strong coupling and better dispersion. Fortunately, the destruction of the filler network does not have any negative effect on the dynamic properties discussed above, and the rolling resistance and wet grip advantages are retained.

Despite the better balance of properties obtainable, the precipitated silica/silane technology has some significant issues. These are partly around raw material costs, but mostly around processing. Reaction of the coupling agent with the filler is most commonly achieved during the compounding process and as a result it is necessary to control mixing conditions very carefully, if the silane is to react efficiently with the filler surface without the sulfide group entering into premature reaction with the polymer (a process known as scorch). Prereacted fillers are now becoming available, but have made little impact so far. The increased hardness of silica also leads to faster machine wear.

Not unexpectedly, the success of precipitated silica in penetrating the tread market spurred the carbon black producers into upgrading their own products and significant advances have been made, reducing but not eliminating their deficiency in rolling resistance. Typical of these products are nanostructured and inversion carbon blacks. Work is continuing to further upgrade carbon black performance.

As might be expected from the size of the market and the cost of the present system, there is considerable interest in finding alternatives to the silica/silane combination. Most of this is proprietary, but it has been reported that fillers including starch, conventional and nanoclays, and additives like Kevlar are showing promise.

Table 7 Comparative results for carbon black and silica fillers in a tire tread formulation