Silica-reinforced natural rubber : use of natural rubber grafted with chemical functionalities as compatibilizer

Epoxidized natural rubber (ENR) is a modified form of NR bearing polar epoxy groups. This work investigates a silica-reinforced NR tire tread compounds with ENR as a compatibilizer. The ENRs consisting of 10, 38 and 51 mol% epoxide are used in a range of 2.5 to 15.0 parts per hundred parts of rubber (phr). The use of ENR as compatibilizer improves the filler-rubber interaction, silica dispersion and tensile strength. Increase of the ENR content and mole% of epoxide groups decreases the filler-filler interaction and improves silica dispersion, as reflected in a lower Mooney viscosity, Payne effect, flocculation rate constant and filler networking factor. With respect to tire performance, the presence of ENR-38 and ENR-51 as compatibilizer improves wet grip but negatively affects rolling resistance due to an increased glass transition temperature of the epoxidized NR. The addition of ENR-51 at 7.5 phr shows the best overall properties, however still somewhat lower when compared to a compound with the commonly employed bis-(triethoxysilylpropyl)tetrasulfide (TESPT) silane as coupling agent. The use of ENR-51 at 7.5 phr with TESPT at 4 wt% relative to silica and sulfur compensation gives vulcanizates with the same levels of tensile strength compared to vulcanizates with TESPT at 8.6 wt% relative to silica. The presence of ENR in the silica-filled NR affects the dynamic mechanical properties. The addition of TESPT to the ENR-compatibilized compounds causes only a small change in glass transition temperature (Tg) and tan δ at 5oC, but lowers tan δ at 60oC. Furthermore, the addition of extra sulfur leads to an increase of Tg and lower tan δ at both 5oC and 60oC as a result of increased crosslink density. SEM micrographs clearly show good silica dispersion in all compatiblized vulcanizates, and the tensile fracture surfaces of the materials show different tear line patterns dependent on their tensile strengths.