Optimization of mixing conditions for silica-reinforced natural rubber tire tread compounds

The dump temperature and mixing interval between rubber, silica, and silane coupling agent for silica-filled natural rubber (NR) tire tread compounds using bis-triethoxysilylpropyl tetrasulfide (TESPT) as silane were optimized. The dump temperature turns out to be the key parameter governing the properties of the silica-filled NR compounds. The increase in viscosity of the compounds by changing the dump temperature from 100 to 150 °C indicates that inevitably some cross-linking of NR occurs by sulfur contained in TESPT, simultaneous with the silanization reaction between silica and silane. However, the viscosity decreases again when dump temperatures above 150 °C are applied, indicating a dominant occurrence of degradation of the NR molecules. The results are in good agreement with bound rubber contents. The overall properties indicate that a dump temperature in the range of 135–150 °C and a silica–silane–rubber mixing interval of 10 min are the most appropriate mixing conditions for silica-filled NR compounds with TESPT as coupling agent.