The challenges of silica-silane reinforcement of natural rubber

In recent years, highly-dispersible silica has become the preferred alternative to carbon-black as reinforcing filler for low rolling-resistance tires. However, the application of this filler system is so far limited to passenger car tires, as their treads contain styrene butadiene rubber (SBR). In contrast to this, truck tires are mainly made from natural rubber (NR); this is the main application of the currently used 11 million tons of natural rubber. unfortunately, the combination of NR with silica and a coupling agent remains a challenge. Natural rubber is a durable, natural resource, but has the disadvantage of containing a variety of non-rubber components such as proteins. An in-rubber study of the interaction of silica with proteins present in natural rubber shows that the latter compete with the coupling agent during the silanization reaction; the presence of proteins makes the silane less efficient for improving dispersion and fillerpolymer coupling, and thus negatively influences the final properties of the rubber material. Furthermore, the protein content influences the rheological properties as well as filler-filler and filler-polymer interactions. Stress strain properties also vary with protein content, as do dynamic properties. With high amounts of proteins present in NR, the interactions between proteins and silica are able to disrupt the silica-silica network and improve silica dispersion. High amounts of proteins reduce the thermal sensitivity of the filler-polymer network formation. The effect of proteins is most pronounced when no silane is used; however, proteins are not able to replace a coupling agent. In order to achieve a good balance of properties, the presence of a coupling agent is essential. Keywords: silica, silane, natural rubber, protein, rolling resistance.