9 - Natural rubber and epoxidized natural rubber in combination with silica fillers for low rolling resistance tires

Abstract

Introducing silica into tire tread rubber compounds offers at least two advantages: a reduction in heat build-up as well as an improvement in mechanical properties, in particular tear strength, cut, chip, and chunking resistance, when compared with the use of carbon black. Silica itself gives a lower degree of reinforcement when compared to carbon black of the same primary particle size due to the different nature of the surface chemistry of the fillers. In general, silica can reinforce better in more polar rubbers when compared to nonpolar rubbers due to stronger silica-to-rubber interactions. The insufficient reinforcement efficiency of silica-filled nonpolar rubbers—general purpose rubbers, for example, natural rubber (NR)—can be improved by using silane coupling agents. The quality of silica-filled rubber compounds significantly depends upon mixing conditions and key ingredients in the compounds. These parameters are crucial and need to be optimized to obtain desirable hydrophobation and micro-dispersion of silica as well as appropriate filler–rubber and rubber–rubber networks in the compounds, leading to the optimum properties of the final vulcanizates. Epoxidized NR (ENR) is often quoted to be suitable for easy-dispersion precipitated silica-reinforced passenger car tire treads compounds, without the need to use a silane coupling agent. The epoxy-moieties on the ENR backbone enable a chemical reaction with the silanol groups on the silica, thereby creating a solid chemical bond between rubber and silica, similar to what the silane coupling agent can achieve. There are several approaches to this technology, each with their own merits and potential success: (1) The full use of ENR instead of NR, whereby the degree of epoxidation of the ENR plays an important dual role: first, there should be a high enough degree of epoxidation to guarantee sufficient reactivity toward the silica; on the opposite side the epoxidation raises the glass transition temperature of the NR, which tends to become too high for tire applications; (2) The use of ENR in small quantities as compatibilizer and/or reactive species between the silica and otherwise pristine NR as the main component. This enables to use ENR with higher epoxide contents without major adverse effects on the glass transition temperature; (3) Epoxidized low molecular weight NR has a potential to improve both processing and vulcanizate properties, due to its plasticizing effect and epoxide–silica interactions; and (4) In all three cases the use of small amounts of silane coupling agents relative to the quantities used for pure NR–silica compounds helps to overcome eventual shortcomings. The present chapter provides a review of the developments over time, the advantages and disadvantages of the various approaches, highlighted on the basis of laboratory-scale compounding and characterizations, like Mooney viscosities, Payne effects, dynamic mechanical analyses, and vulcanized tensile mechanical properties.